An oblique effect of chromatic gratings measured by color-mixture thresholds

Contrast sensitivity is lower for obliquely oriented achromatic gratings th an for vertical or horizontal gratings at high spatial and low temporal fre quencies. Although this response is suggestive of mediation by P-pathway co rtical correlates, no dear sensory (ie class 1) oblique effect has been dem onstrated with isoluminant chromatic stimuli. In the present experiment, a two-alternative forced-choice detection task was used to measure observers' sensitivity to spatiotemporal sinusoids varying in orientation and color c ontrast. A maximum-likelihood method fit ellipses to the thresholds, with t he length of each ellipse taken as a measure of chromatic contrast sensitiv ity at isoluminance, and the width as luminance contrast threshold. A chrom atic oblique effect was observed at about 3 cycles deg(-1) suggesting an or ientation bias within the cortical stream conveying P-cell activity.