Ipsilateral Motor Evoked Potentials Research Articles

IS 21. tDCS effects on ipsilateral motor control: Robbing Peter to pay Paul?

In humans both cerebral hemispheres play an essential role in controlling the upper limb. However, the neurophysiological effects of noninvasive brain stimulation (NBS) of primary motor cortex (M1) have been investigated primarily from motor evoked potentials (MEPs) recorded in contralateral distal hand muscles alone. This may present an incomplete picture. For example, the idea of using NBS after stroke to suppress excitability of the contralesional hemisphere (cM1), or “re-balance” excitability, may be too simplistic, and may lead to undesirable outcomes for some patients. The purpose of this presentation is to show how NBS alters excitability of projections to contralateral and ipsilateral motoneurons (MNs) controlling the proximal upper limb, and in particular, the cortico-reticulo-propriospinal pathway (CRPP). This has implications for the use of NBS as adjuvants to upper limb rehabilitation after stroke. Experiments involved TMS to elicit MEPs in distal and proximal upper limb muscles, M1 cathodal t-DCS (c-tDCS), and sham control. Effects of M1 c-tDCS and sham were compared in healthy participants and patients with upper limb weakness after sub-cortical stroke. M1 c-tDCS modulated ipsilateral and contralateral MEPs in proximal upper limb muscles in a task-dependent manner, at least in part via the CRPP. In stroke patients, cM1 c-tDCS modified selectivity of paretic biceps brachii (BB). Mildly impaired patients showed improved selectivity, whereas moderate-severely impaired patients showed worsened selectivity, and modulation of ipsilateral BB MEPs after cM1 c-tDCS correlated with clinical, neurophysiological and neuroimaging measures. In patients with upper limb weakness after stroke, the role of the cM1 for upper limb control can vary between individuals, depending on extent of damage to the corticospinal pathway. We present a neurophysiological model that explains how the cM1 may gain control of the paretic arm via the CRPP and why suppression of cM1 with NBS can be beneficial for some patients, but detrimental for others. Finally we describe ways to predict whether cM1 suppression may be indicated or contraindicated for an individual patient after stroke.

P 20. Corticobulbar projection and intracortical circuits in the facial motor cortex innervating lower facial muscles in healthy humans

Despite the importance of facial muscles in social life and their frequent and disabling involvement in stroke or movement disorders, conclusive data concerning bilaterality, symmetry and hemisphere dominance in the control of lower facial muscles are not available yet. In addition, the role of intracortical circuits in the modulation of resting and active facial muscles is still debated. (1) To reassess controversy about the symmetry of cortical output to lower facial muscles, arising from the left and right hemisphere. (2) To clarify whether intracortical excitability modulates the ipsi-and contralateral corticobulbar projection, depending on the muscle state and on the stimulated hemisphere. In nine healthy subjects, left and right hemisphere were tested using single and paired pulse TMS methods to probe hot spot, motor evoked potentials (MEPs) latencies and amplitudes as well as short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) in the ipsi-and contralateral depressor anguli oris (DAO) muscles both at rest and during activation at 10% of maximal voluntary contraction. Resting and active motor thresholds (RMT and AMT, respectively) were determined following TMS of both hemispheres. Test and conditioning stimuli were 120% and 70% of motor threshold, respectively, and interstimulus intervals (ISIs) were 2 ms for SICI and 10 ms for ICF. Student’s paired t -test, ANOVA and post hoc t -test have been performed for single paired pulse TMS on the left and right hemispheres. Resting and active MEPs were evoked bilaterally in the relaxed and active DAO following left and right hemisphere stimulations. Student’s paired t test showed no differences between left and right hemisphere as for hot spot, RMT, AMT, MEP amplitudes and onset latencies. In both muscle states, the contralateral MEP had a significantly shorter latency ( p = 0.005) and larger amplitude ( p = 0.02) than the ipsilateral MEP, with no significant differences according to the side of the stimulated hemisphere. Repeated measures ANOVA showed a significant effect of the paired pulse protocol ( p < 0.001) at rest and during voluntary contraction, regardless the side of the stimulated hemisphere. Post hoc analysis showed, in both muscle states and for both hemispheres, significant SICI ( p < 0.01) at 2 ms and ICF ( p < 0.05) at 10 ms. SICI effects were smaller in the ipsilateral respect to the contralateral DAO at rest ( p = 0.003) and during voluntary contraction (p = 0.032); ICF was reduced ipsilaterally only in the resting state ( p = 0.037). The amount of SICI and ICF was significantly smaller during voluntary contraction in the ipsi-and contralateral DAO ( p = 0.026 and p = 0.005, respectively). TMS applied to the left and right facial motor cortex evokes, in the DAO muscle, bilateral and asymmetric MEPs, with contralateral predominance regardless of the stimulated hemisphere. SICI and ICF are present bilaterally at rest and during voluntary activation and show similar extent following stimulation of both hemispheres. In conclusion, corticobulbar responses as well as intracortical excitability of the left and right facial motor cortex innervating lower facial muscles show similar physiological properties. These data may provide further insight into studies of pathologies affecting the facial motor system. This research was founded by Fondazione Italiana Sclerosi Multipla (FISM 2011/R/17).

Training-induced modifications of corticospinal reactivity in severely affected stroke survivors

When permitted access to the appropriate forms of rehabilitation, many severely affected stroke survivors demonstrate a capacity for upper limb functional recovery well in excess of that formerly considered possible. Yet, the mechanisms through which improvements in arm function occur in such profoundly impaired individuals remain poorly understood. An exploratory study was undertaken to investigate the capacity for brain plasticity and functional adaptation, in response to 12-h training of reaching using the SMART Arm device, in a group of severely affected stroke survivors with chronic upper limb paresis. Twenty-eight stroke survivors were enroled. Eleven healthy adults provided normative data. To assess the integrity of ipsilateral and contralateral corticospinal pathways, transcranial magnetic stimulation was applied to evoke responses in triceps brachii during an elbow extension task. When present, contralateral motor-evoked potentials (MEPs) were delayed and reduced in amplitude compared to those obtained in healthy adults. Following training, contralateral responses were more prevalent and their average onset latency was reduced. There were no reliable changes in ipsilateral MEPs. Stroke survivors who exhibited contralateral MEPs prior to training achieved higher levels of arm function and exhibited greater improvements in performance than those who did not initially exhibit contralateral responses. Furthermore, decreases in the onset latency of contralateral MEPs were positively related to improvements in arm function. Our findings demonstrate that when severely impaired stroke survivors are provided with an appropriate rehabilitation modality, modifications of corticospinal reactivity occur in association with sustained improvements in upper limb function.

Differences in Descending Control of External Oblique and Latissimus Dorsi Muscles in Humans: A Preliminary Study

Descending bilateral control of external oblique (EO) and latissimus dorsi (LD) was investigated using transcranial magnetic stimulation. Contralateral (CL) motor evoked potential (MEP) thresholds were lower and latencies were shorter than for ipsilateral (IL) MEPs. Hotspots for EO were symmetrical; this was not the case for LD. The volumes of drive to the left and right muscles were not different. The laterality index was not different between the left and right muscles. The average index for the EO muscles was closer to zero than that for LD, suggesting a stronger IL drive to EO. The symmetry of drive to each muscle did not differ; however, the symmetry of drive varies within a subject for different muscles and between subjects for the same muscle. The findings may be useful in understanding a number of clinical conditions relating to the trunk and also for predicting the outcome of rehabilitative strategies.

Motor Evoked Potentials of Trunk Muscles in Traumatic Brain Injury Patients

ObjectiveTo evaluate the motor innervation of trunk muscles in traumatic brain injury patients.MethodTwenty patients (12 men and 8 women) with traumatic brain injury were enrolled in this study. Their mean age was 41 years. Motor evoked potentials (MEPs) were performed on the motor cortex. Electromyographic activities were recorded from the bilateral rectus abdominis muscles, the external oblique abdominal muscles, and the 4th and 9th thoracic erector spinae muscles. The onset latency and amplitude of contralateral and ipsilateral MEPs were measured. All patients were assessed by the Korean version of the Berg Balance Scale (K-BBS) to investigate the relationship between the frequency of MEPs in trunk muscles and gait ability.ResultsThe mean frequency of ipsilateral MEPs was 23.8% with more damaged hemisphere stimulation, while the contralateral MEPs showed a mean frequency of 47.5% with more damaged hemisphere stimulation in traumatic brain injury patients. The latencies and amplitudes of MEPs obtained from the more damaged hemisphere were not significantly different from those of the less damaged hemisphere. There was no correlation between the manifestation of MEPs in trunk muscles and gait ability.ConclusionThe ipsilateral and contralateral corticospinal pathways to trunk muscles are less likely to be activated in traumatic brain injury patients because of direct injury of the descending corticospinal motor tract or decreased excitability of the corticospinal tract from prefrontal contusion.

Task-Dependent Modulation of Inputs to Proximal Upper Limb Following Transcranial Direct Current Stimulation of Primary Motor Cortex

Cathodal transcranial DC stimulation (c-tDCS) suppresses excitability of primary motor cortex (M1) controlling contralateral hand muscles. This study assessed whether c-tDCS would have similar effects on ipsi- and contralateral M1 projections to a proximal upper limb muscle. Transcranial magnetic stimulation (TMS) of left M1 was used to elicit motor evoked potentials (MEPs) in the left and right infraspinatus (INF) muscle immediately before and after c-tDCS of left M1, and at 20 and 40 min, post-c-tDCS. TMS was delivered as participants preactivated each INF in isolation (left, right) or both INF together (bilateral). After c-tDCS, ipsilateral MEPs in left INF and contralateral MEPs in right INF were suppressed in the left task but not in the bilateral or right tasks, indicative of task-dependent modulation. Ipsilateral silent period duration in the left INF was reduced after c-tDCS, indicative of altered transcallosal inhibition. These findings may have implications for the use of tDCS as an adjunct to therapy for the proximal upper limb after stroke.

Clinical and physiological effects of transcranial electrical stimulation position on motor evoked potentials in scoliosis surgery

BackgroundDuring intraoperative monitoring for scoliosis surgery, we have previously elicited ipsilateral and contralateral motor evoked potentials (MEP) with cross scalp stimulation. Ipsilateral MEPs, which may have comprised summation of early ipsilaterally conducted components and transcallosally or deep white matter stimulated components, can show larger amplitudes than those derived purely from contralateral motor cortex stimulation. We tested this hypothesis using two stimulating positions. We compared intraoperative MEPs in 14 neurologically normal subjects undergoing scoliosis surgery using total intravenous anesthetic regimens.MethodsTrancranial electrical stimulation was applied with both cross scalp (C3C4 or C4C3) or midline (C3Cz or C4Cz) positions. The latter was assumed to be more focal and result in little transcallosal/deep white matter stimulation. A train of 5 square wave stimuli 0.5 ms in duration at up to 200 mA was delivered with 4 ms (250 Hz) interstimulus intervals. Averaged supramaximal MEPs were obtained from the tibialis anterior bilaterally.ResultsThe cross scalp stimulating position resulted in supramaximal MEPs that were of significantly higher amplitude, shorter latency and required lower stimulating intensity to elicit overall (Wilcoxon Signed Rank test, p < 0.05 for all), as compared to the midline stimulating position. However, no significant differences were found for all 3 parameters comparing ipsilaterally and contralaterally recorded MEPs (p > 0.05 for all), seen for both stimulating positions individually.ConclusionsOur findings suggest that cross scalp stimulation resulted in MEPs obtained ipsilaterally and contralaterally which may be contributed to by summation of ipsilateral and simultaneous transcallosally or deep white matter conducted stimulation of the opposite motor cortex. Use of this stimulating position is advocated to elicit MEPs under operative circumstances where anesthetic agents may cause suppression of cortical and spinal excitability. Although less focal in nature, cross scalp stimulation would be most suitable for infratentorial or spinal surgery, in contrast to supratentorial neurosurgical procedures.

Lateralization of forelimb motor evoked potentials by transcranial magnetic stimulation in rats.

To approximate methods for human transcranial magnetic stimulation (TMS) in rats, we tested whether lateralized cortical stimulation resulting in selective activation of one forelimb contralateral to the site of stimulation could be achieved by TMS in the rat. Motor evoked potentials (MEP) were recorded from the brachioradialis muscle bilaterally in adult male anesthetized rats (n=13). A figure-of-eight TMS coil was positioned lateral to midline. TMS intensity was increased stepwise from subthreshold intensities to maximal machine output in order to generate input-output curves and to determine the motor threshold (MT) for brachioradialis activation. In 100% of the animals, selective activation of the contralateral brachioradialis, in the absence of ipsilateral brachioradialis activation was achieved, and the ipsilateral brachioradialis was activated only at TMS intensities exceeding contralateral forelimb MT. With increasing TMS intensity, the amplitudes of both the ipsilateral and contralateral signals increased in proportion to TMS strength. However, the input-output curves for the contralateral and ipsilateral brachioradialis were significantly different (p<0.001) such that amplitude of the ipsilateral MEP was reliably lower than the contralateral signal. We demonstrate that lateralized TMS leading to asymmetric brachioradialis activation is feasible with conventional TMS equipment in anesthetized rats. These data show that TMS can be used to assess the unilateral excitability of the forelimb descending motor pathway in the rat, and suggest that rat TMS protocols analogous to human TMS may be applied in future translational research.

Ipsilateral responses of motor evoked potential correlated with the motor functional outcomes after cortical resection

The aim of this study was to evaluate if ipsilateral motor evoked potential (MEP) elicited by transcranial magnetic stimulation (TMS) could provide neurosurgeons preoperatively with useful information regarding surgical procedure for patients with severe cerebral hemiatrophy or unilateral malformation. Thirteen epilepsy patients with severe cerebral hemiatrophy or unilateral malformation were studied before operation using MEPs recorded on bilateral abductor pollicis brevis (APBs) muscles, elicited by transcranial magnetic stimulation of the motor cortex. Ten subjects served as controls. Results: (1) no ipsilateral MEP responses were recorded in all the 10 healthy subjects; (2) in the 13 patients, the results of MEPs could be divided into four types. Type A: in 3 patients bilateral MEPs were recorded when unaffected hemisphere was stimulated, while no responses were elicited when the affected hemisphere was stimulated. Type B: in another 3 patients, the MEPs were elicited from bilateral APB muscles when the unaffected hemisphere was stimulated, and the contralateral MEP was also elicited when the affected hemisphere was stimulated. Type C: in two patients contralateral MEP was elicited when the unaffected hemisphere was stimulated, while no MEP was induced in APB muscles of either side following the affected hemisphere stimulation. Type D: in the remaining 5 patients, contralateral magnetic MEPs were elicited either when the affected or the unaffected hemisphere was stimulated. Patients of type A, B and C received hemispherectomy showed no significant permanent motor functional deficit. Among the total 8 patients, 7 patients got seizure free after the operation. Patients of type D showed minor muscle strength decrease after localized cortical resection. Three out of 5 patients of type D got seizure free after the operation. Ipsilateral MEP response might be useful for neurosurgeons to plan appropriate surgical procedure which helps avoid post-operative motor deficits.

Ipsilateral Motor Cortical Responses To Tms During Shortening And Lengthening Of The Wrist Flexors

Previous studies showed that responses to TMS of the primary motor cortex (M1) are different during lengthening compared with shortening muscle contractions performed with the contralateral limb. There is also strong evidence to suggest that the ipsilateral M1 is activated during a unilateral task. PURPOSE: To determine if ipsilateral M1 responses to TMS vary according to the type of muscle contraction. METHODS: In Experiment 1, healthy volunteers (n = 12, mean age 22) performed lengthening and shortening isokinetic contractions at 20 deg/s with the left wrist flexors at the same absolute force (20, 40, 60, 80, and 100% of concentric maximal voluntary force, MVC). During the contractions, single-pulse TMS was delivered to the left (ipsilateral) M1 at 120% of resting motor threshold. In Experiment 2, volunteers (n = 10, mean age 24) performed shortening and lengthening contraction at 90% MVC and received paired-pulse TMS in the ipsilateral M1 to evoke intracortical inhibition (ICI, 2 ms) and facilitation (ICF, 10 ms). In both experiments motor evoked potentials (MEPs) were recorded from the resting right flexor carpi radialis (FCR) using surface electrodes. RESULTS: Although the peak-to-peak amplitude of the ipsilateral MEPs was similar (p = 0.132) during shortening (215% SE ± 17.4) and lengthening (238% ± 18), there was a significant contraction type (shortening, lengthening) by contraction intensity (20, 40, 60, 80,100%) interaction (P = 0.025), indicating that M1 output increased more (P < 0.05) with contraction intensity during lengthening than shortening at 80 and 100% MVC. In addition, the contraction type by stimulation condition (Test, ICI, ICF) interaction (F = 6.4, P = 0.008) suggested that ICI diminished more during lengthening (92% ± 11.2) than shortening (69% ±7.2) and rest (53% ± 7.3) and ICF diminished during shortening (116% ±11.2) but increased during lengthening (158% ± 15.4) compared with rest (135% ± 7.3) (all P < 0.05). CONCLUSION: These data are consistent with the hypothesis that ipsilateral responses to TMS vary according to the type of muscle contraction. This specificity is characterized by a generally higher ipsilateral M1 output during lengthening than shortening, including a shift to diminished inhibition and heightened facilitation. Supported in part by NIH R01AG024161

The ipsilateral cortico‐spinal tract is activated after hemiparetic stroke

The presence of a projection from the primary motor cortex to the ipsilateral muscles has been established in human, but whether this pathway contributes to functional recovery after stroke is unclear. We investigated whether the ipsilateral tract is activated in hemiparetic stroke. Motor-evoked potentials (MEPs) were simultaneously recorded from the bilateral trapezius or abductor digiti minimi (ADM) muscles after magnetic stimulation to the motor cortex in 40 acute stroke patients. At rest, ipsilateral trapezius MEPs were recordable in none of the 24 normal controls, and in 38% of the patients after stimulation to the non-affected hemisphere (P < 0.001). With voluntary contraction, ipsilateral trapezius MEPs were elicited in 21% of the normal controls and 73% of the patients (P < 0.001). Ipsilateral ADM MEPs were rarely recordable in both controls (0%) and patients (3%). The presence of ipsilateral trapezius MEPs was associated with less severe paresis in the trapezius (P = 0.04) and deltoid (P = 0.07), but not in the more distal muscles. The ipsilateral cortico-spinal tract is acutely facilitated after stroke in the trunk or proximal muscles, but not in the hand muscles. Activation of such pathway appears to partly compensate motor dysfunction of the trunk/proximal muscles.

Ipsilateral versus contralateral cortical motor projections to a shoulder adductor in chronic hemiparetic stroke: implications for the expression of arm synergies

An increase in ipsilateral descending motor pathway activity has been reported following hemiparetic stroke. In axial muscles, increased ipsilateral cortical activity has been correlated with good recovery whereas in distal arm muscles it is correlated with poor recovery. Currently, little is known about the control of proximal upper limb muscles following stroke. This muscle group is less impaired than the distal arm muscles following stroke, yet contributes to the abnormal motor coordination patterns associated with movements of the arm which can severely impair reaching ability. This study used transcranial magnetic stimulation (TMS) to evaluate the presence and magnitude of ipsilateral and contralateral projections to the pectoralis major (PMJ) muscle in stroke survivors. A laterality index (LI) was used to investigate the relationship between ipsilateral and contralateral projections and strength, clinical impairment level, and the degree of abnormal coordination expressed in the arm. The ipsilateral and contralateral hemispheres were stimulated using 90% TMS intensity while the subject generated shoulder adduction torques in both arms. Motor evoked potentials (MEPs) were measured in the paretic and non-paretic PMJ. The secondary torque at the elbow was measured during maximal adduction as an indicator of the degree of extensor synergy. Ipsilateral MEPs were most common in stroke survivors with moderate to severe motor deficits. The LI was correlated with clinical impairment level (P = 0.05) and the degree of extension synergy expressed in the arm (P = 0.03). The LI was not correlated with strength. These results suggest that increased excitability of ipsilateral pathways projecting to the proximal upper arm may contribute to the expression of the extension synergy following stroke. These findings are discussed in relation to a possible unmasking or upregulation of oligosynaptic cortico-bulbospinal pathways following stroke.

The value of bilateral ipsilateral and contralateral motor evoked potential monitoring in scoliosis surgery

Intraoperative monitoring (IOM) of the motor pathways is a routine procedure for ensuring integrity of corticospinal tracts during scoliosis surgery. We have previously demonstrated presence of ipsilateral motor evoked potentials (MEPs) during IOM for scoliosis surgery, but its significance was uncertain. In this case series, we show concurrent ipsilateral and contralateral MEP amplitude changes obtained with cortical stimulation are of value in reducing false positive observations during IOM. The use of this easily recordable MEP is thus advocated as a diagnostic adjunct to contralateral MEPs for scoliosis and spinal surgery.

Interhemispheric and ipsilateral connections in Parkinson's disease: Relation to mirror movements

Mirror movements (MM) occur in early, asymmetric Parkinson's disease (PD). To examine the pathophysiology of MM in PD, we studied 13 PD patients with MM (PD-MM), 7 PD patients without MM (PD-NM), and 14 normal subjects. Cross-correlogram did not detect common synaptic input to motoneuron pools innervating homologous hand muscles in PD-MM patients. Transcranial magnetic stimulation studies showed no significant difference in ipsilateral motor-evoked potentials between PD-MM patients and normal subjects. The MM side of PD-MM patients showed a slower increase in ipsilateral silent period area with higher level of muscle contraction than the non-MM side and normal subjects. There was less interhemispheric inhibition (IHI) at long interstimulus intervals of 20 to 50 ms in PD-MM than PD-NM. IHI reduced short interval intracortical inhibition in normal subjects and PD-NM, but not in PD-MM. IHI significantly increased intracortical facilitation in PD-MM and PD-NM patients, but not in normal subjects. Our results suggest that MM in PD is due to activation of the contralateral motor cortex. PD-MM patients had reduced transcallosal inhibitory effects on cortical output neurons and on intracortical inhibitory circuits compared to PD-NM patients and controls. These deficits in transcallosal inhibition may contribute to MM in PD patients.

The silent period of the thenar muscles to contralateral and ipsilateral deep brain stimulation

Objective We aimed at characterizing the silent period induced in hand muscles by subcortical stimulation through electrodes implanted on the subthalamic nucleus for deep brain stimulation (STN-DBS). Methods In 10 patients with Parkinson’s disease, we analyzed the inhibitory effects induced in the contralateral and ipsilateral thenar muscles by STN-DBS of varying stimulus intensity and strength of muscle contraction. Results Both, the contralateral silent period (CSP) and the ipsilateral silent period (ISP) were induced by stimuli at an intensity subthreshold for eliciting a contralateral motor evoked potential (MEP) and were composed of two phases. With a stimulus intensity of 120% of active threshold and a strength of 20%, the first CSP had a mean onset latency of 38.0 ± 2.9 ms and a mean duration of 37.7 ms ± 2.8 ms, and the second CSP had a mean onset latency of 90.6 ± 18.5 ms and a mean duration of 53.4 ± 6.3 ms. The first ISP had a mean onset latency of 34.9 ± 4.3 ms and a mean duration of 12.5 ± 3.4 ms, and the second ISP had a mean onset latency of 76.3 ± 10.1 ms and a mean duration of 23.1 ± 9.0 ms. The duration of both phases of the CSP increased with increasing the stimulus intensity and the burst separating the two phases of the CSP increased in size with increasing the strength of muscle contraction or the stimulus intensity. No ipsilateral MEP was observed in any patient at any strength or stimulus intensity. Conclusion Our results indicate that the silent period induced by STN-DBS has specific physiological mechanisms that differ from those of the silent period induced by cortical transcranial magnetic stimulation. Significance The induction of ISP by stimulation of the motor tract at a point caudal to the corpus callosum indicates that non-callosal pathways are capable of generating ISP.

Intraoperative monitoring study of ipsilateral motor evoked potentials in scoliosis surgery

Ipsilateral motor evoked potentials (MEPs) in spinal cord surgery intraoperative monitoring is not well studied. We show that ipsilateral MEPs have significantly larger amplitudes and were elicited with lower stimulation intensities than contralateral MEPs. The possible underlying mechanisms are discussed based on current knowledge of corticospinal pathways. Ipsilateral MEPs may provide additional information on the integrity of descending motor tracts during spinal surgery monitoring.

Lateralized asymmetry of facial motor evoked potentials

There is considerable evidence that emotions are expressed more intensely on the left side of the face. This asymmetry could reflect a right hemisphere advantage in processing emotional material or an asymmetry in corticobulbar motor systems. Transcranial magnetic stimulation (TMS) was used to test for lateralized asymmetry in the cortical control of muscles of facial expression in the lower face. We administered TMS to the motor cortex of 50 subjects during contraction of bilateral orbicularis oris muscles. We analyzed motor evoked potentials (MEPs) with a repeated measures analysis of variance (ANOVA) using hemisphere stimulated and orbicularis oris side recorded as within subject factors. TMS elicited contralateral MEPs in 42 of 50 subjects. Forty of these 42 subjects showed bilateral MEPs. The ANOVA showed a significant main effect of face side, such that MEPs elicited in left face were larger than in right face (p < 0.0001). The analysis also showed a significant interaction between the hemisphere stimulated and face side, such that the difference between contralateral and ipsilateral MEPs with right brain TMS was greater than with left brain TMS (p < 0.0001). The results provide evidence of lateralized asymmetry of corticobulbar projections to muscles of facial expression in the lower face.

Mirror movements and ipsilateral motor evoked potentials in ALS

It can be difficult to detect upper motor neuron (UMN) involvement in early amyotrophic lateral sclerosis (ALS). The present study tested the usefulness of mirror movements (MMs), i.e., contralateral coactivation of hand muscles, as a UMN sign in ALS. Thirty-seven patients with possible, probable or definite ALS and 19 patients with suspected ALS without clinical signs of UMN involvement, 21 disease controls and 15 healthy volunteers were included. MMs were studied clinically, electromyographically and by transcranial magnetic stimulation (TMS), looking for ipsilateral motor evoked potentials (IMEPs). MMs were observed clinically in 15 ALS patients (27%) and electromyographically in 28 (50%). IMEPs in the abductor pollicis brevis muscle following TMS were recorded in 34 (61%) of all ALS patients and in nine (47%) out of 19 patients with suspected ALS, but not in healthy controls. Central motor conduction times were prolonged in 15 ALS patients (27%). It is concluded that MMs, especially if studied by EMG and TMS (IMEPs) can essentially improve detection of UMN involvement, especially in early stages of ALS.

Bilateral responses of upper limb muscles to transcranial magnetic stimulation in human subjects.

Anatomical and behavioural work on primates has shown bilateral innervation of axial and proximal limb muscles, and contralateral control of distal limb muscles. The following study examined if a clear boundary exists between the distal and proximal upper limb muscles that are controlled contralaterally or bilaterally. The right motor cortical area representing the upper limb was stimulated, while surface EMG was recorded bilaterally from various upper limb muscles during rest and phasic voluntary contractions. Peak-to-peak amplitude of motor evoked potential (MEP) was measured for each muscle on both sides. The ratio R = (ipsilateral MEP: contralateral MEP) was calculated for seven pairs of muscles. For each of the seven pairs, R was less than 1.0, implying that for each muscle and subject, the contralateral control is stronger. The boundary where R changed from almost zero to a clearly measurable magnitude depended on the subject. Ipsilateral MEPs from trapezius and pectoralis could be recorded with a small background contraction from almost all subjects; on the other hand, in deltoid and biceps brachii, ipsilateral MEPs were observed only with bimanual phasic contractions. The forearm and hand muscles, in general, did not show any ipsilateral MEPs. Major differences between subjects lay in the presence or the absence of ipsilateral MEPs in biceps brachii and deltoid, without defining a sharp boundary between proximal and distal muscles.

Ipsilateral motor pathway confirmed by combined brain mapping of a patient with hemiparetic stroke: A case report

Kim Y-H, Jang SH, Byun WM, Han BS, Lee K-H, Ahn SH. Ipsilateral motor pathway confirmed by combined brain mapping of a patient with hemiparetic stroke: a case report. Arch Phys Med Rehabil 2004;85:1351–3. This study investigated the motor control pathway using both functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) in a patient with left hemiparesis with an infarction on the posterior limb of the right internal capsule. fMRI was performed using the blood oxygen level-dependent technique at 1.5T with a standard head coil. The motor activation task consisted of hand grasp-release movements in 1-Hz cycles. TMS was performed using a butterfly coil; the intersection of the wings (center of the coil) was applied tangentially to the scalp 1.0cm apart. Stimulation was performed at 100% of maximal output. Motor evoked potentials (MEPs) from both abductor pollicis brevis (APB) muscles were obtained simultaneously. fMRI showed that the unaffected (left) primary sensorimotor cortex (SM1) was activated by movements of the unaffected (right) hand. Conversely, the bilateral SM1 were activated by movements of the affected (left) hand. Brain mapping using TMS showed that ipsilateral MEPs were obtained at the affected (left) APB muscle when the unaffected (left) motor cortex was stimulated. We concluded that the ipsilateral motor pathway from the unaffected motor cortex to the affected hand was present in this patient.