Abstract
Adding an equiluminance color grating to a similar high contrast luminance grating drifting in the same direction increases the motion response (i.e. to null the perceived motion, an opposing luminance grating needs a higher contrast). Such color contribution to motion could either be due to nonlinearities within the luminance pathway or to a color-opponent pathway (L − M) distinct from the luminance pathway (L + M). The fact that nonlinearities following the summation of the L- and M-cone contrast responses (e.g., [L + M]2) would not be sufficient to explain the substantial color contribution to motion has been taken as evidence of a color-opponent motion pathway (L − M). However, our simulations showed that expansive nonlinearities preceding the summation of the L- and M-cone contrast responses (e.g., L2 + M2) would substantially increase the color contribution to motion especially when the color and luminance gratings are in phase (0 or 180 deg) compared to out of phase (90 or 270 deg). To investigate if the color contribution to motion is due to a color-opponent pathway (e.g., L − M) or to early expansive nonlinearities within the luminance pathway (e.g. L2 + M2), we measured the color contribution to motion when superimposing high contrast luminance and color gratings as a function of their relative phase. We found that the color contribution to motion was about two times greater when the luminance and color gratings were in phase compared to when they were out of phase. This color-luminance phase interaction cannot be explained by a color-opponent pathway (L − M) independent of the luminance pathway (L + M). We conclude that early expansive nonlinearities within the luminance pathway substantially increase the color contribution to motion and the fact that adding a color grating to a high contrast luminance grating increases the motion response cannot be taken as evidence of a color-opponent motion pathway distinct from the luminance motion pathway.