Representation of expression and identity by ventral prefrontal neurons

Fast face-selective responses in prefrontal face patches of the macaque

Face processing models propose gradually more complex receptive field properties culminating in invariant representations in anterior inferotemporal cortex (aITC), leading to late socio-emotionally relevant encoding in pre- and orbitofrontal cortex (POC). Top-down facilitation models, however, predict that some lower-level POC neurons respond faster than aITC. To resolve this discrepancy, we recorded from 2,459 neurons in fMRI-defined POC and aITC face patches. POC patches are more heterogeneous, containing smaller fractions of face-selective neurons than aITC and a mixture of responses to faces and non-faces. In one POC patch, face responses are surprisingly fast, outpacing those in aITC. Moreover, its responses correlate inversely with stimulus spatial frequency. Hence, our extensive data, with a large diversity of POC neurons, support both models and suggest one POC face patch might be specialized in fast, low-level face processing, which may enable (partially) invariant face representations during subsequent processing stages in inferotemporal cortex (ITC).

Monkeys can identify pictures from words

Humans learn and incorporate cross-modal associations between auditory and visual objects (e.g., between a spoken word and a picture) into language. However, whether nonhuman primates can learn cross-modal associations between words and pictures remains uncertain. We trained two rhesus macaques in a delayed cross-modal match-to-sample task to determine whether they could learn associations between sounds and pictures of different types. In each trial, the monkeys listened to a brief sound (e.g., a monkey vocalization or a human word), and retained information about the sound to match it with one of 2-4 pictures presented on a touchscreen after a 3-second delay. We found that the monkeys learned and performed proficiently in over a dozen associations. In addition, to test their ability to generalize, we exposed them to sounds uttered by different individuals. We found that their hit rate remained high but more variable, suggesting that they perceived the new sounds as equivalent, though not identical. We conclude that rhesus monkeys can learn cross-modal associations between objects of different types, retain information in working memory, and generalize the learned associations to new objects. These findings position rhesus monkeys as an ideal model for future research on the brain pathways of cross-modal associations between auditory and visual objects.

Neuronal Mechanisms Underlying Face Recognition in Non-human Primates

Humans and primates rely on visual face recognition for social interactions. Damage to specific brain areas causes prosopagnosia, a condition characterized by the inability to recognize familiar faces, indicating the presence of specialized brain areas for face processing. A breakthrough finding came from a non-human primate (NHP) study conducted in the early 2000s; it was the first to identify multiple face processing areas in the temporal lobe, termed face patches. Subsequent studies have demonstrated the unique role of each face patch in the structural analysis of faces. More recent studies have expanded these findings by exploring the role of face patch networks in social and memory functions and the importance of early face exposure in the development of the system. In this review, we discuss the neuronal mechanisms responsible for analyzing facial features, categorizing faces, and associating faces with memory and social contexts within both the cerebral cortex and subcortical areas. Use of NHPs in neuropsychological and neurophysiological studies can highlight the mechanistic aspects of the neuronal circuit underlying face recognition at both the single-neuron and whole-brain network levels.

Anatomo-functional basis of emotional and motor resonance elicited by facial expressions

Simulation theories predict that the observation of other's expressions modulates neural activity in the same centres controlling their production. This hypothesis has been developed by two models, postulating that the visual input is directly projected either to the motor system for action recognition (motor resonance) or to emotional/interoceptive regions for emotional contagion and social synchronization (emotional resonance). Here we investigated the role of frontal/insular regions in the processing of observed emotional expressions by combining intracranial recording, electrical stimulation and effective connectivity. First, we intracranially recorded from prefrontal, premotor or anterior insular regions of 44 patients during the passive observation of emotional expressions, finding widespread modulations in prefrontal/insular regions (anterior cingulate cortex, anterior insula, orbitofrontal cortex and inferior frontal gyrus) and motor territories (Rolandic operculum and inferior frontal junction). Subsequently, we electrically stimulated the activated sites, finding that (i) in the anterior cingulate cortex and anterior insula, the stimulation elicited emotional/interoceptive responses, as predicted by the 'emotional resonance model'; (ii) in the Rolandic operculum it evoked face/mouth sensorimotor responses, in line with the 'motor resonance' model; and (iii) all other regions were unresponsive or revealed functions unrelated to the processing of facial expressions. Finally, we traced the effective connectivity to sketch a network-level description of these regions, finding that the anterior cingulate cortex and the anterior insula are reciprocally interconnected while the Rolandic operculum is part of the parieto-frontal circuits and poorly connected with the former. These results support the hypothesis that the pathways hypothesized by the 'emotional resonance' and the 'motor resonance' models work in parallel, differing in terms of spatio-temporal fingerprints, reactivity to electrical stimulation and connectivity patterns.

Neuronal Population Encoding of Identity in Primate Prefrontal Cortex

The ventrolateral prefrontal cortex (VLPFC) shows robust activation during the perception of faces and voices. However, little is known about what categorical features of social stimuli drive neural activity in this region. Since perception of identity and expression are critical social functions, we examined whether neural responses to naturalistic stimuli were driven by these two categorical features in the prefrontal cortex. We recorded single neurons in the VLPFC, while two male rhesus macaques (Macaca mulatta) viewed short audiovisual videos of unfamiliar conspecifics making expressions of aggressive, affiliative, and neutral valence. Of the 285 neurons responsive to the audiovisual stimuli, 111 neurons had a main effect (two-way ANOVA) of identity, expression, or their interaction in their stimulus-related firing rates; however, decoding of expression and identity using single-unit firing rates rendered poor accuracy. Interestingly, when decoding from pseudo-populations of recorded neurons, the accuracy for both expression and identity increased with population size, suggesting that the population transmitted information relevant to both variables. Principal components analysis of mean population activity across time revealed that population responses to the same identity followed similar trajectories in the response space, facilitating segregation from other identities. Our results suggest that identity is a critical feature of social stimuli that dictates the structure of population activity in the VLPFC, during the perception of vocalizations and their corresponding facial expressions. These findings enhance our understanding of the role of the VLPFC in social behavior.

Multisensory integration in the mammalian brain: diversity and flexibility in health and disease

Multisensory integration (MSI) occurs in a variety of brain areas, spanning cortical and subcortical regions. In traditional studies on sensory processing, the sensory cortices have been considered for processing sensory information in a modality-specific manner. The sensory cortices, however, send the information to other cortical and subcortical areas, including the higher association cortices and the other sensory cortices, where the multiple modality inputs converge and integrate to generate a meaningful percept. This integration process is neither simple nor fixed because these brain areas interact with each other via complicated circuits, which can be modulated by numerous internal and external conditions. As a result, dynamic MSI makes multisensory decisions flexible and adaptive in behaving animals. Impairments in MSI occur in many psychiatric disorders, which may result in an altered perception of the multisensory stimuli and an abnormal reaction to them. This review discusses the diversity and flexibility of MSI in mammals, including humans, primates and rodents, as well as the brain areas involved. It further explains how such flexibility influences perceptual experiences in behaving animals in both health and disease. This article is part of the theme issue 'Decision and control processes in multisensory perception'.

Multisensory interactions of face and vocal information during perception and memory in ventrolateral prefrontal cortex

The ventral frontal lobe is a critical node in the circuit that underlies communication, a multisensory process where sensory features of faces and vocalizations come together. The neural basis of face and vocal integration is a topic of great importance since the integration of multiple sensory signals is essential for the decisions that govern our social interactions. Investigations have shown that the macaque ventrolateral prefrontal cortex (VLPFC), a proposed homologue of the human inferior frontal gyrus, is involved in the processing, integration and remembering of audiovisual signals. Single neurons in VLPFC encode and integrate species-specific faces and corresponding vocalizations. During working memory, VLPFC neurons maintain face and vocal information online and exhibit selective activity for face and vocal stimuli. Population analyses indicate that identity, a critical feature of social stimuli, is encoded by VLPFC neurons and dictates the structure of dynamic population activity in the VLPFC during the perception of vocalizations and their corresponding facial expressions. These studies suggest that VLPFC may play a primary role in integrating face and vocal stimuli with contextual information, in order to support decision making during social communication. This article is part of the theme issue 'Decision and control processes in multisensory perception'.

Functional network properties of the auditory cortex

The auditory system transforms auditory stimuli from the external environment into perceptual auditory objects. Recent studies have focused on the contribution of the auditory cortex to this transformation. Other studies have yielded important insights into the contributions of neural activity in the auditory cortex to cognition and decision-making. However, despite this important work, the relationship between auditory-cortex activity and behavior/perception has not been fully elucidated. Two of the more important gaps in our understanding are (1) the specific and differential contributions of different fields of the auditory cortex to auditory perception and behavior and (2) the way networks of auditory neurons impact and facilitate auditory information processing. Here, we focus on recent work from non-human-primate models of hearing and review work related to these gaps and put forth challenges to further our understanding of how single-unit activity and network activity in different cortical fields contribution to behavior and perception.

Decreased inter-brain synchronization in the right middle frontal cortex in alcohol use disorder during social interaction: An fNIRS hyperscanning study

BACKGROUND

Alcohol use disorder (AUD) is a widespread mental disorder and has thrust a heavy burden on the health system all over the world. Social cognition and function are reported to be impaired in AUD, but its neural mechanism is rarely investigated. The current study attempts to fill this gap.

METHODS

28 subjects with AUD and 36 healthy controls (HC) were recruited in this study and were paired into 14 AUD dyads and 18 HC dyads. The drinking problems, depression, anxiety, and impulsivity of subjects were measured. Each dyad completed cooperation and competition tasks with simultaneous frontal functional near-infrared spectroscopy (fNIRS) hyperscanning recording. The inter-brain synchronization (IBS) in the frontal cortex was calculated for each dyad and compared between AUD and HC. The significantly altered IBS in AUD was correlated with clinical measures to explore possible influencing factors.

RESULTS

The IBS in the right middle frontal cortex was significantly decreased in AUD under both cooperation (t = -2.257, P = 0.028) and competition (t = -2.488, P = 0.016) task. The IBS during the cooperation task in the right middle frontal cortex in AUD was negatively correlated with non-planning impulsivity (r = -0.673, P = 0.006).

LIMITATIONS

This study used cross-sectional data, which limited the causal inference. The synchronization between other brain regions besides the frontal cortex should be further explored in patients with AUD.

CONCLUSION

The current study could provide new insights into the neural mechanism of social dysfunction in AUD and facilitate clinical intervention in future practice.