MCAT Behavioral Sciences Review - Kaplan Test Prep 2021–2022

1.6 Development
A Brief History of Neuropsychology

LEARNING OBJECTIVES

After Chapter 1.6, you will be able to:

· Describe the process of neurulation

· Link the primitive reflexes with the behaviors to which they correspond

· Identify the main themes that dictate stages of motor development in children

The developmental process begins at the moment of conception. Physiological changes are rapid from embryonic to fetal stages, and well into infancy. Children exhibit surprisingly consistent patterns of motor abilities, as well as physiological changes based on age. Understanding these changes and when they occur is important in the discussion of developmental psychology. There are specific periods in development where children are particularly susceptible to environmental factors, called critical periods. Absence of the appropriate environmental factors may result in failure to learn a given skill or trait during the critical period, which may also mean learning that skill later on is difficult or even impossible.

PRENATAL

The development of the nervous system starts with neurulation, at three to four weeks’ gestational age. Neurulation occurs when the ectoderm overlying the notochord begins to furrow, forming a neural groove surrounded by two neural folds, as shown in Figure 1.13. Cells at the leading edge of the neural fold are called the neural crest, and will migrate throughout the body to form disparate tissues, including dorsal root ganglia, melanocytes (pigment-producing cells), and calcitonin-producing cells of the thyroid. The remainder of the furrow closes to form the neural tube, which will ultimately form the central nervous system (CNS). The neural tube has an alar plate, which differentiates into sensory neurons, and a basal plate, which differentiates into motor neurons. Over time, the neural tube invaginates and folds on itself many times; the embryonic brain begins as three swellings (prosencephalon, mesencephalon, rhombencephalon) that become five swellings (telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon) as it becomes the mature brain, as demonstrated in Figure 1.6 earlier in this chapter.

Figure 1.13. Development of the Nervous System

Prenatal development does not occur in a vacuum, of course, but in the mother’s uterus. Within this environment, temperature, chemical balance, orientation of the fetus with respect to gravity, and atmospheric pressure are all carefully controlled and remain relatively constant. The fetus is attached to the uterine wall and placenta by the umbilical cord. The placenta transmits food, oxygen, and water to the fetus while returning water and waste to the mother. Maternal blood supplies many of the proteins and amino acids needed for growth, although the embryo begins to produce its own proteins and amino acids as well.

A variety of external influences can have deleterious effects on the development of the fetus. A number of viruses or bacteria can cross the placenta and cause damage to the developing fetus, including rubella (German measles), which may cause cataracts, deafness, heart defects, and intellectual disability. Other viral infections, such as measles, mumps, hepatitis, influenza, varicella (chickenpox), and herpes, have been linked to various birth defects.

An unfortunate side effect of the revolution in pharmaceutical development is that many drugs that help the mother can have damaging effects on the fetus she carries. The most notorious of these drugs is thalidomide, which was prescribed in the late 1950s and early 1960s to reduce morning sickness. Mothers who took this drug while pregnant often gave birth to babies with missing and malformed limbs and defects of the heart, eyes, ears, digestive tract, and kidneys. Antiepileptic medications are associated with neural tube defects, in which the neural tube fails to close completely, leading to devastating malformations such as spina bifida or anencephaly.

A host of environmental factors and exposures may also affect maturation. Maternal malnutrition is considered to be a leading cause of abnormal development. Protein deficiency can slow growth, lead to intellectual disability, and reduce immunity to disease. Maternal narcotic addiction produces chemically dependent infants who must undergo severe withdrawal after birth. Regular cigarette smoking can lead to slowed growth, increased fetal heart rate, and a greater chance of premature birth. Daily use of alcohol also leads to slowed growth, both physically and psychologically. Finally, prenatal exposure to X-rays has been strongly linked to retardation; defects of the skull, spinal cord, and eyes; cleft palate; and limb deformities.

MOTOR

Although they may seem helpless, infants are equipped with well-developed somatic structures and a broad array of reflexes that help ensure survival. A reflex is a behavior that occurs in response to a given stimulus without higher cognitive input. While motor and startle reflexes exist in adults, infants have a number of primitive reflexes that disappear with age. For example, the rooting reflex is the automatic turning of the head in the direction of a stimulus that touches the cheek—such as a nipple during feeding. Sucking and swallowing when an object is placed in the mouth are also examples of reflexes related to feeding.

Other primitive reflexes may have served an adaptive purpose in earlier stages of human evolution, but are currently used mainly in assessing infant neurological development. By comparing the point in time at which each of these reflexes disappears relative to the established norms, it is possible to tell whether neurological development is taking place in a normal fashion. One such reflex is the Moro reflex, illustrated in Figure 1.14. Infants react to abrupt movements of their heads by flinging out their arms, then slowly retracting their arms and crying. It has been speculated that this reflex may have developed during a time when our prehuman ancestors lived in trees and falling could have been prevented by instinctive clutching. The Moro reflex usually disappears after four months and its continuation at one year is a strong suggestion of developmental difficulties. Asymmetry of the Moro reflex may hint at underlying neuromuscular problems.

Figure 1.14. The Moro ReflexThe infant extends the arms, then slowly withdraws them and cries.

The Babinski reflex causes the toes to spread apart automatically when the sole of the foot is stimulated, as seen in Figure 1.15. The grasping reflex occurs when the infant closes his or her fingers around an object placed in his or her hand. Adults with neurological diseases may exhibit these primitive reflexes, especially in illnesses that cause demyelination (loss of the myelin sheath).

Figure 1.15. The Babinski ReflexThe big toe extends while the other toes fan outward.

Although reflexive behavior dominates the repertoire of the neonate, other behaviors occur as well. Newborn infants also kick, turn, and wave their arms. These uncoordinated, unconnected behaviors form the basis for later, more coordinated movements.

Infants typically develop motor skills at about the same age, in the same order. Due to this pattern, most psychologists and doctors agree that these are innately programmed abilities for human infants. However, the educational richness of the environment has been observed to affect the rate of learning, with more enriched environments promoting quicker development.

Motor skills are broken down into two classes: gross and fine motor skills. Gross motor skills incorporate movement from large muscle groups and whole body motion, such as sitting, crawling, and walking. Fine motor skills involve the smaller muscles of the fingers, toes, and eyes, providing more specific and delicate movement. Fine motor abilities include tracking motion, drawing, catching, and waving.

SOCIAL

In addition to motor skills, social development occurs in infancy and through adolescence. At birth, the parental figure becomes the center of the infant’s world, and as the infant ages, stranger anxiety (a fear and apprehension of unfamiliar individuals) and separation anxiety (a fear of being separated from the parental figure) develop at approximately seven months and one year, respectively. During this time, play style progresses from solitary to onlooker, and at two years develops into parallel play, in which children will play alongside each other without influencing each other’s behavior. At age three, a child has an awareness of his or her gender identity, engages in sex-typed play, and knows his or her full name.

By age five, conformity to peers and romantic feelings for others begin to develop. From ages six through twelve, friend circles tend to be of the same sex without expression of romantic feelings. In the teenage years, children become more self-sufficient, and often express their desire for independence by rebelling against their parents. Cross-gender friendships become more common. Individuals also become more aware of their sexual orientation and sexual relationships begin.

In this chapter we have described several abilities and behaviors that are expected to emerge at particular times in a person's development. These skills are known as developmental milestones. The developmental milestones of the first three years of life are listed in Table 1.4. While this is a general timetable based on averages, most children fall within plus or minus two months of the chart. The goal is not to memorize this chart, but to recognize some themes. For example, gross motor skills progress in a head-to-toe order starting with the ability to lift the head, stabilize the trunk, and finally walking. There is also a correlation between the development of motor skills and proximity to the center of the body, with skills being developed at the core prior to extremities. Social skills move from being parent-oriented to self-oriented to other-oriented. Language skills, discussed in Chapter 4 of MCAT Behavioral Sciences Review, become more complex and structured.

MCAT Concept Check 1.6:

Before you move on, assess your understanding of the material with these questions.

1. Describe the process of neurulation.

2. For each of the primitive reflexes below, briefly describe the observed behavior.

3. What are the two main themes that dictate the stages of motor development in early childhood?

1.

2.