A psychophysical measurement and analysis of motion perception in normal and binocularly deprived monkeys

PURPOSE. To measure psychophysically the thresholds for motion detection in the nasal and temporal directions under monocular viewing conditions in mo nkeys reared under conditions of daily alternating monocular occlusion (AMO ). The hypothesis was that motion perception would be asymmetric with more sensitivity for motion in the nasal direction. METHODS. Three monkeys subjected to AMO (AMO monkeys) and three normal monk eys were studied. All were trained with operant conditioning techniques to discriminate coherent from random motion in a random dot display. The perce ntage of dots in the display that moved either left or right was varied. Th resholds for motion detection of nasally directed and temporally directed s timuli were measured to determine whether the motion perception of AMO monk eys was asymmetric, as predicted. RESULTs. A two-factor analysis of variance revealed a statistically signifi cant difference between treatment groups (normal versus AMO) and directions (nasal versus temporal) and a significant interaction. The interaction was due to a significant difference between nasal and temporal directions for the AMO group, but no significant difference for the normal group. Planned comparisons were performed based on each animal's best eye (eye most sensit ive to nasal motion) and worst eye (eye least sensitive to temporal motion) . No significant differences were found between the two groups in the best eyes' responses to the nasal direction, but the worst eyes' responses in th e temporal direction were significantly poorer in the AMO group. A neural m odel that can account for these findings is based on a Hebbian teacher loca ted in the nucleus of the optic tract that strengthens connections of a sub population of directionally selective cortical neurons. CONCLUSIONS. AMO rearing results in asymmetric motion perception. Threshold s for detecting nasally directed motion are normal, whereas thresholds for detecting temporally directed motion are deficient. These results demonstra te that motion-processing mechanisms in primates exhibit experience-depende nt developmental neural plasticity. The locus of the neural plasticity coul d be a subpopulation of directionally selective neurons in the striate cort ex (V1).