Disparity sensitivity and receptive field incongruity of units in the cat striate cortex

1. Receptive field correspondence and binocular interaction were studied in the cat striate cortex using fundus photography for correction of residual eye drifts. 2. The scatter of incongruities (frequently called receptive field disparities) was found to be equal in horizontal and vertical direction (S.D. 0.5 °, N = 96). Incongruity was not correlated with field orientation. 3. Using binocular stimulation, disparity tuning and binocular facilitation were analysed quantitatively in 46 cells. Seven of these were disparity specific, i.e. showed marked facilitation critically dependent on disparity. 4. With monocular stimulation, three of the seven disparity specific cells could be driven only from one eye; one gave weak responses to both left and right monocular stimulation, and one no monocular responses at all. 5. The disparity specific cells preferred vertical or oblique orientations, and vertical disparity was less critical than horizontal disparity. The disparity tuning curves, plotted with reference to the experimentally determined horopter, showed peaks within 0.5 ° crossed and 0.2 ° uncrossed disparity. The smallest half-widths at half-amplitude were 0.25 °. Some of the curves were asymmetric, or step-like, with a steep slope at zero disparity. 6. It is concluded that stereoscopic depth may be signalled by disparity specific cells in area 17. These cells could account for stereoscopic acuity of a few min arc, if one assumes that depth is encoded in the amount of activity of single units, or in the graded imbalance of activity of antagonistic units. 7. It is argued that the distribution of incongruities does not reflect stereoscopic function because the disparity specific cells often cannot be plotted in both eyes, and most of the units that can be plotted are not disparity selective.