Introduction to Psychological Science: Integrating Behavioral, Neuroscience and Evolutionary Perspectives - William J. Ray 2021
The Neuroscience of Behavior and Experience
✵ 3.1 Describe the basic elements of the nervous system and their connection to behavior and experience.
✵ 3.2 Describe the development and evolution of the brain.
✵ 3.3 List the key structures of the brain.
✵ 3.4 Outline the neuroscience methods that are being used to observe activity in the brain.
✵ 3.5 Describe the role that genetics play in helping us understand our psychological processes.
In 1848, 25-year-old Phineas Gage was a railroad construction supervisor in Vermont. Part of his job was to prepare the charges to blast rocks so that the railroad tracks could be laid. After a hole was drilled in the rock, it was filled by gunpowder and sand, which was then tamped down with a long iron rod. On one occasion, Phineas Gage did not realize that the sand had not been added and began to drop the iron rod into the hole. As the rod went into the rock, a spark ignited the gunpowder and it exploded, sending the fine-pointed 13-pound iron rod through his face, skull, and brain ( Figure 3-1 ). The rod landed some 30 feet away.
Figure 3-1 Depiction of the rod in Phineas Gage’s brain.
After being momentarily stunned, Gage amazingly regained full consciousness. He was taken back to his boarding house by cart and was even able to walk upstairs with help. He was even able to joke and laugh with the local doctor who was treating him. Soon after that he became unconscious and stayed that way for two weeks. Surprisingly, over time Phineas Gage was able to recover from his physical injuries. He continued to be able to speak and perform everyday motor processes. His intelligence and ability to learn new information remained as before the accident.
However, his personality showed such a drastic change that his coworkers said he was “no longer Gage.” Whereas he was a mild-mannered person before the accident, afterward he was prone to angry outbursts. He would make a plan but then change his mind. A person who previously had a good sense of money management, he now lacked this ability. Gage also started ignoring social conventions and frequently used profanity. The accident had clearly influenced his emotional processing.
This case study, as described by his physician, Dr. Harlow, has helped scientists understand how brain damage can influence social and emotional processes seen in other types of mental disorders (Harlow, 1868). This knowledge has helped later scientists to consider which areas of the brain might be involved in mental disorders that show deficits in social and emotional processing. The importance of this case has also prompted a number of modern reexaminations of the evidence concerning Phineas Gage (Damasio, Grabowski, Frank, Galaburda, & Damasio, 1994; Van Horn, Irimia, Torgerson, Chambers, Kikinis, & Toga, 2012). The iron rod and Gage’s skull now reside at the Medical Museum at Harvard University.
How Does the Nervous System Enable Behavior and Experience?
Sir Charles Sherrington, the 1930s Nobel Prize winner for his groundbreaking work on reflexes, suggested that we view the brain as an enchanted loom. He was referring to how the cells in the brain become active and form patterns, only to change these patterns as new processes become important. This metaphor helps us to realize that the brain is never constant but always changing. In fact, Sherrington saw these patterns as a dance (Figure 3-2).
Figure 3-2 The dance of the neurons—sometimes one is more active than another.
Every experienced feeling or thought involves different aspects of our nervous system. The same is true of behavior. For example, we make predictions as to how to catch a ball in the air or when to swerve the car to miss a pothole on the road. This happens when our nervous system, composed of networks of neurons that convey information throughout our bodies, is activated. We have also come to the realization that our brain not only processes information but also makes predictions about what will happen next (Allen & Friston, 2018; Friston, 2018).
Our nervous system can be divided into two major components (Figure 3-3). The first division is the central nervous system (CNS), which is composed of the brain and spinal cord. The second division is the peripheral nervous system (PNS). The peripheral nervous system includes the somatic nervous system, which sends and receives information to and from the brain. This system allows us to pick up a water bottle, find our keys in a backpack, or go for a run. This system also permits our brains to send information to and receive information from internal organs. For example, when we see a bear in the woods, our heart rate speeds up as we prepare to avoid danger. The chapter on stress and health will describe the peripheral nervous system (PNS) and how the body reacts to fear and stress, while this chapter will focus on the central nervous system and its major component, the brain. Many of the topics covered in this chapter, such as parts of the brain, neurotransmitters, and the function of neurons, are available in two-minute videos (www.neurochallenged.com).
Figure 3-3 Divisions of the nervous system.
Santiago Ramón y Cajal (1852—1934), often considered the father of modern neuroscience, was the first to theorize that the nervous system is made up of individual cells. He identified these cells as separate components of the nervous system. These specialized cells are neurons, single nerve cells that can transmit information to other neurons. Neurons are central to all brain processes, and are the basis for the brain’s communication with the rest of the body.
Neurons—all 86 billion of them—play important roles in learning as well as in transferring information in the brain (Azevedo et al., 2009). If you turn your head, specific neurons are active in your brain. If you feel excited, other neurons are active. Even the act of remembering the fact that there are 86 billion neurons in your brain involves another set of neurons.
Amazingly, the neuron has remained the basic building block of organisms over millions and even billions of years of evolution. Almost all living types of organisms, except some like the sponge or slime mold, have neurons. Neurons are found in the nervous systems of animals ranging from insects to humans. In general, humans have 100,000 neurons in a cubic millimeter of the brain, which is one millimeter on each side (Fried, Rutishauser, Cerf, & Kreiman, 2014).
Figure 3-4 Scale showing size of a millimeter. In a box with a millimeter on each side, there can be 100,000 neurons.
Although neurons come in a variety of sizes and shapes, they share some basic characteristics (see Figure 3-5). The cell body contains a nucleus, which includes deoxyribonucleic acid (DNA) and other substances, and also mitochondria that are involved in supplying energy. The axon (from the Greek word for axle) comes from the cell body, or soma, and is involved in conveying information to other cells. Axons can be fairly short—as found in the human brain—or four or five feet in length, such as those that go from the spinal cord to your arms and legs. There are axons in blue whales that are half the length of a basketball court. Bundles of axons are the nerves we refer to when discussing nerve damage in the arms or legs. The dendrites (from the Greek word for tree) are attached to the soma and receive information from other cells.
Figure 3-5 Structure of a neuron. The basic parts of the neuron are the dendrites that connect to the cell body, the axon, and the terminals off of the axon.
We can also describe neurons in terms of their function.
✵ Motor neurons are involved in moving our muscles, which allows us to walk, throw a ball, or send a text.
✵ Sensory neurons help us to see, feel, and hear the world. Other senses, such as taste and smell, also involve sensory neurons. These neurons send information to the brain that allows us to experience the world. Motor and sensory neurons transfer information to the brain through the spinal cord.
✵ Finally, neurons that relay information from other neurons are referred to as interneurons, which create circuits to process information in the brain. Most of our actions, such as picking up and drinking a cup of coffee, use all three types of neurons. We look at the cup and through our hands determine how much force to use as we pick up the cup. We then taste the coffee through our taste buds and smell.
Besides neurons, another component of the brain is glia. Glia cells are different from neurons. Glia cells outnumber neurons by two to ten times in the brains of humans. During the development of the human brain, glia cells are involved in directing neurons to their location and connections. Although the role of glia cells in cognitive processes has been largely ignored, new research suggests that glia does play a part (Bilbo & Stevens, 2017; Koob, 2009; Perea, Sur, & Araque, 2014). Glia cells influence how neurons interact with one another. They also form networks that communicate differently than neurons. A third component of the brain is arteries and vesicles that are part of the blood system in the brain. Glucose and oxygen are components of blood and supply energy to the brain. As you will see, being able to measure blood flow in specific areas of the brain can give us insight into how the brain works.
The dendrites receive information from other neurons, which end at different locations on the dendrites. Although illustrations in textbooks usually show only a few connections between neurons, there are generally thousands of these connections. The terminal branche, also referred to as axon terminals or terminal buttons, from these other neurons do not actually touch but make a biochemical connection through a small gap filled with fluid. That small gap filled with fluid is referred to as a synapse. These biochemical connections can release molecules that influence the transmission of information. Synaptic transmission can either be influenced by chemical agents called neurotransmitters or electrical ions to convey information.
A neuron is surrounded by a membrane, which separates it from the fluid in which it exists. In the walls of the membrane are small pores called channels. These channels can open and close depending on the internal and external chemical structure. Inside the neuron there is a high concentration of ions of potassium (K+) and other substances. In the fluid surrounding the cell are ions of sodium (Na+) and others. The electrochemical nature of the inside and outside of the membrane creates a battery-like situation with a resting potential. The resting neuron has an electrical difference of about -50 to -70 mV (millivolts) between itself and the fluid that surrounds it. That is, when the neuron is negative, it is in a resting state. When sufficient information arrives at the dendrites and results in an electrochemical change, then, at a critical point, the neuron becomes positive and an electrical potential is created.
As more of these electrical changes add together, the size of the electrical potential is increased. At a critical point, an electrical signal—called an action potential—is produced at a location near the cell body. The brain receives, analyzes, and conveys information by using action potentials. When you see something, hear something, feel something, and even smell something, action potentials are involved. Regardless of the stimuli, each action potential is the same. What determines the experience that you have is the pathway involved. Visual pathways result in seeing, auditory pathways result in hearing, and so forth.
Figure 3-6 Depiction of the structures and processes of synapses.
More specifically, there is a rapid rise in sodium ion (Na+) permeability in the membrane, which results in the large positive change in the electrical potential. This is followed by a slower rise in potassium ion (K+) permeability resulting in a return to the stable electrical state. This same mechanism moves the action potential along the length of the axon. That is, small spaces along the axon referred to as the nodes of Ranvier renew the action potential as it moves along the axon.
Figure 3-7 Action potential changes over time. These changes allow information to be passed throughout the brain.
When ions are added together in this way to produce the action potential, we refer to it as excitatory. It can also happen that the charged ions from the dendrites combine in a manner that prevents the firing of the action potential. This is referred to as inhibitory.
After the electrical signal is produced, the action potential then travels quickly down the axon. It travels only in one direction. The speed at which the action potential travels down the axon depends on two factors. The first is the width of the axon. Action potentials travel faster in axons with larger diameters. And the second factor is whether the axon is covered with an insulating material called the myelin sheath. Action potentials travel faster in axons surrounded by this myelin sheath, which is made from glia cells. This helps different brain areas to communicate with one another. Without a myelin sheath, the action potential may move at the rate of 1 meter per second. With a myelin sheath, the speed is 10 to 100 times faster. Thus, an axon with a larger diameter and wrapped in myelin will have the fastest conduction times.
An action potential is referred to as an “all or none” signal. This is because it is only above a critical value that an action potential is produced. Below that critical value, no electrical activity is sent down the axon. Following the production of an action potential there is a brief period in which the cell cannot fire another action potential. This is referred to as the refractory period.
In the chemical synapse, neurotransmitters play a critical role (Spitzer, 2017). Neurotransmitters are chemical molecules of a specific shape that influence how information is transmitted from one neuron to another. Neurotransmitters can transmit signals from one neuron to another and in this way are excitatory. Neurotransmitters can also decrease the information flow, and in this way are inhibitory. Most neurons utilize more than one type of neurotransmitter for their functioning, and the amount of time each neurotransmitter has influence depends on the neurotransmitter. To date, more than 100 different neurotransmitters have been identified. Many of the medications used to treat psychological disorders, such as anxiety, work by influencing neurotransmitters. They have been classified both in terms of structure and function.
Structure of Neurotransmitters
Classifying neurotransmitters in terms of size (Purves et al., 2008) results in two broad categories. The first type is small molecule neurotransmitters that tend to be involved in rapid synaptic functions. They are often composed of single amino acids. One of these small molecule neurotransmitters, which is excitatory, is glutamate. Glutamate is the most important neurotransmitter in terms of normal brain function. In abnormal conditions, the firing of rapid glutamate neurons can lead to seizures in a number of areas of the brain. The other is the inhibitory neurotransmitter GABA (gamma-aminobutyric acid). Drugs that increase the amount of GABA available are used to treat such disorders as anxiety. The second type of neurotransmitter in terms of size are larger protein molecules referred to as neuropeptides. These can be made up of 3 to 36 amino acids. Neuropeptides tend to be involved in slower ongoing synaptic functions. Endorphins that help your body to not experience pain are an example of neuropeptides.
Function of Neurotransmitters
Neurotransmitters can also be categorized into three broad groups by function (Nadeau et al., 2004). The first category includes those neurotransmitters such as glutamate and GABA, which mediate communication between neurons. The second category includes neurotransmitters such as opioid peptides in the pain system that influences the communication of this information. And the third category includes neurotransmitters such as dopamine, adrenaline, noradrenaline, and serotonin that influence the activity of large populations of neurons. In later chapters, you will learn about the function of neurotransmitters in greater detail. For example, when you expect something pleasant to happen such as experiences related to food, sex, or drugs, dopamine is involved. If, however, you were to see a bear, then adrenaline would prepare you to quickly run.
The Synaptic Gap
Passing information from one neuron to another involves a number of steps. The first step is that neurotransmitters need to be created and stored. When the conditions described previously occur, an action potential travels down the axon to the terminal. This allows a neurotransmitter to be released into the synaptic gap between the two neurons. The neurotransmitter then binds with specific proteins in the next neuron. This either increases (excitatory) or decreases (inhibitory) the possibility that the next neuron will create an action potential. Afterwards, the gap between the two neurons must be made neutral by a variety of mechanisms including making the neurotransmitter inactive, having the neurotransmitter taken up by the first neuron (referred to as reuptake), and removing the neurotransmitter from the gap between the two neurons.
Table 3-1 Some representative neurotransmitters.
Involved in mood, sleep, arousal, aggression, depression, obsessive-
compulsive disorder, and alcoholism.
Contributes to movement control and promotes reinforcing effects
of food, sex, and abused drugs; involved in schizophrenia and
A hormone released during stress. Functions as a neurotransmitter
in the brain to increase arousal and attentiveness to events in the
environment; involved in depression.
A stress hormone related to norepinephrine; plays a minor role as a
neurotransmitter in the brain.
The principal excitatory neurotransmitter in the brain and spinal
cord. Vitally involved in learning and implicated in schizophrenia.
Gamma-aminobutyric acid (GABA)
The predominant inhibitory neurotransmitter. Its receptors respond
to alcohol and the class of tranquilizers called benzodiazepines.
Deficiency in GABA or receptors is one cause of epilepsy.
Neuromodulators that reduce pain and enhance reinforcement.
Figure 3-8 The role of neurotransmitters in synaptic processes. Neurotransmitters can increase activity of the neuron or decrease activity depending on the neurotransmitter in the space between two neurons.
Action at the synaptic gap determines how you experience the world. As you will see in later chapters, touching your finger, hearing a sound, or having light come into your eyes results in specific types of receptors setting up the conditions for the generation of action potentials. These action potentials result in information being passed from neuron to neuron. This allows your brain to create the sensations you experience as reality.
Some of these experiences involve a pathway using only a few neurons. Being startled by a loud sound or touching a hot stove are examples of processes that use short pathways. More voluntary and complex processes such as reading this page and examining the diagrams use a much longer series of neuronal connections.
Frequency determines how action potentials encode information. That is, a loud sound would be encoded by a series of action potentials from the cells sensitive to sound intensity. A soft sound would result in fewer action potentials being fired. When observed in relation to a stimulus, action potentials are also referred to as spikes, and a number of spikes over time are referred to as spike trains. Understanding the nature of spikes and how they relate to information in the brain has been an important question since the beginning of the 20th century when they were first recorded (see Rieke, Warland, van Steveninck, & Bialek, 1999 for an overview).
The early work on spike trains was performed at the University of Cambridge in the UK by Lord Edgar Adrian (1889—1977) who won the Nobel Prize for his work on the function of neurons (Adrian, 1928). This work led to three important observations. First, although there are a variety of sensory systems (for example, vision, audition, and touch), the neurons connected to them all produce similar action potentials to external stimuli. This universality is seen across a variety of species. Second, the rate of spiking increases as the stimulus becomes larger. This can serve as a measure of intensity. And, third, if a given stimulus is continued for a long period of time, the spiking decreases.
As you will see in a number of other chapters in this book, action potentials and their frequency help to determine how you experience your world. An increased number of spike trains result in a greater experience of intensity in areas such as brightness and loudness. Fewer action potentials in a given period result in a less intense experience. Further, if we experience a consistent set of sounds such as a bell tower on a campus or the sounds of a train, our sensory system produces fewer action potentials over time. This results in our ignoring what was once an obvious experience.
How neurons work together is an important topic in brain research. Donald Hebb (1904—1985), like Freud before him, considered brain connections to be closely connected with previous experience (Hebb, 1949). Previous experience helps to develop connections between neurons, and the repetition of those behaviors strengthens these connections. We now know that neurons actually grow additional connections with other neurons as learning takes place. Our brains are not static. A network is simply a group of neurons that becomes active under certain conditions. Networks allow basic human processes such as learning, memory, thinking, planning, feeling, and decision making to take place (Bassett & Mattar, 2017; Ito, Hearne, Mill, Cocuzza, & Cole, 2020).
Hebb’s work suggested a link between the physical processes of neurons and psychological processes. Networks allow our brains to efficiently process information (Bassett & Sporns, 2017; Laughlin & Sejnowski, 2003; Sporns, 2011; Logothetis, 2015; Rosenberg, Finn, Scheinost, Constable, & Chun, 2017). Overall, cortical networks are influenced by experience and designed to be efficient in terms of connections between neurons in the network. This efficiency allows for less use of energy. One way energy is conserved is through not having every neuron connect with every other neuron.
How are neurons connected in a network? The answer may seem strange. Neurons are neither totally random in their connections with other neurons nor totally patterned. It appears that neurons are connected to one another in the same way that all humans on this planet are socially connected. In the 1960s, the social psychologist Stanley Milgram (Travers & Milgram, 1969) asked the question, “What is the probability that any two people randomly selected from a large population of individuals such as the United States would know each other?” They answered this question by giving an individual a letter addressed to another person somewhere in the United States. This individual was to send the letter to someone he knew who might know the other person. In turn this person was to send the letter to someone she knew who might know the person. Surprisingly, it only required five or six different people for the letter to go from the first individual to the final individual. This phenomenon has been referred to as the small world problem and later the phrase six degrees of separation became part of common language.
It turns out that neurons, like humans, can be connected to one another in similar ways. Various studies have shown that the neurons in the brain can be considered within a small world framework (Sporns, 2011). Neurons have numerous short distance local connections, which taken together, can be considered as a hub or module. From these hubs there are more long-distance connections to other hubs.
Local hubs can be made up of neurons that connect with one another over very short distances. Such connections are seen in gray matter. Gray matter appears dark and can be seen in the outer shell of the brain. Gray matter contains the synapses, dendrites, cell bodies, and local axons of neurons. Some 60% of gray matter is composed of axons and dendrites. Underlying this are the axons that transfer information throughout the brain. Their myelin sheaths are lighter in color and thus these areas are referred to as white matter. Myelin is made up of fats and proteins and wrap around axons like insulation does around electrical cables and results in an increased speed in information transmissions. About 44% of the human brain is white matter. White matter generally represents longer connections between neurons. This allows for cortical networks over larger areas of the brain.
The Brain’s Default Network
What does your brain do when you are just sitting and waiting or daydreaming or talking to yourself? This is a question that is just now beginning to be explored. In psychology, most of the research you read about involves a person doing something. Reacting to emotional pictures or solving cognitive problems are common examples. In these cases, one’s attention is focused on a task in the external world.
Different networks of your brain are involved when you perform different types of tasks. In the same way that the brain is organized to process spatial and verbal material differently and involve different cortical networks, it also appears that different circuits are involved with internal versus external information. A variety of studies have examined brain-imaging procedures in which individuals performed internal tasks versus external tasks. However, we all know that even without an external task to do, our mind is constantly working. It jumps from one thought to another. William James called this process the stream of consciousness (James, 1890). Recent researchers refer to this process as mind wandering.
Those neural networks that are active during internal processing have come to be referred to as the brain’s default or intrinsic network (see Bucker, Andrews-Hanna, & Schacter, 2008; Brucker & DiNicola, 2019; Raichle, 2015 for overviews). The default network is separate from, but can be understood as similar to, other networks such as those involved in visual perception or motor activities. It is made up of a set of interacting brain regions that include the medial prefrontal cortex, the posterior cingulate cortex, and the inferior parietal lobe. The default network can be seen in brain imaging when individuals are not engaged in any active task. Engaging in an active task reduces the activity of the default network.
Different Networks Are Involved in Different Tasks
In addition to the default network, the executive and salience networks have been identified (Menon, 2011). The central executive network is involved in performing such tasks as planning, goal setting, directing attention, performing, inhibiting the management of actions, and the coding of representations in working memory (see Eisenberg & Berman, 2010 for an overview). These are sometimes referred to as frontal lobe tasks since damage to the frontal areas of the brain compromise performance in these tasks. These tasks are also referred to as executive functions since they are involved in planning, understanding new situations, and having cognitive flexibility. The salience network, as the name implies, is involved in monitoring and noting important changes in biological and cognitive systems. That is, a process that is salient is something that stands out. There are also other networks involved with vision, audition, and other sensory experiences.
Hormones and the Endocrine System
Neurons and neurotransmitters are not the only chemical communications that can affect behavior and experience. The endocrine system is a system of glands located throughout our body. The pituitary gland is a pea-sized gland located at the base of the brain, just below the hypothalamus. The hypothalamus controls the glandular system by affecting the pituitary. The pituitary gland is sometimes called the master gland since it affects other glands of the endocrine system (Figure 3-9). The endocrine glands release biochemical substances into the bloodstream called hormones that change the physiology and behavior of specific organs as they move throughout the body. These hormones can stimulate our immune system, influence male and female sex characteristics, affect blood sugar levels, and perform a number of other functions.
Figure 3-9 Glands of the endocrine system that secrete hormones into our blood.
Hormones have a direct effect on our experiences and behaviors. This includes our basic human functions such as eating, emotional responses, including anxiety and depression, sexuality, and the use of energy. They can affect how well we are able to flee from what is perceived as a dangerous situation, to how we experience closeness with our family members. In fact, hormones and neurotransmitters are very similar. If the biochemical substance is released into the gap between neurons in the brain, it is referred to as a neurotransmitter. If it is released into the blood supply, it is referred to as a hormone. Neurotransmitters tend to be fast acting as they facilitate or inhibit the action potential and thus process information in the brain. In contrast, hormones take longer to produce effects. Changes produced by hormones are in the time frame of seconds to days, depending on the particular hormone involved. Hormones and the endocrine system will be discussed further in the chapter covering stress.
What Are the Parts of the Brain?
Neuroscientists share a common conviction—there is something unusual about the human brain that leads to our abilities to perform a variety of tasks (Northcutt & Kaas, 1995; Preuss & Kaas, 1999). The human brain contains billions of neurons and more than 100,000 kilometers of interconnections (Azevedo et al., 2009; Hofman, 2001). It is estimated that in mammals, a given neuron would directly connect to at least 500 and at times thousands of other neurons. This suggests that there are at least 50 trillion or more different connections in the human brain.
Regardless of how exact this estimate may be, we still come away with the conclusion that the human brain is extremely complex. Let’s begin with some simple terms. Structures closer to the front of the brain are referred to as anterior, whereas those closer to the back are called posterior. You will also see the Latin terms rostral and caudal used in the same way. You can remember that the speaker’s rostrum is in the front of the room if you want a way to remember this difference.
The brain appears symmetrical from the top with left and right hemispheres. Structures closer to the midline dividing the left and right hemispheres are referred to as medial, whereas those farther away from the midline are called lateral.
Looking at the left hemisphere from the side, we can describe four lobes of the brain (see Figure 3-10).
Figure 3-10 The left hemisphere of the brain from the side.
The frontal lobe, located at the front of the cortex, is involved in planning, higher order cognitive processes such as thinking and problem solving, as well as moral and social judgments. Scientists also refer to the frontal part of the frontal lobe as the prefrontal cortex (PFC). The beginning of this chapter told the story of Phineas Gage, whose frontal lobe was damaged by the iron rod. After that, he had difficulties following a plan and making appropriate social decisions. There is a cavity referred to as the central sulcus that separates the frontal lobe from the parietal lobe. In the area of the frontal lobe along the central sulcus is a strip of cortex referred to as the motor strip. Different parts of this strip correspond to movements of different parts of the body. As can be seen in Figure 3-11, larger parts of the motor strips are involved in movement of the hands and face than other parts of the body.
Figure 3-11 Cerebral cortex and associated body regions. The size of the body part in the figure represents the brain area devoted to that part.
The parietal lobe, which is toward the back and at the top of the cortex, is involved in spatial processes such as knowing where you are in space and performing spatial problems. Behind the central sulcus is a strip of cortex in the parietal lobe referred to as the somatosensory cortex (see Figure 3-11). It is this strip that is associated with receiving sensations from various parts of the body. Thus, you experience the feel of your coffee cup through the somatosensory area and then use the corresponding motor process to pick it up and drink from it. Like the motor strip, the size of the area involved in sensations from various parts of the body reflects the sensitivity of that body part. Our hands and face are much more sensitive than our back, and these areas have more cortical representation on the somatosensory cortex than does the back.
✵ The occipital lobe is located near the back of the brain and toward the bottom. The occipital lobe is involved with the processing of visual information and receives information from our eyes, which you will learn about in the chapter on sensation and perception.
✵ Below the frontal and parietal lobes is the temporal lobe. Looking at the brain, you can see that the frontal and temporal lobes are separated by a deep groove that is called the Sylvian fissure. The temporal lobe receives information from our ears and is involved in hearing as well as aspects of language. Other parts of the temporal lobe are involved in the naming of objects from visual information processed in the occipital lobes.
The Limbic System
During the 1930s, James Papez suggested that a particular brain network was associated with emotional experience and expression (Papez, 1937). Later, Paul MacLean introduced the term limbic, which comes from the Latin limbus, which means border. Papez believed that in the same way that the occipital lobe processes visual information, the limbic system processes emotional processes. Today, scientists see this system involved in emotion, motivation, memory, and other related processes.
Figure 3-12 Structures and areas in the limbic system. The limbic system is associated with emotional processing.
Initially, Papez saw the limbic system being composed of parts of the thalamus, the hypothalamus, the cingulate gyrus, and the hippocampus. The thalamus functions as a relay center between subcortical structures and the cortex. Except for the experience of smell, information from all sensory systems passes through the thalamus. The hypothalamus is involved in a number of metabolic processes including hormones that influence organs throughout the body. Responses to stress and danger involve the hypothalamus. It is also involved in a variety of basic processes such as hunger, thirst, and temperature control. Neurons in the hypothalamus become active when you are thirsty or when glucose levels are low. The cingulate gyrus is involved in a variety of emotional and motivational processes including pain and cognitive control. The hippocampus is a critical part of our memory system. You can actually see changes in the hippocampus if you learn to navigate a complex town, as described in the box: The World Is Your Laboratory—Changing the Brain through Experience. Additionally, individuals with smaller hippocampi are more at risk for post-traumatic stress disorder (PTSD), and those with larger structures are shown to recover faster (Apfel et al., 2011).
Later formulations of the limbic system include the amygdala. The term “amygdala” comes from the Latin word for “almond,” which reflects its shape. Initial brain-imaging studies suggested that the amygdala was involved in the experience of negative emotions such as fear or anger including aggression. The amygdala is also involved in stress related disorders such as post-traumatic stress disorder (PTSD). In individuals with PTSD, a mildly fearful stimulus produces an overreactive amygdala.
Additional research has shown the amygdala to be active whenever any emotional information that is related to one’s self is processed. Neuronal connections within the amygdala and its connections to the hippocampus may also be changed in situations of severe stress (Roozendaal, McEwen, & Chattarji, 2009). In this manner, humans would have a heightened memory of severe stress situations. The manner in which these changes relate to the development of anxiety disorders is being studied.
The World Is Your Laboratory—Changing the Brain through Experience
Eric Kandel set out to understand how learning and memory were represented in the brain. For his discoveries, he won the Nobel Prize in 2000. But that is getting ahead of the story. Since no one knew how learning and memory took place in the brain, Kandel needed to choose a simple organism that could learn. For his research, he chose the California sea slug Aplysia. Although this sea slug is complicated in many ways (it can be both male and female as needed), it does have a simple reflexive behavior—the withdrawal of its gill. The gill is an external organ that the sea slug uses to breathe. Touching the gill causes it to withdraw.
It turns out that this gill reflex is similar to a number of responses in humans. For example, the first time you hear a clock tower chiming or a plane flying over your house, it seems loud. However, after a time, your response decreases as the sound is repeated. This process is referred to as habituation.
Kandel and his colleagues focused on a critical synapse that is located between the neuron that brought sensory information related to being touched and the motor neuron that caused the gill to withdraw. They discovered that with repeated stimulation, habituation took place and the sensory neuron released less of a neurotransmitter. That neurotransmitter was glutamate—the major excitatory neurotransmitter in our brain. Less of the neurotransmitter glutamate reduces the chance that an action potential will be produced.
The opposite of habituation is sensitization. For example, if you receive a painful shock, then the next shock would result in a greater response in a number of different systems. The shock could, for example, make a noise seem louder. Kandel and his colleagues found that sensitization results in a greater level of glutamate at the synapse. Thus, this increases the chance that an action potential will be produced. In other studies, Kandel and his colleagues showed that when experiences become part of long-term learning and memory, the connections at the synapse are increased. This shows that with learning the brain connections change (see Kandel, 2006 for an overview).
Does learning also change the human brain? Katherine Woollett and Eleanor Maguire at University College London decided to tackle this question. To become a London taxi driver, you must learn all of the obscure and non-structured roads of the city. As you can see from the map of London (Figure 3-13), this is not an easy task.
Figure 3-13 Map of London showing the approximately 25,000 irregular streets. How long do you think it would take you to learn these streets?
Source: Woollett and Maguire (2011).
These researchers performed brain-imaging studies of a group of volunteers at the beginning of their training as well as brain-imaging studies of a control group of non-taxi drivers. At the beginning of the study, the two groups showed no difference in the hippocampus, which is the structure of the brain involved in memory.
After three or four years, brain imaging, along with some tests of memory, was again conducted on these two groups. The researchers found that those who passed the test and became taxi drivers had a greater volume of gray matter in the hippocampus. Since gray matter reflects neurons, this suggests that new nerve cells were created in the brain as new learning was taking place. Long-term learning does change the brain (Woollett & Maguire, 2011). If you want to change your brain, learn something new.
Thought Question: What experiences do you think have changed your brain the most?
The Brain Stem
The brain stem includes the midbrain, pons, and medulla. These structures are involved in basic life functions, including breathing, digestion, levels of arousal, pleasure, and physiological processes such as heart rate and blood pressure. Pons and midbrain processes are also associated with eye movements, including the rapid eye movements (REM) seen during sleep. Posture and certain facial expressions are also related to these structures. Movements controlled by the brain stem tend to be whole body movements rather than the finer hand movements connected with neocortical control. Across species, movements related to the brain stem tend to be more of an instinctual nature, such as those seen in a startled organism.
Although not part of the brain stem, one important structure related to movement is the basal ganglia, which are located deep within the cerebral hemispheres. Parkinson’s disease, which is characterized by tremors, is associated with the degeneration of neurons in the basal ganglia. It is reduction of the neurotransmitter dopamine in the basal ganglia that is associated with Parkinson’s disease. Serotonin, another neurotransmitter, was seen in lower levels in the medulla of infants who died of sudden infant death syndrome (SIDS) (Duncan et al., 2010).
Involved in the coordination of movement, the cerebellum (little brain) is located at the base of the skull (Cerminara, Lang, Sillitoe, & Apps, 2015; Schmahmann, Guell, Stoodley, & Halko, 2019). It receives inputs from almost all areas of the cortex and allows for the smooth movement of our bodies. When the cerebellum is damaged, a person’s movements can appear jerky and uncontrolled. It is also involved in the automation of tasks such as learning to ride a bicycle, typing, or playing a musical instrument. One important aspect of cerebellar control is the timing of motor movement as would be required when you play a musical instrument.
The cerebellum, with its connections to our brain and body, allows for an intricate set of feedback processes. When you decide to shoot a basketball at a hoop, your neocortical motor areas send out messages to your arms and legs. This same information goes to the cerebellum. Once you have made the shot, feedback from limbs and the visual system goes back to the cerebellum. With this information, it is possible to correct your next shot based on the performance of the previous one.
Although it was historically thought that the cerebellum was only involved in movement, more recent research suggests that it is also involved in cognitive processes (King, Hernandez-Castillo, Poldrack, Ivry, & Diedrichsen, 2019; Wagner & Luo, 2020). For example, with its connections to the prefrontal cortex, activity is seen in the cerebellum when learning simple rules for performing a task (Balsters & Ramnani, 2011). Every day we make these types of decisions mainly outside of awareness, such as when we stop and go in relation to traffic lights. Another study performed brain imaging on individuals born in 1936. Brain imaging of these individuals in their 60s was correlated with cognitive abilities. Scanning the frontal areas and the cerebellum showed that gray matter volume in the cerebellum predicted general intelligence (Hogan et al., 2011).
The cerebellum interacts with many different parts of the brain. Pathways to and from the neocortex suggest that a closed loop system includes inputs and outputs to the cerebellum. This allows for coordinated control of motor functions such as playing a musical instrument or throwing darts. In addition to motor movements, cerebellum activity is also related to attention, executive control, language, working memory, learning, pain, emotion, and addiction (Strick, Dum, & Fiez, 2009; Wagner & Luo, 2020).
Figure 3-14 Brain stem and cerebellum.
The Spinal Cord
The hind brain is directly connected to the spinal cord. As the second structure of the central nervous system (the brain is the first) the spinal cord contains fi ber tracts within a cavity surrounded by bone through which information from all parts of the body is taken to the brain (ascending tracts). Information is also taken from the brain to the muscles and internal organs (descending tracts). Damage to the fiber tracts in the spinal cord can result in individuals being unable to move their bodies. The higher up on the spinal cord that the damage occurs, the greater the paralysis.
Besides controlling muscles, information to and from internal organs goes through the fiber tracts in the spinal cord (see Figure 3-15). Not only does the spinal cord move information to and from the brain, it is also able to produce simple reflexes. What if you touch a hot stove? You may have noticed you actually move your hand before response (withdraw your hand) is produced without involving the brain.
Figure 3-15 Spinal cord and connection to the internal organs
✵ What are the primary structures of the brain, and what is the primary function of each?
✵ What is the primary function of the limbic system? What are the five major structures that make up the limbic system, and what is the primary function of each?
✵ Research has shown that the brains of California sea slugs and London taxi drivers were changed by experience. What evidence did the researchers present to support their conclusions?
✵ What structures are part of the brain stem and what is the primary function of each?
✵ The cerebellum interacts with many different parts of the brain and the body and is critical for performing a variety of tasks. Describe four of these tasks that are dependent on the functioning of the cerebellum.
✵ How is the functioning of the spinal cord related to the brain?
How Do We Investigate the Brain?
There are a number of ways we can measure the energy used by the brain. This, in turn, helps us to understand the fascinating manner in which the brain is involved in behavior and experience. With 86 billion neurons and 50 to 200 trillion connections between neurons in the human brain, understanding these connections on a neuronal level would be an impossible task. However, scientists have been able to use the manner in which neurons work together as a window into their function.
Measuring Electrical and Magnetic Activity
Our brain is always active, even when we sleep. This constant activity includes electrical and magnetic changes, and measuring these changes allows researchers to better understand how the brain processes information and which networks are involved.
Electroencephalography (EEG) records the electrical activity of the brain at the level of the synapse (Nunez & Srinivasan, 2006). The EEG was first demonstrated in humans by the German psychiatrist Hans Berger in 1924, who initially measured EEG from his teenage children. His work was published five years later (Berger, 1929). It has been used to signal changes in sleep states as well as perceptual, cognitive, and emotional processes (Cohen, 2017). EEG is the product of the changing excitatory and inhibitory currents at the synapse. Action potentials contribute very little to the EEG. However, since changes at the synapse do influence the production of action potentials, there is an association of EEG with spike trains (Mazzoni et al., 2010; Whittingstall & Logothetis, 2009).
Since the neurons of the brain and their connections are constantly active, EEG can be measured during both wake and sleep. EEG can be measured with only two electrodes or as a high-density array of more than 200 electrodes (see Figure 3-16).
Figure 3-16 Small child wearing high-density array EEG cap with her mother.
Figure 3-17 Patterns of EEG activity ranging from high to low frequency activity.
Some aspects of the EEG may appear almost random while other fluctuations appear periodic. Using signal-processing techniques, it is possible to determine the major frequency and amplitude seen in the signal. Amplitude refers to how large the signal is, and frequency to how fast the signal cycles measured in cycles per second or Hertz (Hz). Some EEG patterns are extremely reliable and can be visually observed (as would have been required in the days before computer analysis). These patterns have been identified by Greek letters such as α (alpha), ß (beta), and θ (theta). Alpha activity in the 8—12 Hz range was the first pattern of EEG activity Hans Berger noted.
Over the years researchers have noticed that specific patterns of EEG activity are associated with a variety of psychological states. When an individual is relaxed with his or her eyes closed, high amplitude regular activity is seen in the EEG at a frequency of 8—12 Hz called Alpha activity. If the person begins to perform some mental activity such as mental arithmetic, lower amplitude EEG is seen at a higher frequency above 20 Hz, referred to as Beta activity. Delta and Theta activity can be seen during sleep.
Magnetoencephalography (MEG) uses a SQUID (superconducting quantum interference device) to detect the small amount of magnetic activity that results from the activity of neurons. As shown in Figure 3-18, the person simply puts her head in a device that contains magnetic sensors.
Figure 3-18 Woman sitting in MEG.
MEG signals are similar to EEG signals but have one important advantage. This advantage stems from the fact that magnetic fields are not distorted when they pass through the cortex and the skull. This makes it possible to be more accurate with MEG in noting where the signal arises within the brain.
In the 1980s a great step forward in brain imaging was made with the introduction of magnetic resonance imaging (MRI). MRI offers a static image of brain structure. This is accomplished by having the person lie on a horizontal bed that is moved into the scanner (see Figure 3-19). It is often used for scanning the brain, although it can be used to show any body part. The basic technique is to align the water molecules in the body with an extremely strong field. Radiofrequency current is then used to reflect the structure of the organs of the body. Figure 3-20 shows an MRI image of the brain of a person looking to your left.
Figure 3-19 Person lying in MRI.
Figure 3-20 MRI image of the brain.
By using software, it is possible to reconstruct any area of the brain. This allows researchers to determine the size of different structures in the brain. They can then compare these between groups of people or changes within a person as in the case of London taxi drivers. Often researchers will reconstruct images of the brain as if they were taking a “slice” at different distances from the front or top of the brain.
It is also possible to use the MRI to measure cortical connections in the brain, which is referred to as diffusion tensor imaging (DTI). DTI is available with most MRI imaging systems. It is a procedure for showing fiber tracts (white matter) in the brain. This information can then be visualized by color coding it, as shown in Figure 3-21. This allows the mapping of white matter connections in the brain. In this figure, the connections between different parts of the brain can be seen.
Figure 3-21 Mapping white matter connections in the brain using color coding. These represent the myelin sheath connections in the brain.
Often DTI is used to note connections in individual regions of the brain in terms of a specific research question. If you were interested in memory changes following concussion, for example, you might map the connections with the hippocampus. The brain can also be considered as a whole and the connections with all areas mapped. Analyzing the connections of the whole brain has been referred to as the connectome. That is, the connectome is a map of neural connections in the brain.
Currently, ambitious projects to understand the connections in the brain are being developed by the National Institutes of Health in the United States (www.nih.gov/science/brain) and the countries of the European Union (www.humanbrainproject.eu). Some see great potential in the connectome projects (see Seung, 2012 for an overview). Since our experiences help to determine our brain connections, even identical twins with the same genes could have different experiences and brain connections. Describing whole brain connections can be valuable not only in understanding brain differences but also in creating ways to change these connections in order to treat disorders.
One such study looked at the connectome of males and females (Ingalhalikar et al., 2014). As shown in Figure 3-22, females showed more connections between the hemispheres of the brain, whereas males showed more connections within each hemisphere. These researchers suggest that female brains are structured to facilitate communication between analytical and intuitive processing modes, whereas male brains are structured to facilitate connectivity between perception and coordinated action.
Figure 3-22 Brain networks of males (top) and females (bottom). Connections within the hemisphere are shown in blue and connections between hemispheres are shown in red.
Source: Ingalhalikar et al. (2014).
Measuring Blood Flow Changes
The human brain weighs only about 3 pounds but uses some 20% of all the energy consumed by our body (see Magistretti, 2009 for an overview). Energy is available to the brain by a complex system of blood that circulates in the brain. For the brain to function efficiently, it needs blood with oxygen and glucose. By measuring changes in blood flow it is possible to estimate which areas of the brain are involved in processing information. The two major techniques are fMRI and PET.
Functional Magnetic Resonance Imaging
In the 1990s the development of functional magnetic resonance imaging (fMRI) helped to create the field of cognitive neuroscience. Whereas MRI shows brain structure, fMRI is able to reflect brain function. With fMRI, it was now possible to measure processing in the brain as well as which brain areas are involved. fMRI is possible because blood flow increases in active areas of the cortex. That is, if running through math problems in your head, the areas of the brain related to arithmetic require energy that is supplied by the blood. Specifically, hemoglobin, which carries oxygen in the bloodstream, has different magnetic properties before and after oxygen is absorbed. Thus, by measuring the ratio of hemoglobin with and without oxygen, the fMRI is able to map changes in cortical blood and infer neuronal activity.
Measurements using fMRI are made by having a person lie on his or her back inside a device that measures changes in blood oxygen levels. Initially a structural image of the brain is created. A common procedure is then to take a baseline in which the person just relaxes. Following this, specific tasks are performed. The fMRI response recorded during the tasks is subtracted from that recorded during baseline. This shows the specific areas of the brain that are involved in performing a task. This information is then placed on the image of the brain. The color used reflects the amount of activity seen in a particular brain area. Figure 3-23 shows fMRI images reflecting activity of internal brain structures.
Figure 3-23 FMRI images—color reflects the amount of activity seen in a particular brain area.
In using fMRI, it is necessary to compare activity under one condition with activity under another condition. What if you were interested in the concept of self? You could ask if a person would react differently to his or her own face as compared to a friend’s face or to his or her own voice as compared to a friend’s voice. To answer this question, it would be important to establish how your brain reacts when it sees a picture or hears a voice. Figure 3-24 shows brain activity when a person saw a picture or heard a voice compared to a resting baseline (Kaplan, Aziz-Zadeh, Uddin, & Iacoboni, 2008).
Figure 3-24 Viewing pictures and hearing voices compared with resting baseline. The orange yellow areas represent more brain activity.
Kaplan, Aziz-Zadeh, Uddin, & Iacoboni (2008).
Positron Emission Tomography
Positron emission tomography (PET) is a technique that measures the blood flow in the brain that reflects cognitive processing. In other words, PET systems measure variations in cerebral blood flow that are correlated with brain activity. It is through blood flow that the brain obtains the oxygen and glucose from which it gets its energy. By measuring changes in blood flow in different brain areas, it is possible to infer which areas of the brain are more or less active during particular tasks. Blood flow using PET is measured after participants inhale, or are injected with a tracer (a radioactive isotope) that travels in the bloodstream and is recorded by the PET scanner (a gamma ray detector).
The general procedure is to make a measurement during a control task that is subtracted from the reading taken during an experimental task. It takes some time to make a PET reading that reduces its value in terms of measuring cognitive processes that change quickly. However, it is possible to determine specific areas of the brain that are active during slower changing types of processing. This is referred to as temporal resolution, or the speed at which brain changes can be measured. Further, since PET can measure almost any molecule that can be radioactively labeled, it can be used to answer specific questions about neurotransmitters.
Some of PET’s main disadvantages include the expense involved in creating radioactive agents and the risk involved in the injection of radioactive tracers. Also, PET has limited temporal resolution. Due to risks associated with exposure to the radioactive tracer elements in a PET study, participants typically do not participate in more than one study per year, which limits the degree to which short-term treatment changes can be studied. With the development of fMRI, PET is no longer the technique of choice for research studies. However, it does offer an advantage for studying specific receptors such as dopamine receptors in the brain, which are particularly active in those with a drug addiction or inactive in those with Parkinson’s disorder.
In summary, there are a variety of neuroscience techniques for measuring physiological changes in your body. There are advantages and disadvantages to each of these techniques. For example, measuring heart rate is easy and inexpensive. Measuring blood flow in the brain is more complicated and much more expensive. These different techniques are presented in Table 3-2.
Table 3—2 Pros and cons of different neuroscience techniques.
Reflects quick changes in the brain, inexpensive, not invasive, safe, little discomfort
Difficult to know which brain areas produced the EEG
Reflects quick changes in the brain, not invasive, safe, no discomfort
Basic equipment is expensive
MRI and fMRI
More exact location of structure and activity, safe, little discomfort
Basic equipment is expensive, cannot be used with people who have any metal in their body (heart pacemaker or metal pins), fMRI not able to measure short-term changes in the brain
Able to measure specific neurotransmitters
Basic equipment is expensive, injection of radioactive tracers limits number of scans per year, not able to measure short-term changes in the brain
There are a number of trade-offs that researchers must consider when choosing a brain-imaging technique. It begins with the research question you are asking. If you wanted to know whether the areas of the brain associated with memory, such as the hippocampus, are larger or smaller in London taxi drivers, then you would want a measure of structure. If you wanted to know whether those with autism quickly viewed different emotional faces in a different way, then you would want a measure that reflects changes in brain processes.
One important question is how fast a particular technique can measure change. This is referred to as temporal resolution. EEG and MEG, for example, can measure quick changes in the brain on the millisecond level. PET, on the other hand, can only record changes that take place in a period of a few minutes or more. Another consideration is spatial resolution—that is, what size of brain area can a technique measure? PET and fMRI are better able to pinpoint the location of activity in the brain, whereas with EEG it is less possible to know specifically where in the brain the activity came from. The relationship between spatial and temporal resolution is shown in Figure 3-25. Temporal resolution is important for using brain activity to control artificial limbs as described in the box: Applying Psychological Science—Using Brain Activity to Move Limbs.
Figure 3-25 Spatial and temporal resolution of imaging techniques.
Source: Meyer-Linderberg (2010, p. 194).
Applying Psychological Science—Using Brain Activity to Move Limbs
In the 1960s, researchers were interested in whether or not individuals could learn to discern changes in internal processes such as EEG, heart rate, and muscle potentials. One application of this research was for clinical, or treatment, purposes and was referred to as biofeedback. For example, controlling muscle potentials could aid in the treatment of headaches. Likewise, if an individual could learn to control blood flow to the hands, then disorders such as Raynaud’s disease, which produces cold and numb hands or feet, could be treated.
Another set of researchers asked if individuals could learn to control their EEG (Wolpaw & Wolpaw, 2012; McFarland & Wolpaw, 2017). It would be somewhat amazing if you could play a video game with your brain. This is exactly what they were able to do. Initially, EEG was used to move objects on a computer screen. With further advances, the person was able to make more refined moves.
Using cortical signals to control devices in this way is known as brain—machine interface (BMI). In the last 20 years, a number of research labs have sought to train individuals to control mechanical devices such as artificial limbs. For those who had lost a limb, the artificial limb could be attached to the person. People who were paralyzed could learn to control an external device that could feed the person or move objects. Researchers wanted to know which aspects of EEG, for example, should be used and how we could train someone to control them. Another question was whether we can determine how the brain signals a movement to a muscle and then use this to control an artificial limb.
In June 2014, a 29-year-old paraplegic man literally kicked off soccer’s World Cup competition in Brazil. EEG from the man’s scalp went to the external movement device on the man’s legs. Through intensive training, he learned to imagine leg movements, which produced the EEG that was then computer-analyzed to control the device that moved his legs.
Figure 3-26 Learning to move an artificial limb.
Figure 3-27 Paraplegic kicks soccer ball at 2014 World Cup.
The goal of brain—machine interface is to give individuals without the ability to control their limb that ability through their brain activity. One approach is to implant electrodes in the brain. The signals from these electrodes are processed and sent wire-lessly to the person’s limbs, which can initiate movements. Today, many individuals have implanted pacemakers that keep their heartbeat both regular and able to respond to an increase in production. Perhaps tomorrow, we will see more individuals being able to control problem limbs with brain interfaces.
Thought Question: Using a brain—machine interface to allow a man with paraplegia to kick a soccer ball to open the World Cup is truly a monumental accomplishment. If you were a researcher in this area, what task would you study that might be more significant to an individual’s day-to-day life? Why would you choose that task?
✵ Describe six types of brain-imaging techniques currently being used, and identify a purpose for which each is especially valuable.
✵ What are some of the trade-offs that researchers and clinicians must consider when choosing a brain-imaging technique? What questions help inform their decision?
✵ What is brain—machine interface (BMI)? What are some ways it has been used to extend the power of an individual’s brain to enhance functioning in his or her body?
How Does the Brain Develop and Evolve?
Our brains represent a long evolutionary history in which different structures and functions have developed at different periods of time. For example, the experience of pain is suggested to have developed before our sense of feeling social rejection. Although most animals, including humans, show similarities in their patterns of development, differences in structure and the connection between these brain structures allow different species to display different abilities. Humans, for example, show language performance not seen in other species. Overall, the development of the brain follows genetic processes, but environmental factors can play a role at any point after conception. Since humans continue to show brain development following birth, environmental factors continue to be an important component of brain development.
From Neural Tube to Brain
The central nervous system, which includes the brain and spinal cord, develops during the first month of pregnancy. At about 18 days post conception, a process begins that results in the creation of the neural tube from which the brain and spinal cord develop.
Structurally, the most anterior end of the neural tube becomes the forebrain, the mid-brain, the cerebellum, and the hindbrain. The remaining part of the tube becomes the spinal cord (see Figure 3-28). The forebrain includes the two cerebral hemispheres, the thalamus and the hypothalamus. The midbrain contains structures that have secondary roles in vision, hearing, and movement. The hindbrain is composed of the pons and the medulla. The cerebellum is located at the back and below the forebrain. These structures will be described in detail later. In general, the areas of the hindbrain are involved in sleep, arousal, and movement. The hindbrain is continuous with the spinal cord. In fact, you may not be able to tell where the hindbrain ends and the spinal cord begins. Figure 3-29 shows the development of the neural tube into the forebrain, the midbrain, the cerebellum, and the hindbrain.
Figure 3-28 Neural tube. The neural tube is the beginning of our brain and spinal cord.
Source: Allman (2000, p. 53).
Figure 3-29 The development of the brain from the neural tube. Figure shows the brain at 3 weeks after conception, 7 weeks after conception, 11 weeks after conception, and at birth.
Different parts of the tube expand differently in different animals under the control of specific genes. For example, as compared to other organisms, primates, including humans, have an extremely large neocortex that develops from the most anterior part of the neural tube. Within mammals, there are extreme differences not only in number of neurons, but also in the surface area of the neocortex. For example, the surface area of a macaque monkey brain is some 100 times that of the mouse. The human brain has about 1,000 times more surface area than the mouse and about 10 times more than the monkey (Blinkov & Glezer, 1968).
Brain Structure Evolved According to Necessity
Along with the expansion of the surface area of the brain during evolution, there was also an increase and expansion of areas involved in processing information. Humans have more than 30 areas devoted to visual processing whereas rodents have only 5. As evolution produced greater diversity in cortical structure, there remained a similarity in the manner in which the nervous system functioned. The basic unit of signal transmission is the axon and the action potential that is generated within it.
Mammals have evolved a brain structure different from other organisms—the six-layered neocortex (see Figure 3-30). Each of these layers has a distinct neurological organization and connections. The bottom layers (five and six) have neurons that project to subcortical structures. The next layer (four) contains local circuit neurons. The next two layers (two and three) have neurons that project to other cortical structures. Layer one primarily contains dendrites and almost no cell bodies. Although consistent in structure, among mammals there is great variation in cortical size and organization.
Figure 3-30 Development of the six layers of the brain over time. As we age, our brain develops a more complicated structure.
This variation is not just in size but also in area devoted to different types of cortical processing and greater network connectivity. As seen in Figure 3-31, connections between neurons increase during a child’s early life.
Figure 3-31 There is an increase in the connections between neurons during the early years of a human child’s life.
Source: Gilmore, Knickmeyer, and Gao (2018).
Neurons are also created in humans after birth. A somewhat strange part of this story is that as an adult you actually have fewer neurons than you did as an infant. This may seem surprising since neurons are some of the longest living cells in the body. However, this is a normal condition that is connected with a variety of processes. In infancy, use is important. For example, in your first year of life, you could recognize any sound used in any language around the world. However, this ability is lost if that language is not part of your environment. Likewise, if a child is born unable to hear, the normal initial babbling behaviors will be produced during the first year of life but then lost. These types of disuse and loss take place on the neuronal level. Unused neurons die. Use it or lose it is the brain’s way of being.
Separate cortical areas are devoted to processing specific types of information such as color or motion, which are later combined to give us a coherent image. It is thought that this increase in size allows for additional information processing to occur. However, humans do not have the largest brain, nor do we have a brain with the greatest number of folds or convolutions. Whales, dolphins, porpoises, and elephants all have larger brains than humans.
It should be noted that as brains become larger across species, they also tend to change in terms of internal organization (Striedter, 2005). This change in internal organization allows for new cortical networks, which have evolved to process novel capacities. Language in humans would be one such example. If a brain increases in size, there must also be an increase in energy and ways to supply that energy. Although our brain is only about 2% of our body weight, it uses some 20% of the energy. Further, as a brain evolves to solve the challenges of life, the complexity of the neuronal networks and processing times becomes limited by this very complexity and the related processing time (Hofman, 2014).
We are just beginning to understand the manner in which human brain evolution is built upon a vast history of organic development. Since our understanding is incomplete, scientists have developed large conceptual frameworks upon which to build testable hypotheses. Paul MacLean developed one of the major programs for understanding cortical and behavioral processes from an evolutionary perspective in the 20th century (MacLean, 1990). Examining fossil records along with brains from a variety of organisms, MacLean suggested that our current brain can be viewed as having the features of three basic evolutionary formations—reptiles, early mammals, and recent mammals.
MacLean’s formulation, which is referred to as the triune brain, suggests that through rich interconnections, our brains can process a variety of information in three somewhat independent, although not autonomous, ways (see Figure 3-32). MacLean (1973) emphasized that the three brains intermesh and function together.
Figure 3-32 The triune brain. Each part is different in terms of structure and function.
The first level MacLean referred to as the reptilian brain, the name of which suggests that the basic structure is similar to that found in reptiles. This level includes the brain stem and cerebellum, and processes major life requirements such as breathing, temperature regulation, and sleep—wake cycles regulated through the brain stem. This level of processing represents fairly structured behavioral patterns involving basic activities such as territoriality, courtship, and hunting, and also includes patterned displays related to these behaviors.
The second level of the triune brain is paleomammalian, which includes the limbic system and its involvement in emotional processing. The third level of the triune brain is neomammalian and is related to the neocortex and thalamic structures. As the name implies, the neocortex and its related structures is the “new brain.” This level is generally associated with problem solving, executive control, and an orientation toward the external world with an emphasis on linguistic functions. From an evolutionary perspective, it is the level of the neocortex that would be most influenced by cultural processes and new learning. It would also be at this level that processes involving self-control and self-regulation become apparent.
Although MacLean describes these three levels separately in an evolutionary sense, it is important to recognize the rich interconnections. This allows for information exchange between the structures that compose the various levels. In fact, MacLean notes the manner in which neocortical structures may be involved in regulating emotional and instinctual functions. Structurally, an interesting recent finding in neuroscience research is that there are more inhibitory pathways from the higher brain areas to the lower ones than vice versa, which would allow for more behavioral flexibility than that available to more primitive organisms. This gives us a means to inhibit more basic or primitive responses.
✵ What are the three different types of material in the brain, and what is their function?
✵ “The brain works in terms of one basic element, the neuron.” What are the different parts that form the structure of the neuron, and what roles do they play?
✵ What are the defining characteristics of neurotransmitters in terms of structure and function?
✵ What are the steps involved in a neuron passing information on to other neurons, and how is that information encoded?
✵ What are white matter and gray matter? How are they related to different types of connectivity in the brain?
✵ How is the brain’s default or intrinsic network different from the central executive and salience networks in terms of function and region of the brain?
✵ How does the central nervous system, including the brain and spinal cord, develop in the human embryo? How is the development process the same, and different, in other species?
✵ Paul MacLean proposed that our current brain can be viewed as having the features of three basic evolutionary formations—reptiles, early mammals, and recent mammals. What did he call his schema of the brain, and what were its primary characteristics?
What Is the Link Between Heredity, Behavior, and Experience?
Genetic terminology has permeated popular culture. Court cases or police investigations that reference deoxyribonucleic acid (DNA) appear daily in our newsfeeds, and forensic crime procedurals dominate the television networks. In the field of medicine, new discoveries can connect a person’s genetic makeup to certain diseases. However, in newspaper and television descriptions of genetic discoveries, the complex turning on and off of our genes is often ignored. It is easy to believe that if a person has a particular gene, then whatever activity is connected with that gene will be evident. This could be a negative experience such as cancer or alcoholism. It could also be a positive experience such as having perfect pitch or having a strong immune system. However, it is just not that simple.
Genes do not exist to make people or animals sick or to give an individual special talents. The manner in which we become sick or display talents is a complex process that involves our genes, but that also involves a variety of environmental factors. Except for blood type, genes display very few traits without a complex interaction with the environment or experience. The link between heredity, behavior, and experience or environment is not a simple one. Genes influence behaviors and behaviors can also influence gene processes. The environment we are in also plays a critical role.
In considering the role of genetic and environmental factors in behavior and experience, we can recall Darwin’s reminder that “how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life.” That is, genetic and environmental factors are mutually dependent upon one another. In fact, what you experience can determine if a gene influences your behavior (gene turns on) or not (gene turns off).
Environments can determine which genes turn on and off. The National Aeronautics and Space Administration (NASA) in 2019 released results of an astronaut who spent a year in space while his twin brother remained on Earth (https://www.nasa.gov/twins-study). Both twins were astronauts and thus had similar experiences previous to the study. Spending time in space changed the way that specific genes turned on and off (Garrett-Bakelman et al., 2019). Although the twin who spent time in space retained the basic genetic structure as the one who remained home, about 7% of his genes did turn on and off in different ways. Turning a gene on or off can influence behavior and experience. The majority of the genes that did change during the space flight returned to normal functioning within six months after returning to Earth. Thus, the environment can influence genetic expression.
The opposite is also the case. Your genes can influence the type of environments you seek. That is, if you are a person who seeks stimulation, then you are more likely to have a certain set of genes different from those who are happy reading a book at home. In seeking stimulation, you look for environments that allow you to go mountain climbing or sky diving.
Numerous examples of the close fitting of genes and environment also exist in nature, such as a particular butterfly, the Bicyclus anyana, which is brightly colored if born in the rainy season, but gray if born in the dry season. The side-blotched lizard of the Mojave Desert will also have different colorings depending on its environment. It will be black if it is raised in a habitat of black lava flow and lighter if raised in light desert sand. The advantage of this tight coupling with the environment is that it offers a means of protection against predators.
Genes, the most basic unit of heredity, carry the instructions that direct the expression of particular traits and behaviors in a complicated process. As you will see, a number of factors influence which genetic programs are expressed.
A basic distinction in terminology is made between the genotype (genetic material) and the phenotype (organism’s observable characteristics). The genotype consists of what is inherited through the sperm and the egg at the moment of conception. The phenotype represents the observed traits of the individual including structure, physiology, and behavior. The focus of psychology has largely been the study of the phenotype. Current research also seeks to describe how genes are involved.
Genetics Help Explain Evolutionary Change
Genetic transmission is how evolution “works.” In his theory of natural selection, the English naturalist Charles Darwin (1809—1882) stressed variations in heritable traits and the manner in which different environments place specific traits at an advantage. Darwin originally showed this with the finches in the Galápagos Islands. In times of plenty, the seeds and nuts from the trees in which the finches ate were well developed and hard. Thus, the birds with strong beaks were able to obtain food. In times of less food, those birds with longer peaks were able to find food that had fallen between rocks. Those birds that survive are able to pass their traits on.
Another example are peacocks that have brightly colored large tails. How did they get these? The female peahen chooses which male peacock to mate with in terms of the quality of the characteristics of his tail. Thus, by choosing males with large and colorful tails, this trait is passed on. After a number of generations, peacock tails became larger with certain colors. We now know that the vibrancy of the color also reflects the health of the peacock as well as the underlying genetics. By choosing the male with the highest quality tail, the female is also choosing the male with better genes. What Charles Darwin did not know at the time he was writing was the specific units involved in this process.
Long strands of genetic material, including genes, are called chromosomes. Different organisms have different numbers of chromosomes. In humans there are 23 separate pairs of chromosomes making 46 in all. Each chromosome has a unique appearance. The chromosomes are numbered in approximate order of size with 1 being the largest and 22 being the smallest. These 22 are referred to as autosomes. The remaining chromosome pair is called the sex chromosome in that this chromosome differs in males and females. Females carry two copies of the X chromosome while males carry one X and one Y. By treating cells with particular dyes, chromosomes can be seen under a microscope. They appear as long, banded, cylinder-like structures that are pinched in at one point along their length (see Figure 3-33).
Figure 3-33 Normal human karyotype with autosomes and sex chromosomes. Each chromosome is associated with particular physiological processes.
The image of chromosomes after being isolated, stained, and examined with a microscope is referred to as a karyotype. Chromosomes are typically laid out in order of size in pairs with the sex chromosomes being shown last. Thus, a human male would be 46 XY and a human female 46 XX. Through research, each of the chromosomes has been associated with different physiological processes, particularly in terms of genes.
The Job of the Gene
All of us begin as a single fertilized egg, which by the time of birth has given us a body with some trillion cells. These cells can be differentiated both in terms of structure and function. For instance, cells in your muscles do different things from those in your brain. However, each of the cells, regardless of function, has a nucleus that contains the genome. A human genome is a set of some 20,500 genes. Other organisms have different numbers of genes, although the basic mechanism is similar across species. In fact, many of our genes are also found in other organisms and perform similar functions.
The job of a gene is to lay out the process by which a particular protein is made. In other words, each gene is able to encode a protein. Proteins are involved in a variety of processes. Functionally, proteins in the form of enzymes are able to make metabolic events speed up, whereas structural proteins are involved in building body parts. Similar proteins in insects are involved in creating such structures as spider webs, and butterfly wings. Proteins are diverse and complex and found in the foods we eat as well as made by our cells. Proteins serve as signals for changes in cell activity. Proteins are also involved in health and disease as well as development and aging. Proteins do the work of the body and genes influence their production.
Although the cells in the body carry the full set of genetic information, only a limited amount is expressed at any one time related to the function of the cell. That is to say, although a large variety of proteins could be produced at any one time, there is selectivity as to what is produced related to internal and external conditions. Further, the location of the genes makes a difference in that cells in your brain produce different proteins from those in your muscles, liver, heart, and so forth.
Figure 3-34 Structure of a cell. In every cell there is a nucleus that contains our DNA.
Before continuing, let’s think about some of the implications for a gene encoding a protein.
✵ First, a gene does not produce a protein constantly but in the context of a complex physiological system influenced both by internal bodily processes and events from the external environment. Thus, we can think of a gene being turned on (produce the protein) or turned off (do not produce the protein) relative to specific events. The bottom line is that just because a person has a specific gene does not mean that it will necessarily be expressed.
✵ Second, the environment in which a person develops and lives plays an important role in gene expression. Even identical twins with the same genotype can display different phenotypes if their environmental conditions differ during their development. For example, if one was to grow up in a high mountain range and the other in a below sea level desert, important physiological differences such as lung capacity and function would be apparent. Thus, as we will see there are few factors in terms of human processes that can be explained totally by genetic factors alone. It is also equally true that few human processes can be explained totally by the environment.
✵ Third, we should remember that there are few human factors of interest to psychologists that can be explained totally by a single gene. Rather, what we view in human processes represents not only a complex interaction between a person’s genetic makeup and the environment, but also a complex expression of many genes.
✵ Fourth, thinking of a gene’s ability to encode a protein, we should also realize that we should not speak of a gene producing a behavior. Rather, we must remember that a gene encodes a protein that can have implications for actions, feelings, and thoughts. Although there are implications, it does not necessarily follow that the behavioral outcome is the result of a genetic plan alone.
The production of proteins by genes is a complex process that involves a variety of levels, especially in humans. Even moving to the simpler level of insects, there are still a number of steps involved in most behavioral processes. For example, if the appropriate genes in a spider do not produce certain proteins, then through a number of steps the web will end up misshapen. Given the various mechanisms involved, it is not accurate to talk about a gene for misshapen spider webs. Thus, when we hear or read about the gene for misshapen spider webs or any of the other traits, we must consider what protein was produced under what conditions as well as the steps required. At this point it will be useful to move to a structural level in our consideration of genetics. Let’s begin with DNA.
DNA and RNA
Deoxyribonucleic acid (DNA) provides information necessary to produce proteins. Ribonucleic acid (RNA) transmits genetic information from DNA to proteins produced by the cell. We can think of proteins as a link between the genotype and the phenotype. The question arises as to how we get from information, which is what DNA and RNA is all about, to actual physiological changes. This is accomplished in two steps:
1. The information in DNA is encoded in RNA.
2. This information in RNA determines the sequence of amino acids, which are the building blocks of proteins.
DNA, which is the information storage molecule, transfers information to RNA, which is the information transfer molecule, to produce a particular protein. Further, change in the rate at which RNA is transcribed controls the rate at which genes produce proteins. The expression rate of different genes in the same genome may vary from 0 to approximately 100,000 proteins per second. Thus, not only do genes produce proteins, but they do so at different rates. The crucial question becomes what causes a gene to turn on or turn off.
In a classic work, Jacques Monod and François Jacob in the 1960s came to an initial understanding of how genes turn on and off. The particular organism they were studying was the bacterium Escherichia coli. This organism can switch from a diet of sugar (glucose) to milk sugar (lactose) if glucose becomes scarce in its environment.
Figure 3-35 DNA, RNA, and the production of proteins. DNA, which is the information storage molecule, transfers information to RNA, which is the information transfer molecule, to produce a particular protein.
To make this happen, these bacteria must produce an enzyme that breaks down lactose. Monod and Jacob discovered that an environmental event—the presence of lactose and the absence of glucose—would cause the genes to turn on and the enzymes to be produced. When this environmental condition was not present, these enzyme-producing genes were inhibited and thus not allowed to turn on.
Said in the language of genes, the transcription of the DNA into RNA and the resulting proteins was repressed in the environmental condition of lactose presence and glucose absence. What this tells us is that genes are designed to perform specific tasks depending on the presence of certain internal or external environmental conditions. Even the determination of larger organizational structures, such as whether an organism becomes male or female in some species, relates directly to the environmental conditions during its development and the genes that are turned on and off. For example, eggs hatched in warm water produces males in alligators but females in turtles.
In summary, genes form the blueprint that determines what an organism is to become. They are found in every cell of the body. Within each gene, DNA—the information storage molecule—transfers information to RNA—the information transfer molecule—to produce (or encode) a particular protein. The location of the genes in the body makes a difference in that cells in the brain produce different proteins from those in the muscles, or liver, or heart. A gene is turned on (produces the protein) or turned off (does not produce the protein) relative to specific events.
In terms of evolution, one of the real scientific contributions of DNA analysis has been the realization of how similar organisms are in terms of DNA structure. This suggests that all organic life on Earth involves similar building blocks in terms of DNA. Humans have DNA, chimps have DNA, bacteria have DNA, and even flowers have DNA. Given that all organisms have DNA, it is possible to compare how similar different organisms are, and from this make inferences about common ancestors. We know, for example, that the structure of DNA in chimpanzees and humans is about 97% the same. However, how the genes are expressed makes a real difference.
Whereas DNA science began with Watson and Crick and the discovery of DNA structure, it has come of age with the publication of the map of the human genome in 2003. Genes are particular stretches of chromosomes that contain all of the heritable bits of information necessary to produce a new individual. On any one chromosome, there can be anywhere from a few hundred to almost 3,000 genes (Lander et al., 2001). In 1990, the Human Genome Project was launched with the goal of cataloging the genetic information found in humans. It was completed in 2003 with the identity of all of the approximately 20,500 human genes. The research related to the project can be seen online (http://www.ornl.gov/hgmis/).
Figure 3-36 Chromosomes, genes, and DNA. In the center of every cell in our body, we have a complete set of genes (DNA). There is also another set of genes passed on from your mother referred to as mitochondrial DNA (mtDNA), which supplies energy to the cell.
Patterns of Inheritance Vary
Genes form the blueprint to describe who you are to become. They determine many physical characteristics such as eye color. Although genes can produce variation over evolutionary history, a majority of our genes have not changed. This is why all humans have two eyes and one nose and one mouth, for example. However, perhaps a fourth of all genes allow for variation, which results in differences between us in physical and psychological characteristics. What makes things interesting is that each of these genes can have pairs in which one is slightly different from the other.
The technical name for the unique molecular form of the same gene is an allele. It has been estimated that of our approximately 20,500 genes, some 6,000 exist in different versions or alleles (Zimmer, 2001). Thus, in each sperm or egg, the alleles inherited from the mother may express different characteristics from those from the father.
It is these structural differences that determine many of your characteristics from the shape of your face to the size of your fingers to whether or not your earlobes are attached. For example, although it is a single gene that determines the development of your earlobes, slight structural differences in this gene determine whether they develop attached or not. Being able to touch your tongue to your nose or curling your tongue are also related to the alleles of two different genes.
The study of genetics began with the work of Gregor Mendel (1822—1884). Although unknown to each other, at about the same time Darwin was examining heritable traits in finches, Gregor Mendel was describing the initial mechanisms by which genes work. During the latter part of the 19th century, the work of Darwin and Mendel and their followers occupied parallel tracks. However, both emphasized variation and factors that influenced it. It was the publication of Dobzhansky’s Genetics and the Origins of Species in 1937 that brought together the natural history of organisms with an understanding of genetics. These two lines of research came together in what is now referred to as the modern synthesis (Huxley, 1942; Mayr, 1942).
Curious about how plants obtain atypical characteristics, Mendel performed a series of experiments with the garden pea plant. Peas are a self-fertilizing plant, which means that the male and female aspects needed for reproduction develop in different parts of the same flower. Therefore, successive generations of peas are similar to their parents in terms of particular traits such as their height or the color of their flowers.
Mendel found that when combining peas with white flowers with those with purple flowers, the next generation had all purple flowers. Allowing this generation to self-fertilize brought forth plants that had purple flowers but also some that had white flowers. Mendel explained these findings by suggesting that a plant inherits information from each parent, the male and female aspect. Mendel was hypothesizing that information must be conveyed. He further suggested that one unit of information could be dominant in comparison to the other. The non-dominant one we now call recessive. In this case, the unit of information that coded for purple would be dominant.
Mendel did not know about genes, but hypothesized the existence of a specific structure he called elements. From his experiments, he determined the basic principle that there are two elements of heredity for each trait (for example, color in the previous example). Mendel also assumed that one of these elements can dominate the other and if it is present then the trait will be present. Mendel suggested that these elements can also be non-dominant, or recessive. For the trait to appear, both of these non-dominant elements must be present. These ideas are referred to as Mendel’s first law or the law of segregation.
If your eyes are blue, then you can guess that both of your parents have blue eyes since the allele for blue eyes is recessive. Put in today’s language, Mendel suggested that variants of a specific gene exist that account for variations in inherited characteristics and that an organism receives one of these from each parent. Traits are inherited depending on whether the allele is dominant or recessive. The allele for blue eyes is recessive so you need to receive the same allele from both your mother and father to have blue eyes. However, if you receive the gene associated with brown eyes from one parent and the gene associated with blue eyes from the other, you will have brown eyes since the brown eye allele is dominant.
Mendel also realized that the inheritance of the gene of one trait is not affected by the inheritance of the gene for another trait. In the previous example illustrating the inheritance of eye color, those factors influencing color do not affect height, and vice versa. That is, the probability for each occurs separately. This fact is known as Mendel’s second law or the law of independent assortment.
As genetic research progressed in the 20th century, it became clear that it was necessary to go beyond the two laws suggested by Mendel to a more complex understanding of how traits are passed from generation to generation. For example, if two genes are located close to one another on the same chromosome, then the result is different from that predicted by Mendel’s second law. Additionally, it was initially assumed that one day genes would be able to explain the development of psychopathology (mental disorders), especially schizophrenia. However, after decades of research, it is clear that simple genetic explanations will not be forthcoming. Mental disorders disrupt a variety of cognitive, emotional, and motor processes that develop over a person’s lifetime and are guided by thousands of genes (see Plomin, 2018 for an overview).
In addition to the genetics that Mendel studied, there is also inheritance that is sex-linked. Males and females have partially different genomes—the sex chromosomes are inherited differently for males and females. Human females have two X chromosomes and males have an X and a Y chromosome. One implication of this is that genes on the Y chromosome are expressed only in males. Another implication is that some genes on the X chromosome may be expressed at a higher level in females. Besides physical sexual organs, researchers are just beginning to understand which characteristics of males and females are influenced by genes on the X and Y chromosomes. Given that females have two X chromosomes, they transmit an X chromosome to all of their offspring whether they are males or females. Thus, everyone receives an X chromosome from their mothers. Males, on the other hand, transmit an X chromosome to their offspring who are daughters, but not to their sons. Thus, sons cannot inherit an allele on the X chromosome from their fathers. These male offspring receive only a Y chromosome.
Since the X and Y chromosomes are different and do not contain the same identical genes, this sets up the possibility for transmission of traits that can be different for males and females. One implication is that recessive traits on the X chromosome will be expressed in males, but not as frequently in females. One common X-linked recessive trait is color blindness. The most common form results in the individual being unable to see red or green as distinct colors. Since this is caused by a recessive allele on the X chromosome, females would need to inherit the allele on both X chromosomes for them to be color blind, whereas males, having only one X chromosome, would only need to inherit one recessive allele.
Sex-linked traits also have different patterns of inheritance throughout generations for males and females. Since fathers do not pass X chromosomes to their sons, a color-blind father and a non-color-blind mother are not likely to have a color-blind son. They might, however, pass the allele on to their daughter whose son could be color blind. That is to say, the son would not be color blind but would be a carrier of the color-blind allele, whereas the grandson would be color blind. In this manner the trait would skip a generation.
Figure 3-37 Color-blind test used with children. The ability to see the animals in the picture requires that the person can see red and green as separate from gray.
Behavior and Environment Influence Gene Expression
In terms of behavior and experience, changes caused by the production of proteins can be transitory. For example, touching a cat’s whiskers causes changes in gene expression in the cells of the sensory cortex of the brain (Mack & Mack, 1992). This is just a momentary change. Changes can also be long term. As we will see when we discuss development, turning on one set of genes may have lasting influence on the ability of other genes to produce specific proteins. For example, when a songbird first hears the specific song of its species, a particular set of genes comes into play that when once set, determines the song produced by that bird for its entire life. This process has been mapped by a number of researchers (Mello, Vicario, & Clayton, 1992). Likewise, raising mice in an enriched environment, that is, one with lots of toys and stimulation, will cause increased gene expression in genes that are associated with learning and memory (Rampon et al., 2000).
How do we know which genes are involved? In this study, the genes of mice in enriched environments were compared with those of control mice who did not have this experience. Another way to know which genes are involved in a process is to actually change the genes in a particular organism. So called “knockout” mice are genetically engineered mice in which particular genes have been turned off by breeding them in specific ways. Research shows that simple genetic changes made experimentally in animals can result in protein changes that influence social behavior. Some examples of such behaviors are increased fear and anxiety, increased grooming, hyperactivity, and even increased alcohol consumption when stressed.
One striking example of the manner in which cultural and environmental change can be mapped along with genetic change is lactose tolerance (Tishkoff et al., 2006). Lactose is a sugar found in milk. The infants of most mammals, including humans, can no longer digest milk once weaned. Infants are able to digest lactose due to certain enzymes in their mothers’ milk that work with the infant’s own genes to break down the lactose. A single dominant gene is involved in the production of the enzyme that breaks the milk sugar down. Without the enzyme, anyone drinking milk can feel sick, including cramps, diarrhea, and nausea. However, some humans currently living in Europe, India, and sub-Saharan Africa and their descendants continue to be able to drink milk into adulthood. How did this happen?
It appears that when cattle were first domesticated some 9,000 years ago, humans began to eat their meat and drink their milk. Some of the people living at that time had a mutation that kept the gene involved in producing the enzyme, which allowed the individual to digest milk sugar permanently, switched on. Given that these individuals did not become sick from drinking milk, they had a certain advantage, particularly in difficult times such as droughts.
In fact, it has been estimated that individuals with the mutation that allowed for milk drinking into adulthood were able to produce almost ten times as many descendants as those without it (Tishkoff et al., 2006). This is demonstrated today by the fact that almost all of the citizens of the Netherlands and Sweden, the area where cattle were originally domesticated in Europe, are lactose tolerant. Further, it is thought that lactose tolerance developed independently in different parts of the world. Overall, this shows the manner in which cultural and environmental changes can produce a shift in the genetics of a specific population.
Environmental couplings may also promote health and well-being. With some genetic disorders, such as phenylketonuria (PKU), simply changing the environmental conditions in terms of the types of food a person eats can actually change the negative outcomes of the disorder. Even more surprising is that if you are a mouse, what your mother ate before you were born can influence the color of your fur and the diseases you are susceptible to. This is the result of epigenetic changes.
Epigenetic Processes Respond to Changing Environments
According to Mendelian genetics, genes are not changed by experience. What is passed on, except in the case of damage to the gene, is exactly the same gene that was received by the organism from its parents. This was known as the central dogma of molecular biology as described by neuroscientist Francis Crick, who discovered the structure of DNA. He stated that the information flow was one-directional (Crick, 1970). That is, it went from the gene to the protein. What came to be called reverse translation was seen as impossible. Thus, the gene could not be influenced or affected by changes in proteins, and, by implication, experience could not change DNA. This was the basic view from the 1950s until very recently.
However, the story has become more complicated. Researchers examining how genes turn on and off in relation to environmental factors discovered that internal genetic processes could make genes easier or harder to turn on or off. These processes that determine which genes turn on and off could themselves be passed on to the next generation. The factors that turn genes on and off are complex but largely influenced by the environment of the organism. In short, DNA is covered by proteins. Changes in these proteins and the other chemicals associated with them can influence the manner in which nearby genes are turned on or off. Even more surprising was that the tendency for a gene to be turned on easily or not could be passed on to the next generation. Thus, although DNA itself could not be influenced by the environment, it was possible for the environment to influence future generations through its changes to those processes that turn genes on and off.
This possibility of another form of inheritance came to be called epigenetic inheritance (see Bagot et al., 2014; Hallgrímsson & Hall, 2011; Nestler, 2011; O’Donnell & Meaney, 2020 for overviews). Instead of actually changing the gene itself, epigenetic modifications mark a gene. This alters how it is turned on and off. DNA is wrapped around clusters of proteins called histones. These are further bundled into chromosomes. Being tightly packed keeps genes in an inactive state by preventing access to processes that turn genes on. When action is needed, a section of DNA unfurls and the gene turns on. Whether a segment is relaxed and able to be activated or condensed, resulting in no action, is influenced by epigenetic marks or tags (see Figure 3-38). As a tag, histone acetylation tends to promote gene activity and is called a writer. Histone methylation and DNA methylation tend to inhibit it and are called erasers.
Figure 3-38 Epigenetic changes alter gene activity. When the genes are tightly wrapped, they are more difficult to turn on. When they are not, they are easier to turn on.
Source: Nestler (2011).
Figure 3-39 A mother rat taking care of her pups. This information can be passed on in the form of epigenetic mechanisms.
Tags help an organism respond to a changing environment. The environment itself can influence these writer and eraser tags. Some tags last a short time while others can last a lifetime. In a now classic study, researchers observed that some rat mothers displayed high levels of nurturing behavior, licking and grooming their pups, whereas others were less diligent (Weaver et al., 2004; Meaney, 2010; Miller, 2010). Behaviorally, the offspring of the more active mothers were less anxious and produced less stress hormone when disturbed than pups cared for by more passive mothers. Further, the females raised by nurturing mothers became nurturing mothers themselves.
The intriguing part of this study was that the offspring of the rat mothers who showed more licking and grooming differed in epigenetic factors. Pups raised by passive mothers showed more DNA methylation than aggressively groomed pups. The pup differences were related to the animal’s response to the stress hormone cortisol. This excessive methylation was detected in a brain region involved in learning and memory called the hippocampus. Activation of the receptor in the hippocampus actually signals the body to slow production of a brain substance related to stress. The epigenetic-related response exacerbates the stress response in the animals. This makes the animals more anxious and fearful. Further, these traits persist throughout their lifetime. Overall, attentive mothers cause the methyl marks to be removed. Inattentive mothers, on the other hand, cause methyl marks to be added. Thus, rats inherit certain behaviors based on experience. The genes had not been changed, but the tags were.
Fathers, including human fathers, can also influence their offspring. It has been shown that a mouse will develop a diabetes-like disease if her father’s diet before her conception was high in fat (Skinner, 2010). Also, if a mouse father is overweight, then gene activity in the pancreas of his offspring is abnormal (Ng et al., 2010). Since the pancreas makes insulin, which regulates blood sugar, then this may set up the possibility of future diabetes. The opposite is also the case. If the father’s diet results in an underweight condition, then genes in the liver associated with fat and cholesterol synthesis were shown to be more active in their offspring (Carone, 2010). Another study examined human fathers who smoked early in life and determined this was associated with his sons being heavier in weight at age 9 (Pembrey et al., 2006). Stress-related factors can also influence epigenetic processes. For example, the experience of violent events is associated with decreased DNA methylation across human generations (Serpeloni, Nätt, Assis, Wieling, & Elbert, 2019).
Overall, epigenetic research suggests that behavior and environmental experiences at critical periods could later influence characteristics for future generations. Current health research related to such disorders as diabetes and cancer, as well as types of psychopathology, is suggestive of such a relationship (Bagot, 2014; Katsnelson, 2010; O’Donnell & Meaney, 2020). Both addiction and depression have been shown to have an epigenetic component (Nestler, 2011, 2014). Extreme childhood abuse is related to a number of negative outcomes including suicide. Those who committed suicide were shown to have changes in epigenetic mechanisms (see Labonte & Turecki, 2010 for an overview). There is also research to suggest that long-term physical exercise can not only increase cognitive functions of the brain and reduce the possibilities of neurological disorders, but that exercise can influence future generations through epigenetic mechanisms (Fernandes, Arida, & Gomez-Pinilla, 2017).
Thus, epigenetic inheritance, which involves tags or marks that determine when genes are turned off or on, offers a parallel track to traditional Mendelian inheritance for influencing phenotypes. One new area of research uses identical twins to study specific epigenetic mechanisms with the goal of determining how genetic and environmental factors influence epigenetics (for example, Bell & Spector, 2011; Tan, 2020). This approach may offer better insight into the expression of complex traits as seen in both normal and psychopathological processes. One example of epigenetics is described in the box: Real World Psychology—Dutch Winter and Epigenetics.
Real World Psychology—Dutch Winter and Epigenetics
In the 1940s, much of Europe was experiencing hardships from World War II and from especially cold winter weather. In the Netherlands, there was an active resistance movement directed against the Nazi occupation. In November of 1944, the Nazi occupiers of Holland sought to punish the population for war resistance activity. To do this, the food supply was cut such that the mean caloric intake went from approximately 2,000 calories a day to less than 800. When the Allies liberated Holland some seven months later, food supplies returned to normal.
This terrible event was referred to as Hongerwinter (hunger winter) in which 18,000 people starved to death. Since everyone had their food supplies cut and reinstated at exactly the same time, it is possible to see the effects of this experience. Like much of Europe, hospitals in Holland collected health data from mothers and their children. Using birth data from the hospitals during the period of food deprivation, it was possible to determine which mothers became pregnant after the food was limited and which mothers were already pregnant at this time. Thus, some fetuses would be undernourished at the beginning of the pregnancy and others only at the end of the pregnancy. Those fetuses that were undernourished late in pregnancy had a reduced birth weight. Those fetuses that were undernourished early in pregnancy had a normal birth weight. Even with early malnutrition, these infants were able to catch up in body weight.
The close connection between an infant and its environment influences a number of developmental processes. The body of an infant is structured to live in the same environment that it experienced as a fetus. In periods of famine, one would want a physiological system that could survive in times of less food. However, a different system would be ideal for times of plenty. In the Dutch winter of 1944, there was a brief period of intense deprivation, in an otherwise well-nourished population. This resulted in a mismatch between the infant and his or her environment.
Not only did scientists study the birth weight of those born in the aftermath of the Dutch Winter, but they also followed these individuals throughout their lives. What they found was that those babies who were born small remained that way for the rest of their lives and showed less obesity than the general population. On the other hand, those infants born with a normal body weight were later shown to develop obesity and insulin resistance. Later tests also showed these individuals had higher plasma glucose concentrations after a standard glucose tolerance test. Thus, malnutrition early in pregnancy left these individuals susceptible to future obesity and diabetes.
There is also an epigenetic aspect to the Dutch Winter. With the excellent data collected from parents of the Dutch Winter and their children and grandchildren, it has been possible to explore the consequences of experiencing the famine. In the Dutch Winter, those women who were malnourished in the first trimester had normal weight babies but their grandchildren tended to be heavier. If on the other hand the women were malnourished in the third trimester, their babies were underweight but their grandchildren were of normal weight. Further, some 60 years later researchers looked for epigenetic differences. They did this by comparing the individuals who were in the womb during the Dutch Winter to their older and younger siblings who were born in normal periods of food availability. What was found was that the siblings did not show differences in DNA methylation of particular genes, whereas the Dutch Winter children did. What this and other research suggests is that it is possible for an environmental event to be transmitted to later generations through epigenetic mechanisms (see Ahmed, 2010 and Carey, 2012 for overviews).
Thought Question: What does the Dutch Winter story teach us about how the environment influences developmental processes? What epigenetic changes were found?
Behavioral genetics is the study of genetic and environmental contributions to behavior (see Knopik, Neiderhiser, DeFries, & Plomin, 2017; Plomin, 2018 for overviews). This is an exciting field of research today, and a fundamental question addressed is the manner in which genes and the environment work together to shape behavior. What if you were raised by a different set of parents, would you be different? What if you had an older brother or sister rather than a younger one? What if you were not an only child? In each case you would have grown up with different life experiences. Although interrelated, psychologists interested in behavioral genetics seek to quantify the amount of variance that can be attributed to genetic influences and the amount attributed to environmental influences. One of the main approaches used in behavioral genetics is twin studies.
Figure 3-40 Identical twins usually look alike and may display similar preferences.
Twins offer an occurrence in nature that allows for understanding critical factors related to genetic influences. This is largely based on the fact that there are two types of twins. Monozygotic (MZ) twins are identical twins resulting from the zygote (fertilized egg) dividing during the first two weeks of gestation. Because they both come from the same egg, their genes are identical. Dizygotic (DZ) twins, on the other hand, result from the situation in which two different eggs are fertilized by two different spermatozoa. These individuals are called fraternal twins since their shared genes are approximately 50%, which is the same as that between any two siblings. DZ twins can be either same sex or opposite sex, whereas MZ twins must always be the same sex. By comparing the psychological traits of MZ and DZ twins it is possible to obtain an estimate of heritability.
A classic research design is to compare the responses of MZ twins with DZ twins on particular behavioral traits such as intelligence or personality characteristics (Bouchard, 2013; Johnson, Turkheimer, Gottesman, & Bouchard, 2010; Segal, 2012). Since it is assumed that both DZ and MZ twins would have had similar environmental influences in their family, then differences between MZ and DZ twins would be seen to be the result of genetic influences.
Gottesman (1991) has studied schizophrenia with this design. In these studies, a particular MZ twin had a 48% chance of developing schizophrenia if the other twin also did. In DZ twins there was only a 17% chance. Figure 3-41 shows the risk of developing schizophrenia based on another person with schizophrenia related to you.
Figure 3-41 The risk of developing schizophrenia based on how another person with schizophrenia is related to you.
Statistically, researchers examine the degree to which twins are identical to each other as a function of genetic influences and environmental influences. That is to say, you create correlation coefficients for MZ twins and for DZ twins. This tells you how similar each type of twin is on a particular trait. From this, it is possible to determine the percentage of contribution to the trait that comes from environmental influences and the percentage of contribution that comes from genetic influences. For example, personality factors such as extraversion have been shown to have a 50% contribution of genetic factors and a 50% contribution of environmental factors. Although we use the term identical twins, they can be different, as described in the box: Myths and Misconceptions—Identical Twins: Identical Brains.
Myths and Misconceptions—Identical Twins: Identical Brains
Identical twins look the same. There are even stories about identical twins who grew up apart going into the ocean on vacation in the same way—by walking in backwards. While it is true that identical twins come from the same egg and sperm, after that there are a number of ways they can become different.
One of the first ways they can be different is when they are in the womb. Although we think of life in the womb being the same for both of them, that is not actually the case. Through a variety of situations, one of the twins may receive more nutrients than the other. Even at the beginning, their DNA can be reproduced in slightly different ways. In the process of turning genes on and off, a single gene can produce different proteins. These proteins, in turn, can influence other processes that can lead to differences between the two twins.
One exciting finding in the study of genes is the so-called jumping gene. These jumping genes, also known as transposable elements, were discovered more than 50 years ago by Barbara McClintock at Cold Spring Harbor Laboratory in New York. As the name implies, these genes move from one location on the genome to another. As such, they can move within and between chromosomes. In their new location, these genes sometimes have no effect at all. However, in some cases they can activate the genes in the new location and influence individual cells in the brain. This in turn can create differences in brain function between individuals, even identical twins.
Given that identical twins may have different brain processes, which include behavior, cognition, and even reaction to stress, this may help to explain how one twin may remain disease-free, and the other not. Even mice who have been bred to be genetically identical and environmentally treated in the same manner will show differences in learning ability and responses to fear and stress. Further, novel situations and exercise in mice, as well as humans, produce new growth of nerve cells and connections in the brain. An intriguing idea is that as we learn new information that increases brain connections, we also increase the activity of jumping genes (see Gage & Muotri, 2012 for an overview). This, in turn, would allow us to solve new problems in different ways. Thus, each person remains unique in his or her brain activity, even identical twins.
Thought Question: What are the different ways identical twins can be different?
Another important type of study is the adoption study. This is the situation where DZ and MZ twins have been raised apart. During World War II, for example, children from England were often sent to Canada to allow them to grow up in a less dangerous situation. At times, the twins were split up and raised by different parents. Since the children were raised in different environments, this made it possible to better determine the environmental and genetic influences.
Since 1979, in the United States a series of twins who were separated in infancy and reared apart have been studied by researchers at the University of Minnesota (see Bouchard, Lykken, McGue, Segal, & Tellegen, 1990; Segal, 2012 for overviews). In work with more than 100 pairs of twins, these researchers found that about 70% of the variance in intelligence quotient (IQ) could be associated with genetic factors. Later studies have supported this finding. However, environmental factors such as poverty can greatly reduce the role of genetics as related to IQ. Although it is not surprising to find IQ or temperament to have genetic associations, it was intriguing to see that the leisure time interests of each twin in a pair were similar whether the twins were reared together or reared apart.
✵ Many people think that if a person has a particular gene, then whatever trait or activity is connected with that gene will be seen. What is a better description of the relationship between a gene and the trait or activity to which it is connected?
✵ What is the role of the environment in turning genes on and off? Give some specific examples.
✵ What are the concepts of genotype and phenotype, and how are they related?
✵ “The job of a gene is to lay out the process by which a particular protein is made.” What are four implications of this statement?
✵ What do genes do and how and where do they do it? What are the roles of DNA and RNA in that process?
✵ How do we know that genes change behavior? What kinds of research have been done with animals to identify the specific genes involved?
✵ Describe the human genome. In what ways is it similar to, and in what ways different from, the genome of other animals?
✵ How does the process of sexual reproduction ensure that you will not be a clone of either of your parents or even a 50—50 combination of the two of them? What are the mathematical odds that you ended up with the specific combination of chromosomes that you did?
✵ What are the two important principles of Mendelian genetics? What evidence led Mendel to their discovery? Give two examples of how Mendel’s laws have been modified since he proposed them.
✵ How does epigenetic inheritance work? Describe the steps in the process using the study on mother rats and their nurturing behavior. What other examples can you cite?
✵ What is the overall goal of behavioral genetics research? What are the three primary types of research designs used in behavioral genetics research?
Learning Objective 1: Describe the basic element of the brain and its connection to behavior and experience.
The basic element of the brain is the neuron that is connected to other neurons. Since the human brain has been estimated to contain 86 billion neurons and more than 100,000 kilometers of interconnections, scientists have analyzed them in terms of networks using the small world framework. Neurons have numerous short distance local connections that taken together can be considered as a hub or module. From these hubs there are more long-distance connections to other hubs. Three specific networks have been examined in terms of psychology—the default network (also called the intrinsic network), the central executive network, and the salience network.
Learning Objective 2: List the key structures of the brain.
First, the brain has four lobes—frontal lobe (involved in planning, higher order cognitive processes, and moral and social judgments); parietal lobe (involved in spatial processes); occipital lobe (involved in the processing of visual information); and temporal lobe (involved in hearing as well as aspects of language). Two other sections are the motor strip (associated with movements of different parts of the body) and the somatosensory cortex (associated with receiving sensations from various parts of the body). Second, the limbic system is considered to be an important evolutionary system that evolved for the processing of emotional material. Five major structures composed the limbic system: parts of the thalamus, hypothalamus, cingulate gyrus, hippocampus, and amygdala. Third, the brain stem is traditionally considered to include the midbrain, pons, and medulla. These structures are involved in basic life functions. Movements related to the brain stem tend to be more of an instinctual nature. Fourth, the cerebellum, located at the base of the skull, is involved in the coordination of movement and in the automation of physical and cognitive tasks. It interacts with many different parts of the brain using an intricate set of feedback processes.
Learning Objective 3: Discuss the neuroscience methods that are being used to observe activity in the brain.
Scientists have been able to use the manner in which neurons work as a window into their function. A variety of techniques for observing activity in the brain have been developed. Currently, the major types of brain-imaging techniques are EEG, MEG, PET, and fMRI. There are a number of trade-offs that researchers and clinicians must consider when choosing a brain-imaging technique. It begins with the research or clinical question one is asking whether the appropriate measure is one of structure (spatial resolution) or how fast a process can be measured (temporal resolution).
Learning Objective 4: Describe how the networks of the brain process information.
Networks allow our brains to efficiently process information. Overall, cortical networks are influenced by experience and designed to be efficient in terms of connections between neurons in the network. This efficiency allows for less use of energy. One way energy is conserved is through not having every neuron connect with every other neuron. Neurons are neither totally random in their connections with other neurons nor totally patterned. It appears that neurons are connected to one another in the same way that all humans on this planet are socially connected. It turns out that neurons, like humans, can be connected to one another in similar ways.
Various studies have shown that the neurons in the brain can be considered within a small world framework (Sporns, 2011). Neurons have numerous short-distance local connections that, taken together, can be considered as a hub or module. From these hubs, there are more long-distance connections to other hubs. Such connections are seen in gray matter. Gray matter appears dark and can be seen in the outer shell of the brain. Gray matter contains the synapses, dendrites, cell bodies, and local axons of neurons. Some 60% of gray matter is composed of axons and dendrites.
Underlying this are the axons that transfer information throughout the brain. Their myelin sheaths are lighter in color and thus these areas are referred to as white matter. Myelin is made up of fats and proteins and wrap around axons like insulation does around electrical cables and results in an increased speed in information transmissions. About 44% of the human brain is white matter. White matter generally represents longer connections between neurons. This allows for cortical networks over larger areas of the brain.
Those neural networks that are active during internal processing have come to be referred to as the brain’s default or intrinsic network. Overall, the default network is involved during internal or private considerations that do not require processing external sensory information. In fact, it appears as if there is a negative correlation between activities in the default network versus networks associated with processing information from the environment. That is, when someone begins some cognitive activity, then new networks associated with that task become active and the default mode network becomes less active. Overall, this suggests that separate brain mechanisms evolved for dealing with information involving the external environment as opposed to considerations internal to the person.
In addition to the default network, the executive and salience networks have been identified (Menon, 2011). The central executive network is involved in performing such tasks as planning, goal setting, directing attention, performing, inhibiting the management of actions, and the coding of representations in working memory (see Eisenberg & Berman, 2010, for an overview). These tasks are also referred to as executive functions since they are involved in planning, understanding new situations, and having cognitive flexibility. The salience network, as the name implies, is involved in monitoring and noting important changes in biological and cognitive systems. There are also networks involved with vision, audition, and other sensory experiences.
Learning Objective 5: Describe the development and evolution of the brain.
The human central nervous system (CNS), which includes the brain and spinal cord, begins to develop during the first month of pregnancy with the creation of the neural tube. The most anterior end of the neural tube becomes the forebrain, the midbrain, the cerebellum, and the hindbrain. The remaining part of the tube becomes the spinal cord. Different parts of the tube expand differently in different animals under the control of specific genes. Along with the expansion of the surface area of the brain during evolution, there was also an increase and expansion of areas involved in processing information. As evolution produced greater diversity in cortical structure, there remained a similarity in the manner in which the nervous system functioned. Mammals have evolved a brain structure different from other organisms—the six-layered neocortex. Although consistent in structure, among mammals there is great variation in cortical size and organization in terms of area devoted to different types of cortical processing and greater network connectivity.
We are just beginning to understand the manner in which human brain evolution is built upon a vast history of organic development. In the 20th century, Paul MacLean examined fossil records along with brains from a variety of organisms and proposed that our current human brain can be viewed as having the features of three basic evolutionary formations—reptiles, early mammals, and recent mammals. MacLean’s formulation—called the triune brain—suggests that through rich interconnections our brains can process a variety of information in three somewhat independent, although not autonomous, ways. In effect, these three brains—reptilian, paleomammalian, and neomammalian—intermesh and function together.
Learning Objective 6. Describe the role that genetics and evolution play in helping us understand our psychological processes.
Genes form the blueprint that determines what an organism is to become. They are found on chromosomes in every cell of the body. Within each gene, DNA—the information storage molecule—transfers information to RNA—the information transfer molecule—to produce (or encode) a particular protein. The location of the genes in the body makes a difference in that cells in the brain produce different proteins from those in the muscles, or liver, or heart. A gene is turned on (produces the protein) or turned off (does not produce the protein) relative to specific events. A basic distinction in terminology is made between the genotype (what is inherited through the sperm and the egg at the moment of conception) and the phenotype (the observed characteristics of the individual, including body structure, physiology, and behavior). The focus of psychology has largely been the study of the phenotype. In terms of evolution, one of the real scientific contributions of DNA analysis has been the realization of how similar organisms are in terms of DNA structure. Different organisms have different numbers of chromosomes—humans have 23 separate pairs of chromosomes. Males and females have partially different genomes—the sex chromosomes are inherited differently for males and females. Since the X and Y chromosomes are different and do not contain the same identical genes, this sets up the possibility for transmission of traits that can be different for males and females. Sex-linked traits also have different patterns of inheritance throughout generations for males and females.
1. The connectivity in the brain has been described by some as “an enchanted loom” or a “dance.” What characteristics of that connectivity led them to use those metaphors? What metaphor would you use?
2. The human brain is more accessible to science than it has ever been. Brain imaging is a window into the structure and function of the brain. Brain—machine interfaces extend brain functioning to control devices, for example, moving prosthetic limbs directly with thoughts. Another example of these interfaces is deep brain stimulation where electrodes are implanted in specific areas of the brain to regulate abnormal impulses in treating a number of neurological conditions. With all of this capability, what do you think is the next critical brain research area for psychology?
3. How does the small world framework from social science help us understand how neurons are connected in a network? What implications does this have for the transmission of information within a network and across networks?
4. Mammals have evolved a brain structure different from other organisms—the six-layered neocortex. Although consistent in structure, among mammals there is great variation in cortical size and organization. Does the species with the largest brain win? What are the implications for increase in overall size as well as organization and connectivity?
5. Describe the way in which Paul MacLean mapped evolutionary history onto the structure and functioning of the human brain through his theoretical conception of the triune brain. What are the primary implications of this mapping for interpreting previous research results and suggesting future research questions?
6. How have the discoveries of epigenetic inheritance and mitochondrial inheritance enriched our understanding and added to the complexity of Mendel’s initial theory of genetic inheritance?
For Further Reading
✵ Andreasen, N. (2001). Brave New Brain. New York: Oxford University Press.
✵ Andreasen, N. (2005). The Creative Brain. New York: Dana Foundation.
✵ Dingman M. (2019). Your Brain, Explained. Boston, MA: Nicholas Brealey Publishing.
✵ Eagleman, D. (2011). Incognito: The Secret Lives of the Brain. New York: Pantheon.
✵ Nesse, R. (2019). Good Reasons for Bad Feelings. New York: Dutton.
✵ Plomin, R. (2018). Blueprint: How DNA Makes Us Who We Are. Cambridge, MA: MIT Press.
✵ Ramachandran, V. (2011). The Tell-tale Brain. New York: Norton.
✵ Riddley, M. (2000). Genome, the Autobiography of a Species in 23 Chapters. New York: HarperCollins Publishers.
✵ Sapolsky, R. (2017). Behave, the Biology of Humans at Our Best and Worst. New York: Penguin Press.
✵ Brain-to-brain interface—http://video.techbriefs.com/video/Brain-to-Brain-Interface-Demons;medical
✵ Two-minute videos—www.neurochallenged.com
✵ NIMH brain—www.nih.gov/science/brain
✵ European brain project—www.humanbrainproject.eu
✵ NASA twin study—https://www.nasa.gov/twins-study
✵ Genome project—http://www.ornl.gov/hgmis/
Key Terms and Concepts
central executive network
central nervous system (CNS)
default or intrinsic network
deoxyribonucleic acid (DNA)
diffusion tensor imaging (DTI)
dizygotic (DZ) twins
functional magnetic resonance imaging (fMRI)
magnetic resonance imaging (MRI)
monozygotic (MZ) twins
peripheral nervous system (PNS)
positron emission tomography (PET)
ribonucleic acid (RNA)
small world framework