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. T he 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 select ive stimulation of the S-cones in man, a 3 cd/m(2) blue light was superimpo sed 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 lever o ver a circular area of variable size. The test stimuli were presented at 12 .5 degrees retinal eccentricity. Within the test-stimulus spectral band, se nsitivity was that of Stiles' pi(1) mechanism. Increasing stimulus area red uced more the decrement threshold than the increment threshold, and Ricco's area was larger for luminance decrements (0.8-2 degrees) than for incremen ts (0.6-0.9 degrees). Experiments with red-on-red stimuli confirmed that th e large summation area and stimulus-polarity-dependent spatial summation ar e 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. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.