Mapping motor representations with positron emission tomography

A PET exploration of the neural mechanisms involved in reciprocal imitation

Imitation is a natural mechanism involving perception-action coupling which plays a central role in the development of understanding that other people, like the self, are mental agents. PET was used to examine the hemodynamic changes occurring in a reciprocal imitation paradigm. Eighteen subjects (a) imitated the actions of the experimenter, (b) had their actions imitated by the experimenter, (c) freely produced actions, or (d) freely produced actions while watching different actions made by the experimenter. In a baseline condition, subjects simply watched the experimenter's actions. Specific increases were detected in the left STS and in the inferior parietal cortex in conditions involving imitation. The left inferior parietal is specifically involved in producing imitation, whereas the right homologous region is more activated when one's own actions are imitated by another person. This pattern of results suggests that these regions play a specific role in distinguishing internally produced actions from those generated by others.

Organization of inputs from cingulate motor areas to basal ganglia in macaque monkey

The cingulate motor areas reside within regions lining the cingulate sulcus and are divided into rostral and caudal parts. Recent studies suggest that the rostral and caudal cingulate motor areas participate in distinct aspects of motor function: the former plays a role in higher-order cognitive control of movements, whereas the latter is more directly involved in their execution. Here, we investigated the organization of cingulate motor areas inputs to the basal ganglia in the macaque monkey. Identified forelimb representations of the rostral and caudal cingulate motor areas were injected with different anterograde tracers and the distribution patterns of labelled terminals were analysed in the striatum and the subthalamic nucleus. Corticostriatal inputs from the rostral and caudal cingulate motor areas were located within the rostral striatum, with the highest density in the striatal cell bridges and the ventrolateral portions of the putamen, respectively. There was no substantial overlap between these input zones. Similarly, a certain segregation of input zones from the rostral and caudal cingulate motor areas occurred along the mediolateral axis of the subthalamic nucleus. It has also been revealed that corticostriatal and corticosubthalamic input zones from the rostral cingulate motor area considerably overlapped those from the presupplementary motor area, while the input zones from the caudal cingulate motor area displayed a large overlap with those from the primary motor cortex. The present results indicate that a parallel design underlies motor information processing in the cortico-basal ganglia loop derived from the rostral and caudal cingulate motor areas.

Grasp with hand and mouth: a kinematic study on healthy subjects

Neurons involved in grasp preparation with hand and mouth were previously recorded in the premotor cortex of monkey. The aim of the present kinematic study was to determine whether a unique planning underlies the act of grasping with hand and mouth in humans as well. In a set of four experiments, healthy subjects reached and grasped with the hand an object of different size while opening the mouth (experiments 1 and 3), or extending the other forearm (experiment 4), or the fingers of the other hand (experiment 5). In a subsequent set of three experiments, subjects grasped an object of different size with the mouth, while opening the fingers of the right hand (experiments 6-8). The initial kinematics of mouth and finger opening, but not of forearm extension, was affected by the size of the grasped object congruently with the size effect on initial grasp kinematics. This effect was due neither to visual presentation of the object, without the successive grasp motor act (experiment 2) nor to synchronism between finger and mouth opening (experiments 3, 7, and 8). In experiment 9 subjects grasped with the right hand an object of different size while pronouncing a syllable printed on the target. Mouth opening and sound production were affected by the grasped object size. The results of the present study are discussed according to the notion that in an action each motor act is prepared before the beginning of the motor sequence. Double grasp preparation can be used for successive motor acts on the same object as, for example, grasping food with the hand and ingesting it after bringing it to the mouth. We speculate that the circuits involved in double grasp preparation might have been the neural substrate where hand motor patterns used as primitive communication signs were transferred to mouth articulation system. This is in accordance with the hypothesis that Broca's area derives phylogenetically from the monkey premotor area where hand movements are controlled.

Functional brain mapping of monkey tool use

When using a tool, we can perceive a psychological association between the tool and the body parts-the tool is incorporated into our "body-image." During tool use, visual response properties of bimodal (tactile and visual) neurons in the intraparietal area of the monkey's cerebral cortex were modified to include the hand-held tool. Visual properties of the monkey intraparietal neurons may represent the body-image in the brain. We explored tool use-induced activation within the intraparietal area and elsewhere in alert monkey brain using positron emission tomography (PET). Tool use-related activities compared with the control condition (simple-stick manipulation) revealed a significant increase in cerebral blood flow in the corresponding intraparietal region, basal ganglia, presupplementary motor area, premotor cortex, and cerebellum. These tool use-specific areas may participate in maintaining and updating the body-image for the precise guidance of a hand-held rake onto a distant reward.

Motor and visual imagery as two complementary but neurally dissociable mental processes

Recent studies indicate that covert mental activities, such as simulating a motor action and imagining the shape of an object, involve shared neural representations with actual motor performance and with visual perception, respectively. Here we investigate the performance, by normal individual and subjects with a selective impairment in either motor or visual imagery, of an imagery task involving a mental rotation. The task involved imagining a hand in a particular orientation in space and making a subsequent laterality judgement. A simple change in the phrasing of the imagery instructions (first-person or third-person imagery) and in actual hand posture (holding the hands on the lap or in the back) had a strong impact on response time (RT) in normal subjects, and on response accuracy in brain-damaged subjects. The pattern of results indicates that the activation of covert motor and visual processes during mental imagery depends on both top-down and bottom-up factors, and highlights the distinct but complementary contribution of covert motor and visual processes during mental rotation.

Identification of multiple nonprimary motor cortical areas with simple movements

The human cortex reportedly contains at least five nonprimary motor areas: in the frontolateral convexity, the dorsal and ventral premotor cortex (PMd and PMv), and in the frontomesial wall, the presupplementary and supplementary motor areas (pre-SMA and SMA), and the rostral, dorsal and ventral cingulate areas (CMAr, CMAd, and CMAv). Activation of these regions in neuroimaging studies has been generally associated either with the performance of complex motor tasks or with reorganization occurring with motor recovery in the presence of pathology. Recent evidence from neuroimaging studies suggests that the same areas are activated with well controlled simple movements in healthy subjects providing support to the observation that their contribution may be more quantitative rather than exclusively specific to a certain aspect of motor behaviour. An important consequence of this observation is that activation of multiple nonprimary motor areas during simple motor tasks should not be considered unique to patients with upper or lower motoneuron lesions but rather as a normal physiological process.

An fMRI study of imagined self-rotation

In the present study, functional magnetic resonance imaging was used to examine the neural mechanisms involved in the imagined spatial transformation of one's body. The task required subjects to update the position of one of four external objects from memory after they had performed an imagined self-rotation to a new position. Activation in the rotation condition was compared with that in a control condition in which subjects located the positions of objects without imagining a change in self-position. The results indicated similar networks of activation to other egocentric transformation tasks involving decisions about body parts. The most significant area of activation was in the left posterior parietal cortex. Other regions of activation common among several of the subjects were secondary visual, premotor, and frontal lobe regions. These results are discussed relative to motor and visual imagery processes as well as to the distinctions between the present task and other imagined egocentric transformation tasks.

Different brain correlates for watching real and virtual hand actions

We investigated whether observation of actions reproduced in three-dimensional virtual reality would engage perceptual and visuomotor brain processes different from those induced by the observation of real hand actions. Participants were asked to passively observe grasping actions of geometrical objects made by a real hand or by hand reconstructions of different quality in 3D virtual reality as well as on a 2D TV screen. We found that only real actions in natural environment activated a visuospatial network including the right posterior parietal cortex. Observation of virtual-reality hand actions engaged prevalent visual perceptual processes within lateral and mesial occipital regions. Thus, only perception of actions in reality maps onto existing action representations, whereas virtual-reality conditions do not access the full motor knowledge available to the central nervous system.

The attentive homunculus: now you see it, now you don't

The nature of the neural system that directs our attention toward selective items in the extrapersonal world is a longstanding and interesting puzzle. The ability to image the human brain at work non-invasively using positron-emission tomography or functional magnetic resonance has provided the means to investigate this issue. In this article, I review the contributions of brain imaging toward the characterization of attentional control in the human brain. The majority of experiments to date have investigated visual spatial orienting. A consistent pattern of brain areas has been revealed, comprising most notably the posterior parietal cortex around the intraparietal sulcus and frontal regions including the frontal eye fields. The brain areas implicated in the control of visual spatial attention were noted to resemble those involved in the control of eye movements, and direct experimental comparisons supported a tight link between the two systems. The findings suggested a sensible view of the attentional 'homunculus' as a distributed neural system related to the control of eye movements. Eye movements form perhaps the most basic orienting response, and can be shifted rapidly and efficiently based on multiple frames of reference. Some attention experiments using objects and features instead of spatial locations as the target of selection also obtained similar patterns of parietal-frontal activations, rendering further support to this view of the attentional control system. Some recent experiments, however, have cautioned against a premature conclusion regarding the ubiquity of the attentional control system revealed by studies of visual spatial attention. Different parietal and frontal regions become engaged when attention is shifted along non-spatial dimensions, such as when attention is directed toward a particular motor act or toward a specific point in time. In these cases, the neural system resembles those involved in the control of limb movements. The attentional homunculus thus begins to dissolve. The alternative view suggested is that attentional control may be a property of specialized parietal-frontal systems that transform perception into action. Future studies will be needed to validate this view of attention, or to provide a more mature understanding of its true nature.

Reactivation of motor brain areas during explicit memory for actions

Recent functional brain imaging studies have shown that sensory-specific brain regions that are activated during perception/encoding of sensory-specific information are reactivated during memory retrieval of the same information. Here we used PET to examine whether verbal retrieval of action phrases is associated with reactivation of motor brain regions if the actions were overtly or covertly performed during encoding. Compared to a verbal condition, encoding by means of overt as well as covert activity was associated with differential activity in regions in contralateral somatosensory and motor cortex. Several of these regions were reactivated during retrieval. Common to both the overt and covert conditions was reactivation of regions in left ventral motor cortex and left inferior parietal cortex. A direct comparison of the overt and covert activity conditions showed that activation and reactivation of left dorsal parietal cortex and right cerebellum was specific to the overt condition. These results support the reactivation hypothesis by showing that verbal-explicit memory of actions involves areas that are engaged during overt and covert motor activity.

The attentional role of the left parietal cortex: the distinct lateralization and localization of motor attention in the human brain

It is widely agreed that visuospatial orienting attention depends on a network of frontal and parietal areas in the right hemisphere. It is thought that the visuospatial orienting role of the right parietal lobe is related to its role in the production of overt eye movements. The experiments reported here test the possibility that other parietal regions may be important for directing attention in relation to response modalities other than eye movement. Specifically, we used positron emission tomography (PET) to test the hypothesis that a 'left' parietal area, the supramarginal gyrus, is important for attention in relation to limb movements (Rushworth et al., 1997; Rushworth, Ellison, & Walsh, in press). We have referred to this process as 'motor attention' to distinguish it from orienting attention. In one condition subjects spent most of the scanning period covertly attending to 'left' hand movements that they were about to make. Activity in this first condition was compared with a second condition with identical stimuli and movement responses but lacking motor attention periods. Comparison of the conditions revealed that motor attention-related activity was almost exclusively restricted to the 'left' hemisphere despite the fact that subjects only ever made ipsilateral, left-hand responses. Left parietal activity was prominent in this comparison, within the parietal lobe the critical region for motor attention was the supramarginal gyrus and the adjacent anterior intraparietal sulcus (AIP), a region anterior to the posterior parietal cortex identified with orienting attention. In a second part of the experiment we compared a condition in which subjects covertly rehearsed verbal responses with a condition in which they made verbal responses immediately without rehearsal. A comparison of the two conditions revealed verbal rehearsal-related activity in several anterior left hemisphere areas including Broca's area. The lack of verbal rehearsal-related activity in the left supra-marginal gyrus confirms that this area plays a direct role in motor attention that cannot be attributed to any strategy of verbal mediation. The results also provide evidence concerning the importance of ventral premotor (PMv) and Broca's area in motor attention and language processes.

Cortical correlates of gesture processing: clues to the cerebral mechanisms underlying apraxia during the imitation of meaningless gestures

The clinical test of imitation of meaningless gestures is highly sensitive in revealing limb apraxia after dominant left brain damage. To relate lesion locations in apraxic patients to functional brain activation and to reveal the neuronal network subserving gesture representation, repeated H2(15O)-PET measurements were made in seven healthy subjects during a gesture discrimination task. Observing paired images of either meaningless hand or meaningless finger gestures, subjects had to indicate whether they were identical or different. As a control condition subjects simply had to indicate whether two portrayed persons were identical or not. Brain activity during the discrimination of hand gestures was strongly lateralized to the left hemisphere, a prominent peak activation being localized within the inferior parietal cortex (BA40). The discrimination of finger gestures induced a more symmetrical activation and rCBF peaks in the right intraparietal sulcus and in medial visual association areas (BA18/19). Two additional foci of prominent rCBF increase were found. One focus was located at the left lateral occipitotemporal junction (BA 19/37) and was related to both tasks; the other in the pre-SMA was particularly related to hand gestures. The pattern of task-dependent activation corresponds closely to the predictions made from the clinical findings, and underlines the left brain dominance for meaningless hand gestures and the critical involvement of the parietal cortex. The lateral visual association areas appear to support first stages of gesture representation, and the parietal cortex is part of the dorsal action stream. Finger gestures may require in addition precise visual analysis and spatial attention enabled by occipital and right intraparietal activity. Pre-SMA activity during the perception of hand gestures may reflect engagement of a network that is intimately related to gesture execution.

Neural Simulation of Action: A Unifying Mechanism for Motor Cognition

Paradigms drawn from cognitive psychology have provided new insight into covert stages of action. These states include not only intending actions that will eventually be executed, but also imagining actions, recognizing tools, learning by observation, or even understanding the behavior of other people. Studies using techniques for mapping brain activity, probing cortical excitability, or measuring the activity of peripheral effectors in normal human subjects and in patients all provide evidence of a subliminal activation of the motor system during these cognitive states. The hypothesis that the motor system is part of a simulation network that is activated under a variety of conditions in relation to action, either self-intended or observed from other individuals, will be developed. The function of this process of simulation would be not only to shape the motor system in anticipation to execution, but also to provide the self with information on the feasibility and the meaning of potential actions.

The brain that plays music and is changed by it

Playing a musical instrument demands extensive procedural and motor learning that results in plastic reorganization of the human brain. These plastic changes seem to include the rapid unmasking of existing connections and the establishment of new ones. Therefore, both functional and structural changes take place in the brain of instrumentalists as they learn to cope with the demands of their activity. Neuroimaging techniques allow documentation of these plastic changes in the human brain. These plastic changes are fundamental to the accomplishment of skillful playing, but they pose a risk for the development of motor control dysfunctions that may give rise to overuse syndromes and focal, task-specific dystonia.

Effect of subjective perspective taking during simulation of action: a PET investigation of agency

Perspective taking is an essential component in the mechanisms that account for intersubjectivity and agency. Mental simulation of action can be used as a natural protocol to explore the cognitive and neural processing involved in agency. Here we took PET measurements while subjects simulated actions with either a first-person or a third-person perspective. Both conditions were associated with common activation in the SMA, the precentral gyrus, the precuneus and the MT/V5 complex. When compared to the first-person perspective, the third-person perspective recruited right inferior parietal, precuneus, posterior cingulate and frontopolar cortex. The opposite contrast revealed activation in left inferior parietal and somatosensory cortex. We suggest that the right inferior parietal, precuneus and somatosensory cortex are specifically involved in distinguishing self-produced actions from those generated by others.

Integrating cognitive psychology, neurology and neuroimaging

In the last decade, there has been a dramatic increase in research effectively integrating cognitive psychology, functional neuroimaging, and behavioral neurology. This new work is typically conducting basic research into aspects of the human mind and brain. The present review features as examples of such integrations two series of studies by the author and his colleagues. One series, employing object recognition, mental motor imagery, and mental rotation paradigms, clarifies the nature of a cognitive process, imagined spatial transformations used in shape recognition. Among other implications, it suggests that when recognizing a hand's handedness, imagining one's body movement depends on cerebrally lateralized sensory-motor structures and deciding upon handedness depends on exact match shape confirmation. The other series, using cutaneous, tactile, and auditory pitch discrimination paradigms, elucidates the function of a brain structure, the cerebellum. It suggests that the cerebellum has non-motor sensory support functions upon which optimally fine sensory discriminations depend. In addition, six key issues for this integrative approach are reviewed. These include arguments for the value and greater use of: rigorous quantitative meta-analyses of neuroimaging studies; stereotactic coordinate-based data, as opposed to surface landmark-based data; standardized vocabularies capturing the elementary component operations of cognitive and behavioral tasks; functional hypotheses about brain areas that are consistent with underlying microcircuitry; an awareness that not all brain areas implicated by neuroimaging or neurology are necessarily directly involved in the associated cognitive or behavioral task; and systematic approaches to integrations of this kind.

A history and review of quantitative electroencephalography in traumatic brain injury

The electroencephalogram (EEG) is a physiologic measure of cerebral function that has been used by some to assess coma and prognosticate survival and global outcome after traumatic brain injury (TBI). Surface recordings of the brain's electrical activity reveal distinct patterns that indicate injury severity, depth of unconsciousness, and patient survival. The data produced with traditional qualitative studies, however, does not allow resolution and quantification of the wave frequency spectrum present in the brain. As a result, conventional EEG typically has only been used for gross and qualitative analyses and is not practical for use in long-term patient monitoring or as a sophisticated prognostic tool. One area of investigation that is working to address the limitations of conventional EEG has been the development and implementation of Fourier Transform (FT) EEG which resolves and quantifies frequency bands present in the brain. When FT analysis is applied to EEG, it provides concurrent and continuous monitoring, resolution, and quantification of all frequencies emitted. This review discusses the history and significance of conventional EEG and provides a review of how FT-EEG, commonly referred to as Quantitative EEG (QEEG), is being used in the clinical setting. The specific applications and significance of QEEG methods regarding treatment of patients with TBI are discussed in detail. The advantages, disadvantages, and future directions of QEEG in TBI are also discussed.

Preserved Adjustment but Impaired Awareness in a Sensory-Motor Conflict following Prefrontal Lesions

Control of action occurs at different stagesof the executive process, in particular at those of sensory-motor integration and conscious monitoring. The aim of this study was to determine the implication of the prefrontal cortex in the control of action. For that purpose, we compared the performance of 15 patients with frontal lobe lesions and 15 matched controls on an experimental paradigm generating a conflict between the action planned and the sensory-motor feedback. Subjects had to trace a sagittal line witha stylus on a graphic tablet. The hand was hidden by a mirror on which the traced line, processed by a computer, was projected. Without informing the subjects, the line traced was modified by introducing a bias to the right, which increased progressively from 2° to 42°. To succeed the task, subjects had to modify their motor program and deviate their hand in the opposite direction. The sensory-motor adjustment to the bias was evaluated by the surface between the line traced and the ideal line to compensate for the deviation. The awareness of the conflict was measured by the angle of the bias at which subjects expressed the feeling that the line they traced was not the same as the line they saw. The deviation was similarly compensated for by patients and controls until24°. Then 14 controls but only3 patients were aware of a conflict. After that, the variability of performance increased significantly for the unaware patients. These results suggest that the prefrontal cortex is required at the level of conscious monitoring of actions, but not at the level of sensory-motor integration.

Within grasp but out of reach: evidence for a double dissociation between imagined hand and arm movements in the left cerebral hemisphere

What roles are played by the cerebral hemispheres in planning object-oriented reaching and grasping movements? In an attempt to address this question, we compared the abilities of the left and right hemispheres of commissurotomy patient J.W. to imagine hand manipulation (i.e., grasp) or arm transportation (i.e., reach) movements. A graphically rendered manipulandum (dowel) was briefly presented to the left (LVF) or right (RVF) visual fields in a variety of different orientations. In the grasp selection task (experiment 1), J.W. was required to determine which side of a dowel his thumb would be on if he were to engage the stimulus in a power grip using either his dominant (right) or non-dominant hand. In the reach selection task (experiment 3), J.W. judged which end his elbow would be on if he treated the dowel as an armrest for his dominant or non-dominant forearm. No actual movements were allowed in either task. Movements selected in the imagery tasks were compared with those chosen during actual motor control under comparable circumstances. These comparisons revealed a left hemisphere advantage for representing grasping movements involving the right hand, and reaching movements involving the left arm. The right hemisphere, by contrast, displayed moderate accuracy when representing grasping movements with the left hand, but appeared incapable of imagining reaching movements with either arm. The double dissociation between imagery for hand and arm movements in the left cerebral hemispere is consistent with the hypothesis that grasping and reaching components of prehension involve dissociable planning mechanisms.

Functional organization of the lateral premotor cortex: fMRI reveals different regions activated by anticipation of object properties, location and speed

Previous studies have provided evidence that the lateral premotor cortex (PMC) is involved in representations triggered by attended sensory events. However, while the functional specificity of subregions of this large cortical structure has been intensively investigated in the monkey, little is known about functional differences within human lateral premotor areas. In the present study, functional magnetic resonance imaging was used to investigate if attending to object-specific (O), spatial (S), or temporal (T) properties of the same sensory event, i.e. moving objects, involves different premotor areas. We found a frontoparietal 'prehension network' comprising the pre-supplementary motor area (preSMA), the ventral PMC, and the left anterior intraparietal sulcus (aIPS) to be activated independently of the attended stimulus property, but most intensively during object-related attention. Moreover, several areas were exclusively activated according to the attended stimulus property. Particularly, different PMC regions responded to the Object (O) task (left superior ventrolateral PMC), the Spatial (S) task (dorsolateral PMC), and the Timing (T) task (frontal opercular cortex (FOP)). These results indicate that the representation of different stimulus dimensions engage distinct premotor areas and, therefore, that there is a functional specificity of lateral premotor subregions.

Neural correlates of simple and complex mental calculation

Some authors proposed that exact mental calculation is based on linguistic representations and relies on the perisylvian language cortices, while the understanding of proximity relations between numerical quantities implicates the parietal cortex. However, other authors opposed developmental arguments to suggest that number sense emerges from nonspecific visuospatial processing areas in the parietal cortex. Within this debate, the present study aimed at revealing the functional anatomy of the two basic resolution strategies involved in mental calculation, namely arithmetical fact retrieval and actual computation, questioning in particular the respective role of language and/or visuospatial cerebral areas. Regional cerebral blood flow was measured with positron emission tomography while subjects were at rest (Rest), read digits (Read), retrieved simple arithmetic facts from memory (i.e., 2 x 4, Retrieve), and performed mental complex calculation (i.e., 32 x 24, Compute). Compared to Read, Retrieve engaged a left parieto-premotor circuit representing a developmental trace of a finger-counting representation that mediates, by extension, the numerical knowledge in adult. Beside this basic network, Retrieve involved a naming network, including the left anterior insula and the right cerebellar cortex, while it did not engage the perisylvian language areas, which were deactivated as compared to Rest. In addition to this retrieval network, Compute specifically involved two functional networks: a left parieto-frontal network in charge of the holding of the multidigit numbers in visuospatial working memory and a bilateral inferior temporal gyri related to the visual mental imagery resolution strategy. Overall, these results provide strong evidence of the involvement of visuospatial representations in different levels of mental calculation.

Selective impairment of verb processing associated with pathological changes in Brodmann areas 44 and 45 in the motor neurone disease-dementia-aphasia syndrome

We report six patients with clinically diagnosed and electrophysiologically confirmed motor neurone disease (MND), in whom communication problems were an early and dominant feature. All patients developed a progressive non-fluent aphasia culminating in some cases in complete mutism. In five cases, formal testing revealed deficits in syntactic comprehension. Comprehension and production of verbs were consistently more affected those that of nouns and this effect remained stable upon subsequent testing, despite overall deterioration. The classical signs of MND, including wasting, fasciculations and severe bulbar symptoms, occurred over the following 6-12 months. The behavioural symptoms ranged from mild anosognosia to personality change implicating frontal-lobe dementia. In three cases, post-mortem examination has confirmed the clinical diagnosis of MND-dementia. In addition to the typical involvement of motor and premotor cortex, particularly pronounced pathological changes were observed in the Brodmann areas 44 (Broca's area) and 45. The finding of a selective impairment of verb/action processing in association with the dementia/aphasia syndrome of MND suggests that the neural substrate underlying verb representation is strongly connected to anterior cortical motor systems.

Functional anatomy of execution, mental simulation, observation, and verb generation of actions: a meta-analysis

There is a large body of psychological and neuroimaging experiments that have interpreted their findings in favor of a functional equivalence between action generation, action simulation, action verbalization, and perception of action. On the basis of these data, the concept of shared motor representations has been proposed. Indeed several authors have argued that our capacity to understand other people's behavior and to attribute intention or beliefs to others is rooted in a neural, most likely distributed, execution/observation mechanism. Recent neuroimaging studies have explored the neural network engaged during motor execution, simulation, verbalization, and observation. The focus of this metaanalysis is to evaluate in specific detail to what extent the activated foci elicited by these studies overlap.

Abnormalities in the awareness and control of action

Much of the functioning of the motor system occurs without awareness. Nevertheless, we are aware of some aspects of the current state of the system and we can prepare and make movements in the imagination. These mental representations of the actual and possible states of the system are based on two sources: sensory signals from skin and muscles, and the stream of motor commands that have been issued to the system. Damage to the neural substrates of the motor system can lead to abnormalities in the awareness of action as well as defects in the control of action. We provide a framework for understanding how these various abnormalities of awareness can arise. Patients with phantom limbs or with anosognosia experience the illusion that they can move their limbs. We suggest that these representations of movement are based on streams of motor commands rather than sensory signals. Patients with utilization behaviour or with delusions of control can no longer properly link their intentions to their actions. In these cases the impairment lies in the representation of intended movements. The location of the neural damage associated with these disorders suggests that representations of the current and predicted state of the motor system are in parietal cortex, while representations of intended actions are found in prefrontal and premotor cortex.

The neural-cognitive basis of the Jamesian stream of thought

William James described the stream of thought as having two components: (1) a nucleus of highly conscious, often perceptual material; and (2) a fringe of dimly felt contextual information that controls the entry of information into the nucleus and guides the progression of internally directed thought. Here I examine the neural and cognitive correlates of this phenomenology. A survey of the cognitive neuroscience literature suggests that the nucleus corresponds to a dynamic global buffer formed by interactions between different regions of the brain, while the fringe corresponds to a set of mechanisms in the frontal and medial temporal lobes that control the contents of this global buffer. A consequence of this account is that there might be conscious imagistic representations that are not part of the nucleus. I argue that phenomenology can be linked to psychology and neuroscience and a meaningful way that illuminates both.

Broca's region subserves imagery of motion: a combined cytoarchitectonic and fMRI study

Broca's region in the dominant cerebral hemisphere is known to mediate the production of language but also contributes to comprehension. Here, we report the differential participation of Broca's region in imagery of motion in humans. Healthy volunteers were studied with functional magnetic resonance imaging (fMRI) while they imagined movement trajectories following different instructions. Imagery of right-hand finger movements induced a cortical activation pattern including dorsal and ventral portions of the premotor cortex, frontal medial wall areas, and cortical areas lining the intraparietal sulcus in both cerebral hemispheres. Imagery of movement observation and of a moving target specifically activated the opercular portion of the inferior frontal cortex. A left-hemispheric dominance was found for egocentric movements and a right-hemispheric dominance for movement characteristics in space. To precisely localize these inferior frontal activations, the fMRI data were coregistered with cytoarchitectonic maps of Broca's areas 44 and 45 in a common reference space. It was found that the activation areas in the opercular portion of the inferior frontal cortex were localized to area 44 of Broca's region. These activations of area 44 can be interpreted to possibly demonstrate the location of the human analogue to the so-called mirror neurones found in inferior frontal cortex of nonhuman primates. We suggest that area 44 mediates higher-order forelimb movement control resembling the neuronal mechanisms subserving speech.

Compatibility between observed and executed finger movements: comparing symbolic, spatial, and imitative cues

Intuitively, one can assume that imitating a movement is an easier task than responding to a symbolic stimulus like a verbal instruction. Support for this suggestion can be found in neuropsychological research as well as in research on stimulus-response compatibility. However controlled experimental evidence for this assumption is still lacking. We used a stimulus-response compatibility paradigm to test the assumption. In a series of experiments, it was tested whether observed finger movements have a stronger influence on finger movement execution than a symbolic or spatial cue. In the first experiment, we compared symbolic cues with observed finger movements using an interference paradigm. Observing finger movements strongly influenced movement execution, irrespective of whether the finger movement was the relevant or the irrelevant stimulus dimension. In the second experiment, effects of observed finger movements and spatial finger cues were compared. The observed finger movement dominated the spatial finger cue. A reduction in the similarity of observed and executed action in the third experiment led to a decrease of the influence of observed finger movement, which demonstrates the crucial role of the imitative relation of observed and executed action for the described effects. The results are discussed in relation to recent models of stimulus-response compatibility. Neurocognitive support for the strong relationship between movement observation and movement execution is reported.

Representation of manipulable man-made objects in the dorsal stream

We used fMRI to examine the neural response in frontal and parietal cortices associated with viewing and naming pictures of different categories of objects. Because tools are commonly associated with specific hand movements, we predicted that pictures of tools, but not other categories of objects, would elicit activity in regions of the brain that store information about motor-based properties. We found that viewing and naming pictures of tools selectively activated the left ventral premotor cortex (BA 6). Single-unit recording studies in monkeys have shown that neurons in the rostral part of the ventral premotor cortex (canonical F5 neurons) respond to the visual presentation of graspable objects, even in the absence of any subsequent motor activity. Thus, the left ventral premotor region that responded selectively to tools in the current study may be the human homolog of the monkey canonical F5 area. Viewing and naming tools also selectively activated the left posterior parietal cortex (BA 40). This response is similar to the firing of monkey anterior intraparietal neurons to the visual presentation of graspable objects. In humans and monkeys, there appears to be a close link between manipulable objects and information about the actions associated with their use. The selective activation of the left posterior parietal and left ventral premotor cortices by pictures of tools suggests that the ability to recognize and identify at least one category of objects (tools) may depend on activity in specific sites of the ventral and dorsal visual processing streams.

The voluntary control of motor imagery. Imagined movements in individuals with feigned motor impairment and conversion disorder

The ability to volitionally control motor imagery was investigated by comparing the chronometry of real and imagined movements in a patient (AB) with conversion disorder who presented with paralysis of the left arm and hand and in a patient (MM) with an actual injury to the left arm. Control experiments investigated voluntary control of motor imagery in a group of healthy individuals who feigned a motor impairment with one limb and in one group who were instructed to move carefully and slowly. The visually guided pointing task was used to investigate the speed for accuracy trade-offs that occur as target size is varied for both real and imagined performance. In the healthy individuals, the speed for accuracy trade-off for both real and imagined performance on the motor task conformed to Fitts' law provided both the speed and accuracy of movements was emphasised. In MM, real and imagined performance was also within normal limits despite considerable pain and discomfort. In AB and in subjects feigning a motor impairment, motor task performance with the affected limb was slow and did not conform to Fitts' law. However, although imagined performance with the affected limb was generally slower than with the unaffected limb, it did conform to Fitts' law. These results suggest subjects cannot anticipate the effects of an actual limb injury. Furthermore, while they are able to control the general duration of imagined movements they have little voluntary control over their relative timing.

Verb retrieval in brain-damaged subjects: 1. Analysis of stimulus, lexical, and conceptual factors

Verb retrieval for action naming was assessed in 53 brain-damaged subjects by administering a standardized test with 100 items. The goal of the study was to gain further insight into the nature of verb processing impairments by investigating the influence of several kinds of stimulus, lexical, and conceptual factors on the subjects' performance at the level of group tendencies and also at the level of individual differences. (1) Stimulus factors: visual complexity, familiarity, image agreement, and one vs. two pictures (which corresponds to ongoing vs. completed actions); (2) lexical factors: name agreement, verb frequency, and whether the root of the target verb has a homophonous noun; (3) conceptual factors: whether the action is done with the hand or the body, whether the action involves one or two core participants, whether the undergoer of the action has a change of internal state, whether the undergoer has a change of spatial location, and whether the actor makes use of an instrument in carrying out the action. The subjects were divided into an impaired group (n = 19) and an unimpaired group (n = 34) on the basis of their overall performance on the test. For both groups of subjects, verb retrieval was significantly affected by the following factors: familiarity, image agreement, name agreement, homophonous noun, and undergoer change of location. These results indicate that, at the level of group analysis, some factors have a stronger influence on verb retrieval for action naming than others. Moreover, the finding that the two groups exhibited the same general pattern of factor sensitivity suggests that although the processing efficiency of the mechanisms that subserve verb retrieval is degraded in the impaired group, the basic functional properties of these mechanisms may not be qualitatively very different from those of the unimpaired group. Further analyses were conducted at the level of individual subjects and revealed a considerable amount of variation with regard to factor sensitivity. Many patterns of associations and dissociations of factors were found across the subjects, which suggests that the task of retrieving verbs for naming actions is quite complex and that different subjects can be influenced by different properties of both the stimuli and the target verbs.

Impairment of motor imagery in putamen lesions in humans

Patients with putamen or cortical lesions participated in a first- and third-person movement imagery task, each primarily engaging kinesthetic and visual imagery. The subjects were instructed to imagine themselves (first-person task) and a third party (third-person task) performing a sequence of three movements and to choose from a set of four photos the end posture resulting from the movements. The results demonstrated that, limb-specific imagery was impaired in both putamen and cortical lesions, in the first-, but not third-person task. Moreover, more than half of the errors made by cortical patients were with respect to the first movement, a finding consistent with motor cortex involvement in memory processes. Taken overall, the results provide evidence that the basal ganglia as well as cortical structures play an important role in the neural network mediating motor imagery.

The role of lateral occipitotemporal junction and area MT/V5 in the visual analysis of upper-limb postures

Humans, like numerous other species, strongly rely on the observation of gestures of other individuals in their everyday life. It is hypothesized that the visual processing of human gestures is sustained by a specific functional architecture, even at an early prelexical cognitive stage, different from that required for the processing of other visual entities. In the present PET study, the neural basis of visual gesture analysis was investigated with functional neuroimaging of brain activity during naming and orientation tasks performed on pictures of either static gestures (upper-limb postures) or tridimensional objects. To prevent automatic object-related cerebral activation during the visual processing of postures, only intransitive postures were selected, i. e., symbolic or meaningless postures which do not imply the handling of objects. Conversely, only intransitive objects which cannot be handled were selected to prevent gesture-related activation during their visual processing. Results clearly demonstrate a significant functional segregation between the processing of static intransitive postures and the processing of intransitive tridimensional objects. Visual processing of objects elicited mainly occipital and fusiform gyrus activity, while visual processing of postures strongly activated the lateral occipitotemporal junction, encroaching upon area MT/V5, involved in motion analysis. These findings suggest that the lateral occipitotemporal junction, working in association with area MT/V5, plays a prominent role in the high-level perceptual analysis of gesture, namely the construction of its visual representation, available for subsequent recognition or imitation.

Internally driven vs. externally cued movement selection: a study on the timing of brain activity

Brain imaging studies in man and single cell recordings in monkey have suggested that medial supplementary motor areas (SMA) and lateral pre-motor areas (PMA) are functionally dissociated concerning their involvement in internally driven and externally cued movements. This dichotomy, however, seems to be relative rather than absolute. Here, we searched for further evidence of relative differences and aimed to determine by what aspect of brain activity (duration, strength, or both) these might be accounted for. Event-related potentials (ERPs) were recorded while healthy, right-handed subjects selected one of three possible right hand digit movements based either on 'internal' choice or 'external' cues. The results obtained from ERP mapping suggest that movement selection evokes the same electrical brain activity patterns in terms of surface potential configurations in the same order and at the same strength independent of the selection mode. These identical configurations, however, differed in their duration. Combined with the results of a distributed source localization procedure, our data are suggestive of longer lasting activity in SMA during the 'internal' and longer lasting activity in PMA during the 'external' condition. Our results confirm previous findings in showing that SMA and PMA are distinctively involved in the two tasks and that this functional dichotomy is relative rather than absolute but indicate that such a dissociation can result from differences in duration rather than pure strength of activation.

Changes in breathing during observation of effortful actions

Respiration and heart rates were recorded in normal subjects watching effortful actions produced by an actor in front of them. Subjects remained immobile throughout. Two experiments were performed. In experiment 1, subjects watched a weight-lifting performance, either static or dynamic, with increasing weights. In experiment 2, they watched a walking/running performance on a treadmill moving at increasing speed. In both experiments, no change was found in observers' heart rate. By contrast, consistent changes were found in respiration rate. These changes tended to follow the exercise rhythm of the actor, specially during accelerated running (from 2.5 to 10 km/h) where respiration rate increased linearly with speed of the treadmill. Average maximum increase ranged between 25 and 30% above resting rate. This finding demonstrates activation of central mechanisms related to action performance during observation of effortful actions. It could represent a basis for understanding and imitating actions performed by other people.

Brain dystrophin, neurogenetics and mental retardation

Duchenne muscular dystrophy (DMD) and the allelic disorder Becker muscular dystrophy (BMD) are common X-linked recessive neuromuscular disorders that are associated with a spectrum of genetically based developmental cognitive and behavioral disabilities. Seven promoters scattered throughout the huge DMD/BMD gene locus normally code for distinct isoforms of the gene product, dystrophin, that exhibit nervous system developmental, regional and cell-type specificity. Dystrophin is a complex plasmalemmal-cytoskeletal linker protein that possesses multiple functional domains, autosomal and X-linked homologs and associated binding proteins that form multiunit signaling complexes whose composition is unique to each cellular and developmental context. Through additional interactions with a variety of proteins of the extracellular matrix, plasma membrane, cytoskeleton and distinct intracellular compartments, brain dystrophin acquires the capability to participate in the modulatory actions of a large number of cellular signaling pathways. During neural development, dystrophin is expressed within the neural tube and selected areas of the embryonic and postnatal neuraxis, and may regulate distinct aspects of neurogenesis, neuronal migration and cellular differentiation. By contrast, in the mature brain, dystrophin is preferentially expressed by specific regional neuronal subpopulations within proximal somadendritic microdomains associated with synaptic terminal membranes. Increasing experimental evidence suggests that in adult life, dystrophin normally modulates synaptic terminal integrity, distinct forms of synaptic plasticity and regional cellular signal integration. At a systems level, dystrophin may regulate essential components of an integrated sensorimotor attentional network. Dystrophin deficiency in DMD/BMD patients and in the mdx mouse model appears to impair intracellular calcium homeostasis and to disrupt multiple protein-protein interactions that normally promote information transfer and signal integration from the extracellular environment to the nucleus within regulated microdomains. In DMD/BMD, the individual profiles of cognitive and behavioral deficits, mental retardation and other phenotypic variations appear to depend on complex profiles of transcriptional regulation associated with individual dystrophin mutations that result in the corresponding presence or absence of individual brain dystrophin isoforms that normally exhibit developmental, regional and cell-type-specific expression and functional regulation. This composite experimental model will allow fine-level mapping of cognitive-neurogenetic associations that encompass the interrelationships between molecular, cellular and systems levels of signal integration, and will further our understanding of complex gene-environmental interactions and the pathogenetic basis of developmental disorders associated with mental retardation.

Motor control: Mechanisms of motor equivalence in handwriting

Handwriting is a classic example of how the details of movement can be scale and plane invariant: letter forms reflecting personal style are unchanged, whether one is writing on a piece of paper, on a blackboard or in the sand using the foot. Recent research points to a role for the parietal cortex in such motor equivalence.

Covert visual spatial orienting and saccades: overlapping neural systems

We used functional magnetic resonance imaging (fMRI) to investigate the functional anatomical relationship between covert orienting of visual spatial attention and execution of saccadic eye movements. Brain areas engaged by shifting spatial attention covertly and by moving the eyes repetitively toward visual targets were compared and contrasted directly within the same subjects. The two tasks activated highly overlapping neural systems and showed that common parietal and frontal regions are more activated during the covert task than the overt oculomotor condition. The possible nature of the relationship between these two operations is discussed.

Recognising a hand by grasp

The present study aimed to demonstrate that motor representations are used to recognise biological stimuli. In three experiments subjects were required to judge laterality of hands and forearms presented by pictures. The postures of the hands were those assumed when holding a small, medium and large sphere. In experiment 1, the sphere held in hand was presented, whereas in experiment 2 it was absent. In experiment 3, the same images, showing holding-a-sphere hands, as in experiment 1 were presented, but without forearm. In all experiments one finger of each hand could be absent. In experiment 1 recognition time was longer for those hand postures for which the corresponding grasping motor acts required more accuracy. This was confirmed by a control experiment (experiment 4), in which subjects actually grasped the spheres. Absence of fingers did not influence right-left hand recognition. However, the absence of target object in experiment 2, and of forearm in experiment 3 reduced the effects of the type of holding on hand laterality recognition. The results of the present study indicate that grasp representations are used to recognise hand laterality. In particular, the visual description of how hand and object interact in space (the opposition space [M.A. Arbib, Programs, schemas and neural networks for control of hand movement: beyond the RS frameworks, in: M. Jeannerod (Ed.), Attention and Performance XIII: Motor Representation and Control, Lawrence Erlbaum, Hillsdale, NJ, 1990, 111-138; M.A. Arbib, T. Iberall, D. Lyons, Coordinated control programs for movements of the hand, in: A.W. Goodman, I. Darian-Smith (Eds.), Hand function and the neocortex, Springer, Berlin, 1985, pp. 135-170]) and the anchoring of the hand to the agent are the features of the grasp representations used in hand-recognition processes. The data are discussed according to the more general notion that motor representations are automatically extracted in the process of intuiting situations, or people's intentions. These motor representations, which are compared with those of other people, contain concrete information on the actions (the motor program) by which a situation is created and on the aim of the agents executing those actions.

Representation of actions in rats: the role of cerebellum in learning spatial performances by observation

Experimental evidence demonstrates that cerebellar networks are involved in spatial learning, controlling the acquisition of exploration strategies without blocking motor execution of the task. Action learning by observation has been considered somehow related to motor physiology, because it provides a way of learning performances that is almost as effective as the actual execution of actions. Neuroimaging studies demonstrate that observation of movements performed by others, imagination of actions, and actual execution of motor performances share common neural substrates and that the cerebellum is among these shared areas. The present paper analyzes the effects of observation in learning a spatial task, focusing on the cerebellar role in learning a spatial ability through observation. We allowed normal rats to observe 200 Morris water maze trials performed by companion rats. After this observation training, "observer" rats underwent a hemicerebellectomy and then were tested in the Morris water maze. In spite of the cerebellar lesion, they displayed no spatial defects, exhibiting exploration abilities comparable to controls. When the cerebellar lesion preceded observation training, a complete lack of spatial observational learning was observed. Thus, as demonstrated already for the acquisition of spatial procedures through actual execution, cerebellar circuits appear to play a key role in the acquisition of spatial procedures also through observation. In conclusion, the present results provide strong support for a common neural basis in the observation of actions that are to be reproduced as well as in the actual production of the same actions.

Functional brain areas used for the lifting of objects using a precision grip: a PET study

Positron emission tomography (PET) was performed in 10 normal volunteers to investigate regional cortical and subcortical activation induced by the lifting of an object repetitively using a precision grip between the index finger and thumb. Data were obtained for three object weights (4, 200 and 600 g) and a resting condition. Grip and lift forces on a similar object and the activity of selected muscles in the hand, arm and shoulder were also recorded in separate lifting trials. A comparison between all movement conditions and the resting condition revealed significant activation of the primary motor (M1), primary sensory (S1), dorso-caudal premotor (PM), caudal supplementary motor (SMA) and cingulate motor (CMA) cortices contralateral to the hand used. On the ipsilateral side, activation of the M1, caudal SMA and inferior parietal cortex (BA 40) was also found. In the subcortical areas, the bilateral hemispheres and right vermis of the cerebellum, left basal ganglia and thalamus were activated. Behavioral adaptation to a heavier object weight was revealed in a nearly proportional increase of both grip and lift forces, prolonged force application period and a higher level of hand and arm muscle activities. An increase in the rCBF associated with these changes was noted in several cortical and subcortical areas. However, consistent object weight-dependent activation was observed only in the M1/S1 contralateral to the hand used.

Thinking ahead: the case for motor imagery in prospective judgements of prehension

How similar are judgements concerning how we expect to perform an action, to how we actually behave? The veracity of such prospective action judgements, and the mechanisms by which they are computed, was explored in a series of tasks that involved either grasping (MC conditions) or thinking about grasping (PJ conditions) a dowel presented in various orientations. PJs concerning limits of comfortable hand supination and pronation when turning a dowel in the picture plane were highly consistent with values obtained during actual hand rotation (Exp. 1). The same was true for judgements regarding the level of awkwardness involved in adopting a prescribed grip (e.g. overhand with right hand) for dowels in various picture plane orientations (Exp. 2). When allowed to select the most natural grip (overhand versus underhand) or hand (left versus right) for engaging dowels in these orientations, subjects preferred virtually identical responses in both PJ and MC conditions. In both instances, they consistently chose the least awkward response options. As would be expected for actual movements, PJs involving awkward hand postures had longer response times (RTs), and were less accurate. Likewise, latencies for both grip and hand judgements tended to increase as a function of the angular distance between the current positions of subjects' hands, and the orientation of the chosen posture. Together, these findings are consistent with a the hypothesis that PJs involve mentally simulated actions, or motor imagery. These results suggest that motor imagery does not depend on the existence of a completed premotor plan (Jeannerod, 1994), but may instead be involved in the planning process itself. A provisional model for the involvement of imagery in motor planning is outlined, as are a set of criteria for evaluating claims of the involvement of motor imagery in problem solving.