Desynchronization and synchronization of central 20-Hz rhythms associated with voluntary muscle relaxation: a magnetoencephalographic study

On Stopping Voluntary Muscle Relaxations and Contractions: Evidence for Shared Control Mechanisms and Muscle State-Specific Active Breaking

Control of the body requires inhibiting complex actions, involving contracting and relaxing muscles. However, little is known of how voluntary commands to relax a muscle are cancelled. Action inhibition causes both suppression of muscle activity and the transient excitation of antagonist muscles, the latter being termed active breaking. We hypothesized that active breaking is present when stopping muscle relaxations. Stop signal experiments were used to compare the mechanisms of active breaking for muscle relaxations and contractions in male and female human participants. In experiments 1 and 2, go signals were presented that required participants to contract or relax their biceps or triceps muscle. Infrequent Stop signals occurred after fixed delays (0-500 ms), requiring that participants cancelled go commands. In experiment 3, participants increased (contract) or decreased (relax) an existing isometric finger abduction depending on the go signal, and cancelled these force changes whenever Stop signals occurred (dynamically adjusted delay). We found that muscle relaxations were stopped rapidly, met predictions of existing race models, and had Stop signal reaction times that correlated with those observed during the stopping of muscle contractions, suggesting shared control mechanisms. However, stopped relaxations were preceded by transient increases in electromyography (EMG), while stopped contractions were preceded by decreases in EMG, suggesting a later divergence of control. Muscle state-specific active breaking occurred simultaneously across muscles, consistent with a central origin. Our results indicate that the later stages of action inhibition involve separate excitatory and inhibitory pathways, which act automatically to cancel complex body movements.SIGNIFICANCE STATEMENT The mechanisms of how muscle relaxations are cancelled are poorly understood. We showed in three experiments involving multiple effectors that stopping muscle relaxations involves transient bursts of EMG activity, which resemble cocontraction and have onsets that correlate with Stop signal reaction time. Comparison with the stopping of matched muscle contractions showed that active breaking was muscle state specific, being positive for relaxations and negative for contractions. The two processes were also observed to co-occur in agonist-antagonist pairs, suggesting separate pathways. The rapid, automatic activation of both pathways may explain how complex actions can be stopped at any stage of their execution.

A score to map the lateral nonprimary motor area: Multispectrum intrinsic brain activity versus cortical stimulation

OBJECTIVE

Multispectrum electrocorticographic components are critical for mapping the nonprimary motor area (NPMA). The objective of this study was to derive and validate a reliable scoring system for electrocorticography-based NPMA mapping (NPMA score) to replace electrical cortical stimulation (ECS) during brain surgery.

METHODS

We analyzed 14 consecutive epilepsy patients with subdural electrodes implanted in the frontal lobe at Kyoto University Hospital. The NPMA score was retrospectively derived from multivariate analysis in the derivation group (patients = 7, electrodes = 713, during 2010-2013) and validated in the validation group (patients = 7, electrodes = 772, during 2014-2017). We assessed the accuracy and reliability of the score relative to ECS in determining the NPMA and predicting postoperative functional outcomes.

RESULTS

Multivariate analysis in the derivation group led to an 8-point score for predicting ECS-based NPMA (1 point for anatomical localization of the electrode and 1 or 2 points for movement-related electrocorticographic components regardless of somatotopy in very slow cortical potential shifts [<0.5 Hz], 40-80-Hz band power increase, and 8-24-Hz band power decrease), which was validated in the validation group. The area under the receiver operating characteristic curve (AUC) was 0.89 in the derivation group. Good prediction (specificity = 94%, sensitivity = 100%) and discrimination (AUC = 0.87) were reproduced in the validation group. Overall, higher NPMA scores identified 2 patients with postoperative deficits after frontal lobe resection.

SIGNIFICANCE

The NPMA score is reliable for NPMA mapping, potentially replacing ECS. It is a potential prognostic marker for postoperative functional deficits.

Use of imperceptible wrist vibration to modulate sensorimotor cortical activity

Peripheral sensory stimulation has been used as a method to stimulate the sensorimotor cortex, with applications in neurorehabilitation. To improve delivery modality and usability, a new stimulation method has been developed in which imperceptible random-frequency vibration is applied to the wrist concurrently during hand activity. The objective of this study was to investigate effects of this new sensory stimulation on the sensorimotor cortex. Healthy adults were studied. In a transcranial magnetic stimulation (TMS) study, resting motor threshold, short-interval intracortical inhibition, and intracortical facilitation for the abductor pollicis brevis muscle were compared between vibration on vs. off, while subjects were at rest. In an electroencephalogram (EEG) study, alpha and beta power during rest and event-related desynchronization (ERD) for hand grip were compared between vibration on vs. off. Results showed that vibration decreased EEG power and decreased TMS short-interval intracortical inhibition (i.e., disinhibition) compared with no vibration at rest. Grip-related ERD was also greater during vibration, compared to no vibration. In conclusion, subthreshold random-frequency wrist vibration affected the release of intracortical inhibition and both resting and grip-related sensorimotor cortical activity. Such effects may have implications in rehabilitation.

Visuomotor coordination and cortical connectivity of modular motor learning

The ability to transfer sensorimotor skill components to new actions and the capacity to use skill components from whole actions are characteristic of the adaptability of the human sensorimotor system. However, behavioral evidence suggests complex limitations for transfer after combined or modular learning of motor adaptations. Also, to date, only behavioral analysis of the consequences of the modular learning has been reported, with little understanding of the sensorimotor mechanisms of control and the interaction between cortical areas. We programmed a video game with distorted kinematic and dynamic features to test the ability to combine sensorimotor skill components learned modularly (composition) and the capacity to use separate sensorimotor skill components learned in combination (decomposition). We examined motor performance, eye-hand coordination, and EEG connectivity. When tested for integrated learning, we found that combined practice initially performed better than separated practice, but differences disappeared after integrated practice. Separate learning promotes fewer anticipatory control mechanisms (depending more on feedback control), evidenced in a lower gaze leading behavior and in higher connectivity between visual and premotor domains, in comparison with the combined practice. The sensorimotor system can acquire motor modules in a separated or integrated manner. However, the system appears to require integrated practice to coordinate the adaptations with the skill learning and the networks involved in the integrated behavior. This integration seems to be related to the acquisition of anticipatory mechanism of control and with the decrement of feedback control.

Motor imagery of voluntary muscle relaxation of the foot induces a temporal reduction of corticospinal excitability in the hand

The object of this study was to clarify how the motor imagery of foot muscle relaxation influences corticospinal excitability for the ipsilateral hand. Twelve participants volitionally relaxed their right foot from a dorsiflexed position (actual relaxation), or imaged the same movement (imagery relaxation) in response to an auditory cue. Transcranial magnetic stimulation (TMS) was delivered to the hand area of the left primary motor cortex at different time intervals after an auditory cue. 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 during both actual relaxation and imagery relaxation as compared with those of the resting control. A correlation of MEP amplitude between actual relaxation and imagery relaxation was observed. Our findings indicate that motor imagery of muscle relaxation of the foot induced a reduction of corticospinal excitability in the ipsilateral hand muscles. This effect is likely produced via the same mechanism that functions during actual muscle relaxation.

Modulation of motor cortex inhibition during motor imagery

Motor imagery (MI) is similar to overt movement, engaging common neural substrates and facilitating the corticomotor pathway; however, it does not result in excitatory descending motor output. Transcranial magnetic stimulation (TMS) can be used to assess inhibitory networks in the primary motor cortex via measures of 1-ms short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI), and late cortical disinhibition (LCD). These measures are thought to reflect extrasynaptic GABA_A tonic inhibition, postsynaptic GABA_B inhibition, and presynaptic GABA_B disinhibition, respectively. The behavior of 1-ms SICI, LICI, and LCD during MI has not yet been explored. This study aimed to investigate how 1-ms SICI, LICI, and LCD are modulated during MI and voluntary relaxation (VR) of a target muscle. Twenty-five healthy young adults participated. TMS was used to assess nonconditioned motor evoked potential (MEP) amplitude, 1-ms SICI, 100- (LICI₁₀₀) and 150-ms LICI, and LCD in the right abductor pollicis brevis (APB) and right abductor digiti minimi during rest, MI, and VR of the hand. Compared with rest, MEP amplitudes were facilitated in APB during MI. SICI was not affected by task or muscle. LICI₁₀₀ decreased in both muscles during VR but not MI, whereas LCD was recruited in both muscles during both tasks. This indicates that VR modulates postsynaptic GABA_B inhibition, whereas both tasks modulate presynaptic GABA_B inhibition in a non-muscle-specific way. This study highlights further neurophysiological parallels between actual and imagined movement, which may extend to voluntary relaxation.NEW & NOTEWORTHY This is the first study to investigate how 1-ms short-interval intracortical inhibition, long-interval intracortical inhibition, and late cortical disinhibition are modulated during motor imagery and voluntary muscle relaxation. We present novel findings of decreased 100-ms long-interval intracortical inhibition during voluntary muscle relaxation and increased late cortical disinhibition during both motor imagery and voluntary muscle relaxation.

Effect of muscle relaxation in the foot on simultaneous muscle contraction in the contralateral hand

We investigated the effects of foot muscle relaxation and contraction on muscle activities in the hand on both ipsilateral and contralateral sides. The subjects sat in an armchair with hands in the pronated position. They were able to freely move their right/left hand and foot. They performed three tasks for both ipsilateral (right hand and right foot) and contralateral limb coordination (left hand and right foot for a total of six tasks). These tasks involved: (1) wrist extension from a flexed (resting) position, (2) wrist extension with simultaneous ankle dorsiflexion from a plantarflexed (resting) position, and (3) wrist extension with simultaneous ankle relaxation from a dorsiflexed position. The subjects performed each task as fast as possible after hearing the start signal. Reaction time for the wrist extensor contraction (i.e. the degree to which it preceded the motor reaction time), as observed in electromyography (EMG), became longer when it was concurrently done with relaxation of the ankle dorsiflexor. Also, the magnitude of EMG activity became smaller, as compared with activity when wrist extensor contraction was done alone or with contraction of the ankle dorsiflexor. These effects were observed not only for the ipsilateral hand, but also for the contralateral hand. Our findings suggest that muscle relaxation in one limb interferes with muscle contraction in both the ipsilateral and contralateral limbs.

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.

Time Course of Corticospinal Excitability and Intracortical Inhibition Just before Muscle Relaxation

Using transcranial magnetic stimulation (TMS), we investigated how short-interval intracortical inhibition (SICI) was involved with transient motor cortex (M1) excitability changes observed just before the transition from muscle contraction to muscle relaxation. Ten healthy participants performed a simultaneous relaxation task of the ipsilateral finger and foot, relaxing from 10% of their maximal voluntary contraction (MVC) force after the go signal. In the simple reaction time (RT) paradigm, single or paired TMS pulses were randomly delivered after the go signal, and motor evoked potentials (MEPs) were recorded from the right first dorsal interosseous (FDI) muscle. We analyzed the time course prior to the estimated relaxation reaction time (RRT), defined here as the onset of voluntary relaxation. SICI decreased in the 80-100 ms before RRT, and MEPs were significantly greater in amplitude in the 60-80 ms period before RRT than in the other intervals in single-pulse trials. TMS pulses did not effectively increase RRT. These results show that cortical excitability in the early stage, before muscle relaxation, plays an important role in muscle relaxation control. SICI circuits may vary between decreased and increased activation to continuously maintain muscle relaxation during or after a relaxation response. With regard to M1 excitability dynamics, we suggest that SICI also dynamically changes throughout the muscle relaxation process.

Effects of muscle relaxation on sustained contraction of ipsilateral remote muscle

The objective of this study was to clarify the temporal change of muscle activity during relaxation of ipsilateral remote muscles. While participants maintained a constant right wrist extensor isometric force, they dorsiflexed the ipsilateral ankle from resting position or relaxed from dorsiflexed position in response to an audio signal. The wrist extensor force magnitude increased in the 0–400 msec period after the onset of foot contraction compared to that of the resting condition (P < 0.05). On the other hand, wrist extensor force magnitude and electromyographic (EMG) activity decreased in the 0–400 msec period after the onset of ankle dorsiflexion compared to that of the resting condition (P < 0.05). Our findings suggest that foot muscle relaxation induces temporal reduction in hand muscle EMG activity and force magnitude.

Somato-motor inhibitory processing in humans: evidence from neurophysiology and neuroimaging

Motor execution processing has been examined using an index of behavioral performance such as reaction times, kinetics, and kinematics. However, difficulties have been associated with the study of motor inhibitory processing because of the absence of actual behavioral performance. Therefore, non-invasive neurophysiological and neuroimaging methods including electroencephalography, magnetoencephalography, transcranial magnetic stimulation, and functional magnetic resonance imaging have been used to investigate neural processes in the central nervous system. We mainly reviewed research on somato-motor inhibitory processing based on data obtained by using these techniques, which can examine 'when', 'where, and 'how' motor inhibition occurs in the brain. Although to date a number of studies have used these techniques separately, few studies have utilized them in a comprehensive manner. In this review, we provide evidence that combining neurophysiological and neuroimaging methods should contribute to our understanding of how executive and inhibitory functions are implemented.

The ups and downs of β oscillations in sensorimotor cortex

Since the first descriptions of sensorimotor rhythms by Berger (1929) and by Jasper and Penfield (1949), the potential role of beta oscillations (~13-30 Hz) in the brain has been intensely investigated. We start this review by showing that experimental studies in humans and monkeys have reached a consensus on the facts that sensorimotor beta power is low during movement, transiently increases after movement end (the "beta rebound") and tonically increases during object grasping. Recently, a new surge of studies exploiting more complex sensorimotor tasks including multiple events, such as instructed delay tasks, reveal novel characteristics of beta oscillatory activity. We therefore proceed by critically reviewing also this literature to understand whether modulations of beta oscillations in task epochs other than those during and after movement are consistent across studies, and whether they can be reconciled with a role for beta oscillations in sensorimotor transmission. We indeed find that there are additional processes that also strongly affect sensorimotor beta oscillations, such as visual cue anticipation and processing, fitting with the view that beta oscillations reflect heightened sensorimotor transmission beyond somatosensation. However, there are differences among studies, which may be interpreted more readily if we assume multiple processes, whose effects on the overall measured beta power overlap in time. We conclude that beta oscillations observed in sensorimotor cortex may serve large-scale communication between sensorimotor and other areas and the periphery.

Writing's shadow: corticospinal activation during letter observation

We can recognize handwritten letters despite the variability among writers. One possible strategy is exploiting the motor memory of orthography. By using TMS, we clarified the excitatory and inhibitory neural circuits of the motor corticospinal pathway that might be activated during the observation of handwritten letters. During experiments, participants looked at the handwritten or printed single letter that appeared in a random order. The excitability of the left and right primary motor cortex (M1) was evaluated by motor-evoked potentials elicited by single-pulse TMS. Short interval intracortical inhibition (SICI) of the left M1 was evaluated using paired-pulse TMS. F waves were measured for the right ulnar nerve. We found significant reduction of corticospinal excitability only for the right hand at 300-400 msec after each letter presentation without significant changes in SICI. This suppression is likely to be of supraspinal origin, because of no significant alteration in F-wave amplitudes. These findings suggest that the recognition of handwritten letters may include the implicit knowledge of "writing" in M1. The M1 activation associated with that process, which has been shown in previous neuroimaging studies, is likely to reflect the active suppression of the corticospinal excitability.

Seeing touch and pain in a stranger modulates the cortical responses elicited by somatosensory but not auditory stimulation

Viewing other's pain inhibits the excitability of the motor cortex and also modulates the neural activity elicited by a concomitantly delivered nociceptive somatosensory stimulus. As the neural activity elicited by a transient nociceptive stimulus largely reflects non nociceptive-specific, multimodal neural processes, here we tested, for the first time, whether the observation of other's pain preferentially affects the brain responses elicited by nociceptive stimulation, or instead similarly modulates those elicited by stimuli belonging to a different sensory modality. Using 58-channel electroencephalography (EEG), we recorded the cortical responses elicited by laser and auditory stimulation during the observation of videoclips showing either noxious or non-noxious stimulation of a stranger's hand. We found that the observation of other's pain modulated the cortical activity consisting in an event-related desynchronization in the β band (β ERD), and elicited by nociceptive laser stimuli, but not by auditory stimuli. Using three different source analysis approaches, we provide converging evidence that such modulation affected neural activity in the contralateral primary sensorimotor cortex. The magnitude of this modulation correlated well with a subjective measure of similarity between the model's hand and the onlooker's representation of the hand. Altogether, these findings demonstrate that the observation of other's pain modulates, in a somatosensory-specific fashion, the cortical responses elicited by nociceptive stimuli in the sensorimotor cortex contralateral to the stimulated hand.

Cortical activities associated with voluntary movements and involuntary movements

Recent advance in non-invasive techniques including electrophysiology and functional neuroimaging has enabled investigation of control mechanism of voluntary movements and pathophysiology of involuntary movements in human. Epicortical recording with subdural electrodes in epilepsy patients complemented the findings obtained by the non-invasive techniques. Before self-initiated simple movement, activation occurs first in the pre-supplementary motor area (pre-SMA) and SMA proper bilaterally with some somatotopic organisation, and the lateral premotor area (PMA) and primary motor cortex (M1) mainly contralateral to the movement with precise somatotopic organisation. Functional connectivity among cortical areas has been disclosed by cortico-cortical coherence, cortico-cortical evoked potential, and functional MRI. Cortical activities associated with involuntary movements have been studied by jerk-locked back averaging and cortico-muscular coherence. Application of transcranial magnetic stimulation helped clarifying the state of excitability and inhibition in M1. The sensorimotor cortex (S1-M1) was shown to play an important role in generation of cortical myoclonus, essential tremor, Parkinson tremor and focal dystonia. Cortical myoclonus is actively driven by S1-M1 while essential tremor and Parkinson tremor are mediated by S1-M1. 'Negative motor areas' at PMA and pre-SMA and 'inhibitory motor areas' at peri-rolandic cortex might be involved in the control of voluntary movement and generation of negative involuntary movements, respectively.

β-Band correlates of the fMRI BOLD response

Oscillatory activity in the β-band (15-30 Hz) has been studied in detail in the sensorimotor cortex. It has been postulated that β-activity acts as a localized gating of cortical activity. Here, the induced oscillatory response in the β-band is measured by magnetoencephalography, and the hemodynamic response is measured by fMRI. We assess the linearity of the responses to stimuli of varying duration in the primary motor cortex and to a sinusoidal drifting grating of varying contrast amplitude and drift frequency in the visual cortex. In this way, we explore the nature of β-oscillations and their relationship with hemodynamic effects. Excellent spatial colocalization of BOLD and β-activity in both central and lateral (MT) visual areas and sensorimotor areas suggests that the two are intimately related. In contrast to the BOLD response, the level of β-desynchronization is not modulated by stimulus contrast or by stimulus duration, consistent with a gating role. The amplitude of β-desynchronization in the central visual area is however modulated by drift frequency, and this seems to parallel the modulation in BOLD amplitude at the same location.

Measuring functional connectivity using MEG: methodology and comparison with fcMRI

Functional connectivity (FC) between brain regions is thought to be central to the way in which the brain processes information. Abnormal connectivity is thought to be implicated in a number of diseases. The ability to study FC is therefore a key goal for neuroimaging. Functional connectivity (fc) MRI has become a popular tool to make connectivity measurements but the technique is limited by its indirect nature. A multimodal approach is therefore an attractive means to investigate the electrodynamic mechanisms underlying hemodynamic connectivity. In this paper, we investigate resting state FC using fcMRI and magnetoencephalography (MEG). In fcMRI, we exploit the advantages afforded by ultra high magnetic field. In MEG we apply envelope correlation and coherence techniques to source space projected MEG signals. We show that beamforming provides an excellent means to measure FC in source space using MEG data. However, care must be taken when interpreting these measurements since cross talk between voxels in source space can potentially lead to spurious connectivity and this must be taken into account in all studies of this type. We show good spatial agreement between FC measured independently using MEG and fcMRI; FC between sensorimotor cortices was observed using both modalities, with the best spatial agreement when MEG data are filtered into the β band. This finding helps to reduce the potential confounds associated with each modality alone: while it helps reduce the uncertainties in spatial patterns generated by MEG (brought about by the ill posed inverse problem), addition of electrodynamic metric confirms the neural basis of fcMRI measurements. Finally, we show that multiple MEG based FC metrics allow the potential to move beyond what is possible using fcMRI, and investigate the nature of electrodynamic connectivity. Our results extend those from previous studies and add weight to the argument that neural oscillations are intimately related to functional connectivity and the BOLD response.

Oscillatory cortical activity related to voluntary muscle relaxation: Influence of normal aging

OBJECTIVE

In this study we aimed to investigate if there are age-related differences in cortical oscillatory activity induced by self-paced muscular pure relaxation in comparison with muscle contraction as reference movement.

METHODS

Event-related (de)synchronization (ERD/ERS) have been recorded related to voluntary muscle contraction and relaxation in 10 young and 10 elderly right-handed healthy subjects. The muscle relaxation task consisted in a voluntary relaxation of maintained wrist extension without any overt, associated muscle contraction. The muscle contraction task corresponded to a self-initiated brief wrist extension.

RESULTS

In elderly subjects compared to young ones, mu and beta ERD preceding muscular relaxation was more widespread, beginning significantly earlier over contralateral frontocentral and parietocentral regions (p<0.05) as well as over ipsilateral regions (p<0.05). The beta synchronization was significantly attenuated (p<0.05).

CONCLUSIONS

These results suggest an alteration of inhibitory motor systems and an altered post-movement somesthetic inputs processing with normal aging. These alterations were accompanied by compensatory mechanisms.

SIGNIFICANCE

These age-related alterations during different phases of muscle relaxation could participate to explain global sensorimotor slowing observed with normal aging.

What is the Bereitschaftspotential?

Since discovery of the slow negative electroencephalographic (EEG) activity preceding self-initiated movement by Kornhuber and Deecke [Kornhuber HH, Deecke L. Hirnpotentialänderungen bei Willkurbewegungen und passiven Bewegungen des Menschen: Bereitschaftspotential und reafferente Potentiale. Pflugers Archiv 1965;284:1-17], various source localization techniques in normal subjects and epicortical recording in epilepsy patients have disclosed the generator mechanisms of each identifiable component of movement-related cortical potentials (MRCPs) to some extent. The initial slow segment of BP, called 'early BP' in this article, begins about 2 s before the movement onset in the pre-supplementary motor area (pre-SMA) with no site-specificity and in the SMA proper according to the somatotopic organization, and shortly thereafter in the lateral premotor cortex bilaterally with relatively clear somatotopy. About 400 ms before the movement onset, the steeper negative slope, called 'late BP' in this article (also referred to as NS'), occurs in the contralateral primary motor cortex (M1) and lateral premotor cortex with precise somatotopy. These two phases of BP are differentially influenced by various factors, especially by complexity of the movement which enhances only the late BP. Event-related desynchronization (ERD) of beta frequency EEG band before self-initiated movements shows a different temporospatial pattern from that of the BP, suggesting different neuronal mechanisms for the two. BP has been applied for investigating pathophysiology of various movement disorders. Volitional motor inhibition or muscle relaxation is preceded by BP quite similar to that preceding voluntary muscle contraction. Since BP of typical waveforms and temporospatial pattern does not occur before organic involuntary movements, BP is used for detecting the participation of the 'voluntary motor system' in the generation of apparently involuntary movements in patients with psychogenic movement disorders. In view of Libet et al.'s report [Libet B, Gleason CA, Wright EW, Pearl DK. Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential). The unconscious initiation of a freely voluntary act. Brain 1983;106:623-642] that the awareness of intention to move occurred much later than the onset of BP, the early BP might reflect, physiologically, slowly increasing cortical excitability and, behaviorally, subconscious readiness for the forthcoming movement. Whether the late BP reflects conscious preparation for intended movement or not remains to be clarified.

Cortical mechanisms of unilateral voluntary motor inhibition in humans

While motor control is very often a goal-oriented event, little is known about the mechanisms underlying the termination of motor performance. To investigate what type of cortical activation underlies the muscle relaxation required to terminate the act, we performed single- and double-pulse transcranial magnetic stimulation (TMS) studies during voluntary muscle relaxation in nine normal volunteers. Subjects maintained a weak isometric contraction of the right first dorsal interosseous muscle (FDI), and either increased the level of contraction (Contraction), terminated the contraction (Relaxation), or maintained it (No-go) depending on a visual cue. Motor evoked potentials (MEP) and the silent period (SP) were recorded from the FDI during motor activity. To measure intra-cortical inhibition (ICI), we also performed double-pulse TMS, applying subthreshold conditioning stimuli at interstimulus intervals of 2 ms. When single-pulse TMS was given just prior to muscle relaxation (-21 to -70 ms), the MEP was reduced while the SP was unchanged. Intra-cortical inhibition was smaller just prior to the muscle relaxation. Unilateral voluntary muscle relaxation may not be associated with activation of the intracortical inhibitory system, but rather with the possible excitation of the corticospinal system, which can inhibit motoneurons disynaptically. These findings suggest that multiple inhibitory mechanisms act in diverse ways to achieve motor inhibition.

How do children prepare to react? Imaging maturation of motor preparation and stimulus anticipation by late contingent negative variation

Both the motor system and the frontal executive control system show a late maturation in humans which continues into school-age and even adolescence. We investigated the maturation of preparation processes towards a fast motor reaction in 74 healthy right-handed children aged 6 to 18 years and analyzed the topography of the late component of contingent negative variation (lCNV) in a 64-electrode high density sensor array. While adolescents from about 12 years on showed a bilaterally distributed centro-parietal maximum like adults do, younger children almost completely missed the negativity over the left central area contralaterally to the side of the anticipated movement. The reason, as revealed by current source density, was that only adolescents showed significant evoked activity of the left pre-/primary motor and supplementary/cingulate motor areas, while in contrast both age groups displayed significant current sinks over the right (ipsilateral) centro-temporal area and right posterior parietal cortex. Spatio-temporal source analysis confirmed that negativity over the right posterior parietal area could not be explained by a projection via volume conduction from frontal areas involved in motor preparation but represented an independent component with a different maturational course most likely related to sensory attention. Significant event-related desynchronization of alpha-power over the contralateral sensorimotor cortex was found in the younger age group, indicating that also 6- to 11-year-old children were engaged in motor preparation. Thus, the missing current sink over the contalateral sensorimotor cortex during late CNV in 6- to 11-year-old children might reflect the immaturity of a specific subcomponent of the motor preparation system which is related to evoked (late CNV) but not induced activity (alpha-ERD).

Relaxation from a voluntary contraction is preceded by increased excitability of motor cortical inhibitory circuits

Termination of a muscle contraction is as important a part of movement as muscle activation yet the mechanisms responsible are less well understood. In the present experiments we examined the possible role of intracortical inhibitory circuits in terminating a 20% maximum isometric contraction of the first dorsal interosseous muscle (FDI) in eight healthy subjects. Subjects performed the task simultaneously with both hands and received single or pairs (at an interstimulus interval of 3 ms to evaluate short interval intracortical inhibition, SICI) of transcranial magnetic stimuli (TMS) via a focal coil over the motor hand area of the left hemisphere at different times before and after the onset of relaxation. The amplitude of the motor-evoked potential (MEP) following a single or a pair of TMS pulses was measured in the right FDI and plotted relative to the onset of relaxation as estimated from the surface electromyogram (EMG) of the left FDI. MEPs were larger during contraction than after relaxation whereas SICI was absent during contraction and reappeared after relaxation. We found that in all subjects, the time course of MEP changes during relaxation was closely fitted by a Boltzmann sigmoidal curve which allowed us to estimate the mean MEP amplitudes as well as the ratio of the amplitudes after single or pairs of TMS pulses (i.e.%SICI) at any time in the task. The data showed that the amplitude of MEPs to single pulse TMS had started to decline at about the same time as the onset of EMG silence. Furthermore, the size of the MEPs evoked by paired pulses decreased up to 30 ms beforehand. The latter suggests that an increase in SICI occurs prior to the onset of MEP changes, and hence that increased cortical inhibition may play a role in suppressing corticospinal excitability during relaxation. A subsidiary experiment showed that the time relations of changes in SICI and MEP were unchanged by a period of 10 min training on the task.

Alpha and beta oscillatory activity during a sequence of two movements

OBJECTIVE

We studied movement-related electroencephalographic oscillatory changes in the alpha and beta range during a sequence of two movements in 7 healthy volunteers, in order to investigate the relationship between these changes and each component in the sequence.

METHODS

The sequence consisted of a wrist active extension-passive flexion followed by a first and second finger pincer. A total of 10.5 s sweeps were recorded using the level of surface electromyographic (EMG) activity in wrist extensors as trigger, including a 7.5 s pre-stimulus. The sweeps were also realigned manually offline using as trigger the end of the first EMG burst, or the beginning of the second movement. An index of the changes in non-phase-locked energy in the 7-37 Hz range was obtained by averaging single-sweep time-frequency transforms.

RESULTS

The duration of each of the movements in the sequence and the relationship between them were compatible with the use of two different motor programmes in the sequence. In the beta band, a decrease in energy (event-related desynchronisation, ERD) began 1.5 s before the onset of the first movement, and was sustained until the end of the second movement. No energy increases were observed until the end of the second movement. In the alpha band, the ERD began 0.5 seconds before the first movement and was sustained throughout the recording.

CONCLUSION

These findings suggest that the beta-event-related synchronisation is related to the end of the whole motor process, and not to the end of each motor programme.

Movement rate effect on activation and functional coupling of motor cortical areas

We investigated changes in the activation and functional coupling of bilateral primary sensorimotor (SM1) and supplementary motor (SMA) areas with different movement rates in eight normal volunteers. An auditory-cued repetitive right-thumb movement was performed at rates of 0.5, 0.75, 1, 2, 3, and 4 Hz. As a control condition, subjects listened to pacing tones with no movements. Electroencephalogram (EEG) was recorded from 28 scalp electrodes and electromyogram was obtained from the hand muscles. The event-related changes in EEG band-power (ERpow: activation of each area) and correlation (ERcor: functional coupling between each pair of cortical areas) were computed every 32 ms. Modulations of ERpow and ERcor were inspected in alpha (8-12 Hz) and beta (16-20 Hz) bands. Motor cortical activation and coupling was greater for faster movements. With increasing movement rate, the timing relationship between movement and tone switched from synchronization (for 0.5-1 Hz) to syncopation (for 3-4 Hz). The results suggested that for slow repetitive movements (0.5-1 Hz), each individual movement is separately controlled, and EEG activation and coupling of the motor cortical areas were immediately followed by transient deactivation and decoupling, having clear temporal modulation locked to each movement. In contrast, for fast repetitive movements (3-4 Hz), it appears that the rhythm is controlled and the motor cortices showed sustained EEG activation and continuous coupling.