Nonlinear spatial summation and the contrast gain control of cat retinal ganglion cells.

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Abstract
How responses to visual stimuli at spatially separated locations were combined by cat retinal ganglion cells was studied. The temporal signal which modulated the stimuli was a sum of sinusoids. Fourier analysis of the ganglion cell impulse train yielded 1st order responses at the modulation frequencies, and 2nd order responses at sums and differences of the input frequencies. Spatial stimuli were spots in the center and periphery of the cell''s receptive field. Four conditions of stimulation were used: center alone, periphery alone, center and periphery in phase and center and periphery out of phase. The effective 1st order response of the center was defined as the response due to center stimulation in the presence of periphery stimulation, but independent of the relative phases of the 2 regions. Likewise, the effective 1st order response of the periphery was defined as the response due to periphery in the presence of center stimulation, but independent of the relative phases of the 2 regions. These effective responses may be calculated by addition and subtraction of the measured responses to the combined stimuli. There was a consistent difference between the 1st order frequency kernel of the effective center and the 1st order kernel of the center alone. The amplitudes of the effective center responses were diminished at low frequencies of modulation compared to the isolated center responses. Alos, the phase of the effective center''s response to high frequencies was advanced. Such non-linear interaction occurred in all ganglion cells, X or Y, but the effects were larger in Y cells. In addition to spatially uniform stimuli in the periphery, spatial grating patterns were also used. These peripheral gratings affec d the 1st order kernel of the center even though the peripheral gratings produced no 1st order responses by themselves. The temporal properties of the non-linear interaction of center and periphery were probed by modulation in the periphery with single sinusoids. The most effective temporal frequencies for producing non-linear summation were: 4-15 Hz when all the visual stimuli were spatially uniform and 2-8 Hz when spatial grating patterns were used in the periphery. The characteristics of non-linear spatial summation observed in these experiments are explained by the properties of the contrast gain control mechanism previously postulated.