Passage II

SOCIAL SCIENCE: This passage is adapted from Morton Hunt's The Story of Psychology (©1993 by Morton Hunt). In the passage, the term maturation refers to the process of growth and development, and the term perceptual ability refers to the capacity to recognize something through the senses (sight, smell, touch, etc.).

Much maturation research is concerned with physical skills and physical attributes, and adds little to our knowledge of the growth of the mind. But research on the development of perceptual abilities begins to provide solid factual answers to the ancient central question of psychology: How much is due to nature and how much to nurture (or, in developmental terms, to maturation and to learning)?

The work has been focused on early infancy, when perceptual abilities evolve rapidly; its aim is to discover when each new ability first appears, the assumption being that at its first appearance, the new ability arises not from learning but from maturation of the optic nervous structures and especially of that part of the brain cortex where visual signals are received and interpreted.

Much has been learned by simply watching infants. What, exactly, do very young infants see? Since we cannot ask them what they see, how can we find out?

In 1961, the psychologist Robert Fantz devised an ingenious method of doing so. He designed a stand in which, on the bottom level, the baby lies on her back, looking up. A few feet above is a display area where the experimenter puts two large cards, each containing a design—a white circle, a yellow circle, a bull's-eye, a simple sketch of a face. The researcher, peering down through a tiny peephole, can watch the movement of the baby's eyes and time how long they are directed at one or the other of each pair of patterns. Fantz found that at two months babies looked twice as long at a bull's-eye as at a circle of solid color, and twice as long at a sketch of a face as at a bull's-eye. Evidently, even a two-month-old can, distinguish major differences and direct her gaze toward what she finds more interesting.

Using this technique, developmental psychologists have learned a great deal about what infants see and when they begin to see it. 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 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, 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 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.

Exactly how maturation and experience interact in the brain tissues to produce such developmental changes is becoming clear from neuroscience research. Microscopic examination of the brains of infants shows that as the brain triples in size during the first two years of life, a profusion of dendrites (branches) grow from its neurons and make contact with one another.

By the time a human is twelve, the brain has an estimated hundred trillion synapses (connections between nerve cells). 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.

Why should nature have done that? Why should perceptual development be possible only at a critical period and not later? It does not make evolutionary sense for the organism to be permanently impaired in sensory performance just because it fails to have the proper experiences at specific times in its development. But some brain researchers say that there is an offsetting advantage: the essential experiences are almost always available at the right time, and they fine-tune the brain structure so as to provide far more specific perceptual powers than could result from genetic control of synapse formation.

With that, the vague old terms nature and nurture take on precise new meaning. Now, after so many centuries of speculation, we catch the first glimpse of how mind is constructed out of matter by experience.