Sensitivity to spatial phase at equiluminance

We have measured sensitivity for discriminating the spatial phase of multi-harmonic and two-harmonic patterns modulated either in luminance or in chromaticity (red-green). The multi-harmonic patterns were either highpass squarewaves, lines or ramps. For all patterns, contrast thresholds for discriminating 0 from 180 deg phase were similar to those for discriminating -90 from 90 deg, for luminance or chromatic modulation (or both). For all types of multi-harmonic patterns, the ratio of contrast thresholds for the phase discrimination to that for pattern detection was the same for luminance and chromatic modulation, and for combinations of both. Similarly, phase thresholds, the minimum detectable differences in phase (about a mean 0 deg), were the same for chromatic and luminance patterns, provided that contrast was scaled to equate detection thresholds of the patterns. Similar results were observed for simple three-harmonic patterns (f + 2f + 3f), and for (f + 2f) two-harmonic patterns. Strangely, however, two-harmonic patterns of f + 3f (first two terms of square-wave) of moderate to high spatial frequency did show a two-fold advantage for luminance over colour, as Troscianko and Harris (1988) have previously reported (Vision Research, 28, 1041-1049), possibly because the two harmonics have a greater separation in frequency. However, for most classes of patterns, sensitivity for spatial phase is as good for chromatic as for luminance modulation, suggesting that similar sorts of mechanisms operate under these two conditions.