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 fi nding that patterns consisting of pairs of drifting gratings modulated along different cardinal axes appear to slip with respect to one anot her, while the same patterns appear as a single coherent plaid if the modulation directions of the patterns are rotated by 45 deg in color s pace [Krauskopf & Farell (1990), Nature, 348, 328-331], A forced-choic e procedure was used in which observers were asked to choose which of two successively presented patterns appeared less coherent, The patter ns consisted of pairs of drifting gratings; the direction of modulatio n of one of the gratings was fixed and that of the other varied, For e xample, an estimate of an individual's isoluminant plane could be obta ined by fixing the modulation of one grating in the luminance directio n and finding the elevation of the modulation of the other grating tha t resulted in minimum perceived coherence, We found it important to ta ke into consideration individual differences in the tilt of the isolum inant 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 quantitat ive estimates of the cardinal directions, However, the most important result was the inadequacy of the generalization that patterns appear c oherent 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 rejec ted. Along with other data the results point to the existence of highe r-order mechanisms tuned to different isoluminant chromatic directions .