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

- Longitudinal studies of children with temporary visual deprivation from cataracts reveals the role of early visual input in normal development.
- Early visual deprivation causes later sleeper effects.
- Visual experience early in life is necessary for the later development of both low level and high level vision.
- Compensation for early visual deprivation may involve recruitment of alternative neural networks.
- The effects of early visual deprivation are altered by competition between the eyes for neural connections and possibly, cross-modality competition.

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.