Facilitatory effect of thinking about movement on motor-evoked potentials to transcranial magnetic stimulation of the brain

The effects of physical training versus combined action observation and motor imagery in conjunction with physical training on upper-extremity performance

INTRODUCTION

Combined action observation and motor imagery training (AO+MI training), which involves motor imagery during action observation and physical training, has been attracting attention as an effective strategy for learning motor skills. However, little has been reported on the effects of AO+MI training. In the present study, we compared the effects of AO+MI training to the effects of physical training on upper-extremity performance.

MATERIALS AND METHODS

Ninety-six healthy participants were randomly assigned to either the control group or the experimental group. Sport stacking, which is often used to evaluate upper-extremity performance, was adopted for the task. The experiment was scheduled for three days. The training was 20 min per day. The control group performed only physical training, while the experimental group performed four 5-min AO+MI training sessions. Time taken to complete a sport stacking try (task completion time) was defined as the index of speed of upper-extremity performance and number of fallen cups as the index of its accuracy. The outcomes within each group and between the two groups were compared.

RESULTS

Both AO+MI training and physical training showed reduced task completion time and increased number of fallen cups. There were no significant differences in the degree of changes between the groups.

CONCLUSION

Results from the present study showed that AO+MI training and physical training had almost the same influence on upper-extremity performance in the early stages of learning sport stacking. This result suggests that AO+MI training may be an effective and low-burden training method for participants.

On the characterization of cognitive tasks using activity-specific short-lived synchronization between electroencephalography channels

Accurate characterization of brain activity during a cognitive task is challenging due to the dynamically changing and the complex nature of the brain. The majority of the proposed approaches assume stationarity in brain activity and disregard the systematic timing organization among brain regions during cognitive tasks. In this study, we propose a novel cognitive activity recognition method that captures the activity-specific timing parameters from training data that elicits maximal average short-lived pairwise synchronization between electroencephalography signals. We evaluated the characterization power of the activity-specific timing parameter triplets in a motor imagery activity recognition framework. The activity-specific timing parameter triplets consist of latency of the maximally synchronized signal segments from activity onset Δt, the time lag between maximally synchronized signal segments τ, and the duration of the maximally synchronized signal segments w. We used cosine-based similarity, wavelet bi-coherence, phase-locking value, phase coherence value, linearized mutual information, and cross-correntropy to calculate the channel synchronizations at the specific timing parameters. Recognition performances as well as statistical analyses on both BCI Competition-III dataset IVa and PhysioNet Motor Movement/Imagery dataset, indicate that the inter-channel short-lived synchronization calculated using activity-specific timing parameter triplets elicit significantly distinct synchronization profiles for different motor imagery tasks and can thus reliably be used for cognitive task recognition purposes.

Motor imagery enhances corticospinal transmission mediated by cervical premotoneurons in humans

Imaging movement has positive effects on the reacquisition of motor functions after damage to the central nervous system. This study shows that motor imagery facilitates oligosynaptic corticospinal excitation that is mediated via cervical premotoneurons, which may be important for motor recovery in monkeys and humans. Current findings highlight how this imagery might be a beneficial tool for movement disorders through effects on premotoneuron circuitry.

Effects of Music Reading on Motor Cortex Excitability in Pianists: A Transcranial Magnetic Stimulation Study

Neurophysiological studies suggest that music reading facilitates sensorimotor cortex. The aim of this study was to evaluate (1) whether in pianists, reading notes in bass and treble clef selectively enhances right and left primary motor cortex (M1) excitability; and (2) whether reading notes played with the thumb or little finger selectively modulates the excitability of specific muscles. Twenty musicians (11 pianists, 9 non-pianists) participated. Transcranial magnetic stimulation (TMS) was applied while subjects read the bass or the treble clef of sheets music and during the observation of a blank staff (baseline). When pianists read the treble clef, the excitability of the left M1 was higher compared to that recorded in the right M1. Moreover, in the treble clef condition motor evoked potentials (MEPs) induced by TMS of the left M1 were higher when pianists read notes to be played with the 5° finger (little finger) with respect to 1° finger (thumb) notes, whereas in the bass clef condition TMS of the right M1 induced higher MEPs for 1° finger note compared to 5° finger notes. No significant modulation was observed in non-pianists. These data support the view that music reading may induce specific inter- and intra-hemispheric modulation of the motor cortex excitability.

Visual and kinesthetic modes affect motor imagery classification in untrained subjects

The understanding of neurophysiological mechanisms responsible for motor imagery (MI) is essential for the development of brain-computer interfaces (BCI) and bioprosthetics. Our magnetoencephalographic (MEG) experiments with voluntary participants confirm the existence of two types of motor imagery, kinesthetic imagery (KI) and visual imagery (VI), distinguished by activation and inhibition of different brain areas in motor-related α- and β-frequency regions. Although the brain activity corresponding to MI is usually observed in specially trained subjects or athletes, we show that it is also possible to identify particular features of MI in untrained subjects. Similar to real movement, KI implies muscular sensation when performing an imaginary moving action that leads to event-related desynchronization (ERD) of motor-associated brain rhythms. By contrast, VI refers to visualization of the corresponding action that results in event-related synchronization (ERS) of α- and β-wave activity. A notable difference between KI and VI groups occurs in the frontal brain area. In particular, the analysis of evoked responses shows that in all KI subjects the activity in the frontal cortex is suppressed during MI, while in the VI subjects the frontal cortex is always active. The accuracy in classification of left-arm and right-arm MI using artificial intelligence is similar for KI and VI. Since untrained subjects usually demonstrate the VI imagery mode, the possibility to increase the accuracy for VI is in demand for BCIs. The application of artificial neural networks allows us to classify MI in raising right and left arms with average accuracy of 70% for both KI and VI using appropriate filtration of input signals. The same average accuracy is achieved by optimizing MEG channels and reducing their number to only 13.

Motor Imagery of Speech: The Involvement of Primary Motor Cortex in Manual and Articulatory Motor Imagery

Motor imagery refers to the phenomenon of imagining performing an action without action execution. Motor imagery and motor execution are assumed to share a similar underlying neural system that involves primary motor cortex (M1). Previous studies have focused on motor imagery of manual actions, but articulatory motor imagery has not been investigated. In this study, transcranial magnetic stimulation (TMS) was used to elicit motor-evoked potentials (MEPs) from the articulatory muscles [orbicularis oris (OO)] as well as from hand muscles [first dorsal interosseous (FDI)]. Twenty participants were asked to execute or imagine performing a simple squeezing task involving a pair of tweezers, which was comparable across both effectors. MEPs were elicited at six time points (50, 150, 250, 350, 450, 550 ms post-stimulus) to track the time course of M1 involvement in both lip and hand tasks. The results showed increased MEP amplitudes for action execution compared to rest for both effectors at time points 350, 450 and 550 ms, but we found no evidence of increased cortical activation for motor imagery. The results indicate that motor imagery does not involve M1 for simple tasks for manual or articulatory muscles. The results have implications for models of mental imagery of simple articulatory gestures, in that no evidence is found for somatotopic activation of lip muscles in sub-phonemic contexts during motor imagery of such tasks, suggesting that motor simulation of relatively simple actions does not involve M1.

Neural activity related to volitional regulation of cortical excitability

To date there exists no reliable method to non-invasively upregulate or downregulate the state of the resting human motor system over a large dynamic range. Here we show that an operant conditioning paradigm which provides neurofeedback of the size of motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS), enables participants to self-modulate their own brain state. Following training, participants were able to robustly increase (by 83.8%) and decrease (by 30.6%) their MEP amplitudes. This volitional up-versus down-regulation of corticomotor excitability caused an increase of late-cortical disinhibition (LCD), a TMS derived read-out of presynaptic GABAB disinhibition, which was accompanied by an increase of gamma and a decrease of alpha oscillations in the trained hemisphere. This approach paves the way for future investigations into how altered brain state influences motor neurophysiology and recovery of function in a neurorehabilitation context.

Dynamic Changes in Upper-Limb Corticospinal Excitability during a 'Pro-/Anti-saccade' Double-Choice Task

Under natural behavioral conditions, visually guided eye movements are linked to direction-specific modulations of cortico-spinal system (CSS) excitability in upper-limb muscles, even in absence of a manual response. These excitability changes have been shown to be compatible with a covert motor program encoding a manual movement toward the same target of the eyes. The aim of this study is to investigate whether this implicit oculo-manual coupling is enforced following every saccade execution or it depends on the behavioral context. Twenty-two healthy young adults participated in the study. Single-pulse transcranial magnetic stimulation was applied to the motor cortex at nine different time epochs during a double-choice eye task, in which the decision to execute a prosaccade or an antisaccade was made on the color of a peripheral visual cue. By analyzing the amplitude of the motor evoked potentials (MEP) in three distal muscles of the resting upper-limb, a facilitation peak of CSS excitability was found in two of them at 120 ms before the eyes begin to move. Furthermore, a long-lasting, generalized reduced corticomotor excitability develops following the eye response. Finally, a quite large modulation of MEP amplitude, depending on the direction of the saccade, is observed only in the first dorsal interosseous muscle, in a narrow time window at about 150 ms before the eye movement, irrespective of the type of the ocular response (pro-/anti-saccade). This change in CSS excitability is not tied up to the timing of the occurrence of the visual cue but, instead, appears to be tightly time-related to the saccade onset. Observed excitability changes differ in many respects from those previously reported with different behavioral paradigms. A main finding of our study is that the implicit coupling between eye and hand motor systems is contingent upon the particular motor set determined by the cognitive aspects of the performed oculomotor task. In particular, the direction-specific modulation in CSS excitability described in this study appears to be related to perceptual and decision-making processes rather than representing an implicit upper-limb motor program, coupled to the saccade execution.

Clinical Factors Underlying the Inter-individual Variability of the Resting Motor Threshold in Navigated Transcranial Magnetic Stimulation Motor Mapping

Correctly determining individual's resting motor threshold (rMT) is crucial for accurate and reliable mapping by navigated transcranial magnetic stimulation (nTMS), which is especially true for preoperative motor mapping in brain tumor patients. However, systematic data analysis on clinical factors underlying inter-individual rMT variability in neurosurgical motor mapping is sparse. The present study examined 14 preselected clinical factors that may underlie inter-individual rMT variability by performing multiple regression analysis (backward, followed by forward model comparisons) on the nTMS motor mapping data of 100 brain tumor patients. Data were collected from preoperative motor mapping of abductor pollicis brevis (APB), abductor digiti minimi (ADM), and flexor carpi radialis (FCR) muscle representations among these patients. While edema and age at exam in the ADM model only jointly reduced the unexplained variance significantly, the other factors kept in the ADM model (gender, antiepileptic drug intake, and motor deficit) and each of the factors kept in the APB and FCR models independently significantly reduced the unexplained variance. Hence, several clinical parameters contribute to inter-individual rMT variability and should be taken into account during initial and follow-up motor mappings. Thus, the present study adds basic evidence on inter-individual rMT variability, whereby some of the parameters are specific to brain tumor patients.

Imagery Improves Upper Extremity Motor Function in Chronic Stroke Patients: A Pilot Study

After medical stabilization, stroke patients receive rehabilitation during a period that is considered subacute. The traditional view of neurologic rehabilitation is that it reduces impairment and minimizes disability. However, intensive rehabilitation is expensive and there are limited and diminishing financial resources to pay for therapy after stroke. Imagery improves motor learning and performance. Studies report increased blood flow, electromyographic activity during imagery, and subsequent changes in organization of the motor cortex in stroke patients following imagery. Since these events usually precede reacquisition of function, it has been suggested that imagery may be beneficial in reacquisition of function following stroke. To test this, 8 chronic stroke patients with right-arm hemiparesis were provided a four-week program combining occupational therapy and imagery (OT + I), while 8 controls received a program consisting only of OT. Scores on the upper extremity section of the Fugl-Meyer Assessment of Sensorimotor Recovery indicated that those receiving OT + I exhibited significantly more improved function than those receiving OT ( F [1,14] −14.71; p < .05). Findings suggest that imagery may be an inexpensive, noninvasive compliment to OT for hemiparesis in stroke. The researchers encourage further investigation of OT + I in stroke.

Mental imagery-induced attention modulates pain perception and cortical excitability

Background

Mental imagery is a powerful method of altering brain activity and behavioral outcomes, such as performance of cognition and motor skills. Further, attention and distraction can modulate pain-related neuronal networks and the perception of pain. This exploratory study examined the effects of mental imagery-induced attention on pressure pain threshold and cortical plasticity using transcranial magnetic stimulation (TMS). This blinded, randomized, and parallel-design trial comprised 30 healthy right-handed male subjects. Exploratory statistical analyses were performed using ANOVA and t-tests for pain and TMS assessments. Pearson’s correlation was used to analyze the association between changes in pain threshold and cortical excitability.

Results

In the analysis of pain outcomes, there was no significant interaction effect on pain between group versus time. In an exploratory analysis, we only observed a significant effect of group for the targeted left hand (ANOVA with pain threshold as the dependent variable and time and group as independent variables). Although there was only a within-group effect of mental imagery on pain, further analyses showed a significant positive correlation of changes in pain threshold and cortical excitability (motor-evoked potentials via TMS).

Conclusions

Mental imagery has a minor effect on pain modulation in healthy subjects. Its effects appear to differ compared with chronic pain, leading to a small decrease in pain threshold. Assessments of cortical excitability confirmed that these effects are related to the modulation of pain-related cortical circuits. These exploratory findings suggest that neuronal plasticity is influenced by pain and that the mental imagery effects on pain depend on the state of central sensitization.

Nonphysiological factors in navigated TMS studies; confounding covariates and valid intracortical estimates

Brain stimulation is used to induce transient alterations of neural excitability to probe or modify brain function. For example, single-pulse transcranial magnetic stimulation (TMS) of the motor cortex can probe corticospinal excitability (CSE). Yet, CSE measurements are confounded by a high level of variability. This variability is due to physical and physiological factors. Navigated TMS (nTMS) systems can record physical parameters of the TMS coil (tilt, location, and orientation) and some also estimate intracortical electric fields (EFs) on a trial-by-trial basis. Thus, these parameters can be partitioned with stepwise regression.

PURPOSE

The primary objective was to dissociate variance due to physical parameters from variance due to physiological factors for CSE estimates. The secondary objective was to establish the predictive validity of EF estimates from spherical head models.

HYPOTHESIS

Variability of physical parameters of TMS predicts CSE variability.

METHODS

Event-related measurements of physical parameters were analyzed in stepwise regression. Partitioned parameter variance and predictive validity were compared for a target-controlled and a nontarget-controlled experiment. A control experiment (preinnervation) confirmed the validity of linear data analysis. A bias-free model quantified the effect of divergence from optimum.

RESULTS

Partitioning physical parameter variance reduces CSE variability. EF estimates from spherical models were valid. Post hoc analyses showed that even small physical fluctuations can confound the statistical comparison of CSE measurements.

CONCLUSIONS

It is necessary to partition physical and physiological variance in TMS studies to make confounded data interpretable. The spatial resolution of nTMS is <5 mm and the EF-estimates are valid.

Inadequate modulation of excitability with voluntary dorsiflexion in Parkinson's disease

BACKGROUND

Freezing phenomenon at onset of movement causes gait disturbance in Parkinson's disease (PD), but the pathophysiology is unclear. We studied motor property at onset of dorsiflexion in PD.

METHODS

In 9 patients with PD and 8 normal subjects, motor evoked potential was recorded from the tibialis anterior muscle under 3 conditions: at rest, during tonic contraction, and at onset of contraction. Motor threshold, size of motor evoked potential and the relationship between the intensity of transcranial magnetic stimulation, and the size of motor evoked potentials (recruitment gain) were examined.

RESULTS

Motor threshold decreased with voluntary contraction in both PD and normal subjects, but the threshold at rest and during tonic contraction was lower in Parkinson's disease. The size of motor evoked potential with maximal stimulus intensity increased with voluntary contraction in both groups; this tendency was more pronounced in normal subjects. The recruitment gain during contraction was steeper than at rest in normal subjects. However, there was no such increase in PD.

CONCLUSIONS

There was no increase in recruitment gain with voluntary contraction in PD, which was obvious in normal subjects, especially at onset of voluntary contraction. Modulation of motor excitability at onset of voluntary contraction was impaired in PD.

Binocular vision enhances a rapidly evolving affordance priming effect: behavioural and TMS evidence

Extensive research has suggested that simply viewing an object can automatically prime compatible actions for object manipulation, known as affordances. Here we explored the generation of covert motor plans afforded by real objects with precision ('pinchable') or whole-hand/power ('graspable') grip significance under different types of vision. In Experiment 1, participants viewed real object primes either monocularly or binocularly and responded to orthogonal auditory stimuli by making precision or power grips. Pinchable primes facilitated congruent precision grip responses relative to incongruent power grips, and vice versa for graspable primes, but only in the binocular vision condition. To examine the temporal evolution of the binocular affordance effect, participants in Experiment 2 always viewed the objects binocularly but made no responses, instead receiving a transcranial magnetic stimulation pulse over their primary motor cortex at three different times (150, 300, 450ms) after prime onset. Motor evoked potentials (MEPs) recorded from a pinching muscle were selectively increased when subjects were primed with a pinchable object, whereas MEPs from a muscle associated with power grips were increased when viewing graspable stimuli. This interaction was obtained both 300 and 450ms (but not 150ms) after the visual onset of the prime, characterising for the first time the rapid development of binocular grip-specific affordances predicted by functional accounts of the affordance effect.

The supplementary motor area exerts a tonic excitatory influence on corticospinal projections to phrenic motoneurons in awake humans

Introduction

In humans, cortical mechanisms can interfere with autonomic breathing. Respiratory-related activation of the supplementary motor area (SMA) has been documented during voluntary breathing and in response to inspiratory constraints. The SMA could therefore participate in the increased resting state of the respiratory motor system during wake (i.e. "wakefulness drive to breathe").

Methods

The SMA was conditioned by continuous theta burst magnetic stimulation (cTBS, inhibitory) and 5 Hz conventional rTMS (5 Hz, excitatory). The ensuing effects were described in terms of the diaphragm motor evoked response (DiMEPs) to single-pulse transcranial magnetic stimulation over the motor cortex. DiMEPs were recorded at baseline, and at 3 time-points ("post1", "post2", "post3") up to 15 minutes following conditioning of the SMA.

Results

cTBS reduced the amplitude of DiMEPs from 327.5±159.8 µV at baseline to 243.3±118.7 µV, 217.8±102.9 µV and 240.6±123.9 µV at post 1, post 2 and post 3, respectively (F = 6.341, p = 0.002). 5 Hz conditioning increased the amplitude of DiMEPs from 184.7±96.5 µV at baseline to 270.7±135.4 µV at post 3 (F = 4.844, p = 0.009).

Conclusions

The corticospinal pathway to the diaphragm can be modulated in both directions by conditioning the SMA. This suggests that the baseline respiratory activity of the SMA represents an equipoise from which it is possible to move in either direction. The resting corticofugal outflow from the SMA to phrenic motoneurones that this study evidences could putatively contribute to the wakefulness drive to breathe.

Effects of parietal TMS on visual and auditory processing at the primary cortical level -- a concurrent TMS-fMRI study

Accumulating evidence suggests that multisensory interactions emerge already at the primary cortical level. Specifically, auditory inputs were shown to suppress activations in visual cortices when presented alone but amplify the blood oxygen level-dependent (BOLD) responses to concurrent visual inputs (and vice versa). This concurrent transcranial magnetic stimulation-functional magnetic resonance imaging (TMS-fMRI) study applied repetitive TMS trains at no, low, and high intensity over right intraparietal sulcus (IPS) and vertex to investigate top-down influences on visual and auditory cortices under 3 sensory contexts: visual, auditory, and no stimulation. IPS-TMS increased activations in auditory cortices irrespective of sensory context as a result of direct and nonspecific auditory TMS side effects. In contrast, IPS-TMS modulated activations in the visual cortex in a state-dependent fashion: it deactivated the visual cortex under no and auditory stimulation but amplified the BOLD response to visual stimulation. However, only the response amplification to visual stimulation was selective for IPS-TMS, while the deactivations observed for IPS- and Vertex-TMS resulted from crossmodal deactivations induced by auditory activity to TMS sounds. TMS to IPS may increase the responses in visual (or auditory) cortices to visual (or auditory) stimulation via a gain control mechanism or crossmodal interactions. Collectively, our results demonstrate that understanding TMS effects on (uni)sensory processing requires a multisensory perspective.

Facilitation of corticospinal excitability during motor imagery of wrist movement with visual or quantitative inspection of EMG activity

The present study investigated facilitation of corticospinal excitability during motor imagery of wrist movement with visual or quantitative inspection of background electromyographic (EMG) activity. Ten healthy participants imagined wrist extension from a first-person perspective in response to a start cue. Transcranial magnetic stimulation was delivered to the motor cortex 2 sec. after the start cue. EMG signals were recorded from the extensor carpi radialis muscle. Trials with background EMG activity were discarded based on visual inspection. Both motor-evoked potential (MEP) and background EMG amplitudes increased during motor imagery. The amount of increase in MEP amplitude was positively correlated with the amount of increase in background EMG amplitude during motor imagery. The statistically significant increase in MEP amplitude during motor imagery disappeared when the effect of muscle activity was statistically eliminated or after trials with background EMG activity were discarded based on strict quantitative criteria. Facilitation of corticospinal excitability during motor imagery of wrist movement depends partially on muscle activity. Discarding background EMG activity during motor imagery based on visual inspection is not sufficient to equalize background EMG amplitude between resting and motor imagery. Discarding trials with background EMG activity through strict quantitative criteria is useful to equalize background EMG amplitude between at rest and during motor imagery.

Viewing objects and planning actions: on the potentiation of grasping behaviours by visual objects

How do humans interact with tools? Gibson (1979) suggested that humans perceive directly what tools afford in terms of meaningful actions. This "affordances" hypothesis implies that visual objects can potentiate motor responses even in the absence of an intention to act. Here we explore the temporal evolution of motor plans afforded by common objects. We presented objects that have a strong significance for action (pinching and grasping) and objects with no such significance. Two experimental tasks involved participants viewing objects presented on a computer screen. For the first task, they were instructed to respond rapidly to changes in background colour by using an apparatus mimicking precision and power grip responses. For the second task, they received stimulation of their primary motor cortex using transcranial magnetic stimulation (TMS) while passively viewing the objects. Muscular responses (motor evoked potentials: MEPs) were recorded from two intrinsic hand muscles (associated with either a precision or power grip). The data showed an interaction between type of response (or muscle) and type of object, with both reaction time and MEP measures implying the generation of a congruent motor plan in the period immediately after object presentation. The results provide further support for the notion that the physical properties of objects automatically activate specific motor codes, but also demonstrate that this influence is rapid and relatively short lived.

The spinal substrate of the suppression of action during action observation

We have previously demonstrated that the forelimb representations of the primary motor and somatosensory cortices, as well as several premotor and parietal areas, are activated by both action-execution and action-observation, indicating that the spectator mentally simulates the observed action. Moreover, several studies demonstrated repeatedly that corticospinal excitability is modulated during action observation, providing evidence of an activation of the observer's motor system. However, evidence for the involvement of the spinal cord in action observation is controversial. The aim of the present study was to explore whether and how action-observation affects the spinal cord. To this end, we analyzed the spinal cord of eight monkeys (Macaca mulatta) trained to either execute reaching-to-grasp movements or observe the experimenter performing the same movements. Observation of grasping induced a bilateral decrease of glucose consumption in the spinal forelimb representation, whereas execution of grasping induced an increase of glucose utilization in the same area, ipsilaterally to the grasping hand. The depression of overall activity in the cervical enlargement of the spinal cord for action-observation may explain the suppression of overt movements, despite the activation of the observer's motor system.

Motor excitability during imagination and observation of foot dorsiflexions

To explore the effects of motor imagery (MI) and action observation (AO) of foot movements on motor excitability. Fifteen healthy subjects were studied at rest, during MI of foot dorsiflexions and during watching a video of foot dorsiflexions. Transcranial magnetic stimulation was used to explore corticospinal and intracortical excitability by comparing amplitudes of motor-evoked potentials during the different conditions. F waves were recorded to test the spinal motoneuronal excitability. MI and AO increased corticospinal excitability, but MI was more effective than AO. During MI, intracortical inhibition was reduced. Intracortical facilitation and spinal motoneuronal excitability remained unchanged. Excitability increases were similar for the right and the left leg when recording from the side the subjects had focused their MI on. However, MI of left foot dorsiflexions did not increase excitability in the right tibial anterior muscle. MI and AO of foot dorsiflexions enhance motor excitability. MI induced a disinhibition in the motor cortex. The lack of excitability increase during MI of contralateral foot movements might be related to the alternating movement pattern during walking. MI and AO effects could support the restitution of motor deficits in neurological diseases with impaired motor excitability.

Modulation of interhemispheric inhibition by volitional motor activity: an ipsilateral silent period study

Brief interruption of voluntary EMG in a hand muscle by focal transcranial magnetic stimulation (TMS) of the ipsilateral primary motor cortex (M1), the so-called ipsilateral silent period (ISP), is a measure of interhemispheric motor inhibition. However, little is known about how volitional motor activity would modulate the ISP. Here we tested in 30 healthy adults to what extent and under what conditions voluntary activation of the stimulated right M1 by moving the left hand strengthens interhemispheric inhibition as indexed by an enhancement of the ISP area in the maximally contracting right first dorsal interosseous (FDI). Left index finger abduction, already at low levels of contraction, significantly enhanced the ISP compared to left hand at rest. Even imagination of left index finger movement enhanced the ISP compared to rest or mental calculation. This enhancement occurred in the absence of motor-evoked potential amplitude modulation in the left FDI, thus excluding a non-specific contribution from an increase in right M1 corticospinal excitability. Contraction of the left extensor indicis, but not contraction of more proximal left upper limb or left or right lower limb muscles also enhanced the ISP. A reaction time experiment showed that the ISP enhancement developed at a late stage of movement preparation just before or at movement onset. Interhemispheric inhibition of the motor-evoked potential as tested by a bifocal paired-pulse TMS protocol and thought to be mediated via a neuronal circuit different to the ISP was not enhanced when tested under identical motor task conditions. Finally, ISP enhancement by contraction of the left FDI correlated inversely with EMG mirror activity in the right FDI during phasic abductions of the left index finger. Our findings strongly suggest that voluntary M1 activation by real or imagined movement of the contralateral hand increases interhemispheric motor inhibition of the opposite M1. This phenomenon shows substantial topographical, temporal and neuronal circuit specificity, and has functional significance as it probably plays a pivotal role in suppressing mirror activity.

Differential effect of linguistic and non-linguistic pen-holding tasks on motor cortex excitability

Writing and drawing are unique human activities. They are complex high-precision actions, which involve not only the motor system but also various cognitive systems, such as attention, short-term memory, action control, and language. In relation to motor control, the study of writing and drawing is of great interest as they provide insight in the interaction between motor control processes and the concurrent non-motor processes. Although sharing similar motor and mechanical demands, writing and drawing involve different levels of linguistic/semantic load and thus may be associated with different modulation of motor cortical excitability. Here, we have used transcranial magnetic stimulation to study separately activation of excitatory and inhibitory mechanisms of the motor cortex during performance of writing and drawing acts as well as during simple pen-squeezing task. While cortical excitatory mechanisms appeared to be saturated by the pure motor demands of the tasks, and thus not amenable to modulation by the tasks' linguistic load, variation in cortical inhibitory activity was the main vehicle for differential modulation of motor cortical excitability by linguistic demands of the tasks. The results of this study highlight the importance of cortical inhibitory mechanisms in the physiology of higher cognitive activities. They also provide further evidence that the task specific modulation of the excitability of the motor cortex goes beyond motor complexity of the task and is also dependant on associated cognitive components.

Effect of motion imagery to counter rest-induced suppression of F-wave as a measure of anterior horn cell excitability

OBJECTIVE

To test if motor imagery prevents the rest-induced suppression of anterior horn cell excitability.

METHODS

Ten healthy subjects underwent two separate experiments, each consisting of stimulating the median nerve 100 times and recording F-waves from abductor pollicis brevis (APB) in three consecutive sessions: (1) after muscle exercise to standardize the baseline, (2) after immobilization of APB for 3h and (3) after muscle exercise to check recovery. We instructed the subject to volitionally relax APB in experiment 1 (relaxation task), and to periodically simulate thumb abduction without actual movement in experiment 2 (imagery task).

RESULTS

F-wave persistence and amplitude declined after relaxation task and recovered quickly after exercise, but changed little with imagery task. F-wave latencies showed no change when analyzed individually. The frequency distribution of collective F-waves recorded from all subjects remained the same after relaxation task, but showed a shift toward longer latencies after imagery task.

CONCLUSIONS

Mental imagery without overt motor output suffices to counter the effect of sustained volitional muscle relaxation, which would, otherwise, cause a reversible reduction in anterior horn cell excitability.

SIGNIFICANCE

This finding documents the importance of central drive for spinal excitability, which affects F-wave studies of a paretic muscle.

Pursuit eye movements involve a covert motor plan for manual tracking

When we make an aiming movement toward a moving visual object, eye-hand coupling is of paramount importance for accurate motor performance. Some studies have suggested that both gaze and manual tracking control systems are driven by a common command signal. However, it has never been demonstrated that a motor plan for the arm is produced even when the object is tracked by the eyes alone. By applying transcranial magnetic stimulation to the motor cortex, we show for the first time that ocular tracking is linked to an overall decrease in the excitability of the motor control system of the relaxed upper limb, as estimated from the amplitude of the motor evoked potentials recorded in contralateral hand and wrist muscles. Furthermore, this reduced excitability is modulated in a manner compatible with a subthreshold neural activation encoding a manual tracking response to the same target pursued by the eyes. In addition, excitability changes are contingent on upper-limb posture, because they are present only with a pronated forearm and not with a supinated hand position. We provide direct evidence that, if the arm is held in a congruent postural configuration, tracking a moving object always entails a coordinated motor plan, which involves both gaze and hand movements. Active inhibitory mechanisms are activated to prevent an overt arm movement, whenever a manual tracking is not requested. Our data provide strong evidence in favor of the existence of a common drive to both eye and hand tracking systems.

Assessing the temporal reproducibility of human esophageal motor-evoked potentials to transcranial magnetic stimulation

BACKGROUND

Although the electrophysiological properties and reproducibility of somatic limb motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) are well characterized, little is known about the reproducibility of MEPs for viscerosomatic structures such as the esophagus.

AIM

To determine the temporal reproducibility of esophageal MEPs to TMS.

METHODS

MEPs to TMS were recorded from the proximal esophagus, using a swallowed catheter housing a pair of electrodes, in eight healthy subjects at five stimulus intensities (SI) (motor threshold [MT] to 20% above MT). For each SI, 20 consecutive TMS stimuli at 5-second intervals were delivered over a single scalp site (dominant hemisphere at site exhibiting MT at lowest SI) and repeated 40 and 80 minutes thereafter. MEP amplitudes and latencies were measured, and means were sequentially calculated for each SI and then log-transformed. The repeatability coefficients (RC) for the three time points were calculated across each set of 20 stimuli and presented as an exponential ratio.

RESULTS

Best RC (amplitude/latency) were achieved at 120% SI relative to MT, being 1.8/1.2 (optimal = 1.0). For lower intensities of 115%, 110%, 105%, and 100% SI, the RC were 2.1/1.2, 2.1/1.1, 2.4/1.2, and 2.6/1.4, respectively. For all SI, the greatest reductions in RC occurred over the first 10 stimuli, with little additional gain beyond this number.

CONCLUSIONS

Latencies of esophageal MEP to TMS across intensities are highly reproducible, whereas amplitudes are more stimulus intensity-dependent, being most reliable and reproducible at the highest stimulus strengths.

SIGNIFICANCE

Using careful parameters, TMS can be used reliably in future studies of viscerosomatic structures, although the size of the response variability needs to be taken into account when assessing changes in cortico-fugal activity.

Facilitation and reciprocal inhibition by imagining thumb abduction

It is well known that motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) of the motor cortex are facilitated by voluntary muscle contraction. We evaluated the effects of imagination of movements on MEP latencies of agonist and antagonist muscles in the hand using TMS. Twenty-two healthy volunteers were studied. TMS delivered at rest and while imagining tonic abduction of the right thumb. MEPs were recorded in response to magnetic stimulation over the scalp and cervical spine (C7-T1), and central motor conduction times (CMCT) were calculated. MEPs were recorded from right abductor pollicis brevis muscle (APB) and adductor pollicis muscle (AP) simultaneously. Imagination of abduction resulted in a shortened latency of MEPs in the APB muscle, and a prolonged latency in the AP muscle. But the imagination caused no significant change in the latency of MEPs elicited by stimulation over the cervical spine. The changes of the CMCT may account for these latency changes with imagination of movement. These findings indicate that imagination of thumb abduction facilitates motoneurons of agonist muscle and has an inhibitory effect on those of antagonist muscle (reciprocal inhibition).

Electrophysiology of action representation

We continuously act on objects, on other individuals, and on ourselves, and actions represent the only way we have to manifest our own desires and goals. In the last two decades, electrophysiological experiments have demonstrated that actions are stored in the brain according to a goal-related organization. The authors review a series of experimental data showing that this "vocabulary of motor schemata" could also be used for non-strictly motor purposes. In the first section, they present data from monkey experiments describing the functional properties of inferior premotor cortex and, in more detail, the properties of visuomotor neurons responding to objects and others' actions observation (mirror neurons). In the second section, human data are reviewed, with particular regard to electrophysiological experiments aiming to investigate how action representations are stored and addressed. The specific facilitatory effect of motor imagery, action/object observation, and speech listening on motor excitability shown by these experiments provides strong evidence that the motor system is constantly involved whenever the idea of an action is evoked.

Depression of diaphragm motor cortex excitability during mechanical ventilation

The effect of mechanical ventilation on the diaphragm motor cortex remains unknown. We assessed the effect of mechanical ventilation on diaphragm motor cortex excitability by measuring the costal and crural diaphragm motor-evoked potential (MEP) elicited by single and paired transcranial magnetic stimulation. In six healthy subjects, MEP recruitment curves of the costal and crural diaphragms were assessed at relaxed end expiration during spontaneous breathing [baseline tidal volume (Vtbaseline)] and isocapnic volume cycled ventilation delivered noninvasively (NIV) at three different levels of tidal volume (Vtbaseline, Vtbaseline + 5 ml/kg liters, and Vtbaseline + 10 ml/kg liters). The costal and crural diaphragm response to peripheral stimulation of the right phrenic nerve was not reduced by NIV. NIV reduced the costal and crural MEP amplitude during NIV ( P < 0.0001) with the maximal reduction at Vtbaseline + 5 ml/kg. Response to paired TMS showed that NIV (Vtbaseline + 5 ml/kg) significantly increased the sensitivity of the cortical motoneurons to facilitatory (>9 ms) interstimulus intervals ( P = 0.002), suggesting that the diaphragm MEP amplitude depression during NIV is related to neuromechanical inhibition at the level of motor cortex. Our results demonstrate that mechanical ventilation directly inhibits central projections to the diaphragm.

Integrated technology for evaluation of brain function and neural plasticity

The study of neural plasticity has expanded rapidly in the past decades and has shown the remarkable ability of the developing, adult, and aging brain to be shaped by environmental inputs in health and after a lesion. Robust experimental evidence supports the hypothesis that neuronal aggregates adjacent to a lesion in the sensorimotor brain areas can take over progressively the function previously played by the damaged neurons. It definitely is accepted that such a reorganization modifies sensibly the interhemispheric differences in somatotopic organization of the sensorimotor cortices. This reorganization largely subtends clinical recovery of motor performances and sensorimotor integration after a stroke. Brain functional imaging studies show that recovery from hemiplegic strokes is associated with a marked reorganization of the activation patterns of specific brain structures. To regain hand motor control, the recovery process tends over time to bring the bilateral motor network activation toward a more normal intensity/extent, while overrecruiting simultaneously new areas, perhaps to sustain this process. Considerable intersubject variability exists in activation/hyperactivation pattern changes over time. Some patients display late-appearing dorsolateral prefrontal cortex activation, suggesting the development of "executive" strategies to compensate for the lost function. The AH in stroke often undergoes a significant "remodeling" of sensory and motor hand somatotopy outside the "normal" areas, or enlargement of the hand representation. The UH also undergoes reorganization, although to a lesser degree. Although absolute values of the investigated parameters fluctuate across subjects, secondary to individual anatomic variability, variation is minimal with regards to interhemispheric differences, due to the fact that individual morphometric characters are mirrored in the two hemispheres. Excessive interhemispheric asymmetry of the sensorimotor hand areas seems to be the parameter with highest sensitivity in describing brain reorganization after a monohemispheric lesion, and mapping motor and somatosensory cortical areas through focal TMS, fMRI, PET, EEG, and MEG is useful in studying hand representation and interhemispheric asymmetries in normal and pathologic conditions. TMS and MEG allow the detection of sensorimotor areas reshaping, as a result of either neuronal reorganization or recovery of the previously damaged neural network. These techniques have the advantage of high temporal resolution but also have limitations. TMS provides only bidimensional scalp maps, whereas MEG, even if giving three-dimensional mapping of generator sources, does so by means of inverse procedures that rely on the choice of a mathematical model of the head and the sources. These techniques do not test movement execution and sensorimotor integration as used in everyday life. fMRI and PET may provide the ideal means to integrate the findings obtained with the other two techniques. This multitechnology combined approach is at present the best way to test the presence and amount of plasticity phenomena underlying partial or total recovery of several functions, sensorimotor above all. Dynamic patterns of recovery are emerging progressively from the relevant literature. Enhanced recruitment of the affected cortex, be it spared perilesional tissue, as in the case of cortical stroke, or intact but deafferented cortex, as in subcortical strokes, seems to be the rule, a mechanism especially important in early postinsult stages. The transfer over time of preferential activation toward contralesional cortices, as observed in some cases, seems, however, to reflect a less efficient type of plastic reorganization, with some aspects of maladaptive plasticity. Reinforcing the use of the affected side can cause activation to increase again in the affected side with a corresponding enhancement of clinical function. Activation of the UH MI may represent recruitment of direct (uncrossed) corticospinal tracts and relate more to mirror movements, but it more likely reflects activity redistribution within preexisting bilateral, large-scale motor networks. Finally, activation of areas not normally engaged in the dysfunctional tasks, such as the dorsolateral prefrontal cortex or the superior parietal cortex in motor paralysis, might reflect the implication of compensatory cognitive strategies. An integrated approach with technologies able to investigate functional brain imaging is of considerable value in providing information on the excitability, extension, localization, and functional hierarchy of cortical brain areas. Deepening knowledge of the mechanisms regulating the long-term recovery (even if partial), observed for most neurologic sequelae after neural damage, might prompt newer and more efficacious therapeutic and rehabilitative strategies for neurologic diseases.

Mental practice: a promising restorative technique in stroke rehabilitation

Upper limb hemiparesis is a common, yet debilitating, result of stroke. It has long been known that mental practice, when combined with physical practice, improves motor learning and performance. Recent studies also indicate that massed use of the affected arm results in cortical reorganizations and correlative functional improvements. During mental practice, there are widespread activations of neural and muscular mechanisms as if the arm were actually being used. This article introduces mental practice as a form of massed practice, reviews the bases for mental practice as a potent restorative technique, and presents data suggesting mental practice as a restorative technique for upper limb hemiparesis.

Transcranial magnetic stimulation and epilepsy

Transcranial magnetic stimulation has been used to study generalized and focal epilepsies for more than a decade. The technique appears safe and has yielded important information about the mechanisms underlying epilepsy. Transcranial magnetic stimulation findings differ depending on the epilepsy syndrome, lending support to the concept that there are distinct pathophysiologies underlying each condition. In most studies of generalized epilepsies, transcranial magnetic stimulation has indicated a state of relative hyperexcitability of excitatory cortical interneurons and possibly inhibitory interneurons as well, which can be reversed through the actions of anticonvulsant medications. Transcranial magnetic stimulation studies in patients with a seizure focus in the motor cortex indicate increased cortical excitability and reduced inhibition, but in patients with seizure foci located elsewhere the findings are similar to those in generalized epilepsies. Transcranial magnetic stimulation has also been used to study the mode of action of anticonvulsants and may prove to be a useful means of testing the potential for new drugs to act as anticonvulsants. Repetitive transcranial magnetic stimulation may prove to have a therapeutic role by producing long-lasting cortical inhibition after a train of impulses.

Focal enhancement of motor cortex excitability during motor imagery: a transcranial magnetic stimulation study

OBJECTIVES

In order to learn more about the physiology of the motor cortex during motor imagery, we evaluated the changes in excitability of two different hand muscle representations in the primary motor cortex (M1) of both hemispheres during two imagery conditions.

MATERIALS AND METHODS

We applied focal transcranial magnetic stimulation (TMS) over each M1, recording motor evoked potentials (MEPs) from the contralateral abductor pollicis brevis (APB) and first dorsal interosseus (FDI) muscles during rest, imagery of contralateral thumb abduction (C-APB), and imagery of ipsilateral thumb abduction (I-APB). We obtained measures of motor threshold (MT), MEP recruitment curve (MEP-rc) and F waves.

RESULTS

Motor imagery compared with rest significantly decreased the MT and increased MEPs amplitude at stimulation intensities clearly above MT in condition C-APB, but not in condition I-APB. These effects were not significantly different between right and left hemisphere. MEPs simultaneously recorded from the FDI, which was not involved in the task, did not show facilitatory effects. There were no significant changes in F wave amplitude during motor imagery compared with rest.

CONCLUSIONS

Imagery of unilateral simple movements is associated with increased excitability only of a highly specific representation in the contralateral M1 and does not differ between hemispheres.

Transcranial magnetic stimulation coregistered with MRI: a comparison of a guided versus blind stimulation technique and its effect on evoked compound muscle action potentials

INTRODUCTION AND METHODS

Compound muscle action potentials (CMAPs) elicited by transcranial magnetic stimulation (TMS) are characterized by enormous variability, even when attempts are made to stimulate the same scalp location. This report describes the results of a comparison of the spatial errors in coil placement and resulting CMAP characteristics using a guided and blind TMS stimulation technique. The former uses a coregistration system, which displays the intersection of the peak TMS induced electric field with the cortical surface. The latter consists of the conventional placement of the TMS coil on the optimal scalp position for activation of the first dorsal interossei (FDI) muscle.

RESULTS

Guided stimulation resulted in significantly improved spatial precision for exciting the corticospinal projection to the FDI compared to blind stimulation. This improved precision of coil placement was associated with a significantly increased probability of eliciting FDI responses. Although these responses tended to have larger amplitudes and areas, the coefficient of variation between guided and blind stimulation induced CMAPs did not significantly differ.

CONCLUSION

The results of this study demonstrate that guided stimulation improves the ability to precisely revisit previously stimulated cortical loci as well as increasing the probability of eliciting TMS induced CMAPs. Response variability, however, is due to factors other than coil placement.

Optimization of facilitation related to threshold in transcranial magnetic stimulation

OBJECTIVES

To attain the standardized procedure for optimal facilitation, we analyzed motor-evoked potential (MEP) responses to the degree of voluntary contraction and stimulus intensity.

METHODS

Fifteen normal subjects were included. MEPs were elicited at thenar muscles during rest and at gradual voluntary contraction (MVC), using 10, 30, and 50% of MVC. During rest and each contraction, the excitability threshold at rest (RET) and at contraction (CET) were determined. Consecutive stimuli were applied, with the intensity of ratio increments (110-150% of ET).

RESULTS

The RET showed a remarkable decrease after contraction. Shortening of latency reached a saturation level at 10% of MVC. Amplitude reached a saturation level at 30% of MVC with 62.7+/-8.5% of the maximum output, which is equal to 140% intensity of CET, and 110% of RET. The MEP amplitudes at rest and at 10% MVC were influenced by their ETs, but those measured above 30% of MVC were not related.

CONCLUSIONS

The procedure recommended for optimal facilitation is as follows: to achieve minimal latency of MEPs, a minimal contraction (10% of MVC) with RET intensity is sufficient and for maximal amplitude, a moderate contraction (30% of MVC) with 110% of RET intensity is adequate.

Motor nervous pathway function is impaired after treatment of childhood acute lymphoblastic leukemia: a study with motor evoked potentials

BACKGROUND

The objective was to evaluate whether motor nervous pathways are affected when patients are treated for childhood acute lymphoblastic leukemia (ALL).

PROCEDURE

Thirty-two children with ALL were studied at the end of treatment by means of motor evoked potentials (MEPs) elicited by magnetic stimulation (MS) transcranially and peripherally and underwent a detailed neurological examination. Thirty-two healthy children matched with them for age, sex, and height served as a control group.

RESULTS

The latencies of the MEPs were significantly prolonged along the entire motor nervous pathway in the patients with ALL compared with the healthy controls, indicating demyelination in the thick motor fibres. The MEP amplitudes of the distal extremities elicited by stimulation at the brachial plexus and LV spinal level were significantly lowered in the patients treated for ALL, also indicating anatomical or functional loss of descending motor fibres and/or muscle fibres. The MEP amplitudes elicited by cortical MS showed wider variation and no clear abnormalities were found. Neurological signs and symptoms were common after treatment: 41% of the patients had depressed deep tendon reflexes, 31% had fine motor difficulties and 63% gross motor difficulties, and 34% had dysdiadochokinesia. The conduction delay within the peripheral nerve was related to the post-therapeutic interval after administration of vincristine and the lesions within the CNS to the number of injections of intrathecal methotrexate.

CONCLUSIONS

The present results show adverse effects of the ALL treatment on the entire motor nervous pathways. In our experience, the measurement of MEPs by MS provides an objective, painless, and practical tool for assessing the treatment-related neurotoxicity in both the CNS and the peripheral nerves. These disturbances in the motor nervous pathways at the end of treatment raise the question of the long-term effects of ALL treatment on the motor nerve tracts, and have led us to employ MEPs to study these effects in long-term survivors of ALL.

Are autonomic signals influencing cortico-spinal motor excitability? A study with transcranial magnetic stimulation

In order to investigate the role of visceral afferent inputs flowing along autonomic fibers on corticospinal tract excitability, the variability of Motor Evoked Potentials (MEPs), elicited by Transcranial Magnetic Stimulation (TMS), was analysed during simultaneous monitoring of electrocardiogram (EKG) phases, breathing phases and sudomotor skin responses (SSRs) in a group of 10 healthy subjects. A cascade of at least 60 consecutive magnetic stimuli, with an interstimulus interval randomly varying between 20 and 40 s, was acquired. At the end of the recording session, the subject was asked to make at random five not consecutive self-paced forced inspirations. TMS was carried out at an intensity 10% above motor threshold excitability via a circular coil placed over the motor area of the right hemisphere. MEPs were recorded from the contralateral abductor digiti minimi muscle (ADM). Sudomotor Skin Responses (SSRs) were recorded on both hand palms. MEPs latency and amplitude did not show significant correlation with any of the EKG and respiratory phases. During forced inspiration, a significant latency shortening was found. TMS elicited SSRs, whose amplitudes were not correlated with MEP parameters. During forced inspiration a significant SSR amplitude increment, not correlated with MEP latency shortening, was also observed. These results assign a minor if any role to the considered autonomic parameters in modulating corticospinal motor excitability.

Task-dependent effects on motor-evoked potentials and on the following silent period

The silent period (SP) after transcranial stimulation is used as a diagnostic tool in various central nervous system disorders although no standardized experimental setup has been established. The aim of this study was to demonstrate the influence of an isotonic compared to an isometric experimental condition. The SP after transcranial magnetic brain stimulation in the biceps brachii and brachioradialis muscle was up to 130% longer when elicited during a maintain-position (isotonic) task as compared to a maintain-force task (isometric) when stimulus intensities of 5% to 25% above threshold were used. The mean SP duration in these muscles was positively correlated to the mean contraction time in both tasks. However, no such relationship was observed for the trials within the individual subjects. We speculate that the invariably longer SP of the maintain-position task was due to the different "motor set" which predictively determined the muscle behavior after the stimulus. In the maintain-position trials, the stimulus-induced long-lasting flexion movement is counteracted by a motor set aiming to relax the elbow flexors immediately after the stimulus. In the maintain-force task the contraction twitch is short and a force drop below the preset level must be prevented by a motor set aiming to contract the elbow flexors immediately after the stimulus. The latter may increase the synaptic input to the motoneuron pool and facilitate the reoccurrence of the electromyogram terminating the SP. At high-stimulus intensities the SP duration increased in both tasks, and the task-dependent differences disappeared. Therefore, when using the SP duration for diagnostic purposes, isometric conditions and high-stimulus intensities should be used.

Facilitation of motor evoked potentials (MEPs) in first dorsal interosseous (FDI) muscle is dependent on different motor images

OBJECTIVE

We investigated changes in motor evoked potentials (MEPs) to explain why mental practice can improve motor performance.

METHODS

MEPs were recorded from right and left first dorsal interosseous (FDI) muscles of 9 normal, right-handed subjects during different motor images of index finger movement: (1) rest, (2) flexion, (3) abduction, (4) extension. A paired t test was used to compare differences of stimulus intensities and MEP amplitudes among conditions.

RESULTS

MEP amplitudes significantly increased in both FDI muscles during motor images of flexion and abduction but not of extension. Moreover, MEP amplitudes were larger in flexion than in abduction. These differences were proportional to the amount of real EMG discharge of FDI muscle in the selected direction of index finger movement. With regard to right-left differences, MEP amplitudes in the right FDI muscle were larger than those in the left.

CONCLUSIONS

The primary motor cortex plays a role in the mental representation of motor acts. Furthermore, the amount of corticomotoneuronal cell activity is affected by the different motor images utilizing the same muscle. Right-left difference of MEP amplitude supports the view of left-hemisphere dominance for motor programming as an aspect of normal brain function among right-handers.

Dependence of the transcranially induced silent period on the 'instruction set' and the individual reaction time

OBJECTIVES AND METHODS

We looked for influences of the experimental condition on the silent period (SP) from transcranial motor cortex stimulation and analyzed how the instruction given to the subject, as well as the individual reaction time, might affect the duration of the SP in the biceps brachii muscle.

RESULTS

The duration of the SP was found to critically depend on the subject's voluntary reaction of the target muscle immediately after the stimulus. With low stimulus intensity and low background force, the duration of the silent period was significantly longer in 10 of 13 subjects (P = 0.002) when they were instructed to relax quickly after the stimulus rather than to maintain the the force at a constant level. A significant shortening of the SP (P = 0.02) was observed when the subjects were instructed to perform a rapid contraction of the target muscle in reaction to the cortical stimulus. With low stimulus intensity and high background force, the same influence of the instruction set was found in 6 of 13 subjects. When the subjects were left without precise instruction, the SP duration was unpredictable. In 10 subjects, the SP corresponded to that obtained with the instruction to maintain the force at a constant level. However, in 3 subjects it was prolonged to the value observed in the 'relax' instruction. With greater stimulus intensities, the effect of the instruction set on the SP duration was generally smaller. A significant prolongation was nevertheless found at low background forces with rapid relaxation (P < 0.001), and a significant shortening was found at high background forces with rapid contraction (P < 0.001) after the stimulus. The SP duration observed with 20% of maximal voluntary contraction (MVC) significantly correlated with the individual reaction time. No such correlation was found for the SP obtained with 80% MVC. The SP was slightly longer at 20% MVC, as compared to 80% MVC within each instruction group. This effect was significant (P < 0.05) at low stimulus intensities.

CONCLUSIONS

Therefore, when assessing the SP duration for diagnostic purposes, not only the stimulus intensity but also the background force and the voluntary reaction must be standardized. Furthermore, great stimulus intensities and high background forces should be used to minimise the effects of instruction set and individual reaction time.