Motion at isoluminance: motion dead zones in three-dimensional color space

Under some conditions, moving isoluminant stimuli perceptually slow down or even appear to stop. The purpose of the experiment was to explore the shape of the motion dead zone, the region of color space over which the perception of stopped motion occurs. Subjects viewed a small patch of moving grating (2.3 deg x 2.3 deg, 1.3 cycles/degree, 2.9 deg/s), that was spatially modulated in chromaticity, luminance, or both, presented either foveally or at 2-deg eccentricity. The bars of the grating moved from both edges inward toward the center of the patch. Subjects set perceptual motion boundaries by adjusting the contrast of the luminance-modulation component of the grating. Over most or all of the available gamut of chromatic contrasts, the upper and lower boundaries of the motion dead zone formed two parallel planes near the V lambda-isoluminant plane in three-dimensional color space. The data thus suggest that under the conditions of the experiment, perceptual-motion boundaries are determined largely or entirely by the luminance contrast of the stimulus. The data also provide the most extensive evidence available to date for the additivity of motion photometry.