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

1. The retinal disparity sensitivity of neurones 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 & Wiesel, 1970).2. Two types of disparity-sensitive cells were found, similar to those observed in the monkey by Poggio & Fischer (1977). The first 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 also superimposed. By analogy with the monkey this point was taken to be the fixation point, or 0 degrees . The second type, near and far cells, were most often monocular. They gave their weakest response (which was usually no response at all) at 0 degrees . On one side of 0 degrees the response grew linearly for up to 4 degrees and then remained at the maximum. On the other side of zero, it remained at the minimum for up to several degrees before rising towards the maximum.3. The receptive field organization of several disparity-sensitive cells was examined using the activity profile method of Henry, Bishop & Coombs (1969). The size and strength of the discrete excitatory and inhibitory regions of the receptive fields of a cell could quantitatively account for the shape of its disparity-response curve.4. The laminar distribution of disparity sensitivity as well as of several other receptive field properties in areas 17 and 18 was studied. The organization of the two areas was remarkably similar in many respects. There was a difference, however, in the proportions of the two types of disparity-sensitive cells in the two areas. Area 17 contained many more tuned excitatory cells than near and far cells, while area 18 had the reverse distribution. In addition, the cells in area 18 were sensitive to a much broader range of disparities. While both areas contain disparity-sensitive neurones, these differences suggest that they play different roles in depth vision.5. Recent psychophysical and neurophysiological evidence has led to a new model of stereopsis in which depth is signalled by the pooled activity of large groups of cells (Richards, 1971). The current results are consistent with this model.