INTERACTION BETWEEN CENTER AND SURROUND IN RABBIT RETINAL GANGLION-CELLS

1. The interaction between the center and surround mechanisms of a var iety of rabbit retinal ganglion cell classes was examined in extracell ular single-unit recordings in an isolated eyecup preparation. Ganglio n cell classes studied included ON and OFF brisk sustained and transie nt, ON and OFF sluggish sustained and transient, ON-OFF and ON directi onally selective, orientationally selective, and large field units. Th e surround effects observed were qualitatively similar in all these ga nglion cell classes. 2. The average reponse-versus-contrast functions for stimuli within the ganglion cells' receptive-field centers were re latively linear between threshold and saturation for all ganglion cell classes examined. The major effect of surround stimulation on the cen ter response-versus-contrast function was a reduction in the slope of the linear portion of the curve, rather than a downward, parallel shif t of the function. Stimulation of the surround had no systematically s ignificant effect on the contrast threshold for the center spot, and, when it did have a significant effect, it sometimes decreased, rather than increased the magnitude of the threshold. 3. Step changes in surr ound contrast were most effective when they were made simultaneously w ith step changes in the center; surround inhibition decreased signific antly when it preceded stimulation of the center by >100 ms and was ge nerally ineffective when preceding the center by >500 ms. The decrease in the inhibitory effect of surround stimulation was a monotonic func tion of delay between 0 and 500 ms. 4. Stimulation of the surround by step changes in the contrast of a sine-wave grating annulus produced q ualitatively similar re suits to those obtained for pure luminance mod ulations. This suggests that the surround mechanism observed in these experiments was not due to pure luminance adaptation within the surrou nd. The inhibitory effect of sine-wave gratings in the surround decrea sed monotonically as a function of spatial frequency. 5. Stimulation w ith a spot and an annulus that were both entirely within the ganglion cell's excitatory receptive-held center typically yielded nonadditive summation at contrasts whose linear sum of responses were below satura tion. The effect of an annulus within the receptive-field center on re sponses elicited by a central spot quantitatively resembled the inhibi tion elicited from annuli in the inhibitory surround, after the excita tory center response due to the annulus was taken into account. These results suggest that the inhibition elicited from the surrounds of the ganglion cells in these experiments extended into their receptive-fie ld centers. 6. The strength of inhibition elicited from the surround d eclined with reductions in the background level, both for a given modu lation depth, and for the same absolute modulation amplitude. At the l owest backgrounds used, there was no apparent inhibition of center res ponses by the surround. 7. Particularly for transient ganglion cells, the nature of the interaction between surround and center mechanisms i s generally consistent with nonlinear, division-like models for center -surround interaction, such as contrast gain control, but not with lin ear models in which the surround is subtractively antagonistic to the center.