Color discrimination in the tufted capuchin monkey, Sapajus spp

Abstract

The present study evaluated the efficacy of an adapted version of the Mollon-Reffin test for the behavioral investigation of color vision in capuchin monkeys. Ten tufted capuchin monkeys (Sapajus spp., formerly referred to as Cebus apella) had their DNA analyzed and were characterized as the following: one trichromat female, seven deuteranope dichromats (six males and one female), and two protanope males, one of which was identified as an "ML protanope." For their behavioral characterization, all of the subjects were tested at three regions of the Commission International de l'Eclairage (CIE) 1976 u'v' diagram, with each test consisting of 20 chromatic variation vectors that were radially distributed around the chromaticity point set as the test background. The phenotypes inferred from the behavioral data were in complete agreement with those predicted from the genetic analysis, with the threshold distribution clearly differentiating between trichromats and dichromats and the estimated confusion lines characteristically converging for deuteranopes and the "classic" protanope. The discrimination pattern of the ML protanope was intermediate between protan and deutan, with confusion lines horizontally oriented and parallel to each other. The observed phenotypic differentiation confirmed the efficacy of the Mollon-Reffin test paradigm as a useful tool for evaluating color discrimination in nonhuman primates. Especially noteworthy was the demonstration of behavioral segregation between the "classic" and "ML" protanopes, suggesting identifiable behavioral consequences of even slight variations in the spectral sensitivity of M/L photopigments in dichromats.

Figure 1. Example of the stimulus arrangement adopted in the present study.

A pseudoisochromatic stimulus composed of circles of varying sizes and luminance is shown. The approximately square area with a different hue from background is the target.

Figure 2. Testing phase parameters.

(A) Chromaticity points used as background. (B) Distribution of the 20 test vectors. The CIE 1976 u′v′ coordinates for each background chromaticity are presented. Also shown are the angle and dominant wavelength for each vector tested from the achromatic point (Background 1) of the CIE 1976 u′v′ chromaticity diagram. Negative values refer to vectors that contain “purples and magentas,” which are nonspectral colors and have no dominant wavelength but are described by their complementary dominant wavelength, lying on the opposite side of the diagram.

Figure 3. Spectral sensitivity curves of L and M cone pigments from capuchin monkeys

. The maximum absorption peak of the visual pigments were predicted from the amino acid sequences from exons 3 and 5 of the X-linked opsin.

Figure 4. Example of staircases leading to threshold calculation.

The figures shows the sequences produced by a male monkey in a blue-yellow oriented vector (7) and red-green oriented vector (19). Each point represents the distance, in u′v′ units, between the chromaticity points that define the target and background. The distance was decreased following correct responses and increased following incorrect responses.

Figure 5. Color discrimination thresholds obtained for capuchin monkeys.

The figure shows the thresholds obtained for 20 vectors around the achromatic point of the CIE 1976 u′v′ chromatic diagram (Background 1). The data were grouped according to the phenotypic classification derived from the genetic analysis.

Figure 6. Discrimination ellipses obtained for capuchin monkeys.

The figure shows the best-fitting ellipses derived from the thresholds obtained in the 20 test vectors for each background. Deuteranopes are represented by data from subject M4.