Minimally Distinct Border Judgments and Contrast: Effects of Eccentricity

Precise isolation of individual chromatic mechanisms is useful for investigations of vision. One technique for isolating chromatic pathways is “silent substitution” (modulating colors along specific color directions). The minimally distinct border (MDB) technique can be further used to determine the specific color direction that isolates the Scones (tritan axis). However, macular pigment distribution may render a foveallymeasured tritan line imprecise when stimuli are large or peripheral. We had subjects make MDB judgments at five retinal eccentricities, and compared the results to a model of macular distribution. The model predicted the direction of chromatic rotation but the predicted magnitude was less than observed. We hypothesized that the neural properties that underlie MDB judgments in the fovea may not be valid for the periphery. For example a decrease in L-M sensitivity across the visual field may result in minimum border detection shifted near the L-M axis. We investigated this issue by using a suprathreshold contrast matching task to examine the perceptual scaling of chromatic and luminance as a function of eccentricity. Subjects made contrast matches between Gabor patches across the visual field, comparing chromatic and achromatic stimuli to a foveal achromatic stimulus. Our results show a falloff in L-M and luminance sensitivity, relative to S-cone sensitivity with increasing eccentricity. Our suprathreshold results mirror those of threshold measures for chromatic pathways but not for luminance, consistent with known physiology of magno and parvo luminance pathways. Our results indicate that MDB may not be a valid peripheral indicator of the tritan axis.