SPATIAL ASYMMETRIES IN CAT RETINAL GANGLION-CELL RESPONSES

Enroth-Cugell and Robson (1966) first proposed a classification of ret inal ganglion cells into X cells, which exhibit approximate linear spa tial summation and largely sustained responses, and Y cells, which exh ibit nonlinearities and transient responses. Gaudiano (1992a, 1992b, 1 994) has suggested that the dominant characteristics of both X and Y c ells can be simulated with a single model simply by changing receptive field profiles to match those of the anatomical counterparts of X and Y cells. He also proposed that a significant component of the spatial nonlinearities observed in Y (and sometimes X) cells can result from photoreceptor nonlinearities coupled with push-pull bipolar connection s. Specifically, an asymmetry was predicted in the ganglion cell respo nse to rectangular gratings presented at different locations in the re ceptive field under two conditions: introduction/withdrawal (on-off) o r contrast reversal. When measuring the response to these patterns as a function of spatial phase, the standard difference-of-Gaussians mode l predicts symmetrical responses about the receptive field center, whi le the push-pull model predicts slight but significant asymmetry in th e on-off case only. To test this hypothesis, we have recorded ganglion cell responses from the optic tract fibers of anesthetized cat. The m ean and standard deviations of responses to on-off and contrast-revers ed patterns were compared. We found that all but one of the cells that yielded statistically significant data confirmed the hypothesis. Thes e results largely support the theoretical prediction.