fMRI adaptation reveals mirror neurons in human inferior parietal cortex

Birds, primates, and spoken language origins: behavioral phenotypes and neurobiological substrates

Vocal learners such as humans and songbirds can learn to produce elaborate patterns of structurally organized vocalizations, whereas many other vertebrates such as non-human primates and most other bird groups either cannot or do so to a very limited degree. To explain the similarities among humans and vocal-learning birds and the differences with other species, various theories have been proposed. One set of theories are motor theories, which underscore the role of the motor system as an evolutionary substrate for vocal production learning. For instance, the motor theory of speech and song perception proposes enhanced auditory perceptual learning of speech in humans and song in birds, which suggests a considerable level of neurobiological specialization. Another, a motor theory of vocal learning origin, proposes that the brain pathways that control the learning and production of song and speech were derived from adjacent motor brain pathways. Another set of theories are cognitive theories, which address the interface between cognition and the auditory-vocal domains to support language learning in humans. Here we critically review the behavioral and neurobiological evidence for parallels and differences between the so-called vocal learners and vocal non-learners in the context of motor and cognitive theories. In doing so, we note that behaviorally vocal-production learning abilities are more distributed than categorical, as are the auditory-learning abilities of animals. We propose testable hypotheses on the extent of the specializations and cross-species correspondences suggested by motor and cognitive theories. We believe that determining how spoken language evolved is likely to become clearer with concerted efforts in testing comparative data from many non-human animal species.

Neurobiology of self-awareness in schizophrenia: an fMRI study

Self-awareness (SA) is one of the core domains of higher cortical functions and is frequently compromised in schizophrenia. Deficits in SA have been associated with functional and psychosocial impairment in this patient population. However, despite its clinical significance, only a few studies have examined the neural substrates of self-referential processing in schizophrenia. The aim of this study was to assess self-awareness in schizophrenia using a functional magnetic resonance imaging (fMRI) paradigm designed to elicit judgments of self-reference in a simulated social context. While scanned, volunteers looked at visually-displayed sentences that had the volunteer's own first name (self-directed sentence-stimulus) or an unknown other person's first name (other-directed sentence stimulus) as the grammatical subject of the sentence. The volunteers were asked to discern whether each sentence-stimulus was about the volunteer personally (during a self-referential cue epoch) or asked whether each statement was about someone else (during an other-referential cue epoch). We predicted that individuals with schizophrenia would demonstrate altered functional activation to self- and other-directed sentence-stimuli as compared to controls. Fifteen controls and seventeen schizophrenia volunteers completed clinical assessments and SA fMRI task on a 3T Philips 3.0 T Achieva system. The results showed significantly greater activation in schizophrenia compared to controls for cortical midline structures in response to self- vs. other-directed sentence-stimuli. These findings support results from earlier studies and demonstrate selective alteration in the activation of cortical midline structures associated with evaluations of self-reference in schizophrenia as compared to controls.

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.

Action understanding within and outside the motor system: the role of task difficulty

When we observe actions, we activate parietal and premotor areas that are also recruited when we perform actions ourselves. It has been suggested that this action mirroring is causally involved in the process of action understanding. Alternatively, it might reflect the outcome of action understanding, with the underlying cognitive processes taking place elsewhere. To identify and characterize areas involved in action understanding, we presented participants with point-light displays depicting human actions and engaged them in tasks that required identifying the effector (arm/leg) or the goal of an action. We observed a stronger blood oxygen level-dependent signal during the Goal in comparison to the Effector Task not only in premotor areas, but also in the middle temporal gyrus (MTG) and the anterior ventrolateral prefrontal cortex. In the MTG, the Goal Task led to a signal higher than the Effector Task only when actions were easy to understand, whereas frontal areas showed this difference also when the task was difficult, a finding that is not caused by a ceiling effect. Our results suggest an interplay between temporal and frontal areas that is modulated by task difficulty and thus provide important constraints for biologically plausible models of action understanding.

Viewpoint (In)dependence of Action Representations: An MVPA Study

The discovery of mirror neurons—neurons that code specific actions both when executed and observed—in area F5 of the macaque provides a potential neural mechanism underlying action understanding. To date, neuroimaging evidence for similar coding of specific actions across the visual and motor modalities in human ventral premotor cortex (PMv)—the putative homologue of macaque F5—is limited to the case of actions observed from a first-person perspective. However, it is the third-person perspective that figures centrally in our understanding of the actions and intentions of others. To address this gap in the literature, we scanned participants with fMRI while they viewed two actions from either a first- or third-person perspective during some trials and executed the same actions during other trials. Using multivoxel pattern analysis, we found action-specific cross-modal visual–motor representations in PMv for the first-person but not for the third-person perspective. Additional analyses showed no evidence for spatial or attentional differences across the two perspective conditions. In contrast, more posterior areas in the parietal and occipitotemporal cortex did show cross-modal coding regardless of perspective. These findings point to a stronger role for these latter regions, relative to PMv, in supporting the understanding of others' actions with reference to one's own actions.

The impact of aesthetic evaluation and physical ability on dance perception

The field of neuroaesthetics attracts attention from neuroscientists and artists interested in the neural underpinnings of esthetic experience. Though less studied than the neuroaesthetics of visual art, dance neuroaesthetics is a particularly rich subfield to explore, as it is informed not only by research on the neurobiology of aesthetics, but also by an extensive literature on how action experience shapes perception. Moreover, it is ideally suited to explore the embodied simulation account of esthetic experience, which posits that activation within sensorimotor areas of the brain, known as the action observation network (AON), is a critical element of the esthetic response. In the present study, we address how observers' esthetic evaluation of dance is related to their perceived physical ability to reproduce the movements they watch. Participants underwent functional magnetic resonance imaging while evaluating how much they liked and how well they thought they could physically replicate a range of dance movements performed by professional ballet dancers. We used parametric analyses to evaluate brain regions that tracked with degree of liking and perceived physical ability. The findings reveal strongest activation of occipitotemporal and parietal portions of the AON when participants view movements they rate as both esthetically pleasing and difficult to reproduce. As such, these findings begin to illuminate how the embodied simulation account of esthetic experience might apply to watching dance, and provide preliminary evidence as to why some people find enjoyment in an evening at the ballet.

Subcortical contributions to effective connectivity in brain networks supporting imitation

Using functional magnetic resonance imaging (fMRI), we investigated effective connectivity in brain networks supporting imitation. Despite extensive reports of regional functional specialization underlying action perception, action execution and imitation, our understanding of the potential contribution of subcortical sites is limited, as is our knowledge of how regions displaying functional specialization interact with each other on a system level. While in the scanner, participants performed a simple imitation paradigm with three conditions: Observe trials, in which participants passively viewed a human actor executing a sequence of four finger presses on a keypad; Imitate trials, in which participants imitated the actor's finger presses on a keyboard; and Execute trials, in which participants also executed finger presses but did so based on visuospatial cues in the absence of the actor's hand. Relative to the Execute condition, Imitate trials evoked significantly more activity in superior and inferior parietal lobules (SPL, IPL), posterior superior temporal sulcus (pSTS), and in a ventral aspect of dorsal premotor cortex (PMd). Psychophysiological interaction (PPI) analysis, a means of assessing effective connectivity, revealed significant interactions with regions of cerebellar lobule VII from seeds both in the right pSTS and right SPL, such that activity in these sites was more highly correlated during imitation. A similar interaction was found between right pSTS and left IPL. These results clarify the role of cortical regions supporting action observation, action execution and imitation, and highlight the role the cerebellum may play in facilitating both motor and nonmotor aspects of imitation.

More than one pathway to action understanding

Many believe that the ability to understand the actions of others is made possible by mirror neurons and a network of brain areas known as the action-observation network (AON). Despite nearly two decades of research into mirror neurons and the AON, however, there is little evidence that they enable the inference of the intention of observed actions. Instead, theories of action selection during action execution indicate that a ventral pathway, linking middle temporal gyrus with the anterior inferior frontal gyrus, might encode these abstract features during action observation. Here I propose that action understanding requires more than merely the AON, and might be achieved through interactions between a ventral pathway and the dorsal AON.

Brain regions with mirror properties: a meta-analysis of 125 human fMRI studies

Mirror neurons in macaque area F5 fire when an animal performs an action, such as a mouth or limb movement, and also when the animal passively observes an identical or similar action performed by another individual. Brain-imaging studies in humans conducted over the last 20 years have repeatedly attempted to reveal analogous brain regions with mirror properties in humans, with broad and often speculative claims about their functional significance across a range of cognitive domains, from language to social cognition. Despite such concerted efforts, the likely neural substrates of these mirror regions have remained controversial, and indeed the very existence of a distinct subcategory of human neurons with mirroring properties has been questioned. Here we used activation likelihood estimation (ALE), to provide a quantitative index of the consistency of patterns of fMRI activity measured in human studies of action observation and action execution. From an initial sample of more than 300 published works, data from 125 papers met our strict inclusion and exclusion criteria. The analysis revealed 14 separate clusters in which activation has been consistently attributed to brain regions with mirror properties, encompassing 9 different Brodmann areas. These clusters were located in areas purported to show mirroring properties in the macaque, such as the inferior parietal lobule, inferior frontal gyrus and the adjacent ventral premotor cortex, but surprisingly also in regions such as the primary visual cortex, cerebellum and parts of the limbic system. Our findings suggest a core network of human brain regions that possess mirror properties associated with action observation and execution, with additional areas recruited during tasks that engage non-motor functions, such as auditory, somatosensory and affective components.

Hand shape selection in pantomimed grasping: interaction between the dorsal and the ventral visual streams and convergence on the ventral premotor area

In visually guided grasping, possible hand shapes are computed from the geometrical features of the object, while prior knowledge about the object and the goal of the action influence both the computation and the selection of the hand shape. We investigated the system dynamics of the human brain for the pantomiming of grasping with two aspects accentuated. One is object recognition, with the use of objects for daily use. The subjects mimed grasping movements appropriate for an object presented in a photograph either by precision or power grip. The other is the selection of grip hand shape. We manipulated the selection demands for the grip hand shape by having the subjects use the same or different grip type in the second presentation of the identical object. Effective connectivity analysis revealed that the increased selection demands enhance the interaction between the anterior intraparietal sulcus (AIP) and posterior inferior temporal gyrus (pITG), and drive the converging causal influences from the AIP, pITG, and dorsolateral prefrontal cortex to the ventral premotor area (PMv). These results suggest that the dorsal and ventral visual areas interact in the pantomiming of grasping, while the PMv integrates the neural information of different regions to select the hand posture. The present study proposes system dynamics in visually guided movement toward meaningful objects, but further research is needed to examine if the same dynamics is found also in real grasping.

The predictive mirror: interactions of mirror and affordance processes during action observation

An important question for the study of social interactions is how the motor actions of others are represented. Research has demonstrated that simply watching someone perform an action activates a similar motor representation in oneself. Key issues include (1) the automaticity of such processes, and (2) the role object affordances play in establishing motor representations of others’ actions. Participants were asked to move a lever to the left or right to respond to the grip width of a hand moving across a workspace. Stimulus-response compatibility effects were modulated by two task-irrelevant aspects of the visual stimulus: the observed reach direction and the match between hand-grasp and the affordance evoked by an incidentally presented visual object. These findings demonstrate that the observation of another person’s actions automatically evokes sophisticated motor representations that reflect the relationship between actions and objects even when an action is not directed towards an object.

fMRI repetition suppression: neuronal adaptation or stimulus expectation?

Measurements of repetition suppression with functional magnetic resonance imaging (fMRI adaptation) have been used widely to probe neuronal population response properties in human cerebral cortex. fMRI adaptation techniques assume that fMRI repetition suppression reflects neuronal adaptation, an assumption that has been challenged on the basis of evidence that repetition-related response changes may reflect unrelated factors, such as attention and stimulus expectation. Specifically, Summerfield et al. (Summerfield C, Trittschuh EH, Monti JM, Mesulam MM, Egner T. 2008. Neural repetition suppression reflects fulfilled perceptual expectations. Nat Neurosci. 11:1004-1006) reported that the relative frequency of stimulus repetitions and non-repetitions influenced the magnitude of repetition suppression in the fusiform face area, suggesting that stimulus expectation accounted for most of the effect of repetition. We confirm that stimulus expectation can significantly influence fMRI repetition suppression throughout visual cortex and show that it occurs with long as well as short adaptation durations. However, the effect was attention dependent: When attention was diverted away from the stimuli, the effects of stimulus expectation completely disappeared. Nonetheless, robust and significant repetition suppression was still evident. These results suggest that fMRI repetition suppression reflects a combination of neuronal adaptation and attention-dependent expectation effects that can be experimentally dissociated. This implies that with an appropriate experimental design, fMRI adaptation can provide valid measures of neuronal adaptation and hence response specificity.

Attention, biological motion, and action recognition

Interacting with others in the environment requires that we perceive and recognize their movements and actions. Neuroimaging and neuropsychological studies have indicated that a number of brain regions, particularly the superior temporal sulcus, are involved in a number of processes essential for action recognition, including the processing of biological motion and processing the intentions of actions. We review the behavioral and neuroimaging evidence suggesting that while some aspects of action recognition might be rapid and effective, they are not necessarily automatic. Attention is particularly important when visual information about actions is degraded or ambiguous, or if competing information is present. We present evidence indicating that neural responses associated with the processing of biological motion are strongly modulated by attention. In addition, behavioral and neuroimaging evidence shows that drawing inferences from the actions of others is attentionally demanding. The role of attention in action observation has implications for everyday social interactions and workplace applications that depend on observing, understanding and interpreting actions.

The thing that should not be: predictive coding and the uncanny valley in perceiving human and humanoid robot actions

Using functional magnetic resonance imaging (fMRI) repetition suppression, we explored the selectivity of the human action perception system (APS), which consists of temporal, parietal and frontal areas, for the appearance and/or motion of the perceived agent. Participants watched body movements of a human (biological appearance and movement), a robot (mechanical appearance and movement) or an android (biological appearance, mechanical movement). With the exception of extrastriate body area, which showed more suppression for human like appearance, the APS was not selective for appearance or motion per se. Instead, distinctive responses were found to the mismatch between appearance and motion: whereas suppression effects for the human and robot were similar to each other, they were stronger for the android, notably in bilateral anterior intraparietal sulcus, a key node in the APS. These results could reflect increased prediction error as the brain negotiates an agent that appears human, but does not move biologically, and help explain the 'uncanny valley' phenomenon.

Increased overlap between the brain areas involved in self-other distinction in schizophrenia

Self-awareness impairments are frequently mentioned as being responsible for the positive symptoms of schizophrenia spectrum disorders. However, the neural correlates of self-other distinction in this pathology are still poorly understood. In the present study, we developed an fMRI procedure in order to examine self-other distinction during speech exchange situations. Fifteen subjects with schizophrenia were compared to 15 matched controls. The results revealed an increased overlap between the self and non-self cortical maps in schizophrenia, in the medial frontal and medial parietal cortices, as well as in the right middle temporal cortex and the right inferior parietal lobule. Moreover, these neural structures showed less BOLD amplitude differences between the self and non-self conditions in the patients. These activation patterns were judged to be independent of mirror-like properties, familiarity or body-ownership processing. Significantly, the increase in the right IPL signal was found to correlate positively with the severity of first-rank symptoms, and thus could be considered a “state-marker” of schizophrenia, whereas temporal and medial parieto-frontal differences appear to be “trait-markers” of the disease. Such an increased overlap between self and non-self cortical maps might be considered a neuro-physiological signature of the well established self-awareness impairment in people suffering from schizophrenia.

Empathy: shared circuits and their dysfunctions

Observing another individual acting upon an object triggers cerebral activity well beyond the visual cortex of the observer in areas directly involved in planning and executing actions. This we will call action simulation. Importantly, the brain does not solely simulate the actions of others but also the sensations they feel, and their emotional responses. These simulation mechanisms are most active in individuals who report being very empathic. Simulation may indeed be instrumental for our understanding of the emotional and mental state of people in our sight, and may contribute heavily to the social interactions with our peers by providing a first-person perspective on their inner feelings. Simulation mechanisms are at work at an early stage of social development and might be defective in young individuals with autism spectrum disorders (ASD). However, the results to date regarding ASD are not clearcut, and an equal number of studies report positive and negative findings.

It takes two-skilled recognition of objects engages lateral areas in both hemispheres

Our object recognition abilities, a direct product of our experience with objects, are fine-tuned to perfection. Left temporal and lateral areas along the dorsal, action related stream, as well as left infero-temporal areas along the ventral, object related stream are engaged in object recognition. Here we show that expertise modulates the activity of dorsal areas in the recognition of man-made objects with clearly specified functions. Expert chess players were faster than chess novices in identifying chess objects and their functional relations. Experts' advantage was domain-specific as there were no differences between groups in a control task featuring geometrical shapes. The pattern of eye movements supported the notion that experts' extensive knowledge about domain objects and their functions enabled superior recognition even when experts were not directly fixating the objects of interest. Functional magnetic resonance imaging (fMRI) related exclusively the areas along the dorsal stream to chess specific object recognition. Besides the commonly involved left temporal and parietal lateral brain areas, we found that only in experts homologous areas on the right hemisphere were also engaged in chess specific object recognition. Based on these results, we discuss whether skilled object recognition does not only involve a more efficient version of the processes found in non-skilled recognition, but also qualitatively different cognitive processes which engage additional brain areas.

Selective responses in right inferior frontal and supramarginal gyri differentiate between observed movements of oneself vs. another

The fact that inferior frontal (IFg) and supramarginal (SMg) gyri respond to both self-generated and observed actions has been interpreted as evidence for a perception-action linking mechanism (mirroring). Yet, the brain readily distinguishes between percepts generated by one's own movements vs. those of another. Do IFg and/or SMg respond differentially to these visual stimuli even when carefully matched? We used BOLD fMRI to address this question as participants made repetitive bimanual hand movements while viewing either live visual feedback or perceptually similar, pre-recorded video of an actor. As expected, bilateral IFg and SMg increased activity during both conditions. However, right SMg and IFg responded differentially during live visual feedback vs. matched recordings. These mirror system areas may distinguish self-generated percepts by detecting subtle spatio-temporal differences between predicted and actual sensory feedback and/or visual and somatosensory signals.

The role of the superior temporal sulcus and the mirror neuron system in imitation

It has been suggested that in humans the mirror neuron system provides a neural substrate for imitation behaviour, but the relative contributions of different brain regions to the imitation of manual actions is still a matter of debate. To investigate the role of the mirror neuron system in imitation we used fMRI to examine patterns of neural activity under four different conditions: passive observation of a pantomimed action (e.g., hammering a nail); (2) imitation of an observed action; (3) execution of an action in response to a word cue; and (4) self-selected execution of an action. A network of cortical areas, including the left supramarginal gyrus, left superior parietal lobule, left dorsal premotor area and bilateral superior temporal sulcus (STS), was significantly active across all four conditions. Crucially, within this network the STS bilaterally was the only region in which activity was significantly greater for action imitation than for the passive observation and execution conditions. We suggest that the role of the STS in imitation is not merely to passively register observed biological motion, but rather to actively represent visuomotor correspondences between one's own actions and the actions of others.

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.

On the role of object information in action observation: an fMRI study

Observing other people's actions activates a network of brain regions that is also activated during the execution of these actions. Here, we used functional magnetic resonance imaging to test whether these "mirror" regions in frontal and parietal cortices primarily encode the spatiomotor aspects or the functional goal-related aspects of observed tool actions. Participants viewed static depictions of actions consisting of a tool object (e.g., key) and a target object (e.g., keyhole). They judged the actions either with regard to whether the objects were oriented correctly for the action to succeed (spatiomotor task) or whether an action goal could be achieved with the objects (function task). Compared with a control condition, both tasks activated regions in left frontoparietal cortex previously implicated in action observation and execution. Of these regions, the premotor cortex and supramarginal gyrus were primarily activated during the spatiomotor task, whereas the middle frontal gyrus was primarily activated during the function task. Regions along the intraparietal sulcus were more strongly activated during the spatiomotor task but only when the spatiomotor properties of the tool object were unknown in advance. These results suggest a division of labor within the action observation network that maps onto a similar division previously proposed for action execution.

Knowing beans: human mirror mechanisms revealed through motor adaptation

Human mirror mechanisms (MMs) respond during both performed and observed action and appear to underlie action goal recognition. We introduce a behavioral procedure for discovering and clarifying functional MM properties: blindfolded participants repeatedly move beans either toward or away from themselves to induce motor adaptation. Then, the bias for perceiving direction of ambiguous visual movement in depth is measured. Bias is affected by (a) number of beans moved, (b) movement direction, and (c) similarity of the visual stimulus to the hand used to move beans. This cross-modal adaptation pattern supports both the validity of human MMs and functionality of our testing instrument. We also discuss related work that extends the motor adaptation paradigm to investigate contributions of MMs to speech perception and language comprehension.

Knowing beans: human mirror mechanisms revealed through motor adaptation

An important development in behavioral neuroscience in the past 20 years has been the demonstration that it is possible to stimulate functional recovery after cerebral injury in laboratory animals. Rodent models of cerebral injury provide an important tool for developing such rehabilitation programs. The models include analysis at different levels including detailed behavioral paradigms, electrophysiology, neuronal morphology, protein chemistry, and epigenetics. A significant challenge for the next 20 years will be the translation of this work to improve the outcome from brain injury and disease in humans. Our goal in the article will be to synthesize the multidisciplinary laboratory work on brain plasticity and behavior in the injured brain to inform the development of rehabilitation programs.

Toward a neurobiology of delusions

Delusions are the false and often incorrigible beliefs that can cause severe suffering in mental illness. We cannot yet explain them in terms of underlying neurobiological abnormalities. However, by drawing on recent advances in the biological, computational and psychological processes of reinforcement learning, memory, and perception it may be feasible to account for delusions in terms of cognition and brain function. The account focuses on a particular parameter, prediction error--the mismatch between expectation and experience--that provides a computational mechanism common to cortical hierarchies, fronto-striatal circuits and the amygdala as well as parietal cortices. We suggest that delusions result from aberrations in how brain circuits specify hierarchical predictions, and how they compute and respond to prediction errors. Defects in these fundamental brain mechanisms can vitiate perception, memory, bodily agency and social learning such that individuals with delusions experience an internal and external world that healthy individuals would find difficult to comprehend. The present model attempts to provide a framework through which we can build a mechanistic and translational understanding of these puzzling symptoms.

Neural coding of movement direction in the healthy human brain

Neurophysiological studies in monkeys show that activity of neurons in primary cortex (M1), pre-motor cortex (PMC), and cerebellum varies systematically with the direction of reaching movements. These neurons exhibit preferred direction tuning, where the level of neural activity is highest when movements are made in the preferred direction (PD), and gets progressively lower as movements are made at increasing degrees of offset from the PD. Using a functional magnetic resonance imaging adaptation (fMRI-A) paradigm, we show that PD coding does exist in regions of the human motor system that are homologous to those observed in non-human primates. Consistent with predictions of the PD model, we show adaptation (i.e., a lower level) of the blood oxygen level dependent (BOLD) time-course signal in M1, PMC, SMA, and cerebellum when consecutive wrist movements were made in the same direction (0° offset) relative to movements offset by 90° or 180°. The BOLD signal in dorsolateral prefrontal cortex adapted equally in all movement offset conditions, mitigating against the possibility that the present results are the consequence of differential task complexity or attention to action in each movement offset condition.

Anatomical and temporal architecture of theory of mind: a MEG insight into the early stages

Representation of others' actions and mental states leads to the activation of several brain networks: the mentalizing and the "mirror neuron" systems as well as a "low level" social perception component. However, respective activations of the regions belonging to these networks remain unknown with respect to chronometrical data when static drawing stimuli are presented. To determine anatomical and temporal characteristics of theory of mind processes, magnetic signals were measured in 21 subjects during a validated nonverbal attribution of intentions task. Minimum norm estimation provides chronometric and localization data showing that regions known to be involved in the mentalizing, "mirror neuron" and social perception networks have simultaneous activations between 100 and 700 ms post-stimulus, a period which may be thought as corresponding to early stages of social processes. Among some regions, different profiles as well as modulations regarding experimental conditions suggest functional distinctions between these structures, pleading for a cooperative nature of these networks. While the left temporo-parietal area and superior temporal sulcus seem more specialized in social cues coding, we demonstrate that their right homologues, as well as the right inferior parietal cortex, are preferentially recruited during attribution of intentions stimuli compared to scenarios based on physical causality from 200 to 600 ms.

fMRI activation during observation of others' reach errors

When exposed to novel dynamical conditions (e.g., externally imposed forces), neurologically intact subjects easily adjust motor commands on the basis of their own reaching errors. Subjects can also benefit from visual observation of others' kinematic errors. Here, using fMRI, we scanned subjects watching movies depicting another person learning to reach in a novel dynamic environment created by a robotic device. Passive observation of reaching movements (whether or not they were perturbed by the robot) was associated with increased activation in fronto-parietal regions that are normally recruited in active reaching. We found significant clusters in parieto-occipital cortex, intraparietal sulcus, as well as in dorsal premotor cortex. Moreover, it appeared that part of the network that has been shown to be engaged in processing self-generated reach error is also involved in observing reach errors committed by others. Specifically, activity in left intraparietal sulcus and left dorsal premotor cortex, as well as in right cerebellar cortex, was modulated by the amplitude of observed kinematic errors.

Surface-based information mapping reveals crossmodal vision-action representations in human parietal and occipitotemporal cortex

Many lines of evidence point to a tight linkage between the perceptual and motoric representations of actions. Numerous demonstrations show how the visual perception of an action engages compatible activity in the observer's motor system. This is seen for both intransitive actions (e.g., in the case of unconscious postural imitation) and transitive actions (e.g., grasping an object). Although the discovery of "mirror neurons" in macaques has inspired explanations of these processes in human action behaviors, the evidence for areas in the human brain that similarly form a crossmodal visual/motor representation of actions remains incomplete. To address this, in the present study, participants performed and observed hand actions while being scanned with functional MRI. We took a data-driven approach by applying whole-brain information mapping using a multivoxel pattern analysis (MVPA) classifier, performed on reconstructed representations of the cortical surface. The aim was to identify regions in which local voxelwise patterns of activity can distinguish among different actions, across the visual and motor domains. Experiment 1 tested intransitive, meaningless hand movements, whereas experiment 2 tested object-directed actions (all right-handed). Our analyses of both experiments revealed crossmodal action regions in the lateral occipitotemporal cortex (bilaterally) and in the left postcentral gyrus/anterior parietal cortex. Furthermore, in experiment 2 we identified a gradient of bias in the patterns of information in the left hemisphere postcentral/parietal region. The postcentral gyrus carried more information about the effectors used to carry out the action (fingers vs. whole hand), whereas anterior parietal regions carried more information about the goal of the action (lift vs. punch). Taken together, these results provide evidence for common neural coding in these areas of the visual and motor aspects of actions, and demonstrate further how MVPA can contribute to our understanding of the nature of distributed neural representations.

Normal movement selectivity in autism

It has been proposed that individuals with autism have difficulties understanding the goals and intentions of others because of a fundamental dysfunction in the mirror neuron system. Here, however, we show that individuals with autism exhibited not only normal fMRI responses in mirror system areas during observation and execution of hand movements but also exhibited typical movement-selective adaptation (repetition suppression) when observing or executing the same movement repeatedly. Movement selectivity is a defining characteristic of neurons involved in movement perception, including mirror neurons, and, as such, these findings argue against a mirror system dysfunction in autism.

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.

Observing Learned Object-specific Functional Grasps Preferentially Activates the Ventral Stream

In one popular account of the human visual system, two streams are distinguished, a ventral stream specialized for perception and a dorsal stream specialized for action. The skillful use of familiar tools, however, is likely to involve the cooperation of both streams. Using functional magnetic resonance imaging, we scanned individuals while they viewed short movies of familiar tools being grasped in ways that were either consistent or inconsistent with how tools are typically grasped during use. Typical-for-use actions were predicted to preferentially activate parietal areas important for tool use. Instead, our results revealed several areas within the ventral stream, as well as the left posterior middle temporal gyrus, as preferentially active for our typical-for-use actions. We believe these findings reflect sensitivity to learned semantic associations and suggest a special role for these areas in representing object-specific actions. We hypothesize that during actual tool use a complex interplay between the two streams must take place, with ventral stream areas providing critical input as to how an object should be engaged in accordance with stored semantic knowledge.

Competing mechanisms for mapping action-related categorical knowledge and observed actions

Responses to pictures of famous tennis and soccer athletes are slower when the responding effector is a hand or foot, respectively, indicating that visual recognition of individuals characterized by skilled motor behavior interferes with the motor reactivity of nonproficient observers. By contrast, directly viewing actions induces motor facilitation, suggesting that actions are mapped in the observers' motor system. Here, we used single-pulse Transcranial Magnetic Stimulation to determine 1) whether observing and recognizing the identity of famous tennis and soccer athletes selectively reduce the corticospinal excitability of arm and leg representations (categorization), 2) whether any athlete-related inhibition effect contrasts the facilitation associated with direct action observation (categorization + action), and 3) whether the classic action observation-related facilitation effect is found when viewing "in action" nonathlete models (action). In 3 experiments, we found that amplitude of motor evoked potentials (MEPs) recorded from leg and arm muscles gradually shifted from reduction to facilitation, moving from the categorization to the action observation tasks. Thus, semantic derivation of motor skills is reflected in limb-specific reduction of MEP amplitude, indicating that even abstract action knowledge is embodied in the motor system and that mapping others' actions on the basis of categorization or of their direct observation relies on competing functional mechanisms.

The mirror neuron system: a neural substrate for methods in stroke rehabilitation

Mirror neurons found in the premotor and parietal cortex respond not only during action execution, but also during observation of actions being performed by others. Thus, the motor system may be activated without overt movement. Rehabilitation of motor function after stroke is often challenging due to severity of impairment and poor to absent voluntary movement ability. Methods in stroke rehabilitation based on the mirror neuron system--action observation, motor imagery, and imitation--take advantage of this opportunity to rebuild motor function despite impairments, as an alternative or complement to physical therapy. Here the authors review research into each condition of practice, and discuss the relevance of the mirror neuron system to stroke recovery.