Second-order frequency responses were obtained from cat retinal ganglion cells of the Y type. The cells were stimulated by a spatial sine grating whose contrast was modulated in time by a sum of eight sinusoids. 2. Second-order frequency responses obtained at higher contrasts have a peak amplitude at higher input temporal frequency, and phase shifts, compared to their low-contrast counterparts. 3. This change in shape of the second-order frequency response is a departure from the prediction of the linear/static non-linear/linear sandwich model of the non-linear pathway in the cat retina. The departure is analysed by means of the hypothesis that the two filters of the sandwich model are parametric in contrast. 4. Most of the change in shape of the second-order frequency response with contrast is accounted for in terms of the sandwich model by changes in the transfer characteristics of the filter preceding the static non-linearity. 5. The effect of contrast on the second-order responses of Y cells is qualitatively similar in several ways to the effect of contrast on first-order responses. This suggests that the contrast gain control mechanism acts early in the retina, before linear and non-linear pathways have diverged.
