The development of spatial resolution and contrast sensitivity in naturally strabismic monkeys

1. The development of spatial vision was studied in seven naturally strabismic monkeys. Six monkeys were esotropic; one was exotropic. Spatial resolution and contrast sensitivity were measured monocularly as a function of age. 2. The development of spatial vision was considerably slower in the naturally strabismic monkeys than in normal monkeys. Spatial resolution appeared to be normal near birth, but followed a developmental time course that included at least the first two postnatal years; in normal monkeys, this process is complete within the first year after birth. 3. Initially, resolution was similar of the two eyes in each animal. Thereafter, most animals developed interocular differences in spatial resolution and contrast sensitivity. In some cases these differences persisted throughout the period of study. In other cases, the deficits became smaller over time. 4. An analysis of the contrast data revealed that the functions from all monkeys were of similar shape regardless of the age or degree of amblyopia. The major feature of development in the second year was attributable to improvement of the nonpreferred eyes. 5. The form of the visual deficits found in the naturally strabismic monkeys resemble those seen in strabismic humans. These data lend further support to the suggestion that macaque monkeys provide a good model for visual function in human strabismus. The data also suggest that it may not be appropriate to equate the length of the 'sensitive period' with the time period for normal visual development.