Responses of complex cells in the visual cortex of the cat as a function of the length of moving slits.

(1) As a step towards specifying the spatial selectivity characteristics of complex cells with spatially periodic substructures, we have studied single cell responses to narrow slits of variable length moved across the receptive field in the preferred direction. In general, the length-response curves were linear over a considerable and sometimes full range until an optimal slit length was reached. (2) In those cells in which the rate of rise of the slit length-response functions decreased before the optimal length was reached, at least 3 factors contribute to the shape of the curve. First, the receptive field shapes of some complex cells are more ovoid or rounded than rectangular, and the summation of responses from excitatory zones of varying optimal lengths itself results in a nonlinear slit length-response function at long slit lengths. Second, central regions may contribute more to cell response than do more lateral regions along the length dimension. Third, a nonlinearity in the slit length-response curve may occur in the upper range of slit lengths as a saturation effect because discharge rates may reach 600/sec, which appears to be close to a limiting firing rate. (3) Some cells believed to be complex during preliminary receptive field testing showed weak inhibitory regions beyond the region of the optimal slit length. Many of these cells also displayed periodic average response histograms to moving slits. The extent and magnitude of the inhibition were variable from cell to cell. In terms of receptive field properties, these cells and 'regular' complex cells seem part of a continuum.