Mapping motor representations with positron emission tomography

Prediction and preparation, fundamental functions of the cerebellum

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Factors that influence effect size in 15O PET studies: a meta-analytic review

The PET literature is growing exponentially, creating a need and an opportunity to perform a meta-analytic review consolidating the published information. This study describes the use of effect size as an index in PET studies and discusses how this measure can be used for comparing findings across studies, laboratories, and paradigms. In comparing studies across laboratories it is essential to know how the methods employed affect the results and conclusions drawn. This study also compared effect size for two different methods of tracer delivery in 15O PET studies ([15O]H2O bolus injection versus inhalation of [15O]CO2), whether averaged versus single-scan conditions were used, and the data analytic strategy employed. The effect sizes observed across studies were consistently large with a median effect size of 8.55, indicating that the phenomena investigated in 15O PET studies are strong. The largest peak activation reported in a study was found to be affected by variability in sample size, data analytic strategy, and repeat versus single-scan conditions. However, the impact of these factors was not examined on smaller or less intense peaks. Minimal standards for reporting statistical results are discussed.

Decreased probability of neurotransmitter release underlies striatal long-term depression and postnatal development of corticostriatal synapses

Changes in synaptic efficacy are crucial for the development of appropriate neural circuits and brain information storage. We have investigated mechanisms underlying long-term depression (LTD) at glutamatergic synapses in the striatum, a brain region important in motor performance and cognition, and a target for Huntington and Parkinson diseases. Induction of striatal LTD is dependent on postsynaptic depolarization and calcium influx through L-type channels. Surprisingly, LTD maintenance appears to involve a decrease in the probability of neurotransmitter release from presynaptic terminals as evidenced by increases in paired-pulse facilitation and the coefficient of variation of synaptic responses that are tightly associated with LTD expression. Furthermore, both the apparent probability of neurotransmitter release and the magnitude of LTD decrease concomitantly during postnatal development, consistent with the idea that striatal LTD is involved in a developmental decrease in the probability of neurotransmitter release at corticostriatal synapses. The presynaptic changes that underlie striatal LTD may also be important for motor performance and certain forms of learning and memory.

Motor task difficulty and brain activity: investigation of goal-directed reciprocal aiming using positron emission tomography

Differences in the kinematics and pattern of relative regional cerebral blood flow (rCBF) during goal-directed arm aiming were investigated with the use of a Fitts continuous aiming paradigm with three difficulty conditions (index of difficulty, ID) and two aiming types (transport vs. targeting) in six healthy right-handed young participants with the use of video-based movement trajectory analysis and positron emission tomography. Movement time and kinematic characteristics were analyzed together with the magnitude of cerebral blood flow to identify areas of brain activity proportionate to task and movement variables. Significant differences in rCBF between task conditions were determined by analysis of variance with planned comparisons of means with the use of group mean weighted linear contrasts. Data were first analyzed for the group. Then individual subject differences for the movement versus no movement and task difficulty comparisons were related to each individual subjects' anatomy by magnetic resonance imaging. Significant differences in rCBF during reciprocal aiming compared with no-movement conditions were found in a mosaic of well-known cortical and subcortical areas associated with the planning and execution of goal-directed movements. These included cortical areas in the left sensorimotor, dorsal premotor, and ventral premotor cortices, caudal supplementary motor area (SMA) proper, and parietal cortex, and subcortical areas in the left putamen, globus pallidus, red nucleus, thalamus, and anterior cerebellum. As aiming task difficulty (ID) increased, rCBF increased in areas associated with the planning of more complex movements requiring greater visuomotor processing. These included bilateral occipital, left inferior parietal, and left dorsal cingulate cortices--caudal SMA proper and right dorsal premotor area. These same areas showed significant increases or decreases, respectively, when contrast means were compared with the use of movement time or relative acceleration time, respectively, as the weighting factor. Analysis of individual subject differences revealed a correspondence between the spatial extent of rCBF changes as a function of task ID and the individuals' movement times. As task ID decreased, significant increases in rCBF were evident in the right anterior cerebellum, left middle occipital gyrus, and right ventral premotor area. Functionally, these areas are associated with aiming conditions in which the motor execution demands are high (i.e., coordination of rapid reversals) and precise trajectory planning is minimal. These same areas showed significant increases or decreases, respectively, when contrast means were compared with the use of movement time or relative acceleration time, respectively, as the weighting factor. A functional dissociation resulted from the weighted linear contrasts between larger (limb transport) or smaller (endpoint targeting) type amplitude/target width aiming conditions. Areas with significantly greater rCBF for targeting were the left motor cortex, left intraparietal sulcus, and left caudate. In contrast, those areas with greater rCBF associated with limb transport included bilateral occipital lingual gyri and the right anterior cerebellum. Various theoretical explanations for the speed/accuracy tradeoffs of rapid aiming movements have been proposed since the original information theory hypothesis of Fitts. This is the first report to relate the predictable variations in motor control under changing task constraints with the functional anatomy of these rapid goal-directed aiming movements. Differences in unimanual aiming task difficulty lead to dissociable activation of cortical-subcortical networks. Further, these data suggest that when more precise targeting is required, independent of task difficulty, a cortical-subcortical loop composed of the contralateral motor cortex, intraparietal sulcus, and caudate is activated. This is consistent with the role of motor cortex

Preserved performance by cerebellar patients on tests of word generation, discrimination learning, and attention

Recent theories suggest that the human cerebellum may contribute to the performance of cognitive tasks. We tested a group of adult patients with cerebellar damage attributable to stroke, tumor, or atrophy on four experiments involving verbal learning or attention shifting. In experiment 1, a verb generation task, participants produced semantically related verbs when presented with a list of nouns. With successive blocks of practice responding to the same set of stimuli, both groups, including a subset of cerebellar patients with unilateral right hemisphere lesions, improved their response times. In experiment 2, a verbal discrimination task, participants learned by trial and error to pick the target words from a set of word pairs. When age was taken into account, there were no performance differences between cerebellar patients and control subjects. In experiment 3, measures of spatial attention shifting were obtained under both exogenous and endogenous cueing conditions. Cerebellar patients and control subjects showed similar costs and benefits in both cueing conditions and at all SOAs. In experiment 4, intra- and interdimensional shifts of nonspatial attention were elicited by presenting word cues before the appearance of a target. Performance was substantially similar for cerebellar patients and control subjects. These results are presented as a cautionary note. The experiments failed to provide support for current hypotheses regarding the role of the cerebellum in verbal learning or attention. Alternative interpretations of previous results are discussed.

Functional Neuroanatomy of Recall and Recognition: A PET Study of Episodic Memory

The purpose of this study was to directly compare the brain regions involved in episodic-memory recall and recognition. Changes in regional cerebral blood flow were measured by positron emission tomography while young healthy test persons were either recognizing or recalling previously studied word pairs. Reading of previously nonstudied pairs served as a reference task for subtractive comparisons. Compared to reading, both recall and recognition were associated with higher blood flow (activation) at identical sites in the right prefrontal cortex (areas 47, 45, and 10) and the anterior cingulate. Compared to recognition, recall was associated with higher activation in the anterior cingulate, globus pallidus, thalamus, and cerebellum, suggesting that these components of the cerebello-frontal pathway play a role in recall processes that they do not in recognition. Compared to recall, recognition was associated with higher activation in the right inferior parietal cortex (areas 39, 40, and 19), suggesting a larger perceptual component in recognition than in recall. Contrary to the expectations based on lesion data, the activations of the frontal regions were indistinguishable in recall and recognition. This finding is consistent with the notion that frontal activations in explicit memory tasks are related to the general episodic retrieval mode or retrieval attempt, rather than to specific mechanisms of ecphory (recovery of stored information).

Mental simulation of an action modulates the excitability of spinal reflex pathways in man

The question of whether mental simulation of an action has an effect on the spinal reflex circuits was examined in normal humans. Subjects were instructed either to exert or to mentally simulate a strong or a weak pressure on a pedal with the left or the right foot. Changes in the H- and T-reflexes activated by electrical and mechanical stimuli were measured on both legs during motor performance as well as during mental simulation of the same task. Asynchronous EMG activity of the soleus muscles was simultaneously recorded. Reflex excitability increased during performance of the pressure. It was larger when the H-reflex was triggered in the muscle involved in the task as compared to the contralateral side. Because actual performance modified the tension of the tendon and the location of the stimulus, ipsilateral changes of T-reflex amplitude could not be evaluated. Mental simulation of foot pressure in this condition resulted in a large increase of spinal reflex excitability, which was only slightly weaker than the reflex facilitation associated with the actual performance. Changes in T-reflex amplitude, but not in H-reflex amplitude, depended upon the lateralization and force of the simulated pressure, being larger in the leg involved in the simulation than in the contralateral leg, and larger for a strong than for a weak simulated movement. EMG activity was found to be weakly increased during mental imagery. This increase was significantly, although slightly, modulated by the lateralization and intensity of the imagined movement. However, no correlation was found across subjects between reflex amplitude and the amplitude of EMG activity.

Visuomotor transformations for reaching to memorized targets: a PET study

Positron emission tomography (PET) was used to identify cortical and subcortical regions involved in the control of reaching to visual targets. Regional cerebral blood flow (rCBF) was measured in eight healthy subjects using H2(15)O PET during the performance of three different tasks. All tasks required central fixation while a 400-ms target was flashed every 5 s at a random location around a virtual circle centered on the fixation target. Additional instructions differed according to the task: (i) visual detection of the target without overt responses; (ii) immediate pointing to the most recent target in the sequence, and (iii) pointing to the previous target in the sequence. By design, the two motor tasks differed in the cognitive processing required. In each trial of immediate pointing, the spatial location of only the most recent target needed to be processed. In each trial of pointing to the previous, instead, while the most recent target was stored in memory for the movement of the next trial, the previous target had to be retrieved from memory to direct the current movement. Limb trajectories were comparable between the two motor tasks in terms of most spatiotemporal parameters examined. Significant rCBF increases were identified using analysis of covariance and t statistics. Compared with visual detection there was activation of primary sensorimotor cortex, ventrolateral precentral gyrus, inferior frontal gyrus in the opercular region, supramarginal gyrus, and middle occipital gyrus, all these sites in the hemisphere (left) contralateral to the moving limb, and cerebellar vermis, during both immediate pointing and pointing to the previous. During immediate pointing there was additional activation of left inferior parietal lobule close to the intraparietal sulcus, and when compared with pointing to the previous, dorsolateral prefrontal cortex bilaterally. During pointing to the previous, instead, there was additional activation of supplementary motor cortex, anterior and midcingulate, and inferior occipital gyrus in the left hemisphere; superior parietal lobule, supramarginal gyrus, and posterior hippocampus in the right hemisphere; lingual gyri and cerebellar hemispheres bilaterally; anterior thalamus; and pulvinar. The activation of two partially distinct cerebral networks in these two motor tasks reflects the different nature of signal processing involved. In particular, the specific activation of intraparietal sulcus and prefrontal cortex in immediate pointing appears characteristic of a network for visuospatial working memory. By contrast, the corticolimbic network engaged in pointing to the previous could mediate spatial attention and the sequence of encoding, recording, and decoding of spatial memories required by a dual task with two competing targets.

Representations of graphomotor trajectories in the human parietal cortex: evidence for controlled processing and automatic performance

The aim of this study was to identify the cerebral areas activated during kinematic processing of movement trajectories. We measured regional cerebral blood flow (rCBF) during learning, performance and imagery of right-hand writing in eight right-handed volunteers. Compared with viewing the writing space, increases in rCBF were observed in the left motor, premotor and frontomesial cortex, and in the right anterior cerebellum in all movement conditions, and the increases were related to mean tangential writing velocity. No rCBF increases occurred in these areas during imagery. Early learning of new ideomotor trajectories and deliberately exact writing of letters both induced rCBF increases in the cortex lining the right intraparietal sulcus. In contrast, during fast writing of overlearned trajectories and in the later phase of learning new ideograms the rCBF increased bilaterally in the posterior parietal cortex. Imagery of ideograms that had not been practised previously activated the anterior and posterior parietal areas simultaneously. Our results provide evidence suggesting that the kinematic representations of graphomotor trajectories are multiply represented in the human parietal cortex. It is concluded that different parietal subsystems may subserve attentive sensory movement control and whole-field visuospatial processing during automatic performance.

Primary motor and sensory cortex activation during motor performance and motor imagery: a functional magnetic resonance imaging study

The intensity and spatial distribution of functional activation in the left precentral and postcentral gyri during actual motor performance (MP) and mental representation [motor imagery (MI)] of self-paced finger-to-thumb opposition movements of the dominant hand were investigated in fourteen right-handed volunteers by functional magnetic resonance imaging (fMRI) techniques. Significant increases in mean normalized fMRI signal intensities over values obtained during the control (visual imagery) tasks were found in a region including the anterior bank and crown of the central sulcus, the presumed site of the primary motor cortex, during both MP (mean percentage increase, 2.1%) and MI (0.8%). In the anterior portion of the precentral gyrus and the postcentral gyrus, mean functional activity levels were also increased during both conditions (MP, 1.7 and 1.2%; MI, 0.6 and 0.4%, respectively). To locate activated foci during MI, MP, or both conditions, the time course of the signal intensities of pixels lying in the precentral or postcentral gyrus was plotted against single-step or double-step waveforms, where the steps of the waveform corresponded to different tasks. Pixels significantly (r > 0.7) activated during both MP and MI were identified in each region in the majority of subjects; percentage increases in signal intensity during MI were on average 30% as great as increases during MP. The pixels activated during both MP and MI appear to represent a large fraction of the whole population activated during MP. These results support the hypothesis that MI and MP involve overlapping neural networks in perirolandic cortical areas.

Evidence for facilitation of motor evoked potentials (MEPs) induced by motor imagery

This study examined the extent to which motor imagery can facilitate to specific pools of motoneurons. Motor commands induced by motor imagery were subthreshold for muscle activity and were presumably not associated with any change in background afferent activity. To estimate excitability changes of flexor carpi radialis (FCR) muscle motoneuron in spinal and cortical level, electric stimuli for recording H-reflex and transcranial magnetic stimulation (TMS) for recording motor evoked potentials (MEPs) were used. During motor imagery of wrist flexion, remarkable increases in the amplitude of the MEP of FCR were observed with no change in the H-reflex. Furthermore, facilitation of antagonist (extensor carpi radialis; ECR) was also observed. Therefore, it is concluded that internal motor command can activate precisely cortical excitability with no change in spinal level without recourse to afferent feedback.

Imaging Cognition: An Empirical Review of PET Studies with Normal Subjects

We review PET studies of higher-order cognitive processes, including attention (sustained and selective), perception (of objects, faces, and locations), language (word listening, reading, and production), working memory (phonological and visuo-spatial), semantic memory retrieval (intentional and incidental), episodic memory retrieval (verbal and nonverbal), priming, and procedural memory (conditioning and skill learning). For each process, we identify activation patterns including the most consistently involved regions. These regions constitute important components of the network of brain regions that underlie each function.

Task-involvement and ego-involvement goals during actual and imagined movements: their effects on cognitions and vegetative responses

It has been experimentally proven many times that the mental rehearsal of an activity not only improves motor performance but also has vegetative effects whose magnitude is correlated with the amount of imagined effort. These beneficial effects of mental imagery have been explained in terms of central programming structures capable of anticipating the metabolic demands of the task. Twenty-four subjects were asked to actually perform and also imagine an isometric contraction of the forearm under various goal conditions: a task-involving goal (8 subjects), an ego-involving goal (8 subjects), and no goal (8 subjects). During the contractions, electromyographic potential and heart rate were measured. Afterwards, the subjects were asked to indicate the amount of effort expended under different feedback conditions. The results showed no trace of electromyographic activity during the imagined contractions when the lack of movement was controlled using a force sensor. On the other hand, a significantly faster in heart rate was observed with a task- or ego-involving goal than with no goal, during both actual and imagined contraction. Similarly, as predicted, subjects said they applied less effort in the positive feedback condition, and more effort in the negative feedback condition with an ego-involving goal. The results are discussed in the light of goal theories, while regarding goals not only as serving to anticipate metabolic expenditures but also as promoting a self-image of competence, particularly in threatening, ego-involving situations.

Rehabilitation of the head-injured child: basic research and new technology

The view that brain damage in children is less impairing than equivalent damage in adults is no longer acceptable. However, it is acknowledged that recovery following brain damage, when it does occur, owes much to the plasticity of the brain and that the young brain displays greater plasticity than the mature brain. To maximize brain damage recovery in children we need to focus both on what is known about brain plasticity and how to influence it. Research on environmental enrichment in rats has told us that enforced interaction with a complex environment can both stimulate anatomical and biochemical plasticity and ameliorate some of the behavioural consequences of brain damage. The view that environmental interaction has rehabilitative value also accords with clinical experience. However, the sensory, motor and cognitive consequences of brain damage often conspire to make environmental interaction difficult. One potential solution lies in using computers to generate virtual environments tailored to the precise sensory and motor capacities of the brain-injured child. In this way children may be enabled to benefit from environmental interaction whatever their level of disability. The use of Virtual Reality (VR) in the context of rehabilitation is discussed and relevant work reviewed.

Visuomotor functions of the lateral pre-motor cortex

Recent studies have provided new insights into the visuomotor functions of the dorsal and ventral regions of the lateral pre-motor cortex. Anatomical and physiological investigations in non-human primates have demonstrated that these regions have differing patterns of cortical connectivity and distinctive neuronal responses. Brain-imaging techniques and lesion studies have begun to probe the functions of homologous regions in humans.

Functions and structures of the motor cortices in humans

Humans and non-human primates have several motor areas. Exactly how many is a matter of current debate. A proper parcellation of motor areas must be based on correlated structural and functional differences. Recent studies indicate that the primary motor cortex may be, in reality, two areas (4a and 4p). Similarly, there are undoubtedly two or more cingulate motor areas and perhaps two supplementary motor areas. The homologies between human and monkey brains are striking in some cases, making monkey models of human motor cortices attractive. The doctrine of a strict 'homuncular' somatotopical organization of motor areas will have to be abandoned. The engagement of motor areas in different types of voluntary seems merely a matter of degree of activation rather than exclusive specific contributions.

A Kendama Learning Robot Based on Bi-directional Theory

A general theory of movement-pattern perception based on bi-directional theory for sensory-motor integration can be used for motion capture and learning by watching in robotics. We demonstrate our methods using the game of Kendama, executed by the SARCOS Dextrous Slave Arm, which has a very similar kinematic structure to the human arm. Three ingredients have to be integrated for the successful execution of this task. The ingredients are (1) to extract via-points from a human movement trajectory using a forward-inverse relaxation model, (2) to treat via-points as a control variable while reconstructing the desired trajectory from all the via-points, and (3) to modify the via-points for successful execution. In order to test the validity of the via-point representation, we utilized a numerical model of the SARCOS arm, and examined the behavior of the system under several conditions. Copyright 1996 Elsevier Science Ltd.

Neural representations for action

Goal directed behaviour is often internally generated which implies that the generation of action involves a representational step. One of the challenges of cognitive neuroscience is to discover the neural mechanism that underlies the representation of both intention and goal and fuses them into an integrated action. This paper reviews neurophysiological data gathered from different sorts of paradigms that reveal the presence of underlying neural processes which can be related to representations for action. Taken together, these data lead to the notion of distributed representations stored as patterns of activation networks.

The mental representation of hand movements after parietal cortex damage

Recent neuroimagery findings showed that the patterns of cerebral activation during the mental rehearsal of a motor act are similar to those produced by its actual execution. This concurs with the notion that part of the distributed neural activity taking place during movement involves internal simulations, but it is not yet clear what specific contribution the different brain areas involved bring to this process. Here, patients with lesions restricted to the parietal cortex were found to be impaired selectively at predicting, through mental imagery, the time necessary to perform differentiated finger movements and visually guided pointing gestures, in comparison to normal individuals and to a patient with damage to the primary motor area. These results suggest that the parietal cortex is important for the ability to generate mental movement representations.

'Monkey see, monkey do' cells. Neurophysiology

Cells in the premotor cortex of the macaque monkey respond to the sight of specific hand actions made by either the animal itself or the experimenter. What could be the function of such cells?

Localization of grasp representations in humans by PET: 1. Observation versus execution

Positron emission tomography (PET) was used to localize brain regions that are active during the observation of grasping movements. Normal, right-handed subjects were tested under three conditions. In the first, they observed grasping movements of common objects performed by the experimenter. In the second, they reached and grasped the same objects. These two conditions were compared with a third condition consisting of object observation. On the basis of monkey data, it was hypothesized that during grasping observation, activations should be present in the region of the superior temporal sulcus (STS) and in inferior area 6. The findings in humans demonstrated that grasp observation significantly activates the cortex of the middle temporal gyrus including that of the adjacent superior temporal sulcus (Brodmann's area 21) and the caudal part of the left inferior frontal gyrus (Brodmann's area 45). The possible functional homologies between these areas and the monkey STS region and frontal area F5 are discussed.

Object recognition: the man who mistook his dog for a cat

Neuropsychological studies of people with specific brain lesions have led to the theory that different parts of the brain are responsible for recognizing living and non-living objects. Now there is direct evidence from activity measurements that this is the case.

The neurophysiological basis of motor imagery

Motor imagery may be defined as a dynamic state during which representations of a given motor act are internally rehearsed in working memory without any overt motor output. What neural processes underlie the generation of motor imagery? This paper reviews physiological evidence from measurements of regional brain activity and from measurements of autonomic responses in normal subjects and behavioral observations from brain damaged patients. It is proposed that motor imagery shares neural mechanisms with processes used in motor control. This review emphasizes the importance of the prefrontal cortex and its connections to the basal ganglia in maintaining dynamic motor representations in working memory. This view fits with the general idea that the prefrontal cortex is responsible for the creation and maintenance of explicit representations that guide thought and action.

The role of inhibition in the hierarchical gating of executed and imagined movements

A theory is presented concerning the neuronal mechanisms which may underlie the organisation of imagined versus executed movements. A review is first presented of previous theoretical and experimental evidence suggesting that the brain can use the same mechanisms for the imagination and the execution of movement. In particular the fact that adaptation of the vestibulo-ocular reflex can be obtained by pure mental effort and not solely by conflicting visual and vestibular cues has been suggestive of the fact that the brain could internally simulate conflicts and use the same adaptive mechanisms used when actual sensory cues were in conflict. The saccadic system is taken as a good model for the study of this question because the mechanisms which underlie saccade generation are now partially understood at different levels from the brain stem to the cortex. The central idea of the theory is based upon the fact that, in parallel with the excitatory mechanisms underlying saccade generation, several inhibitory mechanisms in cascade allow the selective modulation and blockage of saccades. Synaptic inhibition is therefore supposed to play a major role in a hierarchical selective gating of saccade execution not at one but at several levels allowing a variety of different types of "imagined movements' some involving only the higher levels some in which the execution is only blocked at the very immediate premotor level. But in all cases the theory proposes that imagination and execution have many mechanisms in common. PET data showing that indeed the same structures are activated in both types of movements support this idea although the final answer will have to be brought by neuronal data.

Imagery and perception-action mediation in imitative actions

This paper describes two lines of research exploring a hypothetical function of imagery in the context of imitative actions: the mediation between perceptual and motor processes. Both experimental approaches, a sequence learning task and a timing imitation task, demonstrate that engagement into imagery as a temporally distinct activity between observation and performance is not required for accurate imitation. Moreover, evidence is provided that generative processes can take place during event observation itself, thus making a separate recoding stage redundant. Nevertheless, in the absence of a visual display, imagery of a movement sequence exerted similar learning effects as physical and observational practice, and visual and motor imagery were found to be equally effective rehearsal strategies for maintenance of temporal information in short-term memory.

Electric and magnetic fields of the brain accompanying internal simulation of movement

Methods of functional brain imaging have been used to identify brain structures which are active during internal simulation of movements (ISM). Between 1977 and 1993 it was consistently reported that the primary motor cortex (MI) is not active during ISM whereas other cortical areas, in particular the supplementary motor area (SMA) are active. ISM was assumed to be a situation of "internal programming'. Brain systems involved in ISM or 'programming' were hypothesized to be superior to and separable from 'executive system' including MI. We have studied electric and magnetic fields of the brain when subjects internally simulated either a single movement or a sequence of movements. Results of the studies are consistent with the assumption that MI is active with ISM. Internally subjects experienced effort which was required to inhibit overt movements during ISM. A recent EEG study showed different patterns of cortical activity with ISM and with movement inhibition suggesting that different brain structures may be active during ISM and movement inhibition [23].

Do imagined and executed actions share the same neural substrate?

This paper addresses the issue of the functional correlates of motor imagery, using mental chronometry, monitoring the autonomic responses and measuring cerebral blood flow in humans. The timing of mentally simulated actions closely mimic actual movement times. Autonomic responses during motor imagery parallel the autonomic responses to actual exercise. Cerebral blood flow increases are observed in the motor cortices involved in the programming of actual movement (i.e. premotor cortex, anterior cingulate, inferior parietal lobule and cerebellum). These three sources of data provide converging support for the hypothesis that imagined and executed actions share, to some extent, the same central structures.

Premotor cortex and the recognition of motor actions

In area F5 of the monkey premotor cortex there are neurons that discharge both when the monkey performs an action and when he observes a similar action made by another monkey or by the experimenter. We report here some of the properties of these 'mirror' neurons and we propose that their activity 'represents' the observed action. We posit, then, that this motor representation is at the basis of the understanding of motor events. Finally, on the basis of some recent data showing that, in man, the observation of motor actions activate the posterior part of inferior frontal gyrus, we suggest that the development of the lateral verbal communication system in man derives from a more ancient communication system based on recognition of hand and face gestures.

Neural correlates of category-specific knowledge

An intriguing and puzzling consequence of damage to the human brain is selective loss of knowledge about a specific category of objects. One patient may be unable to identify or name living things, whereas another may have selective difficulty identifying man-made objects. To investigate the neural correlates of this remarkable dissociation, we used positron emission tomography to map regions of the normal brain that are associated with naming animals and tools. We found that naming pictures of animals and tools was associated with bilateral activation of the ventral temporal lobes and Broca's area. In addition, naming animals selectively activated the left medial occipital lobe--a region involved in the earliest stages of visual processing. In contrast, naming tools selectively activated a left premotor area also activated by imagined hand movements, and an area in the left middle temporal gyrus also activated by the generation of action words. Thus the brain regions active during object identification are dependent, in part, on the intrinsic properties of the object presented.

Functional anatomy of inner speech and auditory verbal imagery

The neural correlates of inner speech and of auditory verbal imagery were examined in normal volunteers, using positron emission tomography (PET). Subjects were shown single words which they used to generate short, stereotyped sentences without speaking. In an inner speech task, sentences were silently articulated, while in an auditory verbal imagery condition, subjects imagined sentences being spoken to them in an another person's voice. Inner speech was associated with increased activity in the left inferior frontal gyrus. Auditory verbal imagery was associated with increases in the same region, and in the left premotor cortex, the supplementary motor area and the left temporal cortex. The data suggest that the silent articulation of sentences involves activity in an area concerned with speech generation, while imagining speech is associated with additional activity in regions associated with speech perception.

The psychobiology of mind-body communication: the complex, self-organizing field of information transduction

The current information revolution in molecular biology has important implications for an new understanding of the phenomenology of mind, memory and behavior as a complex, self-organizing field of information transduction. This paper traces the pathways of information transduction in life processes from the molecular-genetic level to the dynamics of mind and behavior together with suggestions for future research exploring the psychobiology of mind-body communication and its implications for the psychotherapeutic arts of the future.

Mentally simulated movements in virtual reality: does Fitts's law hold in motor imagery?

This study was designed to investigate mentally simulated actions in a virtual reality environment. Naive human subjects (n = 15) were instructed to imagine themselves walking in a three-dimensional virtual environment toward gates of different apparent widths placed at three different apparent distances. Each subject performed nine blocks of six trials in a randomised order. The response time (reaction time and mental walking time) was measured as the duration between an acoustic go signal and a motor signal produced by the subject. There was a combined effect on response time of both gate width and distance. Response time increased for decreasing apparent gate widths when the gate was placed at different distances. These results support the notion that mentally simulated actions are governed by central motor rules.

Mental motor imagery: a window into the representational stages of action

The physiological basis of mental states can be effectively studied by combining cognitive psychology with human neuroscience. Recent research has employed mental motor imagery in normal and brain-damaged subjects to decipher the content and the structure of covert processes preceding the execution of action. The mapping of brain activity during motor imagery discloses a pattern of activation similar to that of an executed action.

Building action repertoires: memory and learning functions of the basal ganglia

Research on the basal ganglia suggests that they are critically involved in building up sequences of behavior into meaningful, goal-directed repertoires. Work on rodents, monkeys and humans suggests that the basal ganglia act as part of a distributed forebrain system that helps to encode such repertoires through behavioral learning, and that is engaged in the expression of such repertoires once they have been internalized. The basal ganglia also may be critical to the expression of innate behavioral routines. Experimental findings on reward-based learning suggest that neural activity in the striatum and substantia nigra, pars compacta changes during behavioral learning. New evidence also suggests extreme specificity in the neural connections interrelating the basal ganglia, cerebral cortex and thalamus. Adaptive control of behavior may centrally depend on these circuits and the evaluator-reinforcement circuits that modulate them.

Neural correlates of mental transformations of the body-in-space

Regional cerebral blood flow was measured with positron emission tomography in human subjects during the performance of a task requiring mental rotation of their hand and a perceptually equivalent control task that did not require such a process. Comparison of the distribution of cerebral activity between these conditions demonstrated significant blood flow increases in the superior parietal cortex, the intraparietal sulcus, and the adjacent rostralmost part of the inferior parietal lobule. These findings demonstrated that, in the human brain, there is a specific system of parietal areas that are involved in mental transformations of the body-in-space.

Mental imagery in the motor context

The working hypothesis of the paper is that motor images are endowed with the same properties as those of the (corresponding) motor representations, and therefore have the same functional relationship to the imagined or represented movement and the same causal role in the generation of this movement. The fact that the timing of simulated movements follows the same constraints as that of actually executed movements is consistent with this hypothesis. Accordingly, many neural mechanisms are activated during motor imagery, as revealed by a sharp increase in tendinous reflexes in the limb imagined to move, and by vegetative changes which correlate with the level of mental effort. At the cortical level, a specific pattern of activation, that closely resembles that of action execution, is observed in areas devoted to motor control. This activation might be the substrate for the effects of mental training. A hierarchical model of the organization of action is proposed: this model implies a short-term memory storage of a 'copy' of the various representational steps. These memories are erased when an action corresponding to the represented goal takes place. By contrast, if the action is incompletely or not executed, the whole system remains activated, and the content of the representation is rehearsed. This mechanism would be the substrate for conscious access to this content during motor imagery and mental training.

Motor imagery: perception or action?

Motor imagery has been studied using subjective, behavioural and physiological methods and this paper reviews theoretical and practical issues from all three viewpoints. Attempts to measure motor imagery on a subjective scale have met with limited success but alternative methods are proposed. Research on mental practice suggests a number of different processes may be needed to explain the variety and variability of effects obtained. Recent studies of spatial and motor working memory signify the importance of a primarily visuo-spatial component in which actions are consciously represented together with a more properly motoric component which must be activated to generate either images or overt actions. Finally the question of whether motor imagery is primarily perceptual or motoric in character does not have a simple neurophysiological answer due to the highly distributed nature of motor control. Nevertheless some of the key mechanisms serving both spatial and motoric components have been provisionally identified.

Current issues in the neuropsychology of image generation

Image generation is the process by which long-term memory knowledge of the visual appearance of objects or scenes is used to create a short-term, percept-like image. In this article I will review recent neuropsychological evidence relevant to two questions about image generation: First, is there a distinct component of the cognitive architecture dedicated to image generation? Second, what brain regions are directly involved with image generation?

Characterizing the local oscillatory content of spontaneous cortical activity during mental imagery

We report on the determination of detailed spectra for simultaneously active sources of spontaneous neuronal activity in humans directly from data recorded with a whole-scalp 122-channel magnetometer array. Subjects rested with eyes open and performed two contrasting mental imagery tasks: the imagination of the self-performance of a motor activity and the silent generation of a chain of words. A novel analysis technique, frequency-domain signal-space projection (FDSSP) was utilized to determine the temporal and spectral characteristics of spontaneous brain activity at specific cortical sites. Although intersubject differences were significant, spectra for individual subjects contained task-dependent features which were reproducible over successive 20-s epochs. This result supports the concept of multiple sources of spontaneous cortical activity and suggests that detailed spectra of localized oscillatory activity obtained non-invasively with magnetoencephalographic arrays may provide a useful characterization of cortical involvement in mental imagery.

Use of implicit motor imagery for visual shape discrimination as revealed by PET

Positron emission tomography (PET) can be used to map brain regions that are active when a visual object (for example, a hand) is discriminated from its mirror form. Chronometric studies suggest that viewers 'solve' this visual shape task by mentally modelling it as a reaching task, implicitly moving their left hand into the orientation of any left-hand stimulus (and conversely for a right-hand stimulus). Here we describe an experiment in which visual and somatic processing are dissociated by presenting right hands to the left visual field and vice versa. Frontal (motor), parietal (somatosensory) and cerebellar (sensorimotor) regions similar to those activated by actual and imagined movement are strongly activated, whereas primary somatosensory and motor cortices are not. We conclude that mental imagery is realized at intermediate-to-high order, modality-specific cortical systems, but does not require primary cortex and is not constrained to the perceptual systems of the presented stimuli.

Parietal control of hand action

Recent advances suggest that neurons of the anterior intraparietal area play a critical role in the visual guidance of hand action. The parietal cortex appears to process in-coming binocular visual signals of the three-dimensional features of objects and matches these signals with the motor signals, which come from the ventral premotor cortex, that will be required for hand manipulation of the object.