The effects of brief unrestricted vision during early monocular form deprivation on the monocular response properties of cortical neurons.

Abstract

Abstract: : Purpose: Monocular form deprivation early in life is known to result in amblyopia. We previously reported that if 4 hours of unrestricted vision were given every day during monocular deprivation periods, amblyopia did not develop, while one or two hours of unrestricted vision were less effective in preventing amblyopia. In this study, we determined whether alterations in the monocular response properties of V1 and V2 neurons are involved in these differential effects of unrestricted vision on amblyopia. Methods: Microelectrode recording experiments were conducted under anesthesia and paralysis in 6 macaque monkeys reared with monocular diffuser between 3 weeks and 4 months of age. The rearing regimen included daily unrestricted vision for one, two or four hours. When the monkeys were two years of age, the spatial contrast sensitivity functions were obtained for the deprived and non–deprived eyes. Upon completion of the behavioral testing, we measured the optimal spatial frequency, spatial resolution, orientation bandwidths, and directional bias of individual V1 and V2 neurons. Results: To our surprise, there were no significant interocular differences in any of these monocular response properties of V1 or V2 units even in those monkeys that exhibited severe deprivation amblyopia (monkeys with one hour of unrestricted vision). However, in V2 (but not in V1) of these severely amblyopic monkeys, we could not find units dominated by the behaviorally amblyopic eye. Conclusions: These results suggest that a severe loss of V2 units dominated by the amblyopic eye is more closely associated with the neural mechanisms underlying the observed amblyopia than altered monocular response properties of V1 or V2 units.

Keywords: amblyopia • visual cortex • electrophysiology: non–clinical