Visual acuity: the role of visual input in inducing postnatal change

Some crude visual abilities are present at birth, and hence, do not depend on visual experience. However, there are substantial and rapid postnatal improvements. For example, the acuity of newborns is 40 times worse than that of normal adults, largely because of retinal immaturities. Between birth and 6 months of age, there is a five-fold increase in acuity, followed by slow improvement to adult levels by 6 years of age. This review examines the role of visual experience in inducing those improvements by comparing the visual development of normal children with that of children treated for congenital cataracts that blocked patterned visual input until the cataracts were removed surgically and the eyes were given a compensatory optical correction. The acuity of children treated for congenital cataracts does not improve before they receive patterned visual input, but then increases rapidly to reach normal limits by 1 year of age. However, the patients later show permanent deficits in acuity, presumably because the initial deprivation caused damage to the visual cortex. Studies of children who developed cataracts after birth indicate that visual input is also necessary to consolidate cortical connections. Moreover, the deleterious effects of visual deprivation are worse if there was also uneven competition between the eyes — because the deprivation was monocular and there was little patching of the non-deprived eye — but only at some points during development. Thus, the development of visual acuity is shaped by experience-dependent and competitive mechanisms that have different temporal parameters.