Measured Motor Evoked Potentials Research Articles

Effects of low-gamma tACS on primary motor cortex in implicit motor learning

In the primary motor cortex (M1), rhythmic activity in the gamma frequency band has been found during movement planning, onset and execution. Although the role of high-gamma oscillatory activity in M1 is well established, the contribution of low-gamma activity is still unexplored. In this study, transcranial alternating current stimulation (tACS) was used with the aim to specifically modulate low-gamma frequency band in M1, during an implicit motor learning task. A 40 Hz-tACS was applied over the left M1 while participants performed a serial reaction time task (SRTT) using their right hand. The task required the repetitive execution of sequential movements in response to sequences of visual stimuli. Sequential blocks were interleaved by a random block, which served as interference to sequence learning. Sham and 1 Hz tACS were used as control. Task performance was examined before, during and after tACS (pre-, online- and post-phase, respectively). Furthermore, cortical reactivity of M1 was assessed in the pre- and post-tACS phases, by measuring motor-evoked potentials (MEPs) elicited by single-pulse transcranial magnetic stimulation (TMS).Compared to sham and pre-tACS, the 40 Hz stimulation applied during SRTT slowed down response times in blocks that required retrieving previously learned sequences, after performing the random block. In addition, M1 cortical reactivity was selectively inhibited following 40 Hz-tACS, as quantified by reduced MEP amplitude. These results show that low-gamma tACS delivered over M1 during motor learning enhanced susceptibility to interference generated by the random sequence (i.e., proactive interference effect). Importantly, only low-gamma stimulation produced long-lasting effects on M1 cortical reactivity.

Time course of bilateral corticospinal tract excitability in the motor-learning process

Although it is known that motor learning changes the corticospinal tract excitability, the time course of bilateral corticospinal tract excitability in the motor-learning process has not been clarified. The study aimed to investigate the time course of bilateral corticospinal tract excitability during the motor-learning process. Sixteen subjects performed 10 trials of the visuomotor tracking task by using their right index finger for one minute. The movement intensity of the visuomotor tracking task ranged from 5%–17% of the maximum index finger abduction force and the movement frequency was 0.5 Hz. To assess bilateral corticospinal excitability, we stimulated the bilateral primary motor cortex with transcranial magnetic stimulation between each trial and measured motor-evoked potential (MEP) from the bilateral first dorsal interosseous (FDI) muscle. Motor performance improved rapidly to the sixth trial (P < 0.05), and motor performance did not change from the seventh to the tenth trial. The MEP amplitude of the right FDI increased significantly from the fifth to the ninth trial relative to that before the task (P < 0.05). Conversely, the MEP amplitude of the left FDI did not change during the motor-learning process. This study revealed that primary motor cortex excitability on the contralateral side to the exercising muscle increased in the late motor learning stage and that primary motor cortex excitability on the ipsilateral side to the exercising muscle did not change in the motor-learning process.

Measuring motor evoked-potentials in children with autism spectrum disorders accompanied with cerebral vein thrombosis following intraarterial heparin flushing

Background: The use of the term "Autism Spectrum Disorder" has led to confusion over this substance. Prior to DSM-V criteria, symptoms of Autism Spectrum Disorder (ASD) refers to the problems in brain vasculature and brain chemistry most likely affect the children behavior, whereas the authors' findings suggest such problems depicting a similar anomaly in cerebral vein thrombosis case (CVT). Recently, the evoked potentials, that demonstrated in CVT, have a possible prognostic value on patients suffering from ASD. This study purposes to measure the motor evoked potentials (MEPs) on patients following the intervention of intraarterial heparin flushing (IAHF). Methods: A descriptive study was conducted on 17 patients admitted in Cerebrovascular Center of RSPAD Gatot Soebroto, Jakarta, diagnosed with ASD presenting CVT. The MEPs value was measured by conforming the IAHF procedure.Results: The MEPs value (amplitude, latency, CMCT) pre and post-IAHF showed an increasing value. Meanwhile, there was a lowering value of latency and CMCT in left cortical participants after IAHF.Conclusion: The group tends to perform more expected positive MEPs changes after IAHF.

Effect of acupuncture at ST36 on motor cortical excitation and inhibition.

BackgroundAcupuncture at Zusanli (ST36) is often used to facilitate motor recovery after stroke. However, the effect of acupuncture at ST36 on motor cortical excitation and inhibition remains unclear. This study aimed to explore the effect of acupuncture at ST36 on motor cortical excitation and inhibition.MethodsTwenty healthy volunteers were recruited to receive acupuncture treatment. We selected the acupoint ST36 and its respective sham point as the experimental acupoint. Transcranial magnetic stimulation (TMS) was used to measure motor‐evoked potentials (MEP) at 7 time points—before acupuncture (Pre), acupuncture (T0), 4 and 8 min after acupuncture (T4; T8), needle removal (T12), 4 and 8 min after needle removal (T16; T20). Simultaneously, paired TMS (pTMS) was employed to measure short‐ and long‐interval intracortical inhibition (SICI [short latency intracortical inhibition]; LICI [long latency intracortical inhibition]), respectively, at three time points—before acupuncture (Pre), acupuncture (T0), needle removal (T12). After removing the acupuncture needle, all subjects were asked to quantify their Deqi sensation using a Gas table.ResultsThe average Deqi sensation score of all subjects during acupuncture at ST36 was higher than that observed at the sham point. With acupuncture at ST36, the MEP amplitude was higher at three time points (T0, T4, T8) than at Pre, although the MEP amplitude tended toward Pre after needle removal. The MEP amplitude was also higher at the same time points (T0, T4, T8) than at the sham point. Furthermore, the Deqi sensation score was correlated with MEP amplitude. With acupuncture at ST36, SICI and LICI at T0 were higher than those at Pre, and SICI and LICI at T0 were higher than those at the sham point.ConclusionAcupuncture at ST36 increased motor cortical excitation and had an effect on the remaining needle phase. Deqi sensation was correlated with MEP amplitude. Acupuncture at ST36 also decreased motor cortical inhibition.

Changes in corticospinal excitability associated with post-error slowing

According to available evidence, after making an erroneous decision people tend to slow down on the next decision. This empirical regularity, known as “post error slowing” (PES), has been traditionally interpreted as the result of a conservative response criterion adopted to avoid future errors and it is supposed to be driven by changes in the excitability of the motor system. However, the consequences of errors have been almost exclusively investigated by means of button-press tasks, which have been criticized because of their limited ecological validity and it is still unclear to what extent errors bias the motor system activity during the planning and the on-line control of complex and realistic goal-directed actions. To overcome these potential limitations, in the present study, we investigated the effect of errors on the preparation and execution of the reach-to-grasp movement, one of the most significant daily life actions. In addition to reaction times (RTs), we measured motor-evoked potential (MEP) to explore the influence of errors on corticospinal (CS) excitability, and we applied kinematical analysis to examine the underlying reorganization of the movement following an error. The results of the present study showed that MEPs tend to be reduced after the failure to reach and grasp an object, supporting the traditional interpretation of PES. Furthermore, in addition to RTs, we found that error-reactivity strategically influences the grasping component of the action, whereas the reaching component appears to be impermeable to PES. These findings demonstrate that the error-reactivity is a strong empirical phenomenon, which spreads into the motor system at the level of both movement preparation and execution, even when more realistic and ecologically valid tasks are used.

The effect of repeated iTBS on brain activities during hand movements

Theta burst stimulation (TBS) is proved to be the most efficient protocol of rTMS inducing neuroplasticity-like aftereffects. TBS-induced aftereffects depend on the pattern of stimulation. The aim of the study was to investigate the aftereffects of iTBS repeated every 5 min for 3 times (repeated iTBS) over the left primary motor cortex (LM1) by means of the amplitudes of motor evoked potential (MEP) evoked from the left (LMEP) and right (RMEP) M1 and their correlation with the brain oscillation. Each subject came for two sessions to have EEG with auditory-triggered shoulder flexion and extension action and MEPs measurements before and after repeated iTBS. Results showed that repeated iTBS suppressed LMEPs (mean change = -26.26%; range from -4.8 to -49.64%, p

Effects of Athermal Shortwave Diathermy Treatment on Somatosensory Evoked Potentials and Motor Evoked Potentials in Rats With Spinal Cord Injury.

A study on shortwave diathermy (SWD) versus no treatment following induced spinal cord injury (SCI) in rats. To investigate the effects of athermal SWD treatment on somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) and hindlimb movements in rats with SCI. SWD has been proven to improve vascular circulation and reduce inflammation. However, there have been few studies on neuroprotective effect of SWD on SCI. Twenty-four female Sprague-Dawley (SD) rats were randomly divided into four groups: sham, SCI, SWD, and intact groups. The SCI model was established using the modified Allen weight-drop method. The SWD group received 15 sessions of athermal SWD treatment over a 3-week period of time at 24 hours after SCI. While the sham group and SCI group received no treatment after surgery. Hindlimb movements were evaluated by the Basso, Beattie, and Bresnahan (BBB) scale before surgery, and on days 1, 7, 14, and 21 after the surgery, respectively. The SEP and MEP measurements were simultaneously performed to detect the responses of neural conduction. The week-by-week BBB scores showed a gradual improvement in the rats of both SCI and SWD groups from the first week to the end of the study; however, the BBB scores of the SWD group were higher than those of the SCI group over the course of 3 weeks. Data from the SEP and MEP measurements showed a significant improvement in the SWD group compared with the SCI group at each time point of observation, with a more prominent increase of amplitude and a more evident reduction of latency. There was a linear correlation between the BBB scores and the latency and amplitude of SEPs or MEPs. Athermal SWD treatment might facilitate the recovery of locomotor function and exert neuroprotective effect on the SCI. N/A.

Effects of Hand Intrinsic Muscle Facilitation and Functional Task Training on Cerebral Motor Evoked Potential after 1 Hz Low-Frequency rTMS in Stroke Patients

The purpose of this study was to investigate the effects of hand intrinsic muscles facilitation and functional task training with 1 Hz repetitive transcranial magnetic stimulation (rTMS) on cerebral motor cortex activity in stroke patients and to investigate the effectiveness of stroke rehabilitation. In this study, 20 adult stroke patients were randomly selected and divided into two groups of 10 each other. In the experimental group, hand intrinsic muscles facilitation and functional task training were performed after 1 Hz low frequency for 20 minutes and simple upper limb task training was performed in the control group. To investigate the changes of cerebral motor cortex activity after intervention, we measured Motor evoked potential (MEP) amplitude and latency. In both groups, MEP amplitude increased and MEP latency decreased after intervention. There was a significant difference between the two groups in MEP amplitude and latency (p 0.01) (p 0.05). Therefore, hand intrinsic muscles facilitation and functional task training with 1 Hz low frequency (rTMS) of stroke patients showed positive results in MEP amplitude and latency change of the injured cerebral cortex after stroke.

Time course of changes in corticospinal excitability induced by motor imagery during action observation combined with peripheral nerve electrical stimulation.

While previous studies assessed corticospinal excitability changes during and after motor imagery (MI) or action observation (AO) combined with peripheral nerve electrical stimulation (ES), we examined, for the first time, the time course of corticospinal excitability changes for MI during AO combined with ES (AO-MI + ES) using transcranial magnetic stimulation to measure motor evoked potentials (MEPs) in healthy individuals. Fourteen healthy volunteers participated in the following three sessions on different days: AO-MI alone, ES alone, and AO-MI + ES. In the AO-MI task, participants imagined squeezing and relaxing a ball, along with the respective actions shown in a movie, while passively holding the ball. We applied ES (intensity, 90% of the motor threshold) to the ulnar nerve at the wrist, which innervates the first dorsal interosseous (FDI) muscle. We assessed the FDI muscle MEPs at baseline and after every 5min of the task for a total of 20min. Additionally, participants completed the Vividness of Movement Imagery Questionnaire-2 (VMIQ-2) at the beginning of the experiment. Compared to baseline, AO-MI + ES significantly increased corticospinal excitability after 10min, while AO-MI or ES alone had no effect on corticospinal excitability after 20min. Moreover, the AO-MI + ES-induced cortical excitability changes were correlated with the VMIQ-2 scores for visual and kinaesthetic imagery. Collectively, our findings indicate that AO-MI + ES induces cortical plasticity earlier than does AO-MI or ES alone and that an individual's imagery ability plays an important role in inducing cortical excitability changes following AO-MI + ES.

The Influence of Conscious and Unconscious Body Threat Expressions on Motor Evoked Potentials Studied With Continuous Flash Suppression.

The observation of threatening expression in others is a strong cue for triggering an action response. One method of capturing such action responses is by measuring the amplitude of motor evoked potentials (MEPs) elicited with single pulse TMS over the primary motor cortex. Indeed, it has been shown that viewing whole body expressions of threat modulate the size of MEP amplitude. Furthermore, emotional cues have been shown to act on certain brain areas even outside of conscious awareness. In the current study, we explored if the influence of viewing whole body expressions of threat extends to stimuli that are presented outside of conscious awareness in healthy participants. To accomplish this, we combined the measurement of MEPs with a continuous flash suppression task. In experiment 1, participants were presented with images of neutral bodies, fearful bodies, or objects that were either perceived consciously or unconsciously, while single pulses of TMS were applied at different times after stimulus onset (200, 500, or 700 ms). In experiment 2 stimuli consisted of neutral bodies, angry bodies or objects, and pulses were applied at either 200 or 400 ms post stimulus onset. In experiment 1, there was a general effect of the time of stimulation, but no condition specific effects were evident. In experiment 2 there were no significant main effects, nor any significant interactions. Future studies need to look into earlier effects of MEP modulation by emotion body stimuli, specifically when presented outside of conscious awareness, as well as an exploration of other outcome measures such as intracortical facilitation.

Sensorimotor mu-alpha power is positively related to corticospinal excitability

BackgroundAlpha (8–14 Hz) oscillatory power is linked to cortical excitability and corresponding modulations of sensory evoked potentials and perceptual detection performance. In somatosensory cortex (S1), negative linear and inverted U-shape relationships exist, whereas its effect on the primary motor cortex (M1) is hardly known. ObjectiveWe used real-time EEG-triggered transcranial magnetic stimulation (TMS) of M1 to characterize the relationship between spontaneous sensorimotor mu-alpha power fluctuations at rest and corticospinal excitability. MethodsIn 16 subjects, mu-alpha power was continuously monitored over the left sensorimotor cortex, and each 10%-percentile bin of the individual mu-alpha power distribution was repeatedly targeted in pseudorandomized order by single-pulse TMS of left M1, measuring motor evoked potentials (MEP) in the contralateral hand. ResultsWe found a weak positive relationship between mu-alpha power and MEP amplitude. ConclusionSensorimotor mu-alpha power may reflect a net facilitation or disinhibition of M1, possibly resulting from mu-alpha based suppression of excitatory and inhibitory input from S1.

A reappraisal of pain-paired associative stimulation suggesting motor inhibition at spinal level

Paired associative stimulation (PAS) can modulate motor excitability and consists of delivering repeated pairs of peripheral sensory stimulation combined with transcranial magnetic stimulation (TMS) over the contralateral motor cortex. Our objective was to evaluate the effect of a PAS protocol using a painful stimulation. Ten healthy volunteers underwent pain-PAS protocol consisting of 90 pairs of nociceptive stimulus over the right hand followed by an image-guided navigated TMS of the contralateral motor cortex, delivered at 0.1Hz over 15minutes. Four distinct inter-stimuli intervals (ISI) were assessed: -40ms, +10ms, +30ms and +50ms with respect to the individual pain-related evoked potential (N2 latency). The impact of pain-PAS was assessed by measuring motor evoked potentials (MEPs) amplitudes before and up to 30minutes after the pain-PAS procedure. For the "N2 latency -40ms" ISI condition, a significant decrease of MEP amplitude was observed in the immediate post-pain-PAS period. For longer ISIs, no significant changes were observed. A motor inhibition effect was produced by repetitive pairs of painful peripheral stimulation and TMS pulse delivered to the motor cortex before the afferent volley reaches the somatosensory cortex. This short ISI falls into a period of time corresponding to the cutaneous silent period, suggesting that the motor inhibition effect produced by pain-PAS could occur at spinal level. The repeated interaction of the nociceptive afferent input and the descending motor corticospinal volley could lead to reduced excitability of homonymous spinal motoneurons.

T119. Strength of peripheral nerve stimulation impacts on the magnitude of synaptic plasticity induced at spinal motoneurons

Paired-associative stimulation (PAS), repeated transcranial magnetic stimulation (TMS) pairing with electrical peripheral nerve stimulation (ES), is one method of synaptic plasticity induction. If these two stimuli are delivered at the timing that descending volleys overlap with peripheral inputs at the spinal motoneurons (SM), long-term potentiation (LTP) like plasticity is induced at the SM synapses (SM-PAS). Here we investigated influences of stimulus intensity of ES on the degree of plasticity. Eight right-handed healthy volunteers participated in this study. Electromyograms were recorded from right first dorsal interosseous muscle (FDI). Peripheral ES was delivered on right ulnar nerve at the wrist. TMS was applied over the left primary motor cortex (M1) hand area, and stimulus intensity was set at 120% resting motor threshold of the right FDI. We measured motor-evoked potentials (MEP) and F-wave latencies in each subject. As for SM-PAS, TMS and ES were delivered at the timing that antidromic volley derived by ES arrives at the postsynaptic terminals 1 ms before an initial descending volley evoked by TMS reaches the presynaptic terminals. A stimulus pairing of TMS with ES was given every 5 s (0.2 Hz) for 15 min (total of 180 pairs). Three different ES intensities were delivered at supramaximal intensity, at three times the sensory threshold (with muscle twitch), and at two times the sensory threshold (without muscle twitch). For evaluating SM synaptic excitability, single-pulse TMS was delivered at cervicomedullary junction level during brief contraction of the right FDI, and cervicomedullary MEP (CMEP) was recorded before and 1 min and every 10 min from 10 to 60 min after the SM-PAS intervention. CMEP increased after SM-PAS at supramaximal ES and at three times the sensory threshold, while SM-PAS at two times the sensory threshold induced no CMEP changes. SM-PAS with supramaximal ES was most effective. SM-PAS at three times the sensory threshold induces sensory-motor associative plasticity constituted of sensory afferent inputs with multiple corticospinal descending volleys. Supramaximal ES stimulates both sensory afferent fibers and motor neuron axons antidromically, and SM-PAS may induce both sensory afferent and motor antidromic input associative plasticity. Antidromic inputs driven by supramaximal ES may augment associative synaptic plasticity at spinal motoneurons.

Intra- and Inter-Regional Priming of Ipsilateral Human Primary Motor Cortex With Continuous Theta Burst Stimulation Does Not Induce Consistent Neuroplastic Effects.

Human responses to non-invasive brain stimulation (NIBS) techniques can be highly variable. Recently, priming protocols involving a conditioning round of NIBS applied to a target brain region prior to the application of a test protocol have shown promise in inducing more reliable effects. We investigated whether intra- or inter-regional priming of the left primary motor cortex (M1) using continuous theta burst stimulation (cTBS) can induce consistent, and reliable modulation of corticospinal excitability. Twenty healthy adults (six males) underwent four cTBS protocols. For intra-regional priming, cTBS was applied twice to the left M1 (M1-M1). For inter-regional M1 priming, cTBS was applied to the ipsilateral (left) dorsal premotor cortex (dPMC-M1), and ipsilateral (left) dorsolateral prefrontal cortex (DLPFC-M1). In the control condition, sham stimulation was applied to left M1, followed by active cTBS also applied to the left M1 (sham-M1). Each round of cTBS was separated by 10 min. Neuroplastic responses were indexed using motor evoked potentials (MEPs) elicited from the left M1 hand region, and measured from the contralateral first dorsal interosseous (right hand). MEP measurements were taken before the first round of cTBS priming, then immediately, 10, 20 and 30 min after the second test round of cTBS. The primary two-way repeated measures ANOVA revealed no significant differences in MEP responses across each condition (no main effects or interaction). Intra- and inter-regional priming of the left M1 using cTBS does not induce consistent neuroplastic effects. Further work is required to identify factors which contribute to such variability in human responses to NIBS.

Effects of Passive Finger Movement on Cortical Excitability.

This study examined the effects of joint angle and passive movement direction on corticospinal excitability. The subjects were 14 healthy adults from whom consent could be obtained. We performed two experiments. In Experiment 1, we measured motor evoked potential (MEP) amplitude, F-wave and M-wave at 0° and 20° adduction during adduction or abduction movement, in the range of movement from 10° abduction to 30° adduction. In Experiment 2, MEPs were measured at static 0° and 20° adduction during passive adduction from 10° adduction to 30° adduction and static 20° adduction. MEP, F-waves and M-waves were recorded from the right first dorsal interosseous (FDI) muscle. Experiment 1 revealed significantly increased MEP amplitude at 0° during passive adduction compared to static 0° (p < 0.01). No other significant differences in MEP, M-wave and F-wave parameters were observed. In Experiment 2, MEP amplitude was significantly higher at 20° adduction during passive adduction compared with static 0° (p < 0.01). Based on these findings, it appears that fluctuations in MEP amplitude values during passive movement are not influenced by joint angle, but rather it is possible that it is due to intracortical afferent facilitation (AF) dependent on afferent input due to the start of movement and interstimulus interval (ISI) of transcranial magnetic stimulation (TMS).

The effect of age on the homotopic motor cortical long-term potentiation-like effect induced by quadripulse stimulation.

The reduction of plasticity with age has been shown by many previous papers in animal experiments. This issue can be studied in humans because several non-invasive brain stimulation techniques induce synaptic plasticity in the human brain. We investigated the influence of individuals' age on the responder rate of the long-term potentiation (LTP)-like effect induced by quadripulse magnetic stimulation (QPS). The participants were 107 healthy volunteers: 53 older participants (Mean±SD 65.0±1.5years) and 54 younger participants (37.2±8.7). The quadripulse stimulation with 5-ms inter-pulse interval (QPS5) was applied over the primary motor cortex (M1). We measured motor evoked potentials (MEPs) before QPS, and at five time points after QPS for up to 25min. In each participant, average MEP amplitude (size) ratios were quantified. We first classified participants as responders and non-responders simply by comparing the size ratio with 1.0 for consistency with previous studies, then as "significant responders", "non-responders", and "opposite responders" for more detailed analysis by comparing the size ratio with the mean and standard deviation of the MEP size ratios of the sham condition. The degree of LTP-like effects induced by QPS5 was significantly smaller in the older group compared to the younger group. Also, the rates of responders and significant responders were lower in the older group (58 and 47%, respectively) compared to the younger group (80 and 76%, respectively). The age of the participants significantly affected the LTP-like effect induced by QPS5, which suggests that brain plasticity decreases with age.

Additive effects of sequential excitatory and inhibitory theta burst stimulation in improving cortical excitability following ischaemic stroke

ABSTRACTPurpose: To study the effects of sequential excitatory and inhibitory theta burst stimulation (TBS) on cortical excitability in patients with subacute ischaemic stroke.Methods: Fifteen patients with first-ever middle cerebral artery ischaemic stroke within last three months were subjected to intermittent TBS over the hand area (M1) of the stroke hemisphere (iTBSSH) followed by continuous TBS over the intact hemisphere M1 (cTBSIH). The study measured motor evoked potential (MEP) amplitude, resting motor threshold (RMT) and contralateral cortical silent period (cCSP) at baseline and after 7 minutes of each TBS session.Results: The mean age of the patients was 62.4 ± 8.0 years. The absolute changes in baseline motor physiological parameters were significantly greater after cTBSIH than after iTBSSH (p = 0.001). Following iTBSSH, there was a significant decrease in mean RMT (7.4%; p < 0.001). Following cTBSIH, there was further 18.3% increase in mean MEP amplitude and decrease in mean RMT (4.7%; p < 0.001) and cCSP (5.6%; p < 0.001) of the stroke hemisphere. Similar changes of smaller magnitude were noted in 12 age-matched controls.Conclusions: The results indicate that combining iTBSSH and cTBSIH can have additive effect in increasing cortical excitability in subacute ischaemic patients with stroke.

P138 Influence of the difference of induced current direction on measurement of corticospinal excitability changes after continuous and intermittent theta burst stimulation

Introduction The after-effects of continuous and intermittent theta burst stimulation (cTBS and iTBS) are highly variable between individuals and only about 50% of subjects respond in expected ways (i.e. depression of motor evoked potentials (MEPs) in cTBS and facilitation in iTBS). TBS is usually applied using a biphasic stimulus pulse. It preferentially activates the brain during the second depolarizing phase of the current in the brain, that is, anterior-posterior (AP) current. In contrast, the after-effects of any plasticity inducing protocols are usually evaluated by a monophasic stimulus pulse with posterior-anterior (PA) current. Here we hypothesized that such difference in magnetic pulse configuration of transcranial magnetic stimulation (TMS) influences variability of TBS effects. Objectives To investigate whether the after-effects of TBS should be clearer if tested with MEPs generated by AP currents or biphasic currents. Methods Twenty-three healthy right-handed volunteers participated in this study. TMS was applied to the hot spot of the right first dorsal interosseous muscle and MEPs were measured at the stimulus intensities set to evoke MEPs of about 1 mV in monophasic PA, monophasic AP, and biphasic currents. Thirty MEPs for each direction were measured before and up to 30 min after cTBS or iTBS (Huang et al., 2005). Responder (i.e. depression in cTBS and facilitation in iTBS) was defined according to the grand average of MEPs. Results In cTBS, responder rates were 23.9% (5/21) in PA current, 38.1% (8/21) in AP current, and 14.2% (3/21) in biphasic current. 42.9% (9/21) of subjects showed opposite responses between PA and AP currents. As for non-responders both in PA and AP currents (47.6%, 10/21), the difference of baseline MEP latencies at rest between monophasic PA and biphasic currents was smaller than the others. In iTBS, responder rates were 60.9% (14/23) in PA current, 65.2% (15/23) in AP current, and 47.8% (11/23) in biphasic current. 47.8% (11/23) of subjects showed opposite responses between PA and AP currents. Conclusion The after-effects of TBS were highly variable regardless of current direction in measurement of MEPs. In cTBS, a tendency to early I wave recruitment in biphasic pulse might relate to MEP facilitation both in PA and AP currents.

P069 The prevalence and functional relevance of ipsilateral and contralateral connections to the proximal upper limb in chronic stroke survivors

Question After stroke proximal upper limb muscles are innervated not only by contralateral but also by ipsilateral connections. These connections can be probed by transcranial magnetic stimulation (TMS) but whether these connections contribute to the functional recovery of upper limb movements is not clear. Methods We therefore established proximal upper limb muscle active motor evoked potentials (MEPs) when delivering 20 stimulations to both the affected and unaffected hemisphere of 19 chronic stroke survivors (mean age: 59 years, SD 10.2 years, 5 female, average onset since stroke 3 years 8 months,SD 2 year 4 months) at 70% maximal TMS stimulator output (Magstim 200 squared device, figure 8 coil). In addition, we measured each individuals Fugl-Meyer upper limb score, their endpoint accuracy and movement speed when performing a reaching protocol in a robotic manipulandum and proximal muscle power. Spearman and Pearson’s correlations were performed to assess correlations of Fugl-Meyer scores and baseline endpoint accuracy respectively, with MEP measures on a subject-by-subject basis. Results When stimulating the affected hemisphere at 70%, active MEPs were evoked in the affected weak upper limb in an average of 49% of stimulations. Responses were also seen in 19% of the stimulations delivered to the unaffected hemisphere via the ipsilateral pathway. There was a clear correlation of the presence of responses when stimulating the affected hemisphere and Fugl-Meyerscore (rho = .579, p. Conclusion In summary the findings in our stroke population support previous studies showing that intact corticospinal connections from the affected hemisphere are associated with good performance and recovery. We further observe that ipsilateral responses from the unaffected hemisphere are more common after stroke but that they are not correlated to function. Whether the relevance of these connections could be altered by targeted interventions in the acute period after stroke, when neuroplasticity is increased, should be further investigated.

P151 Repetitive Transcranial Magnetic Stimulation: A call for better data

Introduction The experimental science to inform application of Repetitive Transcranial Magnetic Stimulation (rTMS) is inconsistent. Evidence is strongly based on measurements of motor-evoked potentials (MEPs) which are indirect measures of cortical changes. It is still unclear whether rTMS achieves its effects through plasticity, changes in neural excitability, control of gene expression or another route. Objectives We approached this work as computational physicists. Our aim was to analyze rTMS literature to identify underlying trends. Specifically: 1. Which parameters (e.g. number of pulses, repetition rate) are important? 2. How does the MEP relate to changes in the cortex? 3. What science must be included in a numerical model of the MEP and rTMS? Materials & methods We identified 92 rTMS publications reporting data from 164 experiments. Often, data were poorly recorded and key information was absent. With non-bursting protocols, we identified only 39 publications (76 experiments) which unambiguously gave the rTMS frequency, the number of pulses applied, the pulse intensity and the effect on the MEP. We used principal component analysis to identify which parameters most affected the MEP. We tried to repeat this for bursting protocols; but were prevented by lack of variation in the reported protocols. We used an existing neural field model to study the effect on the MEP of changes in cortical excitability and connection strengths. Results Results are in line with existing dogma: >5 Hz rTMS leads to an increase in MEP; Conclusion Overall, we need better data. Many experiments are poorly reported. Key information is missing or ambiguous. The MEP is an inconvenient and indirect measure of the effect of rTMS. It is often poorly defined and recorded. One needs to 1. record it and report it rigorously and quantitatively, and 2. develop better numerical models of MEPs. Bursting experiments have focused on too few possibilities (e.g. 5 Hz theta-burst stimulation or quadripulse) to unlock underlying science.