Critical periods re-examined: Evidence from children treated for dense cataracts

Studies of children treated for dense cataracts afford an opportunity to examine the role of visual experience in driving visual perceptual development. Collectively, the data indicate that there are multiple periods during which deprivation can damage visual development, but their timing and duration cannot be predicted from the normal developmental trajectory. For lower level vision, the deficits are worse in the previously deprived eye if the deprivation had been monocular rather than binocular, but for higher level perception, that pattern reverses, perhaps because of cross-modal neural completion during the deprivation. Emerging neuroimaging evidence suggests that the neural underpinnings of vision after early visual deprivation may be abnormal, even when the deprivation ended shortly after birth and normal behavioural performance has been achieved. The implication is that in the baby with normal eyes, despite poor acuity and contrast sensitivity, visual experience at birth sets up the neural architecture for later refinement.