Visual sensitivity at the service of action control in posterior parietal cortex

The posterior parietal cortex (PPC) serves as a crucial hub for the integration of sensory with motor cues related to voluntary actions. Visual input is used in different ways along the dorsomedial and the dorsolateral visual pathways. Here we focus on the dorsomedial pathway and recognize a visual representation at the service of action control. Employing different experimental paradigms applied to behaving monkeys while single neural activity is recorded from the medial PPC (area V6A), we show how plastic visual representation can be, matching the different contexts in which the same object is proposed. We also present data on the exchange between vision and arm actions and highlight how this rich interplay can be used to weight different sensory inputs in order to monitor and correct arm actions online. Indeed, neural activity during reaching or reach-to-grasp actions can be excited or inhibited by visual information, suggesting that the visual perception of action, rather than object recognition, is the most effective factor for area V6A. Also, three-dimensional object shape is encoded dynamically by the neural population, according to the behavioral context of the monkey. Along this line, mirror neuron discharges in V6A indicate the plasticity of visual representation of the graspable objects, that changes according to the context and peaks when the object is the target of one's own action. In other words, object encoding in V6A is a visual encoding for action.

Simple visual stimuli are sufficient to drive responses in action observation and execution neurons in macaque ventral premotor cortex

Neurons responding during action execution and action observation were discovered in the ventral premotor cortex 3 decades ago. However, the visual features that drive the responses of action observation/execution neurons (AOENs) have not been revealed at present. We investigated the neural responses of AOENs in ventral premotor area F5c of 4 macaques during the observation of action videos and crucial control stimuli. The large majority of AOENs showed highly phasic responses during the action videos, with a preference for the moment that the hand made contact with the object. They also responded to an abstract shape moving towards but not interacting with an object, even when the shape moved on a scrambled background, implying that most AOENs in F5c do not require the perception of causality or a meaningful action. Additionally, the majority of AOENs responded to static frames of the videos. Our findings show that very elementary stimuli, even without a grasping context, are sufficient to drive responses in F5c AOENs.

Progressively shifting patterns of co-modulation among premotor cortex neurons carry dynamically similar signals during action execution and observation

Many neurons in the premotor cortex show firing rate modulation whether the subject performs an action or observes another individual performing a similar action. Although such "mirror neurons" have been thought to have highly congruent discharge during execution and observation, many if not most show non-congruent activity. Studies of such neuronal populations have shown that the most prevalent patterns of co-modulation-captured as neural trajectories-pass through subspaces which are shared in part, but in part are visited exclusively during either execution or observation. These studies focused on reaching movements for which low-dimensional neural trajectories exhibit comparatively simple dynamical motifs. But the neural dynamics of hand movements are more complex. We developed a novel approach to examine prevalent patterns of co-modulation during execution and observation of a task that involved reaching, grasping, and manipulation. Rather than following neural trajectories in subspaces that contain their entire time course, we identified time series of instantaneous subspaces, calculated principal angles among them, sampled trajectory segments at the times of selected behavioral events, and projected those segments into the series of instantaneous subspaces. We found that instantaneous neural subspaces generally remained distinct during execution versus observation. Nevertheless, execution and observation could be partially aligned with canonical correlation, indicating some dynamical similarity of the neural representations of different movements relative to one another during execution and observation which may enable the nervous system to recognize corresponding actions performed by the subject or by another individual and/or may reflect social interaction between the two. During action execution, mirror neurons showed consistent patterns of co-modulation both within and between sessions, but other neurons that were modulated only during action execution and not during observation showed considerable variability of co-modulation. We speculate that during execution, mirror neurons carry a consistent forward model of the intended movement, while action-execution only neurons process more variable feedback.

Motor cortex retains and reorients neural dynamics during motor imagery

The most prominent characteristic of motor cortex is its activation during movement execution, but it is also active when we simply imagine movements in the absence of actual motor output. Despite decades of behavioural and imaging studies, it is unknown how the specific activity patterns and temporal dynamics in motor cortex during covert motor imagery relate to those during motor execution. Here we recorded intracortical activity from the motor cortex of two people who retain some residual wrist function following incomplete spinal cord injury as they performed both actual and imagined isometric wrist extensions. We found that we could decompose the population activity into three orthogonal subspaces, where one was similarly active during both action and imagery, and the others were active only during a single task type-action or imagery. Although they inhabited orthogonal neural dimensions, the action-unique and imagery-unique subspaces contained a strikingly similar set of dynamic features. Our results suggest that during motor imagery, motor cortex maintains the same overall population dynamics as during execution by reorienting the components related to motor output and/or feedback into a unique, output-null imagery subspace.

Motor actions are spatially organized in motor and dorsal premotor cortex

Frontal motor areas are central to controlling voluntary movements. In non-human primates, the motor areas contain independent, somatotopic, representations of the forelimb (i.e., motor maps). But are the neural codes for actions spatially organized within those forelimb representations? Addressing this question would provide insight into the poorly understood structure-function relationships of the cortical motor system. Here, we tackle the problem using high-resolution optical imaging and motor mapping in motor (M1) and dorsal premotor (PMd) cortex. Two macaque monkeys performed an instructed reach-to-grasp task while cortical activity was recorded with intrinsic signal optical imaging (ISOI). The spatial extent of activity in M1 and PMd was then quantified in relation to the forelimb motor maps, which we obtained from the same hemisphere with intracortical microstimulation. ISOI showed that task-related activity was concentrated in patches that collectively overlapped <40% of the M1 and PMd forelimb representations. The spatial organization of the patches was consistent across task conditions despite small variations in forelimb use. Nevertheless, the largest condition differences in forelimb use were reflected in the magnitude of cortical activity. Distinct time course profiles from patches in arm zones and patches in hand zones suggest functional differences within the forelimb representations. The results collectively support an organizational framework wherein the forelimb representations contain subzones enriched with neurons tuned for specific actions. Thus, the often-overlooked spatial dimension of neural activity appears to be an important organizing feature of the neural code in frontal motor areas.

Silence and its effects on the autonomic nervous system: A systematic review

This systematic review explores the influence of silence on the autonomic nervous system. The Polyvagal Theory has been used as a reference model to describe the autonomic nervous system by explaining its role in emotional regulation, social engagement, and adaptive physiological responses. PubMed, Scopus, PsycInfo, EMBASE, and Google Scholar were systematically searched up until July 2023 for relevant studies. The literature search yielded 511 results, and 37 studies were eventually included in this review. Silence affects the autonomic nervous system differently based on whether it is inner or outer silence. Inner silence enhances activity of the ventral vagus, favoring social engagement, and reducing sympathetic nervous system activity and physiological stress. Outer silence, conversely, can induce a heightened state of alertness, potentially triggering vagal brake removal and sympathetic nervous system activation, though with training, it can foster inner silence, preventing such activation. The autonomic nervous system response to silence can also be influenced by other factors such as context, familiarity with silence, presence and quality of outer noise, and empathy.

Ultra-high field fMRI identifies an action-observation network in the common marmoset

The observation of others' actions activates a network of temporal, parietal and premotor/prefrontal areas in macaque monkeys and humans. This action-observation network (AON) has been shown to play important roles in social action monitoring, learning by imitation, and social cognition in both species. It is unclear whether a similar network exists in New-World primates, which separated from Old-Word primates ~35 million years ago. Here we used ultra-high field fMRI at 9.4 T in awake common marmosets (Callithrix jacchus) while they watched videos depicting goal-directed (grasping food) or non-goal-directed actions. The observation of goal-directed actions activates a temporo-parieto-frontal network, including areas 6 and 45 in premotor/prefrontal cortices, areas PGa-IPa, FST and TE in occipito-temporal region and areas V6A, MIP, LIP and PG in the occipito-parietal cortex. These results show overlap with the humans and macaques' AON, demonstrating the existence of an evolutionarily conserved network that likely predates the separation of Old and New-World primates.

Activation of Cerebellum, Basal Ganglia and Thalamus During Observation and Execution of Mouth, hand, and foot Actions

Humans and monkey studies showed that specific sectors of cerebellum and basal ganglia activate not only during execution but also during observation of hand actions. However, it is unknown whether, and how, these structures are engaged during the observation of actions performed by effectors different from the hand. To address this issue, in the present fMRI study, healthy human participants were required to execute or to observe grasping acts performed with different effectors, namely mouth, hand, and foot. As control, participants executed and observed simple movements performed with the same effectors. The results show that: (1) execution of goal-directed actions elicited somatotopically organized activations not only in the cerebral cortex but also in the cerebellum, basal ganglia, and thalamus; (2) action observation evoked cortical, cerebellar and subcortical activations, lacking a clear somatotopic organization; (3) in the territories displaying shared activations between execution and observation, a rough somatotopy could be revealed in both cortical, cerebellar and subcortical structures. The present study confirms previous findings that action observation, beyond the cerebral cortex, also activates specific sectors of cerebellum and subcortical structures and it shows, for the first time, that these latter are engaged not only during hand actions observation but also during the observation of mouth and foot actions. We suggest that each of the activated structures processes specific aspects of the observed action, such as performing internal simulation (cerebellum) or recruiting/inhibiting the overt execution of the observed action (basal ganglia and sensory-motor thalamus).

Motor cortex retains and reorients neural dynamics during motor imagery

The most prominent role of motor cortex is generating patterns of neural activity that lead to movement, but it is also active when we simply imagine movements in the absence of actual motor output. Despite decades of behavioral and imaging studies, it is unknown how the specific activity patterns and temporal dynamics within motor cortex during covert motor imagery relate to those during motor execution. Here we recorded intracortical activity from the motor cortex of two people with residual wrist function following incomplete spinal cord injury as they performed both actual and imagined isometric wrist extensions. We found that we could decompose the population-level activity into orthogonal subspaces such that one set of components was similarly active during both action and imagery, and others were only active during a single task typeâ€"action or imagery. Although they inhabited orthogonal neural dimensions, the action-unique and imagery-unique subspaces contained a strikingly similar set of dynamical features. Our results suggest that during motor imagery, motor cortex maintains the same overall population dynamics as during execution by recreating the missing components related to motor output and/or feedback within a unique imagery-only subspace.

Engaging distributed cortical and cerebellar networks through motor execution, observation, and imagery

When we interact with the environment around us, we are sometimes active participants, making directed physical motor movements and other times only mentally engaging with our environment, taking in sensory information and internally planning our next move without directed physical movement. Traditionally, cortical motor regions and key subcortical structures such as the cerebellum have been tightly linked to motor initiation, coordination, and directed motor behavior. However, recent neuroimaging studies have noted the activation of the cerebellum and wider cortical networks specifically during various forms of motor processing, including the observations of actions and mental rehearsal of movements through motor imagery. This phenomenon of cognitive engagement of traditional motor networks raises the question of how these brain regions are involved in the initiation of movement without physical motor output. Here, we will review evidence for distributed brain network activation during motor execution, observation, and imagery in human neuroimaging studies as well as the potential for cerebellar involvement specifically in motor-related cognition. Converging evidence suggests that a common global brain network is involved in both movement execution and motor observation or imagery, with specific task-dependent shifts in these global activation patterns. We will further discuss underlying cross-species anatomical support for these cognitive motor-related functions as well as the role of cerebrocerebellar communication during action observation and motor imagery.

The rediscovered motor-related area 55b emerges as a core hub of music perception

Passive listening to music, without sound production or evident movement, is long known to activate motor control regions. Nevertheless, the exact neuroanatomical correlates of the auditory-motor association and its underlying neural mechanisms have not been fully determined. Here, based on a NeuroSynth meta-analysis and three original fMRI paradigms of music perception, we show that the long-ignored pre-motor region, area 55b, an anatomically unique and functionally intriguing region, is a core hub of music perception. Moreover, results of a brain-behavior correlation analysis implicate neural entrainment as the underlying mechanism of area 55b’s contribution to music perception. In view of the current results and prior literature, area 55b is proposed as a keystone of sensorimotor integration, a fundamental brain machinery underlying simple to hierarchically complex behaviors. Refining the neuroanatomical and physiological understanding of sensorimotor integration is expected to have a major impact on various fields, from brain disorders to artificial general intelligence.

Functional magnetic resonance imaging data acquired during passive listening to music suggest that pre-motor area 55b acts as a core hub of music processing in humans.

Mirror neurons 30 years later: implications and applications

Mirror neurons (MNs) were first described in a seminal paper in 1992 as a class of monkey premotor cells discharging during both action execution and observation. Despite their debated origin and function, recent studies in several species, from birds to humans, revealed that beyond MNs properly so called, a variety of cell types distributed among multiple motor, sensory, and emotional brain areas form a 'mirror mechanism' more complex and flexible than originally thought, which has an evolutionarily conserved role in social interaction. Here, we trace the current limits and envisage the future trends of this discovery, showing that it inspired translational research and the development of new neurorehabilitation approaches, and constitutes a point of no return in social and affective neuroscience.

Decoding point-light displays and fully visible hand grasping actions within the action observation network

Action observation typically recruits visual areas and dorsal and ventral sectors of the parietal and premotor cortex. This network has been collectively termed as extended action observation network (eAON). Within this network, the elaboration of kinematic aspects of biological motion is crucial. Previous studies investigated these aspects by presenting subjects with point-light displays (PLDs) videos of whole-body movements, showing the recruitment of some of the eAON areas. However, studies focused on cortical activation during observation of PLDs grasping actions are lacking. In the present functional magnetic resonance imaging (fMRI) study, we assessed the activation of eAON in healthy participants during the observation of both PLDs and fully visible hand grasping actions, excluding confounding effects due to low-level visual features, motion, and context. Results showed that the observation of PLDs grasping stimuli elicited a bilateral activation of the eAON. Region of interest analyses performed on visual and sensorimotor areas showed no significant differences in signal intensity between PLDs and fully visible experimental conditions, indicating that both conditions evoked a similar motor resonance mechanism. Multivoxel pattern analysis (MVPA) revealed significant decoding of PLDs and fully visible grasping observation conditions in occipital, parietal, and premotor areas belonging to eAON. Data show that kinematic features conveyed by PLDs stimuli are sufficient to elicit a complete action representation, suggesting that these features can be disentangled within the eAON from the features usually characterizing fully visible actions. PLDs stimuli could be useful in assessing which areas are recruited, when only kinematic cues are available, for action recognition, imitation, and motor learning.

Parietal maps of visual signals for bodily action planning

The posterior parietal cortex (PPC) has long been understood as a high-level integrative station for computing motor commands for the body based on sensory (i.e., mostly tactile and visual) input from the outside world. In the last decade, accumulating evidence has shown that the parietal areas not only extract the pragmatic features of manipulable objects, but also subserve sensorimotor processing of others’ actions. A paradigmatic case is that of the anterior intraparietal area (AIP), which encodes the identity of observed manipulative actions that afford potential motor actions the observer could perform in response to them. On these bases, we propose an AIP manipulative action-based template of the general planning functions of the PPC and review existing evidence supporting the extension of this model to other PPC regions and to a wider set of actions: defensive and locomotor actions. In our model, a hallmark of PPC functioning is the processing of information about the physical and social world to encode potential bodily actions appropriate for the current context. We further extend the model to actions performed with man-made objects (e.g., tools) and artifacts, because they become integral parts of the subject’s body schema and motor repertoire. Finally, we conclude that existing evidence supports a generally conserved neural circuitry that transforms integrated sensory signals into the variety of bodily actions that primates are capable of preparing and performing to interact with their physical and social world.

Decoding grip type and action goal during the observation of reaching-grasping actions: A multivariate fMRI study

During execution and observation of reaching-grasping actions, the brain must encode, at the same time, the final action goal and the type of grip necessary to achieve it. Recently, it has been proposed that the Mirror Neuron System (MNS) is involved not only in coding the final goal of the observed action, but also the type of grip used to grasp the object. However, the specific contribution of the different areas of the MNS, at both cortical and subcortical level, in disentangling action goal and grip type is still unclear. Here, twenty human volunteers participated in an fMRI study in which they performed two tasks: (a) observation of four different types of actions, consisting in reaching-to-grasp a box handle with two possible grips (precision, hook) and two possible goals (open, close); (b) action execution, in which participants performed grasping actions similar to those presented during the observation task. A conjunction analysis revealed the presence of shared activated voxels for both action observation and execution within several cortical areas including dorsal and ventral premotor cortex, inferior and superior parietal cortex, intraparietal sulcus, primary somatosensory cortex, and cerebellar lobules VI and VIII. ROI analyses showed a main effect for grip type in several premotor and parietal areas and cerebellar lobule VI, with higher BOLD activation during observation of precision vs hook actions. A grip x goal interaction was also present in the left inferior parietal cortex, with higher BOLD activity during precision-to-close actions. A multivariate pattern analysis (MVPA) revealed a significant accuracy for the grip model in all ROIs, while for the action goal model, significant accuracy was observed only for left inferior parietal cortex ROI. These findings indicate that a large network involving cortical and cerebellar areas is involved in the processing of type of grip, while final action goal appears to be mainly processed within the inferior parietal region, suggesting a differential contribution of the areas activated in this study.

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.

Functional Imaging of the Cerebellum during Action Execution and Observation

We employed the ¹⁴C-deoxyglucose autoradiographic method to map the activity in the cerebellar cortex of rhesus monkeys that performed forelimb movements either in the light or in the dark and of monkeys that observed forelimb movements executed by a human experimenter. The execution of forelimb movements, both in the light and in the dark, activated the forelimb representations in the cerebellar hemispheric extensions of 1) vermian lobules IV-VI and 2) vermian lobule VIIIB, ipsilaterally to the moving forelimb. Activations in the former forelimb representation involved both a paravermal and a lateral hemispheric region. Also, Crus II posterior in the ansiform lobule (the hemispheric expansion of lobule VIIB) was activated bilaterally by execution of movements in the light but not in the dark. Action observation activated the lateral-most region of the forelimb representation in the lateral hemispheric extension of vermian lobules IV-VI, as well as the crus II posterior, bilaterally. Our results demonstrate that the cerebellar cortex, in addition to its involvement in the generation of movement, is also recruited in the perception of observed movements. Moreover, our findings suggest a modularity gradient in the primate cerebellar cortex, which progresses from unimodal (medially) to multimodal (laterally) functional areas.

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.

Great apes' understanding of biomechanics: eye-tracking experiments using three-dimensional computer-generated animations

Visual processing of the body movements of other animals is important for adaptive animal behaviors. It is widely known that animals can distinguish articulated animal movements even when they are just represented by points of light such that only information about biological motion is retained. However, the extent to which nonhuman great apes comprehend the underlying structural and physiological constraints affecting each moving body part, i.e., biomechanics, is still unclear. To address this, we examined the understanding of biomechanics in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes), following a previous study on humans (Homo sapiens). Apes underwent eye tracking while viewing three-dimensional computer-generated (CG) animations of biomechanically possible or impossible elbow movements performed by a human, robot, or nonhuman ape. Overall, apes did not differentiate their gaze between possible and impossible movements of elbows. However, some apes looked at elbows for longer when viewing impossible vs. possible robot movements, which indicates that they may have had knowledge of biomechanics and that this knowledge could be extended to a novel agent. These mixed results make it difficult to draw a firm conclusion regarding the extent to which apes understand biomechanics. We discuss some methodological features that may be responsible for the results, as well as implications for future nonhuman animal studies involving the presentation of CG animations or measurement of gaze behaviors.

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

A novel fully immersive virtual reality environment for upper extremity rehabilitation in patients with stroke

Given the rising incidence of stroke, several technology-driven methods for rehabilitation have recently been developed. Virtual reality (VR) is a promising therapeutic technology among them. We recently developed a neuroscientifically grounded VR system to aid recovery of motor function poststroke. The developed system provides unilateral and bilateral upper extremity (UE) training in a fully immersive virtual environment that may stimulate and activate mirror neurons (MNs) in the brain necessary for UE rehabilitation. Twenty-three participants were randomized to a VR group (n = 12) to receive VR intervention (8 h within 2 weeks) plus 8-h occupational therapy (OT) or a control group (n = 11) to receive time-matched OT alone. Treatment effects on motor recovery and cortical reorganization were investigated using the Barthel Index (BI), Fugl-Meyer Upper Extremity (FM-UE), and resting-state fMRI. Both groups significantly improved BI (P < 0.05), reflecting the recovery of UE motor function. The VR group revealed significant improvements on FM-UE scores (P < 0.05) than the control group. Neural activity increased after the intervention, particularly in the brain areas implicating MNs, such as in the primary motor cortex. Overall, results suggested that using a neuroscientifically grounded VR system might offer additional benefits for UE rehabilitation in patients receiving OT.

Representation of the observer's predicted outcome value in mirror and nonmirror neurons of macaque F5 ventral premotor cortex

In the search for the function of mirror neurons, a previous study reported that F5 mirror neuron responses are modulated by the value that the observing monkey associates with the grasped object. Yet we do not know whether mirror neurons are modulated by the expected reward value for the observer or also by other variables, which are causally dependent on value (e.g., motivation, attention directed at the observed action, arousal). To clarify this, we trained two rhesus macaques to observe a grasping action on an object kept constant, followed by four fully predictable outcomes of different values (2 outcomes with positive and 2 with negative emotional valence). We found a consistent order in population activity of both mirror and nonmirror neurons that matches the order of the value of this predicted outcome but that does not match the order of the above-mentioned value-dependent variables. These variables were inferred from the probability not to abort a trial, saccade latency, modulation of eye position during action observation, heart rate, and pupil size. Moreover, we found subpopulations of neurons tuned to each of the four predicted outcome values. Multidimensional scaling revealed equal normalized distances of 0.25 between the two positive and between the two negative outcomes suggesting the representation of a relative value, scaled to the task setting. We conclude that F5 mirror neurons and nonmirror neurons represent the observer's predicted outcome value, which in the case of mirror neurons may be transferred to the observed object or action.NEW & NOTEWORTHY Both the populations of F5 mirror neurons and nonmirror neurons represent the predicted value of an outcome resulting from the observation of a grasping action. Value-dependent motivation, arousal, and attention directed at the observed action do not provide a better explanation for this representation. The population activity's metric suggests an optimal scaling of value representation to task setting.

Activation of cerebellum and basal ganglia during the observation and execution of manipulative actions

Studies on action observation mostly described the activation of a network of cortical areas, while less investigation focused specifically on the activation and role of subcortical nodes. In the present fMRI study, we investigated the recruitment of cerebellum and basal ganglia during the execution and observation of object manipulation performed with the right hand. The observation conditions consisted in: (a) observation of manipulative actions; (b) observation of sequences of random finger movements. In the execution conditions, participants had to perform the same actions or movements as in (a) and (b), respectively. The results of conjunction analysis showed significant shared activations during both observation and execution of manipulation in several subcortical structures, including: (1) cerebellar lobules V, VI, crus I, VIIIa and VIIIb (bilaterally); (2) globus pallidus, bilaterally, and left subthalamic nucleus; (3) red nucleus (bilaterally) and left thalamus. These findings support the hypothesis that the action observation/execution network also involves subcortical structures, such as cerebellum and basal ganglia, forming an integrated network. This suggests possible mechanisms, involving these subcortical structures, underlying learning of new motor skills, through action observation and imitation.

Movement initiation and grasp representation in premotor and primary motor cortex mirror neurons

Pyramidal tract neurons (PTNs) within macaque rostral ventral premotor cortex (F5) and (M1) provide direct input to spinal circuitry and are critical for skilled movement control. Contrary to initial hypotheses, they can also be active during action observation, in the absence of any movement. A population-level understanding of this phenomenon is currently lacking. We recorded from single neurons, including identified PTNs, in (M1) (n = 187), and F5 (n = 115) as two adult male macaques executed, observed, or withheld (NoGo) reach-to-grasp actions. F5 maintained a similar representation of grasping actions during both execution and observation. In contrast, although many individual M1 neurons were active during observation, M1 population activity was distinct from execution, and more closely aligned to NoGo activity, suggesting this activity contributes to withholding of self-movement. M1 and its outputs may dissociate initiation of movement from representation of grasp in order to flexibly guide behaviour.

Anterior Intraparietal Area: A Hub in the Observed Manipulative Action Network

Current knowledge regarding the processing of observed manipulative actions (OMAs) (e.g., grasping, dragging, or dropping) is limited to grasping and underlying neural circuitry remains controversial. Here, we addressed these issues by combining chronic neuronal recordings along the anteroposterior extent of monkeys’ anterior intraparietal (AIP) area with tracer injections into the recorded sites. We found robust neural selectivity for 7 distinct OMAs, particularly in the posterior part of AIP (pAIP), where it was associated with motor coding of grip type and own-hand visual feedback. This cluster of functional properties appears to be specifically grounded in stronger direct connections of pAIP with the temporal regions of the ventral visual stream and the prefrontal cortex, as connections with skeletomotor related areas and regions of the dorsal visual stream exhibited opposite or no rostrocaudal gradients. Temporal and prefrontal areas may provide visual and contextual information relevant for manipulative action processing. These results revise existing models of the action observation network, suggesting that pAIP constitutes a parietal hub for routing information about OMA identity to the other nodes of the network.

Mirror neurons precede non-mirror neurons during action execution

Mirror neurons are thought to represent an individual's ability to understand the actions of others by discharging as one individual performs or observes another individual performing an action. Studies typically have focused on mirror neuron activity during action observation, examining activity during action execution primarily to validate mirror neuron involvement in the motor act. As a result, little is known about the precise role of mirror neurons during action execution. In this study, during execution of reach-grasp-manipulate movements, we found activity of mirror neurons generally preceded that of non-mirror neurons. Not only did the onset of task-related modulation occur earlier in mirror neurons, but state transitions detected by hidden Markov models also occurred earlier in mirror neuron populations. Our findings suggest that mirror neurons may be at the forefront of action execution.NEW & NOTEWORTHY Mirror neurons commonly are thought to provide a neural substrate for understanding the actions of others, but mirror neurons also are active during action execution, when additional, non-mirror neurons are active as well. Examining the timing of activity during execution of a naturalistic reach-grasp-manipulate task, we found that mirror neuron activity precedes that of non-mirror neurons at both the unit and the population level. Thus mirror neurons may be at the leading edge of action execution.