Coherence, cardinal directions and higher-order mechanisms.

Our initial purpose was to develop a quantitative method of estimating the cardinal directions of color space. The method is based on the finding that patterns consisting of pairs of drifting gratings modulated along different cardinal axes appear to slip with respect to one another, while the same patterns appear as a single coherent plaid if the modulation directions of the patterns are rotated by 45 deg in color space [Krauskopf & Farell (1990). Nature, 348,328-331]. A forced-choice procedure was used in which observers were asked to choose which of two successively presented patterns appeared less coherent. The patterns consisted of pairs of drifting gratings; the direction of modulation of one of the gratings was fixed and that of the other varied. For example, an estimate of an individual's isoluminant plane could be obtained by fixing the modulation of one grating in the luminance direction and finding the elevation of the modulation of the other grating that resulted in minimum perceived coherence. We found it important to take into consideration individual differences in the tilt of the isoluminant plane in color space and in the detectability of targets in the nominal cardinal directions. When this was done we found that reliable measurements could be made. The method effectively provided quantitative estimates of the cardinal directions. However, the most important result was the inadequacy of the generalization that patterns appear coherent when they share similar components along cardinal directions (Krauskopf & Farell, 1990) to account for the new results. The present results suggest that patterns appear not to cohere to the extent that they fail to stimulate common chromatic mechanisms, but the assumption that these mechanisms are tuned only along cardinal axes can be rejected. Along with other data the results point to the existence of higher-order mechanisms tuned to different isoluminant chromatic directions.