Contrasting specializations for facial motion within the macaque face-processing system

Predictions enable top-down pattern separation in the macaque face-processing hierarchy

Distinguishing faces requires well distinguishable neural activity patterns. Contextual information may separate neural representations, leading to enhanced identity recognition. Here, we use functional magnetic resonance imaging to investigate how predictions derived from contextual information affect the separability of neural activity patterns in the macaque face-processing system, a 3-level processing hierarchy in ventral visual cortex. We find that in the presence of predictions, early stages of this hierarchy exhibit well separable and high-dimensional neural geometries resembling those at the top of the hierarchy. This is accompanied by a systematic shift of tuning properties from higher to lower areas, endowing lower areas with higher-order, invariant representations instead of their feedforward tuning properties. Thus, top-down signals dynamically transform neural representations of faces into separable and high-dimensional neural geometries. Our results provide evidence how predictive context transforms flexible representational spaces to optimally use the computational resources provided by cortical processing hierarchies for better and faster distinction of facial identities.

Two what, two where, visual cortical streams in humans

ROLLS, E. T. Two What, Two Where, Visual Cortical Streams in Humans. NEUROSCI BIOBEHAV REV 2024. Recent cortical connectivity investigations lead to new concepts about 'What' and 'Where' visual cortical streams in humans, and how they connect to other cortical systems. A ventrolateral 'What' visual stream leads to the inferior temporal visual cortex for object and face identity, and provides 'What' information to the hippocampal episodic memory system, the anterior temporal lobe semantic system, and the orbitofrontal cortex emotion system. A superior temporal sulcus (STS) 'What' visual stream utilising connectivity from the temporal and parietal visual cortex responds to moving objects and faces, and face expression, and connects to the orbitofrontal cortex for emotion and social behaviour. A ventromedial 'Where' visual stream builds feature combinations for scenes, and provides 'Where' inputs via the parahippocampal scene area to the hippocampal episodic memory system that are also useful for landmark-based navigation. The dorsal 'Where' visual pathway to the parietal cortex provides for actions in space, but also provides coordinate transforms to provide inputs to the parahippocampal scene area for self-motion update of locations in scenes in the dark or when the view is obscured.

Specialized Networks for Social Cognition in the Primate Brain

Primates have evolved diverse cognitive capabilities to navigate their complex social world. To understand how the brain implements critical social cognitive abilities, we describe functional specialization in the domains of face processing, social interaction understanding, and mental state attribution. Systems for face processing are specialized from the level of single cells to populations of neurons within brain regions to hierarchically organized networks that extract and represent abstract social information. Such functional specialization is not confined to the sensorimotor periphery but appears to be a pervasive theme of primate brain organization all the way to the apex regions of cortical hierarchies. Circuits processing social information are juxtaposed with parallel systems involved in processing nonsocial information, suggesting common computations applied to different domains. The emerging picture of the neural basis of social cognition is a set of distinct but interacting subnetworks involved in component processes such as face perception and social reasoning, traversing large parts of the primate brain.

Representational geometry of incomplete faces in macaque face patches

The neural code of faces has been intensively studied in the macaque face patch system. Although the majority of previous studies used complete faces as stimuli, faces are often seen partially in daily life. Here, we investigated how face-selective cells represent two types of incomplete faces: face fragments and occluded faces, with the location of the fragment/occluder and the facial features systematically varied. Contrary to popular belief, we found that the preferred face regions identified with two stimulus types are dissociated in many face cells. This dissociation can be explained by the nonlinear integration of information from different face parts and is closely related to a curved representation of face completeness in the state space, which allows a clear discrimination between different stimulus types. Furthermore, identity-related facial features are represented in a subspace orthogonal to the nonlinear dimension of face completeness, supporting a condition-general code of facial identity.

Causal Manipulation of Gaze-Following in the Macaque Temporal Cortex

Gaze-following, the ability to shift one's own attention to places or objects others are looking at, is essential for social interactions. Single unit recordings from the monkey cortex and neuroimaging work on the human and monkey brain suggest that a distinct region in the temporal cortex, the gaze-following patch (GFP), underpins this ability. Since previous studies of the GFP have relied on correlational techniques, it remains unclear whether gaze-following related activity in the GFP indicates a causal role rather than being just a reverberation of behaviorally relevant information produced elsewhere. To answer this question, we applied focal electrical and pharmacological perturbation to the GFP. Both approaches, when applied to the GFP, disrupted gaze-following if the monkeys had been instructed to follow gaze, along with the ability to suppress it if vetoed by the context. Hence the GFP is necessary for gaze-following as well as its cognitive control.

Towards an optimization of functional localizers in non-human primate neuroimaging with (fMRI) frequency-tagging

Non-human primate (NHP) neuroimaging can provide essential insights into the neural basis of human cognitive functions. While functional magnetic resonance imaging (fMRI) localizers can play an essential role in reaching this objective (Russ et al., 2021), they often differ substantially across species in terms of paradigms, measured signals, and data analysis, biasing the comparisons. Here we introduce a functional frequency-tagging face localizer for NHP imaging, successfully developed in humans and outperforming standard face localizers (Gao et al., 2018). FMRI recordings were performed in two awake macaques. Within a rapid 6 Hz stream of natural non-face objects images, human or monkey face stimuli were presented in bursts every 9 s. We also included control conditions with phase-scrambled versions of all images. As in humans, face-selective activity was objectively identified and quantified at the peak of the face-stimulation frequency (0.111 Hz) and its second harmonic (0.222 Hz) in the Fourier domain. Focal activations with a high signal-to-noise ratio were observed in regions previously described as face-selective, mainly in the STS (clusters PL, ML, MF; also, AL, AF), both for human and monkey faces. Robust face-selective activations were also found in the prefrontal cortex of one monkey (PVL and PO clusters). Face-selective neural activity was highly reliable and excluded all contributions from low-level visual cues contained in the amplitude spectrum of the stimuli. These observations indicate that fMRI frequency-tagging provides a highly valuable approach to objectively compare human and monkey visual recognition systems within the same framework.

The contribution of dynamics to macaque body and face patch responses

Previous functional imaging studies demonstrated body-selective patches in the primate visual temporal cortex, comparing activations to static bodies and static images of other categories. However, the use of static instead of dynamic displays of moving bodies may have underestimated the extent of the body patch network. Indeed, body dynamics provide information about action and emotion and may be processed in patches not activated by static images. Thus, to map with fMRI the full extent of the macaque body patch system in the visual temporal cortex, we employed dynamic displays of natural-acting monkey bodies, dynamic monkey faces, objects, and scrambled versions of these videos, all presented during fixation. We found nine body patches in the visual temporal cortex, starting posteriorly in the superior temporal sulcus (STS) and ending anteriorly in the temporal pole. Unlike for static images, body patches were present consistently in both the lower and upper banks of the STS. Overall, body patches showed a higher activation by dynamic displays than by matched static images, which, for identical stimulus displays, was less the case for the neighboring face patches. These data provide the groundwork for future single-unit recording studies to reveal the spatiotemporal features the neurons of these body patches encode. These fMRI findings suggest that dynamics have a stronger contribution to population responses in body than face patches.

Encoding of dynamic facial information in the middle dorsal face area

Faces in motion reveal a plethora of information through visual dynamics. Faces can move in complex patterns while transforming facial shape, e.g., during the generation of different emotional expressions. While motion and shape processing have been studied extensively in separate research enterprises, much less is known about their conjunction during biological motion. Here, we took advantage of the discovery in brain-imaging studies of an area in the dorsal portion of the macaque monkey superior temporal sulcus (STS), the middle dorsal face area (MD), with selectivity for naturalistic face motion. To gain mechanistic insights into the coding of facial motion, we recorded single-unit activity from MD, testing whether and how MD cells encode face motion. The MD population was highly sensitive to naturalistic facial motion and facial shape. Some MD cells responded only to the conjunction of facial shape and motion, others were selective for facial shape even without movement, and yet others were suppressed by facial motion. We found that this heterogeneous MD population transforms face motion into a higher dimensional activity space, a representation that would allow for high sensitivity to relevant small-scale movements. Indeed, we show that many MD cells carry such sensitivity for eye movements. We further found that MD cells encode motion of head, mouth, and eyes in a separable manner, requiring the use of multiple reference frames. Thus, MD is a bona fide face-motion area that uses highly heterogeneous cell populations to create codes capturing even complex facial motion trajectories.

Bidirectional and parallel relationships in macaque face circuit revealed by fMRI and causal pharmacological inactivation

Although the presence of face patches in primate inferotemporal (IT) cortex is well established, the functional and causal relationships among these patches remain elusive. In two monkeys, muscimol was infused sequentially into each patch or pair of patches to assess their respective influence on the remaining IT face network and the amygdala, as determined using fMRI. The results revealed that anterior face patches required input from middle face patches for their responses to both faces and objects, while the face selectivity in middle face patches arose, in part, from top-down input from anterior face patches. Moreover, we uncovered a parallel fundal-lateral functional organization in the IT face network, supporting dual routes (dorsal-ventral) in face processing within IT cortex as well as between IT cortex and the amygdala. Our findings of the causal relationship among the face patches demonstrate that the IT face circuit is organized into multiple functional compartments.

Cognitive dysfunctions in high myopia: An overview of potential neural morpho-functional mechanisms

Dementia and cognitive impairment (CIM) carry high levels of mortality. Visual impairment (VI) is linked with CIM risk. High myopia (HM) is a chronic disease frequently leading to irreversible blindness. Current opinion has shifted from retinal injury as the cause of HM to the condition being considered an eye-brain disease. However, the pathogenesis of this disease and the manner in which neural structures are damaged are poorly understood. This review comprehensively discusses the relationship between HM, the central nervous system, and CIM, together with the novel concept of three visual pathways, and possible research perspectives.

Local features drive identity responses in macaque anterior face patches

Humans and other primates recognize one another in part based on unique structural details of the face, including both local features and their spatial configuration within the head and body. Visual analysis of the face is supported by specialized regions of the primate cerebral cortex, which in macaques are commonly known as face patches. Here we ask whether the responses of neurons in anterior face patches, thought to encode face identity, are more strongly driven by local or holistic facial structure. We created stimuli consisting of recombinant photorealistic images of macaques, where we interchanged the eyes, mouth, head, and body between individuals. Unexpectedly, neurons in the anterior medial (AM) and anterior fundus (AF) face patches were predominantly tuned to local facial features, with minimal neural selectivity for feature combinations. These findings indicate that the high-level structural encoding of face identity rests upon populations of neurons specialized for local features.

Anterior face patches in the macaque have been assumed to represent face identity in a holistic manner. Here the authors show that the neural encoding of face identity in the anterior medial and anterior fundus face patches are instead driven principally by local features.

Representation of expression and identity by ventral prefrontal neurons

Evidence has suggested that the ventrolateral prefrontal cortex (VLPFC) processes social stimuli, including faces and vocalizations, which are essential for communication. Features embedded within audiovisual stimuli, including emotional expression and caller identity, provide abundant information about an individual's intention, emotional state, motivation, and social status, which are important to encode in a social exchange. However, it is unknown to what extent the VLPFC encodes such features. To investigate the role of VLPFC during social communication, we recorded single-unit activity while rhesus macaques (Macaca mulatta) performed a nonmatch-to-sample task using species-specific face-vocalization stimuli that differed in emotional expression or caller identity. 75% of recorded cells were task-related and of these >70% were responsive during the nonmatch period. A larger proportion of nonmatch cells encoded the stimulus rather than the context of the trial type. A subset of responsive neurons were most commonly modulated by the identity of the nonmatch stimulus and less by the emotional expression, or both features within the face-vocalization stimuli presented during the nonmatch period. Neurons encoding identity were found in VLPFC across a broader region than expression related cells which were confined to only the anterolateral portion of the recording chamber in VLPFC. These findings suggest that, within a working memory paradigm, VLPFC processes features of face and vocal stimuli, such as emotional expression and identity, in addition to task and contextual information. Thus, stimulus and contextual information may be integrated by VLPFC during social communication.

Mesoscopic landscape of cortical functions revealed by through-skull wide-field optical imaging in marmoset monkeys

The primate cerebral cortex is organized into specialized areas representing different modalities and functions along a continuous surface. The functional maps across the cortex, however, are often investigated a single modality at a time (e.g., audition or vision). To advance our understanding of the complex landscape of primate cortical functions, here we develop a polarization-gated wide-field optical imaging method for measuring cortical functions through the un-thinned intact skull in awake marmoset monkeys (Callithrix jacchus), a primate species featuring a smooth cortex. Using this method, adjacent auditory, visual, and somatosensory cortices are noninvasively parcellated in individual subjects with detailed tonotopy, retinotopy, and somatotopy. An additional pure-tone-responsive tonotopic gradient is discovered in auditory cortex and a face-patch sensitive to motion in the lower-center visual field is localized near an auditory region representing frequencies of conspecific vocalizations. This through-skull landscape-mapping approach provides new opportunities for understanding how the primate cortex is organized and coordinated to enable real-world behaviors.

Structural Brain Asymmetries for Language: A Comparative Approach across Primates

Humans are the only species that can speak. Nonhuman primates, however, share some ‘domain-general’ cognitive properties that are essential to language processes. Whether these shared cognitive properties between humans and nonhuman primates are the results of a continuous evolution [homologies] or of a convergent evolution [analogies] remain difficult to demonstrate. However, comparing their respective underlying structure—the brain—to determinate their similarity or their divergence across species is critical to help increase the probability of either of the two hypotheses, respectively. Key areas associated with language processes are the Planum Temporale, Broca’s Area, the Arcuate Fasciculus, Cingulate Sulcus, The Insula, Superior Temporal Sulcus, the Inferior Parietal lobe, and the Central Sulcus. These structures share a fundamental feature: They are functionally and structurally specialised to one hemisphere. Interestingly, several nonhuman primate species, such as chimpanzees and baboons, show human-like structural brain asymmetries for areas homologous to key language regions. The question then arises: for what function did these asymmetries arise in non-linguistic primates, if not for language per se? In an attempt to provide some answers, we review the literature on the lateralisation of the gestural communication system, which may represent the missing behavioural link to brain asymmetries for language area’s homologues in our common ancestor.

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.

The role of temporal cortex in the control of attention

Attention is an indispensable component of active vision. Contrary to the widely accepted notion that temporal cortex processing primarily focusses on passive object recognition, a series of very recent studies emphasize the role of temporal cortex structures, specifically the superior temporal sulcus (STS) and inferotemporal (IT) cortex, in guiding attention and implementing cognitive programs relevant for behavioral tasks. The goal of this theoretical paper is to advance the hypothesis that the temporal cortex attention network (TAN) entails necessary components to actively participate in attentional control in a flexible task-dependent manner. First, we will briefly discuss the general architecture of the temporal cortex with a focus on the STS and IT cortex of monkeys and their modulation with attention. Then we will review evidence from behavioral and neurophysiological studies that support their guidance of attention in the presence of cognitive control signals. Next, we propose a mechanistic framework for executive control of attention in the temporal cortex. Finally, we summarize the role of temporal cortex in implementing cognitive programs and discuss how they contribute to the dynamic nature of visual attention to ensure flexible behavior.

Rapid processing of threatening faces in the amygdala of nonhuman primates: subcortical inputs and dual roles

A subcortical pathway through the superior colliculus and pulvinar has been proposed to provide the amygdala with rapid but coarse visual information about emotional faces. However, evidence for short-latency, facial expression-discriminating responses from individual amygdala neurons is lacking; even if such a response exists, how it might contribute to stimulus detection is unclear. Also, no definitive anatomical evidence is available for the assumed pathway. Here we showed that ensemble responses of amygdala neurons in monkeys carried robust information about open-mouthed, presumably threatening, faces within 50 ms after stimulus onset. This short-latency signal was not found in the visual cortex, suggesting a subcortical origin. Temporal analysis revealed that the early response contained excitatory and suppressive components. The excitatory component may be useful for sending rapid signals downstream, while the sharpening of the rising phase of later-arriving inputs (presumably from the cortex) by the suppressive component might improve the processing of facial expressions over time. Injection of a retrograde trans-synaptic tracer into the amygdala revealed presumed monosynaptic labeling in the pulvinar and disynaptic labeling in the superior colliculus, including the retinorecipient layers. We suggest that the early amygdala responses originating from the colliculo-pulvino-amygdalar pathway play dual roles in threat detection.

Parallel functional subnetworks embedded in the macaque face patch system

During normal vision, our eyes provide the brain with a continuous stream of useful information about the world. How visually specialized areas of the cortex, such as face-selective patches, operate under natural modes of behavior is poorly understood. Here we report that, during the free viewing of movies, cohorts of face-selective neurons in the macaque cortex fractionate into distributed and parallel subnetworks that carry distinct information. We classified neurons into functional groups on the basis of their movie-driven coupling with functional magnetic resonance imaging time courses across the brain. Neurons from each group were distributed across multiple face patches but intermixed locally with other groups at each recording site. These findings challenge prevailing views about functional segregation in the cortex and underscore the importance of naturalistic paradigms for cognitive neuroscience.

Comparative connectomics of the primate social brain

Social interaction is thought to provide a selection pressure for human intelligence, yet little is known about its neurobiological basis and evolution throughout the primate lineage. Recent advances in neuroimaging have enabled whole brain investigation of brain structure, function, and connectivity in humans and non-human primates (NHPs), leading to a nascent field of comparative connectomics. However, linking social behavior to brain organization across the primates remains challenging. Here, we review the current understanding of the macroscale neural mechanisms of social behaviors from the viewpoint of system neuroscience. We first demonstrate an association between the number of cortical neurons and the size of social groups across primates, suggesting a link between neural information-processing capacity and social capabilities. Moreover, by capitalizing on recent advances in species-harmonized functional MRI, we demonstrate that portions of the mirror neuron system and default-mode networks, which are thought to be important for representation of the other's actions and sense of self, respectively, exhibit similarities in functional organization in macaque monkeys and humans, suggesting possible homologies. With respect to these two networks, we describe recent developments in the neurobiology of social perception, joint attention, personality and social complexity. Together, the Human Connectome Project (HCP)-style comparative neuroimaging, hyperscanning, behavioral, and other multi-modal investigations are expected to yield important insights into the evolutionary foundations of human social behavior.

One object, two networks? Assessing the relationship between the face and body-selective regions in the primate visual system

Faces and bodies are often treated as distinct categories that are processed separately by face- and body-selective brain regions in the primate visual system. These regions occupy distinct regions of visual cortex and are often thought to constitute independent functional networks. Yet faces and bodies are part of the same object and their presence inevitably covary in naturalistic settings. Here, we re-evaluate both the evidence supporting the independent processing of faces and bodies and the organizational principles that have been invoked to explain this distinction. We outline four hypotheses ranging from completely separate networks to a single network supporting the perception of whole people or animals. The current evidence, especially in humans, is compatible with all of these hypotheses, making it presently unclear how the representation of faces and bodies is organized in the cortex.

Social prediction modulates activity of macaque superior temporal cortex

The ability to attribute thoughts to others, also called theory of mind (TOM), has been extensively studied in humans; however, its evolutionary origins have been challenged. Computationally, the basis of TOM has been interpreted within the predictive coding framework and associated with activity in the temporoparietal junction (TPJ). Here, we revealed, using a nonlinguistic task and functional magnetic resonance imaging, that activity in a region of the macaque middle superior temporal cortex was specifically modulated by the predictability of social situations. As in human TPJ, this region could be distinguished from other temporal regions involved in face processing. Our result suggests the existence of a precursor for the TOM ability in the last common ancestor of human and Old World monkeys.

Interspecies activation correlations reveal functional correspondences between marmoset and human brain areas

The common marmoset has enormous promise as a nonhuman primate model of human brain functions. While resting-state functional MRI (fMRI) has provided evidence for a similar organization of marmoset and human cortices, the technique cannot be used to map the functional correspondences of brain regions between species. This limitation can be overcome by movie-driven fMRI (md-fMRI), which has become a popular tool for noninvasively mapping the neural patterns generated by rich and naturalistic stimulation. Here, we used md-fMRI in marmosets and humans to identify whole-brain functional correspondences between the two primate species. In particular, we describe functional correlates for the well-known human face, body, and scene patches in marmosets. We find that these networks have a similar organization in both species, suggesting a largely conserved organization of higher-order visual areas between New World marmoset monkeys and humans. However, while face patches in humans and marmosets were activated by marmoset faces, only human face patches responded to the faces of other animals. Together, the results demonstrate that higher-order visual processing might be a conserved feature between humans and New World marmoset monkeys but that small, potentially important functional differences exist.

Joint encoding of facial identity, orientation, gaze, and expression in the middle dorsal face area

The last two decades have established that a network of face-selective areas in the temporal lobe of macaque monkeys supports the visual processing of faces. Each area within the network contains a large fraction of face-selective cells. And each area encodes facial identity and head orientation differently. A recent brain-imaging study discovered an area outside of this network selective for naturalistic facial motion, the middle dorsal (MD) face area. This finding offers the opportunity to determine whether coding principles revealed inside the core network would generalize to face areas outside the core network. We investigated the encoding of static faces and objects, facial identity, and head orientation, dimensions which had been studied in multiple areas of the core face-processing network before, as well as facial expressions and gaze. We found that MD populations form a face-selective cluster with a degree of selectivity comparable to that of areas in the core face-processing network. MD encodes facial identity robustly across changes in head orientation and expression, it encodes head orientation robustly against changes in identity and expression, and it encodes expression robustly across changes in identity and head orientation. These three dimensions are encoded in a separable manner. Furthermore, MD also encodes the direction of gaze in addition to head orientation. Thus, MD encodes both structural properties (identity) and changeable ones (expression and gaze) and thus provides information about another animal's direction of attention (head orientation and gaze). MD contains a heterogeneous population of cells that establish a multidimensional code for faces.

Audiovisual integration in macaque face patch neurons

Primate social communication depends on the perceptual integration of visual and auditory cues, reflected in the multimodal mixing of sensory signals in certain cortical areas. The macaque cortical face patch network, identified through visual, face-selective responses measured with fMRI, is assumed to contribute to visual social interactions. However, whether face patch neurons are also influenced by acoustic information, such as the auditory component of a natural vocalization, remains unknown. Here, we recorded single-unit activity in the anterior fundus (AF) face patch, in the superior temporal sulcus, and anterior medial (AM) face patch, on the undersurface of the temporal lobe, in macaques presented with audiovisual, visual-only, and auditory-only renditions of natural movies of macaques vocalizing. The results revealed that 76% of neurons in face patch AF were significantly influenced by the auditory component of the movie, most often through enhancement of visual responses but sometimes in response to the auditory stimulus alone. By contrast, few neurons in face patch AM exhibited significant auditory responses or modulation. Control experiments in AF used an animated macaque avatar to demonstrate, first, that the structural elements of the face were often essential for audiovisual modulation and, second, that the temporal modulation of the acoustic stimulus was more important than its frequency spectrum. Together, these results identify a striking contrast between two face patches and specifically identify AF as playing a potential role in the integration of audiovisual cues during natural modes of social communication.

Evidence for a Third Visual Pathway Specialized for Social Perception

Existing models propose that primate visual cortex is divided into two functionally distinct pathways. The ventral pathway computes the identity of an object; the dorsal pathway computes the location of an object, and the actions related to that object. Despite remaining influential, the two visual pathways model requires revision. Both human and non-human primate studies reveal the existence of a third visual pathway on the lateral brain surface. This third pathway projects from early visual cortex, via motion-selective areas, into the superior temporal sulcus (STS). Studies demonstrating that the STS computes the actions of moving faces and bodies (e.g., expressions, eye-gaze, audio-visual integration, intention, and mood) show that the third visual pathway is specialized for the dynamic aspects of social perception.

Processing communicative facial and vocal cues in the superior temporal sulcus

Facial and vocal cues provide critical social information about other humans, including their emotional and attentional states and the content of their speech. Recent work has shown that the face-responsive region of posterior superior temporal sulcus ("fSTS") also responds strongly to vocal sounds. Here, we investigate the functional role of this region and the broader STS by measuring responses to a range of face movements, vocal sounds, and hand movements using fMRI. We find that the fSTS responds broadly to different types of audio and visual face action, including both richly social communicative actions, as well as minimally social noncommunicative actions, ruling out hypotheses of specialization for processing speech signals, or communicative signals more generally. Strikingly, however, responses to hand movements were very low, whether communicative or not, indicating a specific role in the analysis of face actions (facial and vocal), not a general role in the perception of any human action. Furthermore, spatial patterns of response in this region were able to decode communicative from noncommunicative face actions, both within and across modality (facial/vocal cues), indicating sensitivity to an abstract social dimension. These functional properties of the fSTS contrast with a region of middle STS that has a selective, largely unimodal auditory response to speech sounds over both communicative and noncommunicative vocal nonspeech sounds, and nonvocal sounds. Region of interest analyses were corroborated by a data-driven independent component analysis, identifying face-voice and auditory speech responses as dominant sources of voxelwise variance across the STS. These results suggest that the STS contains separate processing streams for the audiovisual analysis of face actions and auditory speech processing.

Social interaction networks in the primate brain

Primate brains have evolved to understand and engage with their social world. Much about the structure of this world can be gleaned from social interactions. Circuits for the analysis of and participation in social interactions have now been mapped. Increased knowledge about their functional specializations and relative spatial locations promises to greatly improve the understanding of the functional organization of the primate social brain. Detailed electrophysiology, as in the case of the face-processing network, of local operations and functional interactions between areas is necessary to uncover neural mechanisms and computation principles of social cognition. New naturalistic behavioral paradigms, behavioral tracking, and new analytical approaches for parallel non-stationary data will be important components toward a neuroscientific theory of primates' interactive minds.

Parallel Processing of Facial Expression and Head Orientation in the Macaque Brain

When we move the features of our face, or turn our head, we communicate changes in our internal state to the people around us. How this information is encoded and used by an observer's brain is poorly understood. We investigated this issue using a functional MRI adaptation paradigm in awake male macaques. Among face-selective patches of the superior temporal sulcus (STS), we found a double dissociation of areas processing facial expression and those processing head orientation. The face-selective patches in the STS fundus were most sensitive to facial expression, as was the amygdala, whereas those on the lower, lateral edge of the sulcus were most sensitive to head orientation. The results of this study reveal a new dimension of functional organization, with face-selective patches segregating within the STS. The findings thus force a rethinking of the role of the face-processing system in representing subject-directed actions and supporting social cognition.SIGNIFICANCE STATEMENT When we are interacting with another person, we make inferences about their emotional state based on visual signals. For example, when a person's facial expression changes, we are given information about their feelings. While primates are thought to have specialized cortical mechanisms for analyzing the identity of faces, less is known about how these mechanisms unpack transient signals, like expression, that can change from one moment to the next. Here, using an fMRI adaptation paradigm, we demonstrate that while the identity of a face is held constant, there are separate mechanisms in the macaque brain for processing transient changes in the face's expression and orientation. These findings shed new light on the function of the face-processing system during social exchanges.

Anterior superior temporal sulcus is specialized for non-rigid facial motion in both monkeys and humans

Facial motion plays a fundamental role in the recognition of facial expressions in primates, but the neural substrates underlying this special type of biological motion are not well understood. Here, we used fMRI to investigate the extent to which the specialization for facial motion is represented in the visual system and compared the neural mechanisms for the processing of non-rigid facial motion in macaque monkeys and humans. We defined the areas specialized for facial motion as those significantly more activated when subjects perceived the motion caused by dynamic faces (dynamic faces > static faces) than when they perceived the motion caused by dynamic non-face objects (dynamic objects > static objects). We found that, in monkeys, significant activations evoked by facial motion were in the fundus of anterior superior temporal sulcus (STS), which overlapped the anterior fundus face patch. In humans, facial motion activated three separate foci in the right STS: posterior, middle, and anterior STS, with the anterior STS location showing the most selectivity for facial motion compared with other facial motion areas. In both monkeys and humans, facial motion shows a gradient preference as one progresses anteriorly along the STS. Taken together, our results indicate that monkeys and humans share similar neural substrates within the anterior temporal lobe specialized for the processing of non-rigid facial motion.

The neural mechanisms of face processing: cells, areas, networks, and models

Since its discovery, the face-processing network in the brain of the macaque monkey has emerged as a model system that allowed for major neural mechanisms of face recognition to be identified - with implications for object recognition at large. Populations of face cells encode faces through broad tuning curves, whose shapes change over time. Face representations differ qualitatively across faces areas, and we not only understand the global organization of these specializations, but also some of the transformations between face areas, both feed-forward and feed-back, and the computational principles behind face representations and transformations. Facial information is combined with physical features and mnemonic features in extensions of the core network, which forms an early part of the primate social brain.

Evidence for an attentional priority map in inferotemporal cortex

A fundamental dogma in the cognitive neurosciences is that attention is controlled by parietal and prefrontal areas. Here, we show that an area in the temporal lobe exhibits the properties of a priority map coding the focus of attention. We show this through whole-brain functional magnetic resonance imaging, electrophysiological single-unit recordings, and causal electrical stimulation. This discovery changes our understanding of the organization of visual pathways and the functions of attention networks.

From incoming sensory information, our brains make selections according to current behavioral goals. This process, selective attention, is controlled by parietal and frontal areas. Here, we show that another brain area, posterior inferotemporal cortex (PITd), also exhibits the defining properties of attentional control. We discovered this area with functional magnetic resonance imaging (fMRI) during an attentive motion discrimination task. Single-cell recordings from PITd revealed strong attentional modulation across 3 attention tasks yet no tuning to task-relevant stimulus features, like motion direction or color. Instead, PITd neurons closely tracked the subject’s attention state and predicted upcoming errors of attentional selection. Furthermore, artificial electrical PITd stimulation controlled the location of attentional selection without altering feature discrimination. These are the defining properties of a feature-blind priority map encoding the locus of attention. Together, these results suggest area PITd, located strategically to gather information about object properties, as an attentional priority map.

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

BACKGROUND

Rhesus 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 METHOD

We 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.

RESULTS

The 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.

CONCLUSIONS

The 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.

Searching for Face-Category Representation in the Avian Visual Forebrain

Visual information is processed hierarchically along a ventral ('what') pathway that terminates with categorical representation of biologically relevant visual percepts (such as faces) in the mammalian extrastriate visual cortex. How birds solve face and object representation without a neocortex is a long-standing problem in evolutionary neuroscience, though multiple lines of evidence suggest that these abilities arise from circuitry fundamentally similar to the extrastriate visual cortex. The aim of the present experiment was to determine whether birds also exhibit a categorical representation of the avian face-region in four visual forebrain structures of the tectofugal visual pathway: entopallium (ENTO), mesopallium ventrolaterale (MVL), nidopallium frontolaterale (NFL), and area temporo-parieto-occipitalis (TPO). We performed electrophysiological recordings from the right and left hemispheres of 13 pigeons while they performed a Go/No-Go task that required them to discriminate between two sets of stimuli that included images of pigeon faces. No neurons fired selectively to only faces in either ENTO, NFL, MVL, or TPO. Birds' predisposition to attend to the local-features of stimuli may influence the perception of faces as a global combination of features, and explain our observed absence of face-selective neurons. The implementation of naturalistic viewing paradigms in conjunction with electrophysiological and fMRI techniques has the potential to promote and uncover the global processing of visual objects to determine whether birds exhibit category-selective patches in the tectofugal visual forebrain.

Social Origins of Cortical Face Areas

Recently acquired fMRI data from human and macaque infants provide novel insights into the origins of cortical networks specialized for perceiving faces. Data from both species converge: cortical regions responding preferentially to faces are present and spatially organized early in infancy, although fully selective face areas emerge much later. What explains the earliest cortical responses to faces? We review two proposed mechanisms: proto-organization for simple shapes in visual cortex, and an innate subcortical schematic face template. In addition, we propose a third mechanism: infants choose to look at faces to engage in positively valenced, contingent social interactions. Activity in medial prefrontal cortex during social interactions may, directly or indirectly, guide the organization of cortical face areas.

The Organization and Operation of Inferior Temporal Cortex

Inferior temporal cortex (IT) is a key part of the ventral visual pathway implicated in object, face, and scene perception. But how does IT work? Here, I describe an organizational scheme that marries form and function and provides a framework for future research. The scheme consists of a series of stages arranged along the posterior-anterior axis of IT, defined by anatomical connections and functional responses. Each stage comprises a complement of subregions that have a systematic spatial relationship. The organization of each stage is governed by an eccentricity template, and corresponding eccentricity representations across stages are interconnected. Foveal representations take on a role in high-acuity object vision (including face recognition); intermediate representations compute other aspects of object vision such as behavioral valence (using color and surface cues); and peripheral representations encode information about scenes. This multistage, parallel-processing model invokes an innately determined organization refined by visual experience that is consistent with principles of cortical development. The model is also consistent with principles of evolution, which suggest that visual cortex expanded through replication of retinotopic areas. Finally, the model predicts that the most extensively studied network within IT-the face patches-is not unique but rather one manifestation of a canonical set of operations that reveal general principles of how IT works.

Functional Networks for Social Communication in the Macaque Monkey

All primates communicate. To dissect the neural circuits of social communication, we used fMRI to map non-human primate brain regions for social perception, second-person (interactive) social cognition, and orofacial movement generation. Face perception, second-person cognition, and face motor networks were largely non-overlapping and acted as distinct functional units rather than an integrated feedforward-processing pipeline. Whereas second-person context selectively engaged a region of medial prefrontal cortex, production of orofacial movements recruited distributed subcortical and cortical areas in medial and lateral frontal and insular cortex. These areas exhibited some specialization, but not dissociation, of function along the medio-lateral axis. Production of lipsmack movements recruited areas including putative homologs of Broca's area. These findings provide a new view of the neural architecture for social communication and suggest expressive orofacial movements generated by lateral premotor cortex as a putative evolutionary precursor to human speech.

Selective binding of facial features reveals dynamic expression fragments

The temporal correspondence between two arbitrarily chosen pairs of alternating features can generally be reported for rates up to 3–4 Hz. This limit is however surpassed for specialised visual mechanisms that encode conjunctions of features. Here we show that this 3–4 Hz limit is exceeded for eye gaze and eyebrow pairing, but not for eye gaze and mouth pairing, suggesting combined eye and eyebrow motion constitutes a dynamic expression fragment; a building block of superordinate facial actions.

Task-dependent enhancement of facial expression and identity representations in human cortex

What cortical mechanisms allow humans to easily discern the expression or identity of a face? Subjects detected changes in expression or identity of a stream of dynamic faces while we measured BOLD responses from topographically and functionally defined areas throughout the visual hierarchy. Responses in dorsal areas increased during the expression task, whereas responses in ventral areas increased during the identity task, consistent with previous studies. Similar to ventral areas, early visual areas showed increased activity during the identity task. If visual responses are weighted by perceptual mechanisms according to their magnitude, these increased responses would lead to improved attentional selection of the task-appropriate facial aspect. Alternatively, increased responses could be a signature of a sensitivity enhancement mechanism that improves representations of the attended facial aspect. Consistent with the latter sensitivity enhancement mechanism, attending to expression led to enhanced decoding of exemplars of expression both in early visual and dorsal areas relative to attending identity. Similarly, decoding identity exemplars when attending to identity was improved in dorsal and ventral areas. We conclude that attending to expression or identity of dynamic faces is associated with increased selectivity in representations consistent with sensitivity enhancement.

A dedicated network for social interaction processing in the primate brain

Primate cognition requires interaction processing. Interactions can reveal otherwise hidden properties of intentional agents, such as thoughts and feelings, and of inanimate objects, such as mass and material. Where and how interaction analyses are implemented in the brain is unknown. Using whole-brain functional magnetic resonance imaging in macaque monkeys, we discovered a network centered in the medial and ventrolateral prefrontal cortex that is exclusively engaged in social interaction analysis. Exclusivity of specialization was found for no other function anywhere in the brain. Two additional networks, a parieto-premotor and a temporal one, exhibited both social and physical interaction preference, which, in the temporal lobe, mapped onto a fine-grain pattern of object, body, and face selectivity. Extent and location of a dedicated system for social interaction analysis suggest that this function is an evolutionary forerunner of human mind-reading capabilities.

Face Processing Systems: From Neurons to Real-World Social Perception

Primate face processing depends on a distributed network of interlinked face-selective areas composed of face-selective neurons. In both humans and macaques, the network is divided into a ventral stream and a dorsal stream, and the functional similarities of the areas in humans and macaques indicate they are homologous. Neural correlates for face detection, holistic processing, face space, and other key properties of human face processing have been identified at the single neuron level, and studies providing causal evidence have established firmly that face-selective brain areas are central to face processing. These mechanisms give rise to our highly accurate familiar face recognition but also to our error-prone performance with unfamiliar faces. This limitation of the face system has important implications for consequential situations such as eyewitness identification and policing.

High-Level Prediction Signals in a Low-Level Area of the Macaque Face-Processing Hierarchy

Theories like predictive coding propose that lower-order brain areas compare their inputs to predictions derived from higher-order representations and signal their deviation as a prediction error. Here, we investigate whether the macaque face-processing system, a three-level hierarchy in the ventral stream, employs such a coding strategy. We show that after statistical learning of specific face sequences, the lower-level face area ML computes the deviation of actual from predicted stimuli. But these signals do not reflect the tuning characteristic of ML. Rather, they exhibit identity specificity and view invariance, the tuning properties of higher-level face areas AL and AM. Thus, learning appears to endow lower-level areas with the capability to test predictions at a higher level of abstraction than what is afforded by the feedforward sweep. These results provide evidence for computational architectures like predictive coding and suggest a new quality of functional organization of information-processing hierarchies beyond pure feedforward schemes.

Distinct fMRI Responses to Self-Induced versus Stimulus Motion during Free Viewing in the Macaque

Visual motion responses in the brain are shaped by two distinct sources: the physical movement of objects in the environment and motion resulting from one's own actions. The latter source, termed visual reafference, stems from movements of the head and body, and in primates from the frequent saccadic eye movements that mark natural vision. To study the relative contribution of reafferent and stimulus motion during natural vision, we measured fMRI activity in the brains of two macaques as they freely viewed >50 hours of naturalistic video footage depicting dynamic social interactions. We used eye movements obtained during scanning to estimate the level of reafferent retinal motion at each moment in time. We also estimated the net stimulus motion by analyzing the video content during the same time periods. Mapping the responses to these distinct sources of retinal motion, we found a striking dissociation in the distribution of visual responses throughout the brain. Reafferent motion drove fMRI activity in the early retinotopic areas V1, V2, V3, and V4, particularly in their central visual field representations, as well as lateral aspects of the caudal inferotemporal cortex (area TEO). However, stimulus motion dominated fMRI responses in the superior temporal sulcus, including areas MT, MST, and FST as well as more rostral areas. We discuss this pronounced separation of motion processing in the context of natural vision, saccadic suppression, and the brain's utilization of corollary discharge signals.

SIGNIFICANCE STATEMENT

Visual motion arises not only from events in the external world, but also from the movements of the observer. For example, even if objects are stationary in the world, the act of walking through a room or shifting one's eyes causes motion on the retina. This "reafferent" motion propagates into the brain as signals that must be interpreted in the context of real object motion. The delineation of whole-brain responses to stimulus versus self-generated retinal motion signals is critical for understanding visual perception and is of pragmatic importance given the increasing use of naturalistic viewing paradigms. The present study uses fMRI to demonstrate that the brain exhibits a fundamentally different pattern of responses to these two sources of retinal motion.

Two areas for familiar face recognition in the primate brain

Familiarity alters face recognition: Familiar faces are recognized more accurately than unfamiliar ones and under difficult viewing conditions when unfamiliar face recognition fails. The neural basis for this fundamental difference remains unknown. Using whole-brain functional magnetic resonance imaging, we found that personally familiar faces engage the macaque face-processing network more than unfamiliar faces. Familiar faces also recruited two hitherto unknown face areas at anatomically conserved locations within the perirhinal cortex and the temporal pole. These two areas, but not the core face-processing network, responded to familiar faces emerging from a blur with a characteristic nonlinear surge, akin to the abruptness of familiar face recognition. In contrast, responses to unfamiliar faces and objects remained linear. Thus, two temporal lobe areas extend the core face-processing network into a familiar face-recognition system.

Automated face recognition of rhesus macaques

BACKGROUND

Rhesus macaques are widely used in biomedical research. Automated behavior monitoring can be useful in various fields (including neuroscience), as well as having applications to animal welfare but current technology lags behind that developed for other species. One difficulty facing developers is the reliable identification of individual macaques within a group especially as pair- and group-housing of macaques becomes standard. Current published methods require either implantation or wearing of a tracking device.

NEW METHOD

I present face recognition, in combination with face detection, as a method to non-invasively identify individual rhesus macaques in videos. The face recognition method utilizes local-binary patterns in combination with a local discriminant classification algorithm.

RESULTS

A classification accuracy of between 90 and 96% was achieved for four different groups. Group size, number of training images and challenging image conditions such as high contrast all had an impact on classification accuracy. I demonstrate that these methods can be applied in real time using standard affordable hardware and a potential application to studies of social structure.

COMPARISON WITH EXISTING METHOD(S)

Face recognition methods have been reported for humans and other primate species such as chimpanzees but not rhesus macaques. The classification accuracy with this method is comparable to that for chimpanzees. Face recognition has the advantage over other methods for identifying rhesus macaques such as tags and collars of being non-invasive.

CONCLUSIONS

This is the first reported method for face recognition of rhesus macaques, has high classification accuracy and can be implemented in real time.

Development of the macaque face-patch system

Face recognition is highly proficient in humans and other social primates; it emerges in infancy, but the development of the neural mechanisms supporting this behaviour is largely unknown. We use blood-volume functional MRI to monitor longitudinally the responsiveness to faces, scrambled faces, and objects in macaque inferotemporal cortex (IT) from 1 month to 2 years of age. During this time selective responsiveness to monkey faces emerges. Some functional organization is present at 1 month; face-selective patches emerge over the first year of development, and are remarkably stable once they emerge. Face selectivity is refined by a decreasing responsiveness to non-face stimuli.