Temporal and chromatic properties of motion mechanisms.

We measured threshold contours in color space for detecting drifting sinusoidal gratings over a range of temporal frequencies, and for identifying their direction of motion. Observers were able to correctly identify the direction of motion in all directions of color space, given a sufficiently high contrast. At low temporal frequencies we found differences between luminance and isoluminance conditions; for isoluminance there was a marked threshold elevation for identification when compared to detection. The threshold elevation for identification is dependent on eccentricity as well as on temporal frequency. At high temporal frequencies there were no differences between detection and identification thresholds, or between thresholds for luminance and isoluminance. A quantitative analysis of the threshold contours allowed us to identify two mechanisms contributing to motion: a color-opponent mechanism with a high sensitivity at low temporal frequencies and a luminance mechanism whose relative sensitivity increases with temporal frequency. An analysis of the cone contributions to motion detection and identification showed that L-cones dominated threshold behavior for both detection and identification at high temporal frequencies. There was a weak S-cone input to motion detection and identification at high temporal frequencies.