Evidence for separate pathways for color and luminance detection mechanisms.

We measure threshold versus contrast (TvC) functions for chromatic (red-green) and luminance sine-wave-grating stimuli for (1) the detection of luminance in the presence of color contrast and (2) the detection of color in the presence of luminance contrast. We find that, although these crossed TvC functions both display a dipperlike shape, their facilitation differs from that found for standard uncrossed dipper functions (luminance on luminance or color on color contrast). Their facilitation disappears (cross condition 1) or is reduced (cross condition 2) by randomized presentation of the phase of the test and the mask, and the remaining facilitation (cross condition 2) displays no spatial tuning. We argue that these crossed facilitatory interactions cannot be explained by detection mechanisms with common inputs from color and luminance contrast (a nonindependence of transduction), and we present evidence that instead they reflect the use of local cues in the stimuli. We also measure the luminance-luminance TvC function in the presence of a fixed suprathreshold color contrast. The results demonstrate that, even when the color contrast produces a masking of the luminance thresholds, luminance-luminance facilitation still occurs. Thus the opposing effects of masking and facilitation can occur simultaneously. Furthermore, while luminance-luminance facilitation occurs independently of color contrast, masking can be produced by either contrast. This suggests that masking and facilitation have different underlying origins. Similar results are found for the color detection thresholds in the presence of a luminance pedestal. We conclude that there are separate pathways for the detection of color and luminance contrast, each with no input from the other contrast. We suggest that the cross masking reflects divisive interactions between these pathways that is restricted to high contrasts.