Effect of repetitive transcranial magnetic stimulation applied over the premotor cortex on somatosensory-evoked potentials and regional cerebral blood flow

Effects of inter-stimulus and inter-trial intervals on somatosensory gating

AIM OF THE STUDY

Sensory gating is a human higher cognitive function that serves to suppress excessive sensory information and prevent brain overactivity. To elucidate this function, a paired-pulse stimulation paradigm has been used while recording electroencephalography (EEG), and evaluated as an amplitude ratio of responses to a second stimulus (S2) over responses to the first stimulus (S1). The present study investigated the effects of the inter-stimulus interval (ISI) and inter-trial interval (ITI) on somatosensory gating using somatosensory-evoked potentials (SEPs).

METHODS

In Experiment 1, ISI was set at five conditions: 200, 400, 600, 800, and 1000 ms. In Experiment 2, ITI was set at four conditions: 1, 2, 4, and 8 s.

RESULTS

ISI affected the S2/S1 amplitude ratios of P22 and N27 at C3' and N30 at Fz, and these S2/S1 amplitude ratios decreased the most under the 200 and 400-ms conditions. ITI affected the S2/S1 amplitude ratios of P22, N27, and N60 at C3', and especially, the somatosensory gating did not work under the 1-s condition. These results suggest that not all SEP components are modulated in the same manner with changing ISI and ITI. The effects of ISI and ITI independently affected the somatosensory gating.

CONCLUSIONS

Based on our findings, preferable parameters are 200-400 ms for ISI and 4 s or longer for ITI to evaluate the functional mechanisms on somatosensory gating in SEPs.

Effects of repetitive practice of motor tasks on somatosensory gating

Introduction

During voluntary muscle contraction, the amplitude of the somatosensory evoked potential (SEP) is reduced by inhibiting sensory information from a peripheral nerve supplying the contracted muscle. This phenomenon is called “gating.” We reported that participants with good motor skills indicated strong suppression of somatosensory information. The present study investigated the effects of motor performance improvement following repetitive practice on the SEP amplitude.

Methods

The ball rotation task (BR task) was practiced by 15 healthy participants repetitively. SEPs were recorded before (pre) and after (post) repetitive practice.

Results

The BR task performance was significantly improved and the required muscle activation to perform the task was significantly reduced after the repetitive practice. The degree of gating was not significant between pre and post- for the SEP amplitude. A significant correlation was found between changes in SEP amplitude from pre to post and performance improvement.

Discussion

After repetitive practice, the degree of gating did not change, but the performance of the BR task improved, and the muscle activity required for the BR task decreased. These results suggest that repetitive practice does not change the degree of gating but changes the mechanism of gating. Furthermore, they indicate that suppression of the somatosensory area may play a role in improving task performance.

Modulation of sensorimotor oscillation by sensory gating in the frontal cortex

OBJECTIVES

To reveal that nonprimary motor-related areas located in the upper stream of the sensorimotor area are associated with self-regulated local electroencephalogram changes in the sensorimotor area during motor tasks.

METHODS

Among healthy participants, we measured the gating of somatosensory-evoked potentials (SEPs) in nonprimary motor-related areas and the sensorimotor area, and event-related desynchronisation, which reflects the excitability changes of the neurons localised in the sensorimotor area during motor execution and imagery.

RESULTS

We confirmed significant correlations between beta-band event-related desynchronisation and the somatosensory-evoked potential gating of frontal N30 during motor imagery and execution (motor imagery: r = 0.723; P < 0.05; motor execution: r = 0.873; P < 0.05), and nonsignificant correlations between beta-band event-related desynchronisation and the somatosensory-evoked potential gating of N20 (motor imagery: r = 0.079; P > 0.05; motor execution: r = 0.449; P > 0.05).

CONCLUSIONS

The N30 gating of SEPs, with which the beta-band event-related desynchronisation was associated, implies that they modulate sensory input to the supplementary motor area/premotor cortex during motor tasks, the beta-band self-regulated local electroencephalogram changes in the sensorimotor area related sensory input to the supplementary motor area/premotor cortex, and not to primary sensory area derived from N20 gating. This study suggests that some motor commands are derived from sensory gating in the supplementary motor area/premotor cortex.

Acute Effects of Aerobic Exercise on Somatosensory-Evoked Potentials in Patients with Mild Cognitive Impairment

Mild cognitive impairment (MCI) is becoming a serious problem for developing countries as the lifespan of populations increases. Exercise is known to be clinically beneficial for MCI patients. Somatosensory-evoked potentials (SEPs) may be a potential diagnostic and prognostic marker for this population. The objective of this study was to determine the acute effects of aerobic exercise on SEPs in patients with MCI, to test whether SEPs are sensitive enough to detect improvements in early somatosensory processing. The study had a randomized parallel-group design and included 28 MCI subjects (14 in the experimental group and 14 in the control group). The experimental intervention was 20 min of aerobic exercise using a stationary bicycle. The control intervention involved 20 min of movements and stretches. Subjects were assessed before and after a single intervention session. SEPs were recorded by stimulating the median nerve of the dominant hand. Analysis of normalized SEP peak amplitudes showed that a single session of aerobic activity significantly reduced the N30 peak at the F3 channel (p = 0.03). There were no significant effects of aerobic exercise on SEP peak latencies. The results indicate that 20 min of aerobic exercise has a significant effect on the N30 SEP peak amplitude in MCI patients. The results suggest that aerobic exercise is likely to provide sensory-enriching inputs that enhance sensorimotor integration. Future studies should assess the effects of aerobic exercise on somatosensory processing in progressive stages of Alzheimer’s disease, longer exercise durations, and multiple exercise sessions.

Differences in characteristics of somatosensory evoked potentials between children and adults

The present study examined the characteristics of somatosensory processing in healthy prepubescent children (mean age: 124.9 ± 3.8 months) compared with young adults. Somatosensory evoked potentials at the frontal (Fz) and centroparietal (C3') electrodes were recorded by delivering an electrical stimulus to the right median nerve at a rate of 3 Hz. The characteristics of somatosensory evoked potential waveforms at C3' were markedly different between the two groups, while those at Fz were similar. Specifically, the waveforms at C3' in the children involved not only standard P12, N18, P22, N27, P45, and N60 components, but also additional positive (P3) and negative (N3) components between N27 and P45, which were not found in adults. The amplitude of P22 at C3' was significantly larger in the children than adults, indicating hyper-excitability/responsiveness of neural activity on somatosensory processing. In contrast, the amplitudes of N15 at Fz and N27 at C3' were smaller in the children than adults, suggesting an immature somatosensory system in the children. The peak latencies of P12, N15, and P18 at Fz, and P12 and N18 at C3' were significantly shorter in the children than adults, which was dependent on the height. These results indicate the developing somatosensory processing with hyper- and hypo-excitability of neural activity in prepubescent children.

Effects of whole body skin cooling on human cognitive processing: a study using SEPs and ERPs

The present study investigated the effect of whole body skin cooling on somatosensory ascending processing by utilizing somatosensory-evoked potentials (SEPs) and motor execution, as well as inhibitory processing by event-related potentials (ERPs). Fourteen healthy participants wearing a water-perfused suit performed two sessions (sessions 1 and 2) consisting of SEPs and ERPs with somatosensory Go/No-go paradigms under two conditions (cold stress and control) on different days. In session 2, under the cold stress condition, whole body skin cooling was achieved by circulating 20°C water through the suit for 40 min, whereas 34°C water was perfused in the other sessions. The mean skin temperature decreased from 35.0 ± 0.5°C (session 1) to 30.4 ± 0.9°C (session 2) during whole body skin cooling, but the internal temperature was maintained. Whole body skin cooling delayed the peak latencies of N20, P25, and P45 components at C4' of SEPs (all: P < 0.05). Moreover, the peak latencies of P14, N18, and P22 components at Fz of SEPs and the Go-P300 component of ERPs were delayed (all: P < 0.05). In contrast, the peak amplitudes of all individual components of SEPs as well as N140 and P300 of ERPs remained unchanged. These results suggest that passive whole body skin cooling delays neural activities on somatosensory processing and higher cognitive function.

Non-invasive brain stimulation in the modulation of cerebral blood flow after stroke: A systematic review of Transcranial Doppler studies

OBJECTIVE

Non-invasive brain stimulation (NIBS), such as repetitive TMS (rTMS) and transcranial direct current stimulation (tDCS), are promising neuromodulatory priming techniques to promote task-specific functional recovery after stroke. Despite promising results, clinical application of NIBS has been limited by high inter-individual variability. We propose that there is a possible influence of neuromodulation on cerebral blood flow (CBF), as neurons are spatially and temporally related to blood vessels. Transcranial Doppler (TCD), a clinically available non-invasive diagnostic tool, allows for evaluation of CBF velocity (CBFv). However, little is known about the role of neuromodulation on CBFv.

METHODS

A systematic review of literature to understand the effects of NIBS on CBFv using TCD in stroke was conducted.

RESULTS

Twelve studies fit our inclusion criteria and are included in this review. Our review suggested that CBFv and/or vasomotor reactivity maybe influenced by rTMS dosage (intensity and frequency) and the type of tDCS electrode montage.

CONCLUSION

There is limited evidence regarding the effects of NIBS on cerebral hemodynamics using TCD and the usefulness of TCD to capture changes in CBFv after NIBS is not evident from this review. We highlight the variability in the experimental protocols, differences in the applied neurostimulation protocols and discuss open questions that remain regarding CBF and neuromodulation.

SIGNIFICANCE

TCD, a clinically accessible tool, may potentially be useful to understand the interaction between cortical neuromodulation and CBFv.

Effects of face/head and whole body cooling during passive heat stress on human somatosensory processing

We herein investigated the effects of face/head and whole body cooling during passive heat stress on human somatosensory processing recorded by somatosensory-evoked potentials (SEPs) at C4' and Fz electrodes. Fourteen healthy subjects received a median nerve stimulation at the left wrist. SEPs were recorded at normothermic baseline (Rest), when esophageal temperature had increased by ~1.2°C (heat stress: HS) during passive heating, face/head cooling during passive heating (face/head cooling: FHC), and after HS (whole body cooling: WBC). The latencies and amplitudes of P14, N20, P25, N35, P45, and N60 at C4' and P14, N18, P22, and N30 at Fz were evaluated. Latency indicated speed of the subcortical and cortical somatosensory processing, while amplitude reflected the strength of neural activity. Blood flow in the internal and common carotid arteries (ICA and CCA, respectively) and psychological comfort were recorded in each session. Increases in esophageal temperature due to HS significantly decreased the amplitude of N60, psychological comfort, and ICA blood flow in the HS session, and also shortened the latencies of SEPs (all, P < 0.05). While esophageal temperature remained elevated, FHC recovered the peak amplitude of N60, psychological comfort, and ICA blood flow toward preheat baseline levels as well as WBC. However, the latencies of SEPs did not recover in the FHC and WBC sessions. These results suggest that impaired neural activity in cortical somatosensory processing during passive HS was recovered by FHC, whereas conduction velocity in the ascending somatosensory input was accelerated by increases in body temperature.

Effects of passive heat stress on human somatosensory processing

Herein, we investigated the effects of passive heat stress on human somatosensory processing recorded by somatosensory-evoked potentials (SEPs). Fifteen healthy subjects received a median nerve stimulation at the left wrist under two thermal conditions: Heat Stress and normothermic Time Control. The latencies and amplitudes of P14, N20, P25, N35, P45, and N60 at C4′ and P14, N18, P22, and N30 at Fz were evaluated. Under the Heat Stress condition, SEPs were recorded at normothermic baseline (1st), early in heat stress (2nd), when esophageal temperature had increased by ∼1.0°C (3rd) and ∼2.0°C (4th), and after heat stress (5th). In the Time Control condition, SEPs were measured at the same time intervals as those in the Heat Stress condition. The peak latencies and amplitudes of SEPs did not change early in heat stress. However, the latencies of P14, N20, and N60 at C4′ and P14, N18, and P22 at Fz were significantly shorter in the 4th session than in the 1st session. Furthermore, the peak amplitudes of P25 and N60 at C4′, and P22 and N30 at Fz decreased with increases in body temperature. On the other hand, under the Time Control condition, no significant differences were observed in the amplitudes or latencies of any component of SEPs. These results suggested that the conduction velocity of the ascending somatosensory input was accelerated by increases in body temperature, and hyperthermia impaired the neural activity of cortical somatosensory processing.

Modulation of the N30 generators of the somatosensory evoked potentials by the mirror neuron system

The N30 component of the somatosensory evoked potential is known to be modulated by sensory interference, motor action, movement ideation and observation. We introduce a new paradigm in which the observation task of another person's hand movement triggers the somatosensory stimulus, inducing the N30 response in participants. In order to identify the possible contribution of the mirror neuron network (MNN) to this early sensorimotor processing, we analyzed the N30 topography, the event-related spectral perturbation and the inter-trial coherence on single electroencephalogram (EEG) trials, and we applied swLORETA to localize the N30 sources implicated in the time-frequency domain at rest and during observation, as well as the generators differentiating these two contextual brain states. We found that N30 amplitude increase correlated with increased contralateral precentral alpha, frontal beta, and contralateral frontal gamma power spectrum, and with central and precentral alpha and parietal beta phase-locking of ongoing EEG signals. We demonstrate specific activation of the contralateral post-central and parietal cortex where the angular gyrus (BA39), an important MNN node, is implicated in this enhancement during observation. We conclude that this part of the MNN, involved in proprioceptive processing and more complex body-action representations, is already active prior to somatosensory input and may enhance N30.

Dystonia

Focal dystonias such as writer's cramp or blepharospasm are treatable with botulinum toxin injections and medications, but both therapies provide largely symptomatic relief. Because the basic abnormality in dystonia is at the synaptic level, brain modulating therapies with repetitive transcranial magnetic stimulation (rTMS) may well be able to produce lasting clinical improvement. Low-frequency threshold or subthreshold rTMS over the premotor cortex or anterior cingulate cortex, for hand dystonia and blepharospasm, respectively, could in the future become a more curative treatment, perhaps in conjunction with the current therapies.

Continuous theta burst stimulation of the supplementary motor area: effect upon perception and somatosensory and motor evoked potentials

BACKGROUND

The supplementary motor area (SMA) has been implicated in many aspects of movement preparation and execution. In addition to motor roles, the SMA is responsive to somesthetic stimuli though it is unclear exactly what role the SMA plays in a somatosensory network.

OBJECTIVE/HYPOTHESIS

It is the purpose of this study to assess how continuous theta burst stimulation (cTBS) of the SMA affects both somatosensory (SEPs) and motor evoked potentials (MEPs) and if cTBS leads to alterations in tactile perception thresholds of the index fingertip.

METHODS

In experiment 1, cTBS was delivered over scalp sites FCZ (SMA stimulation) (n = 10) and CZ (control stimulation) (n = 10) in separate groups for 40 s (600 pulses) at 90% of participants' resting motor threshold. For both groups, median nerve SEPs were elicited from the right wrist at rest via electrical stimulation (0.5 ms pulse) before and at 10 min intervals post-cTBS out to 30 min (t = pre, 10, 20, and 30 min). Subjects' perceptual thresholds were assessed at similar time intervals as the SEP data using a biothesiometer (120 Hz vibration). In experiment 2 (n = 10) the effect of cTBS to SMA upon single and paired-pulse MEP amplitudes from the right first dorsal interosseous (FDI) was assessed.

RESULTS

cTBS to scalp site FCZ (SMA stimulation) reduced the frontal N30 SEP and increased tactile perceptual thresholds 30 min post-stimulation. However, parietal SEPs and MEP amplitudes from both single and paired-pulse stimulation were unaffected at all time points post-stimulation. cTBS to stimulation site CZ (control) did not result in any physiological or behavioral changes.

CONCLUSION(S)

These data demonstrate cTBS to the SMA reduces the amplitude of the N30 coincident with an increase in vibration sensation threshold but does not affect primary somatosensory or motor cortex excitability. The SMA may play a significant role in a somatosensory tactile attention network.

Transcranial magnetic stimulation for the treatment of depression

Repeated daily left prefrontal transcranial magnetic stimulation (TMS) was first proposed as a potential treatment for depression in 1993. Multiple studies from researchers around the world since then have repeatedly demonstrated that TMS has antidepressant effects greater than sham treatment, and that these effects are clinically meaningful. A large industry-sponsored trial, published in 2007, resulted in US FDA approval in October 2008. Most recently, a large NIH-sponsored trial, with a more rigorous sham technique, found that a course of treatment (3-5 weeks) was statistically and clinically significant in reducing depression. However, consistently showing statistically and clinically significant antidepressant effects, and gaining regulatory approval, is merely the beginning for this new treatment. As with any new treatment involving a radically different approach, there are many unanswered questions about TMS, and the field is still rapidly evolving. These unanswered questions include the appropriate scalp location, understanding the mechanisms of action, refining the 'dose' (frequency, train, number of stimuli/day and pattern of delivery), understanding whether and how TMS can be combined with medications or talking/exposure therapy, or both, and how to deliver maintenance TMS. This article summarizes the available clinical information, and discusses key areas where more research is needed. TMS reflects a paradigm shift in treating depression. It is a safe, relatively noninvasive, focal brain stimulation treatment that does not involve seizures or implanted wires, and does not have drug-drug interactions or systemic side effects.

Frontal phasic and oscillatory generators of the N30 somatosensory evoked potential

The N30 component of somatosensory evoked potentials has been recognized as a crucial index of brain sensorimotor processing and has been increasingly used clinically. Previously, we have shown that the N30 is accompanied by both an increase of the power spectrum of the ongoing beta-gamma EEG (event related synchronization, ERS) and by a reorganization (phase-locking) of the spontaneous phase of this rhythm (inter-trials coherency, ITC). In order to localize its sources taking into account both the phasic and oscillatory aspects of the phenomenon, we here apply swLORETA methods on averaged signals of the event-related potential (ERP) from a 128 scalp-electrodes array in time domain and also on raw EEG signals in frequency domain at the N30 peak latency. We demonstrate that the two different mechanisms that generate the N30 component power increase (ERS) and phase locking (ITC) across EEG trials are spatially localized in overlapping areas in the precentral cortex, namely the motor cortex (BA4) and the premotor cortex (BA6). From this common region, the generator of the N30 event-related potential expands toward the posterior part of BA4, the anterior part of BA6 and the prefrontal cortex (BA9). These latter areas also present significant ITC sources in the beta-gamma frequency range, but without significant power increase of this rhythm. This demonstrates that N30 results from network activity that depends on distinct oscillating and phasic generators localized in the frontal cortex.

Non-dominant hand movement facilitates the frontal N30 somatosensory evoked potential

Background

Previous literature has shown that the frontal N30 is increased during movement of the hand contralateral to median nerve stimulation. This finding was a result of non-dominant left hand movement in right-handed participants. It is unclear however if the effect depends upon non-dominant hand movement or if this is a generalized phenomenon across the upper-limbs. This study tests the effect of dominant and non-dominant hand movement upon contralateral frontal and parietal somatosensory evoked potentials (SEPs) and further tests if this relationship persists in left hand dominant participants. Median nerve SEPs were elicited from the wrist contralateral to movement in both right hand and left hand dominant participants alternating the movement hand in separate blocks. Participants were required to volitionally squeeze (~ 20% of a maximal voluntary contraction) a pressure-sensitive bulb every ~3 seconds with the hand contralateral to median nerve stimulation. SEPs were continuously collected during the task and individual traces were grouped into time bins relative to movement according to the timing of components of the Bereitschaftspotential. SEPs were then averaged and quantified from both FCZ and CP3/4 scalp electrode sites during both the squeeze task and at rest.

Results

The N30 is facilitated during non-dominant hand movement in both right and left hand dominant individuals. There was no effect for dominant hand movement in either group.

Conclusions

N30 amplitude increase may be a result of altered sensory gating from motor areas known to be specifically active during non-dominant hand movement.

A review of combined TMS-EEG studies to characterize lasting effects of repetitive TMS and assess their usefulness in cognitive and clinical neuroscience

Transcranial magnetic stimulation (TMS) has developed into a powerful tool for studying human brain physiology and brain-behavior relations. When applied in sessions of repeated stimulation, TMS can lead to changes in neuronal activity/excitability that outlast the stimulation itself. Such aftereffects are at the heart of the offline TMS protocols in cognitive neuroscience and neurotherapeutics. However, whether these aftereffects are of applied interest critically depends on their magnitude and duration, which should fall within an experimentally or clinically useful range without increasing risks and adverse effects. In this short review, we survey combined TMS-EEG studies to characterize the TMS-aftereffects as revealed by EEG to contribute to the characterization of the most effective and promising repetitive TMS-parameters. With one session of conventional repetitive TMS (of fixed pulse frequency), aftereffects were consistently comparable in magnitude to EEG-changes reported after learning or with fatigue, and were short-lived (<70 min). The few studies using recently developed protocols (such as theta burst stimulation) suggest comparable effect-size but longer effect-durations. Based on the reviewed data, it is expected that TMS-efficacy can be further promoted by repeating TMS-sessions, by using EEG-gated TMS to tailor TMS to current neuronal state, or by other, non-conventional TMS-protocols. Newly emerging developments in offline TMS research for cognitive neuroscience and neurotherapeutics are outlined.

Movement gating of beta/gamma oscillations involved in the N30 somatosensory evoked potential

Evoked potential modulation allows the study of dynamic brain processing. The mechanism of movement gating of the frontal N30 component of somatosensory evoked potentials (SEP) produced by the stimulation of the median nerve at wrist remains to be elucidated. At rest, a power enhancement and a significant phase-locking of the electroencephalographic (EEG) oscillation in the beta/gamma range (25-35 Hz) are related to the emergence of the N30. The latter was also perfectly identified in presence of pure phase-locking situation. Here, we investigated the contribution of these rhythmic activities to the specific gating of the N30 component during movement. We demonstrated that concomitant execution of finger movement of the stimulated hand impinges such temporal concentration of the ongoing beta/gamma EEG oscillations and abolishes the N30 component throughout their large topographical extent on the scalp. This also proves that the phase-locking phenomenon is one of the main actors for the N30 generation. These findings could be explained by the involvement of neuronal populations of the sensorimotor cortex and other related areas, which are unable to respond to the phasic sensory activation and to phase-lock their firing discharges to the external sensory input during the movement. This new insight into the contribution of phase-locked oscillation in the emergence of the N30 and in its gating behavior calls for a reappraisal of fundamental and clinical interpretation of the frontal N30 component.