Conspecific Faces Research Articles

Laminar organization of visual responses in core and parabelt auditory cortex

Abstract Audiovisual (AV) interaction has been shown in many studies of auditory cortex. However, the underlying processes and circuits are unclear because few studies have used methods that delineate the timing and laminar distribution of net excitatory and inhibitory processes within areas, much less across cortical levels. This study examined laminar profiles of neuronal activity in auditory core (AC) and parabelt (PB) cortices recorded from macaques during active discrimination of conspecific faces and vocalizations. We found modulation of multi-unit activity (MUA) in response to isolated visual stimulation, characterized by a brief deep MUA spike, putatively in white matter, followed by mid-layer MUA suppression in core auditory cortex; the later suppressive event had clear current source density concomitants, while the earlier MUA spike did not. We observed a similar facilitation-suppression sequence in the PB, with later onset latency. In combined AV stimulation, there was moderate reduction of responses to sound during the visual-evoked MUA suppression interval in both AC and PB. These data suggest a common sequence of afferent spikes, followed by synaptic inhibition; however, differences in timing and laminar location may reflect distinct visual projections to AC and PB.

Viewpoint independent face recognition via extrapolation in paper wasps.

Visual recognition of three-dimensional signals, like faces, is challenging because the signals appear different from different viewpoints. A flexible, but cognitively challenging solution is viewpoint independent recognition, where receivers identify signals from novel viewing angles. Here, we use same/different concept learning to test viewpoint independent face recognition in Polistes fuscatus, a wasp that uses facial patterns to individually identify conspecifics. We find that wasps use extrapolation to identify novel views of conspecific faces. For example, wasps identify a pair of pictures of the same wasp as the 'same', even if the pictures are taken from different views (e.g. one face 0° rotation, one face 60° rotation). This result is notable because it provides the first evidence of view invariant recognition via extrapolation in an invertebrate. The results suggest that viewpoint independent recognition via extrapolation may be a widespread strategy to facilitate individual face recognition.

Capuchin monkeys' (Sapajus [Cebus] apella) categorization of photos of unknown male conspecifics suggests attention to fWHR and a dominance bias.

The ability to quickly perceive others' rank minimizes costs by helping individuals behave appropriately when interacting with strangers. Indeed, humans and at least some other species can quickly determine strangers' rank or dominance based only on physical features without observing others' interactions or behavior. Nonhuman primates can determine strangers' ranks by observing their interactions, and some evidence suggests that at least some cues to dominance, such as facial width-to-height ratio (fWHR), are also present in other primates. However, it is unknown whether they can determine strangers' rank simply by looking at their faces, rather than observing their interactions. If so, this would suggest selective pressure across the primates on both cues to dominance and the ability to detect those cues accurately. To address this, we examined the ability of male and female tufted capuchin monkeys (Sapajus [Cebus] apella) to categorize images of the faces of unknown conspecifics (Sapajus from different colonies) and humans (computer-generated and real) as dominant or nondominant based only on still images. Capuchins' categorization of unknown conspecific faces was consistent with fWHR, a cue to dominance, although there was a strong tendency to categorize strangers as dominant, particularly for males. This was true despite the continued correct categorization of known individuals. In addition, capuchins did not categorize human strangers in accordance with external pre-ratings of dominance by independent human raters, despite the availability of the same cue, fWHR. We consider these results in the context of capuchin socio-ecology and what they mean for the evolution of rapid decision-making in social contexts.

Geographic variation in individual face learning based on plasticity rather than local genetic adaptation in Polistes wasps

Abstract Signals and receiver responses often vary across a species’ geographic range. Effective communication requires a match between signal and receiver response, so there is much interest in the developmental mechanisms that maintain this link. Two potential mechanisms are genetic covariance between signal and receiver response and plasticity where individuals adjust their phenotype based on their partner’s phenotype. Here, we test how plasticity contributes to geographic variation in individual face recognition in Polistes fuscatus wasps. Previous work has shown that P. fuscatus from Michigan, USA (MI) have variable facial patterns used for individual recognition, while P. fuscatus from central Pennsylvania, USA (PA) lack variable facial patterns and are unable to learn individual conspecifics. We experimentally altered rearing environment, so wasps were either reared with their own population or in a common garden with wasps from both populations. Then, we tested the wasps’ capacity to learn and remember individual conspecific faces. Consistent with previous work, MI wasps reared with MI wasps were adept at learning conspecific faces, while PA wasps reared with PA wasps were unable to learn conspecific faces. However, MI and PA wasps reared in a common garden developed similar, intermediate capacity for individual face learning. These results indicate that individual face learning in Polistes wasps is highly plastic and responsive to the social environment. Plasticity in receiver responses may be a common mechanism mediating geographic differences in non-sexual signaling systems and may play a role in maintaining links between signals and receiver responses in geographically variable communication systems.

Recognition of animal faces is impaired in developmental prosopagnosia

An on-going debate in psychology and neuroscience concerns the way faces and objects are represented. Domain-specific theories suggest that faces are processed via a specialised mechanism, separate from objects. Developmental prosopagnosia (DP) is a neurodevelopmental disorder in which there is a deficit in the ability to recognize conspecific (human) faces. It is unclear, however, whether prosopagnosia also affects recognition of heterospecific (animal) faces. To address this question, we compared recognition performance with human and animal faces in neurotypical controls and participants with DP. We found that DPs showed deficits in the recognition of both human and animal faces compared to neurotypical controls. In contrast to, we found no group-level deficit in the recognition of animate or inanimate non-face objects in DPs. Using an individual-level approach, we demonstrate that in 60% of cases in which face recognition is impaired, there is a concurrent deficit with animal faces. Together, these results show that DPs have a general deficit in the recognition of faces that encompass a range of configural and morphological structures.

Oxytocin and social gaze during a dominance categorization task in tufted capuchin monkeys.

Visual attention to facial features is an important way that group-living primate species gain knowledge about others. However, where this attention is focused on the face is influenced by contextual and social features, and emerging evidence in Pan species suggests that oxytocin, a hormone involved in forming and maintaining affiliative bonds among members of the same group, influences social attention as measured by eye gaze. Specifically, bonobos tend to focus on conspecifics’ eyes when viewing two-dimensional images, whereas chimpanzees focus more on the edges of the face. Moreover, exogenous oxytocin, which was hypothesized to increase eye contact in both species, instead enhanced this existing difference. We follow up on this to (1) determine the degree to which this Pan pattern generalizes across highly social, cooperative non-ape primates and (2) explore the impact of exogenously administered vs. endogenously released oxytocin in impacting this behavior. To do so, we tracked gaze direction on a computerized social categorization task using conspecific faces in tufted capuchin monkeys (Sapajus [Cebus] apella) after (1) exogenously administering intranasal oxytocin using a nebulizer or (2) inducing an endogenous increase in oxytocin using fur-rubbing, previously validated to increase oxytocin in capuchins. Overall, we did not find a general tendency in the capuchins to look toward the eyes or mouth, but we found that oxytocin was related to looking behavior toward these regions, albeit not in a straightforward way. Considering frequency of looking per trial, monkeys were more likely to look at the eye region in the fur-rubbing condition as compared to either the saline or exogenous oxytocin conditions. However, in terms of duration of looking during trials in which they did look at the eye region, monkeys spent significantly less time looking at the eyes in both oxytocin conditions as compared to the saline condition. These results suggest that oxytocin did not necessarily enhance eye looking in capuchins, which is consistent with the results from Pan species, and that endogenous and exogenous oxytocin may behave differently in their effect on how social attention is allocated.

Social experience drives the development of holistic face processing in paper wasps.

Most recognition is based on identifying features, but specialization for face recognition in some taxa relies on a different mechanism, termed 'holistic processing' where facial features are bound together into a gestalt which is more than the sum of its parts. Although previous work suggests that extensive experience may be required for the development of holistic processing, we lack experiments that test how age and experience interact to influence holistic processing. Here, we test how age and experience influence the development of holistic face processing in Polistes fuscatus paper wasps. Previous work has shown that P. fuscatus use facial patterns to individually identify conspecifics and wasps use holistic processing to discriminate between conspecific faces. We tested face processing in three groups of P. fuscatus: young (1-week-old), older, experienced (2-weeks-old, normal experience), and older, inexperienced (2-weeks-old, 1 week normal social experience and 1 week social isolation). Older, experienced wasps used holistic processing to discriminate between conspecific faces. In contrast, older inexperienced wasps used featural rather than holistic mechanisms to discriminate between faces. Young wasps show some evidence of holistic face processing, but this ability was less refined than older, experienced wasps. Notably, wasps only required 2 weeks of normal experience to develop holistic processing, while previous work suggests that humans may require years of experience. Overall, P. fuscatus wasps rapidly develop holistic processing for conspecific faces. Experience rather than age facilitates the transition between featural and holistic face processing mechanisms.

A deep transfer learning model for head pose estimation in rhesus macaques during cognitive tasks: Towards a nonrestraint noninvasive 3Rs approach

Head orientation is a measure of attention used in behavioral psychological research with non-human primates. It is used across a broad range of disciplines and settings, from the field to the laboratory. Field methods are time consuming with risk of coding bias and visibility issues with free-ranging animals. Laboratory methods may require restraint and use of invasive procedures. Automated systems to measure head orientation in unrestrained animals, that are robust to partial occlusion of the head, would improve coding efficiency and accuracy and provide 3Rs animal welfare benefits. We present a free-to-use deep transfer learning model for non-invasive head pose estimation in unrestrained Macaca mulatta taking part in cognitive experiments. Monkeys housed in social groups were filmed viewing two conspecific face stimuli presented on either side of a video camera. Video frames were manually annotated for three head positions relative to the video camera: ‘left’, ‘center’ and ‘right’. The dataset (total = 8135 images from 26 monkeys) was partitioned into training and testing datasets using a leave-k-out strategy, so that 70% of the images were used in training and 30% were used in testing. We used the VGG16, VGG19, InceptionV3 and Resnet50 as base models to train the proposed head pose classifier. We achieved model accuracy up to 93 %. The head pose estimation model presented here will be of use across contexts ranging from field-based playback experiments to assessment of welfare in zoo and clinical veterinary settings and refinement of neuroscience research practices. Model code with instructions is provided.

\u2018When hunger makes everything better looking!\u2019: The effect of hunger on the aesthetic appreciation of human bodies, faces and objects

BackgroundResearch evidence suggests that physiological state of hunger might affect preference for female body weight, such that hungrier, compared to satiate, men prefer heavier body weight and rate as more attractive heavier female figures. Here, we seek to extend these findings by comparing the effects of fasting and snack on aesthetics judgements of the bodies and faces of conspecifics and of objects in a sample of female and male participants.MethodsForty-four participants (women: n = 21, mean age = 23.70 yrs ± 0.62) provided aesthetic liking judgments of round and slim human bodies, faces and objects, under at least 12 h of overnight fasting and immediately after having eaten a snack (i.e., bananas). An anthropometric measure of adiposity (i.e., Body Mass Index, BMI) was also collected from each observer.ResultsOverall, we found that participants’ aesthetic judgements were higher for slim stimuli compared to round ones. However, after fasting, participants rated round stimuli as more attractive compared to when they had a snack. This hunger-based shift in ratings not only was apparent when stimuli depicted a human body or face, but also when they depicted an object, thus suggesting a general modification of observers’ aesthetic preference related to hunger. Importantly, this effect was modulated by participants’ BMI so that only participants with a high BMI provided higher aesthetic judgements for round stimuli after fasting than after a snack.ConclusionsOur results demonstrated that both the modification of the physiological state and the individual differences in adiposity level of the observers might affect the aesthetic appreciation of the external world.

Staring death in the face: chimpanzees' attention towards conspecific skulls and the implications of a face module guiding their behaviour.

Chimpanzees exhibit a variety of behaviours surrounding their dead, although much less is known about how they respond towards conspecific skeletons. We tested chimpanzees' visual attention to images of conspecific and non-conspecific stimuli (cat/chimp/dog/rat), shown simultaneously in four corners of a screen in distinct orientations (frontal/diagonal/lateral) of either one of three types (faces/skulls/skull-shaped stones). Additionally, we compared their visual attention towards chimpanzee-only stimuli (faces/skulls/skull-shaped stones). Lastly, we tested their attention towards specific regions of chimpanzee skulls. We theorized that chimpanzee skulls retaining face-like features would be perceived similarly to chimpanzee faces and thus be subjected to similar biases. Overall, supporting our hypotheses, the chimpanzees preferred conspecific-related stimuli. The results showed that chimpanzees attended: (i) significantly longer towards conspecific skulls than other species skulls (particularly in forward-facing and to a lesser extent diagonal orientations); (ii) significantly longer towards conspecific faces than other species faces at forward-facing and diagonal orientations; (iii) longer towards chimpanzee faces compared with chimpanzee skulls and skull-shaped stones, and (iv) attended significantly longer to the teeth, similar to findings for elephants. We suggest that chimpanzee skulls retain relevant, face-like features that arguably activate a domain-specific face module in chimpanzees' brains, guiding their attention.

Ketamine disrupts gaze patterns during face viewing in the common marmoset.

Faces are stimuli of critical importance for primates. The common marmoset (Callithrix jacchus) is a promising model for investigations of face processing, as this species possesses oculomotor and face-processing networks resembling those of macaques and humans. Face processing is often disrupted in neuropsychiatric conditions such as schizophrenia (SZ), and thus, it is important to recapitulate underlying circuitry dysfunction preclinically. The N-methyl-d-aspartate (NMDA) noncompetitive antagonist ketamine has been used extensively to model the cognitive symptoms of SZ. Here, we investigated the effects of a subanesthetic dose of ketamine on oculomotor behavior in marmosets during face viewing. Four marmosets received systemic ketamine or saline injections while viewing phase-scrambled or intact videos of conspecifics' faces. To evaluate effects of ketamine on scan paths during face viewing, we identified regions of interest in each face video and classified locations of saccade onsets and landing positions within these areas. A preference for the snout over eye regions was observed following ketamine administration. In addition, regions in which saccades landed could be significantly predicted by saccade onset region in the saline but not the ketamine condition. Effects on saccade control were limited to an increase in saccade peak velocity in all conditions and a reduction in saccade amplitudes during viewing of scrambled videos. Thus, ketamine induced a significant disruption of scan paths during viewing of conspecific faces but limited effects on saccade motor control. These findings support the use of ketamine in marmosets for investigating changes in neural circuits underlying social cognition in neuropsychiatric disorders.NEW & NOTEWORTHY Face processing, an important social cognitive ability, is impaired in neuropsychiatric conditions such as schizophrenia. The highly social common marmoset model presents an opportunity to investigate these impairments. We administered subanesthetic doses of ketamine to marmosets to model the cognitive symptoms of schizophrenia. We observed a disruption of scan paths during viewing of conspecifics' faces. These findings support the use of ketamine in marmosets as a model for investigating social cognition in neuropsychiatric disorders.

Bonobos and chimpanzees preferentially attend to familiar members of the dominant sex

Social animals must carefully track consequential events and opportunities for social learning. However, the competing demands of the social world produce trade-offs in social attention, defined as directed visual attention towards conspecifics. A key question is how socioecology shapes these biases in social attention over evolution and development. Chimpanzees, Pan troglodytes , and bonobos, Pan paniscus , provide ideal models for addressing this question because they have large communities with fission–fusion grouping, divergent sex-based dominance hierarchies and occasional intergroup encounters. Using noninvasive eye-tracking measures, we recorded captive apes’ attention to side-by-side images of familiar and unfamiliar conspecifics of the same sex. We tested four competing hypotheses about the influence of taxonomically widespread socioecological pressures on social attention, including intergroup conflict, dominance, dispersal and mating competition. Both species preferentially attended to familiar over unfamiliar conspecifics when viewing the sex that typically occupies the highest ranks in the group: females for bonobos, and males for chimpanzees. However, they did not demonstrate attentional biases between familiar and unfamiliar members of the subordinate sex. Findings were consistent across species despite differences in which sex tends to be more dominant. These results suggest that sex-based dominance patterns guide social attention across Pan . Our findings reveal how socioecological pressures shape social attention in apes and likely contribute to the evolution of social cognition across primates. • Bonobos and chimpanzees discriminate faces of familiar and unfamiliar conspecifics. • Great apes devote greater social attention to familiar members of the dominant sex. • Species differences in male vs female dominance shape patterns of social attention. • Both selective pressures and socialization likely drive patterns of social attention.

Interspecific two-dimensional visual discrimination of faces in horses (Equus caballus).

In social animals, recognizing conspecifics and distinguishing them from other animal species is certainly important. We hypothesize, as demonstrated in other species of ungulates, that horses are able to discriminate between the faces of conspecifics and the faces of other domestic species (cattle, sheep, donkeys and pigs). Our hypothesis was tested by studying inter-and intra-specific visual discrimination abilities in horses through a two-way instrumental conditioning task (discrimination and reversal learning), using two-dimensional images of faces as discriminative stimuli and food as a positive reward. Our results indicate that 8 out of 10 horses were able to distinguish between two-dimensional images of the faces of horses and images showing the faces of other species. A similar performance was obtained in the reversal task. The horses' ability to learn by discrimination is therefore comparable to other ungulates. Horses also showed the ability to learn a reversal task. However, these results were obtained regardless of the images the tested horses were exposed to. We therefore conclude that horses can discriminate between two dimensional images of conspecifics and two dimensional images of different species, however in our study, they were not able to make further subcategories within each of the two categories. Despite the fact that two dimensional images of animals could be treated differently from two dimensional images of non-social stimuli, our results beg the question as to whether a two-dimensional image can replace the real animal in cognitive tests.

Divergent effects of oxytocin on eye contact in bonobos and chimpanzees

Oxytocin has drawn significant research attention for its role in modulating mammalian social behavior. Despite generally conserved roles, oxytocin can function differently even in closely related species. Previous studies have shown that bonobos and chimpanzees, humans’ two closest relatives, demonstrate considerable behavioral differences, including that bonobos look more at others’ eyes than chimpanzees. Oxytocin is known to increase attention to another’s eyes in many mammalian species (e.g. dogs, monkeys, and humans), yet this effect has not been tested in any nonhuman great ape species. This study examined how intranasally-administered oxytocin affects eye contact in bonobos and chimpanzees using eye tracking. Following administration of either oxytocin or saline control with a nebulizer, chimpanzees (n = 6) and bonobos (n = 5) were shown images of conspecific faces while their eye movement was recorded. Oxytocin changed the eye-looking behavior of bonobos and chimpanzees differently. We found that oxytocin increased eye contact in bonobos but not chimpanzees; while one chimpanzee showed an increase, interestingly, 5 out of 6 chimpanzees showed decreased looking to the eyes compared to the mouth, suggesting moderate eye avoidance. Given the importance of eye contact in their social interactions, our results suggest that oxytocin may play modulatory roles in bonobos’ and chimpanzees’ species-specific social behavior and underscore the importance of oxytocin in hominid social evolution.

Potent and Quick Responses to Conspecific Faces and Snakes in the Anterior Cingulate Cortex of Monkeys.

Appropriate processing of others’ facial emotions is a fundamental ability of primates in social situations. Several moods and anxiety disorders such as depression cause a negative bias in the perception of facial emotions. Depressive patients show abnormalities of activity and gray matter volume in the perigenual portion of the anterior cingulate cortex (ACC) and an increase of activation in the amygdala. However, it is not known whether neurons in the ACC have a function in the processing of facial emotions. Furthermore, detecting predators quickly and taking avoidance behavior are important functions in a matter of life and death for wild monkeys. the existence of predators in their vicinity is life-and-death information for monkeys. In the present study, we recorded the activity of single neurons from the monkey ACC and examined the responsiveness of the ACC neurons to various visual stimuli including monkey faces, snakes, foods, and artificial objects. About one-fourth of the recorded neurons showed a significant change in activity in response to the stimuli. The ACC neurons exhibited high selectivity to certain stimuli, and more neurons exhibited the maximal response to monkey faces and snakes than to foods and objects. The responses to monkey faces and snakes were faster and stronger compared to those to foods and objects. Almost all of the neurons that responded to video stimuli responded strongly to negative facial stimuli, threats, and scream. Most of the responsive neurons were located in the cingulate gyrus or the ventral bank of the cingulate sulcus just above or anterior to the genu of the corpus callosum, that is, the perigenual portion of the ACC, which has a strong mutual connection with the amygdala. These results suggest that the perigenual portion of the ACC in addition to the amygdala processes emotional information, especially negative life-and-death information such as conspecifics’ faces and snakes.

Altered face scanning and arousal after orbitofrontal cortex lesion in adult rhesus monkeys.

To further assess orbitofrontal cortex (OFC) contribution to the processing of socioemotional signals, spontaneous scanning patterns and pupil diameter variations were measured while adult rhesus macaques with either bilateral lesions of OFC areas 11 and 13 (Group O-asp) or sham-operations (Group C) freely viewed pictures of neutral and expressive faces of conspecifics, of other nonhuman primates and humans, and of objects with and without facial features. As compared to Group C, Group O-asp displayed (a) increased overall spontaneous visual exploration and increased scanning of primate neutral faces regardless of species and face orientation (upright/inverted), (b) longer gazes at the eyes of faces and of objects with facial features, and (c) intact ability to discriminate emotional from neutral faces, but (d) altered scanning patterns at emotional macaque faces coupled with (e) increased pupil dilation for conspecific faces according to face emotion and orientation (profile/stare). Thus, the primate OFC appears essential in the attention to and processing of faces, especially attention to the eyes and arousal self-regulation. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

Age categorization of conspecific and heterospecific faces in capuchin monkeys (Sapajus apella).

Across various species, infant faces share various features referred to as "baby schema"(Lorenz, 1942). Assuming that these features are indeed shared among species, it is possible that nonhuman animals may perceive age information in conspecific and heterospecific faces. We tested whether tufted capuchin monkeys (Sapajus apella) would visually categorize age from faces. In Experiment 1, we trained 4 monkeys to discriminate adult and infant faces of conspecifics using a symbolic matching-to-sample procedure. We then tested whether their categorization transferred to faces of other species (i.e., dogs and humans). In Experiment 2, we trained another 2 monkeys on age categorization of heterospecific (human) faces and tested them with conspecific and dog faces, to assess whether conspecific age categorization in Experiment 1 was specific. In Experiment 3, the 4 monkeys from Experiment 1 were trained with human faces, whereas the 2 monkeys from Experiment 2 were trained with conspecific faces; we then tested all 6 monkeys with faces of dogs and other species including New World monkeys, Old World monkeys, apes, and carnivores. During training, the monkeys quickly learned to categorize adult and infant faces of both conspecifics and humans. However, age categorization failed to transfer to different species in the test phase in all 3 experiments. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Face detection in 2- to 6-month-old infants is influenced by gaze direction and species.

Humans detect faces efficiently from a young age. Face detection is critical for infants to identify and learn from relevant social stimuli in their environments. Faces with eye contact are an especially salient stimulus, and attention to the eyes in infancy is linked to the emergence of later sociality. Despite the importance of both of these early social skills-attending to faces and attending to the eyes-surprisingly little is known about how they interact. We used eye tracking to explore whether eye contact influences infants' face detection. Longitudinally, we examined 2-, 4-, and 6-month-olds' (N=65) visual scanning of complex image arrays with human and animal faces varying in eye contact and head orientation. Across all ages, infants displayed superior detection of faces with eye contact; however, this effect varied as a function of species and head orientation. Infants were more attentive to human than animal faces and were more sensitive to eye and head orientation for human faces compared to animal faces. Unexpectedly, human faces with both averted heads and eyes received the most attention. This pattern may reflect the early emergence of gaze following-the ability to look where another individual looks-which begins to develop around this age. Infants may be especially interested in averted gaze faces, providing early scaffolding for joint attention. This study represents the first investigation to document infants' attention patterns to faces systematically varying in their attentional states. Together, these findings suggest that infants develop early, specialized functional conspecific face detection.

Expertise for conspecific face individuation in the human brain

Humans exhibit a marked specialization to process the most experienced facial morphologies. In particular, nonhuman primate faces are poorly discriminated compared to human faces in behavioral tasks. So far however, a clear and consistent marker that quantifies our expertise in human over monkey face discrimination directly from brain activity is lacking. Here, using scalp electroencephalography (EEG), we isolate a direct signature of individuation abilities for human and nonhuman (i.e., macaque faces) primate faces. Human or monkey faces were rapidly presented at a base rate of 12 Hz in upright or inverted orientations while participants performed an orthogonal behavioral task. In each stimulation sequence, eight face images of one individual were used as base stimuli, while images of other individuals were briefly introduced every 9th stimulus to quantify an identity-change response at 1.33 Hz and harmonics (i.e., integer multiples) in the EEG frequency spectrum. The brain response to upright human faces was twice as large as to monkey faces, and reduced following picture-plane inversion for human faces only. This reflects the disruption of high-level face identity discrimination developed for the canonical upright human face. No difference was observed between upright monkey faces and inverted human faces, suggesting non-expert visual processes for those two face formats associated with little experience. In addition, the size of the inversion effect for human, but not monkey faces, was predictive of the expertise effect (i.e., difference between upright human and monkey faces) at the individual level. This result suggests a selective ability to discriminate human faces that does not contribute to the individuation of other unexperienced face morphologies such as monkey faces. Overall, these findings indicate that human expertise for conspecific face discrimination can be isolated and quantified in individual human brains.

A parameterized digital 3D model of the Rhesus macaque face for investigating the visual processing of social cues

BackgroundRhesus macaques are the most popular model species for studying the neural basis of visual face processing and social interaction using intracranial methods. However, the challenge of creating realistic, dynamic, and parametric macaque face stimuli has limited the experimental control and ethological validity of existing approaches. New methodWe performed statistical analyses of in vivo computed tomography data to generate an anatomically accurate, three-dimensional representation of Rhesus macaque cranio-facial morphology. The surface structures were further edited, rigged and textured by a professional digital artist with careful reference to photographs of macaque facial expression, colouration and pelage. ResultsThe model offers precise, continuous, parametric control of craniofacial shape, emotional expression, head orientation, eye gaze direction, and many other parameters that can be adjusted to render either static or dynamic high-resolution faces. Example single-unit responses to such stimuli in macaque inferotemporal cortex demonstrate the value of parametric control over facial appearance and behaviours. Comparison with existing method(s)The generation of such a high-dimensionality and systematically controlled stimulus set of conspecific faces, with accurate craniofacial modelling and professional finalization of facial details, is currently not achievable using existing methods. ConclusionsThe results herald a new set of possibilities in adaptive sampling of a high-dimensional and socially meaningful feature space, thus opening the door to systematic testing of hypotheses about the abundant neural specialization for faces found in the primate.