MOTION MINIMA FOR DIFFERENT DIRECTIONS IN COLOR SPACE

We have used the minimum-motion stimulus of Cavanagh, MacLeod & Anstis [(1987) Journal of the Optical Society of America A, 4, 1428-1438] to examine how signals along different directions in color space interac t in motion perception. Stimuli were pairs of counterphasing gratings combined 90 deg out of phase in both space and time and modulated alon g different color-luminance axes. The axis for one of the gratings was fixed, while the axis for the second was varied so as to null perceiv ed motion in the stimulus. The motion nulls show that observers are se nsitive to motion signals carried by each of the cardinal directions o f color space [an achromatic axis and GM and S-(L + M) chromatic axes] , but that signals along different cardinal axes are not combined to y ield a net direction of motion, Pairing an achromatic and chromatic gr ating resulted in a motion null regardless of the relative or overall contrast of the two gratings, while the null directions for intermedia te axes shifted depending on contrast, This result points to the speci al status of the luminance and chromatic axes, However, our results do not reveal a special pair of axes within the equiluminant plane. When contrasts along the cardinal axes are scaled for equal multiples of t heir respective detection thresholds, the GM and S chromatic contrasts contribute roughly equally to the perceived motion, but are many time s weaker than luminance contrast, Moreover, sensitivity to luminance m otion is little affected by the presence of chromatic contrast, wherea s sensitivity to chromatic motion is strongly masked by either luminan ce or chromatic contrast, These asymmetric interactions suggest that t he motion of the luminance and chromatic components is encoded in qual itatively different ways. (C) 1997 Elsevier Science Ltd.