Spread of electrical activity at cortical level after repetitive magnetic stimulation in normal subjects

In normal subjects, focal repetitive transcranial magnetic stimulation (rTMS) of the hand motor area evokes muscle potentials (MEPs) from muscles in the hand (target muscles) and the arm (non-target muscles). In this study we investigated the mechanisms underlying the spread of MEPs induced by focal rTMS in non-target muscles. rTMS was delivered with a Magstim stimulator and a figure-of-eight coil placed over the first dorsal interosseus (FDI) motor area of the left hemisphere. Trains of 10 stimuli were given at a suprathreshold intensity (120% of motor threshold) and at frequencies of 5, 10 and 20 Hz at rest. Electromyographic (EMG) activity was recorded simultaneously from the FDI (target muscle) and the contralateral biceps muscle and from the FDI muscle ipsilateral to the side of stimulation (non-target muscle). rTMS delivered in trains to the FDI motor area of the left hemisphere elicited MEPs in the contralateral FDI (target muscle) that gradually increased in amplitude over the course of the train. Focal rTMS trains also induced MEPs in the contralateral biceps (non-target muscle) but did so only after the second or third stimulus; like target-muscle MEPs, in non-target muscle MEPs progressively increased in amplitude during the train. At no frequency did rTMS elicit MEPs in the FDI muscle ipsilateral to the site of stimulation. rTMS left the latency of EMG responses in the FDI and biceps muscles unchanged during the trains of stimuli. The latency of biceps MEPs was longer after rTMS than after a single TMS pulse. In conditioning-test experiments designed to investigate the cortical origin of the spread, a single TMS pulse delivered over the left hemisphere at an interstimulus interval (ISI) of 50, 100 and 150 ms reduced the amplitude of the test MEP evoked by a single TMS pulse delivered over the right hemisphere; and a conditioning rTMS train delivered over the left hemisphere increased the amplitude of the test MEP evoked by a single TMS pulse over the right hemisphere. A conditioning rTMS train delivered over the left hemisphere and paired magnetic shocks (test stimulus) at 3 and 13 ms ISIs over the right hemisphere reduced MEP inhibition at the 3-ms ISI but left the MEP facilitation at 13 ms unchanged. Using a control MEP size matched with that observed after a conditioning contralateral rTMS, we found that paired-pulse inhibition remained unchanged. Yet a single TMS conditioning pulse sufficiently strong to evoke a MEP in the contralateral FDI and biceps muscles simultaneously (as rTMS did) left paired-pulse inhibition unchanged. We conclude that the spread of EMG activity to non-target muscles depends on cortical mechanisms, mainly including changes in the excitability of the interneurones mediating intracortical inhibition. Electronic Publication     

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.     

Functional organisation of corticonuclear pathways to motoneurones of lower facial muscles in man

EMG responses were recorded from lower facial muscles (depressor labii inferioris or depressor anguli oris) of 12 normal subjects after magnetic stimulation of the motor cortex. Using a figure-of-eight stimulating coil, the largest responses were obtained from points around 8–10 cm lateral to the vertex. Usually they were bilateral and had the same latency (11–12 ms) on both sides of the face. Patients with complete Bell's palsy had no response in muscles on the same side as the lesion, indicating that the ipsilateral component to cortical stimulation was not the result of recrossing in the periphery of nerve fibres from the contralateral side. Single-unit studies showed that cortical stimulation produced two phases of motoneuronal facilitation: a short-latency (central motor delay from contralateral cortex to the intracranial portion of the facial nerve, 7.6 ms), short-duration (1– to 2-ms duration peak in the post-stimulus time histogram) input, which was more commonly evoked by contralateral than ipsilateral stimulation; and a longer latency (central delay > 15 ms), long-duration input evoked equally well from either hemisphere. The former may represent activity in a predominantly contralateral oligosynaptic corticobulbar pathway; the latter, a polysynaptic indirect (e.g. co-rticotegmento-nuclear) bilateral pathway to lower facial muscles.     

Magnetic stimulation and the crossed–uncrossed difference (CUD) paradigm: selective effects in the ipsilateral and contralateral hemispheres

When a visual target is presented to one hemifield, manual responses made to the target using the ipsilateral hand (uncrossed responses) are faster than responses using the contralateral hand (crossed response), because there is no need for visuomotor information to be transferred between the hemispheres. This difference in response times is termed the crossed–uncrossed difference (CUD) and is a valuable means of estimating interhemispheric transfer time. We aimed to investigate the CUD by applying repetitive transcranial magnetic stimulation (rTMS) over the left and right occipital cortex during a lateralized target-detection task. Eleven neurologically healthy subjects, all right-handed, participated in the study. Relative to sham TMS we increased the CUD, by inhibiting the crossed latencies, but only when rTMS was applied to the hemisphere receiving visual information. These results replicate and extend previous findings and suggest the inhibitory rTMS effect under the crossed condition might be because the weak visual output is unable to activate the crossed pathway.     

Symmetric facilitation between motor cortices during contraction of ipsilateral hand muscles

Using transcranial magnetic stimulation (TMS) over the contralateral motor cortex, motor evoked potentials (MEPs) were recorded from resting abductor pollicis brevis (APB) and first dorsal interosseous (FDI) muscles of eight subjects while they either rested or produced one of six levels of force with the APB ipsilateral to the TMS. F-waves were recorded from each APB at rest in response to median nerve stimulation while subjects either rested or produced one of two levels of force with their contralateral APB. Contraction of the APB ipsilateral to TMS produced facilitation of the MEPs recorded from resting APB and FDI muscles contralateral to TMS but did not modulate F-wave amplitude. Negligible asymmetries in MEP facilitation were observed between dominant and subdominant hands. These results suggest that facilitation arising from isometric contraction of ipsilateral hand muscles occurs primarily at supraspinal levels, and this occurs symmetrically between dominant and subdominant hemispheres. Electronic Publication     

Effort-induced mirror movements. A study of transcallosal inhibition in humans

During sustained, fatiguing maximal voluntary contraction of muscles of one hand, muscles of the other hand gradually become activated also. Such effort-induced mirror movements indicate a decreased ability of the central nervous system (CNS) to selectively control individual muscles. We studied whether altered transcallosal inhibition (TCI) contributed to this phenomenon. TCI was determined in ten healthy subjects by measuring the ipsilateral silent period (iSP) and the contralateral silent period (cSP) during a sustained contraction of the abductor digiti minimi, induced by focal unihemispheric ipsilateral transcranial magnetic stimulation. Mirror movements occurred in all subjects in response to the effort. There was a bilateral increase in cSPs and a parallel increase in the iSP in the contralateral working muscle. In contrast, the iSP in the mirroring muscle remained unchanged, explained by a balance of increased crossed pyramidal inhibition (cSP) and decreased transcallosal inhibition. In finely tuned unimanual movements, mirroring activity of the contralateral hand is suppressed by TCI originating in the working hemisphere. During sustained, effortful contractions, the outflow of the contralateral hemisphere is increased due to reduced TCI. Effort-induced mirror contractions are thus the result of disinhibition of contralateral crossed projections rather than disinhibition of ipsilateral uncrossed pathways.     

Ipsilateral responses of motor evoked potential correlated with the motor functional outcomes after cortical resection

The aim of this study was to evaluate if ipsilateral motor evoked potential (MEP) elicited by transcranial magnetic stimulation (TMS) could provide neurosurgeons preoperatively with useful information regarding surgical procedure for patients with severe cerebral hemiatrophy or unilateral malformation. Thirteen epilepsy patients with severe cerebral hemiatrophy or unilateral malformation were studied before operation using MEPs recorded on bilateral abductor pollicis brevis (APBs) muscles, elicited by transcranial magnetic stimulation of the motor cortex. Ten subjects served as controls. Results: (1) no ipsilateral MEP responses were recorded in all the 10 healthy subjects; (2) in the 13 patients, the results of MEPs could be divided into four types. Type A: in 3 patients bilateral MEPs were recorded when unaffected hemisphere was stimulated, while no responses were elicited when the affected hemisphere was stimulated. Type B: in another 3 patients, the MEPs were elicited from bilateral APB muscles when the unaffected hemisphere was stimulated, and the contralateral MEP was also elicited when the affected hemisphere was stimulated. Type C: in two patients contralateral MEP was elicited when the unaffected hemisphere was stimulated, while no MEP was induced in APB muscles of either side following the affected hemisphere stimulation. Type D: in the remaining 5 patients, contralateral magnetic MEPs were elicited either when the affected or the unaffected hemisphere was stimulated. Patients of type A, B and C received hemispherectomy showed no significant permanent motor functional deficit. Among the total 8 patients, 7 patients got seizure free after the operation. Patients of type D showed minor muscle strength decrease after localized cortical resection. Three out of 5 patients of type D got seizure free after the operation. Ipsilateral MEP response might be useful for neurosurgeons to plan appropriate surgical procedure which helps avoid post-operative motor deficits.     

Cortical motor representation of the ipsilateral hand and arm

We sought to determine whether motor evoked potentials (MEPs) as well as silent periods could be produced in hand and shoulder muscles by transcranial magnetic stimulation (TMS) of the ipsilateral cerebral hemisphere and, if so, whether their cortical representations could be mapped with respect to those of contralateral muscles. In six normal subjects, we delivered ten stimuli each to a grid of sites 1 cm apart on the scalp. The EMG was recorded and averaged from the contralateral first dorsal interosseous (FDI) and risorius (facial) muscles at rest and the ipsilateral FDI muscle, which was voluntarily contracted. In four of these subjects and an additional subject, we used the same mapping technique and recorded from the deltoid muscle on the right and left sides and the contralateral FDI during activation of the ipsilateral deltoid. In all subjects, the cortical representation of the contralateral risorius was anterolateral to that of the FDI. The contralateral deltoid could be activated in only three subjects. In them, its representation was slightly medial to that of the FDI. All subjects had at least one scalp site where TMS produced MEPs in the ipsilateral FDI. Two subjects had rich ipsilateral hand representations with multiple ipsilateral MEP sites. Both had ipsilateral MEP sites near the representation of the contralateral FDI, but the largest ipsilateral MEPs occurred with TMS at more lateral sites, which were near the representation of the contralateral risorius. In these subjects, the ipsilateral deltoid was preferentially activated at sites medial and posterior to those activating the contralateral muscle. Ipsilateral TMS also produced silent periods in the FDI in all subjects. These silent periods were much more frequent than the ipsilateral MEPs and tended to occur with TMS near the representation of the contralateral FDI. The excitatory cortical representation of the ipsilateral arm muscles is accessible to TMS in normal subjects and is different from that of the homologous contralateral muscles. The hand may have two ipsilateral representations, one of which produces silent periods and the other MEPs at the same stimulus intensity.     

Excitability changes in human forearm corticospinal projections and spinal reflex pathways during rhythmic voluntary movement of the opposite limb

Rhythmic movements brought about by the contraction of muscles on one side of the body give rise to phase-locked changes in the excitability of the homologous motor pathways of the opposite limb. Such crossed facilitation should favour patterns of  bimanual coordination in which homologous muscles are engaged simultaneously, and disrupt those in which the muscles are activated in an alternating fashion. In order to examine these issues, we obtained responses to transcranial magnetic stimulation (TMS), to stimulation of the cervicomedullary junction (cervicomedullary-evoked potentials, CMEPs), to peripheral nerve stimulation (H-reflexes and f-waves), and elicited stretch reflexes in the relaxed right flexor carpi radialis (FCR) muscle during rhythmic (2 Hz) flexion and extension movements of the opposite (left) wrist. The potentials evoked by TMS in right FCR were potentiated during the phases of movement in which the left FCR was most strongly engaged. In contrast, CMEPs were unaffected by the movements of the opposite limb. These results suggest that there was systematic variation of the excitability of the motor cortex ipsilateral to the moving limb. H-reflexes and stretch reflexes recorded in right FCR were modulated in phase with the activation of left FCR. As the f-waves did not vary in corresponding fashion, it appears that the phasic modulation of the H-reflex was mediated by presynaptic inhibition of Ia afferents. The observation that both H-reflexes and f-waves were depressed markedly during movements of the opposite indicates that there may also have been postsynaptic inhibition or disfacilitation of the largest motor units. Our findings indicate that the patterned modulation of excitability in motor pathways that occurs during rhythmic movements of the opposite limb is mediated primarily by interhemispheric interactions between cortical motor areas.     

Inter-hemispheric asymmetry of ipsilateral corticofugal projections to proximal muscles in humans

Motor evoked potentials (MEPs) elicited in many proximal or truncal muscles by ipsilateral transcranial magnetic stimulation (TMS) are thought to be mediated by an oligosynaptic corticofugal pathway and not by uncrossed corticospinal collaterals. In the present study, we compared the input–output properties and scalp surface topography of the ipsilateral and contralateral projections to pectoralis major (PM) and latissimus dorsi (LD) in seven healthy subjects. In six subjects, ipsilateral MEPs evoked by stimulation of one hemisphere (dominant ipsilateral hemisphere) were markedly larger in amplitude than the MEPs evoked in the opposite hemisphere (non-dominant ipsilateral hemisphere). The dominant ipsilateral hemisphere MEPs were significantly larger than the non-dominant MEPs (p<0.02) by an average factor of 3.6 and 3.4 times in PM and LD, respectively. Similarly, there was significant asymmetry between hemispheres in the scalp surface area from which ipsilateral MEPs could be evoked. In contrast, contralateral projections were symmetric in both MEP amplitude and area. Neither the right nor left hemisphere was consistently the dominant ipsilateral hemisphere. The ipsilateral centre of gravity (CoG) for PM was located an average of 0.8±0.6 cm posterior to the contralateral CoG, but no significant differences were observed between ipsilateral and contralateral CoGs in LD. These results demonstrate that the excitability of ipsilateral corticofugal projections to PM and LD are asymmetric between hemispheres.     

Delay of the execution of rapid finger movement by magnetic stimulation of the ipsilateral hand-associated motor cortex

We investigated the influence of focal transcranial magnetic stimulation (TMS) of the hand-associated motor cortex on the execution of ipsilateral finger-lifting movements in six humans. In a simple reaction time paradigm, suprathreshold TMS (1.6- to 2.1-fold of the response threshold determined at rest) was performed at intervals of 40, 70, 80, 90, and 100 ms after the auditory "go" signal. Movement onset was measured with an accelerometer. TMS delayed the execution of ipsilateral finger movement when the cortex stimulus preceded the onset of the intended movement by about 25-65 ms. Taking the corticomuscular conduction times to the activated muscles into account, TMS suppressed the output from the motor cortex in a period 6-45 ms after the contralateral motor cortex was stimulated. Such timing would be compatible with an interhemispheric inhibition similar to the previously described ipsilateral inhibition of ongoing tonic motor activity. The delay of the movement was 40 ms. The function of the neuronal structures mediating interhemispheric inhibition might be to suppress the coactivation of the other hand during unilateral finger movements within bimanual motor tasks.     

The origin of activity in the biceps brachii muscle during voluntary contractions of the contralateral elbow flexor muscles

During strong voluntary contractions, activity is not restricted to the target muscles. Other muscles, including contralateral muscles, often contract. We used transcranial magnetic stimulation (TMS) to analyse the origin of these unintended contralateral contractions (termed “associated” contractions). Subjects (n = 9) performed maximal voluntary contractions (MVCs) with their right elbow-flexor muscles followed by submaximal contractions with their left elbow flexors. Electromyographic activity (EMG) during the submaximal contractions was matched to the associated EMG in the left biceps brachii during the right MVC. During contractions, TMS was delivered to the motor cortex of the right or left hemisphere and excitatory motor evoked potentials (MEPs) and inhibitory (silent period) responses recorded from left biceps. Changes at a spinal level were investigated using cervicomedullary stimulation to activate corticospinal paths (n = 5). Stimulation of the right hemisphere produced silent periods of comparable duration in associated and voluntary contractions (218 vs 217 ms, respectively), whereas left hemisphere stimulation caused a depression of EMG but no EMG silence in either contraction. Despite matched EMG, MEPs elicited by right hemisphere stimulation were ∼1.5–2.5 times larger during associated compared to voluntary contractions (P < 0.005). Similar inhibition of the associated and matched voluntary activity during the silent period suggests that associated activity comes from the contralateral hemisphere and that motor areas in this (right) hemisphere are activated concomitantly with the motor areas in the left hemisphere. Comparison of the MEPs and subcortically evoked potentials implies that cortical excitability was greater in associated contractions than in the matched voluntary efforts.     

Different effects of fatiguing exercise on corticospinal and transcallosal excitability in human hand area motor cortex

Following forceful exercise that leads to muscle fatigue, the size of muscle evoked responses (MEPs) generated by transcranial magnetic stimulation (TMS) in the exercised muscle is depressed over a prolonged period. Strong evidence implicates intracortical mechanisms in this depression. As well as evoking MEPs in contralateral muscles, TMS also reduces MEPs evoked in ipsilateral muscles through interhemispheric inhibition mediated by a transcallosal pathway. Here we have sought to determine whether this effect is also depressed after exercise. Using two magnetic stimulators, the aftereffects of unilateral hand muscle exercise on the ability of TMS delivered to the hemisphere that generated the exercise were examined to i) generate MEPs in the exercised hand muscles, and ii) depress MEPs evoked by TMS pulses in contralateral (non-exercised) hand muscles. After exercise there was a significant reduction in the amplitudes of MEPs evoked by TMS in the exercised muscles (p<0.001). However, the same stimuli remained able to depress responses evoked by TMS to the contralateral hemisphere in the non-exercised muscles as effectively as before the exercise. We conclude that unlike the MEPs evoked by corticospinal output, interhemispheric inhibition evoked from the hemisphere that generated the exercise is not depressed after exercise. A similar differential effect on interhemispheric inhibition and corticospinal output has been reported recently for the effects of transcranial direct current (DC) stimulation of the motor cortex. Fatiguing exercise and transcranial DC stimulation may therefore engage similar intracortical mechanisms.     

Bilateral cortical control of the human anterior digastric muscles

Transcranial magnetic stimulation (TCMS) was used to determine the organisation of cortical motor projections to the anterior digastric muscles in 12 normal human subjects. Two distinct types of potentials were evoked in anterior digastric with a figure-of-eight coil. A short-latency (3 ms) response appeared bilaterally on the surface electromyogram (EMG), but only ipsilaterally on intramuscular recordings: this was the result of direct stimulation of the ipsilateral trigeminal motor root. Motor evoked potentials (MEPs) were elicited in the anterior digastric muscles at variable onset latencies of around 10 ms by stimulation of scalp areas antero-lateral to the area for the first dorsal interosseous muscle of the hand. These were evoked bilaterally in relaxed anterior digastric muscles in six of the seven subjects. In the other subject, the responses in the relaxed muscle were exclusively ipsilateral. However, when the anterior digastric muscles were contracted, the responses were bilateral in all subjects. TCMS and spike-triggered averaging revealed that the bilateral responses were not due to the branching of axons from individual digastric motoneurones to muscles on each side. Because the digastric motor nucleus may contain separate populations of ipsi- and contralateral projecting motoneurones, it was necessary to study single motor-unit responses to TCMS to demonstrate a bilateral corticobulbar projection. The responses of 17 single motor units in the anterior digastric muscle to TCMS were recorded. All were activated by contralateral stimulation. Approximately 80% were also activated by ipsilateral TCMS, although one well-characterised motor unit was inhibited by ipsilateral TCMS. When bilateral activation was present, the ipsilateral responses were more secure than the contralateral responses, which may indicate an additional interneurone in the pathway to the contralateral motoneurone. The major conclusions from this study are that (1) the cortical representation of the anterior digastric muscle is antero-lateral to hand muscles; (2) the cortical projection to the anterior digastric muscles is bilateral; (3) the corticobulbar projection is stronger contralaterally than ipsilaterally but may involve at least one additional synapse; and (4) anterior digastric motoneurones do not branch to innervate the muscles bilaterally. Received: 8 March 1999 / Accepted: 16 June 1999     

Hemispheric asymmetry and somatotopy of afferent inhibition in healthy humans

A conditioning electrical stimulus to a digital nerve can inhibit the motor-evoked potentials (MEPs) in adjacent hand muscles elicited by transcranial magnetic stimulation (TMS) to the contralateral primary motor cortex (M1) when given 25-50 ms before the TMS pulse. This is referred to as short-latency afferent inhibition (SAI). We studied inter-hemispheric differences (Experiment 1) and within-limb somatotopy (Experiment 2) of SAI in healthy right-handers. In Experiment 1, conditioning electrical pulses were applied to the right or left index finger (D2) and MEPs were recorded from relaxed first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscles ipsilateral to the conditioning stimulus. We found that SAI was more pronounced in right hand muscles. In Experiment 2, electrical stimulation was applied to the right D2 and MEPs were recorded from ipsilateral FDI, extensor digitorum communis (EDC) and biceps brachii (BB) muscles. The amount of SAI did not differ between FDI, EDC and BB muscles. These data demonstrate inter-hemispheric differences in the processing of cutaneous input from the hand, with stronger SAI in the dominant left hemisphere. We also found that SAI occurred not only in hand muscles adjacent to electrical digital stimulation, but also in distant hand and forearm and also proximal arm muscles. This suggests that SAI induced by electrical D2 stimulation is not focal and somatotopically specific, but a more widespread inhibitory phenomenon.     

Task-dependent control of human masseter muscles from ipsilateral and contralateral motor cortex

Corticotrigeminal projections to human masseter motoneuron pools were investigated with focal transcranial magnetic stimulation (TMS). Responses in left and right masseter muscles were quantified from the surface electromyogram (EMG) during different biting tasks. During bilateral biting, TMS elicited motor evoked potentials (MEPs) in both masseter muscles. On average, the MEP area in the masseter contralateral to the stimulus was 39% larger than in the ipsilateral muscle, despite comparable pre-stimulus EMG in both muscles. MEPs elicited while subjects attempted unilateral activation of one masseter muscle were compared with those obtained in the same muscle during a bilateral bite at an equivalent EMG level. MEPs in the masseter contralateral to the stimulated hemisphere were significantly smaller during unilateral compared with bilateral biting. There was no significant difference in the size of ipsilateral MEPs during ipsilateral and bilateral biting. We conclude that the corticotrigeminal projections to masseter are bilateral, with a stronger contralateral projection. The command for unilateral biting is associated with a reduced excitability of corticotrigeminal neurons in the contralateral, but not the ipsilateral motor cortex. We suggest that this may be accomplished by reduced activity of a population of corticotrigeminal neurons which branch to innervate both masseter motoneuron pools.     

Suppression of the motor cortex by magnetic stimulation of the cerebellum

Conditioning magnetic stimulation of the cerebellum suppresses the motor cortex 5-8 ms later, probably through activation of cerebellar Purkinje cells, which inhibit the dentatothalamocortical pathway. To further characterize this pathway, we examined several factors that may modulate its excitability. We tested the effects of different test motor evoked potential (MEP) amplitudes, voluntary activation of the target muscle, and arm extension that required activation of proximal arm muscles while maintaining relaxation of hand muscles. Surface electromyography was recorded from the right first dorsal interosseous (FDI) muscle. A double-cone coil centered 3 cm lateral to the inion was used for right cerebellar stimulation. The stimulus intensity was set at 5% below the threshold for activation of the corticospinal tract. A figure-of-eight coil was used for left motor cortex stimulation. Interstimulus intervals (ISIs) between 3 and 15 ms were studied. Small test MEPs of about 0.5 mV were markedly inhibited at ISIs of 5-8 ms, but there was much less inhibition for test MEPs of about 2 mV. There was no significant MEP suppression during voluntary activation of the FDI muscle or during right arm extension. Left arm extension did not affect inhibition. Our findings indicate that cerebellar stimulation has a much stronger effect on motor cortex neurons activated near threshold intensities than those activated at higher intensities. Activation of contralateral but not ipsilateral proximal arm muscles during arm extension reduced the excitability of the cerebellothalamocortical projections to the hand area of the motor cortex.     

Corticospinal activation of internal oblique muscles has a strong ipsilateral component and can be lateralised in man

Trunk muscles receive corticospinal innervation ipsilaterally and contralaterally and here we investigate the degree of ipsilateral innervation and any cortical asymmetry in pairs of trunk muscles and proximal and distal limb muscles. Transcranial magnetic stimulation (TMS) was applied to left and right motor cortices in turn and bilateral electromyographic (EMG) recordings were made from internal oblique (IO; lower abdominal), deltoid (D; shoulder) and first dorsal interosseus (1DI; hand) muscles during voluntary contraction in ten healthy subjects. We used a 7-cm figure-of-eight stimulating coil located 2 cm lateral and 2 cm anterior to the vertex over either cortex. Incidence of ipsilateral motor evoked potentials (MEPs) was 85% in IO, 40% in D and 35% in 1DI. Mean (± S.E.M.) ipsilateral MEP latencies were longer (P<0.05; paired t-test) than contralateral MEP latencies (contralateral vs. ipsilateral; IO: 16.1±0.4 ms vs. 19.0±0.5 ms; D: 9.7±0.3 ms vs. 15.1±1.9 ms; 1DI: 18.3±0.6 ms vs. 23.3±1.4 ms), suggesting that ipsilateral MEPs were not a result of interhemispheric current spread. Where data were available, we calculated a ratio (ipsilateral MEP areas/contralateral MEP areas) for a given muscle (IO: n=16; D: n=8; 1DI: n=7 ratios). Mean values for these ratios were 0.70±0.20 (IO), 0.14±0.05 (D) and 0.08±0.02 (1DI), revealing stronger ipsilateral drive to IO. Comparisons of the sizes of these ratios revealed a bias towards one cortex or the other (four subjects right; three subjects left). The predominant cortex showed a mean ratio of 1.21±0.38 compared with 0.26±0.06 in the other cortex (P<0.05). It appears that the corticospinal control of IO has a strong ipsilateral component relative to the limb muscles and also shows hemispheric asymmetry.     

Bilateral small-hand-muscle motor evoked responses in a patient with congenital mirror movements

In a 56 year old patient with congenital mirror movements involving the muscles of the hands and forearms, transcranial electrical stimulation of either the left or right motor-cortex hand area induced bilateral surface electromyographic (EMG) responses in the abductor pollicis brevis (APB) muscles with normal latencies (21 msec). Slight volitional background contraction of the APB muscle contralateral to the stimulated motor cortex enhanced the motor evoked potential (MEP) and shortened the latency of the response by about 3 msec, while no definite facilitation was detected in the ipsilateral APB muscle. Volitional activation of the APB muscle ipsilateral to the stimulated motor cortex failed to enhance the response or shorten the latency of the response in either APB muscle. It is concluded that a fast conducting uncrossed or double crossed corticospinal system plays an important role in the execution of ipsilateral involuntary mirror movements and may be explained by aberrant reorganization of the pyramidal system.     

Demonstration of a second rapidly conducting cortico-diaphragmatic pathway in humans

Functional imaging studies in normal humans have shown that the supplementary motor area (SMA) and the primary motor cortex (PMC) are coactivated during various breathing tasks. It is not known whether a direct pathway from the SMA to the diaphragm exists, and if so what properties it has. Using transcranial magnetic stimulation (TMS) a site at the vertex, representing the diaphragm primary motor cortex, has been identified. TMS mapping revealed a second area 3 cm anterior to the vertex overlying the SMA, which had a rapidly conducting pathway to the diaphragm (mean latency 16.7 ± 2.4 ms). In comparison to the vertex, the anterior position was characterized by a higher diaphragm motor threshold, a greater proportional increase in motor-evoked potential (MEP) amplitude with voluntary facilitation and a shorter silent period. Stimulus–response curves did not differ significantly between the vertex and anterior positions. Using paired TMS, we also compared intracortical inhibition/facilitation (ICI/ICF) curves. In comparison to the vertex, the MEP elicited from the anterior position was not inhibited at short interstimulus intervals (1–5 ms) and was more facilitated at long interstimulus intervals (9–20 ms). The patterns of response were identical for the costal and crural diaphragms. We conclude that the two coil positions represent discrete areas that are likely to be the PMC and SMA, with the latter wielding a more excitatory effect on the diaphragm.