Valdir Filgueiras Pessoa

Color vision perception in the capuchin monkey (Cebus apella): a re-evaluation of procedures using Munsell papers.

In previous experiments based on color vision discrimination of Munsell chips, Pessoa et al. (Behav Brain Res 1997;89:285-8) presented evidence of trichromatic color vision in capuchin monkeys. However, the possibility of subjects using brightness cues was not eliminated. The purpose of the present study was to reassess the color perception in Cebus apella in a similar behavioral paradigm, but using a range of brightness values at each tested hue. We now report that male capuchins show a dichromatic behavior. The results are discussed in terms of the hypothesis of male dichromatism in the New World monkey, the biological role of color vision and cognitive abilities of capuchin monkeys.

The adaptive value of primate color vision for predator detection

The complex evolution of primate color vision has puzzled biologists for decades. Primates are the only eutherian mammals that evolved an enhanced capacity for discriminating colors in the green–red part of the spectrum (trichromatism). However, while Old World primates present three types of cone pigments and are routinely trichromatic, most New World primates exhibit a color vision polymorphism, characterized by the occurrence of trichromatic and dichromatic females and obligatory dichromatic males. Even though this has stimulated a prolific line of inquiry, the selective forces and relative benefits influencing color vision evolution in primates are still under debate, with current explanations focusing almost exclusively at the advantages in finding food and detecting socio‐sexual signals. Here, we evaluate a previously untested possibility, the adaptive value of primate color vision for predator detection. By combining color vision modeling data on New World and Old World primates, as well as behavioral information from human subjects, we demonstrate that primates exhibiting better color discrimination (trichromats) excel those displaying poorer color visions (dichromats) at detecting carnivoran predators against the green foliage background. The distribution of color vision found in extant anthropoid primates agrees with our results, and may be explained by the advantages of trichromats and dichromats in detecting predators and insects, respectively. Am. J. Primatol. 76:721–729, 2014.

Color vision in the black howler monkey (Alouatta caraya).

Electrophysiological and molecular genetic studies have shown that howler monkeys (Alouatta) are unique among all studied platyrrhines: they have the potential to display trichromatic color vision among males and females. This study examined the color discrimination abilities of four howler monkeys (Alouatta caraya) through a series of tasks involving a behavioral paradigm of discrimination learning. The animals were maintained and housed as a group in the Zoological Gardens of Brasília and were tested in their own home cages. Stimuli consisting of pairs of Munsell color chips were presented in random brightness values to assure that discriminations were based on color rather than brightness cues. All the animals (three males, one female) successfully discriminated all the stimulus pairs, including those that would be expected to be difficult for a dichromatic monkey. These results are consistent with the earlier predictions suggesting that howler monkeys are routinely trichromatic.

Colour discrimination learning in black-handed tamarin (Saguinus midas niger)

Colour is one cue that monkeys use for perceptual segregation of targets and to identify food resources. For fruit-eating primates such as Saguinus, an accurate colour perception would be advantageous to help find ripe fruits at distance. The colour vision abilities of black-handed tamarins (Saguinus midas niger) were assessed through a discrimination learning paradigm using Munsell colour chips as stimuli. Pairs of chips were chosen from an early experiment with protan and deutan humans. The monkeys (three males and one female) were tested with stimuli of the same hue, but different brightness values, in order to make sure that discriminations were based on colour rather than brightness cues. The results showed that the female, but not the males, presented an above-chance performance for stimuli resembling hue conditions under which tamarins forage (oranges vs greens). Colour vision in S. m. niger is discussed according to the advantages and disadvantages of dichromatism in daily search for food as well as to aspects regarding polymorphism in New World monkeys.

Color Vision in Leontopithecus chrysomelas: A Behavioral Study

We assessed the color discriminations by golden-headed lion tamarins (Leontopithecus chrysomelas) via a series of tasks involving a behavioral paradigm that maximizes the naturalness of the stimuli. The stimuli were pairs of Munsell color chips used in earlier experiments with human dichromats. We tested 3 male and 3 female monkeys with stimuli of random brightness values in order to assure that discriminations were based on color rather than brightness cues. Results indicate that each male and one female presented random performances for stimuli resembling hue conditions under which tamarins forage: oranges vs. greens. Two females exhibited discriminations consistent with allelic trichromacy. Findings indicate the presence of an M/L cone opsin polymorphism, a condition of most platyrrhines that is characterized by dichromatic and/or trichromatic females and dichromatic males. Interspecific differences in allelic frequency among lion tamarins raises the possibility that habitat fragmentation is affecting heterozygous frequencies, a trend that could impact tamarin foraging efficiency.

Colour discrimination in the black-tufted-ear marmoset (Callithrix penicillata): ecological implications.

The dietary diversity of marmosets is substantial, which may reflect differences in their colour vision. This study examined the colour discrimination ability of a gummivore/insectivore callitrichid, Callithrix penicillata, which inhabits the Brazilian cerrado (bush savanna). A series of ecologically relevant tasks, involving a behavioural paradigm of discrimination learning in semi-natural conditions and the usage of ecologically relevant stimuli, was executed. Three marmosets, 2 males and a female, behaved like human dichromats, showing an impaired performance when orange and green stimuli had to be discriminated. In contrast, 2 females resembled human trichromats, discriminating those kinds of pairs. Our data suggest that Callithrix penicillata presents a polymorphic trichromacy, with dichromatic males and dichromatic or trichromatic females.

Color vision discrimination in the capuchin monkey Cebus apella : evidence for trichromaticity

Primates display significant differences in color vision. The purpose of this study was to assess the ability of capuchin monkeys in discriminating chromatic and achromatic Munsell color chips through behavioral tests. The subjects were trained in a simple and reverse discrimination learning procedure. All subjects were capable of discriminations along five color categories investigated. The results are discussed in terms of the hypothesis of male dichromatism in New World monkeys, the role of color vision in adaptation to feeding ecology, as well as to aspects regarding primate evolution.

Detection of fruit by the Cerrado's marmoset (Callithrix penicillata): modeling color signals for different background scenarios and ambient light intensities

Among placental mammals, only primates have trichromatic color vision, however this is not a uniform condition. Under different genetic status, Old World monkeys have routine trichromacy, while New World monkeys show a visual polymorphism, characterized by obligatory male dichromacy. The ecological role of this genetic difference still remains unclear, but some studies show that dichromats and trichromats appear to have different abilities in detecting colored targets against a background of leaves. The Cerrados marmoset (Callithrix penicillata) is known to forage in brightly illuminated (savanna-like vegetation) and dimly illuminated (forests) environments, exploiting a high amount of dark fruits. Hence, it seems to be a good model for studying the differential advantages enjoyed by each color vision phenotype under natural conditions. Our aim was to verify how the different phenotypes of Cerrados marmoset detect components of their diet, evaluating the existence of differential phenotype advantages. Under two different light conditions, visual signals of naturally consumed fruits were modeled against different backgrounds scenarios. Even though dichromats and trichromats appear to be equally suited for tasks involving fruit detection, phenotype differential advantages are observed in this marmoset. In many conditions trichromats are predicted to perform better than dichromats, but under low ambient light dichromats manage to outperform trichromats in some scenarios. Phenotypes that carry widely spaced and longer M/L pigments enjoy the most advantage. These differential performances of trichromatic phenotypes, together with overdominance selection, seem to explain the maintenance of the tri-allelic system found in callitrichids.

Behavioural Evidence of Sex-Linked Colour Vision Polymorphism in the Squirrel Monkey Saimiri ustus

Squirrel monkeys, like most Neotropical primates, display a sex-linked colour vision polymorphism. Here we assess the colour perception of 8 Saimiri ustus by a behavioural paradigm using Munsell colour chips as discriminating stimuli. A random variation in brightness assured that discriminations were based on colour rather than brightness cues. Results indicate that all males showed random performances when presented with stimuli which, in previous experiments with human colour-blind individuals and dichromatic non-human primates, proved to be difficult to discriminate. Females behaved as trichromats. The different phenotypes in S. ustus may offer diverse advantages in feeding ecology and are in agreement with the existence of vision polymorphism, as described for other species of squirrel monkeys.

Low frequency (0.5Hz) rTMS over the right (non-dominant) motor cortex does not affect ipsilateral hand performance in healthy humans

Reduction of excitability of the dominant primary motor cortex (M1) improves ipsilateral hand function in healthy subjects. In analogy, inhibition of non-dominant M1 should also improve ipsilateral performance. In order to investigate this hypothesis, we have used slow repetitive transcranial magnetic stimulation (rTMS) and the Purdue Pegboard test. Twenty-eight volunteers underwent 10 minutes of either 0.5Hz rTMS over right M1 or sham rTMS (coil perpendicular to scalp). The motor task was performed before, immediately after, and 20 minutes after rTMS. In both groups, motor performance improved significantly throughout the sessions. rTMS inhibition of the non-dominant M1 had no significant influence over ipsilateral or contralateral manual dexterity, even though the results were limited by unequal performance between groups at baseline. This is in contrast to an improvement in left hand function previously described following slow rTMS over left M1, and suggests a less prominent physiological transcallosal inhibition from right to left M1.