Modulation of activity in medial frontal and motor cortices during error observation

We used measures of the human event-related brain potential (ERP) to investigate the neural mechanisms underlying error processing during action observation. Participants took part in two conditions, a task execution condition and a task observation condition. We found that activity in both the medial frontal cortex and the motor cortices, as measured via the error-related negativity and the lateralized readiness potential, respectively, was modulated by the correctness of observed behavior. These data suggest that similar neural mechanisms are involved in monitoring one's own actions and the actions of others.     

Judging hand laterality from my or your point of view: Interactions between motor imagery and visual perspective

Motor imagery tasks (hand laterality judgment) are usually performed with respect to a self-body (egocentric) representation, but manipulations of stimulus features (hand orientation) can induce a shift to other's body (allocentric) reference frame. Visual perspective taking tasks are also performed in self-body perspective but a shift to an allocentric frame can be triggered by manipulations of context features (e.g., another person present in the to-be-judged scene). Combining hand laterality task and visual perspective taking, we demonstrated that both stimulus and context features can modulate motor imagery performance. In Experiment 1, participants judged laterality of a hand embedded in a human or non-human silhouette. Results showed that observing a human silhouette interfered with judgments on “egocentric hand stimuli” (right hand, fingers up). In Experiment 2, participants were explicitly required to judge laterality of a hand embedded in a human silhouette from their own (egocentric group) or from the silhouette's perspective (allocentric group). Consistent with previous results, the egocentric group was significantly faster than the allocentric group in judging fingers-up right hand stimuli. These findings showed that concurrent activation of egocentric and allocentric frames during mental transformation of body parts impairs participants’ performance due to a conflict between motor and visual mechanisms.     

Giving a helping hand: effects of joint attention on mental rotation of body parts

Research on joint attention has addressed both the effects of gaze following and the ability to share representations. It is largely unknown, however, whether sharing attention also affects the perceptual processing of jointly attended objects. This study tested whether attending to stimuli with another person from opposite perspectives induces a tendency to adopt an allocentric rather than an egocentric reference frame. Pairs of participants performed a handedness task while individually or jointly attending to rotated hand stimuli from opposite sides. Results revealed a significant flattening of the performance rotation curve when participants attended jointly (experiment 1). The effect of joint attention was robust to manipulations of social interaction (cooperation versus competition, experiment 2), but was modulated by the extent to which an allocentric reference frame was primed (experiment 3). Thus, attending to objects together from opposite perspectives makes people adopt an allocentric rather than the default egocentric reference frame.     

Motor cortex activation induced by a mirror: evidence from lateralized readiness potentials

Similar motor regions are activated during voluntarily executed or observed movements. We investigated whether observing movements of one's own hand through a mirror will generate activations in the cortical motor regions of both the moving and nonmoving hands. Using the lateralized readiness potential (LRP), an electrophysiological correlate of premotor activation in the primary motor cortex, we recorded evoked responses to movements while subjects were viewing the performing (right) hand through a mirror placed sagittally, giving the impression that the left hand was performing the task. Reliable LRPs were recorded in relation to the seen hand, indicating motor cortex activity in the contralateral hemisphere of the inactive hand while the opposite hand was performing the movement.     

Perceptual and not physical eye contact elicits pupillary dilation

Eye contact is important to share communication during social interactions. However, how accurately humans can perceive the gaze direction of others toward themselves and whether pupils dilate when humans consciously or unconsciously perceive own eyes are looked by others remain unclear. In this study, we examined the relationship between the explicit perception of looking into each other's eyes and the implicit physiological response of pupillary dilation by using an original face-to-face method. We found that humans do not correctly detect the gaze direction of others. Furthermore, one's pupils dilated when one gazed at others’ eyes. Awareness of others’ gaze on one's eyes, rather than the actual focusing of other's gaze on one's eyes, enhanced pupillary dilation. Therefore, physiological responses are caused not when people actually look into each other's gaze, but when the consciousness of other's gaze is activated, which suggests that eye contact often involves one-way communication.     

Hand–object interaction in perspective

We investigated the effect of perspective on the recognition of actions, without using motor preparation. Photographs of a hand wearing a glove were presented as primes, followed by photographs of the same hand interacting with an object. Both primes and targets were shown in egocentric or non-egocentric perspective. Participants had to decide whether or not the hand interacted with the object in a sensible way. In order to increase the similarity between the perceived and the enacted movement, half of the participants were required to wear a glove while responding. We found an advantage of the egocentric over the non-egocentric perspective for targets in the Glove condition. The advantage of the egocentric perspective was present for primes as well, even though the effect was limited to the No Glove condition. Results are discussed in the framework of the recent literature on mirror neurons and body schema.     

Does Parkinson’s disease affect judgement about another person’s action?

The observer’s motor system has been shown to be involved in observing the actions of another person. Recent findings suggest that people with Parkinson’s disease do not show the same motor facilitatory effects when observing the actions of another person. We studied whether Parkinson’s patients were able to make unspeeded judgements about another person’s action. Participants were asked to watch video clips of an actor lifting a box containing different weights (100, 200, 300 or 400 g) and to guess the weight that was being lifted on a 9-point scale. We compared the performance of 16 patients with PD with 16 healthy age-matched controls. Both groups were able to do the task, showing a significant relationship between the real weight and the guessed weight, albeit with a tendency to overestimate the lowest weight and underestimate the heaviest weight. The PD patients, however, showed a reduced slope value. These results show that despite their own motor deficits, PD patients are still able to judge the weight being lifted by another person, albeit with a slight reduction in accuracy. Further research will be required to determine whether PD patients use a motor simulation or a visual compensatory strategy to achieve this.     

Numbers are represented in egocentric space: Effects of numerical cues and spatial reference frames on hand laterality judgements

Convergent findings demonstrate that numbers can be represented according to a spatially oriented mental number line. However, it is not established whether a default organization of the mental number line exists (i.e., a left-to-right orientation) or whether its spatial arrangement is only the epiphenomenon of specific task requirements. To address this issue we performed two experiments in which subjects were required to judge laterality of hand stimuli preceded by small, medium or large numerical cues; hand stimuli were compatible with egocentric or allocentric perspectives. We found evidence of a left-to-right number–hand association in processing stimuli compatible with an egocentric perspective, whereas the reverse mapping was found with hands compatible with an allocentric perspective. These findings demonstrate that the basic left-to-right arrangement of the mental number line is defined with respect to the body-centred egocentric reference frame.     

Imagining others’ handedness: visual and motor processes in the attribution of the dominant hand to an imagined agent

In a previous study, we found that when required to imagine another person performing an action, participants reported a higher correspondence between their own dominant hand and the hand used by the imagined person when the agent was visualized from the back compared to when the agent was visualized from the front. This suggests a greater involvement of motor representations in the back-view perspective, possibly indicating a greater proneness to put oneself in the agent’s shoes in such a condition. In order to assess whether bringing to the foreground the right or left hand of an imagined agent can foster the activation of the corresponding motor representations, we required 384 participants to imagine a person—as seen from the right or left side—performing a single manual action and to indicate the hand used by the imagined person during movement execution. The proportion of right- versus left-handed reported actions was higher in the right-view condition than in the left-view condition, suggesting that a lateral vantage point may activate the corresponding hand motor representations, which is in line with previous research indicating a link between the hemispheric specialization of one’s own body and the visual representation of others’ bodies. Moreover, in agreement with research on hand laterality judgments, the effect of vantage point was stronger for left-handers (who reported a higher proportion of right- than left-handed actions in the right-view condition and a slightly higher proportion of left- than right-handed actions in the left-view condition) than for right-handers (who reported a higher proportion of right- than left-handed actions in both view conditions), indicating that during the mental simulation of others’ actions, right-handers rely on sensorimotor processes more than left-handers, while left-handers rely on visual processes more than right-handers.     

Lateralized readiness potential elicited by undetected visual stimuli

Visual stimuli undetected by normal subjects as a result of masking procedures can nonetheless activate response preparation in motor areas and yield a motor response. An unanswered question is whether the same holds for undetected subliminal stimuli that are not responded to. To answer this question, in this study normal subjects were tested on a simple visual reaction time task with stimuli above, at, or below the psychophysical threshold while the lateralized readiness potential (LRP), i.e. an electrophysiological correlate of premotor activation in the primary motor cortex, was computed. We found a reliable LRP not only for suprathreshold stimuli but also for subthreshold stimuli to which subjects did not respond. The main thrust of this study is that it provides evidence that activation of the motor cortex occurs even with subthreshold visual stimuli and without an overt response.     

Joint action modulates motor system involvement during action observation in 3-year-olds

When we are engaged in a joint action, we need to integrate our partner's actions with our own actions. Previous research has shown that in adults the involvement of one's own motor system is enhanced during observation of an action partner as compared to during observation of an individual actor. The aim of this study was to investigate whether similar motor system involvement is present at early stages of joint action development and whether it is related to joint action performance. In an EEG experiment with 3-year-old children, we assessed the children's brain activity and performance during a joint game with an adult experimenter. We used a simple button-pressing game in which the two players acted in turns. Power in the mu- and beta-frequency bands was compared when children were not actively moving but observing the experimenter's actions when (1) they were engaged in the joint action game and (2) when they were not engaged. Enhanced motor involvement during action observation as indicated by attenuated sensorimotor mu- and beta-power was found when the 3-year-olds were engaged in the joint action. This enhanced motor activation during action observation was associated with better joint action performance. The findings suggest that already in early childhood the motor system is differentially activated during action observation depending on the involvement in a joint action. This motor system involvement might play an important role for children's joint action performance.     

Facilitation and interference components in the joint Simon task

Two experiments were conducted to assess whether the joint Simon effect is composed of facilitation and interference and whether facilitation is increased by a joint spatially compatible practice performed before performing the joint Simon task. In both experiments, participants were required to perform a Simon task along another person. Trials could be corresponding, non-corresponding, and neutral. In Experiment 1, participants performed only the Simon task. In Experiment 2, participants first practiced on a joint spatial compatibility task with a compatible mapping and, after a 5-min delay, transferred to a joint Simon task. Results indicated that the joint Simon effect consisted primarily of interference, which was significantly increased by a spatially compatible practice performed jointly. These results allow us to better define in what ways the presence of the other influences performance, in showing that when participants perform a task along with another individual, they display a disadvantage (i.e., slower RTs) when they have to respond to stimuli appearing on the other agent's side.     

Various levels of plasma brain-derived neurotrophic factor in patients with tinnitus

The investigation of brain areas involved in the human execution/observation matching system (EOM) has been limited to restricted motor actions when using common neuroimaging techniques such as functional magnetic resonance imaging (fMRI). A method which overcomes this limitation is functional near-infrared spectroscopy (fNIRS). In the present study, we explored the cerebral responses underlying action execution and observation during a complex everyday task. We measured brain activation of 39 participants during the performance of object-related reaching, grasping and displacing movements, namely setting and clearing a table, and observation of the same task from different perspectives. Observation of the table-setting task activated parts of a network matching those activated during execution of the task. Specifically, observation from an egocentric perspective led to a higher activation in the inferior parietal cortex than observation from an allocentric perspective, implicating that the viewpoint also influences the EOM during the observation of complex everyday tasks. Together these findings suggest that fNIRS is able to overcome the restrictions of common imaging methods by investigating the EOM with a naturalistic task.     

A system in the human brain for predicting the actions of others

The ability to attribute mental states to others, and therefore to predict others' behavior, is particularly advanced in humans. A controversial but untested idea is that this is achieved by simulating the other person's mental processes in one's own mind. If this is the case, then the same neural systems activated by a mental function should re-activate when one thinks about that function performed by another. Here, using functional magnetic resonance imaging (fMRI), we tested whether the neural processes involved in preparing one's own actions are also used for predicting the future actions of others. We provide compelling evidence that areas within the action control system of the human brain are indeed activated when predicting others' actions, but a different action sub-system is activated when preparing one's own actions.     

Far from action-blind: Representation of others' actions in individuals with Autism

It has been suggested that theory of mind may rely on several precursors including gaze processing, joint attention, the ability to distinguish between actions of oneself and others, and the ability to represent goal-directed actions. Some of these processes have been shown to be impaired in individuals with autism, who experience difficulties in theory of mind. However, little is known about action representation in autism. Using two variants of a spatial compatibility reaction time (RT) task, we addressed the question of whether high-functioning individuals with autism have difficulties in controlling their own actions and in representing those of others. Participants with autism showed automatic response activation and had no difficulties with response inhibition. When two action alternatives were distributed among pairs of participants, participants with autism represented a co-actor's task, showing the same pattern of results as the matched control group. We discuss the possibility that in high-functioning individuals with autism, the system matching observed actions onto representations of one's own actions is intact, whereas difficulties in higher-level processing of social information persist.     

Brain activity associated with recognition of appropriate action selection based on allocentric perspectives

We investigated brain activity associated with recognition of appropriate action selection based on allocentric perspectives using functional magnetic resonance imaging. The participants observed video clips in which one person (responder) passed one of three objects after a request by a second person (requester). The requester was unable to see one of the three objects because it was occluded by another object. Participants were asked to judge the appropriateness of the responder's action selection based on the visual information from the requester's perspective (i.e., allocentric perspective), not the responder's perspective (i.e., egocentric perspective). The experimental factors included the congruency of request interpretation and the appropriateness of action selection. The results showed that brain regions including the right temporo-parieto-occipital (TPO) junction and the left inferior parietal lobule (IPL) were more activated when the interpretation of the requested object differed between the egocentric and allocentric perspectives than when it was the same (the effect of incongruency for consistency). On the other hand, greater activation was found in the right dorsolateral prefrontal cortex (DLPFC) when the incongruency effect was compared only between the conditions of appropriate action selection (the interaction effect). These results suggest that both the TPO junction and IPL are involved in obtaining visual information from the allocentric perspective when visual information based on only the egocentric perspective is insufficient to interpret another person's request. The right DLPFC is likely related to this process to override the interference of action selection based on the egocentric perspective.     

Internalizing agency of self-action: perception of one's own hand movements depends on an adaptable prediction about the sensory action outcome

Extensive work on learning in reaching and pointing tasks has demonstrated high degrees of plasticity in our ability to optimize goal-directed motor behavior. However, studies focusing on the perceptual awareness of our own actions during motor adaptation are still rare. Here we present the first simultaneous investigation of sensorimotor adaptation on both levels, i.e., action and action perception. We hypothesized that self-action perception relies on internal predictions about the sensory action outcome that are updated in a way similar to that of motor control. Twenty human subjects performed out-and-back pointing movements that were fed back visually. Feedback was initially presented in spatiotemporal correspondence with respect to the actual finger position, but later rotated by a constant angle. When distorted feedback was applied repetitively, subjects' perceived pointing direction shifted in the direction of the trajectory rotation. A comparable perceptual reinterpretation was observed in control trials without visual feedback, indicating that subjects learned to predict the new visual outcome of their actions based on nonvisual, internal information. The perception of the world, however, remained unchanged. The changes in perception of one's own movements were accompanied by adaptive changes in motor performance of the same amount, i.e., a secondary motor compensation opposite to the direction of the imposed visual rotation. Our results show that the perception of one's own actions depends on adaptable internal predictions about the sensory action outcome, allowing us to attribute new sensory consequences of our actions to our own agency. Furthermore, they indicate that the updated sensory prediction can be used to optimize motor control.     

Motor activation prior to observation of a predicted movement

Previous research has shown that some of the same motor regions are activated both when performing and when observing a movement. Here we demonstrate in human subjects that such motor activity also occurs prior to observing someone else's action. This suggests that the mere knowledge of an upcoming movement is sufficient to excite one's own motor system, enabling people to anticipate, rather than react to, others' actions.     

Perspective taking eliminates differences in co-representation of out-group members�� actions

Coordinated action relies on shared representations between interaction partners: people co-represent actions of others in order to respond appropriately. However, little is known about the social factors that influence shared representations. We investigated whether actions performed by in-group and out-group members are represented differently, and if so, what role perspective-taking plays in this process. White participants performed a joint Simon task with an animated image of a hand with either white or black skin tone. Results of study I demonstrated that actions performed by in-group members were co-represented while actions of out-group members were not. In study II, it was found that participants co-represented actions of out-group members when they had read about an out-group member and to take his perspective prior to the actual experiment. Possible explanations for these findings are discussed.     

The functions of the medial premotor cortex

Monkeys with medial premotor cortex (MPC) lesions are impaired on a simple learned task that requires them to raise their arm at their own pace. However, they can succeed on this task if they are given tones to guide performance. In the externally paced task the tones could aid performance in several ways. They tell the animal when to act (trigger), they remind the animal that food is available and so motivate (predictor), and they remind the animal of what to do (instruction). Monkeys with MPC lesions can respond quickly to visual cues (experiment 1), and they can respond as well as normal monkeys when there is no immediate trigger (experiment 2). They are also quick to relearn a task in which external cues tell them what to do (experiment 5). However, they are poor at selecting between movements on a simple motor sequence task (experiment 3), and they are poor at changing between two movements (experiment 4). On these tasks there were cues to act as triggers and predictors, but there were no external instructions. We conclude that the reason why animals with MPC lesions perform better with external cues is that these cues act as instructions. The cues prompt retrieval of the appropriate action. This is true whether the task requires the animal to perform one action (experiments 1 and 2) or to select between actions (experiments 3 and 4).