Effects of Extensive Psychophysical Testing on Stereoacuity and Disparity Sensitivity of V1 and V2 Neurons in Macaque Monkeys

Abstract

Abstract: : Purpose: Strabismus early in life is known to disrupt binocular vision development and previous investigators have suggested that after the "end" of a critical period, the visual cortex is not sufficiently plastic to be modified and, thus, there is no possibility for functional recovery. However, an increasing body of evidence suggests that the functional connections in higher order visual areas mature later than in V1 and show substantial plasticity for much longer periods of time. In this study, using operant psychophysical methods, we investigated whether extensive stereoscopic threshold testing in mature monkeys that had experienced early strabismus resulted in improved cortical disparity sensitivity compared to untrained monkeys. Methods: Infant macaque monkeys were reared wearing goggles that contained a pair of base–in prisms. Behavioral testing began around 2 years of age. At the conclusion of psychophysical testing microelectrode recording experiments were conducted under anesthesia and paralysis. Results: We found that at the beginning of psychophysical testing, stereoacuity of our strabismic monkeys was about 50 times worse (50 arcmin) than that in normal controls (less than 1 arcmin). After extensive testing for 2 years, their stereoacuities improved to 5–15 arcmin. Microelectrode recording experiments in V1 and V2 of these monkeys indicated that V2 neurons, but not V1 neurons, of strabismic monkeys that had undergone visual testing showed a modest but significantly higher disparity sensitivity than monkeys that did not receive visual testing. Conclusions:Together, our results suggest that the observed stereoacuity improvement may largely reflect extensive perceptual learning by neurons in higher order visual areas to maximally utilize residual local disparity information that remained in V1 and V2.

Keywords: strabismus • binocular vision/stereopsis • electrophysiology: non-clinical