Spatial summation of blue-on-yellow light increments and decrements in human vision

In the primate retina, blue-OFF cells are less numerous than blue-ON cells but no psychophysical equivalent of this asymmetry has been found so far. The hypothesis put forward in the present study is that the ON–OFF asymmetry should manifest itself in the size and effectiveness of spatial summation of S-cone signals of opposite polarity. To test this hypothesis upon selective stimulation of the S-cones in man, a 3 cd/m 2 blue light was superimposed on a 300 cd/m 2 yellow background and the test stimulus consisted in a luminance increment or decrement of the blue light from its steady level over a circular area of variable size. The test stimuli were presented at 12.5° retinal eccentricity. Within the test-stimulus spectral band, sensitivity was that of Stiles’ π 1 mechanism. Increasing stimulus area reduced more the decrement threshold than the increment threshold, and Ricco’s area was larger for luminance decrements (0.8–2°) than for increments (0.6–0.9°). Experiments with red-on-red stimuli confirmed that the large summation area and stimulus-polarity-dependent spatial summation are specific for the isolated S-cone signals. The sign-dependency of spatial summation is probably a psychophysical correlate of the asymmetry of the ON- and OFF- visual pathways receiving S-cone input.