Early non-specific modulation of corticospinal excitability during action observation

Evaluations of dyadic synchrony: observers' traits influence estimation and enjoyment of synchrony in mirror-game movements

While evidence abounds that motor synchrony is a powerful form of 'social glue' for those involved, we have yet to understand how observers perceive motor synchrony: can observers estimate the degree of synchrony accurately? Is synchrony aesthetically pleasing? In two preregistered experiments (n = 161 each), we assess how accurately observers can estimate the degree of synchrony in dyads playing the mirror game, and how much observers enjoy watching these movements. We further assess whether accuracy and enjoyment are influenced by individual differences in self-reported embodied expertise (ability to reproduce movements, body awareness, body competence), psychosocial resources (extraversion, self-esteem), or social competencies (empathy, autistic traits), while objectively controlling for the degree of measured synchrony and complexity. The data revealed that observers' estimated synchrony with poor accuracy, showing a tendency to underestimate the level of synchrony. Accuracy for low synchrony improved with increasing body competence, while accuracy for high synchrony improved with increasing autistic traits. Observers' enjoyment of dyadic movements correlated positively with the degree of measured synchrony, the predictability of the movements, and the observer's empathy. Furthermore, very low enjoyment was associated with increased body perception. Our findings indicate that accuracy in perceiving synchrony is closely linked to embodiment, while aesthetic evaluations of action hinge on individual differences.

Augmented action observation: Theory and practical applications in sensorimotor rehabilitation

Sensory feedback is a fundamental aspect of effective motor learning in sport and clinical contexts. One way to provide this is through sensory augmentation, where extrinsic sensory information are associated with, and modulated by, movement. Traditionally, sensory augmentation has been used as an online strategy, where feedback is provided during physical execution of an action. In this article, we argue that action observation can be an additional effective channel to provide augmented feedback, which would be complementary to other, more traditional, motor learning and sensory augmentation strategies. Given these similarities between observing and executing an action, action observation could be used when physical training is difficult or not feasible, for example during immobilization or during the initial stages of a rehabilitation protocol when peripheral fatigue is a common issue. We review the benefits of observational learning and preliminary evidence for the effectiveness of using augmented action observation to improve learning. We also highlight current knowledge gaps which make the transition from laboratory to practical contexts difficult. Finally, we highlight the key areas of focus for future research.

The social relevance and the temporal constraints of motor resonance in humans

In humans, motor resonance effects can be tracked by measuring the enhancement of corticospinal excitability by action observation. Uncovering factors driving motor resonance is crucial for optimizing action observation paradigms in experimental and clinical settings. In the present study, we deepen motor resonance properties for grasping movements. Thirty-five healthy subjects underwent an action observation task presenting right-hand grasping movements differing from their action goal. Single-pulse transcranial magnetic stimulation was applied over the left primary motor cortex at 100, 200, or 300 ms from the onset of the visual stimulus depicting the action. Motor-evoked potentials were recorded from four muscles of the right hand and forearm. Results show a muscle-specific motor resonance effect at 200 ms after movement but selectively for observing a socially relevant grasp towards another human being. This effect correlates with observers' emotional empathy scores, and it was followed by inhibition of motor resonance at 300 ms post-stimulus onset. No motor resonance facilitation emerged while observing intransitive hand movement or object grasping. This evidence highlights the social side of motor resonance and its dependency on temporal factors.

Can static Rorschach stimuli perceived as in motion affect corticospinal excitability?

It has been proposed that seeing human movement or activity (M), while trying to say what the static Rorschach inkblot design look like, is accompanied by Mirror Neuron System (MNS)-like mirroring activity in the brain. The present study aimed to investigate whether the Rorschach cards eliciting M responses could affect the excitability of the motor cortex by recording motor evoked potentials (MEPs) elicited by single-pulse TMS over the primary motor cortex (M1). We hypothesized that Rorschach inkblot stimuli triggering the viewer's experience of human movement would increase corticospinal excitability. Twenty-one healthy volunteers (15 women) participated in the preliminary experiment, while another different sample of twenty-two healthy participants (11 women) ranging in age from 21 to 41 years was enrolled in the main experiment. Our results showed that the Rorschach cards known to be associated with a high number of M responses elicited human movement both as automatic internal sensations and as verbal production of responses involving human movement. However, contrary to our hypothesis, the reported internal feeling of human movement had no corresponding physiological counterpart, as the amplitude of MEPs did not increase. Possible and innovative explanations for the involvement of bottom-up and top-down processes were provided.

Is there a relationship between EEG and sTMS neurophysiological markers of the putative human mirror neuron system?

The mirror neuron system (MNS) has been theorized to play a neurobiological role in a number of social cognitive abilities and is commonly indexed putatively in humans via interpersonal motor resonance (IMR) and mu suppression. Although both indices are thought to measure similar neuronal populations (i.e., "mirror neurons"), it has been suggested that these methods are unrelated, and therefore, incompatible. However, prior studies reporting no relationships were typically conducted in small and underpowered samples. Thus, we aimed to investigate this potential association in a large sample of neurotypical adults (N = 116; 72 females). Participants underwent transcranial magnetic stimulation (TMS), electromyography (EMG), and electroencephalography (EEG) during the observation of videos of actors performing grasping actions in order to index IMR and mu suppression (in beta, lower alpha, and upper alpha bandwidths). A series of linear regressions revealed no associations between IMR and each of the mu suppression bandwidths. Supplementary Bayesian analyses provided further evidence in favor of the null (B₀₁  = 8.85-8.93), providing further support for no association between the two indices of MNS activity. Our findings suggest that these two measures may indeed be unrelated indices that perhaps assess different neurophysiological aspects of the MNS. These results have important implications for future studies examining the MNS.

Early Right Motor Cortex Response to Happy and Fearful Facial Expressions: A TMS Motor-Evoked Potential Study

The ability to rapidly process others' emotional signals is crucial for adaptive social interactions. However, to date it is still unclear how observing emotional facial expressions affects the reactivity of the human motor cortex. To provide insights on this issue, we employed single-pulse transcranial magnetic stimulation (TMS) to investigate corticospinal motor excitability. Healthy participants observed happy, fearful and neutral pictures of facial expressions while receiving TMS over the left or right motor cortex at 150 and 300 ms after picture onset. In the early phase (150 ms), we observed an enhancement of corticospinal excitability for the observation of happy and fearful emotional faces compared to neutral expressions specifically in the right hemisphere. Interindividual differences in the disposition to experience aversive feelings (personal distress) in interpersonal emotional contexts predicted the early increase in corticospinal excitability for emotional faces. No differences in corticospinal excitability were observed at the later time (300 ms) or in the left M1. These findings support the notion that emotion perception primes the body for action and highlights the role of the right hemisphere in implementing a rapid and transient facilitatory response to emotional arousing stimuli, such as emotional facial expressions.

Dissociating embodiment and emotional reactivity in motor responses to artworks

Perceiving art is known to elicit motor cortex activation in an observer's brain. This motor activation has often been attributed to a covert approach response associated with the emotional valence of an art piece (emotional reaction hypothesis). However, recent accounts have proposed that aesthetic experiences could be grounded in the motor simulation of actions required to produce an art piece and of the sensorimotor states embedded in its subject (embodied aesthetic hypothesis). Here, we aimed to test these two hypotheses by assessing whether motor facilitation during artwork perception mirrors emotional or motor simulation processes. To this aim, we capitalized on single pulse transcranial magnetic stimulation revealing a two-stage motor coding of emotional body postures: an early, non-specific activation related to emotion processing and a later action-specific activation reflecting motor simulation. We asked art-naïve individuals to rate how much they liked a series of pointillist and brushstroke canvases; photographs of artistic gardens served as control natural stimuli. After an early (150 ms) or a later (300 ms) post-stimulus delay, motor evoked potentials were recorded from wrist-extensor and finger muscles that were more involved in brushstroke- and pointillist-like painting, respectively. Results showed that observing the two canvas styles did not elicit differential motor activation in the early time window for either muscle, not supporting the emotional reaction hypothesis. However, in support of the embodied aesthetic hypothesis, we found in the later time window greater motor activation responses to brushstroke than pointillist canvases for the wrist-extensor, but not for the finger muscle. Furthermore, this muscle-selective facilitation was associated with lower liking ratings of brushstroke canvases and with greater empathy dispositions. These findings support the claim that simulation of the painter's movements is crucial for aesthetic experience, by documenting a link between motor simulation, dispositional empathy, and subjective appreciation in artwork perception.

Is the Putative Mirror Neuron System Associated with Empathy? A Systematic Review and Meta-Analysis

Theoretical perspectives suggest that the mirror neuron system (MNS) is an important neurobiological contributor to empathy, yet empirical support is mixed. Here, we adopt a summary model for empathy, consisting of motor, emotional, and cognitive components of empathy. This review provides an overview of existing empirical studies investigating the relationship between putative MNS activity and empathy in healthy populations. 52 studies were identified that investigated the association between the MNS and at least one domain of empathy, representing data from 1044 participants. Our results suggest that emotional and cognitive empathy are moderately correlated with MNS activity, however, these domains were mixed and varied across techniques used to acquire MNS activity (TMS, EEG, and fMRI). Few studies investigated motor empathy, and of those, no significant relationships were revealed. Overall, results provide preliminary evidence for a relationship between MNS activity and empathy. However, our findings highlight methodological variability in study design as an important factor in understanding this relationship. We discuss limitations regarding these methodological variations and important implications for clinical and community translations, as well as suggestions for future research.

Do Gaze Behaviours during Action Observation Predict Interpersonal Motor Resonance?

Interpersonal motor resonance (IMR) is a common putative index of the mirror neuron system (MNS), a network containing specialised cells that fire during both action execution and observation. Visual content inputs to the MNS, however, it is unclear whether visual behaviours mediate the putative MNS response. We aimed to examine gaze effects on IMR during action observation. Neurotypical adults (N = 99; 60 female) underwent transcranial magnetic stimulation, electromyography, and eye-tracking during the observation of videos of actors performing grasping actions. IMR was measured as a percentage change in motor evoked potentials (MEPs) of the first dorsal interosseous muscle during action observation relative to baseline. MEP facilitation was observed during action observation, indicating IMR (65.43%, SE = 11.26%, P < 0.001). Fixations occurring in biologically relevant areas (face/hand/arm) yielded significantly stronger IMR (81.03%, SE = 14.15%) than non-biological areas (63.92%, SE = 14.60, P = 0.012). This effect, however, was only evident in the first of four experimental blocks. Our results suggest that gaze fixation can modulate IMR, but this may be affected by the salience and novelty of the observed action. These findings have important methodological implications for future studies in both clinical and healthy populations.

Differential Motor Facilitation During Action Observation in Followers and Leaders on Instagram

High power and high socioeconomic status individuals have been found to exhibit less motor system activity during observation of another individual’s behavior. In the modern world, the use of online social networks for social interaction is increasing, and these social networks afford new forms of social status hierarchy. An important question is whether social status in an online setting affects social information processing in a way that resembles the known effects of real-world status on such processing. Using transcranial magnetic stimulation (TMS), we examined differences in motor cortical output during action observation between Instagram “leaders” and “followers.” Instagram Leaders were defined as individuals who have more followers than they are following, while Instagram Followers were defined as individuals who have fewer followers than they follow. We found that Followers exhibited increased Motor-evoked Potential (MEP) facilitation during action observation compared to Leaders. Correlational analyses also revealed a positive association between an individual’s Instagram follower/following ratio and their perceived sense of online status. Overall, the findings of this study provide some evidence in favor of the idea that our online sense of status and offline sense of status might be concordant in terms of their effect on motor cortical output during action observation.

Statement of Significance: This study highlights the importance of examining the effects of online status on motor cortical output during action observation, and more generally alludes to the importance of understanding online and offline status effects on social information processing.

Uncertainty in contextual and kinematic cues jointly modulates motor resonance in primary motor cortex

Contextual information accompanying others' actions modulates "motor resonance", i.e., neural activity within motor areas that is elicited by movement observation. One possibility is that we weigh and combine such information in a Bayesian manner according to their relative uncertainty. Therefore, contextual information becomes particularly useful when others' actions are difficult to discriminate. It is unclear, however, whether this uncertainty modulates the neural activity in primary motor cortex (M1) during movement observation. Here, we applied single-pulse transcranial magnetic stimulation (TMS) while subjects watched different grasping actions. We operationalized motor resonance as grip-specific modulation of corticomotor excitability measured in the index (FDI) versus the little finger abductor (ADM). We experimentally modulated either the availability of kinematic information ( experiment 1) or the reliability of contextual cues ( experiment 2). Our results indicate that even in the absence of movement kinematics, reliable contextual information is enough to trigger significant muscle-specific corticomotor excitability changes in M1, which are strongest when both kinematics and contextual information are available. These findings suggest that bottom-up mechanisms that activate motor representations as a function of the observed kinematics and top-down mechanisms that activate motor representations associated with arbitrary cues converge in M1. NEW & NOTEWORTHY Our study reveals new neurophysiological insights in support of the Bayesian account of action observation by showing that "motor resonance", i.e., neural activity evoked by observing others' actions, incorporates the uncertainty related to both contextual (prior beliefs) and kinematic (sensory evidence) cues. Notably, we show that muscle-specific modulation of M1 is strongest when context and movement kinematics are available, and it can be elicited even in the absence of movement kinematics.

Autistic traits predict poor integration between top-down contextual expectations and movement kinematics during action observation

Autism is associated with difficulties in predicting and understanding other people’s actions. There is evidence that autistic traits are distributed across a spectrum and that subclinical forms of autistic impairments can also be measured in the typical population. To investigate the association between autistic traits and motor responses to others’ actions, we quantified these traits and measured cortico-spinal excitability modulations in M1 during the observation of actions embedded in congruent, incongruent and ambiguous contexts. In keeping with previous studies, we found that actions observed in congruent contexts elicited an early facilitation of M1 responses, and actions observed in incongruent contexts, resulted in a later inhibition. Correlational analysis revealed no association between autistic traits and the facilitation for congruent contexts. However, we found a significant correlation between motor inhibition and autistic traits, specifically related to social skills and attention to details. Importantly, the influence of these factors was independent from each other, and from the observer’s gender. Thus, results suggest that individuals with higher social deficits and greater detail-processing style are more impaired in suppressing action simulation in M1 when a mismatch between kinematics and context occurs. This points to difficult integration between kinematics and contextual representations in the autistic-like brain.

Investigating Mirror System (MS) Activity in Adults with ASD When Inferring Others' Intentions Using Both TMS and EEG

ASD is associated with mentalizing deficits that may correspond with atypical mirror system (MS) activation. We investigated MS activity in adults with and without ASD when inferring others' intentions using TMS-induced motor evoked potentials (MEPs) and mu suppression measured by EEG. Autistic traits were measured for all participants. Our EEG data show, high levels of autistic traits predicted reduced right mu (8-10 Hz) suppression when mentalizing. Higher left mu (8-10 Hz) suppression was associated with superior mentalizing performances. Eye-tracking and TMS data showed no differences associated with autistic traits. Our data suggest ASD is associated with reduced right MS activity when mentalizing, TMS-induced MEPs and mu suppression measure different aspects of MS functioning and the MS is directly involved in inferring intentions.

Changes in spontaneous overt motor execution immediately after observing others' painful action: two pilot studies

Research has demonstrated that motor control is directly influenced by observation of others’ action, stimulating the mirror neuron system. In addition, there is evidence that both emotion and empathy after observing a painful stimulus affects motor cortical excitability and reaction times. Aim of the present two pilot studies is a) to test for significant influence of observing other’s painful bending of the trunk on execution of the same activity in a self-directed bending action (study 1) and to compare these results with a bending action according to a strict bending protocol (study 2). In addition to study 1, differences between Low Back Pain (LBP) patients versus healthy subjects are tested. Video footage of a (1) neutral, (2) painful, and (3) happy bending action was presented in random order. Changes in flexion–relaxation phenomenon (FRP) of back muscles were studied directly after watching the videos with surface EMG, in study 1 during a self-directed bending action in LBP patients and healthy subjects, in study 2 according to a strict bending protocol. FRP ratios were calculated by a custom-made analysis scheme tested for sufficient reliability prior to both studies. Evoked emotions were measured with an Emotional Questionnaire after each video. A Mixed Model ANOVA was used to test for the effect video and the difference between LBP and healthy subjects on the FRP-rs. Differences in evoked emotion will be tested with a Wilcoxon Signed Rank Test. In study 1, 24 healthy controls and 16 LBP patients FRP-rs were significantly influenced after observing a painful video in all subjects versus a happy and neutral video (p = 0.00). No differences were present between LBP and healthy controls. All subjects experienced more fear after observation of the painful video (p 0.05). In study 2, 6 healthy subjects followed the strict FRP bending protocol for three times after observing each video. No significant changes occurred in FRPs per video compared to FRPs of six healthy subjects carrying out the spontaneous bending activity. Observing a painful action in another person changes motor performance and increases fear in both people with and without back pain, during self-directed trunk flexion, but not during a protocolled trunk flexion.

Mirror and (absence of) counter-mirror responses to action sounds measured with TMS

To what extent is the mirror neuron mechanism malleable to experience? The answer to this question can help characterising its ontogeny and its role in social cognition. Some suggest that it develops through sensorimotor associations congruent with our own actions. Others argue for its extreme volatility that will encode any sensorimotor association in the environment. Here, we added to this debate by exploring the effects of short goal-directed 'mirror' and 'counter-mirror' trainings (a 'mirror' training is defined as the first type of training encountered by the participants) on human auditory mirror motor-evoked potentials (MEPs). We recorded MEPs in response to two tones void of previous motor meaning, before and after mirror and counter-mirror trainings in which participants generated two tones of different pitch by performing free-choice button presses. The results showed that mirror MEPs, once established, were protected against an equivalent counter-mirror experience: they became manifest very rapidly and the same number of training trials that lead to the initial association did not suffice to reverse the MEP pattern. This steadiness of the association argues that, by serving direct-matching purposes, the mirror mechanism is a good solution for social cognition.

Corticospinal excitability is modulated by distinct movement patterns during action observation

It is well established that excitability of the primary motor cortex increases during action observation. However, the modulation of motor cortex excitability during observation of different actions, with distinct movement patterns, is not fully understood. The aim of the current study was to examine time-dependent changes in corticospinal excitability during observation of two actions with different levels of complexity. We developed videos of two distinct actions (a point and a reach-and-grasp), that were matched in video length, action onset, and onset of kinematics. Single-pulse transcranial magnetic stimulation was used to investigate time-dependent changes in primary motor cortex excitability during observation of the two actions. Motor evoked potentials (MEP) were recorded from two intrinsic hand muscles, namely first dorsal interosseous (FDI) and abductor digiti minimi. Results showed no difference in MEP amplitude during observation of a static hand compared to observation of the actions. When comparing the point to the grasp action, there were two key findings showing time-dependent changes in motor cortex excitability: first, greater MEP amplitude in FDI during observation of the point than the grasp action at an early time-point (index finger extension) and second, greater MEP amplitude in FDI during observation of the grasp than the point action at a later time-point (hand opening to form a grasp). These results show that excitability of the primary motor cortex is differentially modulated during observation of a point and grasp action, suggesting that the action observation network is engaged in a time-dependent manner during action observation.

The TMS Motor Map Does Not Change Following a Single Session of Mirror Training Either with Or without Motor Imagery

Both motor imagery and mirror training have been used in motor rehabilitation settings to promote skill learning and plasticity. As motor imagery and mirror training are suggested to be closely linked, it was hypothesized that mirror training augmented by motor imagery would increase corticospinal excitability (CSE) significantly compared to mirror training alone. Forty-four participants were split over two experimental groups. Each participant visited the laboratory once to receive either mirror training alone or mirror training augmented with layered stimulus response training (LSRT), a type of motor imagery training. Participants performed 16 min of mirror training, making repetitive grasping movements paced by a metronome. Transcranial magnetic stimulation (TMS) mapping was performed before and after the mirror training to test for changes in CSE of the untrained hand. Self-reports suggested that the imagery training was effective in helping the participant to perform the mirror training task as instructed. Nonetheless, neither training type resulted in a significant change of TMS map area, nor was there an interaction between the groups. The results from the study revealed no effect of a single session of 16 min of either mirror training or mirror training enhanced by imagery on TMS map area. Despite the negative result of the present experiment, this does not suggest that either motor imagery or mirror training might be ineffective as a rehabilitation therapy. Further study is required to allow disentangling the role of imagery and action observation in mirror training so that mirror training can be further tailored to the individual according to their abilities.

Motor resonance during action observation is gaze-contingent: A TMS study

When we observe others performing an action, visual input to our mirror neuron system is reflected in the facilitation of primary motor cortex (M1), a phenomenon known as 'motor resonance'. However, it is unclear whether this motor resonance is contingent upon our point-of-gaze. In order to address this issue, we collected gaze data from participants as they viewed an intransitive action - thumb abduction/adduction - under four conditions: with natural gaze behaviour (free viewing) and with their gaze fixated on each of three predetermined loci at various distances from the prime mover. In a control condition, participants viewed little finger movements, also with a fixated gaze. Transcranial magnetic stimulation (TMS) was delivered to M1 and motor evoked potentials (MEPs) were recorded from the right abductor pollicis brevis (APB) and right abductor digiti minimi (ADM). Results showed that, relative to a free viewing condition, a fixated point-of-gaze which maximized transfoveal motion facilitated MEPs in APB. Moreover, during free viewing, saccade amplitudes and APB MEP amplitudes were negatively correlated. These findings indicate that motor resonance is contingent on the observer's gaze behaviour and that, for simple movements, action observation effects may be enhanced by employing a fixed point-of-gaze.

Variability in functional brain networks predicts expertise during action observation

Observing an action performed by another individual activates, in the observer, similar circuits as those involved in the actual execution of that action. This activation is modulated by prior experience; indeed, sustained training in a particular motor domain leads to structural and functional changes in critical brain areas. Here, we capitalized on a novel graph-theory approach to electroencephalographic data (Fraiman et al., 2016) to test whether variability in functional brain networks implicated in Tango observation can discriminate between groups differing in their level of expertise. We found that experts and beginners significantly differed in the functional organization of task-relevant networks. Specifically, networks in expert Tango dancers exhibited less variability and a more robust functional architecture. Notably, these expertise-dependent effects were captured within networks derived from electrophysiological brain activity recorded in a very short time window (2s). In brief, variability in the organization of task-related networks seems to be a highly sensitive indicator of long-lasting training effects. This finding opens new methodological and theoretical windows to explore the impact of domain-specific expertise on brain plasticity, while highlighting variability as a fruitful measure in neuroimaging research.

Behavioral and TMS Markers of Action Observation Might Reflect Distinct Neuronal Processes

Transcranial magnetic stimulation (TMS) studies have shown that observing an action induces muscle-specific changes in corticospinal excitability. From a signal detection theory standpoint, this pattern can be related to sensitivity, which here would measure the capacity to distinguish between two action observation conditions. In parallel to these TMS studies, action observation has also been linked to behavioral effects such as motor priming and interference. It has been hypothesized that behavioral markers of action observation could be related to TMS markers and thus represent a potentially cost-effective mean of assessing the functioning of the action-perception system. However, very few studies have looked at possible relationships between these two measures. The aim of this study was to investigate if individual differences in sensitivity to action observation could be related to the behavioral motor priming and interference effects produced by action observation. To this end, 14 healthy participants observed index and little finger movements during a TMS task and a stimulus-response compatibility task. Index muscle displayed sensitivity to action observation, and action observation resulted in significant motor priming+interference, while no significant effect was observed for the little finger in both task. Nevertheless, our results indicate that the sensitivity measured in TMS was not related to the behavioral changes measured in the stimulus-response compatibility task. Contrary to a widespread assumption, the current results indicate that individual differences in physiological and behavioral markers of action observation may be unrelated. This could have important impacts on the potential use of behavioral markers in place of more costly physiological markers of action observation in clinical settings.

The effects of visual stimuli on EEG mu rhythms in healthy adults

[Purpose] Several action observation/imagery training studies have been conducted in patients with limited physical activity showing improvements in motor function. However, most studies compared effects of action observation and imagery, so little is known about the changes caused by subsequent observation of target objects. Moreover, few studies analyzed brain wave changes in the EEG mu rhythm. [Subjects and Methods] Eighteen healthy female adults participated in this study, and were divided into two groups: ‘Visual Stimuli’ and ‘Non-Visual Stimuli’. EEG amplitude in the 8–13 Hz frequency band over the sensorimotor cortex was evaluated. [Results] Significant mu suppression was obtained in the action observation trials. Mu power showed a main effect of visual stimuli, with decreased power during action observation, and increased power post-observation in both conditions. Comparing the ‘Visual Stimuli’ and ‘Non-Visual Stimuli’ conditions during the post-observation period, mu power demonstrated a greater increase in the ‘Non-Visual Stimuli’ condition. Furthermore, mu power was lower post-observation than pre-observation. [Conclusion] These results show the effects of visual input between maintaining target objects and no visual input, and their relevance to modulations of the mirror neuron system. It also suggests that greater visual input may be more effective for cognitive rehabilitation.

Changing ideas about others' intentions: updating prior expectations tunes activity in the human motor system

Predicting intentions from observing another agent's behaviours is often thought to depend on motor resonance - i.e., the motor system's response to a perceived movement by the activation of its stored motor counterpart, but observers might also rely on prior expectations, especially when actions take place in perceptually uncertain situations. Here we assessed motor resonance during an action prediction task using transcranial magnetic stimulation to probe corticospinal excitability (CSE) and report that experimentally-induced updates in observers' prior expectations modulate CSE when predictions are made under situations of perceptual uncertainty. We show that prior expectations are updated on the basis of both biomechanical and probabilistic prior information and that the magnitude of the CSE modulation observed across participants is explained by the magnitude of change in their prior expectations. These findings provide the first evidence that when observers predict others' intentions, motor resonance mechanisms adapt to changes in their prior expectations. We propose that this adaptive adjustment might reflect a regulatory control mechanism that shares some similarities with that observed during action selection. Such a mechanism could help arbitrate the competition between biomechanical and probabilistic prior information when appropriate for prediction.

No evidence for mirror system dysfunction in schizophrenia from a multimodal TMS/EEG study

Dysfunctional mirror neuron systems have been proposed to contribute to the social cognitive deficits observed in schizophrenia. A few studies have explored mirror systems in schizophrenia using various techniques such as TMS (levels of motor resonance) or EEG (levels of mu suppression), with mixed results. This study aimed to use a novel multimodal approach (i.e. concurrent TMS and EEG) to further investigate mirror systems and social cognition in schizophrenia. Nineteen individuals with schizophrenia or schizoaffective disorder and 19 healthy controls participated. Single-pulse TMS was applied to M1 during the observation of hand movements designed to elicit mirror system activity. Single EEG electrodes (C3, CZ, C4) recorded brain activity. Participants also completed facial affect recognition and theory of mind tasks. The schizophrenia group showed significant deficits in facial affect recognition and higher level theory of mind compared to healthy controls. A significant positive relationship was revealed between mu suppression and motor resonance for the overall sample, indicating concurrent validity of these measures. Levels of mu suppression and motor resonance were not significantly different between groups. These findings indicate that in stable outpatients with schizophrenia, mirror system functioning is intact, and therefore their social cognitive difficulties may be caused by alternative pathophysiology.

Direct mapping rather than motor prediction subserves modulation of corticospinal excitability during observation of actions in real time

Motor facilitation refers to the specific increment in corticospinal excitability (CSE) elicited by the observation of actions performed by others. To date, the precise nature of the mechanism at the basis of this phenomenon is unknown. One possibility is that motor facilitation is driven by a predictive process reminiscent of the role of forward models in motor control. Alternatively, motor facilitation may result from a model-free mechanism by which the basic elements of the observed action are directly mapped onto their cortical representations. Our study was designed to discern these alternatives. To this aim, we recorded the time course of CSE for the first dorsal interosseous (FDI) and the abductor digiti minimi (ADM) during observation of three grasping actions in real time, two of which strongly diverged in kinematics from their natural (invariant) form. Although artificially slow movements used in most action observation studies might enhance the observer's discrimination performance, the use of videos in real time is crucial to maintain the time course of CSE within the physiological range of daily actions. CSE was measured at 4 time points within a 240-ms window that best captured the kinematic divergence from the invariant form. Our results show that CSE of the FDI, not the ADM, closely follows the functional role of the muscle despite the mismatch between the natural and the divergent kinematics. We propose that motor facilitation during observation of actions performed in real time reflects the model-free coding of perceived movement following a direct mapping mechanism.

Motor resonance meets motor performance

The aim of the present work is to explore which of two different models better explains facilitation/interference effects when participants have to conditionally react to an observed action with a movement. The Dimensional Overlap model assumes two parallel routes, an automatic route and a rule-based one, that interact only when the stimulus-set and the response-set share some dimensions. In the alternative model, a motor resonance for rule-based reaction, the automatic visuo-motor transformation is always an obligatory step that provides the correct categorization of the observed action as the input for the rule-based route, thus linking the two routes in a serial fashion. We explicitly tested which of the hypotheses fits better the data by asking participants to perform one of two different actions in a two-choice reaction paradigm. In one condition participants were required to perform the opposite action compared to the one they saw (COUNTER task: see A→do B, see B→do A), while in the other they were required to perform two actions that did not share any dimension with the stimulus-set (NEUTRAL task: see A→do C, see B→do D). We predicted an advantage for the NEUTRAL task if the Dimensional Overlap model was correct, while a similar performance was foreseen if the motor resonance-based model was correct. Since the interpretation of these results was not straightforward, we conducted a distributional analysis of participants' response accuracies in order to understand whether a serial or a general parallel model explained better the data. We found clear evidence that participants responded above chance before the motor representation of the action observed was activated. We conclude that two separate systems in the human brain can transform observed actions in own motor representations. One is stimulus-driven, while the second is rule-driven. Likely, their activity is mutually independent along parallel pathways.

Emotional and movement-related body postures modulate visual processing

Human body postures convey useful information for understanding others' emotions and intentions. To investigate at which stage of visual processing emotional and movement-related information conveyed by bodies is discriminated, we examined event-related potentials elicited by laterally presented images of bodies with static postures and implied-motion body images with neutral, fearful or happy expressions. At the early stage of visual structural encoding (N190), we found a difference in the sensitivity of the two hemispheres to observed body postures. Specifically, the right hemisphere showed a N190 modulation both for the motion content (i.e. all the observed postures implying body movements elicited greater N190 amplitudes compared with static postures) and for the emotional content (i.e. fearful postures elicited the largest N190 amplitude), while the left hemisphere showed a modulation only for the motion content. In contrast, at a later stage of perceptual representation, reflecting selective attention to salient stimuli, an increased early posterior negativity was observed for fearful stimuli in both hemispheres, suggesting an enhanced processing of motivationally relevant stimuli. The observed modulations, both at the early stage of structural encoding and at the later processing stage, suggest the existence of a specialized perceptual mechanism tuned to emotion- and action-related information conveyed by human body postures.

Balancing the excitability of M1 circuitry during movement observation without overt replication

Although observation of a movement increases the excitability of the motor system of the observer, it does not induce a motor replica. What is the mechanism for replica suppression? We performed a series of experiments, involving a total of 66 healthy humans, to explore the excitability of different M1 circuits and the spinal cord during observation of simple movements. Several strategies were used. In the first and second experimental blocks, we used several delay times from movement onset to evaluate the time-course modulation of the cortico-spinal excitability (CSE), and its potential dependency on the duration of the movement observed; in order to do this single pulse transcranial magnetic stimulation (TMS) over M1 was used. In subsequent experiments, at selected delay times from movement-onset, we probed the excitability of the cortico-spinal circuits using three different approaches: (i) electric cervicomedullary stimulation (CMS), to test spinal excitability, (ii) paired-pulse TMS over M1, to evaluate the cortical inhibitory-excitatory balance (short intracortical inhibition (SICI) and intracortical facilitation (ICF)], and (iii) continuous theta-burst stimulation (cTBS), to modulate the excitability of M1 cortical circuits. We observed a stereotyped response in the modulation of CSE. At 500 ms after movement-onset the ICF was increased; although the most clear-cut effect was a decrease of CSE. The compensatory mechanism was not explained by changes in SICI, but by M1-intracortical circuits targeted by cTBS. Meanwhile, the spinal cord maintained the elevated level of excitability induced when expecting to observe movements, potentially useful to facilitate any required response to the movement observed.

"Some like it hot": spectators who score high on the personality trait openness enjoy the excitement of hearing dancers breathing without music

Music is an integral part of dance. Over the last 10 years, however, dance stimuli (without music) have been repeatedly used to study action observation processes, increasing our understanding of the influence of observer’s physical abilities on action perception. Moreover, beyond trained skills and empathy traits, very little has been investigated on how other observer or spectators’ properties modulate action observation and action preference. Since strong correlations have been shown between music and personality traits, here we aim to investigate how personality traits shape the appreciation of dance when this is presented with three different music/sounds. Therefore, we investigated the relationship between personality traits and the subjective esthetic experience of 52 spectators watching a 24 min lasting contemporary dance performance projected on a big screen containing three movement phrases performed to three different sound scores: classical music (i.e., Bach), an electronic sound-score, and a section without music but where the breathing of the performers was audible. We found that first, spectators rated the experience of watching dance without music significantly different from with music. Second, we found that the higher spectators scored on the Big Five personality factor openness, the more they liked the no-music section. Third, spectators’ physical experience with dance was not linked to their appreciation but was significantly related to high average extravert scores. For the first time, we showed that spectators’ reported entrainment to watching dance movements without music is strongly related to their personality and thus may need to be considered when using dance as a means to investigate action observation processes and esthetic preferences.

Conscious and Unconscious Representations of Observed Actions in the Human Motor System

Action observation activates the observer's motor system. These motor resonance responses are automatic and triggered even when the action is only implied in static snapshots. However, it is largely unknown whether an action needs to be consciously perceived to trigger motor resonance. In this study, we used single-pulse TMS to study the facilitation of corticospinal excitability (a measure of motor resonance) during supraliminal and subliminal presentations of implied action images. We used a forward and backward dynamic masking procedure that successfully prevented the conscious perception of prime stimuli depicting a still hand or an implied abduction movement of the index or little finger. The prime was followed by the supraliminal presentation of a still or implied action probe hand. Our results revealed a muscle-specific increase of motor facilitation following observation of the probe hand actions that were consciously perceived as compared with observation of a still hand. Crucially, unconscious perception of prime hand actions presented before probe still hands did not increase motor facilitation as compared with observation of a still hand, suggesting that motor resonance requires perceptual awareness. However, the presentation of a masked prime depicting an action that was incongruent with the probe hand action suppressed motor resonance to the probe action such that comparable motor facilitation was recorded during observation of implied action and still hand probes. This suppression of motor resonance may reflect the processing of action conflicts in areas upstream of the motor cortex and may subserve a basic mechanism for dealing with the multiple and possibly incongruent actions of other individuals.

Empathy, autistic traits, and motor resonance in adults with Turner syndrome

Turner syndrome is a genetic condition resulting from the partial or complete absence of an X-chromosome in phenotypic females. Individuals with Turner syndrome often display social difficulties that are reminiscent of those associated with autistic spectrum disorders (ASD), conditions associated with empathy and mirror-neuron system (MNS) deficits. The goal of the present study was (1) to investigate the extent to which adults with Turner syndrome display autistic and empathic traits, and (2) to probe the integrity of the MNS in this neurogenetic disorder. Sixteen individuals with Turner syndrome and 16 age-, sex-, and IQ-matched controls took part in a neuropsychological assessment where the Weschler Abbreviated Scale of Intelligence, the Autism Spectrum Quotient and the Empathy Quotient were administered. Functioning of the MNS was assessed by measuring motor cortex activity with transcranial magnetic stimulation during an action-observation task. Results show that individuals with Turner syndrome do not differ significantly from controls regarding autistic or empathic traits, and present normal functioning of the MNS during action observation. Correlational analysis showed a significant positive relationship between scores on the Empathy Quotient and motor facilitation during action observation, bringing further support to the hypothesis that MNS activity is related to sociocognitive competence.

Transcranial magnetic stimulation reveals two functionally distinct stages of motor cortex involvement during perception of emotional body language

Studies indicate that perceiving emotional body language recruits fronto-parietal regions involved in action execution. However, the nature of such motor activation is unclear. Using transcranial magnetic stimulation (TMS) we provide correlational and causative evidence of two distinct stages of motor cortex engagement during emotion perception. Participants observed pictures of body expressions and categorized them as happy, fearful or neutral while receiving TMS over the left or right motor cortex at 150 and 300 ms after picture onset. In the early phase (150 ms), we observed a reduction of excitability for happy and fearful emotional bodies that was specific to the right hemisphere and correlated with participants’ disposition to feel personal distress. This ‘orienting’ inhibitory response to emotional bodies was also paralleled by a general drop in categorization accuracy when stimulating the right but not the left motor cortex. Conversely, at 300 ms, greater excitability for negative, positive and neutral movements was found in both hemispheres. This later motor facilitation marginally correlated with participants’ tendency to assume the psychological perspectives of others and reflected simulation of the movement implied in the neutral and emotional body expressions. These findings highlight the motor system’s involvement during perception of emotional bodies. They suggest that fast orienting reactions to emotional cues—reflecting neural processing necessary for visual perception—occur before motor features of the observed emotional expression are simulated in the motor system and that distinct empathic dispositions influence these two neural motor phenomena. Implications for theories of embodied simulation are discussed.

Corticospinal excitability modulation during action observation

This study used the transcranial magnetic stimulation/motor evoked potential (TMS/MEP) technique to pinpoint when the automatic tendency to mirror someone else's action becomes anticipatory simulation of a complementary act. TMS was delivered to the left primary motor cortex corresponding to the hand to induce the highest level of MEP activity from the abductor digiti minimi (ADM; the muscle serving little finger abduction) as well as the first dorsal interosseus (FDI; the muscle serving index finger flexion/extension) muscles. A neuronavigation system was used to maintain the position of the TMS coil, and electromyographic (EMG) activity was recorded from the right ADM and FDI muscles. Producing original data with regard to motor resonance, the combined TMS/MEP technique has taken research on the perception-action coupling mechanism a step further. Specifically, it has answered the questions of how and when observing another person's actions produces motor facilitation in an onlooker's corresponding muscles and in what way corticospinal excitability is modulated in social contexts.

Motor cortical processing is causally involved in object recognition

Background

Motor activity during vicarious experience of actions is a widely reported and studied phenomenon, and motor system activity also accompanies observation of graspable objects in the absence of any actions. Such motor activity is thought to reflect simulation of the observed action, or preparation to interact with the object, respectively.

Results

Here, in an initial exploratory study, we ask whether motor activity during observation of object directed actions is involved in processes related to recognition of the object after initial exposure. Single pulse Transcranial Magnetic Stimulation (TMS) was applied over the thumb representation of the motor cortex, or over the vertex, during observation of a model thumb typing on a cell-phone, and performance on a phone recognition task at the end of the trial was assessed. Disrupting motor processing over the thumb representation 100 ms after the onset of the typing video impaired the ability to recognize the phone in the recognition test, whereas there was no such effect for TMS applied over the vertex and no TMS trials. Furthermore, this effect only manifested for videos observed from the first person perspective. In an additional control condition, there was no evidence for any effects of TMS to the thumb representation or vertex when observing and recognizing non-action related shape stimuli.

Conclusion

Overall, these data provide evidence that motor cortical processing during observation of object-directed actions from a first person perspective is causally linked to the formation of enduring representations of objects-of-action.

A transcranial magnetic stimulation study of the effect of visual orientation on the putative human mirror neuron system

Mirror neurons are a class of motor neuron that are active during both the performance and observation of behavior, and have been implicated in interpersonal understanding. There is evidence to suggest that the mirror response is modulated by the perspective from which an action is presented (e.g., egocentric or allocentric). Most human research, however, has only examined this when presenting intransitive actions. Twenty-three healthy adult participants completed a transcranial magnetic stimulation experiment that assessed corticospinal excitability whilst viewing transitive hand gestures from both egocentric (i.e., self) and allocentric (i.e., other) viewpoints. Although action observation was associated with increases in corticospinal excitability (reflecting putative human mirror neuron activity), there was no effect of visual perspective. These findings are discussed in the context of contemporary theories of mirror neuron ontogeny, including models concerning associative learning and evolutionary adaptation.

Temporal dynamics of motor cortex excitability during perception of natural emotional scenes

Although it is widely assumed that emotions prime the body for action, the effects of visual perception of natural emotional scenes on the temporal dynamics of the human motor system have scarcely been investigated. Here, we used single-pulse transcranial magnetic stimulation (TMS) to assess motor excitability during observation and categorization of positive, neutral and negative pictures from the International Affective Picture System database. Motor-evoked potentials (MEPs) from TMS of the left motor cortex were recorded from hand muscles, at 150 and 300 ms after picture onset. In the early temporal condition we found an increase in hand motor excitability that was specific for the perception of negative pictures. This early negative bias was predicted by interindividual differences in the disposition to experience aversive feelings (personal distress) in interpersonal emotional contexts. In the later temporal condition, we found that MEPs were similarly increased for both positive and negative pictures, suggesting an increased reactivity to emotionally arousing scenes. By highlighting the temporal course of motor excitability during perception of emotional pictures, our study provides direct neurophysiological support for the evolutionary notions that emotion perception is closely linked to action systems and that emotionally negative events require motor reactions to be more urgently mobilized.

Relationships between non-pathological dream-enactment and mirror behaviors

Dream-enacting behaviors (DEBs) are behavioral expressions of forceful dream images often occurring during sleep-to-wakefulness transitions. We propose that DEBs reflect brain activity underlying social cognition, in particular, motor-affective resonance generated by the mirror neuron system. We developed a Mirror Behavior Questionnaire (MBQ) to assess some dimensions of mirror behaviors and investigated relationships between MBQ scores and DEBs in a large of university undergraduate cohort. MBQ scores were normally distributed and described by a four-factor structure (Empathy/Emotional Contagion, Behavioral Imitation, Sleepiness/Anger Contagion, Motor Skill Imitation). DEB scores correlated positively with MBQ total and factor scores even with social desirability, somnambulism and somniloquy controlled. Emotion-specific DEB items correlated with corresponding emotion-specific MBQ items, especially crying and smiling. Results provide preliminary evidence for cross-state relationships between propensities for dream-enacting and mirror behaviors--especially behaviors involving motor-affective resonance--and our suggestion that motor-affective resonance mediates dream-enactment imagery during sleep and emotional empathy during waking.

Timecourse of mirror and counter-mirror effects measured with transcranial magnetic stimulation

The human mirror system has been the subject of much research over the past two decades, but little is known about the timecourse of mirror responses. In addition, it is unclear whether mirror and counter-mirror effects follow the same timecourse. We used single-pulse transcranial magnetic stimulation to investigate the timecourse of mirror and counter-mirror responses in the human brain. Experiment 1 demonstrated that mirror responses can be measured from around 200 ms after observed action onset. Experiment 2 demonstrated significant effects of counter-mirror sensorimotor training at all timepoints at which a mirror response was found in Experiment 1 (i.e. from 200 ms onward), indicating that mirror and counter-mirror responses follow the same timecourse. By suggesting similarly direct routes for mirror and counter-mirror responses, these results support the associative account of mirror neuron origins whereby mirror responses arise as a result of correlated sensorimotor experience during development. More generally, they contribute to theorizing regarding mirror neuron function by providing some constraints on how quickly mirror responses can influence social cognition.

Vicarious motor activation during action perception: beyond correlational evidence

Neurophysiological and imaging studies have shown that seeing the actions of other individuals brings about the vicarious activation of motor regions involved in performing the same actions. While this suggests a simulative mechanism mediating the perception of others' actions, one cannot use such evidence to make inferences about the functional significance of vicarious activations. Indeed, a central aim in social neuroscience is to comprehend how vicarious activations allow the understanding of other people's behavior, and this requires to use stimulation or lesion methods to establish causal links from brain activity to cognitive functions. In the present work, we review studies investigating the effects of transient manipulations of brain activity or stable lesions in the motor system on individuals' ability to perceive and understand the actions of others. We conclude there is now compelling evidence that neural activity in the motor system is critical for such cognitive ability. More research using causal methods, however, is needed in order to disclose the limits and the conditions under which vicarious activations are required to perceive and understand actions of others as well as their emotions and somatic feelings.