SELECTIVITY OF CAT AREA-18 NEURONS FOR DIRECTION AND SPEED IN DEPTH

Fifty-eight area 18 cells recorded in anesthetized and paralyzed cats were tested for selectivity for direction in depth after their monocular velocity characteristics and static disparity profile were determined. Direction in depth was produced by changing the speed and direction in the two eyes, but keeping the speed along axes in depth constant. Forty-two cells were completely investigated, which means that direction in depth selectivity was tested at least at two different position disparities and two different bar speeds. Seven out of the 42 cells were accepted as direction in depth selective. The 16 remaining cells were incompletely tested. Only one of them was direction in depth selective at the disparity and speed tested and shared all the properties of the seven completely tested direction in depth selective cells. Therefore we estimated that 8/58, i.e., 14% of the area 18 cells are direction in depth selective. The direction in depth selective cells are a very homogeneous class: they all belong to the S family, are velocity tuned, monocular, prefer orientations close to vertical, and have a broad inhibitory or an unmodulated position disparity profile. Direction in depth selectivity arises both from monocular properties and binocular interactions. These binocular interaction profiles can be symmetric or asymmetric. The change of these interaction profiles with changes in base speed can be summarized as changes in level of inhibition for the axes corresponding to equal speed in the two eyes on one hand and changes in the slope of the inhibition gradient centered on these axes of equal speed on the other hand. Nineteen of the 58 cells were tested for selectivity for speed along trajectories in depth. All five direction in depth selective cells tested were also tuned to speed in depth. This suggests that area 18 contributes to the elaboration of the 3D velocity map. A wiring diagram that accounts for the binocular interactions underlying direction selectivity in depth is presented.