We measure threshold for a vertical test grating superimposed on a fixed-contrast horizontal background grating of the same spatial and temporal frequency. The rate of change of this threshold with increasing contrast of the background grating is a measure of the contrast gain of the responding mechanism. Large slopes (high contrast gains) occur when spatial frequency is low and temporal frequency is high; small slopes (low contrast gains) occur when both spatial and temporal frequencies are low and when spatial frequency is high. This division of the spatiotemporal frequency domain into low- and high-gain regions is consistent with the transient/sustained dichotomy found in previous psychophysical studies. Furthermore, our results suggest that the mechanism responsible for detecting low spatial frequencies has a gain characteristic similar to that of cat retina Y cells and that the mechanism responsible for detecting high spatial frequencies has a gain characteristic similar to that of cat retina X cells, as found by Shapley and Victor [ J. Physiol. (London) 285, 275– 298 ( 1978)].
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