DETECTING AND DISCRIMINATING THE DIRECTION OF MOTION OF LUMINANCE ANDCOLOR GRATINGS

Human observers were required to discriminate the direction of motion of vertically moving, 1 c/deg luminance and colour gratings. The grati ngs had different contrasts and moved at temporal frequencies between 0.5 and 32 Hz. Sensitivity [the reciprocal of the contrast at which pe rformance reached 75% correct in a temporal two-alternative forced-cho ice (2 AFC) discrimination task] was a band-pass function of temporal frequency for luminance gratings, and a low-pass function of temporal frequency for colour gratings. Further, when colour contrast was expre ssed in terms of the modulation in cone excitation produced by the sti mulus, sensitivity to colour gratings was greater than sensitivity to luminance gratings at frequencies below 2 Hz. On the other hand, at te mporal frequencies above 4 Hz, sensitivity to colour gratings was comp arable with sensitivity to luminance gratings of double the temporal f requency. Detection sensitivity was measured for luminance and colour gratings of 1, 4 and 16 Hz. With either colour or luminance gratings, detection thresholds were very similar to those for direction-of-motio n discrimination. This result confirms findings of Mullen and Boulton [(1992) Vision Research, 32, 483-488] and Cavanagh and Anstis [(1991) Vision Research, 31, 2109-2148], but is different from that reported b y Lindsey and Teller [(1990) Vision Research, 30, 1751-17611 who used a smaller stimulus seen in a parafoveal region and found that motion d iscrimination thresholds were higher than detection threshold for colo ur gratings. We repeated our threshold measurements using parafoveal v iewing conditions similar to those used by Lindsey and Teller (1990). We found that, although for luminance gratings detection thresholds we re very close to direction-discrimination thresholds, for colour grati ngs, they were lower. The result is in qualitative agreement with Lind sey and Teller (1990). Our results suggest that low-level, or ''first- order'' motion mechanisms are not as sensitive to chromatic gratings a s are colour-detection mechanisms.