A causal role for frontal cortico-cortical coordination in social action monitoring

Hypothalamic neuronal activation in non-human primates drives naturalistic goal-directed eating behavior

Maladaptive feeding behavior is the primary cause of modern obesity. While the causal influence of the lateral hypothalamic area (LHA) on eating behavior has been established in rodents, there is currently no primate-based evidence available on naturalistic eating behaviors. We investigated the role of LHA GABAergic (LHAGABA) neurons in eating using chemogenetics in three macaques. LHAGABA neuron activation significantly increased naturalistic goal-directed behaviors and food motivation, predominantly for palatable food. Positron emission tomography and magnetic resonance spectroscopy validated chemogenetic activation. Resting-state functional magnetic resonance imaging revealed that the functional connectivity (FC) between the LHA and frontal areas was increased, while the FC between the frontal cortices was decreased after LHAGABA neuron activation. Thus, our study elucidates the role of LHAGABA neurons in eating and obesity therapeutics for primates and humans.

[The neural basis of face-to-face communication: exploring transmission and sharing through neuroimaging]

Effective human communication is a complex process that involves transmitting and sharing information, ideas, and attitudes between two or more individuals. Researchers need to explore both transmission and sharing concepts to understand the neural basis of communication. Face-to-face communication refers to changing someone's mental state by sharing information, ideas, or attitudes. This type of communication is characterized by "mutual predictability." Scientists are working to clarify the neural basis of communication by studying how inter-individual synchronization of behavior and neural activity occurs during face-to-face communication.

The role of dorsal premotor cortex in joint action inhibition

Behavioral interpersonal coordination requires smooth negotiation of actions in time and space (joint action-JA). Inhibitory control may play a role in fine-tuning appropriate coordinative responses. To date, little research has been conducted on motor inhibition during JA and on the modulatory influence that premotor areas might exert on inhibitory control. Here, we used an interactive task in which subjects were required to reach and open a bottle using one hand. The bottle was held and stabilized by a co-actor (JA) or by a mechanical holder (vice clamp, no-JA). We recorded two TMS-based indices of inhibition (short-interval intracortical inhibition-sICI; cortical silent period-cSP) during the reaching phase of the task. These reflect fast intracortical (GABAa-mediated) and slow corticospinal (GABAb-mediated) inhibition. Offline continuous theta burst stimulation (cTBS) was used to interfere with dorsal premotor cortex (PMd), ventral premotor cortex (PMv), and control site (vertex) before the execution of the task. Our results confirm a dissociation between fast and slow inhibition during JA coordination and provide evidence that premotor areas drive only slow inhibitory mechanisms, which in turn may reflect behavioral co-adaptation between trials. Exploratory analyses further suggest that PMd, more than PMv, is the key source of modulatory drive sculpting movements, according to the socio-interactive context.

Frontal-Auditory Cortical Interactions and Sensory Prediction During Vocal Production in Marmoset Monkeys

The control of speech and vocal production involves the calculation of error between the intended vocal output and the resulting auditory feedback. Consistent with this model, recent evidence has demonstrated that the auditory cortex is suppressed immediately before and during vocal production, yet is still sensitive to differences between vocal output and altered auditory feedback. This suppression has been suggested to be the result of top-down signals containing information about the intended vocal output, potentially originating from motor or other frontal cortical areas. However, whether such frontal areas are the source of suppressive and predictive signaling to the auditory cortex during vocalization is unknown. Here, we simultaneously recorded neural activity from both the auditory and frontal cortices of marmoset monkeys while they produced self-initiated vocalizations. We found increases in neural activity in both brain areas preceding the onset of vocal production, notably changes in both multi-unit activity and local field potential theta-band power. Connectivity analysis using Granger causality demonstrated that frontal cortex sends directed signaling to the auditory cortex during this pre-vocal period. Importantly, this pre-vocal activity predicted both vocalization-induced suppression of the auditory cortex as well as the acoustics of subsequent vocalizations. These results suggest that frontal cortical areas communicate with the auditory cortex preceding vocal production, with frontal-auditory signals that may reflect the transmission of sensory prediction information. This interaction between frontal and auditory cortices may contribute to mechanisms that calculate errors between intended and actual vocal outputs during vocal communication.

Neuromodulation of the Left Inferior Frontal Cortex Affects Social Monitoring during Motor Interactions

Motor interactions require observing and monitoring a partner's performance as the interaction unfolds. Studies in monkeys suggest that this form of social monitoring might be mediated by the activity of the ventral premotor cortex (vPMc), a critical brain region in action observation and motor planning. Our previous fMRI studies in humans showed that the left vPMc is indeed recruited during social monitoring, but its causal role is unexplored. In three experiments, we applied online anodal or cathodal transcranial direct current stimulation over the left lateral frontal cortex during a music-like interactive task to test the hypothesis that neuromodulation of the left vPMc affects participants' performance when a partner violates the agent's expectations. Participants played short musical sequences together with a virtual partner by playing one note each in turn-taking. In 50% of the trials, the partner violated the participant's expectations by generating the correct note through an unexpected movement. During sham stimulation, the partner's unexpected behavior led to a slowdown in the participant's performance (observation-induced posterror slowing). A significant interaction with the stimulation type showed that cathodal and anodal transcranial direct current stimulation induced modulation of the observation-induced posterror slowing in opposite directions by reducing or enhancing it, respectively. Cathodal stimulation significantly reduced the effect compared to sham stimulation. No effect of neuromodulation was found when the partner behaved as expected or when the observed violation occurred within a context that was perceptually matched but noninteractive in nature. These results provide evidence for the critical causal role that the left vPMc might play in social monitoring during motor interactions, possibly through the interplay with other brain regions in the posterior medial frontal cortex.

Dissociation of vicarious and experienced rewards by coupling frequency within the same neural pathway

Vicarious reward, essential to social learning and decision making, is theorized to engage select brain regions similarly to experienced reward to generate a shared experience. However, it is just as important for neural systems to also differentiate vicarious from experienced rewards for social interaction. Here, we investigated the neuronal interaction between the primate anterior cingulate cortex gyrus (ACCg) and the basolateral amygdala (BLA) when social choices made by monkeys led to either vicarious or experienced reward. Coherence between ACCg spikes and BLA local field potential (LFP) selectively increased in gamma frequencies for vicarious reward, whereas it selectively increased in alpha/beta frequencies for experienced reward. These respectively enhanced couplings for vicarious and experienced rewards were uniquely observed following voluntary choices. Moreover, reward outcomes had consistently strong directional influences from ACCg to BLA. Our findings support a mechanism of vicarious reward where social agency is tagged by interareal coordination frequency within the same shared pathway.

Chemogenetic dissection of a prefrontal-hypothalamic circuit for socially subjective reward valuation in macaques

The value of one's own reward is affected by the reward of others, serving as a source for envy. However, it is not known which neural circuits mediate such socially subjective value modulation. Here, we chemogenetically dissected the circuit from the medial prefrontal cortex (MPFC) to the lateral hypothalamus (LH) while male macaques were presented with visual stimuli that concurrently signaled the prospects of one's own and others' rewards. We found that functional disconnection between the MPFC and LH rendered animals significantly less susceptible to others' but not one's own reward prospects. In parallel with this behavioral change, inter-areal coordination, as indexed by coherence and Granger causality, decreased primarily in the delta and theta bands. These findings demonstrate that the MPFC-to-LH circuit plays a crucial role in carrying information about upcoming other-rewards for subjective reward valuation in social contexts.

Non-shared coding of observed and executed actions prevails in macaque ventral premotor mirror neurons

According to the mirror mechanism the discharge of F5 mirror neurons of a monkey observing another individual performing an action is a motor representation of the observed action that may serve to understand or learn from the action. This hypothesis, if strictly interpreted, requires mirror neurons to exhibit an action tuning that is shared between action observation and execution. Due to insufficient data it remains contentious if this requirement is met. To fill in the gaps, we conducted an experiment in which identical objects had to be manipulated in three different ways in order to serve distinct action goals. Using three methods, including cross-task classification, we found that at most time points F5 mirror neurons did not encode observed actions with the same code underlying action execution. However, in about 20% of neurons there were time periods with a shared code. These time periods formed a distinct cluster and cannot be considered a product of chance. Population classification yielded non-shared coding for observed actions in the whole population, which was at times optimal and consistently better than shared coding in differentially selected subpopulations. These results support the hypothesis of a representation of observed actions based on a strictly defined mirror mechanism only for small subsets of neurons and only under the assumption of time-resolved readout. Considering alternative concepts and recent findings, we propose that during observation mirror neurons represent the process of a goal pursuit from the observer's viewpoint. Whether the observer's goal pursuit, in which the other's action goal becomes the observer's action goal, or the other's goal pursuit is represented remains to be clarified. In any case, it may allow the observer to use expectations associated with a goal pursuit to directly intervene in or learn from another's action.

Error observation as a window on performance monitoring in social contexts? A systematic review

Living in a social world requires social monitoring, i.e., the ability to keep track of others' actions and mistakes. Here, we demonstrate the good reliability of the behavioral and neurophysiological indexes ascribed to social monitoring. We also show that no consensus exists on the cognitive bases of this phenomenology and discuss three alternative hypotheses: (i) the direct matching hypothesis, postulating that observed errors are processed through automatic simulation; (ii) the attentional hypothesis, considering errors as unexpected events that take resources away from task processing; and (iii) the goal representation hypothesis, which weighs social error monitoring depending on how relevant the other's task is to the observer's goals. To date, evidence on the role played by factors that could help to disentangle these hypotheses (e.g., the human vs. non-human nature of the actor, the error rate, and the reward context) is insufficient, although the goal representation hypothesis seems to receive more support. Theory-driven experimental designs are needed to enlighten this debate and clarify the role of error monitoring during interactive exchanges.

The dopaminergic system supports flexible and rewarding dyadic motor interactive behaviour in Parkinson's Disease

Studies indicate that the dopaminergic system (DAS) supports individual flexible behaviour. While flexibility is quintessential to effective dyadic motor interactions, whether DAS mediates adaptations of one's own motor behaviour to that of a partner is not known. Here, we asked patients with Parkinson's Disease (PD) to synchronize their grasping movements with those of a virtual partner in conditions that did (Interactive) or did not (Cued) require to predict and adapt to its actions. PD performed the task during daily antiparkinsonian treatment ('On' condition) or after drug-withdrawal ('Off' condition). A group of healthy individuals also served as control group. In the Interactive condition, PDs performed better and found the interaction more enjoyable when in 'On' than in 'Off' condition. Crucially, PD performance in the 'On' condition did not differ from that of healthy controls. This pattern of results hints at the key role of the DAS in supporting the flexible adaptation of one's own actions to the partner's during motor interactions.

Visualizing advances in the future of primate neuroscience research

Future neuroscience and biomedical projects involving non-human primates (NHPs) remain essential in our endeavors to understand the complexities and functioning of the mammalian central nervous system. In so doing, the NHP neuroscience researcher must be allowed to incorporate state-of-the-art technologies, including the use of novel viral vectors, gene therapy and transgenic approaches to answer continuing and emerging research questions that can only be addressed in NHP research models. This perspective piece captures these emerging technologies and some specific research questions they can address. At the same time, we highlight some current caveats to global NHP research and collaborations including the lack of common ethical and regulatory frameworks for NHP research, the limitations involving animal transportation and exports, and the ongoing influence of activist groups opposed to NHP research.

Cognitive genomics of learning delay and low level of social performance monitoring in macaque

Cognitive skills and the underlying neural architecture are under the influence of genetics. Cognitive genomics research explores the triadic relationship between genes, brain, and cognition, with its major strategy being genotype-driven. Here we show that an inverse strategy is feasible to identify novel candidate genes for particular neuro-cognitive phenotypes in macaques. Two monkeys, originally involved in separate psychological studies, exhibited learning delay and low levels of social performance monitoring. In one monkey, mirror neurons were fewer compared to controls and mu suppression was absent in the frontal cortex. The other monkey showed heightened visual responsiveness in both frontal cortex and dopamine-rich midbrain, with a lack of inter-areal synchronization. Exome analyses revealed that the two monkeys were most likely cousins and shared variants in MAP2, APOC1, and potentially HTR2C. This phenotype-driven strategy in cognitive genomics provides a useful means to clarify the genetic basis of phenotypic variation and develop macaque models of neuropsychiatric disorders.

Medial and orbital frontal cortex in decision-making and flexible behavior

The medial frontal cortex and adjacent orbitofrontal cortex have been the focus of investigations of decision-making, behavioral flexibility, and social behavior. We review studies conducted in humans, macaques, and rodents and argue that several regions with different functional roles can be identified in the dorsal anterior cingulate cortex, perigenual anterior cingulate cortex, anterior medial frontal cortex, ventromedial prefrontal cortex, and medial and lateral parts of the orbitofrontal cortex. There is increasing evidence that the manner in which these areas represent the value of the environment and specific choices is different from subcortical brain regions and more complex than previously thought. Although activity in some regions reflects distributions of reward and opportunities across the environment, in other cases, activity reflects the structural relationships between features of the environment that animals can use to infer what decision to take even if they have not encountered identical opportunities in the past.

Dissecting the Prefrontal Network With Pathway-Selective Manipulation in the Macaque Brain—A Review

Macaque monkeys are prime animal models for studying the neural mechanisms of decision-making because of their close kinship with humans. Manipulation of neural activity during decision-making tasks is essential for approaching the causal relationship between the brain and its functions. Conventional manipulation methods used in macaque studies are coarse-grained, and have worked indiscriminately on mutually intertwined neural pathways. To systematically dissect neural circuits responsible for a variety of functions, it is essential to analyze changes in behavior and neural activity through interventions in specific neural pathways. In recent years, an increasing number of studies have applied optogenetics and chemogenetics to achieve fine-grained pathway-selective manipulation in the macaque brain. Here, we review the developments in macaque studies involving pathway-selective operations, with a particular focus on applications to the prefrontal network. Pathway selectivity can be achieved using single viral vector transduction combined with local light stimulation or ligand administration directly into the brain or double-viral vector transduction combined with systemic drug administration. We discuss the advantages and disadvantages of these methods. We also highlight recent technological developments in viral vectors that can effectively infect the macaque brain, as well as the development of methods to deliver photostimulation or ligand drugs to a wide area to effectively manipulate behavior. The development and dissemination of such pathway-selective manipulations of macaque prefrontal networks will enable us to efficiently dissect the neural mechanisms of decision-making and innovate novel treatments for decision-related psychiatric disorders.

Two-monkey fMRI setup for investigating multifaceted aspects of social cognition and behavior involving a real-live conspecific

While brain research over the past decades has shed light on the neural correlates of social cognition and behavior in human and non-human primates, most of this research has been performed in virtual settings requiring subjects to observe pictures or recorded videos instead of observing or interacting with another real-live individual. Here we present a two-monkey fMRI setup, allowing examining whole brain responses in macaque monkeys while they observe or interact face-to-face with another real-live conspecific. We tested this setup by comparing overall brain responses during observation of conspecific hand actions in a virtual (observation of recorded videos of actions) or live context (observation of a real-live conspecific performing actions). This dyadic monkey fMRI setup allows examining brain-wide responses in macaque monkeys during different aspects of social behavior, including observation of real-live actions and sensations, social facilitation, joint-attention and social interactions.

Lateral Habenula Responses During Eye Contact in a Reward Conditioning Task

For many animals, social interaction may have intrinsic reward value over and above its utility as a means to the desired end. Eye contact is the starting point of interactions in many social animals, including primates, and abnormal patterns of eye contact are present in many mental disorders. Whereas abundant previous studies have shown that negative emotions such as fear strongly affect eye contact behavior, modulation of eye contact by reward has received scant attention. Here we recorded eye movement patterns and neural activity in lateral habenula while monkeys viewed faces in the context of Pavlovian and instrumental conditioning tasks. Faces associated with larger rewards spontaneously elicited longer periods of eye contact from the monkeys, even though this behavior was not required or advantaged in the task. Concurrently, lateral habenula neurons were suppressed by faces signaling high value and excited by faces signaling low value. These results suggest that the reward signaling of lateral habenula may contribute to social behavior and disorders, presumably through its connections with the basal ganglia.

Midfrontal Theta tACS Facilitates Motor Coordination in Dyadic Human-Avatar Interactions

Synchronous interpersonal motor interactions require moment-to-moment prediction and proactive monitoring of the partner's actions. Neurophysiologically, this is highlighted by an enhancement of midfrontal theta (4-7 Hz) oscillations. In this study, we explored the causal role of midfrontal theta for interpersonal motor interactions using transcranial alternating current stimulation (tACS). We implemented a realistic human-avatar interaction task in immersive virtual reality where participants controlled a virtual arm and hand to press a button synchronously with a virtual partner. Participants completed the task while receiving EEG-informed theta (Experiment 1) or beta (control frequency, Experiment 2) tACS over the frontal midline, as well as sham stimulation as a control. Results showed that midfrontal theta tACS significantly improved behavioral performance (i.e., reduced interpersonal asynchrony) and participants' motor strategies (i.e., increased movement times and reduced RTs), whereas beta tACS had no effect on these measures. These results suggest that theta tACS over frontal areas facilitates action monitoring and motor abilities supporting interpersonal interactions.

Double viral vector intersectional approaches for pathway-selective manipulation of motor functions and compensatory mechanisms

Selective manipulation of particular subcomponent of neural circuits is crucial for understanding the functional architecture of neural systems and for development of the future therapeutic strategies against neurological disorders. In this article, I review how the intersectional approaches with double viral vector technique was introduced for the pathway-selective manipulation of spinal circuits. In this technique, a retrograde gene transfer vector is injected into the terminal area of the targeted neurons and an anterograde vector is injected at the location of their somata. Either by using the Tet-transactivator or Cre-loxP system, the experimenter can chemogenetically or optogenetically manipulate the transmission of the target pathway originated from the double-infected neurons. This technique was first developed for manipulation of spinal cord interneurons in the macaque monkeys by selective expression of tetanus neurotoxin and successfully affected the dexterous hand movements. Currently, this technique is widely used on a variety of neural pathways both in rodents and primates in combination with a variety of retrograde vectors and a variety of optogenetic and chemogenetic tools. The advantage of this technique is that it is not necessary to generate transgenic animals. Knowledge of the cell-type specific promotors is not needed. Manipulation is achieved primarily by injection of two viral vectors based on the anatomical knowledge and it is applicable in a variety of animal species including primates. The pros, cons and future direction of this technique are discussed.

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.

Live agent preference and social action monitoring in the macaque mid-superior temporal sulcus region

During live social interactions, there is increased demand for mentalizing about others to cope with otherwise unpredictable manifestations of their behavior. Anatomically, the middle superior temporal sulcus (mid-STS) region is hypothesized to be the macaque homologue of the human temporoparietal junction (TPJ), a key node in the mentalizing network. However, whether the macaque mid-STS is the functional homologue of the human TPJ is unknown. Here, we provide single-neuron evidence that the two areas share similar properties in social cognitive functions despite differences in anatomical landmarks. Our findings demonstrate that mid-STS neurons have a preference for task performance with live over video-recorded partners and encode errors in the prediction of partners’ actions, both aspects being cardinal features of the human TPJ.

Mentalizing, the ability to infer the mental states of others, is a cornerstone of adaptive social intelligence. While functional brain mapping of human mentalizing has progressed considerably, its evolutionary signature in nonhuman primates remains debated. The discovery that the middle part of the macaque superior temporal sulcus (mid-STS) region has a connectional fingerprint most similar to the human temporoparietal junction (TPJ)—a crucial node in the mentalizing network—raises the possibility that these cortical areas may also share basic functional properties associated with mentalizing. Here, we show that this is the case in aspects of a preference for live social interactions and in a theoretical framework of predictive coding. Macaque monkeys were trained to perform a turn-taking choice task with another real monkey partner sitting directly face-to-face or a filmed partner appearing in prerecorded videos. We found that about three-fourths of task-related mid-STS neurons exhibited agent-dependent activity, most responding selectively or preferentially to the partner’s action. At the population level, activities of these partner-type neurons were significantly greater under live-partner compared to video-recorded–partner task conditions. Furthermore, a subset of the partner-type neurons responded proactively when predictions about the partner’s action were violated. This prediction error coding was specific to the action domain; almost none of the neurons signaled error in the prediction of reward. The present findings highlight unique roles of the macaque mid-STS at the single-neuron level and further delineate its functional parallels with the human TPJ in social cognitive processes associated with mentalizing.

Neuronal Circuits for Social Decision-Making and Their Clinical Implications

Social living facilitates individual access to rewards, cognitive resources, and objects that would not be otherwise accessible. There are, however, some drawbacks to social living, particularly when competing for scarce resources. Furthermore, variability in our ability to make social decisions can be associated with neuropsychiatric disorders. The neuronal mechanisms underlying social decision-making are beginning to be understood. The momentum to study this phenomenon has been partially carried over by the study of economic decision-making. Yet, because of the similarities between these different types of decision-making, it is unclear what is a social decision. Here, we propose a definition of social decision-making as choices taken in a context where one or more conspecifics are involved in the decision or the consequences of it. Social decisions can be conceptualized as complex economic decisions since they are based on the subjective preferences between different goods. During social decisions, individuals choose based on their internal value estimate of the different alternatives. These are complex decisions given that conspecifics beliefs or actions could modify the subject's internal valuations at every choice. Here, we first review recent developments in our collective understanding of the neuronal mechanisms and circuits of social decision-making in primates. We then review literature characterizing populations with neuropsychiatric disorders showing deficits in social decision-making and the underlying neuronal circuitries associated with these deficits.

Dedicated Representation of Others in the Macaque Frontal Cortex: From Action Monitoring and Prediction to Outcome Evaluation

Social neurophysiology has increasingly addressed how several aspects of self and other are distinctly represented in the brain. In social interactions, the self–other distinction is fundamental for discriminating one’s own actions, intentions, and outcomes from those that originate in the external world. In this paper, we review neurophysiological experiments using nonhuman primates that shed light on the importance of the self–other distinction, focusing mainly on the frontal cortex. We start by examining how the findings are impacted by the experimental paradigms that are used, such as the type of social partner or whether a passive or active interaction is required. Next, we describe the 2 sociocognitive systems: mirror and mentalizing. Finally, we discuss how the self–other distinction can occur in different domains to process different aspects of social information: the observation and prediction of others’ actions and the monitoring of others’ rewards.

Largely shared neural codes for biological and nonbiological observed movements but not for executed actions in monkey premotor areas

The neural processing of others' observed actions recruits a large network of brain regions (the action observation network, AON), in which frontal motor areas are thought to play a crucial role. Since the discovery of mirror neurons (MNs) in the ventral premotor cortex, it has been assumed that their activation was conditional upon the presentation of biological rather than nonbiological motion stimuli, supporting a form of direct visuomotor matching. Nonetheless, nonbiological observed movements have rarely been used as control stimuli to evaluate visual specificity, thereby leaving the issue of similarity among neural codes for executed actions and biological or nonbiological observed movements unresolved. Here, we addressed this issue by recording from two nodes of the AON that are attracting increasing interest, namely the ventro-rostral part of the dorsal premotor area F2 and the mesial pre-supplementary motor area F6 of macaques while they 1) executed a reaching-grasping task, 2) observed an experimenter performing the task, and 3) observed a nonbiological effector moving in the same context. Our findings revealed stronger neuronal responses to the observation of biological than nonbiological movement, but biological and nonbiological visual stimuli produced highly similar neural dynamics and relied on largely shared neural codes, which in turn remarkably differed from those associated with executed actions. These results indicate that, in highly familiar contexts, visuo-motor remapping processes in premotor areas hosting MNs are more complex and flexible than predicted by a direct visuomotor matching hypothesis.

Chemogenetic dissection of the primate prefronto-subcortical pathways for working memory and decision-making

The primate prefrontal cortex (PFC) is situated at the core of higher brain functions via neural circuits such as those linking the caudate nucleus and mediodorsal thalamus. However, the distinctive roles of these prefronto-subcortical pathways remain elusive. Combining in vivo neuronal projection mapping with chemogenetic synaptic silencing, we reversibly dissected key pathways from dorsolateral part of the PFC (dlPFC) to the dorsal caudate (dCD) and lateral mediodorsal thalamus (MDl) individually in single monkeys. We found that silencing the bilateral dlPFC-MDl projections, but not the dlPFC-dCD projections, impaired performance in a spatial working memory task. Conversely, silencing the unilateral dlPFC-dCD projection, but not the unilateral dlPFC-MDl projection, altered preference in a decision-making task. These results revealed dissociable roles of the prefronto-subcortical pathways in working memory and decision-making, representing the technical advantage of imaging-guided pathway-selective chemogenetic manipulation for dissecting neural circuits underlying cognitive functions in primates.

Local and system mechanisms for action execution and observation in parietal and premotor cortices

The action observation network (AON) includes a system of brain areas largely shared with action execution in both human and nonhuman primates. Yet temporal and tuning specificities of distinct areas and of physiologically identified neuronal classes in the encoding of self and others’ action remain unknown. We recorded the activity of 355 single units from three crucial nodes of the AON, the anterior intraparietal area (AIP), and premotor areas F5 and F6, while monkeys performed a Go/No-Go grasping task and observed an experimenter performing it. At the system level, during task execution, F6 displays a prevalence of suppressed neurons and signals whether an action has to be performed, whereas AIP and F5 share a prevalence of facilitated neurons and remarkable target selectivity; during task observation, F5 stands out for its unique prevalence of facilitated neurons and its stronger and earlier modulation than AIP and F6. By applying unsupervised clustering of spike waveforms, we found distinct cell classes unevenly distributed across areas, with different firing properties and carrying specific visuomotor signals. Broadly spiking neurons exhibited a balanced amount of facilitated and suppressed activity during action execution and observation, whereas narrower spiking neurons showed more mutually facilitated responses during the execution of one’s own and others’ action, particularly in areas AIP and F5. Our findings elucidate the time course of activity and firing properties of neurons in the AON during one’s own and others’ action, from the system level of anatomically distinct areas to the local level of physiologically distinct cell classes.

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F6 neurons show a prevalence of suppressed activity, encoding whether to act

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Area F5 and AIP share a prevalence of facilitated neurons and target selectivity

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Across-areas, waveform-based clustering distinguished three neuronal classes

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Narrow-spiking neurons exhibit mutual modulation during self and others’ action

The Role of the Medial Prefrontal Cortex in Moderating Neural Representations of Self and Other in Primates

As a frontal node in the primate social brain, the medial prefrontal cortex (MPFC) plays a critical role in coordinating one's own behavior with respect to that of others. Current literature demonstrates that single neurons in the MPFC encode behavior-related variables such as intentions, actions, and rewards, specifically for self and other, and that the MPFC comes into play when reflecting upon oneself and others. The social moderator account of MPFC function can explain maladaptive social cognition in people with autism spectrum disorder, which tips the balance in favor of self-centered perspectives rather than taking into consideration the perspective of others. Several strands of evidence suggest a hypothesis that the MPFC represents different other mental models, depending on the context at hand, to better predict others' emotions and behaviors. This hypothesis also accounts for aberrant MPFC activity in autistic individuals while they are mentalizing others.

From Observed Action Identity to Social Affordances

Others' observed actions cause continuously changing retinal images, making it challenging to build neural representations of action identity. The monkey anterior intraparietal area (AIP) and its putative human homologue (phAIP) host neurons selective for observed manipulative actions (OMAs). The neuronal activity of both AIP and phAIP allows a stable readout of OMA identity across visual formats, but human neurons exhibit greater invariance and generalize from observed actions to action verbs. These properties stem from the convergence in AIP of superior temporal signals concerning: (i) observed body movements; and (ii) the changes in the body-object relationship. We propose that evolutionarily preserved mechanisms underlie the specification of observed-actions identity and the selection of motor responses afforded by them, thereby promoting social behavior.

Socially relative reward valuation in the primate brain

Reward valuation in social contexts is by nature relative rather than absolute; it is made in reference to others. This socially relative reward valuation is based on our propensity to conduct comparisons and competitions between self and other. Exploring its neural substrate has been an active area of research in human neuroimaging. More recently, electrophysiological investigation of the macaque brain has enabled us to understand neural mechanisms underlying this valuation process at single-neuron and network levels. Here I show that shared neural networks centered at the medial prefrontal cortex and dopamine-related subcortical regions are involved in this process in humans and nonhuman primates. Thus, socially relative reward valuation is mediated by cortico-subcortically coordinated activity linking social and reward brain networks.