Equivalent luminance contrast of red-green drifting stimuli: dependency on luminance-color interactions and on the psychophysical task.

The equivalent luminance contrast (EqLC) of red-green drifting stimuli was assessed by three independent methods. The first method [method (a)] consisted in adjusting the luminance contrast of a yellow, equichromatic stimulus to match the direction-discrimination performances that were obtained with a red-green, equiluminant stimulus. The second method [method (b)] was a replica of the standard motion-cancellation technique proposed by Cavanagh et al. [J. Opt. Soc. Am. A 1, 894 (1984)]. The third method [method (c)] consisted in adjusting the luminance contrast of the same yellow, equichromatic stimulus as in method (a) to match the perceived speed of the red-green, equiluminant stimulus. The three estimated EqLC's are all different. It is argued that differences between EqLC's assessed by means of methods (a) and (b) result from unbalanced interactions between the chromatic and achromatic, directional-sensitive mechanisms and that differences between EqLC's assessed by means of methods (a) and (c) reveal unequal transfer efficiencies from the directional to the speed-processing stages in the chromatic and achromatic pathways.