BEHAVIORAL-STUDIES OF THE SENSITIVE PERIODS OF DEVELOPMENT OF VISUAL FUNCTIONS IN MONKEYS

The age-dependent effects of monocular form deprivation on psychophysically determined visual functions were investigated in rhesus monkeys. Monocular form deprivation was initiated at various ages 1 to 25 months and maintained for 18 months. The effects of form deprivation varied across the visual functions evaluated. Absolute scotopic sensitivity was depressed by form deprivation initiated only at 1 or 2 months of age. Photopic, increment-threshold spectral sensitivity functions showed alterations in sensitivity levels for monkeys treated at 5 months of age or earlier, with no effect thereafter. Monocular form deprivation at 1 to 5 months of age resulted in profound deficits in spatial modulation sensitivity. The effects of spatial vision decreased systematically as the age of onset was delayed from age 6 to 18 months. Finally, binocular summation measures revealed an absence of binocular vision even for the monkey form deprived at 25 months of age, i.e. the experimental treatment series failed to define the upper limit of the age-range for the effects of monocular form deprivation on binocular summation mechanisms. Comparisons of the age-dependent effects of monocular form deprivation across the various functions demonstrated that the sensitive period, i.e. the period of life during which a visual function may be altered by monocular form deprivation, was different for each of the psychophysical measures of visual function.