1. Evidence was obtained that the impulse/quantum (I/Q) ratio of the central response mechanism of retinal ganglion cells in the cat is controlled by the steady effective retinal flux of the background. 2. One experiment revealed that the I/Q ratio was decreased as the area of an adapting spot, of constant illumination, was increased. The curve relating the I/Q ratio to background flux was the same regardless of the size of the adapting spot. 3. The effective central summing area of many retinal ganglion cells was determined. For the same cells, the transition level (Enroth‐Cugell & Shapley, 1973) of the impulse/quantum curve was also measured. Diffuse illumination at the transition level was inversely proportional to the effective summing area, when variation in dark‐adapted sensitivity between cells was taken into account. 4. Therefore, retinal ganglion cells with large central summing areas are more light‐adapted by any given diffuse background than cells with small centres.
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