The lateral spread of light adaptation in cat horizontal cell responses

To investigate the sites of light adaptation processes in the mammalian distal retina, we studied the lateral spread of adaptation signals in cone-driven cat horizontal (H-) cell responses. The size of the adaptation pool is compared to the receptive field for H-cell responses. H-cell activity was recorded intracellularly in the optically intact, in vivo eye. It is demonstrated that light adaptation as measured in H-cells is not a strictly local process. Background light falling outside a central test region effectively modulates the responses to a small test light, flashed on the receptive field center. The integration area for adaptation signals was quantitatively compared to the H-cell receptive field size by measuring the desensitizing effect of background light on the responses to a small centered test spot, as a function of background spot size. The area-adaptation function is comparable to the area—response function but has a slightly smaller length constant. Light adaptation in H-cell responses, therefore, reveals spread of adaptation over a large distance and is probably mediated through lateral interactions in the H-cell network rather than in the cones.