Motor map reliability and aging: a TMS/fMRI study

This study compared the reliability of motor maps over 3 sessions from both neuronavigated transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) data between younger and older adults. Seven younger (ages 19–31) and seven older (ages 64–76) adults participated in three joint TMS/fMRI assessment sessions separated by 7 or 14 days. Sessions involved mapping of the right first dorsal interosseous muscle using single-pulse TMS immediately followed by block-design fMRI scanning involving volitional right-hand index finger to thumb oppositional squeeze. Intersession reliability of map volume, evaluated by intraclass correlation and Jaccard Coefficient between testing sessions, was more consistent for younger adults in both fMRI and TMS. A positive correlation was evidenced between fMRI and TMS map volumes and Jaccard Coefficients indicating spatial consistency across sessions between the two measures. Comparisons of map reliability between age groups showed that younger adults have more stable motor maps in both fMRI and TMS. fMRI and TMS maps show consistency across modalities. Future interpretation of motor maps should attempt to account for potential increased variability of such mapping in older age groups. Despite these age group differences in reliability, fMRI and TMS appear to offer consistent and complementary information about cortical representation of the first dorsal interosseous muscle.     

Plasticity in corticomotor control of the human tongue musculature induced by tongue-task training

Transcranial magnetic stimulation (TMS) has been used to assess characteristics of the corticomotor control of the jaw muscles, but less is known about the cortical control of the human tongue and its modification by training. The aim of the present study was to determine the effect of training humans in a novel tongue-protrusion task for 1 week on corticomotor excitability as assessed by changes in electromyographic activity elicited in the tongue musculature by TMS, and in the tongue cortical motor map revealed by TMS. Eleven healthy subjects participated. Stimulus-response curves were generated from the motor evoked potentials (MEPs) recorded in the tongue musculature and, from the first dorsal interosseos (FDI) muscle as a control, at three time periods: at baseline, immediately after the 1-week training period, and at 2-weeks follow-up. In addition, the corticomotor representations of the tongue and FDI muscles were mapped on a 1 x 1 cm scalp grid. The tongue-training task required each subject to protrude the tongue onto a force transducer placed in front of the subject, and consisted of a relax-protrude-hold-relax cycle lasting 12.5 s with 1 N as the target at the hold phase. The subjects repeated this task for 60 min every day for 1 week. All subjects reported moderate levels of fatigue in the tongue during the first training day; however, these subjective reports decreased during the week (ANOVA P<0.001), and the subjects showed a progressive increase in their ability to perform the task successfully ( P<0.001). The threshold for evoking MEPs by TMS in the tongue musculature was significantly decreased after the last training day compared with baseline and the 2-weeks follow-up ( P<0.001). The amplitude of the MEPs in the tongue musculature was significantly increased at higher intensities of TMS after the last training day but returned to baseline values at the 2-weeks follow-up (P = 0.005). No significant effect of the training on MEPs in the FDI was observed (P = 0.493). Analysis of the corticomotor topographic maps revealed a significant ( P<0.05) increase in excitability and, hence, the cortical area from which TMS could evoke MEPs in the tongue, although the center of gravity representation for the tongue or FDI muscles remained stable. The present findings suggest that a specific and reversible plasticity of the corticomotor excitability related to tongue muscle control can be induced when humans learn to perform successfully a novel tongue task.     

The influence of correlated afferent input on motor cortical representations in humans

Animal models reveal that correlated afferent inputs are a powerful driver of sensorimotor cortex reorganisation. Recently we developed a stimulation paradigm, which evokes convergent afferent input from two hand muscles and induces reorganisation within human motor cortex. Here we investigated whether this reorganisation is characterised by expansion and greater overlap of muscle representation zones, as reported in animal models. Using transcranial magnetic stimulation, we mapped the motor representation of the right first dorsal interosseous (FDI), abductor digiti minimi (ADM) and abductor pollicis brevis (APB) in 24 healthy subjects before and after 1 h of (1) associative stimulation to FDI and ADM motor points, (2) associative stimulation to digits II and V (3) a control condition employing non-correlated stimulation of FDI and ADM motor points. Motor point associative stimulation induced a significant increase in the number of active sites in all three muscles and volume in FDI and ADM. Additionally, the centre of gravity of the FDI and ADM maps shifted closer together. Similar changes were not observed following digital associative stimulation or motor point non-associative stimulation. These novel findings provide evidence that convergent input induces reorganisation of the human motor cortex characterised by expansion and greater overlap of representational zones.     

Symmetric corticospinal excitability and representation of vastus lateralis muscle in right‐handed healthy subjects

The purpose of this study was to determine the size and location of the representations of the anterior thigh muscles on the human motor cortex in the dominant and non‐dominant hemispheres. Motor‐evoked potentials (MEPs) induced by transcranial magnetic stimulation were recorded from the right and left vastus lateralis (rVL, lVL) muscles. A total of ten right‐handed healthy volunteers participated in the study. In a single session experiment, we investigated VL muscle corticospinal excitability (motor threshold, MEP size, short interval intracortical inhibition, intracortical facilitation) and cortical representation (map area, volume, and location) in the dominant and non‐dominant hemispheres. The motor threshold, MEPs, and intracortical excitability did not differ significantly between the hemispheres (P > 0.05). Furthermore, no difference between sides was found in the location of VL motor representation (mediolateral and anteroposterior axis) or in map area and volume (P > 0.05). Vastus lateralis muscle corticospinal excitability and cortical map were symmetrical in right‐handed subjects. Future studies on patients with unilateral lower extremity injuries could examine side‐to‐side plastic reorganization in corticomotor output and map location in both hemispheres. Clin. Anat. 27:1053–1057, 2014. © 2014 Wiley Periodicals, Inc.     

Dissociation of cortical areas responsible for evoking excitatory and inhibitory responses in the small hand muscles

Summary Noninvasive transcranial magnetic stimulation (TMS) of the brain using a focal eight-shaped coil with 100% stimulation output was performed in eleven healthy subjects to find out if excitatory and inhibitory responses in the small hand muscles could be dissociated. Motor evoked potentials (MEP) as well as silent periods (SP) were recorded from the right abductor pollicis brevis (APB), and first dorsal interosseus (FDI) muscles at rest and during weak voluntary contraction. Mapping of the cortical representation area was performed over different scalp locations on the left hemisphere. The cortical representation maps for ABP and FDI recorded during contraction covered much larger area and were more elongated in the anterior-posterior than in the medial-lateral direction compared to maps obtained during relaxation. The distribution maps for SPs covered larger scalp areas compared to the maps of MEPs obtained during voluntary contraction. Also during voluntary contraction the locations for evoking the longest SPs were not identical to locations for evoking the peak MEP amplitudes; the longest SPs were observed during stimulation of more medial and frontal locations compared to peak MEPs. Interestingly, stimulation of some locations resulted in the appearance of an isolated MEP without the following SP and in other locations an isolated SP was recorded. The areas for evoking isolated MEPs were in the center, whereas the areas for isolated SPs were located in the periphery of the map. Features such as exclusive locations for MEPs and SPs, and different locations for peak MEP amplitudes and longest SPs, suggest dissociation of the excitatory and inhibitory cortical processes evoked by transcranial magnetic stimulation during voluntary contraction.The authors would like to thank Milan R. Dimitrijevic, M.D., D.Sc., for his advice. This work was supported by The Vivian L. Smith Foundation for Restorative Neurology, Houston, Texas.     

Motor plasticity induced by synchronized thumb and foot movements

 We used focal transcranial magnetic stimulation to examine the effects of 120 synchronized thumb and foot movements on the motor output map of the right abductor pollicis brevis muscle (APB) (experiment 1). To evaluate the performance, the latencies between the onset of the electromyographic activity (EMG) of the two muscles were measured. As control, 120 asynchronous thumb and foot movements were performed (experiment 2). Exclusively in experiment 1, the center of gravity (CoG) of the output map moved medially in the direction of the foot representation area (mean 7 mm, P<0.05) and returned into its original location within 1 h. In experiment 2, the CoG remained unchanged (mean displacement, 0.68 mm into a lateral direction; not significant). The effect in experiment 1 was independent of an improvement in performance. We conclude that a short-lasting training of synchronous movements induces modulations of motor output maps which probably occur due to interactions between hand and foot representation areas in the motor cortex. Received: 12 January 1998 / Accepted: 16 October 1998     

Cortical excitability and motor task in man: an investigation of the wrist extensor motor area

Task-related changes in the corticospinal excitation of the right extensor carpi radialis (ECR) muscle were investigated in 16 healthy human subjects. The subjects were asked to perform a tonic isometric wrist extension or to clench their hand around a manipulandum, thereby coactivating the antagonistic wrist muscles. At matched levels of background EMG in the ECR muscle, transcranial magnetic stimulation (TMS) was applied through a figure-of-eight coil at 20-30 sites spaced 1 cm apart over the hand area of the left motor cortex. The cortical maps of the representation of the ECR muscle constructed in this way did not change between the two motor tasks. Nevertheless, for all investigated cortical sites TMS evoked a smaller motor evoked potential (MEP) in the ECR muscles during hand clenching than during wrist extension. A similar decrease in the short-latency peak in the poststimulus time histogram (PSTH) of single ECR motor units to TMS during hand clenching was found in seven subjects (number of motor units = 35). In contrast, short-latency peaks in the PSTH evoked by electrical stimulation of the motor cortex had a similar size during the two tasks (number of motor units = 9; two subjects). Already the initial 0.5-1.0 ms of the short-latency peak evoked by TMS was depressed during hand clenching, which suggests that decreased excitability of corticospinal cells with monosynaptic projections onto ECR motor units was involved. This decreased excitability was not explained by increased intracortical inhibition, which was found to be of a similar size during hand clenching and wrist extension. The task-related changes in the efficiency of the motor cortex output are discussed in relation to the function of the wrist antagonist muscles in handling and gripping tasks.     

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.     

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.     

The contribution of fast corticospinal input to the voluntary activation of proximal muscles in normal subjects and in stroke patients

Although it is well known that the corticospinal system exerts more influence over distal (hand and fingers) than proximal (elbow and shoulder) upper limb muscles, differences in the importance of this system for voluntary activation of these muscle groups have not been demonstrated directly. Two investigations were carried out to provide a quantitative comparison of the contribution of fast corticospinal inputs to voluntary activity in proximal and distal muscles of normal subjects. The first study confirmed that the rate of increase in the amplitude of EMG responses to transcranial magnetic stimulation (TMS) with voluntary activation of the muscles was significantly greater in a hand muscle (first dorsal interosseous, 1DI) than in biceps, which was in turn greater than that for deltoid. The second study demonstrated that this result reflected a genuine difference in corticospinal influence over these muscles and was not due to differences in the pattern and type of motor unit recruitment in proximal vs distal muscles. The voluntary activation of a pair of low-threshold single motor units (SMUs) in 1DI and deltoid was compared with their response to TMS. In both muscles only a small amount of additional effort was required to recruit the second SMU; increments were typically within 1% of maximum voluntary contraction, as assessed from EMG measurements. Subjects were asked to voluntarily discharge the lower threshold SMU at a steady rate, and then the threshold of responses of this SMU and that of the second unit to TMS were determined. In 1DI, only small increments in TMS intensity above the threshold for the first SMU were required to activate the second unit [mean 1.4% maximum stimulator output (MSO), SD±1.0%, n=7 subjects]. In contrast, in deltoid a significantly greater intensity increase was needed (mean 6%, SD±1.2%, MSO n=7, P<0.001). Similar results were obtained when TMS thresholds of motor unit pairs were assessed in relaxed subjects. These experiments support the hypothesis that the fast corticospinal input that can be activated by TMS is of greater importance for the voluntary activation of hand than of shoulder muscles. This hypothesis served as a basis for testing deltoid responses in three stroke patients. In two patients smaller responses to TMS were obtained on the affected side than on the unaffected side during the production of equivalent voluntary contractions, suggesting that the patients achieved these contractions using inputs other than the fast corticospinal elements excited by TMS. Received: 9 June 1998 / Accepted: 16 June 1999     

Facilitation of picture naming by focal transcranial magnetic stimulation of Wernicke’s area

On the basis of an evolutionary concept of language it was postulated that activation of the motor systems for arm movements, which are phylogenetically older, should facilitate language processes. In aphasic subjects picture naming can be improved by a concomitant movement of the dominant arm. In the present study it was investigated whether a similar facilitation can be observed in normal subjects by studying the effects of transcranial magnetic stimulation (TMS) on picture naming latencies. Suprathreshold focal TMS was applied to the left motor cortex for proximal arm muscles in right-handed subjects. The effects were compared with TMS of Wernicke’s area. While TMS of the motor cortex and the non-dominant temporal lobe had no facilitatory effects, TMS of Wernicke’s area decreased picture naming latencies significantly when TMS preceded picture presentation by 500 or 1000 ms. The observed effects depended on the intensity of the stimulus used. While clearly present with intensities of 35% and 55% of maximum output the facilitation disappeared with higher stimulation intensities. It is concluded that focal magnetic stimulation is able to facilitate lexical processes due to a general preactivation of language-related neuronal networks when delivered over Wernicke’s area. Received: 11 March 1997 / Accepted: 2 March 1998     

Passive basilar membrane vibrations in gerbil neonates: mechanical bases of cochlear maturation

Reorganisation of the motor cortex may occur after limb amputation or spinal cord injury. In humans, transcranial magnetic stimulation (TMS) shows expansion of motor cortical representations of muscles proximal to the injury. Similarly, ischaemic block of the hand can increase acutely the representation of the biceps muscle, measured by increased biceps motor potentials evoked by TMS. It is thought that this increase occurs at the expense of the cortical representation of the paralysed and deafferented hand muscles but this has never been investigated. To study what changes occur in the cortical representation of the hand muscles during ischaemic block, a tungsten microelectrode was inserted into the ulnar or median nerve above the elbow and the size of the neural potential elicited by TMS in fascicles supplying the hand was measured in seven subjects. Prior to ischaemia, TMS evoked EMG responses in the intrinsic hand muscles. In the nerve, a brief motor potential preceded the response in the muscle and was followed by a contraction-induced sensory potential. During 40 min of ischaemia produced by a blood pressure cuff inflated around the forearm to 210 mmHg, the EMG response to TMS and the sensory potential from the hand were progressively blocked. However, the motor neural evoked potential showed a significant increase in amplitude during the ischaemic period (30.5 %, P = 0.005). The increase in the neural potential suggests that output to the hand evoked from the cortex by TMS was not decreased by ischaemic block. Thus, we conclude that the increased response of biceps to TMS during distal ischaemia is not accompanied by a corresponding decrease in the motor cortical representation of the hand.     

Induction of persistent changes in the organisation of the human motor cortex

Motor learning must involve changes in the organisation of the brain, and it seems axiomatic that afferent signals generated during repeated motor practice contribute to this. In this study, motor-point stimulation of the first dorsal interosseous (FDI) muscle was paired with transcranial magnetic stimulation of the human motor cortex on three successive days to determine whether repeated stimulation sessions result in enduring reorganisation of the motor cortex. This repeated "dual" stimulation induced significant changes in the excitability of the motor cortex together with expansion of the area of scalp from which these responses were elicited. The expansion in muscle representation was accompanied by large movements in the centre of gravity (CoG), suggesting a true reorganisation of the underlying cortical representational zone. The changes persisted for at least 2 days following the last stimulation session. It is concluded that repeated dual stimulation is capable of inducing long-lasting reorganisation within the motor cortex. These changes may be similar in nature to those seen in the motor cortex during motor learning. Moreover, these observations suggest that it may be possible to induce the motor cortex of patients who have suffered strokes to reorganise in a way that improves the voluntary control of the weakened muscles.     

The timing and intensity of transcranial magnetic stimulation,and the scalp site stimulated,as variables influencing motor sequence performance in healthy subjects

Objective: The increasing therapeutic use of transcranial magnetic stimulation (TMS) in disorders of cortical excitability raises the need for reliable stimulus variables. Stimulation of cortical motor areas influences motor programming and execution. We investigated the effects of TMS delivered over various cortical motor areas during the reaction time (RT) on the execution of sequential rapid arm movements in healthy subjects. Methods: Subjects performed a five-submovement (S1–S5) motor sequence mainly involving upper limb proximal muscles. RT and movement time (MT) were measured. We delivered late (close to movement onset) and early (close to the go signal) TMS over the primary motor area (M1-FDI hot-spot for the first dorsal interosseus, M1-D hot-spot for the deltoid muscle), the premotor (PM) area, and the supplementary motor area (SMA), using subthreshold and suprathreshold intensity, single and triple pulses. Results: The motor sequence showed a characteristic pattern of submovement duration, S2–S3–S4 being faster than S1 and S5. Late TMS prolonged RT only when high-intensity pulses were delivered over M1-FDI. Single- and triple-pulse TMS over M1-D or M1-FDI significantly prolonged MT with a dose-related effect. Suprathreshold triple-pulse TMS over the PM—but not over the SMA—also lengthened the MT but did not change RT. Early triple-pulse TMS reduced the RT independently from the stimulus intensity and scalp site. SMA and PM—but not M1-D—stimulation also reduced the MT. Single-pulse TMS over the SMA, despite being delivered through a double-cone coil, did not change RT or MT. Conclusions: TMS-induced changes in the kinematics of a sequential arm movement depend closely on the timing of TMS interference, the scalp site stimulated, and the intensity (and number) of stimuli delivered. Late TMS interference inhibits, whereas early interference facilitates, motor performance. The cortical motor region most sensitive to TMS-induced inhibition is that below the scalp site for M1-FDI. In contrast, TMS-induced facilitation has no strict topographic organization. Particularly for MT (although inhibitory and facilitatory effects both depend on stimulation at high intensities) intensity is less crucial than timing of interference and scalp site.     

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     

Localization of diaphragm motor cortical representation and determination of corticodiaphragmatic latencies by using magnetic stimulation in normal adult human subjects

The aims of this study were to identify the motor cortical representation of the diaphragm and to assess the corticodiaphragmatic pathway from both hemispheres. Specially designed bipolar surface electrodes were used to record the ipsilateral and contralateral compound motor evoked potentials (CMEPs) of the diaphragm after transcranial magnetic stimulation (TMS) of the motor cortex. In addition, the response to cervical magnetic stimulation of the phrenic nerve roots, effected using a figure-of-eight magnetic coil, was also recorded. The study involved 30 normal adult male volunteers. The average point of optimal excitability (POE) was determined to be 3.7 cm lateral to the mid-sagittal plane and 0.89 cm anterior to the preauricular plane. The largest response was obtained at a stimulus coil orientation of 0–90°. The TMS of either hemisphere produced CMEPs in the contralateral and ipsilateral diaphragm muscles. TMS of either hemisphere elicited CMEPs that had significantly greater amplitudes and shorter latencies from the contralateral muscles compared with the ipsilateral response (P<0.0001). The central motor conduction time of the crossed tract (8.8 ms) was significantly shorter than that of the uncrossed tract (12.2 ms). No significant interhemispheric differences were recorded. The recorded CMEPs recorded in response to TMS were facilitated during volitional inspiration. Phrenic nerve latency was 5.7 ms and 5.6 ms for the right and left phrenic nerves, respectively, with no significant difference between these values. Both bilateral crossed and uncrossed corticospinal connections to the diaphragm were usually present, with the crossed tract predominating. The technique used in this study may be useful for investigations into the function and integrity of central and peripheral pathway of the diaphragm muscles in various neurological disorders. Electronic Publication     

Long-lasting depression of motor-evoked potentials to transcranial magnetic stimulation following exercise

We used transcranial magnetic stimulation to study the modulation of motor cortex excitability after rapid repetitive movements. Eleven healthy subjects aged 24–32 years were evaluated. Serial motor-evoked potential (MEP) recordings were performed from the right thenar eminence every 5 min for a period of 20 min at rest and for a period of 35 min after repetitive abduction-adduction of the thumb at maximal frequency for 1 min. All subjects presented distinct changes in MEP amplitude after exercise with an approximately 55% mean maximal decrease compared with basal conditions and complete recovery 35 min after the end of the exercise. The time course of MEP amplitude changes presented the following trend: (1) a rapid decrease phase within the first 5 min; (2) a maximal depression phase of 10 min duration (from the 5th to the 15th min); and (3) a slow recovery phase. No significant modifications in post-exercise MEP amplitude were found in ipsilateral non-exercised muscles. In order to determine the level where these changes take place, we recorded the M and F waves induced by median nerve stimulation at the wrist (all subjects) and MEPs in response to transcranial electrical stimulation (five subjects) at rest and during the decrease and maximal depression phases. None of these tests were significantly affected by exercise, indicating that the motor cortex was the site of change. Evaluation of maps of cortical outputs to the target muscle, performed in four subjects, showed an approximately 40% spatial reduction in stimulation sites evoking a motor response during the maximal depression phase. These data prove that exercise induces a reversible, long-standing depression of cortical excitability, probably related to intracortical presynaptic modulation, which transitorily reduces the motor representation area.     

The effect of transcranial magnetic stimulation and peripheral nerve stimulation on corticomuscular coherence in humans

Cortex and muscle show coupled oscillations in the 15–35 Hz frequency band during voluntary movements. To obtain evidence of the neuronal network responsible for this rhythmicity we investigated the effect of transcranial magnetic stimulation (TMS) and peripheral nerve stimulation on the coupling between eletcroencephalographic (EEG) activity recorded from the scalp over the motor cortex and electromyographic (EMG) activity recorded from the tibialis anterior (TA) muscle in 15 healthy human subjects. TMS over the leg area at intensities between 0.95 and 1.1 × threshold for a motor evoked potential (MEP) in the TA increased corticomuscular coherence in the 15–35 Hz frequency band. This effect lasted on average for 300 ms, but could last up to 600–800 ms in some subjects. Stimulation of motor nerves from the ankle muscles suppressed corticomuscular coherence in the 15–35 Hz frequency range between leg area EEG and TA EMG for a period up to 600–800 ms. In addition, increased coherence around 10 Hz was observed for a period up to 250 ms after the stimulation. Stimulation of motor nerves in the arm and motor nerves from the ankle muscles in the other leg had no effect. The findings indicate that TMS has direct access to the neuronal circuitry in the motor cortex, which generates the corticomuscular coherence. This effect was caused either by direct activation of corticospinal cells or by activation of local neuronal circuitries in the motor cortex. The effects of peripheral nerve stimulation suggest that an alternative rhythm generator may entrain the cortical cells into a lower 10 Hz rhythm and disrupt the 15–35 Hz rhythm.     

Hemispheric asymmetry in memory-guided pointing during single-pulse transcranial magnetic stimulation of human parietal cortex

Dorsal posterior parietal cortex (PPC) has been implicated through single-unit recordings, neuroimaging data, and studies of brain-damaged humans in the spatial guidance of reaching and pointing movements. The present study examines the causal effect of single-pulse transcranial magnetic stimulation (TMS) over the left and right dorsal posterior parietal cortex during a memory-guided "reach-to-touch" movement task in six human subjects. Stimulation of the left parietal hemisphere significantly increased endpoint variability, independent of visual field, with no horizontal bias. In contrast, right parietal stimulation did not increase variability, but instead produced a significantly systematic leftward directional shift in pointing (contralateral to stimulation site) in both visual fields. Furthermore, the same lateralized pattern persisted with left-hand movement, suggesting that these aspects of parietal control of pointing movements are spatially fixed. To test whether the right parietal TMS shift occurs in visual or motor coordinates, we trained subjects to point correctly to optically reversed peripheral targets, viewed through a left-right Dove reversing prism. After prism adaptation, the horizontal pointing direction for a given visual target reversed, but the direction of shift during right parietal TMS did not reverse. Taken together, these data suggest that induction of a focal current reveals a hemispheric asymmetry in the early stages of the putative spatial processing in PPC. These results also suggest that a brief TMS pulse modifies the output of the right PPC in motor coordinates downstream from the adapted visuomotor reversal, rather than modifying the upstream visual coordinates of the memory representation.     

Cortical mapping and laminar analysis of the cutaneous and proprioceptive inputs from the rat foreleg: an extra- and intra-cellular study

Summary The foreleg proprioceptive and cutaneous representations, in the Sm cortex of urethane-anesthetized rats was studied. Natural or electrical stimulations and stretches of single forearm muscles were used. Multiunitary, unitary or intra-cellular recordings were performed in the contra-lateral Sm cortex. The aims of the study were: 1 — to compare the proprioceptive and cutaneous maps 2 — to analyse the characteristics of the unitary responses and 3 — to study the laminar distribution of cutaneous and muscular inputs. It is shown that: 1 — the proprioceptive and cutaneous representations overlapped, except in the anterior part where only proprioceptive (mainly articular) responses were obtained. The representation of each stretched muscle extended over the whole cutaneous area, showing a total overlap between inputs from these muscles. 2–46% of the intracellularly recorded cells (n=215) responded to peripheral stimulation, and 30.7% were influenced by (at least) muscle stretch. The majority of excited cells showed cross-modal covergence, and among neurons responding to muscle stretch, 60% received inputs from the two muscles stretched. Two categories of EPSPs were found, and four neurons responded to cutaneous or muscular stimulation with a burst. 19% of the responding cells were inhibited by peripheral — mainly cutaneous — stimulations. 3 —Excited neurons were recorded in all layers, with just over half located in layer IV, whereas IPSPs were obtained mainly in layer V. The cells excited by cutaneous and muscular inputs (convergent neurons) were preponderant in layers IV to VI. This work shows that the cutaneous and muscular inputs reach the same area in Sm cortex, and that a majority of excited cells are convergent. The results are not in favor of an area 3a (by analogy with cats and monkeys) in the rat.