Spatial characteristics of the contrast gain control in the cat's retina.

Impulse rate was recorded from X- and Y-type ganglion cells in the cat's retina. The stimuli were stationary gratings for which luminance varied sinusoidally with distance across the stimulus, and amplitude varied sinusoidally in time. Y cell fundamental and second harmonic responses recorded at medium to high spatial frequencies advanced in phase with increasing contrast, an effect attributable to the contrast gain control. As spatial frequency increased to the highest value capable of evoking a second harmonic response, the phase of this response became retarded, indicating that the contrast gain control was losing its effect. This result showed that the spatial resolution of the contrast gain control is very close to that of the Y cell's rectifying subunits. The observations strongly suggest that the contrast gain control has its effect early in retinal image processing.