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INTRODUCTION
The human visual system is sensitive to the effects of depriving vision only during a limited period of time in childhood. This critical period is importane in the study and treatment of ambi y op ia. Prior to rhe Nobel Prize-winning studies of Hubel and Wiesel, the existence of the sensitive period of visual development was not known. Physicians did not realize that monocular deprivation during the sensitive period would result in potentially permanent changes in the visual system causing poor vision. In the 1950s, it was considered hopeless to obtain useful vision following the removal of a monocular cataract in an infant. Investigators were puzzled as to why the removal of a cataract from an adult usually resulted in a good visual result, while the same operation m a young child tesulted in such poor vision. Before the early effects of visual deprivation were understood, cataracts in infants were often not removed until the child was older than 6 months of age. Now, with the known importance of the sensitive period of visual development, cataracts are usually removed during the first few weeks of life with significantly improved outcomes.1
VISUAL DEVELOPMENT
For normal visual development to occur during the critical period, an infants visual system requires propet stimulation. During fetal development, differentiation and organization of the visual system are likely guided by intrinsic control mechanisms. At birth, the process is not complete and must undergo considerable development and modification. Unlike the prenatal period, environmental factors and visual experience influence die process m postnatal life. During the sensitive period, depriving vision can interrupt normal visual development.
Much of our information about the sensitive period of visual development comes from basic science experimental work in the field of neuroscience. Little basic science data are available on the human sensitive period, although some individual autopsy cases have been reported.2 However, an understanding of the animal research done on the critical period is helpful in understanding the sensitive period of humans.
Animal Research Involving Critical Period
Early studies by Hubel and Wiesel on the sensitive period of visual development involved monocular deprivation in kittens and evaluating the anatomical and physiological changes of the lateral geniculate nucleus (LGN) and cortex. Their first series of experiments involved monocular...