A behavioral advantage for the face pareidolia illusion in peripheral vision

Investigation of visual illusions helps us understand how we process visual information. For example, face pareidolia, the misperception of illusory faces in objects, could be used to understand how we process real faces. However, it remains unclear whether this illusion emerges from errors in face detection or from slower, cognitive processes. Here, our logic is straightforward; if examples of face pareidolia activate the mechanisms that rapidly detect faces in visual environments, then participants will look at objects more quickly when the objects also contain illusory faces. To test this hypothesis, we sampled continuous eye movements during a fast saccadic choice task-participants were required to select either faces or food items. During this task, pairs of stimuli were positioned close to the initial fixation point or further away, in the periphery. As expected, the participants were faster to look at face targets than food targets. Importantly, we also discovered an advantage for food items with illusory faces but, this advantage was limited to the peripheral condition. These findings are among the first to demonstrate that the face pareidolia illusion persists in the periphery and, thus, it is likely to be a consequence of erroneous face detection.

Oxytocin differentially modulates the early neural responses to faces and non-social stimuli

Oxytocin (OT) alters social cognition partly through effects on the processing and appraisal of faces. It is debated whether the hormone also impacts the processing of other, non-social, visual stimuli. To this end, we conducted a randomized, counter-balanced, double-blind, placebo (PL)-controlled within-subjects' electro-encephalography (EEG) study with cismale participants (to control for gender dimorphic hormonal effects; n = 37). Participants received intranasal OT (24IU) and completed a one-back task viewing emotional (fearful/ happy) and neutral faces, and threat (snakes/spiders) and non-threat (mushrooms/flowers) non-social stimuli. OT differentially impacted event-related potentials (ERP)s to faces and non-social stimuli. For faces regardless of emotion, OT evoked greater occipital N1 and anterior P1 amplitudes at ∼155 ms than after PL, and lead to sustained differences over anterior, bilateral parietal and occipital sites from 205 ms onwards. For all non-social stimuli, OT evoked greater right parietal N1 amplitudes, and later only impacted threat stimuli over right parietal and occipital sites. None of these OT-induced modulations was related to individual anxiety levels. This pattern of results indicates that OT differentially modulates the processing of faces and non-social stimuli, and that the hormone's effect on visual processing and cognition does not occur as a function of non-clinical levels of anxiety.

Real Face Value: The Processing of Naturalistic Facial Expressions in the Macaque Inferior Temporal Cortex

For primates, expressions of fear are thought to be powerful social signals. In laboratory settings, faces with fearful expressions have reliably evoked valence effects in inferior temporal cortex. However, because macaques use so called "fear grins" in a variety of different contexts, the deeper question is whether the macaque inferior temporal cortex is tuned to the prototypical fear grin, or to conspecifics signaling fear? In this study, we combined neuroimaging with the results of a behavioral task to investigate how macaques encode a wide variety of fearful facial expressions. In Experiment 1, we identified two sets of macaque face stimuli using different approaches; we selected faces based on the emotional context (i.e., calm vs. fearful), and we selected faces based on the engagement of action units (i.e., neutral vs. fear grins). We also included human faces in Experiment 1. Then, using fMRI, we found that the faces selected based on context elicited a larger valence effect in the inferior temporal cortex than faces selected based on visual appearance. Furthermore, human facial expressions only elicited weak valence effects. These observations were further supported by the results of a two-alternative, forced-choice task (Experiment 2), suggesting that fear grins vary in their perceived pleasantness. Collectively, these findings indicate that the macaque inferior temporal cortex is more involved in social intelligence than commonly assumed, encoding emergent properties in naturalistic face stimuli that transcend basic visual features. These results demand a rethinking of theories surrounding the function and operationalization of primate inferior temporal cortex.

Inactivation of face-selective neurons alters eye movements when free viewing faces

During free viewing, faces attract gaze and induce specific fixation patterns corresponding to the facial features. This suggests that neurons encoding the facial features are in the causal chain that steers the eyes. However, there is no physiological evidence to support a mechanistic link between face-encoding neurons in high-level visual areas and the oculomotor system. In this study, we targeted the middle face patches of the inferior temporal (IT) cortex in two macaque monkeys using an functional magnetic resonance imaging (fMRI) localizer. We then utilized muscimol microinjection to unilaterally suppress IT neural activity inside and outside the face patches and recorded eye movements while the animals free viewing natural scenes. Inactivation of the face-selective neurons altered the pattern of eye movements on faces: The monkeys found faces in the scene but neglected the eye contralateral to the inactivation hemisphere. These findings reveal the causal contribution of the high-level visual cortex in eye movements.

Predictive extrapolation effects can have a greater impact on visual decisions, while visual adaptation has a greater impact on conscious visual experience

Human vision is shaped by historic and by predictive processes. The lingering impact of visual adaptation, for instance, can act to exaggerate differences between past and present inputs, whereas predictive processes can promote extrapolation effects that allow us to anticipate the near future. It is unclear to what extent either of these effects manifest in changes to conscious visual experience. It is also unclear how these influences combine, when acting in concert or opposition. We had people make decisions about the sizes of inputs, and report on levels of decisional confidence. Tests were either selectively subject to size adaptation, to an extrapolation effect, or to both of these effects. When these two effects were placed in opposition, extrapolation had a greater impact on decision making. However, our data suggest the influence of extrapolation is primarily decisional, whereas size adaptation more fully manifests in changes to conscious visual awareness.

Preliminary evidence of an increased susceptibility to face pareidolia in postpartum women

As primates, we are hypersensitive to faces and face-like patterns in the visual environment, hence we often perceive illusory faces in otherwise inanimate objects, such as burnt pieces of toast and the surface of the moon. Although this phenomenon, known as face pareidolia, is a common experience, it is unknown whether our susceptibility to face pareidolia is static across our lifespan or what factors would cause it to change. Given the evidence that behaviour towards face stimuli is modulated by the neuropeptide oxytocin (OT), we reasoned that participants in stages of life associated with high levels of endogenous OT might be more susceptible to face pareidolia than participants in other stages of life. We tested this hypothesis by assessing pareidolia susceptibility in two groups of women; pregnant women (low endogenous OT) and postpartum women (high endogenous OT). We found evidence that postpartum women report seeing face pareidolia more easily than women who are currently pregnant. These data, collected online, suggest that our sensitivity to face-like patterns is not fixed and may change throughout adulthood, providing a crucial proof of concept that requires further research.

Inactivation of face selective neurons alters eye movements when free viewing faces

During free viewing, faces attract gaze and induce specific fixation patterns corresponding to the facial features. This suggests that neurons encoding the facial features are in the causal chain that steers the eyes. However, there is no physiological evidence to support a mechanistic link between face encoding neurons in high-level visual areas and the oculomotor system. In this study, we targeted the middle face patches of inferior temporal (IT) cortex in two macaque monkeys using an fMRI localizer. We then utilized muscimol microinjection to unilaterally suppress IT neural activity inside and outside the face patches and recorded eye movements while the animals free viewing natural scenes. Inactivation of the face selective neurons altered the pattern of eye movements on faces: the monkeys found faces in the scene but neglected the eye contralateral to the inactivation hemisphere. These findings reveal the causal contribution of the high-level visual cortex in eye movements.

Significance

It has been shown, for more than half a century, that eye movements follow distinctive patterns when free viewing faces. This suggests causal involvement of the face-encoding visual neurons in the eye movements. However, the literature is scant of evidence for this possibility and has focused mostly on the link between low-level image saliency and eye movements. Here, for the first time, we bring causal evidence showing how face-selective neurons in inferior temporal cortex inform and steer eye movements when free viewing faces.

A database of heterogeneous faces for studying naturalistic expressions

Facial expressions are thought to be complex visual signals, critical for communication between social agents. Most prior work aimed at understanding how facial expressions are recognized has relied on stimulus databases featuring posed facial expressions, designed to represent putative emotional categories (such as 'happy' and 'angry'). Here we use an alternative selection strategy to develop the Wild Faces Database (WFD); a set of one thousand images capturing a diverse range of ambient facial behaviors from outside of the laboratory. We characterized the perceived emotional content in these images using a standard categorization task in which participants were asked to classify the apparent facial expression in each image. In addition, participants were asked to indicate the intensity and genuineness of each expression. While modal scores indicate that the WFD captures a range of different emotional expressions, in comparing the WFD to images taken from other, more conventional databases, we found that participants responded more variably and less specifically to the wild-type faces, perhaps indicating that natural expressions are more multiplexed than a categorical model would predict. We argue that this variability can be employed to explore latent dimensions in our mental representation of facial expressions. Further, images in the WFD were rated as less intense and more genuine than images taken from other databases, suggesting a greater degree of authenticity among WFD images. The strong positive correlation between intensity and genuineness scores demonstrating that even the high arousal states captured in the WFD were perceived as authentic. Collectively, these findings highlight the potential utility of the WFD as a new resource for bridging the gap between the laboratory and real world in studies of expression recognition.

A broadly tuned network for affective body language in the macaque brain

Body language is a powerful tool that we use to communicate how we feel, but it is unclear whether other primates also communicate in this way. Here, we use functional magnetic resonance imaging to show that the body-selective patches in macaques are activated by affective body language. Unexpectedly, we found these regions to be tolerant of naturalistic variation in posture as well as species; the bodies of macaques, humans, and domestic cats all evoked a stronger response when they conveyed fear than when they conveyed no affect. Multivariate analyses confirmed that the neural representation of fear-related body expressions was species-invariant. Collectively, these findings demonstrate that, like humans, macaques have body-selective brain regions in the ventral visual pathway for processing affective body language. These data also indicate that representations of body stimuli in these regions are built on the basis of emergent properties, such as socio-affective meaning, and not just putative image properties.

Clutter Substantially Reduces Selectivity for Peripheral Faces in the Macaque Brain

According to a prominent view in neuroscience, visual stimuli are coded by discrete cortical networks that respond preferentially to specific categories, such as faces or objects. However, it remains unclear how these category-selective networks respond when viewing conditions are cluttered, i.e., when there is more than one stimulus in the visual field. Here, we asked three questions: (1) Does clutter reduce the response and selectivity for faces as a function of retinal location? (2) Is the preferential response to faces uniform across the visual field? And (3) Does the ventral visual pathway encode information about the location of cluttered faces? We used fMRI to measure the response of the face-selective network in awake, fixating macaques (two female, five male). Across a series of four experiments, we manipulated the presence and absence of clutter, as well as the location of the faces relative to the fovea. We found that clutter reduces the response to peripheral faces. When presented in isolation, without clutter, the selectivity for faces is fairly uniform across the visual field, but, when clutter is present, there is a marked decrease in the selectivity for peripheral faces. We also found no evidence of a contralateral visual field bias when faces were presented in clutter. Nonetheless, multivariate analyses revealed that the location of cluttered faces could be decoded from the multivoxel response of the face-selective network. Collectively, these findings demonstrate that clutter blunts the selectivity of the face-selective network to peripheral faces, although information about their retinal location is retained.SIGNIFICANCE STATEMENT Numerous studies that have measured brain activity in macaques have found visual regions that respond preferentially to faces. Although these regions are thought to be essential for social behavior, their responses have typically been measured while faces were presented in isolation, a situation atypical of the real world. How do these regions respond when faces are presented with other stimuli? We report that, when clutter is present, the preferential response to foveated faces is spared but preferential response to peripheral faces is reduced. Our results indicate that the presence of clutter changes the response of the face-selective network.

Assessing the perception of face pareidolia in children (Homo sapiens), rhesus monkeys (Macaca mulatta), and capuchin monkeys (Sapajus apella)

Face pareidolia is the misperception of a face in an inanimate object and is a common feature of the face detection system in humans. Whereas there are many similarities in how humans and nonhuman animals such as monkeys perceive and respond to faces, it is still unclear whether other species also perceive certain nonface stimuli as faces. We presented a novel computerized task to capuchin monkeys (Sapajus apella), rhesus monkeys (Macaca mulatta), and preschool-aged children (Homo sapiens). This task trained subjects to choose faces over nonface images, and then presented pareidolia images with nonface images. All species selected faces most often on trials that included face images. However, only children selected pareidolia images at levels above chance. These results indicate that while children report perceiving face pareidolia, monkeys do not. These species differences could be due to human-unique experiences that result in an increased aptitude for anthropomorphizing objects with face-like patterns. It could also be due to monkeys showing a greater reliance on stimulus features rather than global, holistically organized cues that faces provide. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

The cortical and subcortical correlates of face pareidolia in the macaque brain

Face detection is a foundational social skill for primates. This vital function is thought to be supported by specialized neural mechanisms; however, although several face-selective regions have been identified in both humans and nonhuman primates, there is no consensus about which region(s) are involved in face detection. Here, we used naturally occurring errors of face detection (i.e. objects with illusory facial features referred to as examples of 'face pareidolia') to identify regions of the macaque brain implicated in face detection. Using whole-brain functional magnetic resonance imaging to test awake rhesus macaques, we discovered that a subset of face-selective patches in the inferior temporal cortex, on the lower lateral edge of the superior temporal sulcus, and the amygdala respond more to objects with illusory facial features than matched non-face objects. Multivariate analyses of the data revealed differences in the representation of illusory faces across the functionally defined regions of interest. These differences suggest that the cortical and subcortical face-selective regions contribute uniquely to the detection of facial features. We conclude that face detection is supported by a multiplexed system in the primate brain.

Single-Unit Recordings Reveal the Selectivity of a Human Face Area

The exquisite capacity of primates to detect and recognize faces is crucial for social interactions. Although disentangling the neural basis of human face recognition remains a key goal in neuroscience, direct evidence at the single-neuron level is limited. We recorded from face-selective neurons in human visual cortex in a region characterized by functional magnetic resonance imaging (fMRI) activations for faces compared with objects. The majority of visually responsive neurons in this fMRI activation showed strong selectivity at short latencies for faces compared with objects. Feature-scrambled faces and face-like objects could also drive these neurons, suggesting that this region is not tightly tuned to the visual attributes that typically define whole human faces. These single-cell recordings within the human face processing system provide vital experimental evidence linking previous imaging studies in humans and invasive studies in animal models.SIGNIFICANCE STATEMENT We present the first recordings of face-selective neurons in or near an fMRI-defined patch in human visual cortex. Our unbiased multielectrode array recordings (i.e., no selection of neurons based on a search strategy) confirmed the validity of the BOLD contrast (faces-objects) in humans, a finding with implications for all human imaging studies. By presenting faces, feature-scrambled faces, and face-pareidolia (perceiving faces in inanimate objects) stimuli, we demonstrate that neurons at this level of the visual hierarchy are broadly tuned to the features of a face, independent of spatial configuration and low-level visual attributes.

A shared mechanism for facial expression in human faces and face pareidolia

Facial expressions are vital for social communication, yet the underlying mechanisms are still being discovered. Illusory faces perceived in objects (face pareidolia) are errors of face detection that share some neural mechanisms with human face processing. However, it is unknown whether expression in illusory faces engages the same mechanisms as human faces. Here, using a serial dependence paradigm, we investigated whether illusory and human faces share a common expression mechanism. First, we found that images of face pareidolia are reliably rated for expression, within and between observers, despite varying greatly in visual features. Second, they exhibit positive serial dependence for perceived facial expression, meaning an illusory face (happy or angry) is perceived as more similar in expression to the preceding one, just as seen for human faces. This suggests illusory and human faces engage similar mechanisms of temporal continuity. Third, we found robust cross-domain serial dependence of perceived expression between illusory and human faces when they were interleaved, with serial effects larger when illusory faces preceded human faces than the reverse. Together, the results support a shared mechanism for facial expression between human faces and illusory faces and suggest that expression processing is not tightly bound to human facial features.

Using FACS to trace the neural specializations underlying the recognition of facial expressions: A commentary on Waller et al. (2020)

In the recent review by Waller et al. (2020) the authors discuss how the Facial Action Coding System (FACS) can be used to study the evolution of facial behaviors. This is a timely and thought-provoking review which highlights the numerous ways in which FACS could be used to compare the mechanisms responsible for the production of facial behaviors across species. We propose that FACS could also be used to study the recognition of facial behaviors in nonhuman subjects where one of the key challenges is finding suitable stimuli that convey different emotions. By using FACS-rated images in awake neuroimaging experiments, researchers could accurately identify the brain mechanisms responsible for recognizing expressions across mammalian species. This approach would reveal neural homologs and deepen our understanding of how nonverbal social communication has evolved.

Rapid and dynamic processing of face pareidolia in the human brain

The human brain is specialized for face processing, yet we sometimes perceive illusory faces in objects. It is unknown whether these natural errors of face detection originate from a rapid process based on visual features or from a slower, cognitive re-interpretation. Here we use a multifaceted approach to understand both the spatial distribution and temporal dynamics of illusory face representation in the brain by combining functional magnetic resonance imaging and magnetoencephalography neuroimaging data with model-based analysis. We find that the representation of illusory faces is confined to occipital-temporal face-selective visual cortex. The temporal dynamics reveal a striking evolution in how illusory faces are represented relative to human faces and matched objects. Illusory faces are initially represented more similarly to real faces than matched objects are, but within ~250 ms, the representation transforms, and they become equivalent to ordinary objects. This is consistent with the initial recruitment of a broadly-tuned face detection mechanism which privileges sensitivity over selectivity.

The impact of stimulus size and orientation on individual face coding in monkey face-selective cortex

Face-selective neurons in the monkey temporal cortex discharge at different rates in response to pictures of different individual faces. Here we tested whether this pattern of response across single neurons in the face-selective area ML (located in the middle Superior Temporal Sulcus) tolerates two affine transformations; picture-plane inversion, known to decrease the average response of face-selective neurons and the other, stimulus size. We recorded the response of 57 ML neurons in two awake and fixating monkeys. Face stimuli were presented at two sizes (10 and 5 degrees of visual angle) and two orientations (upright and inverted). Different faces elicited distinct patterns of activity across ML neurons that were reliable (i.e., predictable with a classifier) within a specific size and orientation condition. Despite observing a reduction in the average response magnitude of face-selective neurons to inverted faces, compared to upright faces, classifier performance was above chance for both upright and inverted faces. While decoding was largely preserved across changes in stimulus size, a classifier trained with one orientation condition and tested on the other did not lead to performance above chance level. We conclude that different individual faces can be decoded from patterns of responses in the monkey area ML regardless of orientation or size, but with qualitatively different patterns of responses for upright and inverted faces.

Face Pareidolia in the Rhesus Monkey

Face perception in humans and nonhuman primates is rapid and accurate [1-4]. In the human brain, a network of visual-processing regions is specialized for faces [5-7]. Although face processing is a priority of the primate visual system, face detection is not infallible. Face pareidolia is the compelling illusion of perceiving facial features on inanimate objects, such as the illusory face on the surface of the moon. Although face pareidolia is commonly experienced by humans, its presence in other species is unknown. Here we provide evidence for face pareidolia in a species known to possess a complex face-processing system [8-10]: the rhesus monkey (Macaca mulatta). In a visual preference task [11, 12], monkeys looked longer at photographs of objects that elicited face pareidolia in human observers than at photographs of similar objects that did not elicit illusory faces. Examination of eye movements revealed that monkeys fixated the illusory internal facial features in a pattern consistent with how they view photographs of faces [13]. Although the specialized response to faces observed in humans [1, 3, 5-7, 14] is often argued to be continuous across primates [4, 15], it was previously unclear whether face pareidolia arose from a uniquely human capacity. For example, pareidolia could be a product of the human aptitude for perceptual abstraction or result from frequent exposure to cartoons and illustrations that anthropomorphize inanimate objects. Instead, our results indicate that the perception of illusory facial features on inanimate objects is driven by a broadly tuned face-detection mechanism that we share with other species.

Face familiarity promotes stable identity recognition: exploring face perception using serial dependence

Studies suggest that familiar faces are processed in a manner distinct from unfamiliar faces and that familiarity with a face confers an advantage in identity recognition. Our visual system seems to capitalize on experience to build stable face representations that are impervious to variation in retinal input that may occur due to changes in lighting, viewpoint, viewing distance, eye movements, etc. Emerging evidence also suggests that our visual system maintains a continuous perception of a face's identity from one moment to the next despite the retinal input variations through serial dependence. This study investigates whether interactions occur between face familiarity and serial dependence. In two experiments, participants used a continuous scale to rate attractiveness of unfamiliar and familiar faces (either experimentally learned or famous) presented in rapid sequences. Both experiments revealed robust inter-trial effects in which attractiveness ratings for a given face depended on the preceding face's attractiveness. This inter-trial attractiveness effect was most pronounced for unfamiliar faces. Indeed, when participants were familiar with a given face, attractiveness ratings showed significantly less serial dependence. These results represent the first evidence that familiar faces can resist the temporal integration seen in sequential dependencies and highlight the importance of familiarity to visual cognition.

Who is the Usual Suspect? Evidence of a Selection Bias Toward Faces That Make Direct Eye Contact in a Lineup Task

The speed and ease with which we recognize the faces of our friends and family members belies the difficulty we have recognizing less familiar individuals. Nonetheless, overconfidence in our ability to recognize faces has carried over into various aspects of our legal system; for instance, eyewitness identification serves a critical role in criminal proceedings. For this reason, understanding the perceptual and psychological processes that underlie false identification is of the utmost importance. Gaze direction is a salient social signal and direct eye contact, in particular, is thought to capture attention. Here, we tested the hypothesis that differences in gaze direction may influence difficult decisions in a lineup context. In a series of experiments, we show that when a group of faces differed in their gaze direction, the faces that were making eye contact with the participants were more likely to be misidentified. Interestingly, this bias disappeared when the faces are presented with their eyes closed. These findings open a critical conversation between social neuroscience and forensic psychology, and imply that direct eye contact may (wrongly) increase the perceived familiarity of a face.

Faces in Context: Does Face Perception Depend on the Orientation of the Visual Scene?

The mechanisms held responsible for familiar face recognition are thought to be orientation dependent; inverted faces are more difficult to recognize than their upright counterparts. Although this effect of inversion has been investigated extensively, researchers have typically sliced faces from photographs and presented them in isolation. As such, it is not known whether the perceived orientation of a face is inherited from the visual scene in which it appears. Here, we address this question by measuring performance in a simultaneous same–different task while manipulating both the orientation of the faces and the scene. We found that the face inversion effect survived scene inversion. Nonetheless, an improvement in performance when the scene was upside down suggests that sensitivity to identity increased when the faces were more easily segmented from the scene. Thus, while these data identify congruency with the visual environment as a contributing factor in recognition performance, they imply different mechanisms operate on upright and inverted faces.

The impact of orientation filtering on face-selective neurons in monkey inferior temporal cortex

Faces convey complex social signals to primates. These signals are tolerant of some image transformations (e.g. changes in size) but not others (e.g. picture-plane rotation). By filtering face stimuli for orientation content, studies of human behavior and brain responses have shown that face processing is tuned to selective orientation ranges. In the present study, for the first time, we recorded the responses of face-selective neurons in monkey inferior temporal (IT) cortex to intact and scrambled faces that were filtered to selectively preserve horizontal or vertical information. Guided by functional maps, we recorded neurons in the lateral middle patch (ML), the lateral anterior patch (AL), and an additional region located outside of the functionally defined face-patches (CONTROL). We found that neurons in ML preferred horizontal-passed faces over their vertical-passed counterparts. Neurons in AL, however, had a preference for vertical-passed faces, while neurons in CONTROL had no systematic preference. Importantly, orientation filtering did not modulate the firing rate of neurons to phase-scrambled face stimuli in any recording region. Together these results suggest that face-selective neurons found in the face-selective patches are differentially tuned to orientation content, with horizontal tuning in area ML and vertical tuning in area AL.

The effect of face inversion for neurons inside and outside fMRI-defined face-selective cortical regions

It is widely believed that face processing in the primate brain occurs in a network of category-selective cortical regions. Combined functional MRI (fMRI)-single-cell recording studies in macaques have identified high concentrations of neurons that respond more to faces than objects within face-selective patches. However, cells with a preference for faces over objects are also found scattered throughout inferior temporal (IT) cortex, raising the question whether face-selective cells inside and outside of the face patches differ functionally. Here, we compare the properties of face-selective cells inside and outside of face-selective patches in the IT cortex by means of an image manipulation that reliably disrupts behavior toward face processing: inversion. We recorded IT neurons from two fMRI-defined face-patches (ML and AL) and a region outside of the face patches (herein labeled OUT) during upright and inverted face stimulation. Overall, turning faces upside down reduced the firing rate of face-selective cells. However, there were differences among the recording regions. First, the reduced neuronal response for inverted faces was independent of stimulus position, relative to fixation, in the face-selective patches (ML and AL) only. Additionally, the effect of inversion for face-selective cells in ML, but not those in AL or OUT, was impervious to whether the neurons were initially searched for using upright or inverted stimuli. Collectively, these results show that face-selective cells differ in their functional characteristics depending on their anatomicofunctional location, suggesting that upright faces are preferably coded by face-selective cells inside but not outside of the fMRI-defined face-selective regions of the posterior IT cortex.

A comparative study of face processing using scrambled faces

It is a widespread assumption that all primate species process faces in the same way because the species are closely related and they engage in similar social interactions. However, this approach ignores potentially interesting and informative differences that may exist between species. This paper describes a comparative study of holistic face processing. Twelve subjects (six chimpanzees Pan troglodytes and six rhesus monkeys Macaca mulatta) were trained to discriminate whole faces (faces with features in their canonical position) and feature-scrambled faces in two separate conditions. We found that both species tended to match the global configuration of features over local features, providing strong evidence of global precedence. In addition, we show that both species were better able to generalize from a learned configuration to an entirely novel configuration when they were first trained to match feature-scrambled faces compared to when they were trained with whole faces. This result implies that the subjects were able to access local information easier when facial features were presented in a scrambled configuration and is consistent with a holistic processing hypothesis. Interestingly, these data also suggest that, while holistic processing in chimpanzees is tuned to own-species faces, monkeys have a more general approach towards all faces. Thus, while these data confirm that both chimpanzees and rhesus monkeys process faces holistically, they also indicate that there are differences between the species that warrant further investigation.

The organization of conspecific face space in nonhuman primates

Humans and chimpanzees demonstrate numerous cognitive specializations for processing faces, but comparative studies with monkeys suggest that these may be the result of recent evolutionary adaptations. The present study utilized the novel approach of face space, a powerful theoretical framework used to understand the representation of face identity in humans, to further explore species differences in face processing. According to the theory, faces are represented by vectors in a multidimensional space, the centre of which is defined by an average face. Each dimension codes features important for describing a face's identity, and vector length codes the feature's distinctiveness. Chimpanzees and rhesus monkeys discriminated male and female conspecifics' faces, rated by humans for their distinctiveness, using a computerized task. Multidimensional scaling analyses showed that the organization of face space was similar between humans and chimpanzees. Distinctive faces had the longest vectors and were the easiest for chimpanzees to discriminate. In contrast, distinctiveness did not correlate with the performance of rhesus monkeys. The feature dimensions for each species' face space were visualized and described using morphing techniques. These results confirm species differences in the perceptual representation of conspecific faces, which are discussed within an evolutionary framework.

The composite face effect in chimpanzees (Pan troglodytes) and rhesus monkeys (Macaca mulatta)

The accurate recognition of individuals is a cornerstone of social exchange in primates. Current approaches to the study of individual recognition, however, are focused on human performance and only occasionally reference comparative research for support. A number of studies have suggested that humans can easily recognize thousands of individual faces and that this perceptual expertise is supported by holistic processing, a phenomenon best demonstrated by the composite face effect (CFE). Recent advances have indicated that the CFE in humans is robust to changes in contrast polarity (positive or negative). Here we apply a two alternative forced choice match-to-sample paradigm across two species of nonhuman primate, chimpanzees (Pan troglodyte) and rhesus monkeys (Macaca mulatta). We find strong evidence that chimpanzees and rhesus monkeys experience interference from holistic processing in the positive contrast condition. While there is some indication that the chimpanzees experienced a CFE in the negative contrast condition, there was no evidence of a CFE in the corresponding rhesus monkey data. These results suggest that, while holistic processing is a general mechanism, rhesus monkeys are more sensitive to contrast-polarity than humans.