BINOCULAR SPATIAL PHASE TUNING CHARACTERISTICS OF NEURONS IN THE MACAQUE STRIATE CORTEX

We employed microelectrode recording techniques to study the sensitivi ty of individual neurons in the striate cortex of anesthetized and par alyzed monkeys to relative interocular image disparities and to determ ine the effects of basic stimulus parameters on these cortical binocul ar interactions. The visual stimuli were drifting sine wave gratings. After the optimal stimulus orientation, spatial frequency, and directi on of stimulus movement were found, the cells' disparity tuning charac teristics were determined by measuring responses as a function of the relative interocular spatial phase of dichoptic grating pairs. No atte mpts were made to assess absolute position disparities or horizontal d isparities relative to the horopter. The majority (similar to 70%) of simple cells were highly sensitive to interocular spatial phase dispar ities, particularly neurons with balanced ocular dominances. Simple ce lls typically demonstrated binocular facilitation at the optimal phase disparity and binocular suppression for disparities 180 degrees away. Fewer complex cells were phase selective (similar to 40%); however, t he range of disparity selectivity in phase-sensitive complex cells was comparable with that for simple cells. Binocular interactions in non- phase-sensitive complex cells were evidenced by binocular response amp litudes that differed from responses to monocular stimulation. The deg ree of disparity tuning was independent of a cell's optimal orientatio n or the degree of direction tuning. However, disparity-sensitive cell s tended to have narrow orientation tuning functions and the degree of disparity tuning was greatest for the optimal stimulus orientations. Rotating the stimulus for one eye 90 degrees from the optimal orientat ion usually eliminated binocular interactions. The effects of phase di sparities on the binocular response amplitude were also greatest at th e optimal spatial frequency. Thus a cell's sensitivity to absolute pos ition disparities reflects its spatial tuning characteristics, with ce lls sensitive to high spatial frequencies being capable of signaling v ery small changes in image disparity. On the other hand, stimulus cont rast had relatively little effect on a cell's disparity tuning, becaus e response saturation occurred at the same contrast level for all rela tive interocular phase disparities. Thus, as with orientation tuning, a cell's optimal disparity and the degree of disparity selectivity wer e invariant with contrast. Overall, the results show that sensitivity to interocular spatial phase disparities is a common property of stria te neurons. A cell's disparity tuning characteristics appear to largel y reflect its monocular receptive field properties and the interocular balance between excitatory and inhibitory inputs. However, distinct f unctional classes of cortical neurons could not be discriminated on th e basis of disparity sensitivity alone.