Invariants of spatial summation for S (short wavelength) cone vision

Spatial summation in the human visual system was studied as a function of retinal eccentricity upon selective stimulation of the short-wavelength sensitive cones. The area of complete spatial summation (Ricco's area) was found to increase with retinal eccentricity while the threshold of stimuli equal in size with Ricco's area remained constant. Comparisons with known morphology of the small bistratified retinal ganglion cells, the only cells known to be excited by S-one ON stimulation, showed that Ricco's area included 2-4 such cells and is up to 1.5 times larger than the dendritic field of a single cell. These relationships were relatively constant within the eccentricity range tested (5-20 deg along the temporal horizontal meridian) and might be the source of threshold invariance of stimuli matching Ricco's area.