Studying the effects “visual deprivation” is one way of demonstrating the contribution of experience, pigeons have been raised for a period of time with their eyelids shut or with translucent hoods over their heads. When these restrictions are removed, the birds assume unusual postures as though they are disoriented in space. They find it impossible to avoid bumping into obstacles, similar perceptual abnormalities have been observed in kittens, rabbits, and chimpanzees reared in darkness and in fish brought from dark pools into lighted aquariums. In one of the earliest of these studies carried out by Riesen. Chimpanzees were raised in total darkness for sixteen months.
When they were brought into the light, these annuals behaved as though they were blind. And they were blind or almost blind because on effect of living in darkness from birth is a degeneration of the retina. The visual system requires some stimulation, if it is to develop, and experience contributes in this basic way. Later research, mostly with kittens, has shown that “visual deprivation” disturbs the functioning of the cells in the “visual cortex”.
Since these cells respond to such stimuli as “vertical” and “horizontal” “edges”, we might expect that depriving the kittens of stimulation of these specific types would cause specific “perceptual disabilities”. This expectation has been confirmed. Kittens raised in a cylinder with walls consisting entirely of vertical stripes developed both behavioural and neurological impairments.
Although they showed normal kittenish interest in a stick held vertically before them, they ignored the same stick when it was held “horizontally”. Neurologically, there was a sharp decrease in the number of cortical cells responding to horizontal stimuli. We now know that there is a critical period for the development of visual perception. If kittens were deprived of visual stimulation for only three days between the age of four and five weeks, they have extensive behavioural and neurological abnormalities. The damage is greater with greater deprivation. If adult cats are deprived of visual stimulation, however, their visual systems are unaffected.
Not only in animals but in human beings too, experience shapes our perceptual development. Although researchers do not deprive humans of normal visual stimulation, this sometimes happens naturally or as a consequence of medical treatment. After eye surgery, the operated eye is usually patched. If this happens to a child in the first year of life (a condition called strabismus), he or she will not have binocular depth perception in later life, even if the strabismus is corrected by surgery. Likewise if astigmatism (an optical defect that prevents horizontal and vertical contours to be in focus at the same moment) is not corrected early in life, individual will later have low acuity for one orientation.
These facts suggest that there is a critical period early in the development of the visual system similar to that in animals; if stimulation is restricted during this period, the system will not develop normally. The critical period is much longer in humans than in animals; it may last as long as eight years, but the greatest vulnerability occurs during the first two years of life. None of these facts indicate that we have to learn to perceive; but they do show that stimulation is essential for the maintenance and development of the perceptual capacity present at birth. Learning appears to play a role in the development of capacities that require perceptual integration, it is essential to our ability to recognise things and to explain the effects of “context” and “expectation”. In conclusion one may ask, “Is perception innate or learned?” The evidence indicates that we are horn with considerable perceptual capacity which is shaped and developed by learning. One way to conceptualise this join role of heredity and environment is that the lowest levels in the perceptual system are intact at birth and the higher levels develop as a result of learning.