Transcranial magnetic stimulation and cortical evoked potentials: A TMS/EEG co-registration study

Resistance training induces supraspinal adaptations: evidence from movement-related cortical potentials

Early effects of a resistance training program include neural adaptations at multiple levels of the neuraxis, but direct evidence of central changes is lacking. Plasticity exhibited by multiple supraspinal centers following training may alter slow negative electroencephalographic activity, referred to as movement-related cortical potentials (MRCP). The purpose of this study was to determine whether MRCPs are altered in response to resistance training. Eleven healthy participants (24.6 +/- 3.5 years) performed 3 weeks of explosive unilateral leg extensor resistance training. MRCP were assessed during 60 self-paced leg extensions against a constant nominal load before and after training. Resistance training was effective (P < 0.001) in increasing leg extensor peak force (+22%), rate of force production (+32%) as well as muscle activity (iEMG; +47%, P < 0.05). These changes were accompanied by several MRCP effects. Following training, MRCP amplitude was attenuated at several scalp sites overlying motor-related cortical areas (P < 0.05), and the onset of MRCP at the vertex was 28% (561 ms) earlier. In conclusion, the 3-week training protocol in the present study elicited significant strength gains which were accompanied by neural adaptations at the level of the cortex. We interpret our findings of attenuated cortical demand for submaximal voluntary movement as evidence for enhanced neural economy as a result of resistance training.

Methodology for combined TMS and EEG

The combination of transcranial magnetic stimulation (TMS) with simultaneous electroencephalography (EEG) provides us the possibility to non-invasively probe the brain's excitability, time-resolved connectivity and instantaneous state. Early attempts to combine TMS and EEG suffered from the huge electromagnetic artifacts seen in EEG as a result of the electric field induced by the stimulus pulses. To deal with this problem, TMS-compatible EEG systems have been developed. However, even with amplifiers that are either immune to or recover quickly from the pulse, great challenges remain. Artifacts may arise from the movement of electrodes, from muscles activated by the pulse, from eye movements, from electrode polarization, or from brain responses evoked by the coil click. With careful precautions, many of these problems can be avoided. The remaining artifacts can be usually reduced by filtering, but control experiments are often needed to make sure that the measured signals actually originate in the brain. Several studies have shown the power of TMS-EEG by giving us valuable information about the excitability or connectivity of the brain.

Safety, ethical considerations, and application guidelines for the use of transcranial magnetic stimulation in clinical practice and research

This article is based on a consensus conference, which took place in Certosa di Pontignano, Siena (Italy) on March 7-9, 2008, intended to update the previous safety guidelines for the application of transcranial magnetic stimulation (TMS) in research and clinical settings. Over the past decade the scientific and medical community has had the opportunity to evaluate the safety record of research studies and clinical applications of TMS and repetitive TMS (rTMS). In these years the number of applications of conventional TMS has grown impressively, new paradigms of stimulation have been developed (e.g., patterned repetitive TMS) and technical advances have led to new device designs and to the real-time integration of TMS with electroencephalography (EEG), positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Thousands of healthy subjects and patients with various neurological and psychiatric diseases have undergone TMS allowing a better assessment of relative risks. The occurrence of seizures (i.e., the most serious TMS-related acute adverse effect) has been extremely rare, with most of the few new cases receiving rTMS exceeding previous guidelines, often in patients under treatment with drugs which potentially lower the seizure threshold. The present updated guidelines review issues of risk and safety of conventional TMS protocols, address the undesired effects and risks of emerging TMS interventions, the applications of TMS in patients with implanted electrodes in the central nervous system, and safety aspects of TMS in neuroimaging environments. We cover recommended limits of stimulation parameters and other important precautions, monitoring of subjects, expertise of the rTMS team, and ethical issues. While all the recommendations here are expert based, they utilize published data to the extent possible.

Repetitive Transcranial Magnetic Stimulation Affects behavior by Biasing Endogenous Cortical Oscillations

A governing assumption about repetitive transcranial magnetic stimulation (rTMS) has been that it interferes with task-related neuronal activity – in effect, by “injecting noise” into the brain – and thereby disrupts behavior. Recent reports of rTMS-produced behavioral enhancement, however, call this assumption into question. We investigated the neurophysiological effects of rTMS delivered during the delay period of a visual working memory task by simultaneously recording brain activity with electroencephalography (EEG). Subjects performed visual working memory for locations or for shapes, and in half the trials a 10-Hz train of rTMS was delivered to the superior parietal lobule (SPL) or a control brain area. The wide range of individual differences in the effects of rTMS on task accuracy, from improvement to impairment, was predicted by individual differences in the effect of rTMS on power in the alpha-band of the EEG (∼10 Hz): a decrease in alpha-band power corresponded to improved performance, whereas an increase in alpha-band power corresponded to the opposite. The EEG effect was localized to cortical sources encompassing the frontal eye fields and the intraparietal sulcus, and was specific to task (location, but not object memory) and to rTMS target (SPL, not control area). Furthermore, for the same task condition, rTMS-induced changes in cross-frequency phase synchrony between alpha- and gamma-band (>40 Hz) oscillations predicted changes in behavior. These results suggest that alpha-band oscillations play an active role cognitive processes and do not simply reflect absence of processing. Furthermore, this study shows that the complex effects of rTMS on behavior can result from biasing endogenous patterns of network-level oscillations.

Reproducibility of TMS-Evoked EEG responses

Navigated transcranial magnetic stimulation combined with electroencephalography (nTMS-EEG), allows noninvasive studies of cortical excitability and connectivity in humans. We investigated the reproducibility of nTMS-EEG in seven healthy subjects by repeating left motor and prefrontal cortical stimulation with a 1-week interval. TMS was applied at three intensities: 90, 100, and 110% of subjects' motor threshold (MT). The TMS-compatible neuronavigation system guaranteed precise repositioning of the stimulation coil. The responses were recorded by a 60-channel whole head TMS-compatible EEG amplifier. A high overall reproducibility (r > 0.80) was evident in nTMS-EEG responses over both hemispheres for both motor and prefrontal cortical stimulation. The results suggest that nTMS-EEG is a reliable tool for studies investigating cortical excitability changes in the test-retest designs.

Consensus paper: combining transcranial stimulation with neuroimaging

In the last decade, combined transcranial magnetic stimulation (TMS)-neuroimaging studies have greatly stimulated research in the field of TMS and neuroimaging. Here, we review how TMS can be combined with various neuroimaging techniques to investigate human brain function. When applied during neuroimaging (online approach), TMS can be used to test how focal cortex stimulation acutely modifies the activity and connectivity in the stimulated neuronal circuits. TMS and neuroimaging can also be separated in time (offline approach). A conditioning session of repetitive TMS (rTMS) may be used to induce rapid reorganization in functional brain networks. The temporospatial patterns of TMS-induced reorganization can be subsequently mapped by using neuroimaging methods. Alternatively, neuroimaging may be performed first to localize brain areas that are involved in a given task. The temporospatial information obtained by neuroimaging can be used to define the optimal site and time point of stimulation in a subsequent experiment in which TMS is used to probe the functional contribution of the stimulated area to a specific task. In this review, we first address some general methodologic issues that need to be taken into account when using TMS in the context of neuroimaging. We then discuss the use of specific brain mapping techniques in conjunction with TMS. We emphasize that the various neuroimaging techniques offer complementary information and have different methodologic strengths and weaknesses.

Somatosensory-evoked potentials and cortical activities evoked by magnetic stimulation on acupoint in human

Two acupuncture manipulations are clinically used: manual manipulation and electrical acupuncture. There is little published on the EEG changes during magnetic stimulation on an acupuncture site. In this study, EEG data in response to magnetic stimulation on HeGu (LI 4) acupoint were measured to determine whether magnetic acupoint stimulation might modulate ongoing EEG or not. Eighteen healthy volunteers (13 male, 5 female) 20 to 35 years old were chosen in this experiment, with consent obtained before the study. The highest evoked potential was recorded in FCZ electrode, at about 140-170 ms (P150) after acupoint stimulation, but not mock point stimulation. Comparison of the somatosensory-evoked potentials in response to acupoint stimulation and mock point stimulation showed that P150 was specific to acupoint stimulation. With regard to the location of P150 in the human brain, we suggest that magnetic stimulation on HeGu acupoint would affect specific brain areas compared with the mock point. The difference in the anatomical structure of acupoint and non-acupoint may explain the specific acupoint-brain correlation, and P150 may be a characteristic activation in response to acupoint afferent.

Modulation of cortical oscillatory activity during transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) can transiently modulate cortical excitability, with a net effect depending on the stimulation frequency (< or =1 Hz inhibition vs. > or =5 Hz facilitation, at least for the motor cortex). This possibility has generated interest in experiments aiming to improve deficits in clinical settings, as well as deficits in the cognitive domain. The aim of the present study was to investigate the on-line effects of low frequency (1 Hz) TMS on the EEG oscillatory activity in the healthy human brain, focusing particularly on the outcome of these modulatory effects in relation to the duration of the TMS stimulation. To this end, we used the event-related desynchronization/synchronization (ERD/ERS) approach to determine the patterns of oscillatory activity during two consecutive trains of sham and real TMS. Each train of stimulation was delivered to the left primary motor cortex (MI) of healthy subjects over a period of 10 min, while EEG rhythms were simultaneously recorded. Results indicated that TMS induced an increase in the power of brain rhythms that was related to the period of the stimulation, i.e. the synchronization of the alpha band increased with the duration of the stimulation, and this increase was inversely correlated with motor-evoked potentials (MEPs) amplitude. In conclusion, low frequency TMS over primary motor cortex induces a synchronization of the background oscillatory activity on the stimulated region. This induced modulation in brain oscillations seems to increase coherently with the duration of stimulation, suggesting that TMS effects may involve short-term modification of the neural circuitry sustaining MEPs characteristics.

Cortical inhibition in motor and non-motor regions: a combined TMS-EEG study

A number of studies using paired pulse transcranial magnetic stimulation (TMS) have demonstrated that cortical inhibition (CI) of the motor cortex can be recorded and also gauged through surface electromyography. However, recording CI from other brain regions that are more directly related with the pathophysiology of some neurologic and psychiatric disorders (e.g., dorsolateral prefrontal cortex (DLPFC) in schizophrenia) was previously fraught with technical difficulties. This study was therefore designed to examine, through a combination of TMS with EEG, whether CI could be measured directly from the motor cortex, DLPFC, and another non-motor region. To index CI, long interval cortical inhibition (LICI; a TMS paradigm) was used in the motor cortex and DLPFC in 14 healthy subjects, and in the parietal lobe in 5 of those subjects. In the motor cortex, LICI resulted in a significant suppression in mean cortical evoked activity on EEG (37.31 +/- 47.51%). In the DLPFC, LICI resulted in a significant suppression (32.45 +/- 47.86%) in mean cortical evoked activity and did not correlate with LICI in the motor cortex although they did not significantly differ. In the parietal lobe, LICI resulted in significant suppression (47.76 +/- 44.70%) in mean cortical evoked activity. In conclusion, CI in the dorsolateral prefrontal cortex, motor cortex and parietal cortex were similar at 120% of motor threshold. These data suggest that CI can be recorded by combining TMS with EEG and may facilitate future research attempting to ascertain the role of CI in the pathophysiology of several neurologic and psychiatric disorders.

Cortical plasticity induced by transcranial magnetic stimulation during wakefulness affects electroencephalogram activity during sleep

BACKGROUND

Sleep electroencephalogram (EEG) brain oscillations in the low-frequency range show local signs of homeostatic regulation after learning. Such increases and decreases of slow wave activity are limited to the cortical regions involved in specific task performance during wakefulness. Here, we test the hypothesis that reorganization of motor cortex produced by long-term potentiation (LTP) affects EEG activity of this brain area during subsequent sleep.

METHODOLOGY/PRINCIPAL FINDINGS

By pairing median nerve stimulation with transcranial magnetic stimulation over the contralateral motor cortex, one can potentiate the motor output, which is presumed to reflect plasticity of the neural circuitry. This paired associative stimulation increases M1 cortical excitability at interstimulus intervals of 25 ms. We compared the scalp distribution of sleep EEG power following paired associative stimulation at 25 ms to that following a control paradigm with 50 ms intervals. It is shown that the experimental manipulation by paired associative stimulation at 25 ms induces a 48% increase in amplitude of motor evoked potentials. This LTP-like potentiation, induced during waking, affects delta and theta EEG power in both REM and non-REM sleep, measured during the following night. Slow-wave activity increases in some frontal and prefrontal derivations and decreases at sites neighboring and contralateral to the stimulated motor cortex. The magnitude of increased amplitudes of motor evoked potentials by the paired associative stimulation at 25 ms predicts enhancements of slow-wave activity in prefrontal regions.

CONCLUSIONS/SIGNIFICANCE

An LTP-like paradigm, presumably inducing increased synaptic strength, leads to changes in local sleep regulation, as indexed by EEG slow-wave activity. Enhancement and depression of slow-wave activity are interpreted in terms of a simultaneous activation of both excitatory and inhibitory circuits consequent to the paired associative stimulation at 25 ms.

Navigated TMS combined with EEG in mild cognitive impairment and Alzheimer's disease: a pilot study

Our aim was to assess the potential of navigated transcranial magnetic stimulation (TMS)-evoked electroencephalographic (EEG) responses in studying neuronal reactivity and cortical connectivity in Alzheimer's disease (AD) and in mild cognitive impairment (MCI). We studied 14 right-handed subjects: five patients with AD, five patients with MCI and four healthy controls. Fifty TMS-pulses at an intensity of 110% of individually determined motor threshold were delivered to the hand area of primary motor cortex (M1) with navigated brain stimulation (NBS). Spreading of primary NBS-evoked neuronal activity was monitored with a compatible 60-channel EEG, and analyzed in time, frequency and spatial-domains. We found significantly reduced TMS-evoked P30 (time-locked response 30 ms after the magnetic stimulation) in the AD subjects. This reduction was seen in the temporo-parietal area ipsilateral to stimulation side as well as in the contralateral fronto-central cortex corresponding to the sensorimotor network, which is anatomically interconnected with the stimulated M1. In addition, there was a significant decrease in the N100 amplitude in the MCI subjects when compared with the control subjects. Thus, the combination of NBS and EEG revealed prominent changes in functional cortical connectivity and reactivity in the AD subjects. This pilot study suggests that the method may provide a novel tool for examining the degree and progression of dementia.

Efficient reduction of stimulus artefact in TMS-EEG by epithelial short-circuiting by mini-punctures

OBJECTIVE

We aimed at comparing the effects of two different electrode-to-skin contact preparation techniques on the stimulus artefact induced by transcranial magnetic stimulation (TMS) in electroencephalography (EEG) signals.

METHODS

Six healthy subjects participated in a combined navigated brain stimulation (NBS) and EEG study. Electrode contacts were first prepared in the standard way of rubbing the skin using a wooden stick with a cotton tip. The location of hand motor area and the motor threshold (MT) was determined for each subject. Then, the TMS-induced artefact was measured at 60%, 80%, 100% and 120% of the MT. Subsequently, the epithelium under the electrode contacts was electrically short-circuited by puncturing with custom-made needles and the stimulation sequences were replicated. The artefact was compared between the preparation techniques.

RESULTS

The TMS-induced artefact was significantly reduced after puncturing. In addition, the size and duration of the artefact depended on the applied stimulation intensity. The reduction of the artefact was largest in electrodes at and close to the stimulation site.

CONCLUSIONS

Mini-puncturing technique enables more accurate analysis of TMS-induced short-latency phenomena in EEG during NBS, and it may aid in the examination of the short distance neural connectivity beneath and close to the stimulation site.

SIGNIFICANCE

This study describes a practical skin preparation method that significantly improves the utility of TMS-EEG method in studying short-latency cortical connectivity.

Effects of alcohol on TMS-evoked N100 responses

TMS combined with simultaneous EEG is a novel brain imaging tool allowing investigation local excitability of human cortex. As alcohol acts through increasing function of A-type gamma-aminobutyric acid receptors and attenuating the function of glutaminergic NMDA-receptors-related excitation, we tested whether TMS-evoked N100 response which is thought to reflect cortical inhibitory processes, might be affected by alcohol. Ten healthy subjects ingested alcohol (0.8 g/kg) and EEG responses from 60 channels before and after alcohol ingestion were recorded after left motor-cortex stimulation. Alcohol almost abolished TMS-evoked N100 response. Control experiments with a piece of plastic placed between the head and coil to exclude auditory artefacts were conducted. Alcohol effects were similar when EEG responses from control experiments were subtracted from real-TMS. Alcohol-induced decrease was similar at ipsilateral, contralateral and frontal EEG sites suggesting that alcohol may change cortico-cortical connectivity of motor cortex. Alternative explanation is that alcohol has overall suppression effect on motor cortex. N100 may provide a useful marker of neural inhibition of human cortex for drug research.

Focal stimulation of the posterior parietal cortex increases the excitability of the ipsilateral motor cortex

Paired-pulse transcranial magnetic stimulation (TMS) has been applied as a probe to test functional connectivity within distinct cortical areas of the human motor system. Here, we tested the interaction between the posterior parietal cortex (PPC) and ipsilateral motor cortex (M1). A conditioning TMS pulse over the right PPC potentiates motor evoked-potentials evoked by a test TMS pulse over the ipsilateral motor cortex, with a time course characterized by two phases: an early peak at 4 ms interstimulus interval (ISI) and a late peak at 15 ms ISI. Activation of this facilitatory pathway depends on the intensity of stimulation, because the effects are induced with a conditioning stimulus of 90% resting motor threshold but not at lower or higher intensities. Similar results were obtained testing the ipsilateral interaction in the left hemisphere with a slightly different time course. In control experiments, we found that activation of this facilitatory pathway depends on the direction of induced current in the brain and is specific for stimulation of the caudal part of the inferior parietal sulcus (cIPS) site, because it is not observed for stimulation of adjacent scalp sites. Finally, we found that by using poststimulus time histogram analysis of single motor unit firing, the PPC conditioning increases the excitability of ipsilateral M1, enhancing the relative amount of late I wave input recruited by the test stimulus over M1, suggesting that such interaction is mediated by specific interneurons in the motor cortex. The described facilitatory connections between cIPS and M1 may be important in a variety of motor tasks and neuropsychiatric disorders.

Artifact correction and source analysis of early electroencephalographic responses evoked by transcranial magnetic stimulation over primary motor cortex

Analyzing the brain responses to transcranial magnetic stimulation (TMS) using electroencephalography (EEG) is a promising method for the assessment of functional cortical connectivity and excitability of areas accessible to this stimulation. However, until now it has been difficult to analyze the EEG responses during the several tens of milliseconds immediately following the stimulus due to TMS-induced artifacts. In the present study we show that by combining a specially adapted recording system with software artifact correction it is possible to remove a major part of the artifact and analyze the cortical responses as early as 10 ms after TMS. We used this methodology to examine responses of left and right primary motor cortex (M1) to TMS at different intensities. Based on the artifact-corrected data we propose a model for the cortical activation following M1 stimulation. The model revealed the same basic response sequence for both hemispheres. A large part of the response could be accounted for by two sources: a source close to the stimulation site (peaking approximately 15 ms after the stimulus) and a midline frontal source ipsilateral to the stimulus (peaking approximately 25 ms). In addition the model suggests responses in ipsilateral temporo-parietal junction areas (approximately 35 ms) and ipsilateral (approximately 30 ms) and middle (approximately 50 ms) cerebellum. Statistical analysis revealed significant dependence on stimulation intensity for the ipsilateral midline frontal source. The methodology developed in the present study paves the way for the detailed study of early responses to TMS in a wide variety of brain areas.

Excitation threshold of the motor cortex estimated with transcranial magnetic stimulation electroencephalography

The excitation threshold of the human motor cortex was estimated on the basis of electroencephalographic responses evoked by transcranial magnetic stimulation. The hand area of the primary motor cortex was stimulated at 10 intensities, for seven healthy individuals. The four dominant peaks of the overall brain response could be reliably determined when stimulation was intense enough to induce a cortical electric field of approximately 33-44 mV/mm. This may be estimated as the threshold for evoking measurable brain activity by motor-cortex transcranial magnetic stimulation. The remarkably low threshold reflects the excellent sensitivity of the combination of transcranial magnetic stimulation and electroencephalography for the study of neuronal function of the cortex.

The Interaction of Brain Regions during Visual Search Processing as Revealed by Transcranial Magnetic Stimulation

Although it has long been known that right posterior parietal cortex (PPC) has a role in certain visual search tasks, and human motion area V5 is involved in processing tasks requiring attention to motion, little is known about how these areas may interact during the processing of a task requiring the speciality of each. Using transcranial magnetic stimulation (TMS), this study first established the specialization of each area in the form of a double dissociation; TMS to right PPC disrupted processing of a color/form conjunction and TMS to V5 disrupted processing of a motion/form conjunction. The key finding of this study is, however, if TMS is used to disrupt processing of V5 at its critical time of activation during the motion/form conjunction task, concurrent disruption of right PPC now has a significant effect, where TMS at PPC alone does not. Our findings challenge the conventional interpretation of the role of right PPC in conjunction search and spatial attention.