The detection of a luminance-defined Gabor is improved by two high contrast, aligned, flanking Gabors, an effect termed collinear facilitation. We investigate whether this facilitation also occurs for isoluminant chromatic stimuli, and whether it can occur for chromatic targets with luminance flanks and vice versa. We measured collinear facilitation for Gabor stimuli (0.75 cpd, 1 octave bandwidth) of three different contrast types: achromatic, red-green that isolates the L/M-cone opponent mechanism, and blue-yellow that isolates the S-cone opponent mechanism. Three conditions were investigated: (1) target and flanks all of the same contrast type and spatial phase; (2) target and flanks of the same contrast type but opposite phases (0° and 180°); and (3) target and flanks of different contrast types (chromatic with achromatic contrast) and two opposite phase combinations. We find that a similar degree of collinear facilitation occurs for the isoluminant chromatic stimuli as for the achromatic stimuli, and all exhibit phase dependency. Facilitation did not occur, however, between chromatic and achromatic target and flanking stimuli. This suggests that at the level of collinear facilitation, the chromatic and the achromatic postreceptoral mechanisms have their own spatial interactions that are segregated from one another.
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