Perceptual scaling of suprathreshold chromatic increments and decrements using Maximum Likelihood Difference Scaling

Abstract

Color contrast provides rich information about the world. In the present study, we produce perceptual scales for suprathreshold color contrasts of different major directions in cone contrast space. In particular, we compared cone isolating increments and decrements (L±, M±, S±), as well as achromatic increments and decrements (A±), equiluminant R and G, and increments and decrements along the color direction that modulates L and M cones equally with S cones unmodulated (L=M±). The method we used is Maximum Likelihood Difference Scaling (MLDS; Knoblauch & Maloney, 2008), a fast way of producing interval perceptual scales. For each color direction, seven contrast values were linearly sampled between the observer’s threshold and maximum contrast of the monitor. In each trial, four practiced observers compared two pairs of 1.5°x1.5° squares. The squares were flashed for 0.33 seconds. All four squares were of the same color direction but with four different contrasts. Observers chose the pair in which the two squares looked more different, or in other words, the pair that had a larger perceptual interval. To traverse all interval pairs generated by 7 contrast values, 35 trials were measured for each color direction in a given session, and repeated three times over sessions. The MLDS results are nicely fit with a Michaelis-Menten function, producing a decelerating curve shape for each tested color direction; this function is an estimate of the contrast transducer that may underlie contrast discrimination (Foley & Legge, 1981). The general pattern was slightly more curvature for increments than decrements in most color directions. Achromatic increment scales are the most nonlinear of all directions we examined, consistent with processing by a mechanism summing cone signals that are processed less linearly for increments than decrements. Our results indicate that MLDS is a good method for perceptual scaling of suprathreshold chromatic increments and decrements.