Failure of direction discrimination at detection threshold for both fast and slow chromatic motion

Separate pathways have recently been proposed for "fast" and "slow" motion, whose properties differ in the way that color contrast is processed [see N ature (London) 367, 268 (1994); Trends Neurosci. 19, 394 (1966); and Vision Res. 36, 1281 (1996) and 35, 1547 (1995)]. One reported difference is that for slow motion the direction of chromatic stimuli cannot be determined at detection threshold, whereas at higher temporal rates detection and direct ion discrimination threshold coincide. Using a carefully designed psychophy sical procedure, we measured simultaneously the thresholds for detection, d irection discrimination, and color identification for isoluminant red-green and achromatic Gabor patches (1.5 cpd), over the range of visible temporal frequencies (1-16 Hz). We find that the color of both the red-green and th e achromatic targets can be identified at detection threshold, indicating e ffective isolation of the luminance and the red-green mechanisms at all sti mulus speeds. For the achromatic mechanism, direction discrimination was al ways possible at detection threshold. For the red-green mechanism, we find that direction discrimination thresholds are significantly greater than det ection thresholds at all stimulus speeds. This result calls into question m odels of chromatic motion processing that are dichotomized along the lines of stimulus speed. (C) 1998 Optical Society of America [S0740-3232(98)01012 -6].