The development of the human brain occurs rapidly in the first years of life and continues at a slower pace into adolescence. The major steps involved in brain development, both before and after birth, play important roles in psychological development.
The Cerebral Cortex
The cerebral cortex is a thin, flat sheet of cells at the outer surface of the brain. Understanding the development of this part of the brain is important for understanding psychological development, as it is FIGURE 1 thought that the cortex underlies humans' complex intellectual abilities. The cortex is divided into four lobes: the occipital, parietal, temporal, and frontal lobes, as illustrated in Figure 1. In all of these lobes the cortex consists of six layers of cells, and each of the six layers is made up of particular types of cells and connections to and from other cells. In adults, the cortical lobes can be divided even further into areas that specialize in different functions, such as language and movement.
Development of the Cerebral Cortex
Neurons (the cells of the cortex that are involved in processing information) are formed before birth during the sixth to eighteenth weeks after conception. In the cerebral cortex, neurons find their way to the correct position by moving along the long fibers of radial glia cells, which are like ropes extending from the inner to the outer surface of the brain. The length that neurons must travel is especially long for those that will end up in the frontal lobes, and this may increase the likelihood that they will end up in the incorrect position and disrupt information processing. Schahram Akbarian and his colleagues suggested in a 1993 paper that such errors might contribute to schizophrenia.
Once neurons have traveled to their final positions, they begin to differentiate or take on their mature characteristics. One aspect of differentiation is the growth and branching of dendrites. The dendrites of a neuron are like antennae that pick up signals from many other neurons and, if the circumstances are right, pass the signal down the axon and on to other neurons. The pattern of branching of dendrites is important because it affects the amount and type of signals the neuron receives. During development one change that occurs is an increase in size and complexity of neurons' dendritic trees. For example, by adulthood the length of the dendrites of neurons in the frontal cortex can increase to more than thirty times their length at birth. A second aspect of differentiation that occurs in most neurons is myelination. Myelin is a fatty sheath that forms around neurons and helps them transmit signals more quickly. Myelin begins to form around neurons before birth and continues to do so even into adulthood in some areas of the cortex.
The points of communication between neurons are called synapses, and these begin to form in the brain in the early weeks of gestation. The generation of synapses occurs at different times in different cortical areas. For example, the maximum density of synapses is reached at about four months in the visual cortex but not until about twenty-four months after birth in the prefrontal cortex. This pattern parallels behavioral development, where functions of the visual cortex (such as 3-D vision) develop earlier than some functions of the prefrontal cortex (such as planning for the future).
At the same time that the brain is growing and increasing in size and complexity, regressive events are also occurring. One example is the elimination of synapses. During the process of synapse formation, the number of synapses increases above the level observed in the adult and remains at this level for some time. Then, synapses are eliminated until the adult number is reached. For example, in certain parts of the visual cortex the density of synapses per neuron reaches a peak of about 150 percent of the adult level at about age four months then starts to decrease at the end of the first year of life to reach the adult level by about age four. The timing of this process is different for different areas of cortex. In the frontal cortex, the peak level is reached at about one year of age, and it then slowly declines to reach adult levels sometime in adolescence. This loss of synapses does not reduce the range of behaviors but may be related to the stabilization of important networks of neurons in the brain.
The adult brain has a very large number of dendritic branches and synapses between cells that are organized in a very specific way. There simply is not enough space in the human genome to specifically encode all of this information. Instead of being only a passive "readout" of genetic information, normal development of the brain depends in part on the activity of the neurons themselves. Even while the baby is still in the womb, neuronal activity (the electrical firing of cells) is very important. For example, it has been discovered that the rhythmical waves of firing of groups of receptor cells in the eyes play an important role in helping to structure some parts of the brain involved in vision. This activity cannot be a response to visual input, since the eyes are closed at this age. Instead, it appears that one part of the nervous system can create a kind of "virtual environment" specifically to aid the formation of other, later developing, parts.
The Effect of Experience on Brain Development
Once a baby is born, the external world can begin to influence the activity of neurons and thereby the pattern of brain development. According to Mark Johnson and his colleagues, for example, newborns less than one hour old tend to orient their heads and eyes to look at faces more often than many other complex patterns. This reaction is like a reflex and may well be controlled not by the cortex but by evolutionarily older, subcortical parts of the brain. All of this staring at faces serves a critical purpose in providing the necessary input for training some of the slower-developing "higher" brain areas within the cerebral cortex. Thus, infants themselves play an important and active role in determining the subsequent organization of the cerebral cortex.
One way that experience affects brain development is by determining which synapses are retained during the process of synapse elimination. Useful synapses are kept, while surplus ones are lost. This type of learning through selective synapse elimination is thought to happen only at certain points in development. This means that there are some types of learning that may only occur during certain points in development, sometimes called sensitive or critical periods. If certain synaptic connections are not laid down early in life, they are less likely to become established later in life. For example, some children are born with cataracts (a clouding of the lens that prevents patterned light from reaching the eye's receptor cells) and experience visual deprivation during the first months of life until the cataracts are treated. These children, even when tested years after vision has been restored, show some difficulties in face recognition, according to a study by Daphne Maurer and her colleagues. Thus, visual experience in the first months of life appears critical for the ability to recognize faces and cannot be replaced even by years of later experience.
The sensitivity of the young brain to the inputs it receives means that different patterns of brain organization can occur in infants with different types of experience. One example is individuals who are deaf from birth and thus do not receive typical auditory inputs. While some aspects of their visual processing remain unchanged, their processing of visual motion and information in the visual periphery are enhanced and reorganized. One interpretation is that there are surplus visual connections that are normally eliminated during development but that, in the absence of auditory input, remain and take over what would normally be auditory cortex.
Of all the cortical areas, the frontal areas appear to develop the slowest, as many functions attributed to the frontal lobe, such as planning for the future, do not mature until adolescence. This does not mean, however, that the frontal lobes are not working early in life and suddenly are "switched on" in adolescence. For example, if a seven-month-old baby watches an object being hidden in one of two locations she can remember a few seconds later where it is hidden. In contrast, a monkey with an injury to the frontal lobe has difficulty with this task. That human infants can perform a task that monkeys with damage to the frontal lobes cannot suggests that the frontal lobe is beginning to work already in young infants. The development of the ability to keep things in mind even when they are not observable may be related to the emergence of infants' objection to separation from the caregiver that often occurs around age seven to nine months. Thus, areas of the cortex that appear to develop late may be functioning in simpler ways earlier in life rather than remaining completely "silent."
Summary and Conclusions
Initially the young brain contains more components and connections than it will in adulthood, and the inputs it receives shape the elimination of this surplus. This provides a way in which individuals can develop in similar ways even if the plan of development is not encoded specifically in the human genome. Different areas of the brain develop at different times, and this is related to the development of their behavioral functions. The infant plays an active role in her own brain development by selectively attending to stimuli, such as faces and speech, that are important for subsequent development.
See also:LANGUAGE DEVELOPMENT
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