2.1 Sensation vs. Perception
Sensation and Perception
After Chapter 2.1, you will be able to:
· Explain the pathway for a stimulus to reach conscious perception
· Connect the common sensory receptors to their functions
· Describe absolute threshold, threshold of conscious perception, and difference threshold
· Explain Weber’s law and signal detection theory
· Describe how sensory adaptation affects a difference threshold
In common parlance, we often use the terms “sensation” and “perception” interchangeably, as synonyms. However, in the field of psychology, these two terms have very specific definitions and are commonly contrasted. Sensation more appropriately aligns with transduction, which means taking the physical, electromagnetic, auditory, and other information from our internal and external environment and converting this information into electrical signals in the nervous system. Sensation is performed by receptors in the peripheral nervous system, which forward the stimuli to the central nervous system in the form of action potentials and neurotransmitters. Sensation can therefore be thought of as a raw signal, which is unfiltered and unprocessed until it enters the central nervous system.
Perception, on the other hand, refers to processing this information within the central nervous system in order to make sense of the information’s significance. The complex manipulations involved in perception include both the external sensory experience and the internal activities of the brain and spinal cord. Perception thus helps us make sense of the world. The difference between sensation and perception is key to the challenge of creating artificial intelligence: we can easily create sensors for robots to pick up information from their environment, but teaching them how to comprehend and respond to that information is far more challenging.
Sensory processing is a common topic on the MCAT; you should not only understand the definitions of these terms, but also be able to apply the concepts herein to your own day-to-day sensory experiences.
Sensory receptors are neurons that respond to stimuli by triggering electrical signals that carry information to the central nervous system. Physical objects outside of the body are referred to as distal stimuli. These objects often produce photons, sound waves, heat, pressure, or other stimuli that directly interact with sensory receptors; these sensory-stimulating byproducts are called proximal stimuli. For example, a campfire is a distal stimulus. The photons that are emitted by the fire, the sounds of crackling and popping, and the energetic gas particles that transfer heat energy are all proximal stimuli. So, proximal stimuli directly interact with and affect the sensory receptors, and thereby inform the observer about the presence of distal stimuli. Sensory receptors may encode multiple aspects of a stimulus. For example, photoreceptors respond to light and can encode not only the brightness of the light, but also its color and shape. The relationship between the physical nature of stimuli and the sensations and perceptions these stimuli evoke is studied in the field of psychophysics.
Distal = in the distance
Proximal = in close proximity
In order to inform the central nervous system, the signals from these stimuli must pass through specific sensory pathways. In each case, different types of receptors—generally nerve endings or specific sensory cells—receive the stimulus, transduce the stimulus into electrical signals, and transmit the data to the central nervous system through sensory ganglia. Ganglia are collections of neuron cell bodies found outside the central nervous system. Once transduction from these sensory ganglia occurs, the electrochemical energy is sent along neural pathways to various projection areas in the brain, which further analyze the sensory input.
Sensory receptors differ from one sense to another. There are over a dozen recognized sensory receptors, but the MCAT is unlikely to test even half of those. The most heavily tested receptors include:
· Photoreceptors: respond to electromagnetic waves in the visible spectrum (sight)
· Mechanoreceptors: respond to pressure or movement. Hair cells, for example, respond to movement of fluid in the inner ear structures (movement, vibration, hearing, rotational and linear acceleration)
· Nociceptors: respond to painful or noxious stimuli (somatosensation)
· Thermoreceptors: respond to changes in temperature (thermosensation)
· Osmoreceptors: respond to the osmolarity of the blood (water homeostasis)
· Olfactory receptors: respond to volatile compounds (smell)
· Taste receptors: respond to dissolved compounds (taste)
Perception, like sensation, is closely tied to the biology and physiology of interpreting the world around us. However, unlike sensation, perception is inextricably linked to experience as well as to internal and external biases. Sensations are relayed to the brain, which perceives the significance of the stimulus. To illustrate the significance of perception, keep in mind that all sensory information is sent to the central nervous system in the form of action potentials, which the central nervous system must then interpret and act upon. For example, the central nervous system must determine whether incoming action potentials from thermoreceptors are indicating whether an object is hot or is cold, and whether that temperature difference is enough to cause us harm. Moreover, the same sensation can produce radically different perceptions in different people, and because these variations must be explained by central nervous system activity, perception is considered a part of psychology.
A good example of the psychological element of perception is a threshold—the minimum amount of a stimulus that renders a difference in perception. For example, the temperature may noticeably change from warm to cool when the sun sets, but subtle fluctuations in temperature throughout the day are generally unnoticeable because they are below the difference threshold. If sound volume increases 10 dB (ten times the sound intensity), the change is usually very obvious; but, if volume increases only 0.1 dB, the change might be too small to detect. There are three main types of thresholds: the absolute threshold, the threshold of conscious perception, and the difference threshold.
On the MCAT, thresholds will frequently be used in conjunction with subjects in studies. Be on the lookout for experimental design questions when thresholds appear in a passage.
The absolute threshold is the minimum of stimulus energy that is needed to activate a sensory system. This threshold is therefore a threshold in sensation, not in perception. While most human sensory systems are extremely sensitive, all systems also have an absolute threshold below which the stimulus will not be transduced into action potentials, and the information will therefore never be sent to the central nervous system. For example, sounds of extremely low intensity may still cause slight vibrations in the sensory receptors of the inner ear, but these vibrations might not be significant enough to open ion channels linked to these sensory receptors. The absolute threshold for sweet taste is a teaspoon of sucrose dissolved in two gallons of water. On a clear, dark night with no other lights shining, the eye can just detect the light of one candle burning thirty miles away. When we are talking about an absolute threshold, we’re talking about how bright, loud, or intense a stimulus must be before it is sensed.
The absolute threshold is the minimum intensity at which a stimulus will be transduced (converted into action potentials).
You already know one of the absolute thresholds from the discussion of sound in Chapter 7 of MCAT Physics and Math Review. Remember that in the equation for sound level is the absolute threshold of normal human hearing.
Threshold of Conscious Perception
It is possible for sensory systems to send signals to the central nervous system without a person perceiving these signals. This lack of conscious perception may be because the stimulus is too subtle to demand our attention, or may last for too brief a duration for the brain to fully process the information. The level of intensity that a stimulus must pass in order to be consciously perceived by the brain is the threshold of conscious perception. By way of contrast, information that is received by the central nervous system but that does not cross this threshold is called subliminal perception. Note the difference between the absolute threshold and the threshold for conscious perception: a stimulus below the absolute threshold will not be transduced, and thus never reaches the central nervous system. A stimulus below the threshold of conscious perception arrives at the central nervous system, but does not reach the higher-order brain regions that control attention and consciousness. Contrary to common thinking, there is actually little practical value to using subliminal perception to sell products.
The Latin word for “thresholds” is limina. Hence, something that is “subliminal” is literally “below threshold.” The threshold referred to in the term “subliminal perception” is the threshold of conscious perception. So, signals that are “subliminal” are strong enough to pass the absolute threshold, but not strong enough to pass the threshold of conscious perception.
A third commonly studied threshold is the difference threshold, sometimes called the just-noticeable difference (jnd) between two stimuli. The difference threshold refers to the minimum change in magnitude required for an observer to perceive that two different stimuli are, in fact, different. If the difference between stimuli is below the difference threshold, the two stimuli will seem to the observer to be the same. For example, imagine two sound waves are played one after the other, the first having frequency 440 Hz and then the second having frequency 441 Hz. These sounds are different. But without formal ear training, most individuals cannot hear the difference. In this range of sound frequencies, the just-noticeable difference for most listeners is about 3 Hz. So, for the average person to hear a difference in pitch, the sound waves need to be 440 Hz and 443 Hz. Below this difference threshold, the two pitches will sound the same.
The previous example illustrates one common experimental technique researchers use to explore the difference threshold. The technique is called psychophysical discrimination testing, or sometimes just discrimination testing. In a common discrimination testing experiment, a participant is presented with a stimulus. The stimulus is then varied slightly and researchers ask the participant to report whether they perceive a change. Often, the difference continues to be increased until the participant reports they notice the change, and this interval is recorded as the just noticeable difference.
Returning to the example of two sounds: The difference between a 440 Hz sound and a 443 Hz sound is just noticeable for most people. But, by using discrimination testing, researchers have discovered that the absolute difference (3 Hz, in this case) is far less important than the percent difference. For this reason, the just noticeable difference is usually reported as a fraction or a percent. To compute this percent, divide the change in stimulus by the magnitude of the original stimulus. In our example, we would compute 3 Hz / 440 Hz = 0.0068 = 0.68%. To illustrate why percentages are used, consider a 1000 Hz sound. An increase of 0.68% results in a sound of frequency 1007 Hz. So, to the average person, the difference in pitch from 1000 Hz to 1007 Hz would be just noticeable. By contrast, the difference from 1000 Hz to 1003 Hz would not be noticeable. While a 3 Hz difference was noticeable in the lower frequency range, that same 3 Hz difference is not noticeable in the higher frequency range.
Ernst Heinrich Weber (1795—1878) is often credited with the observation that difference thresholds are proportional and must be computed as percentages. This idea is therefore often called Weber’s law. Weber’s law applies to the perception of a number of senses, including the perception of loudness and pitch of sounds, the perception of brightness of light, and the perception of weight of objects.
When the MCAT brings up Weber’s law, questions will usually give a numerical relationship and then ask for it to be applied; typically, it simply amounts to applying a ratio.
SIGNAL DETECTION THEORY
Perception of stimuli can also be affected by nonsensory factors, such as experiences (memory), motives, and expectations. Signal detection theory studies how internal (psychological) and external (environmental) factors influence thresholds of sensation and perception. For example, how loud would someone need to yell your name in a crowd to get your attention? The answer depends on environmental factors, like the size of the crowd; social factors, like the makeup of the crowd and your comfort with the individuals around you; psychological factors, like whether or not you are expecting to have your name called; and personality factors, like your level of introversion or extroversion. In signal detection theory, these factors are treated like independent variables. For example, researchers can measure how likely a person is to hear their name called when the person is informed that at some point their name will be called, versus when the person is left uninformed.
A basic signal detection experiment consists of many trials; during each trial, a stimulus (signal) may or may not be presented. Trials in which the signal is presented are called noise trials, whereas those in which the signal is not presented are called catch trials. After each trial, the subject is asked to indicate whether or not a signal was presented. There are therefore four possible outcomes for each trial, as illustrated in Figure 2.1. A hit is a trial in which the signal is presented and the subject correctly perceives the signal; a miss is a trial in which the subject fails to perceive the presented signal. A false alarm is a trial in which the subject indicates that he or she perceives the signal, even though the signal was not presented; a correct negative is a trial in which the subject correctly identifies that no signal was presented. By tracking the rates of these various outcomes, researchers are able to identify factors that influence perception.
Figure 2.1. Possible Outcomes from a Signal Detection Experiment Trial
On the surface, signal detection experiments would appear to be easy tasks—shouldn’t an individual easily be able to tell if he or she perceived something or not? However, consider the thought processes that occur when you’re quietly studying in the library with your phone on silent and you suddenly think you may have heard a buzz. Is my phone ringing? you wonder. You freeze in place and wait for another buzz; even if it doesn’t come, you may still be so convinced you heard a signal that you still check your phone. Perception is not a passive matter!
Our ability to detect a stimulus can change over time through adaptation. Adaptation can have both a physiological (sensory) component and a psychological (perceptual) component. For example, the pupils of the eyes will dilate in the dark and constrict in the light, which illustrates of physiological adaptation. Similarly, in loud environments, small muscles in the middle ear will reflexively contract in order to dampen the vibration of the ossicles, reducing sound intensity. We also adapt to somatosensory stimuli; cold water no longer seems so cold once our bodies “get used to it.” Once we’re dressed, we stop feeling the clothes on our bodies until we have a reason to think about them. Adaptation is one way the mind and body try to focus attention on only the most relevant stimuli, which are usually changes in the environment around us.
MCAT Concept Check 2.1:
Before you move on, assess your understanding of the material with these questions.
1. What is the pathway for a stimulus to reach conscious perception?
2. Match each sensory receptor to its function:
1. Hair cell
3. Olfactory receptor
6. Taste receptor
8. Sense painful or bothersome physical stimuli
9. Sense changes in temperature
10. Sense electromagnetic radiation in the visible range
11. Sense changes in blood concentration
12. Sense volatile chemicals
13. Sense motion of fluid in the inner ear
14. Sense dissolved chemicals
3. For each of the thresholds below, provide a brief description:
o Absolute threshold:
o Threshold of conscious perception:
o Difference threshold:
4. What aspect of thresholds do Weber’s law and signal detection theory focus on?
o Weber’s law:
o Signal detection theory:
5. How does sensory adaptation affect a difference threshold?