Motion at isoluminance: discrimination/detection ratios for moving isoluminant gratings

Subjects viewed a 2.3 x 2.3 deg patch of a moving 1.3 c/deg, 3.75 Hz sinusoidal grating, centered 1.8 deg from fixation. Two-alternative forced-choice contrast thresholds were measured along the luminance axis and 10 chromatic axes at isoluminance for three tasks: detection (D), form discrimination (F), and discrimination of upward from downward motion (M). F/D threshold ratios averaged approx. 1:1 on all axes. M/D ratios were approx. 1:1 on the luminance axis, but varied from 3:1 to indeterminately large with chromatic axis at isoluminance. We conclude that under the present conditions there are large, highly specific losses of direction-of-motion information at isoluminance. The results imply the existence of chromatic channels that are labeled for form but not for direction of motion at threshold. The pattern and significance of variations in M/D ratios within the isoluminant plane is also discussed.