Human beings, unlike many animals, are altricial: they are born with an impoverished set of perceptual and motor skills that make parental care during the first few years of life--and sometimes much longer--essential. Infants are obviously limited in their ability to move in and manipulate their environments, but they also show marked impairments in perception. In this section we will discuss the development of adult-like visual capabilities and the environmental factors that are necessary for it.

At birth, the child is fairly limited in his or her ability to distinguish the combinations of colors and lines that make up the visual world. At a distance of one foot, the infant can distinguish between a gray patch and a striped patch only when the stripes are at least one tenth of any inch thick. (The infant's acuity is determined using the looking-time measure: if a child looks longer at one of the patches than the other, the child is judged able to distinguish between them.) Over the course of the year, the child's acuity improves to one eightieth of an inch; much better, but still nowhere as good people who are six years old or older, who generally can distinguish stripes that are one three-hundredth of an inch thick at a distance of one foot.

What is responsible for this improvement in vision? One answer is that the cones and rods in the retina, the cells that transduce light into neural signals, mature over the course of the first few years. Another, perhaps more important reason is that the brain itself is rapidly developing during that period. As ever- improving visual signals arrive at the primary visual cortex, the cortex itself adapts in order to sensitively and reliably detect differences in those signals. The importance of brain plasticity in the development of visual acuity can be seen in animals that have been deprived of particular types of visual stimulation. Cats, for instance, that have not been exposed to vertical edges during the first few months of life (this can be done by raising them in special environments or with striped blinders) cannot distinguish such edges when they are later exposed to them. Monkeys that have been deprived of vision in one eye during the first few months of life, through the use of sutures or eye patches, fail to develop another important perceptual skill: binocular vision, the ability to perceive depth through the slight differences in retinal stimulation between the two eyes. Perceptual development is obviously largely dependent on the biology of sensory systems, but these examples show that environment has a crucial role to play as well.