On the distinctness of simple and complex cells in the visual cortex of the cat.

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Abstract
The behavior of neurons in cat striate cortex was examined in response to moving sinusoidal gratings and flashed bright and dark lines. The responses were summarized by 3 indices: discreteness was a measure of the degree of separation of inhibitory and excitatory regions in the receptive field; spatial summation ratio showed the degree of spatial summation within each region and relative modulation was a measure of the degree of modulation in the response to a moving grating. Some neurons had receptive fields with completely discrete excitatory and inhibitory regions; others responded equally to stimulus onset and offset throughout their receptive fields; however, some had overlapping excitatory and inhibitory regions. The degree of overlap varied continuously from complete separation to complete overlap. Neurons with discrete receptive fields, had the widths of the regions compared with the width of the bars in a grating of optimum spatial frequency to assess the degree of spatial summation within the regions. Most neurons with discrete receptive fields showed roughly predictable spatial summation, in that the 2 width measures agreed. About 10% of them had receptive field regions that were too large by a factor of over two. The neurons which showed incomplete spatial summation also had considerable overlap of their excitatory and inhibitory regions. The waveforms of the responses to moving gratings of optimal spatial frequency were examined. The degree of modulation in the response was continuously distributed between low values typical of complex cells and high values typical of simple cells; the distribution was not bimodal. The degree of response modulation was closely correlated with the degree to which the excitatory and inhibitory regions in the receptive field were discrete. Both the degree of spatial summation and the degree of response modulation were proposed as means for distinguishing simple and complex cells. The continuity of the distributions of both indices ensured that neither index alone could be used to class all neurons unequivocally. A criterion based on 2 indices did allow classification. Simple and complex cells showed distinctive behavior. Complex cells with distinguishable excitatory and inhibitory regions in their receptive fields were not distinctly different from other complex cells.