Functional synchronization in repetitive bimanual prehension movements.

To examine the mechanisms of functional bimanual synchronization in goal-directed movements, we studied the movement kinematics of motorically unimpaired subjects while they performed repetitive prehension movements (either unimanually or bimanually) to small food items. Compared to unimanual conditions, bimanual movement execution yielded a significantly prolonged mouth contact phase. We hypothesized that this threefold prolongation led to a proper functional synchronization of the movement onsets of both hands at the beginning of each new movement cycle. That these temporal adjustments occurred in the movement phase with maximal haptic input points to the importance of sensory feedback for bimanual coordination. These results are discussed with respect to the important role of sensory feedback in the timing of coordinated bimanual movements. Furthermore, we propose that time-based coordinating schemas, which are implemented by the cerebellum and the posterior parietal cortex using sensory feedback, underlie functional inter-limb coordination.

Hand aperture patterns in prehension.

Although variations in the standard prehensile pattern can be found in the literature, these alternative patterns have never been studied systematically. This was the goal of the current paper. Ten participants picked up objects with a pincer grip. Objects (3, 5, or 7cm in diameter) were placed at 30, 60, 90, or 120cm from the hands' starting location. Usually the hand was opened gradually to a maximum immediately followed by hand closing, called the standard hand opening pattern. In the alternative opening patterns the hand opening was bumpy, or the hand aperture stayed at a plateau before closing started. Two participants in particular delayed the start of grasping with respect to start of reaching, with the delay time increasing with object distance. For larger object distances and smaller object sizes, the bumpy and plateau hand opening patterns were used more often. We tentatively concluded that the alternative hand opening patterns extended the hand opening phase, to arrive at the appropriate hand aperture at the appropriate time to close the hand for grasping the object. Variations in hand opening patterns deserve attention because this might lead to new insights into the coordination of reaching and grasping.

The agent is right: when motor embodied cognition is space-dependent.

The role of embodied mechanisms in processing sentences endowed with a first person perspective is now widely accepted. However, whether embodied sentence processing within a third person perspective would also have motor behavioral significance remains unknown. Here, we developed a novel version of the Action-sentence Compatibility Effect (ACE) in which participants were asked to perform a movement compatible or not with the direction embedded in a sentence having a first person (Experiment 1: You gave a pizza to Louis) or third person perspective (Experiment 2: Lea gave a pizza to Louis). Results indicate that shifting perspective from first to third person was sufficient to prevent motor embodied mechanisms, abolishing the ACE. Critically, ACE was restored in Experiment 3 by adding a virtual "body" that allowed participants to know "where" to put themselves in space when taking the third person perspective, thus demonstrating that motor embodied processes are space-dependent. A fourth, control experiment, by dissociating motor response from the transfer verb's direction, supported the conclusion that perspective-taking may induce significant ACE only when coupled with the adequate sentence-response mapping.

Impaired visual hand recognition in preoperative patients during brachial plexus anesthesia: importance of peripheral neural input for mental representation of the hand.

BACKGROUND: Perceptual illusions described in healthy subjects undergoing regional anesthesia (RA) are probably related to short-term plastic brain changes. We addressed whether performance on an implicit mental rotation task reflects these RA-induced changes in body schema brain representations. Studying these changes in healthy volunteers may shed light on normal function and the central mechanisms of pain. METHODS: Performance pattern was studied in upper limb-anesthetized subjects on a left/right hand judgment task, which is known to involve motor imagery processes relating to hand posture. Three conditions were used: control (i.e., absence of deafferentation), RA (i.e., deafferentation), and vision (i.e., deafferentated limb exposed to view). To limit potential bias such as order effect, the control state was recorded in a randomized manner. RESULTS: All subjects described perceptual illusions of their anesthetized limb. They were slower and less accurate on the task during RA compared with control. Response patterns were similar in all conditions, suggesting sensitivity of performance to arm/hand biomechanical constraints. Vision was associated with an increase in the proportion of correct responses and a reduction of the response times in hand judgment and was accompanied by disappearance of the lateralization of the underlying mental representations, which was identified during RA. CONCLUSIONS: These results suggest the following: (1) the right/left judgment task involves mental simulation of hand movements, (2) underlying mental representations and their neural substrates are subject to acute alterations after RA, and (3) the proprioceptive deficit induced by RA is influenced by the subject's ability to see the anesthetized limb.

Grip force is part of the semantic representation of manual action verbs.

Motor actions and action verbs activate similar cortical brain regions. A functional interference can be taken as evidence that there is a parallel treatment of these two types of information and would argue for the biological grounding of language in action. A novel approach examining the relationship between language and grip force is presented. With eyes closed and arm extended, subjects listened to words relating (verbs) or not relating (nouns) to a manual action while holding a cylinder with an integrated force sensor. There was a change in grip force when subjects heard verbs that related to manual action. Grip force increased from about 100 ms following the verb presentation, peaked at 380 ms and fell abruptly after 400 ms, signalling a possible inhibition of the motor simulation evoked by these words. These observations reveal the intimate relationship that exists between language and grasp and show that it is possible to elucidate online new aspects of sensorimotor interaction.

The sense of agency and its disturbances in schizophrenia: a reappraisal.

How it happens that one can recognise oneself as the source of one's own actions? This process of self-recognition is in fact far from trivial: although it operates covertly and effortlessly, it depends upon a set of mechanisms involving the processing of specific neural signals, from sensory as well as from central origin. In this paper, experimental situations where these signals can be dissociated from each other and where self-recognition becomes ambiguous will be used in healthy subjects and in schizophrenic patients. These situations will reveal that there are two levels of self-recognition, an automatic level for action identification, and a conscious level for the sense of agency, which both rely on the same principle of congruence of the action-related signals. The automatic level provides an immediate signal for controlling and adapting actions to their goal, whereas the conscious level provides information about the intentions, plans and desires of the author of these actions. The contribution of schizophrenic patients is to show that these two levels can be dissociated from each other. Whereas the automatic self-identification is functional in these patients, their sense of agency is deeply impaired: the first rank symptoms, which represent one of the major features of the disease, testify to the loss of the ability of schizophrenic patients to attribute their own thoughts, internal speech, covert or overt actions to themselves.

Putting oneself in the perspective of the other: a framework for self-other differentiation.

Self-other differentiation requires both that one can understand the actions others perform, and that one can attribute these actions to them. Understanding implies that a complete description of the actions of other agents can be available in the brain of the observer. Attributing implies that the agent can be clearly differentiated from the self. The model for the self-other differentiation proposed here can therefore involve two steps: I first displace myself at the location of the person I observe in order to specify her/his location in space. Then, I simulate the action I observe from that person in order to understand what she/he is doing. Assuming that the two operations are more or less synchronous, the action I simulate is automatically attributed to the person I observe, not to myself. The simulation network in the observer's brain overlaps with the execution network in the agent's brain; by contrast, the rotation network is specific to the observer's brain. In a neuroimaging experiment using PET, we compared brain activity during a task where the subjects simulated an action of grasping an object from either first-person or third-person perspectives. This comparison revealed an activation limited to the parieto-occipital junction (area 19), which appeared to be specific to the third-person perspective. The activation of this area is proposed to represent a cue for the self-other differentiation.

Subliminal display of action words interferes with motor planning: a combined EEG and kinematic study.

Recent evidence has shown that processing action-related language and motor action share common neural representations to a point that the two processes can interfere when performed concurrently. To support the assumption that language-induced motor activity contributes to action word understanding, the present study aimed at ruling out that this activity results from mental imagery of the movements depicted by the words. For this purpose, we examined cross-talk between action word processing and an arm reaching movement, using words that were presented too fast to be consciously perceived (subliminally). Encephalogram (EEG) and movement kinematics were recorded. EEG recordings of the "Readiness potential" ("RP", indicator of motor preparation) revealed that subliminal displays of action verbs during movement preparation reduced the RP and affected the subsequent reaching movement. The finding that motor processes were modulated by language processes despite the fact that words were not consciously perceived, suggests that cortical structures that serve the preparation and execution of motor actions are indeed part of the (action) language processing network.

Language-induced motor perturbations during the execution of a reaching movement.

In a recent study Boulenger et al. (2006) found that processing action verbs assisted reaching movement when the word was processed prior to movement onset and interfered with the movement when the word was processed at movement onset. The present study aimed to further corroborate the existence of such cross-talk between language processes and overt motor behaviour by demonstrating that the reaching movement can be disturbed by action words even when the words are presented delayed with respect to movement onset (50 ms and 200 ms). The results are compared to studies that show language-motor interaction in conditions where the word is presented prior to movement onset and are discussed within the context of embodied theories of language comprehension.

Word processing in Parkinson's disease is impaired for action verbs but not for concrete nouns.

Recent studies have demonstrated that processing of action words recruits cortical motor regions that are also involved in the planning and execution of the actions words refer to. The functional role of these regions in word understanding remains, however, to be clarified. The present study investigates this issue by examining the impact of Parkinson's disease (PD) on lexical decision performance for action words, relative to concrete nouns, in a masked priming paradigm. Priming effects for the two word categories were measured in non-demented PD patients off and on dopaminergic treatment, and in healthy participants. Our results revealed that although overall performances did not differ between verbs and nouns, priming effects showed a clear dissociation between word categories. While priming for concrete nouns was not affected by Levodopa intake, it dissociated as a function of treatment for action verbs. No priming was actually obtained for action verbs in PD patients off dopaminergic treatment. Following Levodopa intake, this deficit recovered, however, because priming effects for verbs became comparable to those for concrete nouns and similar to performance of healthy participants. Overall, this study thus brings compelling evidence that processing lexico-semantic information about action words depends on the integrity of the motor system.

Altered processing of sensorimotor feedback in schizophrenia.

Schizophrenia is characterized by an altered sense of the reality, associated with hallucinations and delusions. Some theories suggest that schizophrenia is related to a deficiency of the system that generates information about the sensory consequences of the actions realized by the subject. This system monitors the reafferent information resulting from an action and allows its anticipation. In the present study, we examined visual-event-related potentials (ERPs) generated by a sensorimotor task in 15 patients with schizophrenia and 15 normal controls. The visual feedback from hand movements performed by the subjects was experimentally distorted. Behavioral results showed that patients were impaired in recognizing their own movements. The ERP signal in patients also differed from those of control subjects. In patients, the ERP waveform was affected during the early part of the response (200 ms). This early effect in schizophrenic patients reveals a modified processing of the visual consequence of their actions.

Being oneself.

This paper discusses the difference between self-identification and the self/other differentiation. Self-identification relies on the congruence of self-generated movements and their expected consequences, i.e., on the temporal correlation between the set of signals (command signals, sensory reafferences) that are related to that movement. Only the origin of the movement (i.e., whether it originates from the self or not), not its destination or its goal, is relevant for self-identification. Self/other differentiation requires both that one can understand the actions others perform, and that one can attribute these actions to them. Understanding implies that a complete description of the actions of other agents, from their origin to their end, can be available in the brain of the observer. Attributing implies that the agent can be clearly differentiated from the self. The model for the self/other differentiation proposed here can therefore be described as a "rotate/simulate" model: I first rotate myself at the location of the person I observe in order to specify her location in space. Then, I simulate the action I observe from that person in order to understand what she is doing. Assuming that the two operations are more or less synchronous, the action I simulate is automatically attributed to the person I observe, not to myself. The simulation network in the observer's brain overlaps with the execution network in the agent's brain; by contrast, the rotation network is specific to the observer's brain.

Simulated actions in the first and in the third person perspectives share common representations.

Normal subjects simulated a grasping action with two levels of difficulty of the grasp. In one condition, they simulated the movement from their own, first person perspective (1P). In the other condition, they simulated the same movement made by a person facing them (third person perspective 3P). The time to complete the movement was found to be closely similar in the two conditions. Furthermore, the same difference in simulation time between easy and difficult grasps was retained in the two conditions. These results show that a self-generated and an observed action share the same representation. This representation can be used from different perspectives.

Cross-talk between language processes and overt motor behavior in the first 200 msec of processing.

A recently emerging view sees language understanding as closely linked to sensory and motor processes. The present study investigates this issue by examining the influence of processing action verbs and concrete nouns on the execution of a reaching movement. Fine-grained analyses of movement kinematics revealed that relative to nouns, processing action verbs significantly affects overt motor performance. Within 200 msec after onset, processing action verbs interferes with a concurrent reaching movement. By contrast, the same words assist reaching movement when processed before movement onset. The cross-talk between language processes and overt motor behavior provides unambiguous evidence that action words and motor action share common cortical representations and could thus suggest that cortical motor regions are indeed involved in action word retrieval.

The origin of voluntary action: history of a physiological concept.

The notion that actions are internally represented prior to be executed, and that specific brain areas are devoted to the elaboration of these representations is a challenge for neurophysiology. Questions bearing on the nature of motor centres, on their independence with respect to external stimuli, and on the possibility to monitor or even to perceive their activity are still a matter of debate. Here, we describe a thirty-year exemplary period extending between 1870 and 1900, where critical experiments and clinical observations have contributed to resolve these issues.

Prehension movements in a patient (AC) with posterior parietal cortex damage and posterior callosal section.

Prehension movements of the right hand were recorded in a right-handed man (AC), with an injury to the left posterior parietal cortex (PPC) and with a section of the left half of the splenium. The kinematic analysis of AC's grasping movements in direct and perturbed conditions was compared to that of five control subjects. A novel effect in prehension was revealed--a hemispace effect--in healthy controls only. Movements to the left hemispace were faster, longer, and with a smaller grasp aperture; perturbation of both object position and distance resulted in the attenuation of the direction effect on movement time and the time to velocity peak, with a reverse pattern in the time to maximum grip aperture. Nevertheless, the correlation between transport velocity amplitude and grasp aperture remained stable in both perturbed and non-perturbed movements, reflecting the coordination between reaching and grasping in control subjects. In contrast, transport and grasp, as well as their coordination in both direct and perturbed conditions, were negatively affected by the PPC and splenium lesion in AC, suggesting that transport and grasp rely on two functionally identifiable subsystems.

The motor theory of social cognition: a critique.

Recent advances in the cognitive neuroscience of action have considerably enlarged our understanding of human motor cognition. In particular, the activity of the mirror system, first discovered in the brain of non-human primates, provides an observer with the understanding of a perceived action by means of the motor simulation of the agent's observed movements. This discovery has raised the prospects of a motor theory of social cognition. In humans, social cognition includes the ability to mindread, and many motor theorists of social cognition try to bridge the gap between motor cognition and mindreading by endorsing a simulation account of mindreading. Here, we question the motor theory of social cognition and give reasons for our skepticism.

Neural correlates of action attribution in schizophrenia.

Patients with first-rank symptoms (FRS) of schizophrenia do not experience all of their actions and personal states as their own. FRS may be associated with an impaired ability to correctly attribute an action to its origin. In the present study, we examined regional cerebral blood flow (rCBF) with positron emission tomography during an action-attribution task in a group of patients with FRS. We used a device previously used with healthy subjects that allows the experimenter to modulate the subject's degree of movement control (and thus action attribution) of a virtual hand presented on a screen. In healthy subjects, the activity of the right angular gyrus and the insula cortex appeared to be modulated by the subject's degree of movement control of the virtual hand. In the present study, the schizophrenic patients did not show this pattern. We found an aberrant relationship between the subject's degree of control of the movements and rCBF in the right angular gyrus and no modulation in the insular cortex. The implications of these results for understanding pathological conditions such as schizophrenia are discussed.