HUE SCALING OF ISOLUMINANT AND CONE-SPECIFIC LIGHTS

Using a hue scaling technique, we have examined the appearance of colo red spots produced by shifts from white to isoluminant stimuli along v arious color vectors in order to examine color appearance without the complications of the combined luminance and chromatic stimulation invo lved in most previous hue scaling studies, which have used flashes of monochromatic light. We also used spots lying along cone-isolating vec tors in order to determine what hues would be reported with a change i n activation of only single cone types or of only single geniculate op ponent-cell types, an issue of direct relevance to any model of color vision. We find that: 1. Unique hues do not correspond either to the c hange in activation of single cone types or of single geniculate oppon ent-cell types. This is well known to be the case for yellow and blue, but we find it to be true for red and green as well. 2. These conclus ions are not limited tot he particular white (Illuminant C) used as an adapting background in most of the experiments. Shifts along the same cone-contrast vectors relative to different backgrounds lead to much the same hue percepts, independent of the starting white used. 3. The shifts of the perceptual colors from the geniculate axes are in the di rections, and close to the absolute amounts, predicted by our [De Valo is & De Valois (1993). Vision Research, 33, 1053-1065] multi-stage col or model in which we postulate that the S-opponent cells are added to or subtracted form the M- and L-opponent cells to form the four percep tual color systems. 4. There are distinct asymmetries with respect to the extent to which various hue within each perceptual opponent system deviate from the geniculate opponent-cell axes. Blue is shifted more from the S-LM axis than is yellow; green is shifted more from the L-M axis than is red. There are also symmetries in the angular extent of t he opponent color regions. Blue is seen over a larger range of color v ectors than is yellow, and red over a slightly larger range than green . 5. Such asymmetries are not accounted for by any model that treats r ed-green and yellow-blue each as unitary, mirror-image opponent-color systems. Although red and green are perceptually opponent, the red and green perceptual systems do not appear to be constructed in a mirror- image fashion with respect to input from different cone types or from different geniculate opponent-cell types. The same is true for yellow and blue. (C) 1997 Elsevier Science Ltd. All rights reserved.