The spatial vision of monkeys with experimentally induced strabismus
In the human population, amblyopia is very often associated with strabismus. To investigate the neural basis of strabismic amblyopia, we developed an animal model by producing esotropic strabismus in young infant macaque monkeys (Macaca nemestrina). The aim of this thesis was to characterize the spatial vision of monkeys raised with experimental strabismus. Specifically, we first established the validity of this animal model by comparing the monkeys' deficits in several spatial vision tasks to those of humans with strabismic amblyopia. Second, we tested the specific hypothesis that all the spatial vision deficits of strabismic amblyopes could be explained by a global impairment in their ability to process contrast signals. Finally, we provided a set of quantitative measures of visual performance that will serve as a basis for subsequent physiological investigations.
Using standard operant methods, we measured contrast sensitivity, spatial resolution, Vernier acuity, spatial phase discrimination and suprathreshold contrast discrimination in five strabismic and one normal monkeys. In addition, we used a masking paradigm to obtain information about the characteristics of the visual mechanisms underlying the monkeys' performance in these tasks.
We found that monkeys with experimental strabismus could exhibit deficits in each of the visual functions tested. The deviated eye's performance was impaired compared to the fellow and normal eyes, especially for stimuli with medium to high spatial frequencies. The fellow eyes' performance did not significantly differ from that of the normal eyes. The strabismic monkeys' deficits resembled those of humans with strabismic amblyopia. This similarity in the pattern of deficits validates the use of this animal model for the study of the neural basis of strabismic amblyopia. Finally, for monkeys with severe amblyopia, we found that the deficits in contrast processing could not completely explain their deficits in the other spatial vision tasks. This suggests that in addition to a deficit in contrast processing, one or more additional factors contribute to the poor spatial vision of strabismic amblyopes.