Subthreshold prefrontal repetitive transcranial magnetic stimulation reduces motor cortex excitability

A systematic review of non-motor rTMS induced motor cortex plasticity

Motor cortex excitability can be measured by single- and paired-pulse transcranial magnetic stimulation (TMS). Repetitive transcranial magnetic stimulation (rTMS) can induce neuroplastic effects in stimulated and in functionally connected cortical regions. Due to its ability to non-invasively modulate cortical activity, rTMS has been investigated for the treatment of various neurological and psychiatric disorders. However, such studies revealed a high variability of both clinical and neuronal effects induced by rTMS. In order to better elucidate this meta-plasticity, rTMS-induced changes in motor cortex excitability have been monitored in various studies in a pre-post stimulation design. Here, we give a literature review of studies investigating motor cortex excitability changes as a neuronal marker for rTMS effects over non-motor cortical areas. A systematic literature review in April 2014 resulted in 29 articles in which motor cortex excitability was assessed before and after rTMS over non-motor areas. The majority of the studies focused on the stimulation of one of three separate cortical areas: the prefrontal area (17 studies), the cerebellum (8 studies), or the temporal cortex (3 studies). One study assessed the effects of multi-site rTMS. Most studies investigated healthy controls but some also stimulated patients with neuropsychiatric conditions (e.g., affective disorders, tinnitus). Methods and findings of the identified studies were highly variable showing no clear systematic pattern of interaction of non-motor rTMS with measures of motor cortex excitability. Based on the available literature, the measurement of motor cortex excitability changes before and after non-motor rTMS has only limited value in the investigation of rTMS related meta-plasticity as a neuronal state or as a trait marker for neuropsychiatric diseases. Our results do not suggest that there are systematic alterations of cortical excitability changes during rTMS treatment, which calls into question the practice of re-adjusting the stimulation intensity according to the motor threshold over the course of the treatment.

The EEG correlates of the TMS-induced EMG silent period in humans

Application of magnetic or electrical stimulation to the motor cortex can result in a period of electromyography (EMG) silence in a tonically active peripheral muscle. This period of EMG silence is referred to as the silent period (SP). The duration of SP shows intersubject variability and reflects the integrity of cortical and corticospinal pathways. A non-invasive technique for assessing the duration of SP is the combination of Transcranial Magnetic Stimulation (TMS) with EMG. Utilizing TMS-EMG, several studies have reported on the shortening or lengthening of SP in neuropsychiatric disorders such as schizophrenia, bipolar disorder, depression, obsessive compulsive disorder, epilepsy, Parkinson's disease, and stroke. However, cortical, corticospinal and peripheral components are difficult to disentangle from EMG alone. Here, we use the multimodal neuroimaging technique of TMS-EMG combined with concurrent electroencephalography (EEG) recording to further examine the cortical origin of SP and the cortical oscillatory activity that underlies SP genesis. We demonstrate that the duration of SP is related to the temporal characteristics of the cortical reactivity and the power of delta to alpha oscillations in both local and remote areas ipsilateral and contralateral to the stimulation site, and beta oscillations locally. We illustrate that, compared to EMG, the EEG indices of the SP provide additional information about the brain dynamics and propose that the EEG measures of SP may be used in future clinical and research investigations to more precisely delineate the mechanisms underlying inhibitory impairments.

Dorsolateral prefrontal cortex: a possible target for modulating dyskinesias in Parkinson's disease by repetitive transcranial magnetic stimulation

We studied whether five sessions of 10 Hz repetitive transcranial magnetic stimulation (rTMS treatment) applied over the dorsolateral prefrontal cortex (DLPFC) or the primary motor cortex (MC) in advanced Parkinson's disease (PD) patients would have any effect on L-dopa-induced dyskinesias and cortical excitability. We aimed at a randomised, controlled study. Single-pulse transcranial magnetic stimulation (TMS), paired-pulse transcranial magnetic stimulation, and the Unified Parkinson's Disease Rating Scale (UPDRS parts III and IV) were performed prior to, immediately after, and one week after an appropriate rTMS treatment. Stimulation of the left DLPFC induced a significant motor cortex depression and a trend towards the improvement of L-dopa-induced dyskinesias.

Evaluating the role of prefrontal and parietal cortices in memory-guided response with repetitive transcranial magnetic stimulation

The dorsolateral prefrontal cortex (dlPFC) plays an important role in working memory, including the control of memory-guided response. In this study, with 24 subjects, we used high frequency repetitive transcranial magnetic stimulation (rTMS) to evaluate the role of the dlPFC in memory-guided response to two different types of spatial working memory tasks: one requiring a recognition decision about a probe stimulus (operationalized with a yes/no button press), another requiring direct recall of the memory stimulus by moving a cursor to the remembered location. In half the trials, randomly distributed, rTMS was applied to the dlPFC and in a separate session, the superior parietal lobule (SPL), a brain area implicated in spatial working memory storage. A 10-Hz (3s, 110% of motor threshold) train of TMS was delivered at the onset of the response period. We found that only dlPFC rTMS significantly affected performance, with rTMS of right dlPFC decreasing accuracy on delayed-recall trials, and rTMS of left and right dlPFC decreasing and enhancing accuracy, respectively, on delayed-recognition trials. These findings confirm that the dlPFC plays an important role in memory-guided response, and suggest that the nature of this role varies depending on the processes required for making a response.

Repetitive Transcranial Magnetic Stimulation (rTMS) in the treatment of panic disorder (PD) with comorbid major depression

BACKGROUND

Studies suggest that the dorsolateral prefrontal cortex (DLPFC) participates in neural circuitry that is dysregulated in Panic Disorder (PD) and Major Depressive Disorder (MDD). We tested whether low-frequency repetitive Transcranial Magnetic Stimulation (rTMS) could normalize the overactivity of right frontal regions and thereby improve symptoms.

METHODS

Six patients with PD and comorbid MDD were treated with daily active 1-Hz rTMS to the right DLPFC for 2 weeks in this open-label trial.

RESULTS

Clinical improvements were apparent as early as the first week of treatment. After the second week, 5/6 of patients showed improvements in panic and anxiety, and 4/6 showed a decrease in depression, with sustained improvement at 6 months of follow-up. Right hemisphere resting motor threshold increased significantly after rTMS.

LIMITATIONS

Limitations of this study are the open design and the small sample size.

CONCLUSIONS

Slow rTMS to the right DLPFC resulted in significant clinical improvement and reduction of ipsilateral motor cortex excitability. Replications in larger sample will help to clarify the relevance of this preliminary data and to define the potential role of right DLPFC rTMS in panic with major depression.

Therapeutic and neurophysiologic aspects of transcranial magnetic stimulation in schizophrenia

The use of repetitive transcranial magnetic stimulation (rTMS) in psychiatry provides the therapeutic field with a new tool. Since its introduction in the mid 1980s, the vast majority of studies have focussed on depression. A growing body of evidence suggests that rTMS is effective in the treatment of depression if dorsolateral prefrontal cortex is stimulated. Less is known about its efficacy in schizophrenia. Neuroimaging investigations in schizophrenia suggest abnormalities in the prefrontal and temporoparietal cortex (TPC), which are correlated with psychopathological dimensions. Based on its modulatory effect, rTMS seems to be a promising tool in exploring cortical excitability and reducing auditory hallucinations (AH) and negative symptoms. Neurophysiologic studies of patients suffering from schizophrenia using rTMS indicate high cortical excitability and a lack of transcallosal inhibition. In the therapeutic field, researches provide encouraging results, even though some studies indicate limited benefits. The most promising therapeutic effect seems to be the capability of rTMS to reduce AH if TPC is targeted using slow-frequency. The current paper aims to provide a review of the literature of the use of rTMS in schizophrenia.

Functional lesions and human action monitoring: combining repetitive transcranial magnetic stimulation and event-related brain potentials

OBJECTIVE

Electrophysiological recordings of the error-related negativity (ERN) and functional imaging data point to an involvement of medial frontal cortex (including the anterior cingulate cortex, ACC) and dorsolateral prefrontal cortex (DLPFC) in the detection and correction of performance errors. Here, we studied this network by applying trains of rapid transcranial magnetic stimulation (rTMS) prior to the recording of the ERN.

METHODS

Low-frequency (0.9 Hz) rTMS was applied to medial frontal or lateral frontal regions (different sessions) for 60 s immediately before each 3 min ERN recording in 11 healthy young subjects. The ERN was obtained by multichannel recordings in a typical Eriksen flanker task with instructions calling for immediate error correction in case a performance error was detected by the subject. Event-related potentials were quantified and statistically evaluated using standard methodology.

RESULTS

Compared to a no-stimulation control condition, medial frontal stimulation led to a small but reliable decrease in the number of corrected errors as well as to an attenuation of the ERN and an increase of the subsequent error-positivity (Pe). No effect on these components was seen after lateral frontal stimulation. No reliable effects on the lateralized readiness potential were observed.

CONCLUSIONS

Functional lesions by rTMS appear to interfere with the functions of the medial frontal cortex in error detection and correction.

Transcranial magnetic stimulation: studying motor neurophysiology of psychiatric disorders

RATIONALE

Transcranial magnetic stimulation (TMS) is a noninvasive tool that directly stimulates cortical neurons by inducing magnetic and secondary electric fields. Traditionally TMS has been used to study the motor neurophysiology of healthy subjects and those with neurological disorders.

OBJECTIVE

Given the known motor dysfunctions in many psychiatric disorders supplemental usage of TMS to study the underlying pathophysiology of certain psychiatric disorders and to assess treatment outcomes is underway. Such studies include examination of motor neuronal membrane, corticospinal and intracortical excitability. Our objective is to overview the past findings.

METHODS

We review the past literature that used TMS as an assessment tool in psychiatric disorders such as schizophrenia, mood disorders, Tourette's syndrome, obsessive-compulsive disorder, attention-deficit hyperactivity disorder, and substance abuse.

RESULTS

While the findings are still preliminary due to small sample-size, inconsistent patient population (diagnosis, medication), differences in methodology between research groups, studies restricted to the motor region and possible lack of sensitivity and specificity, the studies are yielding interesting results which could potentially lead to trait- and state-markers of psychiatric disorders.

CONCLUSIONS

Future studies using TMS alone or in combination with other neuroimaging techniques promise to further expand the application of TMS from studies of motor excitability to higher cognitive functions.

Electrical stimulation of the sural nerve partially compensates effects of central fatigue

OBJECTIVES

Depression of motor evoked potentials (MEPs) following transcranial magnetic stimulation (TMS) may be a sign of central motor fatigue. Abnormal fatigue can be observed in MS patients. We have examined whether post-exercise MEP depression can be compensated by application of sensory stimuli prior to TMS.

METHODS

We studied 15 healthy volunteers (aged 21 to 28 years) who were required to perform an exercise protocol of ankle dorsiflexion until force fell below 66% of maximum force. MEPs were recorded from the right tibialis anterior muscle. Prior to TMS, electrical stimuli were applied to the ipsilateral sural nerve with an individual interstimulus interval between 50 to 80 ms.

RESULTS

MEP areas decreased after exercise. When a sensory stimulus was administered MEPs did not change.

DISCUSSION

We conclude that the effects of central fatigue may be--at least partially compensated--by application of sensory stimuli. Sensory stimulation (e.g. by implantation of a neurostimulator) might be a useful therapy for abnormal central fatigue.

Repetitive transcranial magnetic stimulation for the treatment of chronic pain - a pilot study

Invasive electrical stimulation of the motor cortex has been reported to be of therapeutic value in pain control. We were interested whether noninvasive repetitive transcranial magnetic stimulation (rTMS) of the primary motor cortex might also act beneficially. Twelve patients with therapy-resistant chronic pain syndromes (mean age 51.3 +/- 12.6, 6 males) were included in a pilot study. They were treated with rTMS of the corresponding motor cortex area for 20 min (20 Hz, 20 x 2 s trains, intensity 80% of motor threshold) and sham stimulation (sequence-controlled cross-over design). Some of the patients (6/6) had an analgesic effect, but for the whole group, the difference between active and sham stimulation did not reach a level of significance (active rTMS: mean VAS reduction -4.0 +/- 15.6%; sham rTMS: -2.3 +/- 8.8%). Further studies using different rTMS stimulation parameters (duration and frequency of rTMS) or stimulation sites (e.g. anterior cingulate gyrus) are strongly encouraged.

Impaired sensorimotor integration in cervical dystonia: a study using transcranial magnetic stimulation and muscle vibration

The authors studied the effects of sensorimotor integration (corticocortical inhibition and facilitation during muscle vibration [MV]) in dystonic patients. Eleven patients with cervical dystonia and 11 age-matched healthy control subjects were enrolled in the study. They were examined using transcranial magnetic stimulation (TMS) and tonic proprioceptive input (MV). Paired-pulse transcranial magnetic stimulation was done at interstimulus intervals of 3 msec (intracortical inhibition) and 13 msec, the intensity of the conditioning stimulus was 70% of the motor threshold, and the test stimulus was 120%. Motor evoked potentials were recorded from the vibrated extensor carpi radialis muscle and its antagonist, the flexor carpi radialis. Duration of MV trains (80 Hz; amplitude, 0.5 mm) was 4 seconds. The authors found differences between patients and healthy control subjects during MV only. Intracortical inhibition was pronounced significantly only in control subjects, whereas intracortical facilitation was significant in patients only (P < 0.05). Furthermore, the significant reduction of motor evoked potentials at 13-msec interstimulus intervals, which can be found in healthy subjects frequently, was observed in one dystonia patient only. The results of the current study suggest that sensorimotor integration is impaired in cervical dystonia, probably by an altered control of proprioceptive (vibratory) input.

Decrease of middle cerebral artery blood flow velocity after low-frequency repetitive transcranial magnetic stimulation of the dorsolateral prefrontal cortex

OBJECTIVES

Repetitive transcranial magnetic stimulation (rTMS) has been tried therapeutically in a variety of neuropsychiatric disorders. Both, inhibition and activation of cortical areas may be achieved using different stimulation parameters. Using low-frequency rTMS (0.9 Hz), inhibition of cortical areas can be observed.

METHODS

In the present study, 38 right-handed, healthy, normotensive subjects (aged 21-50 years, mean 30.2 years, SD=4.9; 17 women) were enrolled. Twenty-five participants received active rTMS (5 min of 0.9 Hz rTMS, stimulus intensity 90% of motor threshold) of the right dorsolateral prefrontal cortex. Sham stimulation (n=13 subjects) occurred in the same manner as active rTMS, except that the angle of the coil was at 45 degrees off the skull. Simultaneously, ipsilateral and contralateral maximal middle cerebral artery (MCA) flow velocity (and pulsatility index, PI) was monitored using transcranial Doppler sonography.

RESULTS

In the group with active rTMS, maximal MCA flow velocity decreased from a baseline (before rTMS) of 101.6 cm/s (SD=26.0) to a mean of 92.6 cm/s (SD=23.7) immediately after rTMS, T=5.06, P<0.001. This equals a mean decrease of 9.0 cm/s (SD=8.3) or approximately 8.9% of baseline flow. Five and 10 min after rTMS, there was a return to baseline. PI significantly decreased 10 min after rTMS (mean difference -0.05, SD=0.05, T=2.29, P<0.05). In the contralateral MCA, maximal flow velocity tended to increase 10 min after rTMS (mean difference +7.4 cm/s, SD=17.5; T=-2.03, P=0.054). With sham rTMS, no significant changes occurred.

CONCLUSIONS

The results from our study support the hypothesis that low-frequency rTMS may influence cerebral blood flow (CBF) over short periods of time, inducing a temporary decrease of maximal CBF in the ipsilateral MCA followed by an increase in the contralateral MCA.

Transcranial magnetic stimulation: a new tool in the fight against depression

Since its introduction to the clinical realm in 1985, transcranial magnetic stimulation (TMS) has rapidly developed into a tool for exploring central nervous system function in both health and disease. The antidepressant effects of TMS were initially observed in 1993. Since then, a solid body of evidence has accumulated suggesting antidepressant effects for both slow TMS (sTMS) and repetitive TMS (rTMS). This review is divided into four parts. First, it addresses the basic concepts governing TMS, and then, second, it discusses the technical parameters involved in administering TMS. Knowledge of these parameters is necessary for understanding how TMS is administered, and how manipulation of the technique impacts on the results obtained. Third, we review the most relevant studies on the antidepressant effects of sTMS and rTMS published to date. Finally, we discuss cortical excitability and how the understanding of this basic neurophysiological function of cortical neurons can be used for monitoring the effects of TMS. In our discussion, we conclude that the time has arrived for TMS to be offered to depressed patients as a treatment.

A transcranial magnetic stimulation study of inhibitory deficits in the motor cortex in patients with schizophrenia

It has been proposed that inhibitory deficits play a crucial role in the pathophysiological process of schizophrenia as suggested by post-mortem, neuropsychological and neurophysiological evidence. We hypothesised that patients with schizophrenia would demonstrate abnormalities of cortical inhibition in the motor cortex with single and paired pulse transcranial magnetic stimulation (TMS). Patients with DSM-IV schizophrenia (n=22) and normal volunteers (n=21) participated in the study. Electromyographic recordings from the abductor pollicis brevis (APB) muscle were made during focal TMS stimulation to the contra-lateral motor cortex. The threshold intensity to produce a motor response, the size of the motor evoked potential, the duration of the silent period, and the cortical inhibition and facilitation to paired pulse TMS were measured. The patient group demonstrated a reduction in length of the silent period and a reduction in cortical inhibition with paired stimuli. No changes were found in motor threshold, motor evoked potential size, or cortical facilitation. The study demonstrated deficits of cortical inhibition in the motor cortex of patients with schizophrenia. These deficits appear to be of cortical origin. Their relationship to dysfunction in other cortical networks requires further elucidation.

Transcranial magnetic stimulation: no effect on mood with single pulse during learned helplessness

1. Transcranial Magnetic Stimulation (TMS) is suggested to be an effective tool in the treatment of depression. However, the methodology most suitable for clinical application remains unclear. 2. The effect of TMS was tested in a double-blind and placebo-controlled setting on 18 healthy subjects. At the same time an established learned helplessness paradigm was applied to induce dysphoria, which consisted of unsolvable anagrams. 3. Sixty 0.5 Hz stimuli were administered at an intensity of 130% of the subject's motor threshold after the subjects were exposed to the learned helplessness situation. Using a vertically positioned coil, the stimuli were applied to the right or to the left frontal cortex, or on the occipital cortex as a placebo condition. 4. Although dysphoria was successfully induced by unsolvable anagrams, TMS on either of the two frontal locations did not influence mood. This lack of effect may be due to the stimulation characteristics employed here (low TMS intensity, and low frequency). On the other hand, the findings may reflect the neurobiological difference between experimentally induced sad mood and clinical depression.

Repetitive transcranial magnetic stimulation to SMA worsens complex movements in Parkinson's disease

OBJECTIVES

To evaluate the therapeutic potential of repetitive transcranial magnetic stimulation (rTMS) for Parkinson's disease (PD) by delivering stimulation at higher intensity and frequency over longer time than in previous research. Promising beneficial effects on movement during or after rTMS have been reported.

METHODS

Ten patients with idiopathic PD were enrolled in a randomized crossover study comparing active versus sham rTMS to the supplementary motor area (SMA). Assessments included reaction and movement times (RT/MT), quantitative spiral analysis, timed motor performance tests, United Parkinson's Disease Rating Scale (UPDRS), patient self-report and guess as to stimulation condition.

RESULTS

Two of 10 patients could not tolerate the protocol. Thirty to 45 min following stimulation, active rTMS as compared with sham stimulation worsened spiral drawing (P=0.001) and prolonged RT in the most affected limb (P=0.030). No other significant differences were detected.

CONCLUSIONS

We sought clinically promising improvement in PD but found subclinical worsening of complex and preparatory movement following rTMS to SMA. These results raise safety concerns regarding the persistence of dysfunction induced by rTMS while supporting the value of rTMS as a research tool. Studies aimed at understanding basic mechanisms and timing of rTMS effects are needed.

Muscle vibration and prefrontal repetitive transcranial magnetic stimulation

We previously demonstrated that prefrontal subthreshold repetitive transcranial magnetic stimulation (rTMS) may reduce motor cortex excitability. We have now examined whether muscle vibration (MV) can compensate for this depression. We enrolled 25 healthy volunteers (aged 22 to 37 years) who received 5 HZ, 10% subthreshold prefrontal rTMS for 12 s. The extensor carpi radialis muscle was vibrated with an electromagnetic mechanical stimulator with a stimulation frequency of 120 HZ and 0.5 mm amplitude. Motor evoked potentials (MEPs) from the flexor carpi radialis muscle (FCR) following single-pulse transcranial magnetic stimulation (TMS) were recorded at baseline, and after 4, 8, and 12 s. During prefrontal rTMS, MEPs of the FCR exhibited a serial depression (P = 0.001). This effect did not occur during MV. We conclude that rTMS of the prefrontal cortex may inhibit the corticospinal system. This depression may be compensated by MV, suggesting that vibration changes motor cortex excitability. The underlying mechanism might be an input from Ia sensory afferents to the motor and prefrontal cortex.

Effects of somatosensory input on central fatigue: a pilot study

OBJECTIVE

Depression of motor evoked potentials (MEPs) following transcranial magnetic stimulation (TMS) may be a sign of central motor fatigue. As a pilot study, we have examined whether post-exercise MEP depression can be compensated by application of sensory stimuli prior to TMS.

METHODS

We studied 15 healthy volunteers (aged 21-28 years) who were required to perform an exercise protocol of ankle dorsiflexion until force fell below 66% of maximum force. MEPs were recorded from the right tibialis anterior muscle. Prior to TMS, electrical stimuli were applied to the ipsilateral sural nerve with an individual interstimulus interval between 50 and 80 ms.

RESULTS

MEP areas decreased after exercise. When a sensory stimulus was administered MEPs did not change.

CONCLUSION

We conclude that the effects of central fatigue may be influenced by application of sensory stimuli.