Left Median Nerve Stimulation Research Articles

A Novel Technique for Intraoperative Mapping of the Somatosensory Cortex

Intraoperative mapping of the nervous system is used to identify "eloquent" cortical areas. In this technical report, we describe a novel way of mapping the somatosensory cortex so that injury to those critical pathways can be avoided. An 8-year-old female with drug resistant epilepsy presented for resection of a right posterior parietal focal cortical dysplasia. Left median nerve stimulation was used to record somatosensory evoked potentials (SEPs) directly from the somatosensory cortex with a strip electrode. A handheld monopolar electrode was also used to record both the median and tibial SEP. Total intravenous anesthesia with propofol and remifentanil was used. SEP recordings were obtained from a 4-contact strip electrode placed across the central sulcus. A phase reversal was identified and the most likely post central gyrus was noted. With the strip electrode left in place, a monopolar handheld electrode was used to record the median nerve SEPs from different locations on the postcentral gyrus. The tibial nerve was also stimulated to record where the highest amplitude tibial nerve SEP was present. This map was used delineate functionally "eloquent" areas to avoid during surgery. During resection, the median nerve SEP was recorded from the strip electrode continuously. No significant change in the SEP was noted, and the patient awoke without any sensory deficits. Sensory mapping of the cortex is possible with a handheld monopolar electrode. This technique is easy to perform and can help reduce neurological morbidity.

The cerebral haemodynamic response to somatosensory stimulation in preterm newborn lambs is reduced following intrauterine inflammation and dopamine infusion

BackgroundNeurovascular coupling leads to an increase in local cerebral blood flow and oxygenation in response to increased neural activity. Reduced cerebral functional responses may predispose to tissue hypoxia when neural activity is increased. Intrauterine inflammation, identified clinically as chorioamnionitis, is a major contributor to the neuropathology arising after preterm birth. The impact of chorioamnionitis on the preterm cerebral functional haemodynamic response is unknown. Previously, we have reported that somatosensory stimulation produces predominantly positive cerebral haemodynamic responses (i.e., increased cerebral oxygenation) in preterm lambs, which are reduced with dopamine treatment. As preterm infants born after chorioamnionitis often suffer from hypotension and are treated with dopamine, we aimed to investigate how chorioamnionitis with and without dopamine treatment affect the cerebral haemodynamic response in preterm lambs. MethodsAt 119 days of gestation, intrauterine inflammation was induced by intra-amniotic injection of lipopolysaccharide (LPS) in pregnant ewes. At 126–7 days of gestation (term is ~147 days), these LPS-exposed lambs were delivered and mechanically ventilated. The cerebral functional response was assessed by near infrared spectroscopy as changes in cerebral oxy- and deoxyhaemoglobin (ΔoxyHb, ΔdeoxyHb), following left median nerve stimulation of 1.8, 4.8 and 7.8 s durations without dopamine; and 4.8 and 7.8 s stimulations with intravenous dopamine infusion. ResultsStimulation for 1.8, 4.8 and 7.8 s durations led to negative functional responses (decreased ΔoxyHb) in 5 (62.5%), 5 (62.5%) and 4 (50%) of 8 preterm lambs respectively, while other lambs showed positive responses (increased ∆oxyHb). Dopamine infusion increased baseline tissue oxygenation index (TOI), oxyHb and total Hb. In lambs with a positive functional response, dopamine decreased the evoked ΔoxyHb response, increasing the overall incidence of negative cerebral haemodynamic responses. ConclusionsSomatosensory stimulation produced mostly negative responses with decreased cerebral oxygenation in preterm lambs exposed to intrauterine inflammation, contrasting with our previous findings of predominantly positive responses in non-inflamed, control, preterm lambs. Dopamine increased baseline cerebral oxygenation, but further increased the incidence of negative functional responses. Impaired neurovascular coupling leading to intermittent localised tissue hypoxia may therefore contribute to the neuropathy in infants with chorioamnionitis, with the risk of injury exacerbated with dopamine treatment.

The cerebral haemodynamic response to somatosensory stimulation in preterm newborn lambs is reduced with dopamine or dobutamine infusion

BackgroundIn the adult brain, increases in neural activity lead to increases in local blood flow. However, in the preterm neonate, studies of cerebral functional haemodynamics have yielded inconsistent results, including negative responses suggesting decreased perfusion and localised tissue hypoxia, probably due to immature neurovascular coupling. Furthermore, the impact of vasoactive medications, such as dopamine and dobutamine used as inotropic therapies in preterm neonates, on cerebrovascular responses to somatosensory input is unknown. We aimed to characterise the cerebral haemodynamic functional response after somatosensory stimulation in the preterm newborn brain, with and without dopamine or dobutamine treatment. MethodsWe studied the cerebral haemodynamic functional response in 13 anaesthetised preterm lambs, using near infrared spectroscopy to measure changes in cerebral oxy- and deoxyhaemoglobin (ΔoxyHb, ΔdeoxyHb) following left median nerve stimulation using stimulus trains of 1.8, 4.8 and 7.8 s. The 4.8 and 7.8 s stimulations were repeated during dopamine or dobutamine infusion. ResultsStimulation always produced a somatosensory evoked response. Majority of preterm lambs demonstrated positive functional responses (i.e. increased ΔoxyHb) in the contralateral cortex following stimulus trains of all durations. Dopamine increased baseline oxyHb and total Hb, whereas dobutamine increased baseline deoxyHb. Both dopamine and dobutamine reduced the evoked ΔoxyHb responses to 4.8 and 7.8 s stimulations. ConclusionsSomatosensory stimulation increases cerebral oxygenation in the preterm brain, consistent with increased cerebral blood flow due to neurovascular coupling. Notably, our results show that dopamine/dobutamine reduces oxygen delivery relative to consumption in the preterm brain during somatosensory stimulations, suggesting there may be a risk of intermittent localised tissue hypoxia which has clear implications for clinical practice and warrants further investigation.

Cerebral haemodynamic response to somatosensory stimulation in neonatal lambs.

Cerebral haemodynamic response to neural stimulation has been extensively studied in adults, but little is known about cerebral haemodynamic response in the fetal and neonatal brain. The present study describes the cerebral haemodynamic response measured by near infrared spectroscopy to somatosensory stimulation in newborn lambs, in comparison to recent findings in fetal sheep. The cerebral haemodynamic responses in the newborn lamb brain can involve an increase in oxyhaemoglobin (oxyHb), or a decrease of oxyHb suggestive of reduced perfusion and oxygenation. Positive correlations between changes in oxyHb and mean arterial blood pressure were found in newborn but not fetal sheep, which suggests the result is unlikely to be due to immature autoregulation alone. In contrast to adult studies, hypercapnia increased the changes in cerebral blood flow and oxyHb in most of the lambs in response to somatosensory stimulation. The neurovascular coupling response has been defined for the adult brain, but in the neonate non-invasive measurement of local cerebral perfusion using near infrared spectroscopy or blood oxygen level-dependent functional magnetic resonance imaging have yielded variable and inconsistent results, including negative responses suggesting decreased perfusion and localized tissue tissue hypoxia. Also, the impact of permissive hypercapnia (P aC O2>50mmHg) in the management of neonates on cerebrovascular responses to somatosensory input is unknown. Using near infrared spectroscopy to measure changes in cerebral oxy- and deoxyhaemoglobin (ΔoxyHb, ΔdeoxyHb) in eight anaesthetized newborn lambs, we studied the cerebral haemodynamic functional response to left median nerve stimulation using stimulus trains of 1.8, 4.8 and 7.8s. Stimulation always produced a somatosensory evoked response, and superficial cortical perfusion measured by laser Doppler flowmetry predominantly increased following median nerve stimulation. However, with 1.8s stimulation, oxyHb responses in the contralateral hemisphere were either positive (i.e. increased oxyHb), negative, or absent; and with 4.8 and 7.8s stimulations, both positive and negative responses were observed. Hypercapnia increased baseline oxyHb and total Hb consistent with cerebral vasodilatation, and six of seven lambs tested showed increased Δtotal Hb responses after the 7.8s stimulation, among which four lambs also showed increased ΔoxyHb responses. In two of three lambs, the negative ΔoxyHb response became a positive pattern during hypercapnia. These results show that instead of functional hyperaemia, somatosensory stimulation can evoke negative (decreased oxyHb, total Hb) functional responses in the neonatal brain suggestive of decreased local perfusion and vasoconstriction, and that hypercapnia produces both baseline hyperperfusion and increased functional hyperaemia.

Effects of 12 Weeks of Chiropractic Care on Central Integration of Dual Somatosensory Input in Chronic Pain Patients: A Preliminary Study

ObjectiveThe purpose of this preliminary study was to assess whether the dual somatosensory evoked potential (SEP) technique is sensitive enough to measure changes in cortical intrinsic inhibitory interactions in patients with chronic neck or upper extremity pain and, if so, whether changes are associated with changes in pain scores. MethodsThe dual peripheral nerve stimulation SEP ratio technique was used for 6 subjects with a history of chronic neck or upper limb pain. SEPs were recorded after left or right median and ulnar nerve stimulation at the wrist. SEP ratios were calculated for the N9, N13, P14-18, N20-P25, and P22-N30 peak complexes from SEP amplitudes obtained from simultaneous median and ulnar stimulation divided by the arithmetic sum of SEPs obtained from individual stimulation of the median and ulnar nerves. Outcome measures of SEP ratios and subjects’ visual analog scale rating of pains were recorded at baseline, after a 2-week usual care control period, and after 12 weeks of multimodal chiropractic care (chiropractic spinal manipulation and 1 or more of the following: exercises, peripheral joint adjustments/manipulation, soft tissue therapy, and pain education). ResultsA significant decrease in the median and ulnar to median plus ulnar ratio and the median and ulnar amplitude for the cortical P22-N30 SEP component was observed after 12 weeks of chiropractic care, with no changes after the control period. There was a significant decrease in visual analog scale scores (both for current pain and for pain last week). ConclusionThe dual SEP ratio technique appears to be sensitive enough to measure changes in cortical intrinsic inhibitory interactions in patients with chronic neck pain. The observations in 6 subjects revealed that 12 weeks of chiropractic care improved suppression of SEPs evoked by dual upper limb nerve stimulation at the level of the motor cortex, premotor areas, and/or subcortical areas such as basal ganglia and/or thalamus. It is possible that these findings explain one of the mechanisms by which chiropractic care improves function and reduces pain for chronic pain patients.

Rejecting deep brain stimulation artefacts from MEG data using ICA and mutual information

BackgroundRecording brain activity during deep brain stimulation (DBS) using magnetoencephalography (MEG) can potentially help clarifying the neurophysiological mechanism of DBS. The DBS artefact, however, distorts MEG data significantly. We present an artefact rejection approach to remove the DBS artefact from MEG data. New methodsWe developed an approach consisting of four consecutive steps: (i) independent component analysis was used to decompose MEG data to independent components (ICs); (ii) mutual information (MI) between stimulation signal and all ICs was calculated; (iii) artefactual ICs were identified by means of an MI threshold; and (iv) the MEG signal was reconstructed using only non-artefactual ICs. This approach was applied to MEG data from five Parkinson’s disease patients with implanted DBS stimulators. MEG was recorded with DBS ON (unilateral stimulation of the subthalamic nucleus) and DBS OFF during two experimental conditions: a visual attention task and alternating right and left median nerve stimulation. ResultsWith the presented approach most of the artefact could be removed. The signal of interest could be retrieved in both conditions. Comparison with existing methodsIn contrast to existing artefact rejection methods for MEG-DBS data (tSSS and S3P), the proposed method uses the actual artefact source, i.e. the stimulation signal, as reference signal. ConclusionsUsing the presented method, the DBS artefact can be significantly rejected and the physiological data can be restored. This will facilitate research addressing the impact of DBS on brain activity during rest and various tasks.

Towards non-invasive multi-unit spike recordings: Mapping 1 kHz EEG signals over human somatosensory cortex

ObjectiveScalp-derived human somatosensory evoked potentials (SEPs) contain high-frequency oscillations (600Hz; ‘sigma-burst’) reflecting concomitant bursts of spike responses in primary somatosensory cortex that repeat regularly at 600Hz. Notably, recent human intracranial SEP have revealed also 1kHz responses (‘kappa-burst’), possibly reflecting non-rhythmic spiking summed over multiple cells (MUA: multi-unit activity). However, the non-invasive detection of EEG signals at 1kHz typical for spikes has always been limited by noise contributions from both, amplifier and body/electrode interface. Accordingly, we developed a low-noise recording set-up optimised to map non-invasively 1kHz SEP components. MethodsSEP were recorded upon 4Hz left median nerve stimulation in 6 healthy human subjects. Scalp potentials were acquired inside an electrically and magnetically shielded room using low-noise custom-made amplifiers. Furthermore, in order to reduce thermal Johnson noise contributions from the sensor/skin interface, electrode impedances were adjusted to ⩽1kΩ. Responses averaged after repeated presentation of the stimulus (n=4000 trials) were evaluated by spatio-temporal pattern analyses in complementary spectral bands. ResultsThree distinct spectral components were identified: N20 (<100Hz), sigma-burst (450–750Hz), and kappa-burst (850–1200Hz). The two high-frequency bursts (sigma, kappa) exhibited distinct and partially independent spatiotemporal evolutions, indicating subcortical as well as several cortical generators. ConclusionsUsing a dedicated low-noise set-up, human SEP ‘kappa-bursts’ at 1kHz can be non-invasively detected and their scalp distribution be mapped. Their topographies indicate a set of subcortical/cortical generators, at least partially distinct from the topography of the 600Hz sigma-bursts described previously. SignificanceThe non-invasive detection and surface mapping of 1kHz EEG signals presented here provides an essential step towards non-invasive monitoring of multi-unit spike activity.

Intracortical modulation of somatosensory evoked fields during movement: Evidence for selective suppression of postsynaptic inhibition

As accurate finger movements depend on guidance by afferent sensory feedback information, it is of interest to examine how the cortical processing of afferent signals is altered during movement states compared with rest. In the present study we evaluated afferent input to the primary somatosensory cortex (SI) in human subjects performing a finger opposition task. We recorded somatosensory evoked magnetic fields (SEFs) in 6 healthy subjects to stimulation of left and right median nerves in a resting condition and during active right-sided finger movements. At the left SI, the SEFs to right (moving hand) median nerve stimulation showed a selective and robust reduction of the P35m deflection during movement compared with rest, while there were only minor non-significant changes in the other SEF deflections, including N20m, which represents the 1st excitatory cortical event after stimulation. In contrast, at the right SI the SEFs to left (non-moving hand) median nerve stimulation were modified in the opposite direction: the P35m deflection was slightly enhanced during right-sided movement, there being no significant changes in the other deflections. The results thus show that the P35m SEF deflection can be selectively reduced during finger movements of the stimulated hand, and selectively enhanced if the movement is being performed with the fingers of the opposite hand. Because N20m was not changed, the modulation took place at the cortical level rather than in the afferent pathways. As the P35m SEF deflection likely represents postsynaptic IPSPs at SI, the results suggest that postsynaptic inhibition to somatosensory impulses from the moving part of the body is suppressed. Comparison of the present results with recent intracellular studies in behaving mice suggests that the P35m reduction specifically corresponds to a reduction in the activity of parvalbumin-containing fast-spiking inhibitory interneurons during movement. The results provide evidence that precision movements can be executed without this type of cortical postsynaptic inhibition.

Vestibular inputs modulate somatosensory cortical processing

The vestibular system is unique among the senses because of the entirely multisensory nature of its cortical projections. Neuroanatomical and neuroimaging studies show that vestibular stimulation activates somatosensory areas, and particularly the so-called parieto-insular vestibular cortex (PIVC) in the monkey, while deactivating visual areas. Further, recent psychophysical studies showed that vestibular stimulation facilitates detection of electrocutaneous stimuli, suggesting a vestibular-somatosensory perceptual interaction. However, the functional mechanism underlying this perceptual facilitation remains unclear. We therefore recorded somatosensory potentials evoked by left median nerve stimulation, before and immediately after left cold caloric vestibular stimulation (CVS), in a small-scale study of eight healthy volunteers. CVS selectively enhanced the N80 component recorded over both ipsilateral and contralateral somatosensory areas, without significantly affecting earlier or later components. Interestingly, the N80 component has been localised to the parietal operculum, which includes the human homologue of the monkey PIVC, and is thus a prime neuroanatomical candidate for vestibular-somatosensory convergence. As a control, we measured visual evoked potentials to reversing checkerboard patterns and found no effects of vestibular stimulation. This rules out explanations based on indirect effects of vestibular modulations, such as general arousal or supramodal spatial attention. We believe our results provide the first clue linking brain structure to function for the interaction between vestibular and somatosensory systems.

Asymmetric Activation of the Primary Motor Cortex during Observation of a Mirror Reflection of a Hand

Mirror therapy is an effective technique for pain relief and motor function recovery. It has been demonstrated that magnetic 20-Hz activity is induced in the primary motor cortex (M1) after median nerve stimulation and that the amount of the stimulus-induced 20-Hz activity is decreased when the M1 is activated. In the present study, we investigated how the image or the mirror reflection of a hand holding a pencil modulates the stimulus-induced 20-Hz activity in the M1. Neuromagnetic brain activity was recorded from 13 healthy right-handed subjects while they were either viewing directly their hand holding a pencil or viewing a mirror reflection of their hand holding a pencil. The 20-Hz activity in the left or the right M1 was examined after the right or the left median nerve stimulation, respectively, and the suppression of the stimulus-induced 20-Hz in the M1 by viewing directly one hand holding a pencil or by viewing the mirror image of the hand holding a pencil was assumed to indicate the activation of the M1. The results indicated that the M1 innervating the dominant hand was suppressed either by viewing directly the dominant hand holding a pencil or by viewing the mirror image of the non-dominant hand holding a pencil. On the other hand, the M1 innervating the non-dominant hand was activated by viewing the mirror image of the dominant hand holding a pencil, but was not activated by viewing directly the non-dominant hand holding a pencil. The M1 innervating either the dominant or the non-dominant hand, however, was not activated by viewing the hand on the side ipsilateral to the M1 examined or the mirror image of the hand on the side contralateral to the M1 exaimined. Such activation of the M1 might induce some therapeutic effects of mirror therapy.

Stroke Patient with Mirror Movement of the Affected Hand Due to an Ipsilateral Motor Pathway Confirmed by Transcranial Magnetic Stimulation: A Case Report

ABSTRACTA stroke patient with right hemiplegia and mirror movement underwent transcranial magnetic stimulation (TMS) and somatosensory-evoked potential (SEP) testing. The motor-evoked potentials (MEPs) of both abductor pollicis brevis muscles after stimulating the unaffected right hemisphere showed similar latencies, and were potentially produced by corticospinal tracts from the same motor cortex. N20 responses of SEPs were recorded at C4′ after contralateral stimulation of the unaffected left median nerve, but not stimulation of the affected right median nerve. The mirror movements and motor recovery might have utilized an ipsilateral motor pathway between the unaffected hemisphere and the affected hand.

Somatosensory evoked potentials in epileptic children treated with carbamazepine or valproate in monotherapy – a preliminary study

Data from the literature suggest that long-term therapy with various antiepileptic drugs can be responsible for the functional disturbances within the nervous system e.g. peripheral neuropathy and encephalopathy. Useful and non-invasive instruments for evaluation of even subclinical nerve conduction abnormalities in somatosensory tracts are somatosensory evoked potentials (SEPs). The aim of this study was to assess the potentially drug-induced abnormalities in the SEP parameters in epileptic children, treated chronically in monotherapy with one of the two most often used antiepileptic drugs: valproate (VPA) or carbamazepine (CBZ). SEP from left median nerve stimulation were recorded in twenty children with idiopathic/cryptogenic epilepsy treated in monotherapy with CBZ (9 patients) or VPA (11 patients). The mean age of the patients was 13.4 ± 2.9 years (range 7-17 years). The plasma concentrations of the drugs were consistently within therapeutic range. The mean duration of treatment was eight months. The control group consisted of twenty-four age-matched children with tension type headache. The latencies of the components: N9, N13, N20, P25 and the peripheral conduction time (PCT) and central conduction time (CCT) were analyzed. No significant differences in all analyzed SEP parameters between the epileptic and control children were found. Our results indicate that during the first 8 months from the beginning of antiepileptic treatment in children, monotherapy with VPA or CBZ does not induce nerve conduction disturbances within both the peripheral- and the central part of the somatosensory tracts, detected in SEP examination.

Hemispheric asymmetry of hand representation in human primary somatosensory cortex and handedness

Objective To evaluate interhemispheric differences of hand representation in primary somatosensory (SI) and motor (MI) cortices and its relation to handedness. Methods MRI-based EEG dipole source analysis was performed separately for early (P14, N20, P22) and middle/late latency (P30, N60, N110) SEP components after left and right median nerve stimulation. In addition, the location of the MI hand area (Ω region) and handedness were determined. Results Equivalent current dipoles (ECDs) of N20, P30 and N60 SEP components were localized in contralateral SI (area 3b, N20 and P30; area 1, N60), the mean P22 ECD localization was in area 4 of contralateral MI. In contrast to the Ω region and the precentral P22 component, ECDs in both areas 3b and 1 were located more laterally in the right than in the left hemisphere. ECDs in the right SI lay more laterally than the ipsilateral Ω region. Asymmetry in SI was not correlated with handedness. Conclusions The data demonstrate that the location of hand representation shows relevant hemispheric asymmetry in human SI, both in areas 3b and 1. Significance Hemispheric asymmetry in SI must be considered in the studies on cortical reorganization and plasticity in SI as well as for transcranial magnetic stimulation (TMS) over SI.

Cortisol Inhibits Neuroplasticity Induction in Human Motor Cortex

We investigated whether plasticity of human motor cortex (M1) is influenced by time of day, and whether changes in circulating levels of cortisol contribute to this effect. Neuroplasticity was induced using paired associative stimulation (PAS), involving electrical stimulation of left median nerve, paired with transcranial magnetic stimulation over the right M1 25 ms later (90 pairs at 0.05 Hz). Surface EMG was recorded from the left abductor pollicis brevis (APB) and first dorsal interosseous muscle. Cortisol levels were assessed from saliva. Time-of-day modulation of PAS effectiveness was assessed in 25 subjects who were tested twice, at 8:00 A.M. and 8:00 P.M. on separate days. In a second double-blind study, 17 subjects were tested with PAS at 8:00 P.M. on two occasions after administration of oral hydrocortisone (24 mg) or placebo. The motor-evoked potential (MEP) in resting APB increased significantly after PAS in the evening (when endogenous cortisol levels were low), but not in the morning. Oral hydrocortisone prevented facilitation of the APB MEP after PAS, and in the drug study, mean salivary cortisol levels were negatively associated with PAS effectiveness. The GABA(B)-mediated cortical silent period for APB was longer in the morning than in the evening, and was lengthened by PAS and oral hydrocortisone. We conclude that neuroplasticity in human M1 and GABA(B)-dependent intracortical inhibitory systems are influenced by time of day and modified by circulating levels of cortisol.

Factors influencing the magnitude and reproducibility of corticomotor excitability changes induced by paired associative stimulation

Several paired-associative stimulation (PAS) protocols induce neuroplastic changes in human motor cortex (M1). To understand better the inherent variability of responses to PAS, we investigated the effectiveness and reproducibility of two PAS paradigms, and neurophysiological and experimental variables that may influence this. Motor evoked potentials (MEPs) were elicited by transcranial magnetic stimulation (TMS) of right M1, and recorded from surface EMG of left abductor pollicis brevis (APB) and first dorsal interosseous before and after PAS. PAS consisted of electrical stimulation of left median nerve paired with TMS over right M1 25 ms later. Twenty subjects were given one of two PAS protocols: short (132 paired stimuli at 0.2 Hz) or long (90 paired stimuli at 0.05 Hz), and were re-tested with the same protocol on 3 separate occasions, with 11 subjects tested in the morning and 9 in the afternoon. Neurophysiological variables assessed included MEP amplitude, resting and active motor threshold, short-interval intracortical inhibition, intracortical facilitation and cortical silent period duration. The short PAS protocol produced greater APB MEP facilitation (51%) than the long protocol (11%), and this did not differ between sessions. The neurophysiological variables did not consistently predict responses to PAS. Both PAS protocols induced more APB MEP facilitation, and greater reproducibility between sessions, in experiments conducted in the afternoon. The mechanism for this is unclear, but circadian rhythms in hormones and neuromodulators known to influence neuroplasticity warrant investigation. Future studies involving PAS should be conducted at a fixed time of day, preferably in the afternoon, to maximise neuroplasticity and reduce variability.

Effect of theta burst stimulation over the human sensorimotor cortex on motor and somatosensory evoked potentials

Objective To study the after-effect of theta burst stimulation (TBS) over the left sensorimotor cortex on the size of somatosensory as well as motor evoked potentials evoked from both hemispheres in healthy human subjects. Methods We used a continuous TBS paradigm for 40 s (600 pulses) in which a burst of 3 transcranial magnetic stimuli at 50 Hz is repeated at 5 Hz [Huang YZ, Edwards MJ, Rounis E, Bhatia KP, Rothwell JC. Theta burst stimulation of the human motor cortex. Neuron 2005;45:201–6]. Somatosensory evoked potentials (SEPs) following electrical stimulation of right or left median nerve and motor evoked potentials (MEPs) in the right or left first dorsal interosseous (FDI) muscles were recorded before and after TBS over the left motor cortex (M1) or a point 2 cm posterior to left M1. Results Amplitudes of P25/N33 (parietal components) following right median nerve stimulation were significantly increased for at least 53 min after TBS over the left M1, whereas this component was suppressed for 13 min after TBS over a point 2 cm posterior. MEPs in right as well as left FDI muscles were suppressed with a similar time course after TBS over the left M1. Conclusions A single-session of TBS over the sensorimotor cortex can induce a short-lasting change in the size of ipsilateral cortical components of SEPs as well as MEPs evoked from both hemispheres. Significance TBS is an interventional tool that can induce rapid reorganization within cortical somatosensory as well as motor networks in humans.

Spatial dynamics of population activities at S1 after median and ulnar nerve stimulation revisited: An MEG study

In a number of studies, magnetoencephalography (MEG) has been successfully employed in localizing cortical neural population activities after stimulation of peripheral nerves. Little attention has been paid, however, to the spatiotemporal dynamics of these activations within the primary somatosensory cortex (SI). Here we report on the activation sequence at the right SI after left median and ulnar nerve stimulation. The results show that at least three macroscopically separable sources within or near SI are activated within 100 ms after the stimulus, corresponding to the somatosensory evoked field (SEF) deflections N20m, P35m and P60m. As P60m was localized significantly more posteriorly and also tended to be deeper than the two earlier deflections, its underlying source may be more extensive than during N20m and P35m, and it may get contribution from the postcentral gyrus and sulcus, possibly Brodmann areas 1 and 2. The source separation between the neural populations activated by the 2 nerves was 12 mm during N20m, 6 mm during P35m and 4 mm during P60m. Thus, at longer latencies, the centers of gravity of the activations were closer to each other for the 2 nerves. We argue that this reflects spreading of the activation with time from the site of initial excitation to encompass larger and more overlapping neural populations at longer latencies.

Differential generators for N20m and P35m responses to median nerve stimulation

To study the spatial and behavioral dynamics of cortical sources for N20m and P35m at varying stimulus intensities, we measured neuromagnetic cortical responses to left electric median nerve stimulation at the wrist in 17 male healthy adults. The stimulus intensity levels were individually determined according to sensory threshold (ST) for perceiving electric pulses. Using equivalent current dipole (ECD) modeling, we analyzed the peak latencies, amplitudes, and locations of ECDs from 14 subjects for N20m and P35m elicited at 2 ST, 3 ST, and 4 ST. Compared with N20m, P35m was localized 3.3 ± 0.6 mm more superiorly at 2–4 ST, and 2.9 ± 1.2 mm more medially at 3–4 ST. Superimposed over subjects' own MR images, N20m ECDs were localized in the area of 3b contralateral to stimulus side in all 17 subjects at 3 ST, whereas P35m ECDs were localized either in the postcentral (in 14 subjects) or in the precentral areas (in 3 subjects). We found no clear correlation between N20m and P35m in terms of peak latencies as well as the corresponding growth of activation strengths along with stepwise increase in stimulus intensity. Our results imply that the two early SEF components, N20m and P35m, have differential cortical generators, with distinctive neurophysiological behaviors in response to varying stimulus intensity levels.

Effect of transcranial DC sensorimotor cortex stimulation on somatosensory evoked potentials in humans

To study the after-effect of transcranial direct current stimulation (tDCS) over the sensorimotor cortex on the size of somatosensory evoked potentials (SEPs) in humans. SEPs were elicited by electrical stimulation of right or left median nerve at the wrist before and after anodal or cathodal tDCS in 8 healthy subjects. tDCS was applied for 10 min to the left motor cortex at a current strength of 1 mA. Amplitudes of P25/N33, N33/P40 (parietal components) and P22/N30 (frontal component) following right median nerve stimulation were significantly increased for at least 60 min after the end of anodal tDCS, whereas P14/N20, N20/P25 (parietal components) and N18/P22 (frontal component) were unaffected. There was no effect on SEPs evoked by left median nerve stimulation. Cathodal tDCS had no effect on SEPs evoked from stimulation of either arm. Anodal tDCS over the sensorimotor cortex can induce a long-lasting increase in the size of ipsilateral cortical components of SEPs. tDCS can modulate cortical somatosensory processing in humans and might be a useful tool to induce plasticity in cortical sensory processing.

Morphology of somatosensory evoked fields: inter-hemispheric similarity as a parameter for physiological and pathological neural connectivity

The morphology of somatosensory evoked fields (SEF) following electrical separate stimulation of left and right median nerve in 22 healthy volunteers was explored. The analysis of morphological properties of individual SEFs has been performed, in order to quantify an inter-hemispheric correlation coefficient. Despite a high inter-subject variability of the SEF morphologies, a high intra-subject inter-hemispheric shape correlation occurs in the post-stimulus epoch of 100 ms from the stimulus onset. Accordingly, a normative value for the parameter describing the SEF inter-hemispheric correlation coefficient has been calculated. The present work establishes quantitative and normative descriptions of the shape similarity of SEFs from the two hands in the left and right hemispheres. This provides a database for a new parameter, which might be useful in detecting alterations in the primary sensory cortical activation due to possible unilateral alterations of sensory inflow because of either peripheral or central deficits. The described procedure has been successfully applied to a small number of patients affected by unilateral cerebrovascular lesion.