Keeping an eye on the brain: the role of visual experience in monkeys and children.

The quality of visual experience during infancy determines the functional sensitivity and precision of the mature primate visual system. Infant monkeys subjected to monocular form deprivation show a period of critical visual development that, though decreasing in sensitivity, lasts throughout the first 2 years of life. Photopic threshold spectral sensitivity appears to have a briefer critical period, which is essentially complete by 6 months old, whereas scotopic visual functions appear well developed by 3 months old. Binocular visual functions seem to have the longest period of sensitivity to abnormal visual experience because periods of monocular form deprivation initiated during the first 2 years affect visual functions. Viewing the world through prisms, which mimics the condition of strabismus, causes a permanent loss of cortical binocular cells and stereopsis in monkeys. This result explains stereoblindness in children having equivalent clinical histories.