Monophasic Transcranial Magnetic Stimulation Research Articles

High inductance magnetic-core coils have enhanced efficiency in inducing suprathreshold motor response in rats

Objective. Transcranial magnetic stimulation (TMS) coil design involves a tradeoff among multiple parameters, including magnetic flux density (B), inductance (L), induced electric (E) field, focality, penetration depth, coil heating, etc. Magnetic materials with high permeability have been suggested to enhance coil efficiency. However, the introduction of magnetic core invariably increases coil inductance compared to its air-core counterpart, which in turn weakens the E field. Our lab previously reported a rodent-specific TMS coil with silicon steel magnetic core, achieving 2 mm focality. This study aims to better understand the tradeoffs among B, L, and E in the presence of magnetic core. Approach. The magnetic core initially operates within the linear range, transitioning to the nonlinear range when it begins to saturate at high current levels and reverts to the linear range as coil current approaches zero; both linear and nonlinear analyses were performed. Linear analysis assumes a weak current condition when magnetic core is not saturated; a monophasic TMS circuit was employed for this purpose. Nonlinear analysis assumes a strong current condition with varying degrees of core saturation. Main results. Results reveal that, the secondary E field generated by the silicon steel core substantially changed the dynamics during TMS pulse. Linear and nonlinear analyses revealed that higher inductance coils produced stronger peak E fields and longer E field waveforms. On a macroscopic scale, the effects of these two factors on neuronal activation could be conceptually explained through a one-time-constant linear membrane model. Four coils with different B, L, and E characteristics were designed and constructed. Both E field mapping and experiments on awake rats confirmed that inductance could be much higher than previously anticipated, provided that magnetic material possesses a high saturation threshold. Significance. Our results highlight the novel potentials of magnetic core in TMS coil designs, especially for small animals.

The effect of pulse shape in theta-burst stimulation: Monophasic vs biphasic TMS

BackgroundIntermittent theta-burst stimulation (i) (TBS) is a transcranial magnetic stimulation (TMS) plasticity protocol. Conventionally, TBS is applied using biphasic pulses due to hardware limitations. However, monophasic pulses are hypothesised to recruit cortical neurons more selectively than biphasic pulses, predicting stronger plasticity effects. Monophasic and biphasic TBS can be generated using a custom-made pulse-width modulation-based TMS device (pTMS). ObjectiveUsing pTMS, we tested the hypothesis that monophasic iTBS would induce a stronger plasticity effect than biphasic, measured as induced increases in motor corticospinal excitability. MethodsIn a repeated-measures design, thirty healthy volunteers participated in three separate sessions, where monophasic and biphasic iTBS was applied to the primary motor cortex (M1 condition) or the vertex (control condition). Plasticity was quantified as increases in motor corticospinal excitability after versus before iTBS, by comparing peak-to-peak amplitudes of motor evoked potentials (MEP) measured at baseline and over 60 min after iTBS. ResultsBoth monophasic and biphasic M1 iTBS led to significant increases in MEP amplitude. As predicted, linear mixed effects (LME) models showed that the iTBS condition had a significant effect on the MEP amplitude (χ2 (1) = 27.615, p < 0.001) with monophasic iTBS leading to significantly stronger plasticity than biphasic iTBS (t (693) = 2.311, p = 0.021). Control vertex iTBS had no effect. ConclusionsIn this study, monophasic iTBS induced a stronger motor corticospinal excitability increase than biphasic within participants. This greater physiological effect suggests that monophasic iTBS may also have potential for greater functional impact, of interest for future fundamental and clinical applications of TBS.

Optimized monophasic pulses with equivalent electric field for rapid-rate transcranial magnetic stimulation

Objective. Transcranial magnetic stimulation (TMS) with monophasic pulses achieves greater changes in neuronal excitability but requires higher energy and generates more coil heating than TMS with biphasic pulses, and this limits the use of monophasic pulses in rapid-rate protocols. We sought to design a stimulation waveform that retains the characteristics of monophasic TMS but significantly reduces coil heating, thereby enabling higher pulse rates and increased neuromodulation effectiveness. Approach. A two-step optimization method was developed that uses the temporal relationship between the electric field (E-field) and coil current waveforms. The model-free optimization step reduced the ohmic losses of the coil current and constrained the error of the E-field waveform compared to a template monophasic pulse, with pulse duration as a second constraint. The second, amplitude adjustment step scaled the candidate waveforms based on simulated neural activation to account for differences in stimulation thresholds. The optimized waveforms were implemented to validate the changes in coil heating. Main results. Depending on the pulse duration and E-field matching constraints, the optimized waveforms produced 12%–75% less heating than the original monophasic pulse. The reduction in coil heating was robust across a range of neural models. The changes in the measured ohmic losses of the optimized pulses compared to the original pulse agreed with numeric predictions. Significance. The first step of the optimization approach was independent of any potentially inaccurate or incorrect model and exhibited robust performance by avoiding the highly nonlinear behavior of neural responses, whereas neural simulations were only run once for amplitude scaling in the second step. This significantly reduced computational cost compared to iterative methods using large populations of candidate solutions and more importantly reduced the sensitivity to the choice of neural model. The reduced coil heating and power losses of the optimized pulses can enable rapid-rate monophasic TMS protocols.

Physiological symmetry of transcranial magnetic stimulation-evoked EEG spectral features.

Transcranial magnetic stimulation (TMS)-evoked EEG potentials (TEPs) have been used to study the excitability of different cortical areas (CAs) in humans. Characterising the interhemispheric symmetry of TMS-EEG may provide further understanding of structure-function association in physiological and pathological conditions. We hypothesise that, in keeping with the underlying cytoarchitectonics, TEPs in contralateral homologous CAs share similar, symmetric spectral features, whilst ipsilateral TEPs from different CAs diverge in their waveshape and frequency content. We performed single-pulse (<1Hz) navigated monophasic TMS, combined with high-density EEG with active electrodes, in 10 healthy participants. We targeted two bilateral CAs: premotor and motor. We compared frequency power bands, computed Pearson correlation coefficient (R) and Correlated Component Analysis (CorrCA) to detect divergences, as well as common components across TEPs. The main frequency of TEPs was faster in premotor than in motor CAs (p < .05) across all participants. Frequencies were not different between contralateral homologous CAs, whilst, despite closer proximity, there was a significant difference between ipsilateral premotor and motor CAs (p > .5), with frequency decreasing from anterior to posterior CAs. Correlation was high between contralateral homologous CAs and low between ipsilateral CAs. When applying CorrCA, specific components were shared by contralateral homologous TEPs. We show physiological symmetry of TEP spectral features between contralateral homologous CAs, whilst ipsilateral premotor and motor TEPs differ despite lower geometrical distance. Our findings support the role of TEPs as biomarker of local cortical properties and provide a first reference dataset for TMS-EEG studies in asymmetric brain disorders.

Identifying transcranial magnetic stimulation induced EEG signatures of different neuronal elements in primary motor cortex

ObjectiveTo investigate the neuronal elements involved in the activation of corticospinal neurons in the primary motor cortex (M1). MethodsWe studied 10 healthy subjects. Cortical evoked potentials with different components induced by monophasic transcranial magnetic stimulation (TMS) in anterior-posterior and posterior-anterior currents recorded with electroencephalography (EEG) were analyzed. ResultsEEG signatures with P25 and N45 components recorded at the C3 electrode with posterior-anterior current were larger than those with anterior-posterior current, while the signatures with P180 and N280 components recorded at the FC1 electrode with anterior-posterior current were larger than those with posterior-anterior current. The source localization analysis revealed that the cortical evoked potential with anterior-posterior current distributed both in the M1 and premotor cortex while that with posterior-anterior current only located in the M1. ConclusionsWe conclude that the activation of corticospinal pyramidal neurons in the M1 is affected by various neuronal elements including the local intracortical circuits in the M1 and inputs from premotor cortex with different sensitivities to TMS in opposite current directions. SignificanceOur finding helped answer a longstanding question about how the corticospinal pathway from the M1 is functionally organized and activated.

Dissecting the neural underpinnings of the short-latency response to transcranial magnetic stimulation of motor cortex: A computational modeling study

Abstract Objective: Transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) elicits a series of high frequency volleys termed D- and I-waves measured epidurally in the corticospinal tract of awake humans. The D-wave is thought to be caused by direct activation of corticospinal axons, while I-waves are believed to be generated by trans-synaptic activation of corticospinal axons. The cortical circuits and mechanisms involved in the generation of I-waves by TMS of M1 remain unclear. Methods: We implemented a computational model of a cortical column with biophysically-based cortical neurons, following an existing model (Traub 2005). The TMS induced E-field was modeled as intracellular current injections of monophasic TMS pulses into the axon terminals of pyramidal neurons (PN). We constructed dose–response curves of spiking activity averaged across layer 5 (L5) PN as a function of the proportion of activated PN. Results: Model-based responses were compared to I-waves recorded from L5 PN in rodent M1 during TMS (Li 2017), and the network model reproduced I-waves only during the activation of axon terminals of L5 PN in contrast to activation of axon terminals of L2/3 or L6 PN. Inhibitory interneurons in deep layers fired in synchrony during the late phases of I-waves offering a potential explanation for the reduction in magnitude of late I-waves seen in experimental studies after administration of GABA-A agonists. Virtual lesioning of model neurons revealed that recurrent synaptic connections between L5 PN mediated by AMPA receptors play a prominent role in the generation of I-waves. Conclusions: Activation of presynaptic terminal of L5 PN is necessary to generate an I-wave response to TMS. Further, recurrent excitatory synaptic connections between L5 PN play a prominent role in the generation of I-wave response. Significance: Gaining a better understanding of circuit mechanisms of TMS of the M1 could enable optimization of stimulation paradigms. Keywords: Transcranial magnetic stimulation, I-waves, Cortical column model, Motor cortex

M11. ALTERED TRANSCRANIAL MAGNETIC STIMULATION ELECTROENCEPHALOGRAPHIC MARKERS IN SCHIZOPHRENIA

BackgroundCortical inhibition is a neurophysiological process in which cortical gamma-aminobutyric acid (GABA) inhibitory interneurons modulate the activity of pyramidal neurons in the cerebral cortex. Multiple lines of evidence, including neurophysiological and neuropathological, report that individuals with schizophrenia have deficits in cortical inhibition. Combining transcranial magnetic stimulation (TMS) with electroencephalography is a reliable approach to measure inhibitory processes in the cortex. The overall waveform produced by TMS-EEG may index cortical reactivity as a whole and previous investigations have linked the N45 component peak and the N100 component peak with GABA-A and GABA-B inhibitory neurotransmission, respectively. The aim of this study was to stimulate the DLPFC with TMS and examine resultant differences in the TMS-EEG waveform peaks between patients with schizophrenia and healthy subjects. We hypothesized that individuals with schizophrenia will have smaller TMS-evoked potentials, specifically the amplitudes of the N100 and N45 components, those previously related to GABA-ergic inhibition.MethodsWe applied TMS over the left DLPFC and recorded EEG activity in 48 healthy subjects (mean age: 33.8±5.3) and 46 patients with schizophrenia (mean age: 43.3±6.4). Monophasic TMS pulses were administered using a 7-cm figure-of-8 coil, and two Magstim 200 stimulators connected via a Bistim module. Single pulse TMS was administered over the left DLPFC with 100 total pulses, which were delivered every 5s. Resultant waveforms were extracted and analyzed through custom MATLAB scripts. The TMS-evoked potential waveform was examined through Global Mean Field Amplitude (GMFA) analysis of waveform peaks in each the two groups. Normality of the distribution of each variable was assessed and a Mann Whitney U test was then performed for each variable of interest to assess differences between groups.ResultsIndividuals in the schizophrenia group demonstrated smaller measures of cortical inhibition in the DLPFC. Specifically, smaller amplitudes of the N45 (U=724.00, p=0.004) and N100 peaks (U=831.00, p=0.039), although the overall AUC of the waveform did not differ between groups (U=969.00, p=0.307). Further analysis is underway to examine medication and symptom cluster effects.DiscussionThese results demonstrate novel findings of deficits in both GABA-A and GABA-B associated measures of cortical inhibition as indexed by single pulse TMS-EEG. This reinforces previous evidence from different research modalities demonstrating overall GABAergic inhibitory deficits in schizophrenia, and specifically provides new support which confirms recent findings of aberrant GABA-Aergic inhibitory neurotransmission in schizophrenia.

Detecting cortical circuits resonant to high-frequency oscillations in the human primary motor cortex: a TMS-tACS study

Corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) consist of high-frequency bursts (≈667 and ≈333 Hz). However, intracortical circuits producing such corticospinal high-frequency bursts are unknown. We here investigated whether neurons activated by single TMS pulses over M1 are resonant to high-frequency oscillations, using a combined transcranial alternating current stimulation (tACS)-TMS approach. We applied 667, 333 Hz or sham-tACS and, concurrently, we delivered six single-pulse TMS protocols using monophasic or biphasic pulses, different stimulation intensities, muscular states, types and orientations of coils. We recorded motor evoked potentials (MEPs) before, during and after tACS. 333 Hz tACS facilitated MEPs evoked by biphasic TMS through a figure-of-eight coil at active motor threshold (AMT), and by monophasic TMS with anterior-to-posterior-induced current in the brain. 333 Hz tACS also facilitated MEPs evoked by monophasic TMS through a circular coil at AMT, an effect that weakly persisted after the stimulation. 667 Hz tACS had no effects. 333 Hz, but not 667 Hz, tACS may have reinforced the synchronization of specific neurons to high-frequency oscillations enhancing this activity, and facilitating MEPs. Our findings suggest that different bursting modes of corticospinal neurons are produced by separate circuits with different oscillatory properties.

Where and what TMS activates: Experiments and modeling

BackgroundDespite recent developments in navigation and modeling techniques, the type and location of the structures that are activated by transcranial magnetic stimulation (TMS) remain unknown. ObjectiveWe studied the relationships between electrophysiological measurements and electric fields induced in the brain to locate the TMS activation site. MethodsThe active and resting motor thresholds of the first dorsal interosseous muscle were recorded in 19 subjects (7 female, 12 male, age 22 ± 4 years) using anteromedially oriented monophasic TMS at multiple locations over the left primary motor cortex (M1). Structural MR images were used to construct electric field models of each subject's head and brain. The cortical activation site was estimated by finding where the calculated electric fields best explained the coil-location dependency of the measured MTs. ResultsThe experiments and modeling showed individual variations both in the measured motor thresholds (MTs) and in the computed electric fields. When the TMS coil was moved on the scalp, the calculated electric fields in the hand knob region were shown to vary consistently with the measured MTs. Group-level analysis indicated that the electric fields were significantly correlated with the measured MTs. The strongest correlations (R2 = 0.69), which indicated the most likely activation site, were found in the ventral and lateral part of the hand knob. The site was independent of voluntary contractions of the target muscle. ConclusionThe study showed that TMS combined with personalized electric field modeling can be used for high-resolution mapping of the motor cortex.

Synchronizing the transcranial magnetic pulse with electroencephalographic recordings effectively reduces inter-trial variability of the pulse artefact.

BackgroundElectroencephalography (EEG) can capture the cortical response evoked by transcranial magnetic stimulation (TMS). The TMS pulse provokes a large artefact, which obscures the cortical response in the first milliseconds after TMS. Removing this artefact remains a challenge.MethodsWe delivered monophasic and biphasic TMS to a melon as head phantom and to four healthy participants and recorded the pulse artefact at 5 kHz with a TMS-compatible EEG system. Pulse delivery was either synchronized or non-synchronized to the clock of the EEG recording system. The effects of synchronization were tested at 10 and 20 kHz using the head phantom. We also tested the effect of a soft sheet placed between the stimulation coil and recording electrodes in both human and melon.Results & conclusionSynchronizing TMS and data acquisition markedly reduced trial-to-trial variability of the pulse artefact in recordings from the phantom or from the scalp. Reduced trial-to-trial variability was also observed at high sampling frequencies. The use of a soft sheet reduced the variability in recordings on the head phantom, but not in human participants. Effective reduction of the trial-to-trial variability renders it possible to create an artefact template for off-line filtering. Template-based subtraction of the artefact from the EEG signals is a prerequisite to effectively recover the immediate physiological response in the stimulated cortex and inter-connected areas.

Paired Associative Stimulation Targeting the Tibialis Anterior Muscle using either Mono or Biphasic Transcranial Magnetic Stimulation.

Paired associative stimulation (PAS) protocols induce plastic changes within the motor cortex. The objectives of this study were to investigate PAS effects targeting the tibialis anterior (TA) muscle using a biphasic transcranial magnetic stimulation (TMS) pulse form and, to determine whether a reduced intensity of this pulse would lead to significant changes as has been reported for hand muscles using a monophasic TMS pulse. Three interventions were investigated: (1) suprathreshold PAbi-PAS (n = 11); (2) suprathreshold PAmono-PAS (n = 11) where PAS was applied using a biphasic or monophasic pulse form at 120% resting motor threshold (RMT); (3) subthreshold PAbi-PAS (n = 10) where PAS was applied as for (1) at 95% active motor threshold (AMT). The peak-to-peak motor evoked potentials (MEPs) were quantified prior to, immediately following, and 30 min after the cessation of the intervention. TA MEP size increased significantly for all interventions immediately post (61% for suprathreshold PAbi-PAS, 83% for suprathreshold PAmono-PAS, 55% for subthreshold PAbi-PAS) and 30 min after the cessation of the intervention (123% for suprathreshold PAbi-PAS, 105% for suprathreshold PAmono-PAS, 80% for subthreshold PAbi-PAS. PAS using a biphasic pulse form at subthreshold intensities induces similar effects to conventional PAS.

Facilitatory effect of paired-pulse stimulation by transcranial magnetic stimulation with biphasic wave-form

Transcranial magnetic stimulation (TMS) is used to probe corticospinal excitability by stimulating the motor cortex. Our aim was to enhance the effects of biphasic TMS by coupling a suprathreshold test pulse and a following subthreshold priming pulse to induce short-interval intracortical facilitation (SICF), which is conventionally produced with monophasic TMS. Biphasic TMS could potentially induce the SICF effect with better energy-efficiency and with lower stimulus intensities. This would make the biphasic paired-pulses better applicable in patients with reduced cortical excitability.A prototype stimulator was built to produce biphasic paired-pulses. Resting motor thresholds (rMTs) from the right and left hand abductor pollicis brevis muscles, and the right tibialis anterior muscle of eight healthy volunteers were determined using single-pulse paradigm with neuronavigated TMS. The rMTs and MEPs were measured using single-pulses and three paired-pulse setups (interstimulus interval, ISI of 3, 7 or 15ms).The rMTs were lower and MEPs were higher with biphasic paired-pulses compared to single-pulses. The SICF effect was greatest at 3ms ISI. This suggests that the application of biphasic paired-pulses to enhance stimulation effects is possible.

Modifying excitability of trigemino-facial reflex circuits with transcranial direct current stimulation

s / Brain Stimulation 8 (2015) 378e394 384 Background: Responses to a number of different plasticityinducing brain stimulation protocols are highly variable. However there is little data available on the variability of response to 1 Hertz repetitive transcranial magnetic stimulation (1Hz rTMS). Objective: We tested the effects of 1Hz rTMS over the motor cortex on corticospinal excitability. We also examined whether an individual’s response could be predicted frommeasurements of onset latency of motor evoked potential (MEP) following stimulationwith different orientations of monophasic transcranial magnetic stimulation (TMS). Methods: Thirty-two healthy subjects (mean age 30.3 years, SD 11.0; 13 females) participated in a crossover-design. Baseline latency measurements with different coil orientations and MEPs were recorded from the first dorsal interosseous muscle prior to the application of 900 pulses of 1Hz rTMS with two different stimulus intensities, i.e. 90% resting motor threshold (RMT) and 115%RMT. Thirty MEPs were measured every 5 min for up to half an hour after the intervention to assess after-effects on corticospinal excitability. Results: At the group level, there were no significant aftereffects of 1Hz rTMS for both stimulus intensities: about 50% individuals had only a minor or no response to 1Hz rTMS whereas the remainders had a facilitatory effect to both forms of stimulation. There was no significant correlation between the latency difference of MEPs (anterior-posterior stimulation minus latero-medial stimulation) and the response to 1Hz rTMS. Conclusions: The large variability in response to 1Hz rTMS is in line with similar studies using other forms of non-invasive brain stimulation. These results highlight the importance of individual factors influencing the responsiveness to these protocols.

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 5ms (QPS-5) or at 50ms (QPS-50). Same bursts were given every 5s for 30min. 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.

Repetitive transcranial magnetic stimulator with controllable pulse parameters

The characteristics of transcranial magnetic stimulation (TMS) pulses influence the physiological effect of TMS. However, available TMS devices allow very limited adjustment of the pulse parameters. We describe a novel TMS device that uses a circuit topology incorporating two energy storage capacitors and two insulated-gate bipolar transistor (IGBT) modules to generate near-rectangular electric field pulses with adjustable number, polarity, duration, and amplitude of the pulse phases. This controllable pulse parameter TMS (cTMS) device can induce electric field pulses with phase widths of 10–310 µs and positive/negative phase amplitude ratio of 1–56. Compared to conventional monophasic and biphasic TMS, cTMS reduces energy dissipation up to 82% and 57% and decreases coil heating up to 33% and 41%, respectively. We demonstrate repetitive TMS trains of 3000 pulses at frequencies up to 50 Hz with electric field pulse amplitude and width variability less than the measurement resolution (1.7% and 1%, respectively). Offering flexible pulse parameter adjustment and reduced power consumption and coil heating, cTMS enhances existing TMS paradigms, enables novel research applications and could lead to clinical applications with potentially enhanced potency.

Effect of transcranial magnetic stimulation on four types of pressure-programmable valves

Exposure to powerful magnetic fields may alter the settings of programmable ventriculoperitoneal shunt valves or even cause permanent damage to these devices. Transcranial magnetic stimulation (TMS) and magnetic resonance imaging both generate a high-intensity, focal magnetic field. To the best of our knowledge, there is no literature data on the compatibility of TMS with neurosurgical implants. The aim of the present in vitro study was to investigate the effects of TMS on four types of pressure-programmable valves (the Strata 2 from Medtronic, the Polaris from Sophysa, the ProGAV from Miethke, and a cylindrical valve from Codman-Hakim). We used a Magpro X100 stimulator (Medtronic) for monophasic or biphasic TMS via a circular or a figure-of-eight coil. Each valve setting was tested before and after exposure to TMS. Experiment 1: The effect of the coil-valve distance (10, 5, 2.5, and 1 cm) was assessed. Experiment 2: We mimicked in situ stimulation with a human mannequin by placing the valve in a retroauricular position, the TMS circular coil on the apex, and figure-of-eight coil centered over the primary motor area site. Temperature changes were monitored throughout the experiments. Experience 3: TMS-induced valve movements were assessed by using an in-house accelerometric setup. Our results primarily demonstrated that the Strata 2 and Codman-Hakim valves' settings were perturbed by TMS. There was no heating effect for any of the valves. However, TMS induced movements of the Strata 2, Polaris, and ProGAV valves. Experiment 1: The unsetting frequencies observed for the Strata 2 and the Codman-Hakim valve showed an influence of the distance, the coil model, and the magnetic field characteristics, whereas the Polaris and ProGAV's settings remained unchanged. Experiment 2: Unsetting occurred for Strata 2 valve with the circular coil only, whereas the Polaris, ProGAV, and Codman-Hakim valves' settings remained stable. Experiment 3: The Strata 2, Polaris, and ProGAV valves showed high-amplitude oscillations during TMS under all stimulation conditions, whereas the Codman-Hakim valve did not move. Our in vitro experiments showed that TMS can interfere with programmable shunt valves by inducing unsetting or movement. This finding suggests that great care must be taken if applying TMS in hydrocephalic, shunted patients.

Navigated transcranial magnetic stimulation of the primary somatosensory cortex impairs perceptual processing of tactile temporal discrimination

Previous studies indicate that transcranial magnetic stimulation (TMS) with biphasic pulses applied approximately over the primary somatosensory cortex (S1) suppresses performance in vibrotactile temporal discrimination tasks; these previous results, however, do not allow separating perceptual influence from memory or decision-making. Moreover, earlier studies using external landmarks for directing biphasic TMS pulses to the cortex do not reveal whether the changes in vibrotactile task performance were due to action on S1 or an adjacent area. In the present study, we determined whether the S1 area representing a cutaneous test site is critical for perceptual processing of tactile temporal discrimination. Electrical test pulses were applied to the thenar skin of the hand and the subjects attempted to discriminate single from twin pulses. During discrimination task, monophasic TMS pulses or sham TMS pulses were directed anatomically accurately to the S1 area representing the thenar using magnetic resonance image-guided navigation. The subject's capacity to temporal discrimination was impaired with a decrease in the delay between the TMS pulse and the cutaneous test pulse from 50 to 0 ms. The result indicates that S1 area representing a cutaneous test site is involved in perceptual processing of tactile temporal discrimination.

A real electro-magnetic placebo (REMP) device for sham transcranial magnetic stimulation (TMS)

Objective There is growing interest in neuropsychiatry for repetitive transcranial magnetic stimulation (rTMS) as a neuromodulatory treatment. However, there are limitations in interpreting rTMS effects as a real consequence of physiological brain changes or as placebo-mediated unspecific effects, which may be particularly strong in psychiatric patients. This is due to the fact that existing sham rTMS procedures are less than optimal. A new placebo tool is introduced here, called real electro-magnetic placebo (REMP) device, which can simulate the scalp sensation induced by the real TMS, while leaving both the visual impact and acoustic sensation of real TMS unaltered. Methods Physical, neurophysiological and behavioural variables of monophasic and biphasic single-pulse TMS and biphasic 1 Hz and 20 Hz rTMS procedures (at different intensities) were tested in subjects who were expert or naïve of TMS. Results of the real TMS were compared with those induced by the REMP device and with two other currently used sham procedures, namely the commercially available Magstim sham coil and tilting the real coil by 90°. Results The REMP device, besides producing scalp sensations similar to the real TMS, attenuated the TMS-induced electric field (as measured by a dipole probe) to a biologically inactive level. Behaviourally, neither expert nor naïve TMS subjects identified the “coil at 90°” or the “Magstim sham coil” as a real TMS intervention, whilst naïve subjects were significantly more likely to identify the REMP-attenuated TMS as real. Conclusions The “goodness of sham” of the REMP device is demonstrated by physical, neurophysiological, and behavioural results. Significance Such placebo TMS is superior to the available sham procedures when applied on subjects naïve to TMS, as in case of patients undergoing a clinical rTMS trial.

The effect on corticospinal volleys of reversing the direction of current induced in the motor cortex by transcranial magnetic stimulation.

Descending corticospinal volleys were recorded from a bipolar electrode inserted into the cervical epidural space of four conscious human subjects after monophasic transcranial magnetic stimulation over the motor cortex with a figure-of-eight coil. We examined the effect of reversing the direction of the induced current in the brain from the usual posterior-anterior (PA) direction to an anterior-posterior (AP) direction. The volleys were compared with D waves evoked by anodal electrical stimulation (two subjects) or medio-lateral magnetic stimulation (two subjects). As reported previously, PA stimulation preferentially recruited I1 waves, with later I waves appearing at higher stimulus intensities. AP stimulation tended to recruit later I waves (I3 waves) in one of the subjects, but, in the other three, I1 or D waves were seen. Unexpectedly, the descending volleys evoked by AP stimulation often had slightly different peak latencies and/or longer duration than those seen after PA stimulation. In addition the relationship between the size of the descending volleys and the subsequent EMG response was often different for AP and PA stimulation. These findings suggest that AP stimulation does not simply activate a subset of the sites activated by PA stimulation. Some sites or neurones that are relatively inaccessible to PA stimulation may be the low-threshold targets of AP stimulation.