Detecting and discriminating the direction of motion of luminance and colour gratings.

Human observers were required to discriminate the direction of motion of vertically moving, 1 c/deg luminance and colour gratings. The gratings had different contrasts and moved at temporal frequencies between 0.5 and 32 Hz. Sensitivity [the reciprocal of the contrast at which performance reached 75% correct in a temporal two-alternative forced-choice (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 expressed in terms of the modulation in cone excitation produced by the stimulus, sensitivity to colour gratings was greater than sensitivity to luminance gratings at frequencies below 2 Hz. On the other hand, at temporal frequencies above 4 Hz, sensitivity to colour gratings was comparable with sensitivity to luminance gratings of double the temporal frequency. 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-motion 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 by Lindsey and Teller [(1990) Vision Research, 30, 1751-1761] who used a smaller stimulus seen in a parafoveal region and found that motion discrimination thresholds were higher than detection threshold for colour gratings. We repeated our threshold measurements using parafoveal viewing conditions similar to those used by Lindsey and Teller (1990). We found that, although for luminance gratings detection thresholds were very close to direction-discrimination thresholds, for colour gratings, they were lower. The result is in qualitative agreement with Lindsey and Teller (1990). Our results suggest that low-level, or "first-order" motion mechanisms are not as sensitive to chromatic gratings as are colour-detection mechanisms.