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

Abstract

Separate pathways have recently been proposed for “fast” and “slow” motion, whose properties differ in the way that color contrast is processed [see Nature (London)367, 268 (1994); Trends Neurosci.19, 394 (1996); and Vision Res.36, 1281 (1996) and Vision Res.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 direction discrimination threshold coincide. Using a carefully designed psychophysical procedure, we measured simultaneously the thresholds for detection, direction 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 the achromatic targets can be identified at detection threshold, indicating effective isolation of the luminance and the red–green mechanisms at all stimulus speeds. For the achromatic mechanism, direction discrimination was always possible at detection threshold. For the red–green mechanism, we find that direction discrimination thresholds are significantly greater than detection thresholds at all stimulus speeds. This result calls into question models of chromatic motion processing that are dichotomized along the lines of stimulus speed.