Spatial summation of S-cone ON and OFF signals: effects of retinal eccentricity.

We studied spatial summation for S-cone ON and OFF signals as a function of retinal eccentricity in human subjects. S-cone isolation was obtained by the two-colour threshold method of Stiles, modified by adding blue light to the yellow background. Test stimuli were blue light increments or decrements within a circular area of variable size. These were presented for 100 ms at 0 to 20 deg along the horizontal temporal retinal meridian. Ricco's area of complete spatial summation was measured from the threshold vs. area curves. This was nearly constant and approximately the same for both types of stimuli within the 0-5 deg range and increased beyond this range. The decremental area increased faster, suggesting that separate mechanisms, presumably ON and OFF, integrate S-cone increments and decrements. The results appear to provide new evidence for the existence of separate S-cone ON and OFF pathways. We compare the data with known morphology of primate retina and assume that, if S-cone decrements are detected via separate OFF cells, these should differ in density and dendritic field size from the S-cone ON cells, but only in the retinal periphery.