Motion at isoluminance: discrimination/detection ratios and the summation of luminance and chromatic signals.

Under some conditions, direction-of-motion thresholds are elevated with respect to detection thresholds for isoluminant chromatic stimuli. In the present study we investigated the effect of small luminance mismatches on ratios of direction-of-motion thresholds to detection thresholds (M/D ratios). The stimuli were 2 deg x 2 deg patches of a moving 1-cycle/deg, 2.75-deg/s sinusoidal grating, modulated spatially in luminance, chromaticity, or both. M/D ratios were close to 1 except when luminance modulation was < 1%, defined with respect to the individual subject's isoluminance point; in this range M/D ratios varied from 1.5 to 4.4. Sets of thresholds for both detection and motion tasks conformed approximately to ellipses. Thus, although motion processing shows a differential loss of sensitivity at isoluminance, similar summation rules appear to apply to the combination of luminance and chromatic signals for both tasks.