• Choose a category
• All
• Classic-Fiction

By Root 1396 0

when they begin to see it. Some of what the psychologists learned: In the first week infants distinguish light and dark patterns; during the first month they begin to track slowly moving objects; by the second month they begin to have depth perception, coordinate the movement of the two eyes, and differentiate among hues and levels of brightness; by three months they can glance from one object to another, and can distinguish among family members; by four months they focus at varying distances, make increasingly fine distinctions (they look longer at an oblique angle they have not seen before than at an acute angle they have seen a number of times), and begin to recognize the meaning of what they see (they look longer at a normal sketch of a face than at one in which the features have been scrambled and are in unnatural positions); and from four to seven months they achieve stereopsis, recognize that a shape held at different angles is still the same shape, and gain near-adult ability to focus at varying distances.62

A mass of comparable studies have been conducted, over the past half century, on the development of hearing, including the emergence of pitch and volume discrimination, discrimination among voices, and recognition of the direction a sound is coming from.

Exactly how maturation and experience interact in the brain tissues to produce such developmental changes is becoming clear from recent and current neuroscience research. Microscopic examination of the brains of infants who have died shows that in the first months of life a profusion of dendrites (branches) grow from its neurons and make contact with each other, as shown in Figure 19 (see page 433). This burgeoning continues apace; during the first two years of life the brain triples in size and the synaptic connections among the neurons reach astronomical numbers. (The rat’s brain, it has been estimated, forms a quarter of a million synapses—connections between nerve cells—every second during the first month of its life. In the human brain during the first months and years of life the rate of synaptic formation must be very many times greater.)

By the time a human is twelve, the brain has an estimated 164 trillion synapses.63 Those connections are the wiring plan that establishes the brain’s capabilities. Some of the synaptic connections are made automatically by chemical guidance, but others are made by the stimulus of experience during the period of rapid dendrite growth. Lacking such stimulus, the dendrites wither away without forming the needed synapses. Mice reared in the dark develop fewer dendritic spines and synaptic connections in the visual cortex than mice reared in the light, and even when exposed to light never attain normal vision. Kittens reared in a stroboscopic environment, where they see only during flashes of light, fail to develop cortical cells sensitive to movement; when they are grown cats, they see the world as a series of stills. If one eye of a young monkey is kept shut during the critical period, the neurons of that eye never catch up to those of the other eye. Thus, maturation provides—for a limited time—a multitude of potential nerve pathways among which experience makes the choice, “hard-wiring” those circuits needed for perception.64

FIGURE 19

Brain Development: These drawings of neurons in the visual cortex show the flourishing and development of the human brain in the first half-year of life.

Why should nature have done that? Since we can learn all through life—and all learning, at any age, involves the creation of new synaptic connections—why should perceptual development be possible only at a critical period and not later? Apparently, the developing brain’s “use it or lose it” policy is efficient and economical of resources; the growing neurons are preserved by myelinization (which wraps them in a fatty protective sheath), and those sensory connections that are used are further myelinized to make them more permanent. Since the essential experiences are almost always available for carrying out this process at the right