Visual features of an observed agent do not modulate human brain activity during action observation

Mixed emotions to social situations: An fMRI investigation

BACKGROUND

Neuroscience research has generally studied emotions each taken in isolation. However, mixed emotional states (e.g., the co-occurrence of amusement and disgust, or sadness and pleasure) are common in everyday life. Psychophysiological and behavioral evidence suggests that mixed emotions may have response profiles that are distinguishable from their constituent emotions. Yet, the brain bases of mixed emotions remain unresolved.

METHODS

We recruited 38 healthy adults who viewed short, validated film clips, eliciting either positive (amusing), negative (disgusting), neutral, or mixed (a mix of amusement and disgust) emotional states, while brain activity was assessed by functional magnetic resonance imaging (fMRI). We assessed mixed emotions in two ways: first by comparing neural reactivity to ambiguous (mixed) with that to unambiguous (positive and negative) film clips and second by conducting parametric analyses to measure neural reactivity with respect to individual emotional states. We thus obtained self-reports of amusement and disgust after each clip and computed a minimum feeling score (shared minimum of amusement and disgust) to quantify mixed emotional feelings.

RESULTS

Both analyses revealed a network of the posterior cingulate (PCC), medial superior parietal lobe (SPL)/precuneus, and parieto-occipital sulcus to be involved in ambiguous contexts eliciting mixed emotions.

CONCLUSION

Our results are the first to shed light on the dedicated neural processes involved in dynamic social ambiguity processing. They suggest both higher-order (SPL) and lower-order (PCC) processes may be needed to process emotionally complex social scenes.

Neural processing of social interaction: Coordinate-based meta-analytic evidence from human neuroimaging studies

While the action observation and mentalizing networks are considered to play complementary roles in understanding others' goals and intentions, they might be concurrently engaged when processing social interactions. We assessed this hypothesis via three activation-likelihood-estimation meta-analyses of neuroimaging studies on the neural processing of: (a) social interactions, (b) individual actions by the action observation network, and (c) mental states by the mentalizing network. Conjunction analyses and direct comparisons unveiled overlapping and specific regions among the resulting maps. We report quantitative meta-analytic evidence for a "social interaction network" including key nodes of the action observation and mentalizing networks. An action-social interaction-mentalizing gradient of activity along the posterior temporal cortex highlighted a hierarchical processing of interactions, from visuomotor analyses decoding individual and shared intentions to in-depth inferences on actors' intentional states. The medial prefrontal cortex, possibly in conjunction with the amygdala, might provide additional information concerning the affective valence of the interaction. This evidence suggests that the functional architecture underlying the neural processing of interactions involves the joint involvement of the action observation and mentalizing networks. These data might inform the design of rehabilitative treatments for social cognition disorders in pathological conditions, and the assessment of their outcome in randomized controlled trials.

The Mirror Neurons Network in Aging, Mild Cognitive Impairment, and Alzheimer Disease: A functional MRI Study

The aim of the current study is to investigate the integrity of the Mirror Neurons (MN) network in normal aging, Mild Cognitive Impairment (MCI), and Alzheimer disease (AD). Although AD and MCI are considered “cognitive” diseases, there has been increasing recognition of a link between motor function and AD. More recently the embodied cognition hypothesis has also been developed: it postulates that a part of cognition results from the coupling between action and perception representations. MN represent a neuronal population which links perception, action, and cognition, therefore we decided to characterize MN functioning in neurodegenerative cognitive decline. Three matched groups of 16 subjects (normal elderly-NE, amnesic MCI with hippocampal atrophy and AD) were evaluated with a focused neuropsychological battery and an fMRI task specifically created to test MN: that comprised of an observation run, where subjects were shown movies of a right hand grasping different objects, and of a motor run, where subjects observed visual pictures of objects oriented to be grasped with the right hand. In NE subjects, the conjunction analysis (comparing fMRI activation during observation and execution), showed the activation of a bilateral fronto-parietal network in “classical” MN areas, and of the superior temporal gyrus (STG). The MCI group showed the activation of areas belonging to the same network, however, parietal areas were activated to a lesser extent and the STG was not activated, while the opposite was true for the right Broca's area. We did not observe any activation of the fronto-parietal network in AD participants. They did not perform as well as the NE subjects in all the neuropsychological tests (including tests of functions attributed to MN) whereas the MCI subjects were significantly different from the NE subjects only in episodic memory and semantic fluency. Here we show that the MN network is largely preserved in aging, while it appears involved following an anterior-posterior gradient in neurodegenerative decline. In AD, task performance decays and the MN network appears clearly deficient. The preservation of the anterior part of the MN network in MCI could possibly supplement the initial decay of the posterior part, preserving cognitive performance.

Primary somatosensory contribution to action observation brain activity-combining fMRI and cTBS

Traditionally the mirror neuron system (MNS) only includes premotor and posterior parietal cortices. However, somatosensory cortices, BA1/2 in particular, are also activated during action execution and observation. Here, we examine whether BA1/2 and the parietofrontal MNS integrate information by using functional magnetic resonance imaging (fMRI)-guided continuous theta-burst stimulation (cTBS) to perturb BA1/2. Measuring brain activity using fMRI while participants are under the influence of cTBS shows local cTBS effects in BA1/2 varied, with some participants showing decreases and others increases in the BOLD response to viewing actions vs control stimuli. We show how measuring cTBS effects using fMRI can harness this variance using a whole-brain regression. This analysis identifies brain regions exchanging action-specific information with BA1/2 by mapping voxels away from the coil with cTBS-induced, action-observation-specific BOLD contrast changes that mirror those under the coil. This reveals BA1/2 exchanges action-specific information with premotor, posterior parietal and temporal nodes of the MNS during action observation. Although anatomical connections between BA1/2 and these regions are well known, this is the first demonstration that these connections carry action-specific signals during observation and hence, that BA1/2 plays a causal role in the human MNS.

What Do Eye Gaze Metrics Tell Us about Motor Imagery?

Many of the brain structures involved in performing real movements also have increased activity during imagined movements or during motor observation, and this could be the neural substrate underlying the effects of motor imagery in motor learning or motor rehabilitation. In the absence of any objective physiological method of measurement, it is currently impossible to be sure that the patient is indeed performing the task as instructed. Eye gaze recording during a motor imagery task could be a possible way to "spy" on the activity an individual is really engaged in. The aim of the present study was to compare the pattern of eye movement metrics during motor observation, visual and kinesthetic motor imagery (VI, KI), target fixation, and mental calculation. Twenty-two healthy subjects (16 females and 6 males), were required to perform tests in five conditions using imagery in the Box and Block Test tasks following the procedure described by Liepert et al. Eye movements were analysed by a non-invasive oculometric measure (SMI RED250 system). Two parameters describing gaze pattern were calculated: the index of ocular mobility (saccade duration over saccade + fixation duration) and the number of midline crossings (i.e. the number of times the subjects gaze crossed the midline of the screen when performing the different tasks). Both parameters were significantly different between visual imagery and kinesthesic imagery, visual imagery and mental calculation, and visual imagery and target fixation. For the first time we were able to show that eye movement patterns are different during VI and KI tasks. Our results suggest gaze metric parameters could be used as an objective unobtrusive approach to assess engagement in a motor imagery task. Further studies should define how oculomotor parameters could be used as an indicator of the rehabilitation task a patient is engaged in.

Seeing is not feeling: posterior parietal but not somatosensory cortex engagement during touch observation

Observing touch has been reported to elicit activation in human primary and secondary somatosensory cortices and is suggested to underlie our ability to interpret other's behavior and potentially empathy. However, despite these reports, there are a large number of inconsistencies in terms of the precise topography of activation, the extent of hemispheric lateralization, and what aspects of the stimulus are necessary to drive responses. To address these issues, we investigated the localization and functional properties of regions responsive to observed touch in a large group of participants (n = 40). Surprisingly, even with a lenient contrast of hand brushing versus brushing alone, we did not find any selective activation for observed touch in the hand regions of somatosensory cortex but rather in superior and inferior portions of neighboring posterior parietal cortex, predominantly in the left hemisphere. These regions in the posterior parietal cortex required the presence of both brush and hand to elicit strong responses and showed some selectivity for the form of the object or agent of touch. Furthermore, the inferior parietal region showed nonspecific tactile and motor responses, suggesting some similarity to area PFG in the monkey. Collectively, our findings challenge the automatic engagement of somatosensory cortex when observing touch, suggest mislocalization in previous studies, and instead highlight the role of posterior parietal cortex.

Object visibility alters the relative contribution of ventral visual stream and mirror neuron system to goal anticipation during action observation

We used fMRI to study the effect of hiding the target of a grasping action on the cerebral activity of an observer whose task was to anticipate the size of the object being grasped. Activity in the putative mirror neuron system (pMNS) was higher when the target was concealed from the view of the observer and anticipating the size of the object being grasped requested paying attention to the hand kinematics. In contrast, activity in ventral visual areas outside the pMNS increased when the target was fully visible, and the performance improved in this condition. A repetition suppression analysis demonstrated that in full view, the size of the object being grasped by the actor was encoded in the ventral visual stream. Dynamic causal modeling showed that monitoring a grasping action increased the coupling between the parietal and ventral premotor nodes of the pMNS. The modulation of the functional connectivity between these nodes was correlated with the subject's capability to detect the size of hidden objects. In full view, synaptic activity increased within the ventral visual stream, and the connectivity with the pMNS was diminished. The re-enactment of observed actions in the pMNS is crucial when interpreting others' actions requires paying attention to the body kinematics. However, when the context permits, visual-spatial information processing may complement pMNS computations for improved action anticipation accuracy.

Excitability of the primary motor cortex increases more strongly with slow- than with normal-speed presentation of actions

Introduction

The aim of the present study was to investigate how the speed of observed action affects the excitability of the primary motor cortex (M1), as assessed by the size of motor evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS).

Methods

Eighteen healthy subjects watched a video clip of a person catching a ball, played at three different speeds (normal-, half-, and quarter-speed). MEPs were induced by TMS when the model's hand had opened to the widest extent just before catching the ball (“open”) and when the model had just caught the ball (“catch”). These two events were locked to specific frames of the video clip (“phases”), rather than occurring at specific absolute times, so that they could easily be compared across different speeds. MEPs were recorded from the thenar (TH) and abductor digiti minimi (ADM) muscles of the right hand.

Results

The MEP amplitudes were higher when the subjects watched the video clip at low speed than when they watched the clip at normal speed. A repeated-measures ANOVA, with the factor VIDEO-SPEED, showed significant main effects. Bonferroni's post hoc test showed that the following MEP amplitude differences were significant: TH, normal vs. quarter; ADM, normal vs. half; and ADM, normal vs. quarter. Paired t-tests showed that the significant MEP amplitude differences between TMS phases under each speed condition were TH, “catch” higher than “open” at quarter speed; ADM, “catch” higher than “open” at half speed.

Conclusions

These results indicate that the excitability of M1 was higher when the observed action was played at low speed. Our findings suggest that the action observation system became more active when the subjects observed the video clip at low speed, because the subjects could then recognize the elements of action and intention in others.

Neural correlates of grasping

Prehension, the capacity to reach and grasp objects, comprises two main components: reaching, i.e., moving the hand towards an object, and grasping, i.e., shaping the hand with respect to its properties. Knowledge of this topic has gained a huge advance in recent years, dramatically changing our view on how prehension is represented within the dorsal stream. While our understanding of the various nodes coding the grasp component is rapidly progressing, little is known of the integration between grasping and reaching. With this Mini Review we aim to provide an up-to-date overview of the recent developments on the coding of prehension. We will start with a description of the regions coding various aspects of grasping in humans and monkeys, delineating where it might be integrated with reaching. To gain insights into the causal role of these nodes in the coding of prehension, we will link this functional description to lesion studies. Finally, we will discuss future directions that might be promising to unveil new insights on the coding of prehension movements.

The left side of motor resonance

Motor resonance is defined as the internal activation of an observer's motor system, specifically attuned to the perceived movement. In social contexts, however, different patterns of observed and executed muscular activation are frequently required. This is the case, for instance, of seeing a key offered with a precision grip and received by opening the hand. Novel evidence suggests that compatibility effects in motor resonance can be altered by social response preparation. What is not known is how handedness modulates this effect. The present study aimed at determining how a left- and a right-handed actor grasping an object and then asking for a complementary response influences corticospinal activation in left- and right-handers instructed to observe the scene. Transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) were thus recorded from the dominant hands of left- and right-handers. Interestingly, requests posed by the right-handed actor induced a motor activation in the participants' respective dominant hands, suggesting that left-handers tend to mirror right-handers with their most efficient hand. Whereas requests posed by the left-handed actor activated the anatomically corresponding muscles (i.e., left hand) in all the participants, right-handers included. Motor resonance effects classically reported in the literature were confirmed when observing simple grasping actions performed by the right-handed actor. These findings indicate that handedness influences both congruent motor resonance and complementary motor preparation to observed actions.

Involvement of the superior temporal cortex in action execution and action observation

The role of the superior temporal sulcus (STs) in action execution and action observation remains unsettled. In an attempt to shed more light on the matter, we used the quantitative method of (14)C-deoxyglucose to reveal changes in activity, in the cortex of STs and adjacent inferior and superior temporal convexities of monkeys, elicited by reaching-to-grasp in the light or in the dark and by observation of the same action executed by an external agent. We found that observation of reaching-to-grasp activated the components of the superior temporal polysensory area [STP; including temporo-parieto-occipital association area (TPO), PGa, and IPa], the motion complex [including medial superior temporal area (MST), fundus of superior temporal area (FST), and dorsal and ventral parts of the middle temporal area (MTd and MTv, respectively)], and area TS2. A significant part of most of these activations was associated with observation of the goal-directed action, and a smaller part with the perception of arm-motion. Execution of reaching-to-grasp in the light-activated areas TS2, STP partially and marginally, and MT compared with the fixation but not to the arm-motion control. Consequently, MT-activation is associated with the arm-motion and not with the purposeful action. Finally, reaching-to-grasp in complete darkness activated all components of the motion complex. Conclusively, lack of visibility of our own actions involves the motion complex, whereas observation of others' actions engages area STP and the motion complex. Our previous and present findings together suggest that sensory effects are interweaved with motor commands in integrated action codes, and observation of an action or its execution in complete darkness triggers the retrieval of the visual representation of the action.

Can we simulate an action that we temporarily cannot perform?

AIMS OF THE STUDY

The scope of individuals' motor repertoire and expertise influences the way they perceive the actions of others. When observing skilled actions, experts recruit the cortical network subserving action perception (action observation network, AON) to a greater extent than non-experts. However, it remains unknown whether and how a temporary motor injury affects activation within the AON.

MATERIALS AND METHODS

To investigate this issue, brain hemodynamic activity was recorded twice in thirteen national female gymnasts suffering from a lower extremity injury at the onset of the experiment. The gymnasts were scanned one month after the injury and were shown gymnastics routines they were able and temporarily unable to perform. Six months later, after complete recovery, they were scanned again and shown the same routines they were now able to practice.

RESULTS

Results showed: first, that the level of activity within the inferior parietal lobule and MT/V5/EBA (extrastriate body area), areas constitutive of the AON, was independent of the gymnasts' physical condition. Second, when gymnasts were hurt (vs. when recovered), higher activity in the cerebellum was detected.

CONCLUSION

The equal contribution of MT/V5/EBA and inferior parietal lobule during the observation of movements the gymnasts were able or unable to practice suggests respectively that physical provisional incapacity does not interfere with the perceptual processing of body shape and motion information, and that motor expertise may prevent the decay of sensorimotor representations. Higher activations in the cerebellum may suggest that this structure plays a role in dissociating perceived physically feasible movements from those that are provisionally unfeasible.

Mentalizing and the Role of the Posterior Superior Temporal Sulcus in Sharing Others' Embarrassment

The experience of embarrassment provides a highly salient cue for the human moral apparatus. Interestingly, people also experience embarrassment on behalf of others' inappropriate conditions. The perceiver's embarrassment often lacks an equivalent expression of embarrassment in the social counterpart. The present study examines this phenomenon and distinguishes neural circuits involved in embarrassment with and embarrassment for another person's mishaps. Using functional magnetic resonance imaging, we show that the embarrassment on behalf of others engages the temporal pole and the medial prefrontal cortex, central structures of the mentalizing network, together with the anterior insula and anterior cingulate cortex. In contrast, sharing others' embarrassment additionally stimulated the posterior superior temporal sulcus (STS), which exhibited increased functional integration with inferior parietal and insular cortex areas. These findings characterize common neural circuits involved in the embodied representation of embarrassment and further unravel the unique role of the posterior STS in sharing others' affective state.

Cortical regions involved in the observation of bimanual actions

Although we are beginning to understand how observed actions performed by conspecifics with a single hand are processed and how bimanual actions are controlled by the motor system, we know very little about the processing of observed bimanual actions. We used fMRI to compare the observation of bimanual manipulative actions with their unimanual components, relative to visual control conditions equalized for visual motion. Bimanual action observation did not activate any region specialized for processing visual signals related to this more elaborated action. On the contrary, observation of bimanual and unimanual actions activated similar occipito-temporal, parietal and premotor networks. However, whole-brain as well as region of interest (ROI) analyses revealed that this network functions differently under bimanual and unimanual conditions. Indeed, in bimanual conditions, activity in the network was overall more bilateral, especially in parietal cortex. In addition, ROI analyses indicated bilateral parietal activation patterns across hand conditions distinctly different from those at other levels of the action-observation network. These activation patterns suggest that while occipito-temporal and premotor levels are involved with processing the kinematics of the observed actions, the parietal cortex is more involved in the processing of static, postural aspects of the observed action. This study adds bimanual cooperation to the growing list of distinctions between parietal and premotor cortex regarding factors affecting visual processing of observed actions.

Primary somatosensory cortex discriminates affective significance in social touch

Another person's caress is one of the most powerful of all emotional social signals. How much the primary somatosensory cortices (SIs) participate in processing the pleasantness of such social touch remains unclear. Although ample empirical evidence supports the role of the insula in affective processing of touch, here we argue that SI might be more involved in affective processing than previously thought by showing that the response in SI to a sensual caress is modified by the perceived sex of the caresser. In a functional MRI study, we manipulated the perceived affective quality of a caress independently of the sensory properties at the skin: heterosexual males believed they were sensually caressed by either a man or woman, although the caress was in fact invariantly delivered by a female blind to condition type. Independent analyses showed that SI encoded, and was modulated by, the visual sex of the caress, and that this effect is unlikely to originate from the insula. This suggests that current models may underestimate the role played by SI in the affective processing of social touch.

The role of prediction in social neuroscience

Research has shown that the brain is constantly making predictions about future events. Theories of prediction in perception, action and learning suggest that the brain serves to reduce the discrepancies between expectation and actual experience, i.e., by reducing the prediction error. Forward models of action and perception propose the generation of a predictive internal representation of the expected sensory outcome, which is matched to the actual sensory feedback. Shared neural representations have been found when experiencing one's own and observing other's actions, rewards, errors, and emotions such as fear and pain. These general principles of the “predictive brain” are well established and have already begun to be applied to social aspects of cognition. The application and relevance of these predictive principles to social cognition are discussed in this article. Evidence is presented to argue that simple non-social cognitive processes can be extended to explain complex cognitive processes required for social interaction, with common neural activity seen for both social and non-social cognitions. A number of studies are included which demonstrate that bottom-up sensory input and top-down expectancies can be modulated by social information. The concept of competing social forward models and a partially distinct category of social prediction errors are introduced. The evolutionary implications of a “social predictive brain” are also mentioned, along with the implications on psychopathology. The review presents a number of testable hypotheses and novel comparisons that aim to stimulate further discussion and integration between currently disparate fields of research, with regard to computational models, behavioral and neurophysiological data. This promotes a relatively new platform for inquiry in social neuroscience with implications in social learning, theory of mind, empathy, the evolution of the social brain, and potential strategies for treating social cognitive deficits.

Judging roughness by sight--a 7-Tesla fMRI study on responsivity of the primary somatosensory cortex during observed touch of self and others

Observing another person being touched activates our own somatosensory system. Whether the primary somatosensory cortex (S1) is also activated during the observation of passive touch, and which subregions of S1 are responsible for self- and other-related observed touch is currently unclear. In our study, we first aimed to clarify whether observing passive touch without any action component can robustly increase activity in S1. Secondly, we investigated whether S1 activity only increases when touch of others is observed, or also when touch of one's own body is observed. We were particularly interested in which subregions of S1 are responsible for either process. We used functional magnetic resonance imaging at 7 Tesla to measure S1 activity changes when participants observed videos of their own or another's hand in either egocentric or allocentric perspective being touched by different pieces of sandpaper. Participants were required to judge the roughness of the different sandpaper surfaces. Our results clearly show that S1 activity does increase in response to observing passive touch, and that activity changes are localized in posterior but not in anterior parts of S1. Importantly, activity increases in S1 were particularly related to observing another person being touched. Self-related observed touch, in contrast, caused no significant activity changes within S1. We therefore assume that posterior but not anterior S1 is part of a system for sharing tactile experiences with others.

μ-suppression during action observation and execution correlates with BOLD in dorsal premotor, inferior parietal, and SI cortices

The discovery of mirror neurons in the monkey, that fire during both the execution and the observation of the same action, sparked great interest in studying the human equivalent. For over a decade, both functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) have been used to quantify activity in the human mirror neuron system (MNS)-yet, little is still known about how fMRI and EEG measures of the MNS relate to each other. To test the frequent assumption that regions of the MNS as evidenced by fMRI are the origin of the suppression of the EEG μ-rhythm during both action execution and observation, we recorded EEG and BOLD-fMRI signals simultaneously while participants observed and executed actions. We found that the suppression of the μ-rhythm in EEG covaried with BOLD activity in typical MNS regions, inferior parietal lobe (IPL), dorsal premotor (dPM) and primary somatosensory cortex (BA2), during both action observation and execution. In contrast, in BA44, only nonoverlapping voxels correlated with μ-suppression during observation and execution. These findings provide direct support for the notion that μ-suppression is a valid indicator of MNS activity in BA2, IPL, and dPM, but argues against the idea that mirror neurons in BA44 are the prime source of μ-suppression. These results shed light on the neural basis of μ-suppression and provide a basis for integrating more closely the flourishing but often separate literatures on the MNS using fMRI and EEG.

Social neuroscience: mirror neurons recorded in humans

New single-cell recordings show that humans do have mirror neurons, and in more brain regions than previously suspected. Some action-execution neurons were seen to be inhibited during observation, possibly preventing imitation and helping self/other discrimination.

From simulation to reciprocity: the case of complementary actions

A large body of research reports that perceiving body movements of other people activates motor representations in the observer's brain. This automatic resonance mechanism appears to be imitative in nature. However, action observation does not inevitably lead to symmetrical motor facilitation: Mirroring the observed movement might be disadvantageous for successfully performing joint actions. What remains unknown is how we are to resolve the possible conflict between the automatic tendency to "mirror" and the need to perform different context-related complementary actions. By using single-pulse transcranial magnetic stimulation, we found that observation of a double-step action characterized by an implicit complementary request engendered a shift from symmetrical simulation to reciprocity in the participants' corticospinal activity. Accordingly, differential motor facilitation was revealed for the snapshots evoking imitative and complementary gestures despite the fact that the observed type of grasp was identical. Control conditions in which participants observed the same action sequence but in a context not implying a complementary request were included as well. The results provide compelling evidence that when an observed action calls for a nonidentical complementary action, an interplay between the automatic tendency to resonate with what is observed and to implicitly prepare for the complementary action does emerge. In other words, implicit complementary requests might have the ability to draw attention to specific features of the context affording nonidentical responses.

Rapid communication: Automatic priming of grip force following action observation

Research shows that action observation can prime execution. Evidence for this comes from experiments that show that action observation influenced temporal (e.g., speed) or spatial (e.g., peak grasp aperture or trajectory) aspects of executed movement. In the paper presented here, we for the first time show that observation can also prime executed action force. Following observation of force actions, participants executed grip-force responses using a dynamometer, and the data showed that their force was modulated by the condition observed. The findings of the study are discussed in terms of a likely cause of the force modulation effect and potential uses that the effect may have for strength rehabilitation.

Brain regions involved in human movement perception: a quantitative voxel-based meta-analysis

Face, hands, and body movements are powerful signals essential for social interactions. In the last 2 decades, a large number of brain imaging studies have explored the neural correlates of the perception of these signals. Formal synthesis is crucially needed, however, to extract the key circuits involved in human motion perception across the variety of paradigms and stimuli that have been used. Here, we used the activation likelihood estimation (ALE) meta-analysis approach with random effect analysis. We performed meta-analyses on three classes of biological motion: movement of the whole body, hands, and face. Additional analyses of studies of static faces or body stimuli and sub-analyses grouping experiments as a function of their control stimuli or task employed allowed us to identify main effects of movements and forms perception, as well as effects of task demand. In addition to specific features, all conditions showed convergence in occipito-temporal and fronto-parietal regions, but with different peak location and extent. The conjunction of the three ALE maps revealed convergence in all categories in a region of the right posterior superior temporal sulcus as well as in a bilateral region at the junction between middle temporal and lateral occipital gyri. Activation in these regions was not a function of attentional demand and was significant also when controlling for non-specific motion perception. This quantitative synthesis points towards a special role for posterior superior temporal sulcus for integrating human movement percept, and supports a specific representation for body parts in middle temporal, fusiform, precentral, and parietal areas.

One's motor performance predictably modulates the understanding of others' actions through adaptation of premotor visuo-motor neurons

Neurons firing both during self and other's motor behavior (mirror neurons) have been described in the brain of vertebrates including humans. The activation of somatic motor programs driven by perceived behavior has been taken as evidence for mirror neurons' contribution to cognition. The inverse relation, that is the influence of motor behavior on perception, is needed for demonstrating the long-hypothesized causal role of mirror neurons in action understanding. We provide here conclusive behavioral and neurophysiological evidence for that causal role by means of cross-modal adaptation coupled with a novel transcranial magnetic stimulation (TMS)-adaptation paradigm. Blindfolded repeated motor performance of an object-directed action (push or pull) induced in healthy participants a strong visual after-effect when categorizing others' actions, as a result of motor-to-visual adaptation of visuo-motor neurons. TMS over the ventral premotor cortex, but not over the primary motor cortex, suppressed the after-effect, thus localizing the population of adapted visuo-motor neurons in the premotor cortex. These data are exquisitely consistent in humans with the existence of premotor mirror neurons that have access to the action meaning. We also show that controlled manipulation of the firing properties of this neural population produces strong predictable changes in the way we categorize others' actions.

Modulation of cortical motor outputs by the symbolic meaning of visual stimuli

The observation of an action modulates motor cortical outputs in specific ways, in part through mediation of the mirror neuron system. Sometimes we infer a meaning to an observed action based on integration of the actual percept with memories. Here, we conducted a series of experiments in healthy adults to investigate whether such inferred meanings can also modulate motor cortical outputs in specific ways. We show that brief observation of a neutral stimulus mimicking a hand does not significantly modulate motor cortical excitability (Study 1) although, after prolonged exposure, it can lead to a relatively nonspecific modulation (Study 2). However, when such a neutral stimulus is preceded by exposure to a hand stimulus, the latter appears to serve as a prime, perhaps enabling meaning to the neutral stimulus, which then modulates motor cortical excitability in accordance with mirror neuron-driving properties (Studies 2 and 3). Overall results suggest that a symbolic value ascribed to an otherwise neutral stimulus can modulate motor cortical outputs, revealing the influence of top-down inputs on the mirror neuron system. These findings indicate a novel aspect of the human mirror neuron system: an otherwise neutral stimulus can acquire specific mirror neuron-driving properties in the absence of a direct association between motor practice and perception. This significant malleability in the way that the mirror neuron system can code otherwise meaningless (i.e. arbitrarily associated) stimuli may contribute to coding communicative signals such as language. This may represent a mirror neuron system feature that is unique to humans.

Somatosensation in social perception

The discovery of mirror neurons in motor areas of the brain has led many to assume that our ability to understand other people's behaviour partially relies on vicarious activations of motor cortices. This Review focuses the limelight of social neuroscience on a different set of brain regions: the somatosensory cortices. These have anatomical connections that enable them to have a role in visual and auditory social perception. Studies that measure brain activity while participants witness the sensations, actions and somatic pain of others consistently show vicarious activation in the somatosensory cortices. Neuroscientists are starting to understand how the brain adds a somatosensory dimension to our perception of other people.

Mapping the information flow from one brain to another during gestural communication

Both the putative mirror neuron system (pMNS) and the ventral medial prefrontal cortex (vmPFC) are deemed important for social interaction: the pMNS because it supposedly "resonates" with the actions of others, the vmPFC because it is involved in mentalizing. Strictly speaking, the resonance property of the pMNS has never been investigated. Classical functional MRI experiments have only investigated whether pMNS regions augment their activity when an action is seen or executed. Resonance, however, entails more than only "going on and off together". Activity in the pMNS of an observer should continuously follow the more subtle changes over time in activity of the pMNS of the actor. Here we directly explore whether such resonance indeed occurs during continuous streams of actions. We let participants play the game of charades while we measured brain activity of both gesturer and guesser. We then applied a method to localize directed influences between the brains of the participants: between-brain Granger-causality mapping. Results show that a guesser's brain activity in regions involved in mentalizing and mirroring echoes the temporal structure of a gesturer's brain activity. This provides evidence for resonance theories and indicates a fine-grained temporal interplay between regions involved in motor planning and regions involved in thinking about the mental states of others. Furthermore, this method enables experiments to be more ecologically valid by providing the opportunity to leave social interaction unconstrained. This, in turn, would allow us to tap into the neural substrates of social deficits such as autism spectrum disorder.

Smelling odors, understanding actions

Previous evidence indicates that we understand others' actions not only by perceiving their visual features but also by their sound. This raises the possibility that brain regions responsible for action understanding respond to cues coming from different sensory modalities. Yet no studies, to date, have examined if this extends to olfaction. Here we addressed this issue by using functional magnetic resonance imaging. We searched for brain activity related to the observation of an action executed towards an object that was smelled rather than seen. The results show that temporal, parietal, and frontal areas were activated when individuals observed a hand grasping a smelled object. This activity differed from that evoked during the observation of a mimed grasp. Furthermore, superadditive activity was revealed when the action target-object was both seen and smelled. Together these findings indicate the influence of olfaction on action understanding and its contribution to multimodal action representations.

Understanding actors and object-goals in the human brain

When another person takes 10 pounds from your hand, it matters if they are a shopkeeper or a robber. That is, the meaning of a simple, goal-directed action can vary depending on the identity of the actors involved. Research examining action understanding has identified an action observation network (AON) that encodes action features such as goals and kinematics. However, it is not yet known how or where the brain links actor identity to action goal. In the present paper, we used a repetition suppression paradigm during functional magnetic resonance imaging (fMRI) to examine the neural representation of actor identity within the context of object-directed actions. Participants watched video clips of two different actors with two different object-goals. Repeated presentation of the same actor suppressed the blood oxygen level-dependent (BOLD) response in fusiform gyrus and occipitotemporal cortex. In contrast, repeated presentation of an action with the same object-goal suppressed the BOLD response throughout the AON. Our data reveal an extended brain network for understanding other people and their everyday actions that go beyond the traditional action observation network.

ALE meta-analysis of action observation and imitation in the human brain

Over the last decade, many neuroimaging studies have assessed the human brain networks underlying action observation and imitation using a variety of tasks and paradigms. Nevertheless, questions concerning which areas consistently contribute to these networks irrespective of the particular experimental design and how such processing may be lateralized remain unresolved. The current study aimed at identifying cortical areas consistently involved in action observation and imitation by combining activation likelihood estimation (ALE) meta-analysis with probabilistic cytoarchitectonic maps. Meta-analysis of 139 functional magnetic resonance and positron emission tomography experiments revealed a bilateral network for both action observation and imitation. Additional subanalyses for different effectors within each network revealed highly comparable activation patterns to the overall analyses on observation and imitation, respectively, indicating an independence of these findings from potential confounds. Conjunction analysis of action observation and imitation meta-analyses revealed a bilateral network within frontal premotor, parietal, and temporo-occipital cortex. The most consistently rostral inferior parietal area was PFt, providing evidence for a possible homology of this region to macaque area PF. The observation and imitation networks differed particularly with respect to the involvement of Broca's area: whereas both networks involved a caudo-dorsal part of BA 44, activation during observation was most consistent in a more rostro-dorsal location, i.e., dorsal BA 45, while activation during imitation was most consistent in a more ventro-caudal aspect, i.e., caudal BA 44. The present meta-analysis thus summarizes and amends previous descriptions of the human brain networks related to action observation and imitation.