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 evolutio-narily 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 re-

Spinal cord neurons. Neurons must travel from the spot where they are born to arrive at the particular region where they will be used in the mature brain. (Frank Lane Picture Agency/Corbis)

main 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."

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