Temporal and spatial response characteristics of the cat superior colliculus.

We have examined the responses of 72 cells of the cat superior colliculus to drifting gratings of sinusoidal luminance profile as a function of spatial frequency velocity and contrast. Of 72 cells, 66 responded to gratings either by change in mean firing rate only (58/72) or in a temporally modulated pattern in addition to the change in mean firing rate (8/72). The remaining 6 showed no change in discharge rate in response to any of the gratings tested. Many cells (24/72) were inhibited or excited by particular combinations of spatial and temporal frequencies. Some (8/72) demonstrated selective inhibition or excitation to a particular temporal frequency independent of spatial frequency and velocity and could therefore be said to be tuned specifically to temporal frequency. No cells were tuned only to a constant spatial frequency or a constant velocity. (24/72) cells displayed maximum inhibition or excitation only at a particular combination of spatial and temporal frequencies. Some cells (8/72) demonstrated a temporal modulation synchronous with the drifting grating in addition to an elevated mean discharge rate. The change in discharge rates evoked by gratings are generally less than those evoked by presentation of moving small slits or spots of light. Collicular cells often demonstrate a center-surround organization in their response to gratings. The center and surround often differ in their spatial frequency and velocity preferences. Compared to cortical and retinal ganglion cells, individual collicular cells are extremely non-linear. On a cell population basis, however, a linear Fourier analysis on grating response predicts the collicular cells' preference for movement of small objects.