The likelihood of motion capture is more strongly determined by the degree of color contrast of adjacent gratings than by the degree of luminance contrast

Purpose. Ramachandran(1987) showed that a color-defined isoluminant object is induced to move by the motion of a luminance-defined object. The most common explanation for this motion capture is that a weaker motion/location signal from a chromatic edge is overridden by a stronger signal from a luminance-defined edge. The purpose of the current study is to examine whether this is the case. Method.The stimulus consisted of 5 gratings: 1) an isoluminant color grating in a square aperture surrounded by luminance-defined random-dot-noise is presented in the center of the screen; 2) 2 gratings elongated in the vertical direction are located at above and below of the center grating; 3) 2 gratings elongated in the horizontal direction are located to the left and right of the center grating. In half of the trials, the top and bottom gratings were isoluminant while the left and right gratings were luminance-defined. In the other half, the arrangements were switched. The luminance contrast of the luminance-definedgratings and the color contrast of isoluminant gratings were varied randomly. The subjective isoluminant points for each color contrast were measured in pilot experiments and are assigned in the experiment. In each trial, subjects had to judge directions, horizontal or vertical, the central grating moved. Results.The results indicate that the likelihood of the central grating appearing to move in the direction along the isoluminant flanks is higher with larger color contrast of the isoluminant flanks. In contrast, the likelihood is not determined either by the degree of luminance contrast of the luminance-defined flanks or by the degree of the similarity between the color contrast of the central grating and the isoluminant flanks. Conclusion. The results suggest that color contrast that has been overlooked is a strong deterministic factor in motion capture.