Inhibition of ipsilateral motor cortex during phasic generation of low force.

OBJECTIVE: To study the effect of different types of unilateral pinch grips on excitability of the ipsilateral motor cortex. METHODS: In 9 healthy volunteers, transcranial magnetic stimuli (TMS) were applied over one motor cortex while the subjects performed either phasic or tonic ipsilateral pinch grips with different force levels (range 1-40% maximum voluntary contraction, MVC). Motor evoked potentials (MEP) were recorded from the relaxed contralateral first dorsal interosseous muscle (FDI) and were compared to MEPs obtained during muscle relaxation of both hands. In additional experiments, transcranial electrical stimuli (TES) were administered and F waves were recorded after electrical stimulation of the ulnar nerve. RESULTS: Phasic pinch grips with low force (1 and 2% MVC) induced a significant decrease of TMS-induced MEP amplitudes. The effect lasted for about 100 ms after reaching the force level and was similar for both right and left-handed pinch grips. TES-induced MEPs and F waves remained unchanged. In contrast, tonic contractions (20 and 40% MVC) enhanced MEPs in the homologous FDI. CONCLUSIONS: Phasic pinch grips with low force inhibit the motor cortex responsible for the contralateral homologous hand muscle. This effect, which is probably mediated transcallosally, might act at the level of the motor cortex.     

Pre-movement motor excitability is reduced ipsilateral to low force pinch grips

Motor excitability ipsilateral to pinch grips was investigated during the pre-movement period. Subjects performed right-handed phasic pinch grips with 2% or 20% maximum voluntary contraction (MVC) in response to a visual go-signal. Transcranial magnetic stimulation (TMS) was applied over the right motor cortex at various intervals before the go-signal and 100 msec after movement onset. Motor evoked potentials were recorded from the relaxed left first dorsal interosseous muscle. Immediately prior to and during 2% MVC pinch grips, MEP amplitudes were reduced. In contrast, MEPs obtained by transcranial electrical stimulation tended to be increased, indicating that MEP decreases are mediated at a cortical level. Before and during 20% MVC pinch grips MEP amplitudes were enhanced. TMS delayed reaction time if applied close to the go-signal. We conclude that the motor cortex ipsilateral to low force movements is inhibited prior to and during movement.     

Exercise-induced changes of motor excitability with and without sensory block

To explore interactions between the sensory and motor system, we investigated motor excitability changes following a motor exercise with and without an anesthetic block of cutaneous inputs overlying the target muscle. Transcranial magnetic stimulation (TMS) with a focal coil was applied to determine motor output maps, intracortical inhibition (ICI) and intracortical facilitation (ICF) of the first dorsal interosseous muscle (FDI) on both sides. Twelve subjects performed phasic right index finger adductions (frequency: 0.333 Hz) for 30 min. TMS measurements were performed before and after the motor task (Experiment 1). In Experiment 2, median and radial nerve were blocked with Ropivacaine injections at the right wrist prior to the motor exercise. TMS was applied before and after induction of anesthesia and after exercise. In Experiment 3, the same anesthetic block was applied and TMS was performed before and after induction of anesthesia and after additional 30 min of rest. In Experiment 1, right FDI motor output area was enlarged, its center of gravity moved posteriorly, and ICI was reduced after the exercise. In Experiment 2, anesthesia was associated with a shrinkage of right FDI motor output area. After exercise, right FDI motor output area enlarged again but was still significantly smaller than pre-anesthesia. In both experiments, TMS results of left FDI remained unchanged. In Experiment 3, the anesthesia-induced decrease of right FDI motor output area remained unchanged after the period of rest. We conclude that a simple motor task enhanced the cortical representation of the target muscle and reduced intracortical inhibition. An impairment of cutaneous afferents decreased the cortical representation of the target muscle. The decrease of motor excitability induced by the sensory deficit could only partially be reversed by the motor exercise.     

Interhemispheric effects of high and low frequency rTMS in healthy humans.

OBJECTIVE: We investigated whether repetitive transcranial magnetic stimulation (rTMS) applied to the right motor cortex modified the excitability of the unstimulated left motor cortex. METHODS: Interhemispheric effects of 0.5 and 5 Hz subthreshold rTMS over the right motor cortex were examined by single pulse and paired pulse TMS and by transcranial electrical stimulation (TES) applied to the unstimulated left motor cortex. The effects of (a) 1800 pulses real and sham rTMS with 5 Hz, (b) 180 pulses real and sham rTMS with 0.5 Hz and (c) 1800 pulses real rTMS with 0.5 Hz were studied. RESULTS: Following 5 Hz right motor rTMS motor evoked potential (MEP) amplitudes induced by single pulse TMS over the left motor cortex increased significantly. Intracortical inhibition (ICI) and facilitation (ICF) and MEP amplitudes evoked by TES were unchanged. Sham stimulation had no influence on motor cortex excitability. After 180 pulses right motor cortex rTMS with 0.5 Hz a significant decrease of left motor ICF, but no change in single pulse MEP amplitudes was found. A similar trend was observed with 1800 pulses rTMS with 0.5 Hz. CONCLUSIONS: High frequency right motor rTMS can increase left motor cortex excitability whereas low frequency right motor rTMS can decrease it. These effects outlast the rTMS by several minutes. The underlying mechanisms mediating interhemispheric excitability changes are likely to be frequency dependent.     

Unilateral grip fatigue reduces short interval intracortical inhibition in ipsilateral primary motor cortex

ObjectiveThis study was designed to examine whether exhaustive grip exercise of the left hand affected intracortical excitability in ipsilateral motor cortex.MethodsTen healthy male subjects (aged 21–24 years) participated in experiment 1 in which paired-pulse transcranial magnetic stimulation (TMS) was used to test corticospinal and corticocortical excitability in right (relaxed) first dorsal interosseous (FDI) muscle during the recovery period after exhaustive forceful grip exercise of the left hand. Seven of the same subjects participated in experiment 2, in which the intensity of the test stimulus was adjusted so that the amplitude of motor evoked potential (MEP_TEST) was kept constant throughout the measurement.ResultsIn experiment 1, MEP_TEST was slightly reduced from 5 to 15 min after exercise whilst short interval intracortical inhibition (SICI) at interstimulus interval (ISI) of 2 and 3 ms became less effective. Intracortical facilitation (ICF) was unchanged. In experiment 2 when the MEP_TEST was maintained at a constant size there was again no change in ICF, and the reduction in SICI was still present at the same intervals.ConclusionsWe conclude that unilateral exhaustive grip exercise reduced the excitability of the corticospinal output of the ipsilateral motor cortex whilst simultaneously reducing the excitability of SICI. These results would be compatible with the idea that fatigue increases the tonic level of interhemispheric inhibition from the fatigued to the non-fatigued cortex.SignificanceMuscle fatigue to the point of exhaustion has lasting effects on the excitability of intracortical circuits in the non-exercised hemisphere, perhaps via changes in the tonic levels of activity in transcallosal pathways.     

Corticomotor excitability and plasticity following complex visuomotor training in young and old adults

Previous studies with transcranial magnetic stimulation (TMS) have shown that advancing age may influence plasticity induction in human motor cortex (M1), but these changes have been assessed with TMS-induced paradigms or simple motor tasks. The aim of this study was to examine changes in corticospinal excitability and intracortical inhibition as markers of corticomotor plasticity following complex motor training in young and old adults. Electromyographic recordings were obtained from the right first dorsal interosseous (FDI) muscle of 16 young (20-35 years) and 16 older (aged 60-75 years) adults before and after motor skill training. Motor training consisted of three 6-minute blocks of a complex visuomotor task that required matching the metacarpophalangeal (MCP) joint angle of the index finger using abduction-adduction movements. Single- and paired-pulse TMS over the left M1 was used to assess changes in right FDI motor-evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) before and after each training block. Visuomotor tracking performance was diminished in old compared with young adults throughout training. However, improvement in tracking error was similar for young and old adults (7-24% increase in each training block). For young and old adults, motor training increased FDI MEP amplitude (≥ 20%) and reduced the magnitude of SICI (≥ 19%) after each visuomotor training block, reflecting use-dependent plasticity. However, no difference in corticomotor plasticity (change in MEP or SICI) was observed between young and old adults. Further studies are needed to identify the experimental or behavioral factors that might contribute to the maintenance of corticomotor plasticity in older adults.     

Altered dorsal premotor–motor interhemispheric pathway activity in focal arm dystonia

Given the possible role of dorsal premotor cortex (PMd) in the pathophysiology of dystonia, we used transcranial magnetic stimulation (TMS) methods to study PMd and PMd–primary motor cortex (M1) interactions in patients with focal arm dystonia. Here, we tested the connectivity between left PMd and right M1 as well as the intracortical excitability of PMd in 11 right‐handed patients with focal arm/hand dystonia and nine age‐matched healthy controls. The results showed that excitability of the inhibitory connection between PMd and M1 was reduced in patients, but there was no significant difference to healthy subjects in the excitability of the facilitatory connection. A triple stimulation technique in which pairs of TMS pulses are given over PMd and their interaction measured in terms of the effect on the baseline PMd‐M1 connection failed to reveal the usual pattern of interaction between the pairs of PMd stimuli. Indeed, the results in patients were similar to those seen in a group of young healthy subjects after the excitability of PMd had been changed by pretreatment with high‐frequency rTMS. We suggest that reduced transcallosal inhibition from the PMd may be involved in the altered pattern of abnormal muscle contractions of agonists and antagonists (overflow). © 2007 Movement Disorder Society     

Functional plasticity of surround inhibition in the motor cortex during single finger contraction training

We investigated the functional changes in short intracortical inhibitory (SICI) circuits to determine whether surround inhibition is altered during a simple finger movement training. Using an electromyographic (EMG) feedback system linked to a computer monitor, participants practiced sustained index finger abduction by 40% maximum voluntary contraction of the first dorsal interosseous (FDI) while decreasing overflow EMG activity of the abductor digiti minimi (ADM) to less than 5% maximum voluntary contraction. Single transcranial magnetic stimuli (TMS) and paired-pulse TMS were applied to the left primary motor cortex to elicit motor-evoked potentials (MEPs) in the right FDI and ADM before/after training. In addition to recording MEPs from both muscles during voluntary FDI contraction, MEPs were recorded during motor imagery. MEPs from the FDI were not altered by training, indicating no functional changes in SICI circuits associated with the FDI field. In contrast, SICI circuits associated with ADM were significantly strengthened by training, as indicated by reduced baseline EMG activity during both actual FDI contraction and motor imagery and by reduced MEPs in response to post-training TMS. We propose that SICI circuits show functional plasticity during motor training and that surround circuit inhibition of nontarget muscle groups increases in proportion to the acquisition of motor skills.     

Reduced cortical inhibition in first-episode schizophrenia

Disturbances in cortico-cortical and cortico-subcortical circuits in schizophrenia have been described by previous neuroimaging and electrophysiological studies. Transcranial magnetic stimulation (TMS) provides a neurophysiological technique for the measurement of cortical excitability, especially of the motoneural system. Previous studies using paired-pulse TMS to investigate short-interval cortical inhibition (SICI) and intracortical facilitation (ICF), mainly involving chronic schizophrenia patients, have been inconsistent and only one study in first-episode patients has been conducted so far. We assessed SICI (interstimulus interval, ISI, 3 milliseconds, ms) and ICF (ISI 7 ms) in 29 first-episode schizophrenia patients (FE-SZ) with limited exposure to antipsychotic treatment against measures of 28 healthy controls (HC). Amplitudes of motor evoked potentials (MEPs) were measured from the left and right first dorsal interosseus muscle (FDI). The conditioning stimulus was set at 80% intensity of resting motor threshold (RMT) and the test stimulus (TS) was set at an intensity that produced an MEP amplitude of about 1 mV. For SICI conditions, FE-SZ demonstrated significantly higher MEP amplitudes from left motor cortex (right FDI) compared to HC, and for MEPs from right motor cortex (left FDI) a similar trend was observable (FE-SZ 41% vs. HC 21% of TS, p=0.017 for left motor cortex, and FE-SZ 59% vs. HC 31% of TS, p=0.059 for right motor cortex; Mann-Whitney U-test). No significant difference in MEPs could be detected for ICF on either hemisphere. In addition, there was no difference in left and right RMT comparing patients and control subjects. Our result of a reduced SICI in a large sample of well characterized first-episode schizophrenia patients suggests that a GABAergic deficit may be involved in schizophrenic pathophysiology, already early in the disease course, supporting the intracortical dysconnectivity hypothesis.     

Parallel inhibition of cortico-muscular synchronization and cortico-spinal excitability by theta burst TMS in humans

OBJECTIVE: To investigate the after-effects of theta burst TMS (TBS) on cortico-muscular synchronization, and on cortico-spinal excitability, in humans. METHODS: We studied 10 healthy subjects using a continuous paradigm of TBS (cTBS), i.e. 600 pulses in 40s. Before and after the cTBS, coherence function was computed as a measure of cortico-muscular synchronization by recording electroencephalogram (EEG) from 19 scalp sites and electromyogram (EMG) from right first dorsal interosseous (FDI) muscle during the isometric contraction. In a separate experiment, motor-evoked potentials (MEPs) in response to single TMS pulses were recorded from the FDI muscle before and after the cTBS, to measure cortico-spinal excitability. RESULTS: When the cTBS was applied over the left primary motor cortex (M1), the beta-band cortico-muscular coherence for the C3 scalp site, as well as the MEP amplitude significantly decreased in 30-60 min, and then recovered to the original levels in 90-120 min. Neither sham stimulation nor cTBS applied over 2 cm posterior to M1 produced significant effects. CONCLUSIONS: cTBS-over-M1 can inhibit the cortico-muscular synchronization in parallel with the decline of cortico-spinal excitability. SIGNIFICANCE: Our results provide the first evidence that TBS can efficiently alter the functional cortico-muscular coupling in humans.     

Effects of low frequency and low intensity repetitive paired pulse stimulation of the primary motor cortex.

OBJECTIVE: Following a previous report [Bestmann et al. Clin Neurophysiol 2004;115:755-64] that pairs of subthreshold pulses of transcranial magnetic stimulation (TMS) can show temporal summation, we explored whether repeated application of pairs of stimulation could produce long-lasting after effects on the excitability of the human motor cortex. METHODS: Twelve healthy subjects received 25 min repetitive paired pulse magnetic stimulation (paired rTMS) given at a frequency of about 0.6 Hz over the left primary motor cortex (500 paired stimuli in total). The interval between the paired stimuli was 3 ms and the intensity of both stimuli was 80% of active motor threshold. The resting and active motor threshold, MEP recruitment curve, short interval intracortical inhibition (SICI) and facilitation, and the duration of the cortical silent period (SP) were tested for the right first interosseous muscle (FDI) before and two times after the end of 25 min paired rTMS. RESULTS: Prolonged subthreshold paired rTMS produced a significant decrease in excitability in the corticospinal projection to FDI: resting motor threshold was significantly increased and MEP recruitment was significantly decreased, SICI was significantly increased at 2 and 4 ms and the SP was significantly increased in duration. CONCLUSIONS: Prolonged low frequency paired rTMS at subthreshold intensity can modulate cortical excitability by producing inhibitory effects that outlast the period of stimulation.     

Improvement of motor performance and modulation of cortical excitability by repetitive transcranial magnetic stimulation of the motor cortex in Parkinson's disease.

OBJECTIVE: To assess the effects of focal motor cortex stimulation on motor performance and cortical excitability in patients with Parkinson's disease (PD). METHODS: Repetitive transcranial magnetic stimulation (rTMS) was performed on the left motor cortical area corresponding to the right hand in 12 'off-drug' patients with PD. The effects of subthreshold rTMS applied at 0.5 Hz (600 pulses) or at 10 Hz (2000 pulses) using a 'real' or a 'sham' coil were compared to those obtained by a single dose of l-dopa. The assessment included a clinical evaluation by the Unified Parkinson's Disease Rating Scale and timed motor tasks, and a neurophysiological evaluation of cortical excitability by single- and paired-pulse TMS techniques. RESULTS: 'Real' rTMS at 10 or 0.5 Hz, but not 'sham' stimulation, improved motor performance. High-frequency rTMS decreased rigidity and bradykinesia in the upper limb contralateral to the stimulation, while low-frequency rTMS reduced upper limb rigidity bilaterally and improved walking. Concomitantly, 10 Hz rTMS increased intracortical facilitation, while 0.5 Hz rTMS restored intracortical inhibition. CONCLUSIONS: Low- and high-frequency rTMS of the primary motor cortex lead to significant but differential changes in patients with PD both on clinical and electrophysiological grounds. The effects on cortical excitability were opposite to previous observations made in healthy subjects, suggesting a reversed balance of cortical excitability in patients with PD compared to normals. However, the underlying mechanisms of these changes remain to determine, as well as the relationship with clinical presentation and response to l-dopa therapy. SIGNIFICANCE: The present study gives some clues to appraise the role of the primary motor cortex in PD. Clinical improvement induced by rTMS was too short-lasting to consider therapeutic application, but these results support the perspective of the primary motor cortex as a possible target for neuromodulation in PD.     

The effect of continuous theta burst stimulation over premotor cortex on circuits in primary motor cortex and spinal cord

ObjectiveTo understand the effect of continuous theta burst stimulation (cTBS) given to the premotor area, we studied the circuits within the primary motor cortex and spinal cord after cTBS over the dorsal premotor area (PMd).MethodsThree sets of parameters, including corticospinal excitability, short interval intracortical inhibition (SICI) and intracortical facilitation (ICF) and forearm reciprocal inhibition (RI) were tested.ResultsParalleling the effects of cTBS applied directly to the primary motor cortex, cTBS over the left PMd suppressed corticospinal excitability as measured by the change in the size of MEPs evoked by single pulse TMS over primary motor cortex. Premotor cTBS appeared to have a longer lasting, but no more powerful effect on corticospinal excitability than motor cTBS, however, unlike motor cTBS it had no effect on SICI or ICF. Finally, although premotor cTBS had no effect on spinal H-reflexes, it did reduce the third phase of RI between forearm extensor and flexor muscles.ConclusionsPremotor cTBS is a quick and useful way of modulating excitability in cortical and possibly subcortical motor circuits.SignificancePremotor cTBS can be used as an alternative to regular rTMS to evaluate cortical function, motor behaviours and the response to disease therapy.     

Intravenous clomipramine decreases excitability of human motor cortex. A study with paired magnetic stimulation

Several recent reports suggest the possibility of monitoring pharmacological effects on brain excitability through transcranial magnetic stimulation (TMS). In these studies, paired magnetic stimulation has been used in normal subjects and on patients who were taking different antiepileptic drugs. The aim of our study was to investigate motor area excitability on depressed patients after intravenous administration of a single dose of clomipramine, a tricyclic antidepressant. Motor cortex excitability was studied by single and paired transcranial magnetic stimulation (TMS) before and after 4, 8 and 24 h from intravenous administration of 25 mg of clomipramine. Cortical excitability was measured using different TMS parameters: motor threshold (MT), motor evoked potential (MEP) amplitude, duration of cortical silent period (CSP), intracortical inhibition (ICI) and intracortical facilitation (ICF). Spinal excitability and peripheral nerve conduction was measured by F response and M wave. A temporary but significant increase of motor threshold and intracortical inhibition and a decrease of intracortical facilitation were observed 4 h following drug administration. MEP amplitude, cortical silent period, F response and M wave were not significantly affected by drug injection. Our findings suggest that a single intravenous dose of clomipramine can exert a significant but transitory suppression of motor cortex excitability in depressed patients. TMS represents a useful research tool in assessing the effects of motor cortical excitability of neuropsychiatric drugs used in psychiatric disease.     

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.     

Charting the excitability of premotor to motor connections while withholding or initiating a selected movement

In 19 healthy volunteers, we used transcranial magnetic stimulation (TMS) to probe the excitability in pathways linking the left dorsal premotor cortex and right primary motor cortex and those linking the left and right motor cortex during the response delay and the reaction time period while subjects performed a delayed response [symbol 1 (S1) - symbol 2 (S2)] Go-NoGo reaction time task with visual cues. Conditioning TMS pulses were applied to the left premotor or left motor cortex 8 ms before a test pulse was given to the right motor cortex at 300 or 1800 ms after S1 or 150 ms after S2. S1 coded for right-hand or left-hand movement, and S2 for release or stopping the prepared movement. Conditioning of the left premotor cortex led to interhemispheric inhibition at 300 ms post-S1, interhemispheric facilitation at 150 ms post-S2, and shorter reaction times in the move-left condition. Conditioning of the left motor cortex led to inhibition at 1800 ms post-S1 and 150 ms post-S2, and slower reaction times for move-right conditions, and inhibition at 300 and 1800 ms post-S1 for move-left conditions. Relative motor evoked potential amplitudes following premotor conditioning at 150 ms post-S2 were significantly smaller in 'NoGo' than in 'Go' trials for move-left instructions. We conclude that the excitability in left premotor/motor right motor pathways is context-dependent and affects motor behaviour. Thus, the left premotor cortex is engaged not only in action selection but also in withholding and releasing a preselected movement generated by the right motor cortex.     

Stimulus intensity and coil characteristics influence the efficacy of rTMS to suppress cortical excitability.

OBJECTIVE: Low-frequency repetitive transcranial magnetic stimulation (rTMS) can reduce cortical excitability. Here we examined whether inhibitory after effects of low-frequency rTMS are influenced by stimulus intensity, the type of TMS coil and re-afferent sensory stimulation. METHODS: In fifteen healthy volunteers, we applied 900 biphasic pulses of 1Hz rTMS to the left primary motor cortex (M1) at an intensity that was 10% below or 15% above resting motor threshold. For rTMS, we used two different figure-of-eight shaped coils (Magstim or Medtronic coil) attached to the same stimulator. We recorded motor evoked potentials (MEPs) evoked with the same set-up used for rTMS (MEP-rTMS) before and twice after rTMS. Using a different TMS setup, we also applied monophasic pulses to the M1 in order to assess the effects of rTMS on corticospinal excitability, intracortical paired-pulse excitability and the duration of the cortical silent period (CSP). In a control experiment, the same measurements were performed after 15min of 1Hz repetitive electrical nerve stimulation (rENS) of the right ulnar nerve. RESULTS: Analysis of variance revealed an interaction between intensity, coil and time of measurement (p<0.035), indicating that the effect of 1Hz rTMS on MEP-rTMS amplitude depended on the intensity and the type of coil used for rTMS. Suppression of corticospinal excitability was strongest after suprathreshold 1Hz rTMS with the Medtronic coil (p<0.01 for both post-rTMS measurements relative to pre-intervention baseline). Regardless of the type of coil, suprathreshold but not subthreshold rTMS transiently prolonged the CSP and attenuated paired-pulse facilitation. Suprathreshold 1Hz rENS also induced a short-lasting inhibition of MEP-rTMS. CONCLUSIONS: Both the stimulation intensity and the type of TMS coil have an impact on the after effects of 1Hz rTMS. Re-afferent feedback activation may at least in part account for the stronger suppression of corticospinal excitability by suprathreshold 1Hz rTMS. SIGNIFICANCE: These data should be considered when rTMS is used as a therapeutic means.     

Parietal transcranial direct current stimulation modulates primary motor cortex excitability

The posterior parietal cortex is part of the cortical network involved in motor learning and is structurally and functionally connected with the primary motor cortex (M1). Neuroplastic alterations of neuronal connectivity might be an important basis for learning processes. These have however not been explored for parieto‐motor connections in humans by transcranial direct current stimulation (tDCS). Exploring tDCS effects on parieto‐motor cortical connectivity might be functionally relevant, because tDCS has been shown to improve motor learning. We aimed to explore plastic alterations of parieto‐motor cortical connections by tDCS in healthy humans. We measured neuroplastic changes of corticospinal excitability via motor evoked potentials (MEP) elicited by single‐pulse transcranial magnetic stimulation (TMS) before and after tDCS over the left posterior parietal cortex (P3), and 3 cm posterior or lateral to P3, to explore the spatial specificity of the effects. Furthermore, short‐interval intracortical inhibition/intracortical facilitation (SICI/ICF) over M1, and parieto‐motor cortical connectivity were obtained before and after P3 tDCS. The results show polarity‐dependent M1 excitability alterations primarily after P3 tDCS. Single‐pulse TMS‐elicited MEPs, M1 SICI/ICF at 5 and 7 ms and 10 and 15 ms interstimulus intervals (ISIs), and parieto‐motor connectivity at 10 and 15 ms ISIs were all enhanced by anodal stimulation. Single pulse‐TMS‐elicited MEPs, and parieto‐motor connectivity at 10 and 15 ms ISIs were reduced by cathodal tDCS. The respective corticospinal excitability alterations lasted for at least 120 min after stimulation. These results show an effect of remote stimulation of parietal areas on M1 excitability. The spatial specificity of the effects and the impact on parietal cortex–motor cortex connections suggest a relevant connectivity‐driven effect.     

Corticomotor plasticity following unilateral strength training

Introduction: We used transcranial magnetic stimulation (TMS) to investigate 3 weeks of unilateral leg strength training on ipsilateral motor cortex (iM1) excitability, and short-latency intracortical inhibition (SICI). Methods: Right leg dominant participants (n = 14) were randomly divided into either a strength training (ST) or control group. The ST group completed 9 training sessions (4 sets of 6 to 8 repetitions of single right leg squats). Results: We observed a 41% increase in right leg strength, and a 35% increase in strength of the untrained left leg (P < 0.01). There was a significant increase in motor evoked potential (MEP) amplitude recruitment curve for the untrained left leg (P < 0.01). SICI of the iM1 decreased by 21% for the untrained left leg (P < 0.01). Conclusions: The findings provide evidence for corticomotor adaptation for unilateral leg strength training within the iM1 that is modulated by changes in interhemispheric inhibition. Muscle Nerve 46: 384-393, 2012.     

Transcallosal sensorimotor integration: effects of sensory input on cortical projections to the contralateral hand.

OBJECTIVE: Low amplitude vibration of forearm or hand muscles predominantly activates proprioceptive inputs that influence corticospinal projections in a focal manner, increasing output to the stimulated muscle while reducing output to neighbouring muscles. Modulation of contralateral forearm muscles by vibration has also been reported on one occasion. The aim of the current investigation was to investigate the effects of proprioceptive input from a hand muscle on corticospinal excitability, intracortical inhibition (SICI) and interhemispheric inhibition (IHI) targeting the homologous contralateral muscle. METHODS: Transcranial Magnetic Stimulation (TMS) was delivered to the left cortical hand area of 10 healthy subjects and surface electromyography (EMG) recordings taken from the right First Dorsal Interosseus (FDI) and Abductor Digiti Minimi (ADM). The effect of low amplitude vibration of the left FDI on MEP amplitudes, SICI and IHI targeting the right hand was assessed. RESULTS: Vibration of the left FDI caused a significant reduction in MEP amplitudes in the homologous right FDI but not in the right ADM. SICI and IHI targeting both muscles were also significantly increased. CONCLUSIONS: We conclude that proprioceptive input from a hand muscle reduces the corticospinal excitability of the contralateral homologous muscle. The increases in SICI and IHI suggest that at least some of this effect occurs in the cortex ipsilateral to the stimulus and this may be mediated via transcallosal fibres. SIGNIFICANCE: These results suggest that sensory input can modulate excitability in both motor cortices simultaneously, as well as the relationship between them. Interventions which modulate this transcallosal relationship may become useful in disorders where abnormal IHI is a potential therapeutic target.