Suppression of human cortico-motoneuronal excitability during the Stop-signal task

ObjectiveTo investigate whether motor suppression is an active process, and to clarify its somatotopic organization, we investigated cortico-motoneuronal excitability using transcranial magnetic stimulation (TMS) during the Stop-signal task.MethodsSubjects were asked to press a button following a Go cue; a Stop-signal followed the Go cue by a certain time delay in 25% of trials, indicating to subjects that they were not to press the button. TMS was given to the primary motor area of the left or right-hand or leg at variable time delays. Motor evoked potentials (MEPs) were recorded from the hand and leg muscles bilaterally.ResultsWhen TMS was delivered 400 ms after the Go cue, there was significant suppression of the MEPs of the bilateral hand and leg muscles during successful Stop trials, but not during failed Stop trials.ConclusionsThe voluntary stopping of movement in the Stop-signal task is an active process, which likely suppresses not only the cortico-motoneuronal excitability of the task-performing hand, but also causes the widespread suppression of the motor system.SignificanceStudies in the normal physiology of response inhibition would be of help in understanding the pathophysiology of neuro-psychiatric disorders associated with deficits in motor suppression.     

Motor imagery of phasic thumb abduction temporally and spatially modulates corticospinal excitability.

OBJECTIVE: To explore the spatial and temporal characteristics of the modulation of corticospinal and segmental excitability during actual and imagined movement of a single digit. METHODS: Using transcranial magnetic stimulation (TMS), motor evoked potentials (MEPs) were evoked in abductor pollicis brevis (APB) and abductor digiti minimi (ADM) of the dominant hand in 8 subjects, while they either rested, isometrically contracted their thenar muscles in time with a 1 Hz metronome, or imagined doing so. Magnetic stimuli were delivered during the 'on' and 'off' phases of the real and imagined movements. F waves were also recorded from APB and ADM under rest and motor imagery conditions. RESULTS: It was found that both motor imagery and actual movement produced a muscle-specific, temporally modulated increase in corticospinal excitability during the task. The evidence of F-wave modulation was inconclusive. CONCLUSIONS: These results lend further support to the notion that actual movement and motor imagery modulate corticospinal excitability in a similar manner, primarily at the supraspinal level. SIGNIFICANCE: Motor imagery and actual movement appear to modulate motor cortex excitability with a similar degree of spatial and temporal resolution, which supports the use of motor imagery in the rehabilitation of motor function.     

Continuous theta-burst stimulation over primary somatosensory cortex modulates short-latency afferent inhibition

ObjectiveThe present study investigated the effects of continuous theta-burst stimulation (cTBS) over primary somatosensory (SI) and motor (M1) cortices on motor-evoked potentials (MEPs) and short-latency afferent inhibition (SAI).MethodsMEPs and SAI were recorded from the first dorsal interosseous (FDI) muscle of the right hand following 30 Hz cTBS over left-hemisphere SI and M1 delivered to the same participants in separate sessions. Measurements were taken before and up to 60 min following cTBS.ResultsCTBS over M1 suppressed MEPs and did not alter SAI. In contrast cTBS over SI facilitated MEPs and decreased median and digital nerve evoked SAI.ConclusionsThese findings indicate that SAI amplitude is influenced by cTBS over SI but not M1, suggesting an important role for SI in the modulation of this circuit. These data provide further evidence that cTBS over SI versus M1 has opposite effects on corticospinal excitability.SignificanceTo date, plasticity-inducing TMS protocols delivered over M1 have failed to modulate SAI, and the present research continues to support these findings. However, in young adults, cTBS over SI acts to reduce SAI and simultaneously increase corticospinal excitability. Future studies may investigate the potential to modulate SAI via targeting neural activity within SI.     

Exercise-induced strengthening of inter-digital connections in musicians

ObjectiveTo investigate whether finger exercise affects surround inhibition in professional musicians as it was previously observed in non-musicians, we performed a transcranial magnetic stimulation (TMS) study in 13 healthy right-handed professional musicians.MethodsTMS was set to be triggered by self-initiated flexion of the index finger at 3 ms after electromyography onset (self-triggered TMS). Motor evoked potentials (MEPs) of the abductor digiti minimi (ADM) were measured before and at 0, 10, 20 and 30 min after ‘single’ (little finger abduction) and ‘dual’ (both index finger flexion and little finger abduction) exercise at 0.5 Hz for 30 min.ResultsControl and self-triggered MEPs were not different between the two exercise sessions. MEP enhancements were significantly greater in self-triggered TMS than control TMS after single exercise as well as dual exercise.ConclusionThis result demonstrates that MEP enhancement in self-triggered TMS was comparable between two exercise sessions in professional musicians, a result different from that observed in healthy non-musicians. Enhanced self-triggered MEPs after isolated finger exercise suggest that inter-digital cortical connections are strengthened in musicians, presumably due to previous musical training.SignificanceInter-digital cortical connections are strengthened in musicians and are not differently modulated by different types of short-term finger exercise.     

Corticospinal excitability during motor imagery of a simple tonic finger movement in patients with writer's cramp.

Motor imagery (MI) is the mental rehearsal of a motor act without overt movement. Using transcranial magnetic stimulation (TMS), we tested the effect of MI on corticospinal excitability in patients with writer's cramp. In 10 patients with writer's cramp and 10 healthy controls, we applied focal TMS over each primary motor area and recorded motor evoked potentials (MEPs) from contralateral hand and arm muscles while participants imagined a tonic abduction of the index finger contralateral to the stimulated hemisphere. In healthy controls and patients, the MEP amplitude in the relaxed first dorsal interosseus muscle (FDI) showed a muscle-specific increase during MI; however, the increase was less pronounced in patients than in healthy controls. In addition, in patients but not in controls, the MEP amplitude also increased in hand and forearm muscles not involved in the imagined movement. This abnormal spread of facilitation was observed in the affected and unaffected upper limb. MI of simple hand movements is less efficient and less focussed in patients with writer's cramp than it is in normal subjects.     

Dynamical entrainment of corticospinal excitability during rhythmic movement observation: a Transcranial Magnetic Stimulation study

Spontaneous modulations of corticospinal excitability during action observation have been interpreted as evidence for the activation of internal motor representations equivalent to the observed action. Alternatively or complementary to this perspective, growing evidence shows that motor activity during observation of rhythmic movements can be modulated by direct visuomotor couplings and dynamical entrainment. In‐phase and anti‐phase entrainment spontaneously occur, characterized by cyclic movements proceeding simultaneously in the same (in‐phase) or opposite (anti‐phase) direction. Here we investigate corticospinal excitability during the observation of vertical oscillations of an index finger using Transcranial Magnetic Stimulation (TMS). Motor‐evoked potentials (MEPs) were recorded from participants’ flexor and extensor muscles of the right index finger, placed in either a maximal steady flexion or extension position, with stimulations delivered at maximal flexion, maximal extension or mid‐trajectory of the observed finger oscillations. Consistent with the occurrence of dynamical motor entrainment, increased and decreased MEP responses – suggesting the facilitation of stable in‐phase and anti‐phase relations but not an unstable 90° phase relation – were found in participants’ flexors. Anti‐phase motor facilitation contrasts with the activation of internal motor representation as it involves activity in the motor system opposite from activity required for the execution of the observed movement. These findings demonstrate the relevance of dynamical entrainment theories and methods for understanding spontaneous motor activity in the brain during action observation and the mechanisms underpinning coordinated movements during social interaction.     

The facilitatory effects of intermittent theta burst stimulation on corticospinal excitability are enhanced by nicotine

ObjectiveIntermittent theta burst stimulation (iTBS) is increasingly widely used as a means of facilitating corticospinal excitability in the human primary motor cortex. This form of facilitatory plasticity within the stimulated cortex may occur by induction of long term potentiation (LTP). In animal models, agonists of nicotinic acetylcholine receptors have been shown to modulate or induce LTP; we thus sought to test whether nicotine may modulate the effects of iTBS on corticospinal excitability in humans.MethodsA double-blind placebo-controlled cross-over design study was conducted with 10 healthy subjects. iTBS was delivered 60 min after subjects took either 4 mg nicotine or placebo lozenges, and motor-evoked potentials (MEPs) were then recorded for 40 min after the end of stimulation.ResultsIn the placebo arm, iTBS produced an increase in the amplitudes of MEPs which lasted for 5 min. In the nicotine arm, iTBS produced a more pronounced facilitation of MEPs that was still present at 40 min. In a control experiment, nicotine alone had no effect on MEP amplitudes when given in the absence of iTBS.ConclusionsThese data indicate that the effects of iTBS can be enhanced and prolonged by nicotine.SignificanceThese results are consistent with animal models demonstrating nicotinic modulation of facilitatory plasticity, and will be of interest to investigators seeking to enhance artificially induced changes in cortical excitability.     

Behavioural exposure and sleep do not modify corticospinal and intracortical excitability in the human motor system

ObjectiveBehavioural exposure and sleep may bidirectionally modify the excitability of cortical networks including those in the motor cortex. Here we tested whether the excitability of intracortical inhibitory and excitatory networks within the primary motor cortex exhibited changes suggestive of a time of day influence.MethodsShort-interval intracortical inhibition (SICI) and facilitation (ICF), and input–output curves (IO curves) were investigated using transcranial magnetic stimulation (TMS). Recordings were made from the resting right first dorsal interosseous (FDI) muscle in 10 healthy subjects on three occasions: 9 A.M. and 4 P.M. of the same day, and 9 A.M. of the following day.ResultsThere was no significant change in any of the measures across the three assessments.ConclusionsThese findings provide evidence that time of day does not significantly influence corticospinal and intracortical excitability in the primary motor cortex.SignificanceThese results provide no support for the hypothesis that synapses within the motor cortex undergo potentiation due to daytime use and behavioural experiences. Additionally, these findings provide evidence that measurement of motor cortical excitability is not systematically biased by time-of-day dependent variability and thus does not pose a confound in studies assessing corticospinal excitability longitudinally.     

Early non‐specific modulation of corticospinal excitability during action observation

Activity of the primary motor cortex (M1) during action observation is thought to reflect motor resonance. Here, we conducted three studies using transcranial magnetic stimulation (TMS)‐induced motor‐evoked potentials (MEPs) of the first dorsal interosseus muscle (FDI) during action observation to determine: (i) the time course of M1 corticospinal excitability during the observation of a simple finger movement; (ii) the specificity of M1 modulation in terms of type of movement and muscle; and (iii) the relationship between M1 activity and measures of empathy and autistic traits. In a first study, we administered single‐pulse TMS at 30‐ms intervals during the observation of simple finger movements. Results showed enhanced corticospinal excitability occurring between 60 and 90 ms after movement onset. In a second experiment, TMS‐induced MEPs were recorded from the FDI and abductor digiti minimi muscles while pulses were delivered 90 ms after movement onset during observation of simple finger movement and dot movement. Increased corticospinal excitability was restricted to finger movement and was present in both muscles. Finally, in an exploratory experiment, single‐pulse TMS was administered at 30, 90 and 150 ms after movement onset, and participants were asked to complete the Empathy Quotient (EQ) and the Autism Spectrum Quotient (AQ). Correlational analysis revealed a significant link between motor facilitation at 90 ms and the EQ and AQ scores. These results suggest that corticospinal excitability modulation seen at M1 during action observation is the result of a rapid and crude automatic process, which may be related to social functioning.     

Reduced plastic brain responses to repetitive transcranial magnetic stimulation in severe obstructive sleep apnea syndrome

ObjectivesAbnormalities in cortical excitability have been proposed to underlie the pathophysiology of various neurocognitive manifestations of obstructive sleep apnea syndrome (OSAS). Transcranial magnetic stimulation (TMS) provides a noninvasive method for study and modulation of cortical excitability in the human brain, and repetitive TMS (rTMS) has been proven useful for neurophysiologic investigation in various neurologic conditions. We aimed to investigate cortical excitability in patients with OSAS during wakefulness and to determine if rTMS would change the abnormal excitability patterns.MethodsMeasures of motor cortical and corticospinal excitability (resting motor threshold [RMT], motor-evoked potential [MEP] amplitude, and cortical silent period [CSP]) were taken before and after a session of 10-Hz rTMS applied to the motor cortex in 13 individuals with untreated severe OSAS (apnea–hypopnea index [AHI] > 30) and 12 age- and sex-matched healthy controls (HC).ResultsOSAS subjects had a significantly higher RMT (P < .003) and a longer CSP duration (P < .002) compared to HC. No difference was observed between MEP values of OSAS subjects and HC (P > .05). In response to rTMS, the HC group had a significant increase in CSP and MEP values from baseline, which were absent in OSAS subjects.ConclusionsIndividuals with OSAS demonstrated increased motor cortex inhibition, which did not respond to 10-Hz rTMS. As rTMS-induced changes in MEP and CSP involve a separate neurotransmitter system (N-methyl-d-aspartate [NMDA] and gamma-aminobutyric acid [GABA], respectively), these findings suggest a widespread alteration in cortical neurophysiology in severe OSAS subjects that requires clarification with further exploration.     

Motor imagery of foot dorsiflexion and gait: effects on corticospinal excitability

OBJECTIVE: We examined how corticospinal excitability was affected by motor imagery of foot dorsiflexion and motor imagery of gait. METHODS: Transcranial magnetic stimulation was applied over the primary motor cortex of 16 young healthy subjects while they performed imaginary foot dorsiflexions (Experiment I) and imaginary walking (Experiment II). Motor-evoked potentials (MEPs) were recorded from the tibialis anterior (TA) and first dorsal interosseus (FDI). MEPs recorded during motor imagery were compared to those recorded during a matched visual imagery task. RESULTS: Imagined foot dorsiflexions increased MEP areas in both TA and FDI. The increase in TA was stronger than in FDI. Overall, imagined walking did not change MEP areas. However, subjects with larger increases in TA during imagined foot dorsiflexion also showed larger increases in TA during imagined walking. CONCLUSIONS: Imagined foot dorsiflexions increase corticospinal excitability in both a task-related muscle (TA) and a task-unrelated muscle (FDI), with larger increases in the task-related muscle. Imagined gait only increases corticospinal excitability in those subjects with the largest increments during imagined foot dorsiflexion. SIGNIFICANCE: Imagery of a simple lower extremity movement evokes increases in corticospinal excitability. Furthermore, corticospinal effects of a simple motor imagery task can predict corticospinal effects of a more complex motor imagery task involving the same muscle.     

Modulation of corticomotor excitability after maximal or sustainable-rate repetitive finger movement is impaired in Parkinson’s disease and is reversed by levodopa

ObjectivesIn healthy subjects, fatiguing exercises induce a period of post-exercise corticomotor depression (PECD) that is absent in Parkinson’s disease (PD). Our objective is to determine the time-course of corticomotor excitability changes following a 10-s repetitive index finger flexion–extension task performed at maximal voluntary rate (MVR) and a slower sustainable rate (MSR) in PD patients OFF and ON levodopa.MethodsIn 11 PD patients and 10 healthy age-matched controls, motor evoked potentials (MEPs) were recorded from the extensor indicis proprius (EIP) and first dorsal interosseous (FDI) muscles of the dominant arm immediately after the two tasks and at 2-min intervals for 10 min.ResultsIn the OFF condition the PECD was absent in the two test muscles after both the MVR and MSR tasks. In the ON condition finger movement kinematics improved and a period of PECD comparable to that in controls was present after both tasks.ConclusionThe absence of PECD in PD subjects off medication indicates a persisting increase in corticomotor excitability after non-fatiguing repetitive finger movement that is reversed by levodopa.SignificanceDopamine depletion is associated with impaired modulation of corticomotor excitability after non-fatiguing repetitive finger movement.     

Acute effects of lithium on excitability of human motor cortex

ObjectiveLithium has been widely used to treat bipolar affective disorder for over 60 years. Still, its acute effects in human cerebral cortex are poorly understood. This study aimed at investigating the acute effects of lithium on motor cortex excitability as measured by transcranial magnetic stimulation (TMS).MethodsTen healthy young adults participated in a double-blind placebo-controlled randomized crossover study with four sessions, where a single oral dose of lithium carbonate (450 mg, 900 mg, or 1350 mg) or placebo was tested. Focal TMS of the hand area of left motor cortex was used to test resting and active motor thresholds, motor evoked potential input–output curve (MEP IO-curve), slope of the MEP IO-curve and paired-pulse measures of intracortical inhibition and facilitation before, and two and four hours after drug administration.ResultsTwo hours post drug administration, 450 mg of lithium carbonate increased the slope of the MEP IO-curve while 1350 mg tended to decrease it. Lithium had no effect on motor thresholds, or intracortical inhibition or facilitation.ConclusionsThe acute effects of lithium on MEP IO-curve, a marker of corticospinal excitability, are consistent with an inverted U-shaped dose–response relationship.SignificanceFindings are important for our understanding of the therapeutic and toxic effects of lithium on the human central nervous system.     

Effects of water immersion on short- and long-latency afferent inhibition,short-interval intracortical inhibition,and intracortical facilitation

ObjectiveThe aim of the present study was to investigate the effect of water immersion (WI) on short- and long-latency afferent inhibition (SAI and LAI), short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF).MethodsMotor evoked potentials (MEPs) were measured from the first dorsal interosseous (FDI) muscle of fifteen healthy males before, during, and after a 15-min WI at 30 °C up to the axilla. Both SAI and LAI were evaluated by measuring MEPs in response to transcranial magnetic stimulation (TMS) of the left motor cortex following electrical stimulation of the right median nerve (fixed at about three times the sensory threshold) at interstimulus intervals (ISIs) of 20 ms to assess SAI and 200 ms to assess LAI. The paired-pulse TMS paradigm was used to measure SICI and ICF.ResultsBoth SAI and LAI were reduced during WI, while SICI and ICF were not significantly different before, during, and after WI.ConclusionsWI decreased SAI and LAI by modulating the processing of afferent inputs.SignificanceChanges in somatosensory processing and sensorimotor integration may contribute to the therapeutic benefits of WI for chronic pain or movement disorders.     

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.     

Bidirectional modulation of sensory cortical excitability by quadripulse transcranial magnetic stimulation (QPS) in humans

ObjectiveQuadripulse transcranial magnetic stimulation (QPS) is a newly designed patterned repetitive transcranial magnetic stimulation (TMS). Previous studies of QPS showed bidirectional effects on the primary motor cortex (M1), which depended on its inter-stimulus interval (ISI): motor evoked potentials (MEPs) were potentiated at short ISIs and depressed at long ISIs (homotopic effects). These physiological characters were compatible with synaptic plasticity. In this research, we studied effects of QPS on the primary sensory cortex (S1).MethodsOne burst consisted of four monophasic TMS pulses at an intensity of 90% active motor threshold. The ISI of four pulses was set at 5 ms (QPS-5) or at 50 ms (QPS-50). Same bursts were given every 5 s for 30 min. QPS-5 and QPS-50 were performed over three areas (M1, S1 and dorsal premotor cortex (dPMC)). One sham stimulation session was also performed. Excitability changes of S1 were evaluated by timeline of somatosensory evoked potentials (SEPs).ResultsQPS-5 over M1 or dPMC enhanced the P25–N33 component of SEP, and QPS-50 over M1 depressed it. By contrast, QPSs over S1 had no effects on SEPs.ConclusionsQPSs over motor cortices modulated the S1 cortical excitability (heterotopic effects). Mutual connections between dPMC or M1 and S1 might be responsible for these modulations.SignificanceQPSs induced heterotopic LTP or LTD-like cortical excitability changes.     

Corticospinal activation confounds cerebellar effects of posterior fossa stimuli

ObjectiveTo investigate the efficacy of magnetic stimulation over the posterior fossa (PF) as a non-invasive assessment of cerebellar function in man.MethodsWe replicated a previously reported conditioning-test paradigm in 11 healthy subjects. Transcranial magnetic stimulation (TMS) at varying intensities was applied to the PF and motor cortex with a 3, 5 or 7 ms interstimulus interval (ISI), chosen randomly for each trial. Surface electromyogram (EMG) activity was recorded from two intrinsic hand muscles and two forearm muscles. Responses were averaged and rectified, and MEP amplitudes were compared to assess whether suppression of the motor output occurred as a result of the PF conditioning pulse.ResultsCortical MEPs were suppressed following conditioning-test ISIs of 5 or 7 ms. No suppression occurred with an ISI of 3 ms. PF stimuli alone also produced EMG responses, suggesting direct activation of the corticospinal tract (CST).ConclusionsCST collaterals are known to contact cortical inhibitory interneurones; antidromic CST activation could therefore contribute to the observed suppression of cortical MEPs.SignificancePF stimulation probably activates multiple pathways; even at low intensities it should not be regarded as a selective assessment of cerebellar function unless stringent controls can confirm the absence of confounding activity in other pathways.     

Effects of lacosamide and carbamazepine on human motor cortex excitability: A double-blind,placebo-controlled transcranial magnetic stimulation study

PurposeLacosamide (LCM) and carbamazepine (CBZ) are antiepileptic drugs both acting on neuronal voltage-gated sodium channels. Patch-clamp studies demonstrated significant differences in how LCM and CBZ affect neuronal membrane excitability. Despite valuable information patch-clamp studies provide, they also comprise some constraints. For example, little is known about effects of LCM on intracortical synaptic excitability. In contrast, transcranial magnetic stimulation (TMS) can describe drug-induced changes at the system level of the human cerebral cortex.MethodsThe present study was designed to explore dose-depended effects of LCM and effects of CBZ on motor cortex excitability with TMS in a randomized, double-blind, placebo-controlled crossover trial in healthy human subjects. Subjects received 600 mg CBZ, 200 mg LCM, 400 mg LCM or placebo preceding TMS measurements.ResultsCompared to placebo, TMS motor thresholds were significantly increased after carbamazepine and lacosamide, with a trend for a dose dependent effect of lacosamide. Both, carbamazepine and lacosamide did not affect TMS parameters of intracortical synaptic excitability.ConclusionsTMS measurements suggest that lacosamide and carbamazepine predominantly act on neuronal membrane excitability.     

Influence of task-related ipsilateral hand movement on motor cortex excitability.

OBJECTIVE: The time course of the right motor cortex excitability in relation to a task-related voluntary right thumb twitch was studied using sub-threshold transcranial magnetic stimulation (TMS) to the right motor cortex. METHODS: Motor excitability was studied in 8 adult subjects who made a brief right thumb twitch to the predictable omission of every fifth tone in a series of tones 2.5 s apart. This paradigm avoided an overt sensory cue, while allowing experimental control of TMS timing relative to both movement and the cue to move. Motor excitability was characterized by several measures of motor evoked potentials (MEPs) recorded from the left thenar eminence in response to TMS over the right scalp with a 9 cm coil: probability of eliciting MEPs, incidence of MEPs and amplitude of MEPs. RESULTS: All subjects showed suppression of motor excitability immediately following a voluntary right thumb twitch (ipsilateral response), and up to 1 s after it. However, two distinctly different effects on motor excitability were observed before the response: two subjects showed excitation, beginning about 500 ms before response until 300 ms after it, followed by the post-movement suppression; 6 subjects displayed pre-movement suppression, beginning about 600 ms before the response and persisting for the duration. CONCLUSIONS: The net effect of an ipsilateral response on motor cortex can be either inhibitory or excitatory, changing with time relative to the response. These findings are compatible with two separate processes, inhibitory and excitatory, which interact to determine motor excitability ipsilateral to the responding hand.     

Corticospinal excitability is specifically modulated by motor imagery: a magnetic stimulation study

Transcranial magnetic stimulation (TMS) was used to investigate whether the excitability of the corticospinal system is selectively affected by motor imagery. To this purpose, we performed two experiments. In the first one we recorded motor evoked potentials from right hand and arm muscles during mental simulation of flexion/extension movements of both distal and proximal joints. In the second experiment we applied magnetic stimulation to the right and the left motor cortex of subjects while they were imagining opening or closing their right or their left hand. Motor evoked potentials (MEPs) were recorded from a hand muscle contralateral to the stimulated cortex. The results demonstrated that the excitability pattern during motor imagery dynamically mimics that occurring during movement execution. In addition, while magnetic stimulation of the left motor cortex revealed increased corticospinal excitability when subjects imagined ipsilateral as well as contralateral hand movements, the stimulation of the right motor cortex revealed a facilitatory effect induced by imagery of contralateral hand movements only. In conclusion, motor imagery is a high level process, which, however, manifests itself in the activation of those same cortical circuits that are normally involved in movement execution.