Orientation and spatial-frequency discrimination for luminance and chromatic gratings.

We have examined the accuracy of orientation and spatial-frequency discrimination for sine-wave gratings that vary in either luminance or color. The equiluminant chromatic gratings were modulated along either a tritanopic confusion axis (so that they were detectable on the basis of activity in only the short-wavelength-sensitive cones) or an axis of constant short-wavelength-sensitive cone excitation (so that they could be detected on the basis of opposing activity in only the long- and medium-wavelength-sensitive cones). Grating contrasts ranged from the detection threshold to the highest levels that we could produce; the contrasts of the luminance and color patterns were equated for equal multiples of their respective detection thresholds. Discrimination thresholds for all patterns showed a similar dependence on stimulus contrast, rising sharply at low contrasts and becoming nearly asymptotic at moderate contrasts. However, even at threshold contrasts, observers could still reliably discriminate sufficiently large differences in the orientation or spatial frequency of all patterns, and they could also reliably identify the type of variation (luminance or which color) defining the grafting. For most conditions the discrimination thresholds did not differ from the two types of color grafting and reached values as low as 1 deg (orientation) or 4% (spatial frequency). Thus observers were able to make accurate spatial judgments on the basis of either type of chromatic information. However, these thresholds were slightly but consistently higher than the thresholds for comparable luminance graftings. This difference in the color and luminance discrimination thresholds may reflect somewhat coarser orientation and spatial-frequency selectivity in the mechanisms encoding the chromatic patterns.