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At least 145 records · Page 8Linked to original sources
PubMed2003

Focal reduction of intracortical inhibition in the motor cortex by selective proprioceptive stimulation.

The influence of proprioception on motorcortical excitability was assessed by muscle vibration (MV; 80 Hz, 0.5 mm amplitude) of the flexor carpi radialis muscle (FCR) and compared to voluntary contraction and relaxation conditions. Motor thresholds, motor-evoked potentials (MEPs) in response to single pulses of transcranial magnetic stimulation (TMS) and the intracortical inhibition (ICI) and facilitation (ICF) after paired magnetic stimuli were studied. A control experiment using TMS inducing posteriorly directed current was performed. MEPs were recorded simultaneously from the FCR, the extensor carpi radialis, the abductor pollicis brevis and the first dorsal interosseus. In the FCR, MV led to an increase of excitability shown by a decrease of motor threshold, a facilitation of MEPs in response to single-pulse TMS, a reduction of ICI and an increase of ICF. Since especially the ICI and ICF remain unchanged in other recorded muscles, this increase of excitability is specific for the vibrated muscle. With posteriorly directed current the ICI in the FCR was reduced as well, showing an involvement of later I-waves. We suggest that MV induces a focused motorcortical activation which relies on a reduced activity of intracortical inhibitory interneuronal circuits targeting selectively the motorcortical representation of the vibrated muscle.

Adult↗
PubMed1999

Transcranial magnetic stimulation: applications in neuropsychiatry.

In the 1990s, it is difficult to open a newspaper or watch television and not find someone claiming that magnets promote healing. Rarely do these claims stem from double-blind, peer-reviewed studies, making it difficult to separate the wheat from the chaff. The current fads resemble those at the end of the last century, when many were falsely touting the benefits of direct electrical and weak magnetic stimulation. Yet in the midst of this popular interest in magnetic therapy, a new neuroscience field has developed that uses powerful magnetic fields to alter brain activity--transcranial magnetic stimulation. This review examines the basic principles underlying transcranial magnetic stimulation, and describes how it differs from electrical stimulation or other uses of magnets. Initial studies in this field are critically summarized, particularly as they pertain to the pathophysiology and treatment of neuropsychiatric disorders. Transcranial magnetic stimulation is a promising new research and, perhaps, therapeutic tool, but more work remains before it can be fully integrated in psychiatry's diagnostic and therapeutic armamentarium.

Animals↗
PubMed2005

Transcranial magnetic and electrical stimulation compared: does TES activate intracortical neuronal circuits?

OBJECTIVE: To determine whether, and under which conditions, transcranial electrical stimulation (TES) and transcranial magnetic stimulation (TMS) can activate similar neuronal structures of the human motor cortex, as indicated by electromyographic recordings. METHODS: Focal TMS was performed on three subjects inducing a postero-anterior directed current (p-a), TES with postero-anteriorly (p-a) and latero-medially (l-m) oriented electrodes. We analyzed the onset latencies and amplitudes (single-pulse) and intracortical inhibition and excitation (paired-pulse). RESULTS: TMS p-a and TES p-a produced muscle responses with the same onset latency, while TES l-m led to 1.4-1.9 ms shorter latencies. Paired-pulse TMS p-a and TES p-a induced inhibition at short inter-stimulus intervals (ISI) (maximum: 2-3 ms) and facilitation at longer ISIs (maximum: 10 ms). No inhibition but a strong facilitation was obtained from paired-pulse TES l-m (ISIs 1-5 ms). CONCLUSIONS: Our findings support the hypothesis, that current direction is the most relevant factor in determining the mode of activation for both TMS and TES: TMS p-a and TES p-a are likely to activate the corticospinal neurons indirectly. In contrast, TES l-m may preferentially activate the corticospinal fibres directly, distant of the neuronal body. SIGNIFICANCE: TES is a suitable tool to induce intracortical inhibition and excitation.

Adult↗
PubMed2006

Transcranial magnetic stimulation of frontal oculomotor regions during smooth pursuit.

Both the frontal eye fields (FEFs) and supplementary eye fields (SEFs) are known to be involved in smooth pursuit eye movements. It has been shown recently that stimulation of the smooth-pursuit area of the FEF [frontal pursuit area (FPA)] in monkey increases the pursuit response to unexpected changes in target motion during pursuit. In the current study, we applied transcranial magnetic stimulation (TMS) to the FPA and SEF in humans during sinusoidal pursuit to assess its effects on the pursuit response to predictable, rather than unexpected, changes in target motion. For the FPA, we found that TMS applied immediately before the target reversed direction increased eye velocity in the new direction, whereas TMS applied in mid-cycle, immediately before the target began to slow, decreased eye velocity. For the SEF, TMS applied at target reversal increased eye velocity in the new direction but had no effect on eye velocity when applied at mid-cycle. TMS of the control region (leg region of the somatosensory cortex) did not affect eye velocity at either point. Previous stimulation studies of FPA during pursuit have suggested that this region is involved in controlling the gain of the transformation of visual signals into pursuit motor commands. The current results suggest that the gain of the transformation of predictive signals into motor commands is also controlled by the FPA. The effect of stimulation of the SEF is distinct from that of the FPA and suggests that its role in sinusoidal pursuit is primarily at the target direction reversal.

Adult↗
PubMed1978

[Ultrastructural changes in the cerebral cortex following transcranial micropolarization].

Electron microscopy of the cerebral cortex in cats and monkeys following transcranial micropolarization (TCMP) demonstrated ultrastructural changes whose degree was dependent on the electric current intensity and the stimulation period. In the focus of stimulation the current affected the brain tissue directly, different elements of the cerebral cortex showing unegual sensitivity to various TCMP regimens. The glia was the first to respond, then the neuronal bodies, and the last -- the synaptic structures. In the areas distant from the TCMP focus synaptic components altered first. The ultrastructural changes revealed were not of pathological character.

Animals↗
PubMed1996

Activation and suppression of the sternocleidomastoid muscle induced by transcranial magnetic stimulation.

Responses in the stemocleidomastoid muscle (SCM) induced by transcranial magnetic stimulation (TMS) were investigated in 10 healthy subjects. Stimuli were given with the Dantec MagLiteTM magnetic stimulator using a 12.5 cm circular coil with counter-clockwise current direction. Monopolar needle electrodes with isolated shafts were used for simultaneous bilateral electromyographic (EMG) recordings of the SCM. TMS given on either side invariably induced an ipsilateral motor evoked potential of the SCM (SCM-MEP), whereas a contralateral SCM-MEP was just seen in 25% of the performed stimulation series also when maximal intensity was used. The SCM-MEPs recorded ipsilaterally to the side of stimulation had significantly higher maximal amplitudes (P < 0.05) compared to the SCM-MEPs recorded contralaterally (mean +/- S.D.: 1.0 +/- 0.5 and 0.2 +/- 0.1 mV, respectively). These results support the concept of a predominantly ipsilateral control of SCM activation. The contralateral SCM-MEPs tended to have a shorter latency than the ipsilateral SCM-MEPs. The thresholds of the ipsilateral SCM-MEPs were significantly higher (P < 0.01) on the left side than on the right side (mean +/- S.D.: 78 +/- 18 and 60 +/- 16 A/microseconds, respectively), which could be due to the consistent use of counter-clockwise coil current direction. TMS given on either side induced suppression of voluntary SCM activity in all investigated subjects. Responses induced by TMS given ipsilaterally and contralaterally to the voluntary activated muscle did not differ significantly (P > or = 0.05) regarding threshold or latency of the suppression.

Adult↗
PubMed1994

The effect of magnetic coil orientation on the latency of surface EMG and single motor unit responses in the first dorsal interosseous muscle.

We examined the effect of the orientation of a figure-of-eight coil on the latency of surface electromyographic (EMG) responses and the firing pattern of single motor units evoked in the first dorsal interosseous muscle by transcranial magnetic brain stimulation. Two coil positions were used: the coil held on a parasagittal line either with the induced current in the brain flowing in a postero-anterior direction (PA) or with the current flowing latero-medially (LM). The results were compared with those observed after anodal electrical stimulation. LM stimulation produced surface and single unit responses which occurred 0-3 msec earlier than PA stimulation. In many cases responses to LM stimulation had the same latency as those produced by anodal electrical stimulation. Responses evoked by LM stimulation were less affected by changes in motor cortical excitability (cortico-cortical inhibition and transcallosal inhibition) than those to PA stimulation. We suggest that LM stimulation can sometimes stimulate corticospinal fibres directly, at or near the same site as anodal stimulation. In contrast, PA stimulation tends to activate corticospinal fibres trans-synaptically. The difference in stimulation sites may make a comparison of PA and LM stimulation a useful method of localising changes in corticospinal excitability to a cortical level.

Adult↗
PubMed2003

Interference with vision by TMS over the occipital pole: a fourth period.

We investigated the effect of single-pulse transcranial magnetic stimulation (TMS) over the occipital pole on a forced-choice visual letter-identification task. Magnetic stimuli were applied on the midline but with the initial current directed pseudorandomly toward either left or right hemisphere; visual stimuli were presented randomly in either left or right hemifield; magnetic-visual stimulus onset asynchrony varied randomly between 12 values: -500 ms and from -50 ms to +50 ms in 10 ms steps. The data revealed the existence of a hitherto unknown fourth task-interfering TMS effect that was maximal at -10 ms and specific for magnetic stimulus polarity and visual stimulus location. This -10 ms effect cannot be explained by reflex blinking (as the -50 ms effect can) and direct disruption of letter-induced activity (as the +20 ms and +100 ms effects can), but it could be explained by direct disruption of pre-letter activity or indirect disruption of letter-induced activity.

Adult↗
PubMed2006

Half sine, monophasic and biphasic transcranial magnetic stimulation of the human motor cortex.

OBJECTIVE: To compare half sine transcranial magnetic stimuli (TMS) with conventional monophasic and biphasic stimuli, measuring resting and active motor threshold, motor evoked potential (MEP) input/output curve, MEP latency, and silent period duration. METHODS: We stimulated the dominant hand representation of the motor cortex in 12 healthy subjects utilising two different MagPro stimulators to generate TMS pulses of distinct monophasic, half sine and biphasic shape with anteriorly or posteriorly directed current flow. RESULTS: The markedly asymmetric monophasic pulse with a posterior current flow in the brain yielded a higher motor threshold, a less steep MEP input/output curve and a longer latency than all other TMS types. Similar but less pronounced results were obtained with a less asymmetric half sine pulses. The biphasic stimuli yielded the lowest motor threshold and a short latency, particularly with the posterior current direction. CONCLUSIONS: The more asymmetric the monophasic pulse, the stronger the difference to biphasic pulses. The 3rd and 4th quarter cycle of the biphasic waveform make it longer than any other waveform studied here and likely contribute to lowering motor threshold, shortening MEP latency and reversing the influence of current direction. SIGNIFICANCE: This systematic comparison of 3 waveforms and two current directions allows a better understanding of the mechanisms of TMS.

Adult↗
PubMed2005

Remote effects of voluntary teeth clenching on excitability changes of the human hand motor area.

The aim of the present study was to investigate effects of voluntary teeth clenching (VTC) on motor evoked potentials (MEPs) from the first dorsal interosseous (FDI) muscle to transcranial magnetic stimulation (TMS) by different oriented currents (anterior-medially (AM), posterior-laterally (PL)) of the human motor cortex. In active FDI, VTC enhanced MEP responses induced by AM directed current but reduced these responses induced by PL. In relaxed FDI, VTC enhanced MEP responses by AM but had no significant effects on those by PL. Thus, the results suggest that any components produced by AM directed current were enhanced, whereas those by PL directed currents were not affected or reduced. The present evidence indicates that I-waves recorded at the same latency were not completely the same between those produced by AM and PL directed currents. Because VTC had no influence on responses to brainstem electrical stimulation (BES) or F-waves just after the onset of teeth clenching [T. Furubayashi, K. Sugawara, T. Kasai, A. Hayashi, R. Hanajima, Y. Shiio, N.K. Iwara, Y. Ugawa, Remote effects of self-paced teeth clenching on the excitability of hand motor area, Exp. Brain Res., 148 (2003) 261-265], these modulatory effects on MEPs to both AM and PL directed currents must be due to changes of the motor cortical excitability. Thus, we conclude that VTC affects the motor cortical circuits activated by PL and AM directed currents differentially; it facilitates the one and inhibits the other. This is the first demonstration of opposite effects of the same maneuver on MEP responses elicited by AM and PL directed currents.

Adult↗
PubMed2002

Efficacy of repetitive transcranial magnetic stimulation (rTMS) in the treatment of affective disorders.

Transcranial magnetic stimulation (TMS) is a relatively noninvasive technique to interfere with the function of small cortical areas through currents induced by alternating magnetic fields emanating from a handheld coil placed directly above the targeted area. This technique has clear effects on a whole range of measures of brain function and has become an important research tool in neuropsychiatry. More recently, TMS has been studied in psychiatry mainly to assess its putative therapeutic effects in treatment refractory major depression. Most studies indicate that both low-frequency TMS and higher (20 Hz) frequency repetitive TMS may have some antidepressant properties. However, definite therapeutic effects of clinical significance still remain to be demonstrated.

Electromagnetic Phenomena↗
PubMed1991

Optimal transcranial magnetic stimulation sites for the assessment of motor function.

The optimal placement sites for eliciting motor evoked potentials from the abductor digiti minimi and abductor hallucis muscles by means of transcranial magnetic stimulation were determined using a commercially available circular coli. Fifty volunteers were used for the study. The ability to elicit responses was found to be strongly dependent upon the scalp placement of the stimulator coil. The effects of altering the direction of current flow in the coil were tested on two different sets of volunteers: clockwise in one and counterclockwise in the other. It influenced only the locations of the sites which were optimal for eliciting responses from the abductor hallucis muscles and not those which were optimal for eliciting responses from the abductor digiti minimi muscles. Response latencies were found to be significantly dependent only upon volunteers' heights and not on their sex, age, or weight or the stimulus intensities used to elicit responses. No previous studies have defined the optimal scalp placements for eliciting responses from the lower extremities. This information may have clinical importance for making reliable assessments of patients with significantly impaired motor function.

Action Potentials↗
PubMed1989

Investigation of facial motor pathways by electrical and magnetic stimulation: sites and mechanisms of excitation.

A refined technique is described for non invasive examination of the facial motor pathways by stimulation of the extra- and intracranial segment of the facial nerve and the facial motor cortex. Surface recordings from the nasalis muscle rather than from the orbicularis oris muscle were used, since the compound muscle action potential (CMAP) from this muscle showed a more clearly defined onset. Electrical extracranial stimulation of the facial nerve at the stylomastoid fossa in 14 healthy subjects yielded a mean distal motor latency of 3.7 ms (SD 0.46), comparable with reported latencies to the orbicularis oris muscle. Using a magnetic stimulator, transcranial stimulation of the facial nerve was performed. The mechanism of transcranial magnetic facial nerve stimulation was studied using recordings on 12 patients who had facial nerve lesions at different locations, and with intraoperative direct measurements in four patients undergoing posterior fossa surgery. The actual site of stimulation could be localised to the proximal part of the facial canal, and a mean "transosseal conduction time" of 1.2 ms (SD 0.18) was calculated. The cerebrospinal fluid (CSF) played an important role in mediating the magnetically induced stimulating currents. Finally, with transcranial magnetic stimulation of the facial motor cortex, clearly discernible CMAPs could be produced when voluntary activation of several facial muscles was used to facilitate the responses. From this, a central motor conduction time of 5.1 ms was calculated (SD 0.60, 6 subjects).

Action Potentials↗
PubMed1995

Task-related variation in corticospinal output evoked by transcranial magnetic stimulation in the macaque monkey.

1. A volley evoked by transcranial magnetic stimulation (TMS) over the motor cortex was recorded from the medullary pyramid in an awake monkey performing a precision grip task. It was identified as corticospinal using a collision test. 2. The volley latency was 0.50 ms, indicating that it was produced by direct activation of corticospinal neurones. 3. A mean modulation of 13% in the amplitude of this volley was seen during task performance, with the largest volley occurring during the hold phase of the task. A similar pattern of modulation was seen in the EMG responses of hand and forearm muscles to TMS. 4. No comparable modulation was observed in a volley evoked by electrical stimulation of the corticospinal fibres via chronically implanted electrodes in the cerebral peduncle. 5. The results are compatible with direct activation of the corticospinal neurones by TMS at a site close to the soma, with the probability of activation by TMS depending on the current level of cortical excitability.

Animals↗
PubMed2002

A coil design for transcranial magnetic stimulation of deep brain regions.

Noninvasive magnetic stimulation of the human central nervous system has been used in research and the clinic for several years. However, the coils used previously stimulated mainly the cortical brain regions but could not stimulate deeper brain regions directly. The purpose of the current study was to develop a coil to stimulate deep brain regions. Stimulation of the nucleus accumbens and the nerve fibers connecting the prefrontal cortex with the nucleus accumbens was one major target of the authors' coil design. Numeric simulations of the electrical field induced by several types of coils were performed and accordingly an optimized coil for deep brain stimulation was designed. The electrical field induced by the new coil design was measured in a phantom brain and compared with the double-cone coil. The numeric simulations show that the electrical fields induced by various types of coils are always greater in cortical regions (closer to the coil placement); however, the decrease in electrical field within the brain (as a function of the distance from the coil) is markedly slower for the new coil design. The phantom brain measurements basically confirmed the numeric simulations. The suggested coil is likely to have the ability of deep brain stimulation without the need to increase the intensity to levels that stimulate cortical regions to a much higher extent and possibly cause undesirable side effects.

Brain↗
PubMed1990

Quantitative evaluation of hemiparesis with corticomyographic motor evoked potential recorded by transcranial magnetic stimulation.

Corticomyographic motor evoked potentials (MEP) activated by transcranial magnetic stimulation of the motor cortex provide clinicians with an opportunity to evaluate corticospinal motor systems quantitatively and noninvasively. Threshold, amplitude, and latency of the corticomyographic MEP, however, are variable between subjects mainly because current directions and intensities induced by magnetic stimulation cannot be determined precisely due to anatomical variations of subjects. The variability of corticomyographic MEPs has limited the use of corticomyographic MEP for evaluating mild changes in corticospinal motor function. In the present study, we used an internal standard to assess hemiplegia, expressing relative amplitude, latency, and threshold of responses on the paretic side as a function of responses elicited from the intact side (%MEP). Neurological function of paretic muscles, as determined by a muscle maneuver test (MMT), clearly correlated to %MEP threshold, amplitude, and latency. Since corticomyographic MEP are similar when recorded from symmetrical sites on two extremities of normal subjects, %MEP provided a sensitive measure of mild hemiparesis. The %MEP approach revealed abnormal MMT scores of 3 or 4 more frequently than did standard MEP approaches. %MEP amplitude was more sensitive to mild hemiparesis than %MEP latency or %MEP threshold. Since magnetic stimulation with a safe intensity range cannot reliably produce corticomyographic MEP in severely paretic muscles with MMT scores of 2 or less, the MEP appears to be most useful for evaluating mild hemiparesis. This technique should expand significantly the clinical usefulness of corticomyographic MEP in neurosurgical practice.

Adolescent↗
PubMed2006

Magnetic stimulation of human premotor or motor cortex produces interhemispheric facilitation through distinct pathways.

We explored interhemispheric facilitation (IHF) between (a) left and right primary motor cortex (M1) and (b) left dorsal premotor (dPM) and right M1 in 20 right-handed healthy human subjects using a paired pulse transcranial magnetic stimulation (TMS) protocol. A conditioning TMS pulse (CP) applied to left M1 or dPM with an intensity of 80% and 60% active motor threshold (CP(80%AMT) and CP(60%AMT), respectively) was followed by a test pulse (TP) over right M1 induced by anterior-posterior- or posterior-anterior- (TP(AP), TP(PA)) directed currents in the brain at interstimulus intervals (ISIs) of 3-8 and 10 ms. EMG was recorded from left first dorsal interosseous muscle. In the main experimental condition IHF was evoked by CP(80%AMT) over left M1 and TPAP at ISIs of 6 and 8 ms. The same CP(80%AMT) produced IHF at an ISI of 8 ms when applied over left dPM but only with TP(PA). In addition, when CP(60%AMT) was given to M1, IHF was present at an ISI of 6 ms (but not 8 ms) when followed by TP(PA), indicating that IHF elicited over dPM was not caused by current spread of the conditioning pulse to M1. We conclude that IHF can be induced differentially by conditioning M1 and dPM using subthreshold CP. These facilitatory interactions depended on the intensity and ISI of the CP as well as the current flow direction of the TP. We suggest that not only do the CPs activate separate anatomical pathways but also that these pathways project to different populations ofinterneurons in the receiving M1. These may correspond to elements involved in the generation of I3 and I1 waves, respectively.

Adult↗
PubMed2007

The contribution of the human PPC to the orienting of visuospatial attention during smooth pursuit.

Smooth pursuit eye movements function to stabilize the retinal image of small moving targets. In order for those targets to be foveated, however, they must first be "captured" by an attentional mechanism which then interacts with the oculomotor system. Cortical sites involved with producing smooth pursuit overlap with areas known to be involved in directing visuospatial attention, particularly the posterior parietal cortex (PPC). The goal of the current study was to characterize the contributions made by the left and right posterior parietal cortices (lPPC and rPPC) to the interaction between visuospatial attention and the generation of smooth pursuit eye movements. Transcranial magnetic stimulation (TMS) was used to temporarily disrupt each area at different times around target motion onset in a pursuit task that explicitly manipulated the covert orienting of attention. TMS over the lPPC, rPPC and a control site (the vertex) evoked a similar pattern of results, in that the earlier TMS delivery times caused a reduced pursuit latency compared to baseline measures, while TMS immediately prior to target motion onset resulted in latencies slower than baseline. In addition, however, TMS over the lPPC and rPPC (but not the vertex) preferentially influenced the generation of contralateral pursuit, with the lPPC doing so in a relatively time-independent manner, and the rPPC doing so in a time-dependent manner. This pattern of results implies that both the left and right PPC are directly involved in the interaction between attention and smooth pursuit preparation.

Adult↗