Nobuaki Mizuguchi

Combining observation and imagery of an action enhances human corticospinal excitability.

The present study investigated whether combining observation and imagery of an action increased corticospinal excitability over the effects of either manipulation performed alone. Corticospinal excitability was assessed by motor-evoked potentials in the biceps brachii muscle following transcranial magnetic stimulation over the motor cortex during observation, imagery or both. The action utilized was repetitive elbow flexion/extension. Simultaneous observation and imagery of the elbow action facilitated corticospinal excitability as compared to that recorded during observation or imagery alone. However, facilitation due to the combination of observation and imagery was not obtained when the participants imagined the action pattern while they observed the same action presented out of phase. These findings suggest that a combination of observation and imagery can enhance corticospinal excitability. This enhancement depends on phase consistency between the observed and imagined actions.

Influence of touching an object on corticospinal excitability during motor imagery

We investigated whether corticospinal excitability during the imagery of an action involving an external object was influenced by actually touching the object. Corticospinal excitability was assessed by monitoring motor evoked potentials (MEPs) in the first dorsal interosseous muscle following transcranial magnetic stimulation over the motor cortex during imagery of squeezing a ball—with or without passively holding the ball. The MEPs amplitude during imagery when the ball was held was larger than that when the ball was not held. The MEPs amplitude was not modulated just by holding the ball. In the same experimental condition, the somatosensory evoked potentials (SEPs) in response to the stimulation of median nerve were not modulated by motor imagery or by holding the ball. These results suggest that the corticospinal excitability during imagery of squeezing a ball is enhanced with the real touch of the ball, and the enhancement would be caused by some changes along the corticospinal pathway itself and not by the change in responsiveness along the afferent pathway to the primary somatosensory cortex.

Influence of somatosensory input on corticospinal excitability during motor imagery.

Our previous studies showed that corticospinal excitability during imagery of squeezing a foam ball was enhanced by somatosensory input generated by passively holding the ball. In the present study, using the same experimental model, we investigated whether corticospinal excitability was influenced by holding the object with the hand opposite to the imagined hand. Corticospinal excitability was assessed by monitoring motor evoked potentials (MEPs) in the first dorsal interosseous muscle following transcranial magnetic stimulation over the motor cortex during motor imagery. Subjects were asked to imagine squeezing a foam ball with the right hand (experiment 1) or the left hand (experiment 2), while either holding nothing (Null condition), a ball in the right hand (Right condition) or a ball in the left hand (Left condition). The MEPs amplitude during motor imagery was increased, only when the holding hand and the imagined hand were on the same side. These results suggest that performance improvement and rehabilitation exercises will be more effective when somatosensory stimulation and motor imagery are done on the same side.

Effector-independent brain activity during motor imagery of the upper and lower limbs: An fMRI study

We utilized functional magnetic resonance imaging (fMRI) to evaluate the common brain region of motor imagery for the right and left upper and lower limbs. The subjects were instructed to repeatedly imagined extension and flexion of the right or left hands/ankles. Brain regions, which included the supplemental motor area (SMA), premotor cortex and parietal cortex, were activated during motor imagery. Conjunction analysis revealed that the left SMA and inferior frontal gyrus (IFG)/ventral premotor cortex (vPM) were commonly activated with motor imagery of the right hand, left hand, right foot, and left foot. This result suggests that these brain regions are activated during motor imagery in an effector independent manner.

Interaction between simultaneous contraction and relaxation in different limbs

Abstract We investigated the interaction between relaxation and contraction for remote, ipsilateral muscles of the hand and foot (wrist extensor/ankle dorsiflexor). Subjects sat in an armchair and were able to freely move their right hand and foot. They performed eight tasks: (1) wrist extension from a flexed (relaxed) position, (2) ankle dorsiflexion from a plantarflexed (relaxed) position, (3) wrist relaxation from an extended position, (4) ankle relaxation from a moderately dorsiflexed position. The remaining tasks involved (5) simultaneous performance of tasks 1 and 2, (6) 3 and 4, (7) 1 and 4, and (8) 2 and 3. Subjects performed each task as fast as possible after hearing a start signal. They were instructed not to activate antagonistic muscles in the tasks involving relaxation. When contraction of the wrist flexor/ankle dorsiflexor was concurrent with relaxation of the ipsilateral limb (ankle dorsiflexor/wrist extensor), the reaction time of contraction as observed in electromyography (EMG) activities in the wrist extensor/ankle dorsiflexor became longer and the EMG activities became smaller. Our findings suggest that muscle relaxation in one limb interferes with smooth movement of muscle contraction in the ipsilateral limb.

The effect of somatosensory input on motor imagery depends upon motor imagery capability

We investigated that the relationship between motor imagery ability and the effect of tactile input associated with holding a tennis racket on motor imagery of the forehand and backhand swings. The effect was assessed by the time utilized for motor imagery (mental chronometry). Seventeen tennis players imagined forehand and backhand swings with a forehand grip, a backhand grip or while holding nothing. In all cases, imaging the swings took longer than the time taken for a real swing. For imagery of the backhand swing, holding a racket with a backhand grip decreased the imaging time (p < 0.05) as compared to the trials with a forehand grip or while holding nothing. On the other hand, holding the racket with a backhand grip tended to increase the time required for forehand swing imagery. These results suggest that a congruent grip improves, and an incongruent grip deteriorates, motor imagery of the backhand swing. For players who took a longer time in the condition where they held nothing (i.e., poor imaging ability), the effect of a congruent backhand grip was greater (r = 0.67, p < 0.01). However, a congruent forehand grip did not improve motor imagery of the forehand swing. Since 15 of the participants in the present study favored the forehand swing compared to the backhand swing, the participants would have been more familiar with the forehand swing. Thus it would have been easy to vividly imagine the (familiar) forehand swing even when they were not holding a racket. We speculate that tactile input associated with holding a tool improves a vividness of motor imagery of a less familiar movement, especially for those who have poor imaging ability. In the future, it will be important to clarify whether the effect of tactile input associated with holding a tool is dependent upon movement familiarity/performance level.

Activity of right premotor-parietal regions dependent upon imagined force level: an fMRI study

In this study, we utilized functional magnetic resonance imaging (fMRI) to measure blood oxygenation level-dependent (BOLD) signals. This allowed us to evaluate the relationship between brain activity and imagined force level. Subjects performed motor imagery of repetitive right hand grasping with three different levels of contractile force; 10%, 30%, and 60% of their maximum voluntary contraction (MVC). We observed a common activation among each condition in the following brain regions; the dorsolateral prefrontal cortex (DLPFC), ventrolateral prefrontal cortex (VLPFC), supplementary motor area (SMA), premotor area (PM), insula, and inferior parietal lobule (IPL). In addition, the BOLD signal changes were significantly larger at 60% MVC than at 10% MVC in the right PM, the right IPL, and the primary somatosensory cortex (SI). These findings indicate that during motor imagery right fronto-parietal activity increases as the imagined contractile force level is intensified. The present finding that the right brain activity during motor imagery is clearly altered depending on the imagined force level suggests that it may be possible to decode intended force level during the motor imagery of patients or healthy subjects.

Muscle relaxation of the foot reduces corticospinal excitability of hand muscles and enhances intracortical inhibition

The object of this study was to clarify the effects of foot muscle relaxation on activity in the primary motor cortex (M1) of the hand area. Subjects were asked to volitionally relax the right foot from sustained contraction of either the dorsiflexor (tibialis anterior; TA relaxation) or plantarflexor (soleus; SOL relaxation) in response to an auditory stimulus. Single- and paired-pulse transcranial magnetic stimulation (TMS) was delivered to the hand area of the left M1 at different time intervals before and after the onset of TA or SOL relaxation. Motor evoked potentials (MEPs) were recorded from the right extensor carpi radialis (ECR) and flexor carpi radialis (FCR). MEP amplitudes of ECR and FCR caused by single-pulse TMS temporarily decreased after TA and SOL relaxation onset, respectively, as compared with those of the resting control. Furthermore, short-interval intracortical inhibition (SICI) of ECR evaluated with paired-pulse TMS temporarily increased after TA relaxation onset. Our findings indicate that muscle relaxation of the dorsiflexor reduced corticospinal excitability of the ipsilateral hand muscles. This is most likely caused by an increase in intracortical inhibition.

Task-dependent engagements of the primary visual cortex during kinesthetic and visual motor imagery.

Motor imagery can be divided into kinesthetic and visual aspects. In the present study, we investigated excitability in the corticospinal tract and primary visual cortex (V1) during kinesthetic and visual motor imagery. To accomplish this, we measured motor evoked potentials (MEPs) and probability of phosphene occurrence during the two types of motor imageries of finger tapping. The MEPs and phosphenes were induced by transcranial magnetic stimulation to the primary motor cortex and V1, respectively. The amplitudes of MEPs and probability of phosphene occurrence during motor imagery were normalized based on the values obtained at rest. Corticospinal excitability increased during both kinesthetic and visual motor imagery, while excitability in V1 was increased only during visual motor imagery. These results imply that modulation of cortical excitability during kinesthetic and visual motor imagery is task dependent. The present finding aids in the understanding of the neural mechanisms underlying motor imagery and provides useful information for the use of motor imagery in rehabilitation or motor imagery training.

Motor imagery beyond the motor repertoire: Activity in the primary visual cortex during kinesthetic motor imagery of difficult whole body movements

To elucidate the neural substrate associated with capabilities for kinesthetic motor imagery of difficult whole-body movements, we measured brain activity during a trial involving both kinesthetic motor imagery and action observation as well as during a trial with action observation alone. Brain activity was assessed with functional magnetic resonance imaging (fMRI). Nineteen participants imagined three types of whole-body movements with the horizontal bar: the giant swing, kip, and chin-up during action observation. No participant had previously tried to perform the giant swing. The vividness of kinesthetic motor imagery as assessed by questionnaire was highest for the chin-up, less for the kip and lowest for the giant swing. Activity in the primary visual cortex (V1) during kinesthetic motor imagery with action observation minus that during action observation alone was significantly greater in the giant swing condition than in the chin-up condition within participants. Across participants, V1 activity of kinesthetic motor imagery of the kip during action observation minus that during action observation alone was negatively correlated with vividness of the kip imagery. These results suggest that activity in V1 is dependent upon the capability of kinesthetic motor imagery for difficult whole-body movements. Since V1 activity is likely related to the creation of a visual image, we speculate that visual motor imagery is recruited unintentionally for the less vivid kinesthetic motor imagery of difficult whole-body movements.