Contrast sensitivity and vernier acuity in amblyopic monkeys

Human psychophysical studies suggest that strabismic and anisometropic amblyopes may have characteristically different patterns of visual loss. In particular, anisometropic amblyopes often show deficits on spatial localization tasks that scale with their spatial resolution losses, whereas strabismic amblyopes can show localization deficits that are large relative to their losses in spatial resolution. We have compared the performance of non-human primates with experimentally-induced anisometropic and strabismic amblyopia on contrast detection and vernier acuity tasks. The performance of both groups of animals was fundamentally similar: both strabismic and anisometropic monkeys showed deficits in spatial localization that were large relative to their resolution losses, although the animals with the most disproportionate losses were strabismic. We investigated the extent to which contrast sensitivity losses accounted for the vernier acuity deficits. The results showed that, in most cases of either strabismic or anisometropic amblyopia, when the vernier stimuli for each eye were equated in terms of effective contrast, the extent of the vernier acuity deficit was reduced to approximately the extent of the spatial resolution deficit. In two cases, both of strabismic amblyopia, we found that equating the stimuli in this way was not sufficient to make the deficits equal, a pattern that has been described for human strabismic amblyopes.