A comparison of binocular depth mechanisms in areas 17 and 18 of the cat visual cortex

Abstract

The retinal disparity sensitivity of neurons in areas 17 and 18 of the cat visual cortex was examined. The response of each cell to an optimally oriented slit was measured as disparity was varied orthogonally to the receptive field orientation. Eye movements were monitored with a binocular reference cell simultaneously recorded in area 17 (Hubel and Wiesel). Two types of disparity-sensitive cells, similar to those observed in the monkey by Poggio and Fischer were found. The 1st type, tuned excitatory cells, were usually binocular and had a sharp peak in their disparity-response curve. They responded maximally at the disparity that brought their receptive fields into superposition on the tangent screen. This disparity closely coincided with the disparity at which the reference cell''s receptive fields were superimposed. This point was the fixation point or 0.degree.. The 2nd type, near and far cells, were most often monocular. They gave their weakest response (usually no response at all) at 0.degree.. On 1 side of 0.degree. the response grew linearly for up to 4.degree. and then remained at the maximum. On the other side of 0.degree. it remained at the minimum for up to several degrees before rising towards the maximum. The receptive field organization of several disparity-sensitive cells was examined using the activity profile method of Henry, Bishop and Coombs. The size and strength of the discrete excitatory and inhibitory regions of the receptive fields of a cell can quantitatively account for the shape of its disparity-response curve. The laminar distribution of disparity sensitivity and of several other receptive field properties in areas 17 and 18 was studied. The organization of the 2 areas was remarkably similar in many respects. There was a difference in the proportions of the 2 types of disparity-sensitive cells in the 2 areas. Area 17 contained many more tuned excitatory cells than near and far cells; area 18 had the reverse distribution. The cells in area 18 were sensitive to a much broader range of disparities. Both areas contained disparity-sensitive neurons. These differences imply different roles in depth vision. A new model of stereopsis in which depth is signalled by the pooled activity of large groups of cells (Richards) is consistent.