CHROMATIC AND LUMINANCE INTERACTIONS IN SPATIAL CONTRAST SIGNALS

We report VEP studies which delineate interactions between chromatic a nd luminance contrast signals. We examined responses to sinusoidal lum inance gratings undergoing 4-Hz square-wave contrast reversal, upon wh ich standing gratings with various admixtures of luminance and chromat ic contrast were alternately superimposed and withdrawn. The presence of the standing grating induced a VEP component at the fundamental fre quency of the contrast-reversal grating. This VEP component appeared w ithout any appreciable lag, and did not vary in amplitude over the 4 s during which the standing grating was present. The observed fundament al response differed from the fundamental component that would be expe cted from the known interaction between the luminance component of the standing grating with the modulated grating (Bodis-Wollner et al., 19 72; Bobak et al., 1988), in three ways: (1) The fundamental response w as not nulled for standing gratings that were isoluminant or near-isol uminant. (2) The chromatic dependence of the fundamental response impl ied an S-cone input to the interaction. (3) No single mechanism (drive n by a linear combination of cone signals) could account quantitativel y for the size of this response, particularly when the standing gratin g strongly modulated two cones in phase.