Responses of macaque V1 neurons to binocular orientation differences

Interocular differences in orientation occur during binocular viewing of a surface slanted in depth. These orientation disparities could be exploited by the visual system to provide information about surface slant, but gradie nts of positional disparity provide an equally effective means to the same end. We examined the encoding of orientation disparities in V1 neurons that were recorded from two awake fixating monkeys. Monocular orientation selec tivity was measured separately in each eye. Although the preferred monocula r orientation in the left and right eyes was highly correlated (r = 0.98), 19 of 61 cells showed a significant interocular difference in preferred ori entation (IDPO). By itself, an IDPO does not imply a specific binocular sel ectivity for orientation differences. We therefore examined the response to 25 binocular combinations of orientations by pairing each of five orientat ions in one eye with five in the other. Forty-four of 64 neurons showed res ponses that reflected the monocular orientation tuning selectivity; the pre ferred orientation disparity changed when the monocular orientation was cha nged in either eye. The remaining third (20 of 64) responded to a consisten t orientation disparity in a way that was not simply predictable from monoc ular orientation selectivity. However, nearly all of these neurons were sel ective for positional disparity, and several characteristics of the respons es suggest that the apparent selectivity for orientation disparities was ju st a consequence of the positional disparity sensitivity. Neither the data presented here nor previous data from the cat (Blakemore et al., 1972; Nels on et al., 1977) support the idea that a population of neurons early in the visual system has a separate encoding scheme for orientation disparities.