CONTRAST DEPENDENCE OF COLOR AND LUMINANCE MOTION MECHANISMS IN HUMANVISION

CONVENTIONAL VieWS Of visual perception propose a colour-blind pathway conveying motion information and a motion-blind pathway carrying colo ur information1,2. Recent studies show that motion perception is not a lways colour blind, is partially depen dent on attention5,6, can show considerable perceptual slowing around isoluminance7-9 and is contrast -dependent10,11. If there is a single motion pathway, receiving lumina nce and chromatic input, then the dependence of relative perceived vel ocity on relative stimulus contrast should be the same for both lumina nce and chromatic targets. Here we provide a distinctive characterizat ion of the motion mechanisms using a robust velocity-matching task. A relative contrast scale allows direct comparison of the performance wi th luminance and chromatic targets. The results show that the perceive d speed of slowly moving coloured targets at isoluminance has a steep contrast dependence. The perceived speed of slowly moving luminance ta r-ets shows a much lower contrast dependence. At high speeds the contr ast dependence is low for both luminance and isoluminant stimuli, alth ough the behaviour is unlike either of the slow mechanisms. The result s suggest two independent pathways that perceive slowly moving targets : one is luminance-sensitive and the other is colour-sensitive. Fast m ovement is signalled via a single motion pathway that is contrast-inva riant and not colour blind.