SCOTOPIC AND MESOPIC LIGHT ADAPTATION IN CATS RETINA

The effects of light adaptation on retinal on-center ganglion cell firing were measured in the scotopic-mesopic range (10-5-1 cd/m2). Using diffuse adapting stimuli, the maintained discharge rate of all on-center units increases with increasing adapting luminance up to 10-3-10-2 cd/m2. Above this luminance it levels off or decreases. The change of the maintained discharge rate with increasing adapting luminances was related to changes in the receptive field organization of the unit and to changes in retinal sensitivity. The sensitivity of the retina at the different adapting luminances was measured by determining the luminance ΔI of a test spot which elicited a constant criterion ganglion cell response. ΔI increased with increasing adapting luminance not proportional to IA(Weber's law) but proportional to IAn(n varying between 0.45 and 0.75). Correspondingly, by changing the adaptation level, the intensity: response curve of a ganglion cell for small center stimuli was displaced on the intensity-axis by less than the adapting luminance step. The intensity response-curves at the different adaptation levels were parallel and S-shaped in the semilogarithmic coordinates with a straight course over 1.5 log units. The intensity-response relation for small center stimuli was best fitted by a log-function at all adaptation levels. It is concluded that the ganglion cell itself is not involved in the adaptation mechanism. Adaptation and spatial summation of signals occur at different levels of the retina. © 1969 Springer-Verlag.