Experimental evidence that primate trichromacy is well suited for detecting primate social colour signals

Impact of early visual experience on later usage of color cues

Human visual recognition is remarkably robust to chromatic changes. In this work, we provide a potential account of the roots of this resilience based on observations with 10 congenitally blind children who gained sight late in life. Several months or years following their sight-restoring surgeries, the removal of color cues markedly reduced their recognition performance, whereas age-matched normally sighted children showed no such decrement. This finding may be explained by the greater-than-neonatal maturity of the late-sighted children's color system at sight onset, inducing overly strong reliance on chromatic cues. Simulations with deep neural networks corroborate this hypothesis. These findings highlight the adaptive significance of typical developmental trajectories and provide guidelines for enhancing machine vision systems.

Divergences in color perception between deep neural networks and humans

Deep neural networks (DNNs) are increasingly proposed as models of human vision, bolstered by their impressive performance on image classification and object recognition tasks. Yet, the extent to which DNNs capture fundamental aspects of human vision such as color perception remains unclear. Here, we develop novel experiments for evaluating the perceptual coherence of color embeddings in DNNs, and we assess how well these algorithms predict human color similarity judgments collected via an online survey. We find that state-of-the-art DNN architectures - including convolutional neural networks and vision transformers - provide color similarity judgments that strikingly diverge from human color judgments of (i) images with controlled color properties, (ii) images generated from online searches, and (iii) real-world images from the canonical CIFAR-10 dataset. We compare DNN performance against an interpretable and cognitively plausible model of color perception based on wavelet decomposition, inspired by foundational theories in computational neuroscience. While one deep learning model - a convolutional DNN trained on a style transfer task - captures some aspects of human color perception, our wavelet algorithm provides more coherent color embeddings that better predict human color judgments compared to all DNNs we examine. These results hold when altering the high-level visual task used to train similar DNN architectures (e.g., image classification versus image segmentation), as well as when examining the color embeddings of different layers in a given DNN architecture. These findings break new ground in the effort to analyze the perceptual representations of machine learning algorithms and to improve their ability to serve as cognitively plausible models of human vision. Implications for machine learning, human perception, and embodied cognition are discussed.

Sensory collectives in natural systems

Groups of animals inhabit vastly different sensory worlds, or umwelten, which shape fundamental aspects of their behaviour. Yet the sensory ecology of species is rarely incorporated into the emerging field of collective behaviour, which studies the movements, population-level behaviours, and emergent properties of animal groups. Here, we review the contributions of sensory ecology and collective behaviour to understanding how animals move and interact within the context of their social and physical environments. Our goal is to advance and bridge these two areas of inquiry and highlight the potential for their creative integration. To achieve this goal, we organise our review around the following themes: (1) identifying the promise of integrating collective behaviour and sensory ecology; (2) defining and exploring the concept of a 'sensory collective'; (3) considering the potential for sensory collectives to shape the evolution of sensory systems; (4) exploring examples from diverse taxa to illustrate neural circuits involved in sensing and collective behaviour; and (5) suggesting the need for creative conceptual and methodological advances to quantify 'sensescapes'. In the final section, (6) applications to biological conservation, we argue that these topics are timely, given the ongoing anthropogenic changes to sensory stimuli (e.g. via light, sound, and chemical pollution) which are anticipated to impact animal collectives and group-level behaviour and, in turn, ecosystem composition and function. Our synthesis seeks to provide a forward-looking perspective on how sensory ecologists and collective behaviourists can both learn from and inspire one another to advance our understanding of animal behaviour, ecology, adaptation, and evolution.

Assessing color cues of development, breeding status and reproductive condition in captive golden lion tamarins (Leontopithecus rosalia)

Color signals play an important role in intraspecific communication and are well studied in catarrhine primates, which exhibit uniform trichromatic vision that is well suited to detecting such signals. Platyrrhine primates exhibit polymorphic color vision with different individuals possessing different color vision types in most species. Intriguingly, some platyrrhine species exhibit bare faces, which are convergent with those of catarrhines. However, putative functions of bare-faced color signals in platyrrhines remain largely unexplored. We measured facial skin color of five captive golden lion tamarins (Leontopithecus rosalia) using color-calibrated digital photography and modeled these colors to the visual systems of the species. Our results show that facial coloration is different between infant and older adults and varies across reproductive condition, but not between breeding and nonbreeding adults. While preliminary, our study suggests that facial coloration may be involved in sociosexual signaling in golden lion tamarins, and provides intriguing evidence that we hope might stimulate more studies of bare-faced signaling in platyrrhines.

Little information loss with red-green color deficient vision in natural environments

Inherited color vision deficiency affects red-green discrimination in about one in twelve men from European populations. Its effects have been studied mainly in primitive foraging but also in detecting blushing and breaking camouflage. Yet there is no obvious relationship between these specific tasks and vision in the real world. The aim here was to quantify the impact of color vision deficiency by estimating computationally the information available to observers about colored surfaces in natural scenes. With representative independent sets of 50 and 100 hyperspectral images, estimated information was found to be only a little less in red-green color vision deficiency than in normal trichromacy. Colorimetric analyses revealed the importance of large lightness variations within scenes, small redness-greenness variations, and uneven frequencies of different colored surfaces. While red-green color vision deficiency poses challenges in some tasks, it has much less effect on gaining information from natural environments.

Color is necessary for face discrimination in the Northern paper wasp, Polistes fuscatus

Visual individual recognition requires animals to distinguish among conspecifics based on appearance. Though visual individual recognition has been reported in a range of taxa including primates, birds, and insects, the features that animals require to discriminate between individuals are not well understood. Northern paper wasp females, Polistes fuscatus, possess individually distinctive color patterns on their faces, which mediate individual recognition. However, it is currently unclear what role color plays in the facial recognition system of this species. Thus, we sought to test two possible roles of color in wasp facial recognition. On one hand, color may be important simply because it creates a pattern. If this is the case, then wasps should perform similarly when discriminating color or grayscale images of the same faces. Alternatively, color itself may be important for recognition of an image as a "face", which would predict poorer performance on grayscale discrimination relative to color images. We found wasps performed significantly better when discriminating between color faces compared to grayscale versions of the same faces. In fact, wasps trained on grayscale faces did not perform better than chance, indicating that color is necessary for the recognition of an image as a face by the wasp visual system.

Evaluating genital skin color as a putative sexual signal in wild saddleback (Leontocebus weddelli) and emperor (Saguinus imperator) tamarins

Coevolution between signalers and receivers has played a significant role in the diversity of animal signals and sensory systems. Platyrrhines (monkeys in the Americas) exhibit a remarkable color vision polymorphism that may have been selected by both natural and sexual selection, but sociosexual color signaling among platyrrhines has received almost no attention. Here, we study the color of reproductive skin among different reproductive classes in free-ranging female saddleback (Leontocebus weddelli) and emperor (Saguinus imperator) tamarins, modeling color spaces, and contrasts for the different visual systems. We find that the chromatic saturation and luminance of genital color vary between reproductive classes in saddleback tamarins. Chromatic contrast between the vulva and belly is lower in the parous females (PFs) relative to adult but not currently breeding females, while achromatic contrast is higher in PFs in saddleback tamarins relative to nonparous females. However, in emperor tamarins, genital color (saturation, hue, and luminance) does not vary between reproductive classes. Overall, genital skin color variation is present in tamarins and may play a role in sexual signaling. Nevertheless, the patterns are inconsistent between species, suggesting interspecific variation. Future studies should integrate the perceiver's behavioral responses and the physical and social signaling environments into comprehensive studies of communication as well as consider the role and interaction between multiple sensory modalities.

The sensory ecology of primate food perception, revisited

Twenty years ago, Dominy and colleagues published "The sensory ecology of primate food perception," an impactful review that brought new perspectives to understanding primate foraging adaptations. Their review synthesized information on primate senses and explored how senses informed feeding behavior. Research on primate sensory ecology has seen explosive growth in the last two decades. Here, we revisit this important topic, focusing on the numerous new discoveries and lines of innovative research. We begin by reviewing each of the five traditionally recognized senses involved in foraging: audition, olfaction, vision, touch, and taste. For each sense, we provide an overview of sensory function and comparative ecology, comment on the state of knowledge at the time of the original review, and highlight advancements and lingering gaps in knowledge. Next, we provide an outline for creative, multidisciplinary, and innovative future research programs that we anticipate will generate exciting new discoveries in the next two decades.

Stone handling in geladas (Theropithecus gelada): implications for spontaneous drawing-like activity as a playful behavior

Stone handling (SH) is a form of solitary object play widely documented in four species of macaques and most recently also in geladas (Theropithecus gelada). Here, we describe the SH activity of two mother-reared captive gelada males, who combined different behavioral patterns in a sequence that occasionally led to the production of colored marks on a hard surface. Two playful techniques of different complexity emerged spontaneously in the two subjects. In the etching and releasing technique, the stone or pieces of it were repeatedly scraped across either a vertical or horizontal hard surface. In the grindandfinger technique, additional patterns were recruited such as scratching the stone to produce small debris that the subjects manipulated through a thumb–index finger precision grip. Animals selected preferentially hard surfaces when their SH sessions involved patterns that potentially released color and engaged in such patterns for longer and in a repeated way. This evidence suggests the high motivation of the two males to engage in drawing-like behavior that, as it occurs for other forms of play, can be characterized by its autotelic self-rewarding nature. Digital video images related to the article are available at http://www.momo-p.com/showdetail-e.php?movieid=momo220922tg01a   and http://www.momo-p.com/showdetail-e.php?movieid=momo220923tg01a

Variation and heritability of retinal cone ratios in a free-ranging population of rhesus macaques

A defining feature of catarrhine primates is uniform trichromacy-the ability to distinguish red (long; L), green (medium; M), and blue (short; S) wavelengths of light. Although the tuning of photoreceptors is conserved, the ratio of L:M cones in the retina is variable within and between species, with human cone ratios differing from other catarrhines. Yet, the sources and structure of variation in cone ratios are poorly understood, precluding a broader understanding of color vision variability. Here, we report a large-scale study of a pedigreed population of rhesus macaques (Macaca mulatta). We collected foveal RNA and analyzed opsin gene expression using cDNA and estimated additive genetic variance of cone ratios. The average L:M ratio and standard error was 1.03:1 ± 0.02. There was no age effect, and genetic contribution to variation was negligible. We found marginal sex effects with females having larger ratios than males. S cone ratios (0.143:1 ± 0.002) had significant genetic variance with a heritability estimate of 43% but did not differ between sexes or age groups. Our results contextualize the derived human condition of L-cone dominance and provide new information about the heritability of cone ratios and variation in primate color vision.

Perceptual hue, lightness, and chroma are represented in a multidimensional functional anatomical map in macaque V1

Humans perceive millions of colors along three dimensions of color space: hue, lightness, and chroma. A major gap in knowledge is where the brain represents these specific dimensions in cortex, and how they relate to each other. Previous studies have shown that brain areas V4 and the posterior inferotemporal cortex (PIT) are central to computing color dimensions. To determine the contribution of V1 to setting up these downstream processing mechanisms, we studied cortical color responses in macaques-who share color vision mechanisms with humans. We used two-photon calcium imaging at both meso- and micro-scales and found that hue and lightness are laid out in orthogonal directions on the cortical map, with chroma represented by the strength of neuronal responses, as previously shown in PIT. These findings suggest that the earliest cortical stages of vision determine the three primary dimensions of human color perception.

The face is central to primate multicomponent signals

A wealth of experimental and observational evidence suggests that faces have become increasingly important in the communication system of primates over evolutionary time and that both the static and moveable aspects of faces convey considerable information. Therefore, whenever there is a visual component to any multicomponent signal the face is potentially relevant. However, the role of the face is not always considered in primate multicomponent communication research. We review the literature and make a case for greater focus on the face going forward. We propose that the face can be overlooked for two main reasons: first, due to methodological difficulty. Examination of multicomponent signals in primates is difficult, so scientists tend to examine a limited number of signals in combination. Detailed examination of the subtle and dynamic components of facial signals is particularly hard to achieve in studies of primates. Second, due to a common assumption that the face contains “emotional” content. A priori categorisation of facial behavior as “emotional” ignores the potentially communicative and predictive information present in the face that might contribute to signals. In short, we argue that the face is central to multicomponent signals (and also many multimodal signals) and suggest future directions for investigating this phenomenon.

Adaptive Education: Learning and Remembering with a Stone-Age Brain

Educators generally accept that basic learning and memory processes are a product of evolution, guided by natural selection. Less well accepted is the idea that ancestral selection pressures continue to shape modern memory functioning. In this article, I review evidence suggesting that attention to nature's criterion-the enhancement of fitness-is needed to explain fully how and why people remember. Thinking functionally about memory, and adopting an evolutionary perspective in the laboratory, has led to recent discoveries with clear implications for learning in the classroom. For example, our memory systems appear to be tuned to animacy (the distinction between living and nonliving things) which, in turn, can play a role in enhancing foreign language acquisition. Effective learning management systems need to align with students' prior knowledge, skill, and interest levels, but also with the inherent content biases or "tunings" that are representative of all people.

The evolution of primate coloration revisited

Primates are noted for their varied and complex pelage and bare skin coloration but the significance of this diverse coloration remains opaque. Using new updated information, novel scoring of coat and skin coloration, and controlling for shared ancestry, we reexamined and extended findings from previous studies across the whole order and the five major clades within it. Across primates, we found (i) direct and indirect evidence for pelage coloration being driven by protective coloration strategies including background matching, countershading, disruptive coloration, and aposematism, (ii) diurnal primates being more colorful, and (iii) the possibility that pelage color diversity is negatively associated with female trichromatic vision; while (iv) reaffirming avoidance of hybridization driving head coloration in males, (v) darker species living in warm, humid conditions (Gloger’s rule), and (vi) advertising to multiple mating partners favoring red genitalia in females. Nonetheless, the importance of these drivers varies greatly across clades. In strepsirrhines and cercopithecoids, countershading is important; greater color diversity may be important for conspecific signaling in more diurnal and social strepsirrhines; lack of female color vision may be associated with colorful strepsirrhines and platyrrhines; whereas cercopithecoids obey Gloger’s rule. Haplorrhines show background matching, aposematism, character displacement, and red female genitalia where several mating partners are available. Our findings emphasize several evolutionary drivers of coloration in this extraordinarily colorful order. Throughout, we used coarse but rigorous measures of coloration, and our ability to replicate findings from earlier studies opens up opportunities for classifying coloration of large numbers of species at a macroevolutionary scale.

Sensory disruption and sensory inequities in the Anthropocene

Anthropogenic disruptions to animal sensory ecology are as old as our species. But what about the effect on human sensory ecology? Human sensory dysfunction is increasing globally at great economic and health costs (mental, physical, and social). Contemporary sensory problems are directly tied to human behavioral changes and activity as well as anthropogenic pollution. The evolutionary sensory ecology and anthropogenic disruptions to three human senses (vision, audition, olfaction) are examined along with the economic and health costs of functionally reduced senses and demographic risk factors contributing to impairment. The primary goals of the paper are (a) to sew an evolutionary and ecological thread through clinical narratives on sensory dysfunction that highlights the impact of the built environment on the senses, and (b) to highlight structural, demographic, and environmental injustices that create sensory inequities in risk and that promote health disparities.

Colour matters more than shape for chimpanzees' recognition of developmental face changes

Social primates must recognise developmental stages of other conspecifics in order to behave appropriately. Infant faces have peculiar morphological characteristics-relatively large eyes, a small nose, and small mouth-known as baby schema. In addition, the infant faces of many primate species have unique skin coloration. However, it is unclear which features serve as critical cues for chimpanzees to recognise developmental changes in their faces. The present study aimed to investigate the relative contributions of facial shape and colour to age categorisation in chimpanzees. We used a symbolic matching-to-sample task in which chimpanzees were trained to discriminate between adult and infant faces. Then, we tested how their age category judgments transferred to a series of morphed faces which systematically differed in facial shape and colour. Statistical image quantification analysis revealed significant differences both in shape and colour between adult and infant faces. However, we found that facial coloration contributed to age categorisation in chimpanzees more than facial shape. Our results showed that chimpanzees use unique infantile facial coloration as a salient cue when discriminating between adult and infant faces. The display of their developmental stages through facial colour may help chimpanzees to induce appropriate behaviour from other individuals.

Liminal Light and Primate Evolution

The adaptive origins of primates and anthropoid primates are topics of enduring interest to biological anthropologists. A convention in these discussions is to treat the light environment as binary—night is dark, day is light—and to impute corresponding selective pressure on the visual systems and behaviors of primates. In consequence, debate has tended to focus on whether a given trait can be interpreted as evidence of nocturnal or diurnal behavior in the primate fossil record. Such classification elides the variability in light, or the ways that primates internalize light in their environments. Here, we explore the liminality of light by focusing on what it is, its many sources, and its flux under natural conditions. We conclude by focusing on the intensity and spectral properties of twilight, and we review the mounting evidence of its importance as a cue that determines the onset or offset of primate activities as well as the entrainment of circadian rhythms.

Mechanistic insights into the evolution of the differential expression of tandemly arrayed cone opsin genes in zebrafish

The genome of many organisms contains several loci consisting of duplicated genes that are arrayed in tandem. The daughter genes produced by duplication typically exhibit differential expression patterns with each other or otherwise experience pseudogenization. Remarkably, opsin genes in fish are preserved after many duplications in different lineages. This fact indicates that fish opsin genes are characterized by a regulatory mechanism that could intrinsically facilitate the differentiation of the expression patterns. However, little is known about the mechanisms that underlie the differential expression patterns or how they were established during evolution. The loci of green (RH2)- and red (LWS)-sensitive cone opsin genes in zebrafish have been used as model systems to study the differential regulation of tandemly arrayed opsin genes. Over a decade of studies have uncovered several mechanistic features that might have assisted the differentiation and preservation of duplicated genes. Furthermore, recent progress in the understanding of the transcriptional process in general has added essential insights. In this article, the current understanding of the transcriptional regulation of differentially expressed tandemly arrayed cone opsin genes in zebrafish is summarized and a possible evolutionary scenario that could achieve this differentiation is discussed.

Coloration in Mammals

Mammalian colors and color patterns are some of the most diverse and conspicuous traits found in nature and have been widely studied from genetic/developmental and evolutionary perspectives. In this review we first discuss the proximate causes underlying variation in pigment type (i.e., color) and pigment distribution (i.e., color pattern) and highlight both processes as having a distinct developmental basis. Then, using multiple examples, we discuss ultimate factors that have driven the evolution of coloration differences in mammals, which include background matching, intra- and interspecific signaling, and physiological influences. Throughout, we outline bridges between developmental and functional investigatory approaches that help broaden knowledge of mammals' memorable external appearances, and we point out areas for future interdisciplinary research.

Platyrrhine color signals: New horizons to pursue

Like catarrhines, some platyrrhines show exposed and reddish skin, raising the possibility that reddish signals have evolved convergently. This variation in skin exposure and color combined with sex-linked polymorphic color vision in platyrrhines presents a unique, and yet underexplored, opportunity to investigate the relative importance of chromatic versus achromatic signals, the influence of color perception on signal evolution, and to understand primate communication broadly. By coding the facial skin exposure and color of 96 platyrrhines, 28 catarrhines, 7 strepsirrhines, 1 tarsiiform, and 13 nonprimates, and by simulating the ancestral character states for these traits, we provide the first analysis of the distribution and evolution of facial skin exposure and color in platyrrhini. We highlight ways in which studying the presence and use of color signals by platyrrhines and other primates will enhance our understanding of the evolution of color signals, and the forces shaping color vision.

Paradoxical impact of memory on color appearance of faces

What is color vision for? Here we compared the extent to which memory modulates color appearance of objects and faces. Participants matched the colors of stimuli illuminated by low-pressure sodium light, which renders scenes monochromatic. Matches for fruit were not predicted by stimulus identity. In contrast, matches for faces were predictable, but surprising: faces appeared green and looked sick. The paradoxical face-color percept could be explained by a Bayesian observer model constrained by efficient coding. The color-matching data suggest that the face-color prior is established by visual signals arising from the recently evolved L-M cone system, not the older S-cone channel. Taken together, the results show that when retinal mechanisms of color vision are impaired, the impact of memory on color perception is greatest for face color, supporting the idea that trichromatic color plays an important role in social communication.

What is the function of color vision? Here, the authors show that when retinal mechanisms of color are impaired, memory has a paradoxical impact on color appearance that is selective for faces, providing evidence that color contributes to face encoding and social communication.

Desaturation-Induced Brightness in Face Color Perception

The distinctiveness of perception of face from nonface objects has been noted previously. However, face brightness is often confounded with whiteness in the beauty industry; few studies have examined these perceptual differences. To investigate the interactions among face color attributes, we measured the effect of saturation on brightness and whiteness in both uniform color patches and face images to elucidate the relationship between these two perceptions. We found that, at constant luminance, a uniform color patch looked brighter with an increase in saturation (i.e., the Helmholtz-Kohlrausch effect occurred), while in contrast, brightness of a facial skin image looked less bright with increased saturation (i.e., contrary to the Helmholtz-Kohlrausch effect), which suggested this interaction of color attributes was influenced by top-down information. We conclude that this inverse effect of saturation on brightness for face images is not due to face recognition, color range of the skin tone, the luminance distribution, or recognition of human skin but due to the composite interactions of these facial skin factors in higher order recognition mechanisms.

How conspicuous are peacock eyespots and other colorful feathers in the eyes of mammalian predators?

Colorful feathers have long been assumed to be conspicuous to predators, and hence likely to incur costs due to enhanced predation risk. However, many mammals that prey on birds have dichromatic visual systems with only two types of color-sensitive visual receptors, rather than the three and four photoreceptors characteristic of humans and most birds, respectively. Here, we use a combination of multispectral imaging, reflectance spectroscopy, color vision modelling and visual texture analysis to compare the visual signals available to conspecifics and to mammalian predators from multicolored feathers from the Indian peacock (Pavo cristatus), as well as red and yellow parrot feathers. We also model the effects of distance-dependent blurring due to visual acuity. When viewed by birds against green vegetation, most of the feathers studied are estimated to have color and brightness contrasts similar to values previously found for ripe fruit. On the other hand, for dichromat mammalian predators, visual contrasts for these feathers were only weakly detectable and often below detection thresholds for typical viewing distances. We also show that for dichromat mammal vision models, the peacock's train has below-detection threshold color and brightness contrasts and visual textures that match various foliage backgrounds. These findings are consistent with many feathers of similar hue to those studied here being inconspicuous, and in some cases potentially cryptic, in the eyes of common mammalian predators of adult birds. Given that birds perform many conspicuous motions and behaviors, this study suggests that mammalian predators are more likely to use other sensory modalities (e.g., motion detection, hearing, and olfaction), rather than color vision, to detect avian prey. This suggests new directions for future behavioral studies and emphasizes the importance of understanding the influence of the sensory ecology of predators in the evolution of animal coloration.

Social Perception of Facial Color Appearance for Human Trichromatic Versus Dichromatic Color Vision

Typical human color vision is trichromatic, on the basis that we have three distinct classes of photoreceptors. A recent evolutionary account posits that trichromacy facilitates detecting subtle skin color changes to better distinguish important social states related to proceptivity, health, and emotion in others. Across two experiments, we manipulated the facial color appearance of images consistent with a skin blood perfusion response and asked participants to evaluate the perceived attractiveness, health, and anger of the face (trichromatic condition). We additionally simulated what these faces would look like for three dichromatic conditions (protanopia, deuteranopia, tritanopia). The results demonstrated that flushed (relative to baseline) faces were perceived as more attractive, healthy, and angry in the trichromatic and tritanopia conditions, but not in the protanopia and deuteranopia conditions. The results provide empirical support for the social perception account of trichromatic color vision evolution and lead to systematic predictions of social perception based on ecological social perception theory.

Mate choice in the eye and ear of the beholder? Female multimodal sensory configuration influences her preferences

A common assumption in sexual selection studies is that receivers decode signal information similarly. However, receivers may vary in how they rank signallers if signal perception varies with an individual's sensory configuration. Furthermore, receivers may vary in their weighting of different elements of multimodal signals based on their sensory configuration. This could lead to complex levels of selection on signalling traits. We tested whether multimodal sensory configuration could affect preferences for multimodal signals. We used brown-headed cowbird (Molothrus ater) females to examine how auditory sensitivity and auditory filters, which influence auditory spectral and temporal resolution, affect song preferences, and how visual spatial resolution and visual temporal resolution, which influence resolution of a moving visual signal, affect visual display preferences. Our results show that multimodal sensory configuration significantly affects preferences for male displays: females with better auditory temporal resolution preferred songs that were shorter, with lower Wiener entropy, and higher frequency; and females with better visual temporal resolution preferred males with less intense visual displays. Our findings provide new insights into mate-choice decisions and receiver signal processing. Furthermore, our results challenge a long-standing assumption in animal communication which can affect how we address honest signalling, assortative mating and sensory drive.

The Importance of Spatial Visual Scene Parameters in Predicting Optimal Cone Sensitivities in Routinely Trichromatic Frugivorous Old-World Primates

Computational models that predict the spectral sensitivities of primate cone photoreceptors have focussed only on the spectral, not spatial, dimensions. On the ecologically valid task of foraging for fruit, such models predict the M-cone (“green”) peak spectral sensitivity 10–20 nm further from the L-cone (“red”) sensitivity peak than it is in nature and assume their separation is limited by other visual constraints, such as the requirement of high-acuity spatial vision for closer M and L peak sensitivities. We explore the possibility that a spatio-chromatic analysis can better predict cone spectral tuning without appealing to other visual constraints. We build a computational model of the primate retina and simulate chromatic gratings of varying spatial frequencies using measured spectra. We then implement the case study of foveal processing in routinely trichromatic primates for the task of discriminating fruit and leaf spectra. We perform an exhaustive search for the configurations of M and L cone spectral sensitivities that optimally distinguish the colour patterns within these spectral images. Under such conditions, the model suggests that: (1) a long-wavelength limit is required to constrain the L cone spectral sensitivity to its natural position; (2) the optimal M cone peak spectral sensitivity occurs at ~525 nm, close to the observed position in nature (~535 nm); (3) spatial frequency has a small effect upon the spectral tuning of the cones; (4) a selective pressure toward less correlated M and L spectral sensitivities is provided by the need to reduce noise caused by the luminance variation that occurs in natural scenes.

Experimental evidence that primate trichromacy is well suited for detecting primate social colour signals

Primate trichromatic colour vision has been hypothesized to be well tuned for detecting variation in facial coloration, which could be due to selection on either signal wavelengths or the sensitivities of the photoreceptors themselves. We provide one of the first empirical tests of this idea by asking whether, when compared with other visual systems, the information obtained through primate trichromatic vision confers an improved ability to detect the changes in facial colour that female macaque monkeys exhibit when they are proceptive. We presented pairs of digital images of faces of the same monkey to human observers and asked them to select the proceptive face. We tested images that simulated what would be seen by common catarrhine trichromatic vision, two additional trichromatic conditions and three dichromatic conditions. Performance under conditions of common catarrhine trichromacy, and trichromacy with narrowly separated LM cone pigments (common in female platyrrhines), was better than for evenly spaced trichromacy or for any of the dichromatic conditions. These results suggest that primate trichromatic colour vision confers excellent ability to detect meaningful variation in primate face colour. This is consistent with the hypothesis that social information detection has acted on either primate signal spectral reflectance or photoreceptor spectral tuning, or both.