Influence of mental motor imagery on the execution of a finger-to-thumb opposition task.

The present fMRI study compares regional distribution of the cortical activity during the execution of unilateral hand movements (finger-to-thumb opposition) preceded or not by their motor simulation (S + E and E condition, respectively). The results show that, overall, the number and the spatial distribution of activated voxels are both increased in the S + E with respect to the E condition. The motor performance preceded by mental rehearsal is related to selective increase of the cortical activity. Among the motor areas that are found active during the simple motor execution a significant enhancement of functional activation during the S + E condition ipsilateral primary motor regions (M1). The activity increase may be accounted by a sort of neural recruiting that is made possible by the overlapping of cortical networks involved in both motor output and motor imagery. The beneficial effects of "mental practice" on the physical performance may rely to the close temporal association between motor rehearsal and actual performance.

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.

Mental imaging of motor activity in humans.

Motor imagery corresponds to a subliminal activation of the motor system, a system that appears to be involved not only in producing movements, but also in imagining actions, recognising tools and learning by observation, as well as in understanding the behaviour of other people. Recent advances in the field include the use of techniques for mapping brain activity and probing cortical excitability, as well as observation of brain lesioned patients during imaging tasks; these advances provide new insights into the covert aspects of motor activity.

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.

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.

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 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.

[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.

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.

Visual control of reaching movements without vision of the limb. II. Evidence of fast unconscious processes correcting the trajectory of the hand to the final position of a double-step stimulus.

In this study, a visual target was localized by both limb and eye. The experimental procedure provided an opportunity to analyze the limb movement trajectories to the target whose location was displaced during saccades. Absence of visual information about position of the moving limb did not interfere with correction of the trajectory of pointing movements. These corrections reflect the new information about target position that becomes available at the end of the first saccade. Mean localization errors to stationary and to displaced targets were not significantly different. This result suggests that subjects were able to compare visual (retinal + eye position) information about the position of the target with information about the position of their moving limb derived from kinesthesis and/or efference copies of the motor commands. An analysis of velocity profiles indicates that the observed corrections of hand movement to target displacement could not be identified by an inflexion point in the trajectory. None of the subjects reported seeing the target change location. In other words, the motor command was adjustable despite the failure of changes in visual locus to reach consciousness.

Unidirectional habituation of vestibulo-ocular responses by repeated rotational or optokinetic stimulations in the cat.

1. Unilateral habituation of the vestibulo-ocular reflex was produced in adult cats stimulated by repeated unidirectional velocity steps (vestibular training) or by a continuously moving visual surround (optokinetic training). -- 2. Unidirectional vestibular training produced a strong asymmetry of vestibulo-ocular responses (VOR). Responses to velocity steps applied to the "trained" labyrinth were decreased both in gain and in time-constant. This effect generalized to responses to sinusoidal oscillations (0.03 Hz to 0.1 Hz), i.e. to a stimulus not used during training. -- No spontaneous nystagmus was ever observed in spite of the dynamic VOR asymmetry. -- 3. Unilateral vestibular habituation produced by vestibular training appeared to be a long-lasting phenomenon. It was still present 10 days after the end of training. -- 4. Optokinetic responses were not affected by vestibular training. -- 5. Unidirectional optokinetic training produced an increase in the slow phase velocity of optokinetic nystagmus (OKN) by about 25% in both directions. This effect did not persist for more than a few minutes. A marked spontaneous nystagmus was recorded in the dark after each session of optokinetic training, with a slow phase in the direction opposite to the previous OKN. -- 6. VOR in response to velocity steps and to sinusoidal oscillations were decreased unilaterally after optokinetic training. This effect was of short duration, however, and disappeared within the interval between training sessions. This lack of retention contrasted with the prolonged effect of vestibular training.