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.

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.

Possible involvement of primary motor cortex in mentally simulated movement: a functional magnetic resonance imaging study.

The role of the primary motor cortex (M1) during mental simulation of movement is open to debate. In the present study, functional magnetic resonance imaging (fMRI) signals were measured in normal right-handed subjects during actual and mental execution of a finger-to-thumb opposition task with either the right or the left hand. There were no significant differences between the two hands with either execution or simulation. A significant involvement of contralateral M1 (30% of the activity found during execution) was detected in four of six subjects. Premotor cortex (PM) and the rostral part of the posterior SMA were activated bilaterally during motor imagery. These findings support the hypothesis that motor imagery involves virtually all stages of motor control.

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.

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.

[Imagery and its neurological substrate]. / L'imagerie et son substrat neurologique.

What is the nature and the neural substrate of mental representation? This paper reviews findings from experimental psychology demonstrating that visual imagery and perception have similar characteristics. These results suggest that visual imagery and visual perception rely on the same neural substrate. Brain imaging studies as well as clinical observations of neurological patients support this hypothesis. Visual imagery involves visual cortical areas. However, selective visual impairments following damage to the cortical visual system may produce some dissociation between imagery and perception. Similar observations concerning motor imagery are now established in both normals and in neurological patients. Evidence that motor imagery and motor control share some modality specific neural representations are clearly supported by tomographic measurements of cerebral blood flow.

Motor imagery of a lateralized sequential task is asymmetrically slowed in hemi-Parkinson's patients.

We examined seven right-handed, asymmetrical (right side affected) Parkinson's disease patients and seven age-matched controls in a manual finger sequencing test using left and right hands in vision, no vision, and motor imagery conditions. All patients displayed motor asymmetry, favoring the left hand. They also displayed motor imagery asymmetry, mentally simulating movement more slowly with their right affected hand than with their left hand. Additionally, impairment in mental hand rotation correlated significantly with the imagery asymmetry. These data support two related hypotheses: (a) Motor sequence imagery and execution share common neural structures. (b) The frontostriatal system is among these shared structures.

Mapping motor representations with positron emission tomography.

Brain activity was mapped in normal subjects during passive observation of the movements of an 'alien' hand and while imagining grasping objects with their own hand. None of the tasks required actual movement. Shifting from one mental task to the other greatly changed the pattern of brain activation. During observation of hand movements, activation was mainly found in visual cortical areas, but also in subcortical areas involved in motor behaviour, such as the basal ganglia and the cerebellum. During motor imagery, cortical and subcortical areas related to motor preparation and programming were strongly activated. These data support the notion that motor learning during observation of movements and mental practice involves rehearsal of neural pathways related to cognitive stages of motor control.

Vegetative response during imagined movement is proportional to mental effort.

Measurement of cardiac and respiratory activity during mental simulation of locomotion at increasing speed revealed a covariation of heart rate and pulmonary ventilation with the degree of imagined effort. The degree of vegetative activation of a subject mentally running at 12 km/h was comparable to that of a subject actually walking at 5 km/h. This effect cannot be explained by an increase in peripheral (e.g. muscular) metabolic demands. Indeed, oxygen uptake decreased during motor imagery. This finding is suggestive of a commonality of neural structures responsible for mental imagery of movement and those responsible for programming actual movement. In addition, it provides an quantifiable way of testing mental imagery in relation to movement by using easily accessible biological markers.

Sensation of effort and duration of mentally executed actions.

The aim of this study was to examine whether a sensation of effort would arise in subjects requested, by verbal instructions, to mentally perform motor tasks with an internal imagery strategy. Sixteen subjects had to imagine themselves, from a first person perspective, writing a sentence and drawing a Necker's cube either with their right dominant hand or with their left hand, as well as hopping around a square either on their right or left foot. The time needed to mentally execute these actions was measured. The sensation of effort following the mental performance and the difficulty of imaging the tasks were assessed by means of two analog rating scales. The results indicate that the sensation of effort increased across the trials. Furthermore, a negative correlation coefficient (mean r = -0.99) was found between the difficulty to imagining a given motor task and the subjective sensation of effort across the trials. Moreover, the sensation of effort was more pronounced when the tasks involved the non-dominant limb.

The cerebellum participates in mental activity: tomographic measurements of regional cerebral blood flow.

Measurements in man of regional cerebral blood flow (rCBF) have demonstrated a number of cortical and subcortical events coupled to sensory stimulation or motor performance. It has also been shown that local activity changes take place in the cortex during 'pure' mental activity such as motor imagery (unaccompanied by sensory input or motor output). Thus, our group has previously shown that imagination of hand movements gives predominantly a frontal cortical rCBF activation while the corresponding hand movement activates the rolandic hand area mainly. In this paper we report tomographic rCBF measurements with a 133-Xenon SPECT technique during imagined tennis movements and silent counting. Both procedures gave rise to a significant cerebellar activation in addition to cortical rCBF changes. Apparently, the cerebellum may participate in pure mental activity. It possibly plays a role for the temporal organization of neuronal events related to cognition.

Effect of brain and spinal cord injuries on motor imagery.

The timing of mentally executed movements was measured in ten patients with hemiplegia, tetraplegia and paraplegia. In hemiplegic patients a significant difference in mental duration times was found between the paralysed and the normal "represented limb". The paralysed limb was mentally much slower than the healthy one. In contrast, movement times in tetraplegic and paraplegic patients did not differ from those in normal subjects. All patients reported a sensation of subjective effort accompanying the execution of the mental tasks. These observations are compatible with an outflow processing underlying motor imagery.

The timing of mentally represented actions.

The performance of subjects walking blindly to previously inspected visual targets (located at 5, 10 or 15 m from the subjects) was studied in 2 experiments. In Expt. 1, subjects selected as good visual imagers were instructed to build up a mental representation of the target. Then they had to either actually walk or imagine themselves walking to the target. Walking time was measured in both the actual and the mental performance. It was found that subjects took almost exactly the same time in the two conditions. Accuracy of these subjects was also measured in the actual walking task. They were found to make no direction errors and to slightly overshoot target location. Subjects from another, control, group, who received no instructions about visual imagery made much larger errors. In Expt. 2, actual and mental walking times were measured in the same subjects as in Expt. 1, while they carried a 25-kg weight on their shoulders. In this condition, actual walking time was the same as in Expt. 1, although mental walking time was found to increase systematically by about 30%. These results are discussed in terms of the neural parameters encoded in the motor program for actually executing or mentally performing an action.