Task-dependent modulation of excitatory and inhibitory functions within the human primary motor cortex

Cortical plasticity and motor activity studied with transcranial magnetic stimulation

For decades cortical representations of the parts of the body have been considered to be unchangeable. This view has changed radically during the past 20 years using new tools designed to study plasticity in the adult human brain. Transcranial magnetic stimulation (TMS) is a valuable non-invasive technique for exploring the ability of the motor cortex to change during motor skill acquisition. Results obtained with TMS in neurological patients as well as in normal subjects demonstrate that cortical plasticity is a necessity for correct adaptation to the continuously changing environment. Topographical reorganization of the motor cortex depends on the types of movements performed by the subjects. During simple training, the cortical representation is enlarged, and it returns to its initial size when the task is overlearned. These transient modifications characterize simple motor training. Motor skills in which coordination of distal and proximal muscles, precision of the task and spatio-temporal constraints are associated, has a different impact on cortical reorganization. We propose that years of practice of a complex motor skill induces a new cortical topography that must be interpreted as structural plasticity which provides the capacity to execute a plastic behaviour instead of a stereotypical movement. We review the neuronal mechanisms underlying plasticity in different types of movement. We stress new emerging notions, such as overlap of cortical maps, and system dynamics at single neuron and network levels, to explain the reorganization of movement representations that encode motor skill. Dendritic arborizations as functional computing elements, newly generated neurons in adult brain, and plastic architectures of cortical networks operating as distributed functional modules are new hypotheses for structural plasticity.

Transcranial magnetic stimulation over sensorimotor cortex disrupts anticipatory reflex gain modulation for skilled action

Skilled interactions with new environments require flexible changes to the transformation from somatosensory signals to motor outputs. Transcortical reflex gains are known to be modulated according to task and environmental dynamics, but the mechanism of this modulation remains unclear. We examined reflex organization in the sensorimotor cortex. Subjects performed point-to-point arm movements into predictable force fields. When a small perturbation was applied just before the arm encountered the force field, reflex responses in the shoulder muscles changed according to the upcoming force field direction, indicating anticipatory reflex gain modulation. However, when a transcranial magnetic stimulation (TMS) was applied before the reflex response to such perturbations so that the silent period caused by TMS overlapped the reflex processing period, this modulation was abolished, while the reflex itself remained. Loss of reflex gain modulation could not be explained by reduced reflex amplitudes nor by peripheral effects of TMS on the muscles themselves. Instead, we suggest that TMS disrupted interneuronal networks in the sensorimotor cortex, which contribute to reflex gain modulation rather than reflex generation. We suggest that these networks normally provide the adaptability of rapid sensorimotor reflex responses by regulating reflex gains according to the current dynamical environment.

Disinhibition of upper limb motor area by voluntary contraction of the lower limb muscle

It is well known that monosynaptic spinal reflexes and motor evoked potentials following transcranial magnetic stimulation (TMS) are reinforced during phasic and intensive voluntary contraction in the remote segment (remote effect). However, the remote effect on the cortical silent period (CSP) is less known. The purpose of the present study is to determine to what extent the CSP in the intrinsic hand muscle following TMS is modified by voluntary ankle dorsiflexion and to elucidate the origin of the modulation of CSP by the remote effect. CSP was recorded in the right first dorsal interosseous while subjects performed phasic dorsiflexion in the ipsilateral side under self-paced and reaction-time conditions. Modulation of the peripherally-induced silent period (PSP) induced by electrical stimulation of the ulnar nerve was also investigated under the same conditions. In addition, modulation of the CSP was investigated during ischemic nerve block of the lower limb and during application of vibration to the tibialis anterior tendon. The duration of CSP was significantly shortened by phasic dorsiflexion, and the extent of shortening was proportional to dorsiflexion force. Shortening of the CSP duration was also observed during tonic dorsiflexion. In contrast, the PSP duration following ulnar nerve stimulation was not altered during phasic dorsiflexion. Furthermore, the remote effect on the CSP duration was seen during ischemic nerve block of the lower limb and the pre-movement period in the reaction-time paradigm, but shortening of the CSP was not observed during tendon vibration. These findings suggest that phasic muscle contraction in the remote segment results in a decrease in intracortical inhibitory pathways to the corticospinal tract innervating the muscle involved in reflex testing and that the remote effect on the CSP is predominantly cortical in origin.

The origin of activity in the biceps brachii muscle during voluntary contractions of the contralateral elbow flexor muscles

During strong voluntary contractions, activity is not restricted to the target muscles. Other muscles, including contralateral muscles, often contract. We used transcranial magnetic stimulation (TMS) to analyse the origin of these unintended contralateral contractions (termed "associated" contractions). Subjects (n = 9) performed maximal voluntary contractions (MVCs) with their right elbow-flexor muscles followed by submaximal contractions with their left elbow flexors. Electromyographic activity (EMG) during the submaximal contractions was matched to the associated EMG in the left biceps brachii during the right MVC. During contractions, TMS was delivered to the motor cortex of the right or left hemisphere and excitatory motor evoked potentials (MEPs) and inhibitory (silent period) responses recorded from left biceps. Changes at a spinal level were investigated using cervicomedullary stimulation to activate corticospinal paths (n = 5). Stimulation of the right hemisphere produced silent periods of comparable duration in associated and voluntary contractions (218 vs 217 ms, respectively), whereas left hemisphere stimulation caused a depression of EMG but no EMG silence in either contraction. Despite matched EMG, MEPs elicited by right hemisphere stimulation were approximately 1.5-2.5 times larger during associated compared to voluntary contractions (P < 0.005). Similar inhibition of the associated and matched voluntary activity during the silent period suggests that associated activity comes from the contralateral hemisphere and that motor areas in this (right) hemisphere are activated concomitantly with the motor areas in the left hemisphere. Comparison of the MEPs and subcortically evoked potentials implies that cortical excitability was greater in associated contractions than in the matched voluntary efforts.

Transient motor evoked potential suppression following a complex sensorimotor task

OBJECTIVE

To investigate the mechanism involved in the transient suppression of the response to transcranial magnetic stimulation (TMS) following repeated performance of a complex sensorimotor training task (ST).

METHODS

A total of 19 healthy subjects participated in 4 experiments, all involving performance of the grooved pegboard test (GPT). The experiments investigated the effect of the ST on corticospinal and intracortical excitability, spinal excitability and maximal pinch grip force.

RESULTS

Motor evoked potential amplitude decreased significantly following the ST in both muscles tested and this was associated, but not correlated, with a decrease in the time taken to perform the GPT. There was no change in intracortical inhibition or facilitation (tested at interstimulus intervals of 3 and 10 ms, respectively). M wave amplitude was unchanged, as were F wave amplitude, latency and persistence and there was no evidence of muscle fatigue.

CONCLUSIONS

The reduction in corticospinal excitability was short lasting (<10 min) and was not accompanied by changes at the spinal or peripheral level, suggesting that other intracortical circuits may be involved.

SIGNIFICANCE

Repeated performance of motor tasks can result in both short- and long-term modulation of motor cortical excitability. However, the relationship between changes in corticospinal excitability and motor performance is complex and critically dependent upon task type and duration.

Asymmetries of long-latency intracortical inhibition in motor cortex and handedness

Paired-pulse transcranial magnetic stimulation was used in three experiments to measure the properties of long-latency intracortical inhibition (LICI) acting on the relaxed first dorsal interosseus muscle of the left and right hand in right-handed volunteers. The experiments show that LICI is asymmetrical: it emerges more rapidly and is greater in the dominant than non-dominant hand shortly after activation of the LICI circuits, and is greater with low-intensity conditioning stimulus intensities in the dominant than non-dominant hand. These findings suggest that asymmetrical function of long-latency inhibitory circuits in motor cortex might contribute to the asymmetrical dexterity between the hands, possibly through their inhibitory control of the circuits responsible for short-latency inhibition.

Age-related differences in corticospinal control during functional isometric contractions in left and right hands

The purpose of the study was to examine age-related differences in electromyographic (EMG) responses to transcranial magnetic stimulation (TMS) during functional isometric contractions in left and right hands. EMG responses were recorded from the first dorsal interosseus muscle following TMS in 10 young (26.6 +/- 1.3 yr) and 10 old (67.6 +/- 2.3 yr) right-handed subjects. Muscle evoked potentials (MEPs) and silent-period durations were obtained in the left and right hands during index finger abduction, a precision grip, a power grip, and a scissor grip, while EMG was held constant at 5% of maximum. For all tasks, MEP area was 30% (P < 0.001) lower in the left hand of old compared with young subjects, whereas there was no age difference in the right hand. The duration of the EMG silent period was 14% (P < 0.001) shorter in old (150.3 +/- 2.9 ms) compared with young (173.9 +/- 3.0 ms) subjects, and the age differences were accentuated in the left hand (19% shorter, P < 0.001). For all subjects, the largest MEP area (10-12% larger) and longest EMG silent period (8-19 ms longer) were observed for the scissor grip compared with the other three tasks, and the largest task-dependent change in these variables was observed in the right hand of older adults. These differences in corticospinal control in the left and right hands of older adults may reflect neural adaptations that occur throughout a lifetime of preferential hand use for skilled (dominant) and unskilled (nondominant) motor tasks.

Sensorimotor integration in patients with parkinsonian type multisystem atrophy

Sensorimotor integration is an essential feature of the central nervous system that contributes to the accurate performance of motor tasks. Some patients with multiple system atrophy with parkinsonian features (MSAp) exhibit clinical signs compatible with an abnormal central nervous system excitability to somatosensory inputs, such as action myoclonus or enhanced cutaneo-muscular reflexes. To investigate further the site where such dysfunction in sensorimotor integration takes place, we examined the inhibitory effects of a cutaneous afferent volley at two different levels of the motor system in 10 MSAp patients and in 10 age-matched healthy volunteers. Electrical digital nerve stimuli were given as the conditioning stimulus for the motor evoked potentials (MEP) elicited by transcranial magnetic stimulation in hand muscles, and for the blink reflex responses obtained in the orbicularis oculi muscles by supraorbital nerve stimulation. Intervals for the conditioning were 20 to 50 ms for the MEP and 90 to 110 ms for the blink reflex. The MEP was significantly inhibited in test trials in healthy volunteers, reaching a mean of 32% of the baseline values at the ISI of 35 ms. Significant inhibition occurred also in the blink reflex, in which the R2 response was a mean of 12% of baseline values at the ISI of 100 ms. The inhibitory effects were abnormally reduced in 8 patients on the MEP, and in 7 patients on the blink reflex. There were significant group differences between patients and control subjects in the size of the conditioned MEP and blink reflex. These results suggest that sensorimotor integration is abnormal in patients with MSAp in at least two central nervous system sites: the sensorimotor cortex, and the brainstem reticular formation.

Task-specific impairment of motor cortical excitation and inhibition in patients with writer's cramp

Abnormalities in motor cortical excitation and inhibition have been reported in patients with writer's cramp, at rest and during muscle activation. We were interested in whether such abnormalities might be task-specific and depended on the type of movement task used to activate the dystonic hand. We therefore assessed motor-evoked potentials (facilitation/rest MEP amplitude ratio) and duration of the cortical silent period (CSP) from the right first dorsal interosseus (FDI) muscle to transcranial magnetic stimulation (TMS) in 10 patients with writer's cramp and in 10 healthy volunteers performing pincer and power gripping tasks. The mean facilitation/rest MEP amplitude ratio measured during the pincer grip task was significantly larger in dystonic subjects than in controls, but in the power grip condition was similar in the two groups. The CSP measured in the power grip condition was of similar length in normal controls and dystonic subjects, but in the pincer grip condition was significantly shorter in patients than in controls. These results indicate a task-specific impairment of motor cortical excitation and inhibition in writer's cramp.

Motor skill training induces changes in the excitability of the leg cortical area in healthy humans

Training-induced changes in cortical excitability may play an important role in rehabilitation of gait ability in patients with neurological disorders. In this study, we investigated the effect of a 32-min period of motor skill, non-skill and passive training involving the ankle muscles on leg motor cortical excitability in healthy humans. Transcranial magnetic stimulation (TMS) at a range of intensities was applied to obtain a recruitment curve of the motor evoked potentials (MEPs) in the tibialis anterior (TA) muscle before and after training. We also explored the effect of training on inhibitory and facilitatory cortical circuits by using a paired-pulse TMS technique at intervals of 2.5 ms (short-interval intracortical inhibition, SICI) and 8 ms (intracortical facilitation, ICF). During motor skill training, subjects were instructed to make a cursor follow a series of target lines on a computer screen by performing voluntary ankle dorsi- and plantarflexion movements. Non-skill and passive training consisted of repeated voluntary and assisted dorsi- and plantarflexion movements, respectively. Recruitment curves increased significantly after 32 min of motor skill training but not after non-skill and passive training, suggesting that only skill motor training increases motor cortical excitability. Motor skill training was not accompanied by any changes in the recruitment curves of TA MEPs evoked by transcranial electrical stimulation, suggesting that the increased MEPs to TMS was likely caused by changes in excitability at a cortical site. SICI was decreased after 32 min of motor skill training but no changes were observed in ICF. We conclude that similar plastic changes as have previously been reported for the hand motor following motor skill training may also be observed for the leg motor area. The observed plastic changes appeared to be related to the degree of difficulty in the motor task, and may be of relevance for rehabilitation of gait disorders.