Differential effects on motorcortical inhibition induced by blockade of GABA uptake in humans

Disturbed transcallosally mediated motor inhibition in children with attention deficit hyperactivity disorder (ADHD)

OBJECTIVE

The aim of this study was to investigate mechanisms of motor-cortical excitability and inhibition which may contribute to motor hyperactivity in children with attention deficit hyperactivity disorder (ADHD).

METHODS

Using transcranial magnetic stimulation (TMS), involvement of the motor cortex and the corpus callosum was analysed in 13 children with ADHD and 13 sex- and age-matched controls. Contralateral silent period (cSP) and transcallosally mediated ipsilateral silent period (iSP) were investigated.

RESULTS

Resting motor threshold (RMT), amplitudes of motor evoked potentials (MEP) and cSP were similar in both groups whereas iSP-latencies were significantly longer (p<0.05) and their duration shorter (p<0.01) in the ADHD group. For the ADHD group iSP duration tended to increase and iSP latency to decrease with age (n.s.). Conners-Scores did neither correlate with iSP-latencies and -duration nor with children's age.

CONCLUSIONS

The shortened duration of iSP in ADHD children could be explained by an imbalance of inhibitory and excitatory drive on the neuronal network between cortex layer III-the projection site of transcallosal motor-cortical fibers-and layer V, the origin of the pyramidal tract. The longer iSP-latencies might be the result of defective myelination of fast conducting transcallosal fibers in ADHD. iSP may be a useful supplementary diagnostic tool to discriminate between ADHD and normal children.

Interactions between inhibitory and excitatory circuits in the human motor cortex

Cortical activity depends on the balance between excitatory and inhibitory influences. Several different excitatory and inhibitory systems in the human motor cortex can be tested by transcranial magnetic stimulation (TMS). While considerable information is known about these different inhibitory and excitatory phenomena individually, how they are related to each other and how they interact is not well understood. Several recent studies have investigated the interactions between some of these circuits by applying them together. It has been found that short-interval intracortical inhibition (SICI) and long-interval intracortical inhibition (LICI) are mediated by different circuits. LICI appears to inhibit SICI, which may occur through presynaptic GABA(B) receptors. Interhemispheric inhibition elicited by stimulation of the contralateral motor cortex also inhibits SICI and may share inhibitory mechanisms with LICI. Long-interval afferent inhibition induced by median nerve stimulation inhibits LICI but does not interact with SICI. Based on these results, a model of interactions between different inhibitory systems that can be tested and refined in the future is proposed. Further studies of the interaction between different cortical inhibitory and excitatory circuits should improve our understanding of the functional organization of the motor cortex and allow better interpretation of abnormal findings in disease states. It may also be developed into a new way of studying the pathophysiology of diseases and the effects of intervention.

Differential effect of muscle vibration on intracortical inhibitory circuits in humans

Low amplitude muscle vibration (0.5 ms; 80 Hz; duration 1.5 s) was applied in turn to each of three different intrinsic hand muscles (first dorsal interosseus, FDI; abductor pollicis brevis, APB; and abductor digiti minimi, ADM) in order to test its effect on the EMG responses evoked by transcranial magnetic stimulation (TMS). Recordings were also taken from flexor and extensor carpi radialis (FCR and ECR, respectively). We evaluated the amplitude of motor evoked potentials (MEPs) produced by a single TMS pulse, short interval intracortical inhibition and facilitation (SICI and ICF) and long interval intracortical inhibition (LICI). TMS pulses were applied 1 s after the start of vibration with subjects relaxed throughout. Vibration increased the amplitude of MEPs evoked in the vibrated muscle (162 +/- 6 % of MEP with no vibration; mean +/- S.E.M.), but suppressed MEPs in the two non-vibrated hand muscles (72 +/- 9 %). Compared with no vibration (test response reduced to 51 +/- 5 % of control), there was less SICI in the vibrated muscle (test response reduced to 92 +/- 28 % of control) and more in the non-vibrated hand muscles (test response reduced to 27 +/- 5 % of control). The opposite occurred for LICI: compared with the no vibration condition (test response reduced to 33 +/- 6 % control), there was more LICI in the vibrated muscle (test response reduced to 17 +/- 3 % control) than in the non-vibrated hand muscles (test response reduced to 80 +/- 11 % control) even when the intensity of the test stimulus was adjusted to compensate for the changes in baseline MEP. There was no effect on ICF. Cutaneous stimulation of the index finger (80 Hz, 1.5 s duration, twice sensory threshold) had no consistent differential effect on any of the parameters. We conclude that vibratory input from muscle can differentially modulate excitability in motor cortical circuits.

Differential modulation of intracortical inhibition in human motor cortex during selective activation of an intrinsic hand muscle

Paired-pulse transcranial magnetic stimulation (TMS) was used to assess the effectiveness of intracortical inhibition (ICI) acting on corticospinal neurons controlling three intrinsic hand muscles in humans. We hypothesised that the suppression of ICI with selective activation of a muscle would be restricted to corticospinal neurons controlling the muscle targeted for activation. Surface EMG was recorded from abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles of the left hand. Subjects were tested at rest and during weak selective activation of APB or ADM, while they attempted to keep the other muscles relaxed using visual feedback. Paired-pulse TMS was applied with a circular coil oriented to produce antero-posterior (AP) current flow in the right motor cortex (to preferentially evoke I3 waves in corticospinal neurons) and with postero-anterior (PA) currents (to preferentially evoke I1 waves). Paired-pulse TMS was less effective in suppressing the muscle evoked potential (MEP) when the muscle was targeted for selective activation, with both AP and PA stimulation. The mechanism for this includes effects on late I waves, as it was evident with a weak AP test TMS pulse that elicited negligible I1 waves in corticospinal neurons. ICI circuits activated by TMS, which exert their effects on late I waves but do not affect I1 waves, are strongly implicated in this modulation. With AP stimulation, paired-pulse inhibition was not significantly altered for corticospinal neurons controlling other muscles of the same hand which were required to be inactive during the selective activation task. This differential modulation was not seen with PA stimulation, which preferentially activates I1 waves and evokes a MEP that is less influenced by ICI. The observations with AP stimulation suggest that selective activation of a hand muscle is accompanied by a selective suppression of ICI effects on the corticospinal neurons controlling that muscle. The pattern of differential modulation of ICI effectiveness with voluntary activation suggests that the ICI circuits assist the corticospinal system in producing fractionated activity of intrinsic hand muscles.

A transcranial magnetic stimulation study of abnormal cortical inhibition in schizophrenia

Previous research suggests that patients with schizophrenia demonstrate deficits in a range of parameters of motor cortical and cognitive inhibition. I-wave facilitation and long-interval cortical inhibition (LICI) are two paired pulse transcranial magnetic stimulation paradigms that appear to assess aspects of cortical inhibitory function that have not previously been assessed in this patient group. Eighteen patients with schizophrenia (nine medication-free) were compared with eight control subjects. We assessed resting motor threshold (RMT) levels, LICI and I-wave facilitation. RMT levels did not differ between the three groups. There was a significant overall difference in I-wave facilitation levels. Both patient groups as compared with the control group showed increased facilitation. There were no differences between the groups in the measure of LICI. Patients with schizophrenia appear to have increased I-wave facilitation. Increased I-wave facilitation suggests deficient function of cortical inhibitory GABAergic activity. This is consistent with previous research that has found deficient cortical inhibition in patients with schizophrenia.

Responses of single motor units in human masseter to transcranial magnetic stimulation of either hemisphere

The corticobulbar inputs to single masseter motoneurons from the contra- and ipsilateral motor cortex were examined using focal transcranial magnetic stimulation (TMS) with a figure-of-eight stimulating coil. Fine-wire electrodes were inserted into the masseter muscle of six subjects, and the responses of 30 motor units were examined. All were tested with contralateral TMS, and 87 % showed a short-latency excitation in the peristimulus time histogram at 7.0 +/- 0.3 ms. The response was a single peak of 1.5 +/- 0.2 ms duration, consistent with monosynaptic excitation via a single D- or I1-wave volley elicited by the stimulus. Increased TMS intensity produced a higher response probability (n = 13, paired t test, P < 0.05) but did not affect response latency. Of the remaining motor units tested with contralateral TMS, 7 % did not respond at intensities tested, and 7 % had reduced firing probability without any preceding excitation. Sixteen of these motor units were also tested with ipsilateral TMS and four (25 %) showed short-latency excitation at 6.7 +/- 0.6 ms, with a duration of 1.5 +/- 0.3 ms. Latency and duration of excitatory peaks for these four motor units did not differ significantly with ipsilateral vs. contralateral TMS (paired t tests, P > 0.05). Of the motor units tested with ipsilateral TMS, 56 % responded with a reduced firing probability without a preceding excitation, and 19 % did not respond. These data suggest that masseter motoneurons receive monosynaptic input from the motor cortex that is asymmetrical from each hemisphere, with most low threshold motoneurons receiving short-latency excitatory input from the contralateral hemisphere only.

Transcranial magnetic stimulation and epilepsy

Epileptic conditions are characterized by an altered balance between excitatory and inhibitory influences at the cortical level. Transcranial magnetic stimulation (TMS) provides a noninvasive evaluation of separate excitatory and inhibitory functions of the cerebral cortex. In addition, repetitive TMS (rTMS) can modulate the excitability of cortical networks. We review the different ways that TMS has been used to investigate pathophysiological mechanisms and effects of antiepileptic drugs in patients with epilepsy and epileptic myoclonus. The safety of different TMS techniques is discussed too. Finally, we discuss the therapeutic prospects of rTMS in this field.

Methylphenidate facilitates and disinhibits the motor cortex in intact humans

Animal experiments show that motor recovery after focal brain injury is accelerated by the indirect norepinephrine agonist methylphenidate (MPH). The underlying mechanisms are unknown, but an MPH-induced increase in cortical excitability has been advocated. Here, we tested the acute effects of a single oral dose of 40 mg MPH (Ritalin) on motor cortical excitability in eight healthy subjects using focal transcranial magnetic stimulation. MPH increased the slope of the motor evoked potentials (MEP) intensity curve in a hand muscle, reduced short-interval intracortical inhibition, and increased I-wave facilitation. MEP threshold, cortical silent period and measures of spinal and neuromuscular excitability remained unaffected. Findings support the idea that MPH promotes accelerated motor recovery after lesion through facilitation and disinhibition.

Persistent effects of high frequency repetitive TMS on the coupling between motor areas in the human

Repetitive transcranial magnetic stimulation (rTMS) shows promise as a treatment for various movement and psychiatric disorders. How rTMS may have persistent effects on cortical function remains unclear. We hypothesised that it may act by modulating cortico-cortical connectivity. To this end we assessed cortico-cortical coherence before and after high frequency rTMS of the motor cortex. Sixteen healthy subjects received a single train (5 Hz, active motor threshold, 50 stimuli) of rTMS to the left motor hand area. Spectral power and coherence estimates were calculated between different EEG signals at rest and while muscles of the distal upper limb were tonically contracted. Repetitive TMS over the left motor hand area caused a significant decrease in the intrahemispheric EEG-EEG coherence between motor and premotor cortex in the 10.7-13.6 Hz (upper alpha band) lasting a few minutes after stimulation. There was no significant change in interhemispheric EEG-EEG coherence between motor areas. Thus, high frequency rTMS of the motor cortex decreases ipsilateral cortico-cortical intrahemispheric in the upper alpha band.

Transcranial magnetic stimulation in neurology

Transcranial magnetic stimulation (TMS) is a non-invasive tool for the electrical stimulation of neural tissue, including cerebral cortex, spinal roots, and cranial and peripheral nerves. TMS can be applied as single pulses of stimulation, pairs of stimuli separated by variable intervals to the same or different brain areas, or as trains of repetitive stimuli at various frequencies. Single stimuli can depolarise neurons and evoke measurable effects. Trains of stimuli (repetitive TMS) can modify excitability of the cerebral cortex at the stimulated site and also at remote areas along functional anatomical connections. TMS might provide novel insights into the pathophysiology of the neural circuitry underlying neurological and psychiatric disorders, be developed into clinically useful diagnostic and prognostic tests, and have therapeutic uses in various diseases. This potential is supported by the available studies, but more work is needed to establish the role of TMS in clinical neurology.

Direct demonstration of long latency cortico-cortical inhibition in normal subjects and in a patient with vascular parkinsonism

OBJECTIVE

The motor evoked potential to a single suprathreshold transcranial magnetic stimulus (TMS) is suppressed by a preceding stimulus given 100-200 ms before (long latency intracortical inhibition, LICI). The effect is enhanced in patients with Parkinson's disease. Although previous studies have agreed that the effect is cortical, there is disagreement over exactly which cortical mechanisms are involved. The aim of this study was to provide further evidence for cortical involvement in LICI.

METHODS

Recordings of corticospinal volleys evoked by the TMS stimulation were made from electrodes inserted into the cervical epidural space of 4 conscious subjects. Three of the patients had received the electrodes for treatment of lumbo-sacral pain; the other patient had vascular parkinsonism, and had the electrode implanted to evaluate its effect on cerebral blood flow. The number and amplitude of the volleys were compared with and without a conditioning stimulus.

RESULTS

In 3 pain patients, a conditioning stimulus suppressed the later components of the corticospinal volley (I2 and later waves) when the interval between stimuli was 100-150 ms; at 50 ms the responses were enhanced. Early components of the volley were not affected. Inhibition was much more pronounced and involved all descending volleys except the D wave in the patient with vascular parkinsonism.

CONCLUSIONS

LICI, which is conventionally described in EMG recordings, is also evident in recordings of descending corticospinal volleys and appears enhanced in a patient with vascular parkinsonism.

Effects of peripheral sensory input on cortical inhibition in humans

Cortical inhibitory systems play an important role in motor output. The motor cortex can be inhibited by intracortical mechanisms and by peripheral sensory inputs. We examined whether cortical inhibition from peripheral sensory input is mediated through previously identified intracortical inhibitory systems and how these inhibitory systems interact. Two types of intracortical inhibition were assessed by paired-pulse transcranial magnetic stimulation (TMS). Short-interval intracortical inhibition (SICI) was determined with a subthreshold conditioning stimulus (CS) followed by a test stimulus 2 ms later and long-interval intracortical inhibition (LICI) with suprathreshold conditioning and test stimuli 100 ms apart. Cortical inhibition from peripheral sensory input was induced by median nerve stimulation (MNS) of the right hand and followed by a suprathreshold TMS over the left motor cortex 200 ms later. The first set of experiments tested the effects of different test stimulus intensities on SICI, LICI and cortical inhibition induced by median nerve stimulation (MNSI). With higher test stimulus intensities, LICI and MNSI decreased whereas SICI showed a trend towards an increase. The extent of SICI, LICI and MNSI did not correlate. The second experiment assessed the interaction between MNSI and LICI. The results of applying MNSI and LICI simultaneously were compared with MNSI and LICI alone. MNSI was virtually abolished in the presence of LICI and LICI was also significantly decreased in the presence of MNSI. Thus, the effects of MNSI and LICI when applied together were much less than their expected additive effects when applied alone. The degree of interaction between MNSI and LICI was related to the combined strength of MNSI and LICI but not to the strength of LICI alone. The third experiment investigated the interaction between SICI and MNSI. MNSI and SICI were applied together and the results were compared with MNSI and SICI alone. SICI remained unchanged in the presence of MNSI. We conclude that MNSI is mediated by circuits distinct from those mediating LICI or SICI. The MNSI circuits seem to have an inhibitory interaction with the LICI circuits, whereas the SICI and MNSI circuits do not seem to interact.

Topiramate selectively decreases intracortical excitability in human motor cortex

PURPOSE

Topiramate (TPM) is a novel drug with broad antiepileptic effect in children and adults. In vitro studies suggest activity as sodium-channel blocker, as gamma-aminobutyric acid type A (GABAA)-receptor agonist and as non-N-methyl-D-aspartate (NMDA)-glutamate receptor antagonist.

METHODS

With transcranial magnetic stimulation (TMS), we evaluated which of the mechanisms of action of TPM detected in vitro are relevant for the modulation of human motor cortex excitability. In a double-blind, placebo-controlled, crossover study design, we investigated the effect of single oral doses of 50 mg and 200 mg TPM on motor thresholds, cortical silent period (CSP), and on intracortical inhibition (ICI) and intracortical facilitation (ICF) in 20 healthy subjects.

RESULTS

A significant dose-dependent increase of ICI was noticed after 200 mg TPM as compared with placebo at short interstimulus intervals of 2 to 4 ms. TPM had no effect on motor thresholds or the CSP.

CONCLUSIONS

We conclude that a single dose of TPM selectively increases ICI by GABAAergic and/or glutamatergic mechanisms without a relevant influence on measures, depending on ion-channel blockade or GABAB-receptor activity. The decrease of intracortical excitability (as measured by ICI and ICF) caused by TPM may correlate with its lack of proconvulsive potential in idiopathic generalized epilepsy, because drugs without this action or with less pronounced action may exacerbate seizures in this condition.

Cortical and spinal motor excitability during the premovement EMG silent period prior to rapid voluntary movement in humans

(1) To delineate the cortical and spinal excitability during the pre-movement silent period (PMSP), the motor-evoked potentials (MEPs) and H-reflexes were measured for 0-150 ms prior to rapid voluntary movements of 11 healthy subjects. The appearance of PMSP was judged from the electromyogram of the agonist muscle at each rapid movement and the differences of MEPs or H-reflex between the movement which PMSP appeared (PMSP MEPs or PMSP H-reflex) and the movement which did not appear (non-PMSP MEPs or non-PMSP H-reflex) were compared. (2) The mean amplitudes of the PMSP MEPs in the flexor carpi radialis were significantly smaller than the non-PMSP MEPs, whereas the mean amplitude of the PMSP H-reflex was not significantly different from the non-PMSP H-reflex. (3) In the soleus muscle, the change in MEPs and H-reflexes was examined at three periods for 0-150 ms before the rapid movements. The mean amplitudes of PMSP MEPs for the -150- to -100-ms period and for the -50- to -5-ms period were significantly smaller than those of non-PMSP MEPs. On the other hand, the differences of the mean amplitudes between PMSP H-reflex and non-PMSP H-reflex at the three periods were not significant. (4) These findings suggested that the cortical sensitivity to the TMS was decreased during PMSP though the spinal inhibitory mechanisms assess by H-reflex was not altered during the PMSP and that the EMG pause of the PMSP originates not at the spinal level but at the cortical level. The data support the hypothesis that PMSP is preprogrammed in the preparation and initiation process of the voluntary movements aimed at reinforcement of the performance of the subsequent ballistic movements.

Focal clonus elicited by electrical stimulation of the motor cortex in humans

Focal clonic seizures are a frequent epileptic phenomenon. However, there are little data about their pathomechanism. In four patients with focal epilepsy and subdural electrodes, focal clonus was elicited by electrical stimulation of the motor cortex. Three additional patients underwent intraoperative stimulation of the spinal cord. Rhythmic clonic muscle responses were elicited by cortical stimulation with 20-50 Hz. The clonus consisted of simultaneous trains of compound muscle action potentials (CMAP) in agonistic and antagonistic muscles alternating with periods of muscular silence despite continuous stimulation. Clonus frequency decreased from 4.0-8.0 Hz at 50 Hz stimulation to 3.0-3.5 Hz at 20 Hz paralleled by a prolongation of the trains of CMAP. The stimulation frequency correlated with the number of stimuli blocked during relaxation. During the stable stimulation periods, the clonus frequency decreased over time. The number of stimuli which formed a train of CMAP and which were blocked during relaxation increased towards the end of the stimulation periods. Increasing intensity of stimulation at the same frequency converted a clonic to a tonic response. There was always an 1:1 relationship between stimulus and CMAP during spinal cord stimulation. We hypothesize that during cortical stimulation, clonus is elicited by synchronous activation of pyramidal tract (PT) neurons which results in excitation of intracortical GABA(B)ergic interneurons by recurrent axon-collaterals. This leads to stepwise hyperpolarization of PT neurons intermittently suppressing the output of PT neurons despite continuous stimulation. This mechanism can explain our finding that temporal and spatial summation of the stimuli were needed for clonus generation.

Mechanisms of enhancement of human motor cortex excitability induced by interventional paired associative stimulation

Associative stimulation has been shown to enhance excitability in the human motor cortex (Stefan et al. 2000); however, little is known about the underlying mechanisms. An interventional paired associative stimulation (IPAS) was employed consisting of repetitive application of single afferent electric stimuli, delivered to the right median nerve, paired with single pulse transcranial magnetic stimulation (TMS) over the optimal site for activation of the abductor pollicis brevis muscle (APB) to generate approximately synchronous events in the primary motor cortex. Compared to baseline, motor evoked potentials (MEPs) induced by unconditioned, single TMS pulses increased after IPAS. By contrast, intracortical inhibition, assessed using (i) a suprathreshold test TMS pulse conditioned by a subthreshold TMS pulse delivered 3 ms before the test pulse, and (ii) a suprathreshold test TMS pulse conditioned by afferent median nerve stimulation delivered 25 ms before the TMS pulse, remained unchanged when assessed with appropriately matching test stimulus intensities. The increase of single-pulse TMS-evoked MEP amplitudes was blocked when IPAS was performed under the influence of dextromethorphan, an N-methyl-D-aspartate (NMDA) receptor antagonist known to block long-term potentiation (LTP). Further experiments employing the double-shock TMS protocol suggested that the afferent pulse, as one component of the IPAS protocol, induced disinhibition of the primary motor cortex at the time when the TMS pulse, as the other component of IPAS, was delivered. Together, these findings support the view that LTP-like mechanisms may underlie the cortical plasticity induced by IPAS.

The mechanisms of interhemispheric inhibition in the human motor cortex

Transcranial magnetic stimulation can be used to non-invasively study inhibitory processes in the human motor cortex. Interhemispheric inhibition can be measured by applying a conditioning stimulus to the motor cortex resulting in inhibition of the contralateral motor cortex. Transcranial magnetic stimulation can also be used to demonstrate ipsilateral cortico-cortical inhibition in the motor cortex. At least two different ipsilateral cortico-cortical inhibitory processes have been identified: short interval intracortical inhibition and long interval intracortical inhibition. However, the relationship between interhemispheric inhibition and ipsilateral cortico-cortical inhibition remains unclear. This study examined the relationship between interhemispheric inhibition, short interval intracortical inhibition and long interval intracortical inhibition. First, the effect of test stimulus intensity on each inhibitory process was studied. Second, the effects of interhemispheric inhibition on short interval intracortical inhibition and long interval intracortical inhibition on interhemispheric inhibition were examined. Motor evoked potentials were recorded from the right first dorsal interosseous muscle in 11 right-handed healthy volunteers. For interhemispheric inhibition, conditioning stimuli were applied to the right motor cortex and test stimuli to the left motor cortex. For short interval intracortical inhibition and long interval intracortical inhibition, both conditioning stimuli and test stimuli were applied to the left motor cortex. With increasing test stimulus intensities, long interval intracortical inhibition and interhemispheric inhibition decreased, while short interval intracortical inhibition increased. Moreover, short interval intracortical inhibition was significantly reduced in the presence of interhemispheric inhibition. Interhemispheric inhibition was significantly reduced in the presence of long interval intracortical inhibition when matched for test motor evoked potential amplitude but the difference was not significant when matched for test pulse intensity. These findings suggest that both interhemispheric inhibition and long interval intracortical inhibition are predominately mediated by low threshold cortical neurons and may share common inhibitory mechanisms. In contrast, the mechanisms mediating short interval intracortical inhibition are probably different from those mediating long interval intracortical inhibition and interhemispheric inhibition although these systems appear to interact.

Excitability changes in human peripheral nerve axons in a paradigm mimicking paired-pulse transcranial magnetic stimulation

A peripheral nerve model was developed to determine whether changes in axonal excitability could affect the findings in studies of cortical processes using paired-pulse transcranial magnetic stimulation (TMS). The recovery of axonal excitability from a conditioning stimulus smaller than the test stimulus was qualitatively similar to that with suprathreshold conditioning stimuli. There was an initial decrease in excitability, equivalent to refractoriness at conditioning-test intervals < 4 ms, an increase in excitability, equivalent to supernormality, at intervals of 5-20 ms and a second phase of decreased excitability, equivalent to late subnormality at intervals > 30 ms. H reflex studies using conditioning stimuli below threshold for the H reflex established that these excitability changes could be faithfully translated across an excitatory synapse. Changing membrane potential by injecting polarising current altered axonal excitability in a predictable way, and produced results similar to those reported for many disease states using paired-pulse TMS. Specifically, axonal hyperpolarisation produced a smaller decrease in excitability followed by a greater increase in excitability. This study supports the view that changes in excitability of the stimulated axons should be considered before synaptic mechanisms are invoked in the interpretation of findings from paired-pulse TMS studies.

Basic mechanisms of TMS

Transcranial magnetic stimulation (TMS) is now established as an important noninvasive measure for neurophysiologic investigation of the central and peripheral nervous systems in humans. Magnetic stimulation can be used for stimulating peripheral nerves with a similar mechanism of activation as for electrical stimulation. When TMS is applied to the cerebral cortex, however, some features emerge that distinguish it from transcranial electrical stimulation. One of the most important features is designated the D and I wave hypothesis, which is now widely accepted as a mechanism of TMS of the motor cortex. Transcranial electrical stimulation excites the pyramidal tract axons directly, either at the initial segment of the neuron or at proximal internodes in the subcortical white matter, giving rise to D (direct) waves, whereas TMS excites the pyramidal neurons transsynaptically, giving rise to I (indirect) waves. There are still other phenomena with mechanisms that remain to be elucidated. First, not only excitatory effects but also inhibitory effects can be elicited by TMS of the cerebral cortex (e.g., the silent period and intracortical inhibition). The inhibitory effect may also be used to investigate cerebral functions other than the motor cortex, such as the visual, sensory cortices, and the frontal eye field, from which no overt response like the motor evoked potential can be elicited. Second, there is an abundance of intraregional functional connectivities among different cortical areas that can also be revealed by TMS, or TMS in combination with neuroimaging techniques. Last, repetitive transcranial stimulation exerts a lasting effect on brain function even after the stimulation has ceased. With further investigation of the neural mechanisms of TMS, these techniques will open up new possibilities for investigating the physiologic function of the brain as well as opportunities for clinical application.

The effects of subthreshold 1 Hz repetitive TMS on cortico-cortical and interhemispheric coherence

OBJECTIVES

Repetitive transcranial magnetic stimulation (rTMS) shows promise as a treatment for various movement and psychiatric disorders. Just how rTMS may have persistent effects on cortical function remains unclear. We hypothesised that it may act by modulating cortico-cortical and interhemispheric connectivity. To this end we assessed cortico-cortical and interhemispheric coherence before and after low frequency, subthreshold rTMS of the left motor cortex.

METHODS

Fifteen healthy subjects received one train (1Hz, 90% of active motor threshold, 1500 stimuli) of rTMS to the left motor hand area. Spectral power and coherence estimates were calculated between different electroencephalogram (EEG) signals at rest and while muscles of the distal upper limb were tonically contracted.

RESULTS

rTMS over the left motor hand area caused a significant increase in ipsilateral EEG-EEG coherence and in the interhemispheric coherence between motor areas in the alpha band. The effects of rTMS lasted up to 25 min post-stimulation. There was no significant change in EEG-EEG coherence over the hemisphere contralateral to stimulation.

CONCLUSIONS

Low frequency, subthreshold rTMS of the motor cortex increases ipsilateral cortico-cortical and interhemispheric coherence in the alpha band. This may, in part, mediate the inhibitory effects of low frequency rTMS.

Transcranial magnetic stimulation and epilepsy

Transcranial magnetic stimulation has been used to study generalized and focal epilepsies for more than a decade. The technique appears safe and has yielded important information about the mechanisms underlying epilepsy. Transcranial magnetic stimulation findings differ depending on the epilepsy syndrome, lending support to the concept that there are distinct pathophysiologies underlying each condition. In most studies of generalized epilepsies, transcranial magnetic stimulation has indicated a state of relative hyperexcitability of excitatory cortical interneurons and possibly inhibitory interneurons as well, which can be reversed through the actions of anticonvulsant medications. Transcranial magnetic stimulation studies in patients with a seizure focus in the motor cortex indicate increased cortical excitability and reduced inhibition, but in patients with seizure foci located elsewhere the findings are similar to those in generalized epilepsies. Transcranial magnetic stimulation has also been used to study the mode of action of anticonvulsants and may prove to be a useful means of testing the potential for new drugs to act as anticonvulsants. Repetitive transcranial magnetic stimulation may prove to have a therapeutic role by producing long-lasting cortical inhibition after a train of impulses.

Pharmacologic influences on TMS effects

Transcranial magnetic stimulation (TMS) has been used increasingly to probe the physiology of the human cortex. Besides measuring directly the cortical excitability in motor and visual systems, this noninvasive method can be used to study short- and long-term cortical plasticity. One possible method to examine basic mechanisms underlying cortical excitability and plasticity in humans is the combination of TMS and pharmacologic interventions. In this review the author describes TMS paradigms used to study mechanisms of plasticity in the intact human motor system and its excitability using pharmacologic methods.

In vivo study indicating loss of intracortical inhibition in tumor-associated epilepsy

Transcranial magnetic stimulation was performed in 2 patients with focal motor seizures in the right hand caused by a circumscribed tumor process affecting the left precentral gyrus. In both cases, paired-pulse transcranial magnetic stimulation showed a loss of intracortical inhibition for interstimulus intervals of 2 to 4msec that was replaced by an enormous facilitation in the lesioned hand motor cortex. The uniform impairment of inhibitory mechanisms in epileptogenic tumors with different histologies suggests a common, nonspecific cause of tumor-related epileptogenesis.

Effect of volitional inhibition on cortical inhibitory mechanisms

To investigate the effect of volitional inhibition on cortical inhibitory mechanisms, we performed transcranial magnetic stimulation (TMS) studies with a Go/NoGo reaction task in seven healthy subjects. Subjects were asked to extend their right index finger only after Go, but to remain relaxed after NoGo. Single- and paired-pulse TMS were triggered at the average reaction time for the Go response in each subject after Go or NoGo cues. Motor evoked potentials were recorded in the extensor indicis proprius (EIP) and abductor digiti minimi (ADM) muscles of right hand. Paired-pulse TMS with subthreshold conditioning stimuli at interstimulus intervals (ISIs) of 2 ms [short intracortical inhibition (SICI)] and 15 ms [intracortical facilitation (ICF)] and that with suprathreshold conditioning stimuli at ISI of 80 ms [long intracortical inhibition (LICI)] were performed in both Go/NoGo and control conditions. Inhibition of SICI was enhanced in both EIP and ADM after NoGo and was reduced only in EIP after Go. Inhibition of LICI was reduced in both muscles during both conditions, while ICF was not altered. The present results demonstrate that volitional inhibition enhances SICI but reduces LICI nonselectively. These results suggest that these two inhibitory mechanisms act differently during execution and suppression of voluntary movements.

The application of transcranial magnetic stimulation in psychiatry and neurosciences research

OBJECTIVE

Over recent years transcranial magnetic stimulation (TMS) has become widely applied in the study of neuropsychiatric disorders. The aim of this article is to review the application of TMS as an investigative tool and as a potential therapeutic modality in psychiatric disorders.

METHOD

A comprehensive literature review.

RESULTS

When applied as an investigative tool, TMS provides innovative ways to directly study the excitability of the cortex, cortical regional connectivity, the plasticity of brain responses and cognitive functioning in illness and disease states. A number of studies suggest the potential of treatment with TMS in disease states, especially in patients with depression, although difficulties exist with the interpretation of the published literature.

CONCLUSION

TMS has a considerable role in neuropsychiatric research. It appears to have considerable potential as a therapeutic tool in depression, and perhaps a role in several other disorders, although widespread application requires larger trials and establishment of sustained response.

Contribution of GABAergic cortical circuitry in shaping somatosensory evoked scalp responses: specific changes after single-dose administration of tiagabine

OBJECTIVES

To determine whether conventional as well as high-frequency somatosensory evoked potentials (SEPs) to upper limb stimulation are influenced by GABAergic intracortical circuitry.

METHODS

We recorded SEPs from 6 healthy volunteers before and after a single-oral administration of tiagabine. Conventional low-frequency SEPs have been obtained after stimulation of the median nerve, as well as after stimulation of the first phalanx of the thumb, which selectively involves cutaneous finger inputs. Median nerve SEPs have been further analyzed after digital narrow-bandpass filtering, to selectively examine high-frequency responses. Lastly, in order to explain scalp SEP distribution before and after tiagabine administration, we performed the brain electrical source analysis (BESA) of raw data.

RESULTS

After tiagabine administration, conventional scalp SEPs showed a significant amplitude increase of parietal P24, frontal N24 and central P22 components. Similarly, BESA showed a significant strength increase of the second peak of activation of the first two perirolandic dipoles, which are likely to correspond to the N24/P24 and P22 generators. By contrast, no significant changes of high-frequency SEPs were induced by drug intake.

CONCLUSIONS

Our findings support the view that both N24/P24 and P22 SEP components are probably generated by deep spiny cell hyperpolarization, which is strongly increased by inhibitory inputs from GABAergic interneurons. By considering the clear influence of inhibitory circuitry in shaping these SEP components, conventional scalp SEP recording could be useful in the functional assessment of the somatosensory cortex in different physiological and pathological conditions. By contrast, intrinsic firing properties of the cell population generating high-frequency SEP responses are unaffected by the increase of recurrent GABAergic inhibition.

Effects of tiagabine, a gamma-aminobutyric acid re-uptake inhibitor, on normal rat bladder function

PURPOSE

Previous reports have demonstrated the inhibitory effect of exogenous gamma-aminobutyric acid (GABA) on micturition. In the current study we tested whether tiagabine (Sanofi Synthelab., Newcastle-upon Tyne, United Kingdom), a GABA re-uptake inhibitor increasing endogenous GABA concentrations, would affect micturition in awake rats or influence rat detrusor contraction in vitro.

MATERIALS AND METHODS

Nonanesthetized female Sprague-Dawley rats underwent cystometric investigation in a metabolic cage. Micturition was stimulated by infusing saline intravesically. Micturition parameters were recorded and compared before and after drug administration. In vitro the effects of tiagabine on electrical and carbachol induced contractions in bladder strips were investigated. Furthermore, it was studied whether tiagabine interfered with electrically induced release of acetylcholine.

RESULTS

Intravenous administration of 5 and 20 mg. kg.-1 tiagabine in 7 and 9 rats decreased micturition pressure a mean plus or minus standard error of mean of 21% +/- 11% and 42% +/- 9%, and decreased voided volume a mean of 31% +/- 9% and 33% +/- 9%, respectively. At 20 mg. kg.-1 tiagabine intravenously increased post-void residual volume a mean of 300% +/- 120% and decreased bladder capacity a mean of 14% +/- 3%. Tiagabine (100 microg.) intrathecally in 7 rats reduced micturition pressure a mean of 34% +/- 10% and increased bladder capacity a mean of 30% +/- 9% and post-void residual volume a mean of 250% +/- 75%. However, voided volume was not changed. In vitro studies demonstrated that tiagabine attenuated bladder contractions induced by electrical field stimulation to a mean of 69% +/- 6% of controls at 100 microM. but did not affect contractions induced by carbachol. Release studies revealed that tiagabine inhibited electrical induced acetylcholine release to a mean of 82% +/- 5% of controls at 100 microM.

CONCLUSIONS

The current results show that tiagabine has an inhibitory action on rat micturition. The site of action may be central and peripheral.

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.

Functional connectivity of human premotor and motor cortex explored with repetitive transcranial magnetic stimulation

Connections between the premotor cortex and the primary motor cortex are dense and are important in the visual guidance of arm movements. We have shown previously that it is possible to engage these connections in humans and to measure the net amount of inhibition/facilitation from premotor to motor cortex using single-pulse transcranial magnetic stimulation (TMS). The aim of this study was to test whether premotor activation can affect the excitability of circuits within the primary motor cortex (M1) itself. Repetitive TMS (rTMS), which is known to produce effects that outlast the train at the site of stimulation, was given for 20 min at 1 Hz over premotor, primary motor, and sensory areas of cortex at an intensity of 80% of the active motor threshold for the motor hand area. The excitability of some corticocortical connections in M1 was probed by using paired-pulse testing of intracortical inhibition (ICI) and intracortical facilitation (ICF) with a coil placed over the motor cortex hand area. rTMS over the premotor cortex, but not other areas, changed the time course of the ICI/ICF for up to 1 hr afterward without affecting motor thresholds or motor-evoked potential recruitment. The cortical silent period was also shortened. The implication is that rTMS at a site distant from the motor cortex can change the excitability of circuits intrinsic to the motor cortex.

Functional connectivity of human premotor and motor cortex explored with repetitive transcranial magnetic stimulation

Connections between the premotor cortex and the primary motor cortex are dense and are important in the visual guidance of arm movements. We have shown previously that it is possible to engage these connections in humans and to measure the net amount of inhibition/facilitation from premotor to motor cortex using single-pulse transcranial magnetic stimulation (TMS). The aim of this study was to test whether premotor activation can affect the excitability of circuits within the primary motor cortex (M1) itself. Repetitive TMS (rTMS), which is known to produce effects that outlast the train at the site of stimulation, was given for 20 min at 1 Hz over premotor, primary motor, and sensory areas of cortex at an intensity of 80% of the active motor threshold for the motor hand area. The excitability of some corticocortical connections in M1 was probed by using paired-pulse testing of intracortical inhibition (ICI) and intracortical facilitation (ICF) with a coil placed over the motor cortex hand area. rTMS over the premotor cortex, but not other areas, changed the time course of the ICI/ICF for up to 1 hr afterward without affecting motor thresholds or motor-evoked potential recruitment. The cortical silent period was also shortened. The implication is that rTMS at a site distant from the motor cortex can change the excitability of circuits intrinsic to the motor cortex.

Cortical silent period in the tongue induced by transcranial magnetic stimulation

Cortical silent period (SP) of the limb muscles is thought to reflect the cortical excitability. However, the lingual SP has not been examined precisely even in normal subjects. We investigated SP in the tongue induced by transcranial magnetic stimulation (TMS) in 18 controls. Surface electrodes were placed on the lingual dorsum using a bipolar technique. A round coil (13.5 cm in outer diameter) connected with Magstim 200 stimulator was placed on the motor cortex of the tongue, and the intensity of the stimulation was increased stepwise to maximum. SP was detected in all subjects especially at the contralateral side to the stimulated side, without contamination of peripheral SP. The duration of SP depended on the stimulus intensity, while the degrees of muscle contraction did not influence SP. SP of the tongue showed similar characteristics to that of limb muscles. This suggests that SP of lingual muscles can be clinically useful for the evaluation of corticobulbar excitability.

Treatment of painful sensory neuropathy with tiagabine: a pilot study

To evaluate the effect of tiagabine hydrochloride in painful neuropathy in a pilot, open-label study. Painful neuropathy is characterized by preferential involvement of small sensory and autonomic fibers. Tiagabine increases gamma-aminobutyric acid and might enhance the central pain-control mechanisms. Seventeen patients (10 men, 7 women; mean age 51.4 +/- 7.7 y) with chronic painful neuropathy (>6 months) were enrolled in this study. Week 0: All pain medications were discontinued. Weeks 1-4: Dose of tiagabine was increased weekly by 4 mg orally up to 16 mg in week 4. Quantitative sensory testing for vibration, cooling, and heat-pain, and quantitative sudomotor axon reflex test (QSART) were done at week 0 and week 4. The McGill Pain Questionnaire was administered weekly. Nine patients completed the study; 8 patients discontinued the treatment. Baseline pain intensity was 6.2 +/- 3.1 on the McGill Pain Questionnaire scale (0-10 range). Low doses (4-8 mg) of tiagabine reduced pain symptoms by 16-38%, improving surface pain (37.5%), skin sensitivity (32.8%), burning (38.6%), cold (25.4%) and pain sharpness (29%; p <0.03). Dull and deep pain did not improve. Quantitative sensory testing abnormalities diminished with treatment (p <0.02). Autonomic test results did not change. This pilot study evaluated the potential of tiagabine hydrochloride (Gabitril) in treatment of painful sensory neuropathy. Pain symptoms and quantitative sensory test results improved with treatment, especially at low doses of tiagabine (4-8 mg). Higher doses (12-16 mg) were associated with increased number of adverse events. Tiagabine may have potential benefits for treatment of painful neuropathy; however, assessment of its efficacy in a larger study is needed.

Mechanisms of motor-evoked potential facilitation following prolonged dual peripheral and central stimulation in humans

1. Repetitive electrical peripheral nerve or muscle stimulation can induce a lasting increase in the excitability of the corticomotor projection. By pairing peripheral stimulation with transcranial magnetic brain stimulation it is possible to shorten the duration of stimulation needed to induce this effect. This ability to induce excitability changes in the motor cortex may be of significance for the rehabilitation of brain-injured patients. The mechanisms responsible for the increases in excitability have not been investigated thoroughly. 2. Using two paired transcranial magnetic stimuli protocols we investigated the excitability of intracortical inhibitory and excitatory systems before and following a period of repetitive dual muscle and brain stimulation. The dual stimulation consisted of motor point stimulation of first dorsal interosseous (FDI; 10 Hz trains of 1 ms square waves for 500 ms) delivered at one train every 10 s, paired with single transcranial magnetic stimulation given 25 ms after the onset of the train. 3. Following 30 min of dual stimulation, motor-evoked potentials (MEPs) were significantly increased in amplitude. During this period of MEP facilitation there was no significant difference in the level of intracortical inhibition. There was, however, a significant increase in the intracortical facilitation demonstrated with paired magnetic stimuli. The increase in facilitation was seen only at short interstimulus intervals (0.8-2.0 ms). These intervals comprised a peak in the time course of facilitation, which is thought to reflect I wave interaction within the motor cortex. 4. The relevance of this finding to the MEP facilitation seen following dual peripheral and central stimulation is discussed.

Intracortical inhibition and facilitation in human facial motor area: difference between upper and lower facial area

OBJECTIVE

To investigate the intracortical inhibitory and excitatory systems in the motor cortical representation of upper and lower facial muscles.

METHODS

Paired-pulse transcranial magnetic stimulation (TMS) was applied to 7 healthy volunteers, with the interstimulus interval (ISI) between the conditioning stimulus (CS) and test stimulus, varied from 1 to 20 ms. CS was set at 90% of motor threshold. Muscle evoked potentials (MEPs) were recorded from first dorsal interosseus (FDI), orbicularis oculi (o. oculi) and mentalis muscles.

RESULT

TMS evoked MEPs in o. oculi on both ipsi- and contralateral sides in all subjects. In the paired-pulse study, MEP amplitude in the mentalis decreased at short ISIs of 1-3 ms, followed by increases at 12-20 ms. These effects were similar to those in the FDI. O. oculi did not show a distinct inhibitory period at short ISIs and facilitation at long ISIs was detected but was significantly less than in FDI and mentalis. In o. oculi, there was no significant difference between the effects of ipsilateral and contralateral CS on the MEPs.

CONCLUSION

The bi-hemispheric control of volitional movement and the modulation from brainstem projections appear to markedly influence intracortical inhibitory and excitatory systems in the motor cortical representation of o. oculi.

New graphical method to measure silent periods evoked by transcranial magnetic stimulation

OBJECTIVES

Manual methods of measuring duration of cortical silent periods (CSP) evoked by transcranial magnetic stimulation (TMS) depend upon subjective visual estimation of onset and offset. Because of this, the measurements are susceptible to poor rater reliability. We describe a graphical method to measure silent periods with greater precision. The statistical process underlying this new method is simple and particularly suited to signal detection in serially dependent data.

METHODS

TMS-evoked silent periods were recorded in 13 healthy subjects. Two investigators subjectively measured silent period duration on each subject to estimate rater reliability. Using the graphical method, the mean and 99.76% variation limits of pre-stimulus electromyogram (EMG) activity were computed. Each averaged trial was displayed and CSP onset and offset detected when post-stimulus EMG activity moved outside the 99.76% limits.

RESULTS

Maximum variation in silent period duration was 21.8 ms between the two investigators' subjective measurements. Silent period duration measured with the graphical method closely approximated measurements obtained using the manual method. It was possible to automate the procedure.

CONCLUSIONS

This graphical method allowed precise measurement of CSP duration, independent of subjective estimations of onset or offset points. Further studies are necessary to determine if this method can provide a framework for other physiologic measures.

Motor cortex disinhibition in Alzheimer's disease

OBJECTIVES

To explore subclinical disturbances in the motor cortex of patients with Alzheimer's disease (AD).

METHODS

We used transcranial magnetic stimulation in a paired pulse technique to test intracortical inhibition (ICI) and intracortical facilitation in mildly to moderately demented AD patients with a normal neurological examination. Patients were studied before and during treatment with the cholinesterase inhibitor donepezil.

RESULTS

AD patients had a reduced ICI compared to an age-matched control group. The amount of disinhibition correlated with the severity of dementia. Treatment with 10 mg donepezil daily was associated with an increase of ICI.

CONCLUSIONS

The subclinical motor cortex disinhibition in AD patients indicates a functional disturbance, and is probably associated with a cholinergic deficit.

Orally administered atropine enhances motor cortex excitability: a transcranial magnetic stimulation study in human subjects

Oral application of atropine was used to test if a modulation of cholinergic neurotransmission changed motor excitability. Healthy volunteers received either 1 or 2 mg atropine. Paired transcranial magnetic stimulation was used to study intracortical inhibition and intracortical facilitation before, 1 h and 24 h after ingestion of atropine. In addition, the silent period, motor threshold, F wave and motor response amplitudes were measured. The 1 mg dose of atropine induced a loss of intracortical inhibition, the 2 mg dose produced an intracortical disinhibition and enhanced intracortical facilitation. These changes returned to baseline after 24 h. Other electrophysiological parameters remained unchanged. Thus, an antagonist of pre- and postsynaptic muscarinic receptors increased excitability in the human motor cortex in a dose-dependent manner, indicating an influence of the cholinergic system on motor cortex excitation.

Studies of human motor physiology with transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a safe, noninvasive, and painless way to stimulate the human motor cortex in behaving human subjects. When it is applied as a single-pulse, measurements such as central conduction time, motor threshold, silent-period duration, recruitment curve, and mapping of muscle representation can be determined. Paired-pulse TMS is a useful way to examine cortical excitability. Single and paired-pulse TMS have been applied to study plasticity following amputation and cortical excitability in patients with dystonia. Another form of TMS is repetitive TMS (rTMS), with stimuli delivered repeatedly to a single scalp site. High-frequency rTMS can be used to transiently inactivate different cortical areas to study their functions. rTMS can also modulate cortical excitability. At stimulus frequencies higher than 5 Hz, rTMS increases cortical excitability, and stimulation around 1 Hz reduces cortical excitability. Modulation of cortical excitability by rTMS has therapeutic potential in psychiatric and neurological disorders.

Long-term changes in motor cortical organisation after recovery from subcortical stroke

The present study has investigated the long-term changes in the organisation of the corticomotor projection to the hand in a group of subjects who had sustained a subcortical hemispheric stroke up to 15 years previously and had subsequently recovered normal or near-normal motor function. Transcranial magnetic cortical stimulation (TMCS) was employed to map the topography of the primary corticomotor projection to the hand and to obtain measures of cortical motor threshold, long-latency intracortical inhibition and corticospinal conduction. Changes in motor threshold and in motor-evoked potential (MEP) amplitude and latency in keeping with persisting impairment of conduction in the corticospinal pathway were still present in the majority of subjects, whereas the duration of the post-MEP silent period, reflecting the strength of long-latency intracortical inhibition, was usually normal. Topographic shifts in the corticomotor representation relative to the unaffected side were found in the majority of subjects. In some the shifts were in the mediolateral axis suggesting reorganisation within the primary motor cortex, while in the others anteroposterior shifts were present in keeping with recruitment of premotor or postcentral cortex. The present findings indicate that changes in the physiological properties of the corticomotor projection to the hand are frequently present in subjects who have recovered motor function after a subcortical stroke and may persist indefinitely. We postulate that these changes are the result of reorganisation at cortical level and that cortical reorganisation is one of the processes which contribute to motor recovery after a subcortical lesion and which may compensate for persisting impairment of conduction in the corticospinal pathway.

Interactions between two different inhibitory systems in the human motor cortex

Intracortical inhibition in the human motor cortex has been previously demonstrated using paired-pulse transcranial magnetic stimulation (TMS) protocols at short intervals (1-6 ms; short interval intracortical inhibition, SICI) with a subthreshold conditioning pulse preceding a suprathreshold test pulse, and at long intervals (50-200 ms; long interval intracortical inhibition, LICI) with suprathreshold conditioning and test pulses. We investigated whether different circuits mediate these inhibitory phenomena and how they interact. In nine healthy volunteers, we applied TMS to the motor cortex and recorded motor evoked potentials from the first dorsal interosseous muscle. With increasing test pulse strength, LICI decreases but SICI tends to increase. There was no correlation between the degree of SICI and LICI. We tested the interactions between SICI and LICI. SICI was reduced or eliminated in the presence of LICI. Loss of SICI was seen even with a conditioning stimulus too weak to induce significant LICI. Our findings demonstrate that different cell populations mediate SICI and LICI. The results are consistent with the hypothesis that LICI inhibits SICI through presynaptic GABAB receptors. Testing of SICI in the presence of LICI may be a non-invasive way of evaluating inhibitory interactions in the human motor cortex.

Hemispheric asymmetry of ipsilateral motor cortex activation during unimanual motor tasks: further evidence for motor dominance

OBJECTIVES

To test to which extent the increase in ipsilateral motor cortex excitability during unimanual motor tasks shows hemispheric asymmetry.

METHODS

Six right-handed healthy subjects performed one of several motor tasks of different complexity (including rest) with one hand (task hand) while the other hand (non-task hand) was relaxed. Focal transcranial magnetic stimulation was applied to the motor cortex ipsilateral to the task hand and the amplitude of the motor evoked potential (MEP) in the non-task hand was measured. In one session, the task hand was the right hand, in the other session it was the left hand. The effects of motor task and side of the task hand were analyzed. Spinal motoneuron excitability was assessed using F-wave measurements.

RESULTS

Motor tasks, in particular complex finger sequences, resulted in an increase in MEP amplitude in the non-task hand. This increase was significantly less when the right hand rather than the left hand was the task hand. This difference was seen only in muscles homologous to primary task muscles. The asymmetry could not be explained by changes in F-wave amplitudes.

CONCLUSIONS

Hemispheric asymmetry of ipsilateral motor cortex activation either supports the idea that, in right handers, the left motor cortex is more active in ipsilateral hand movements, or alternatively, that the left motor cortex exerts more effective inhibitory control over the right motor cortex than vice versa. We suggest that hemispheric asymmetry of ipsilateral motor cortex activation is one property of motor dominance of the left motor cortex.

Exploration of motor cortex excitability in a diabetic patient with hemiballism-hemichorea

Hemiballism-hemichorea in older patients with hyperglycemia, associated with high signal intensity in the contralateral striatum on T1-weighted magnetic resonance scans, is now an accepted clinical entity. We present an additional patient with this disorder. Using transcranial magnetic stimulation, we show that intracortical inhibition in the motor cortex contralateral to hemiballism-hemichorea is increased. This finding is discussed in the context of current models of basal ganglia-thalamo-cortical connectivity.

Motor cortex excitability in patients with focal epilepsy

We studied the excitability of the motor cortex using, transcranial magnetic stimulation (TMS) in patients with temporal and extratemporal epilepsy. We applied single and paired-pulse TMS to 15 patients with temporal (n = 7), extratemporal (n = 6) and focal epilepsy lateralised to one hemisphere (n = 2). Patients had no antiepileptic drugs in the last 48 h and were seizure free for 4 h prior to testing. We determined the threshold for EMG responses at rest (RMT), the cortically evoked silent period (CSSP) and intracortical inhibition (ICI, intervals of 2-4 ms) and facilitation (ICF, 7-15 ms) and compared the results to those obtained in 17 normal controls. ICI and ICF was reduced in both hemispheres (P < 0.01. ANOVA) compared to the controls. In the hemisphere of seizure origin ('abnormal') there was a reduction of ICF (P < 0.01) and normal ICI, in the 'normal' hemisphere there was a reduced ICI (P < 0.01) and a slight reduction of ICF (P < 0.05). ICF on the 'abnormal' side was reduced (P < 0.05) compared to the 'normal' hemisphere. RMT was increased in two patients, but group comparison of RMT and CSSP showed no significant differences between patients and controls. The results suggest a remote effect of epileptic activity onto the motor cortex leading to an alteration of activity in local inhibitory circuits.

Repetitive transcranial magnetic stimulation causes a short-term increase in the duration of the cortical silent period in patients with Parkinson's disease

In ten patients with Parkinson's disease (PD) and ten age-matched healthy controls, we applied 15 30-s trains of subthreshold 5-Hz repetitive transcranial magnetic stimulation (rTMS) over the primary motor hand area. Ten minutes after rTMS, PD patients showed a significant prolongation of the transcranially evoked silent period (SP) in the contralateral first dorsal interosseus muscle, whereas the SP remained unchanged in healthy subjects. Since the duration of the transcranially evoked SP is a well-established measure of intracortical inhibition, this finding demonstrates that rTMS is capable of inducing a short-term increase in intracortical inhibition in PD. The lack of a prolongation of the SP in healthy controls suggests that PD patients may be particularly susceptible to modulatory effects of rTMS on motocortical inhibition.