Color-detection thresholds in macaque monkeys and humans

Abstract

Macaque monkeys are a model of human color vision. The spectral sensitivity of the three cone classes in the two species is virtually identical, but monkey performance on psychophysical color tasks has not been well characterized. To address this gap in knowledge, we compared color-detection thresholds in three humans and three monkeys using colors sampled along eight directions in the equiluminant plane of cone-opponent color space. All subjects were tested on the identical apparatus with the same 4-alternative-forced-choice task. Targets were 2째 square, centered 2째 from fixation. Unlike previous comparative studies, performance was assessed after thousands of trials exhausting perceptual learning, using targets embedded in luminance noise (0.2째 checks) to mask residual artifacts. At plateau performance, monkeys had lower color-detection thresholds than humans for colors that modulated L-M cones, but not for colors that only modulated S cones. Differences in the cone mosaics across species may account for these results. In particular, humans appear to have higher variability in the ratio of L:M cones, leading to large patches of a given cone type. Humans also have a small gap in S-cone distribution at the fovea, and a less regular S-cone distribution. These differences favor higher spatial acuity in humans compared to monkeys, which is consistent with previous evidence. Theoretically, we expect a tradeoff between spatial acuity and color acuity. Optimal spatial acuity would be achieved by an array of a single cone type (the output of a mixed array cannot be unambiguously assigned to luminance variation across an image). The present results suggest that selective pressures produced higher chromatic sensitivity at the cost of spatial acuity amongst monkeys compared to humans, specifically for the more recently evolved L-M chromatic mechanism. These selective pressures may also account for the dramatically lower rates of color blindness amongst monkeys compared to humans.

Meeting abstract presented at VSS 2014