Sensitivity to colour- and to orientation-carried motion respectively improves and deteriorates under equiluminant background conditions.

This study presents two distinct effects produced by manipulation of the background illumination on the directional sensitivity to colour- and orientation-carried motion. The two motion percepts were produced with two of a class of stimuli extensively used by the first and last authors in apparent-motion studies. The stimuli were designed to produce motion perception by virtue of spatiotemporal matching of (a) colour with orientation systematically mismatched (Colour across Orientation, CxO) and of (b) orientation with colour systematically mismatched (OxC). An increase in background illumination from dark to the equiluminance point (relative to the luminance of the discrete stimulus microelements) entails a significant increase and decrease of directional performances with CxO and OxC stimuli, respectively. It is proposed that these anti-symmetrical background effects have distinct neurophysiological origins. For CxO stimuli, improvement of directional performances at the equiluminant point is presumably due to the inactivation of the inhibitory effect of the luminance-motion pathway on the chromatic-motion pathway. The opposite effect obtained with OxC stimuli, previously referred to as the veto effect (Gorea and Papathomas, 1988 Invest. Ophthal. Vis. Sci. Suppl., 29, 265), is supposed to be entailed by the inactivation of the luminance-oriented mechanism, the only motion sensitive mechanism activated by this stimulus configuration.