We investigated receptive field properties of cat retinal ganglion cells with visual stimuli which were sinusoidal spatial gratings amplitude modulated in time by a sum of sinusoids. Neural resp>onses were analyzed into the Fourier components at the input frequencies and the components at sum and difference frequencies. The first-order frequency response of X cells had a marked spatial phase and spatial frequency dependence which could be explained in terms of linear interactions between center and surround mechanisms in the receptive field. The second-order frequency resp onse of X cells was much smaller than the first-order frequency response at all spatial frequencies. The spatial phase and spatial frequency dep)endence of the first-order frequency response in Y cells in some ways resembled that of X cells. However, the Y first- order response declined to zero at a much lower spatial frequency than in X cells. Furthermore, the second-order frequency response was larger in Y cells; the second-order frequency comp>onents became the dominant part of the response for patterns of high spatial frequency. This implies that the receptive field center md surround mechanisms are physiologically quite different in Y cells from those in X cells, and that the Y cells also receive excitatory drive from an additional nonlinear receptive field mechanism.
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