Near-infrared study of fluctuations in cerebral hemodynamics during rest and motor stimulation: temporal analysis and spatial mapping

We have noninvasively studied the motor cortex hemodynamics in human subjects under rest and motor stimulation conditions using a multichannel near-infrared tissue spectrometer. Our instrument measures optical maps of the cerebral cortex at two wavelengths (758 and 830 nm), with an acquisition time of 160 ms per map. We obtained optical maps of oxy- and deoxy-hemoglobin concentration changes in terms of amplitudes of folding average, power spectrum and coherence at the stimulation repetition frequency, and the phase synchronization index. Under periodic motor stimulation conditions, we observed coherence and frequency or phase synchronization of the local hemodynamic changes with stimulation. Our main findings are the following: (1) The amplitude of the hemodynamic response to the motor stimulation is comparable to the amplitude of the fluctuations at rest. (2) The spatial patterns of the oxy- and deoxy-hemoglobin responses to the stimulation are different. (3) The hemodynamic response to stimulation shows a spatial localization and a level of phase synchronization with the motor stimulation that depends on the stimulation period.     

Cortical excitability changes following grasping exercise augmented with electrical stimulation

Rehabilitation with augmented electrical stimulation can enhance functional recovery after stroke, and cortical plasticity may play a role in this process. The purpose of this study was to compare the effects of three training paradigms on cortical excitability in healthy subjects. Cortical excitability was evaluated by analysing the input–output relationship between transcranial magnetic stimulation intensity and motor evoked potentials (MEPs) from the flexor muscles of the fingers. The study was performed with 25 healthy volunteers who underwent 20-min simulated therapy sessions of: (1) functional electrical stimulation (FES) of the finger flexors and extensors, (2) voluntary movement (VOL) with sensory stimulation, and (3) therapeutic FES (TFES) where the electrical stimulation augmented voluntary activation. TFES training produced a significant increase in MEP magnitude throughout the stimulation range, suggesting an increase in cortical excitability. In contrast, neither the FES nor voluntary movement alone had such an effect. These results suggest that the combination of voluntary effort and FES has greater potential to induce plasticity in the motor cortex and that TFES might be a more effective approach in rehabilitation after stroke than FES or repetitive voluntary training alone.     

Effects of voluntary contraction on descending volleys evoked by transcranial electrical stimulation over the motor cortex hand area in conscious humans

 The spinal volleys evoked by electric anodal and cathodal stimulation over the cerebral motor cortex hand area were recorded from a bipolar electrode inserted into the cervical epidural space of two conscious human subjects. We measured the size of volleys elicited by electric stimulation at active motor threshold and at 3% of maximum stimulator output above this value with subjects at rest and during maximum voluntary contraction of the contralateral first dorsal interosseous muscle. Surface EMG activity was recorded at the same time. Electrical anodal stimulation evoked a single negative wave that we termed D-wave in analogy with data in experimental animals. Cathodal stimulation evoked a single negative wave with a latency of 0.2 ms longer than the D-wave recruited by anodal stimulation. At both intensities tested, voluntary contraction did not modify the amplitude of the descending waves. We conclude that changes in cortical excitability induced by voluntary activity do not modify the corticospinal volley evoked by electric stimulation and that the D-waves evoked by both anodal and cathodal electric stimulation are probably initiated several nodes distant to the cell body. Received: 9 September 1998 / Accepted: 21 October 1998     

Motor evoked potential depression following repetitive central motor initiation

Prior reports have described a transient and focal decline in transcranial magnetic stimulation (TMS)-induced motor evoked potential (MEP) amplitude following fatiguing motor tasks. However, the neurophysiological causes of this change in MEP amplitude are unknown. The aim of this study was to determine whether post-task depression of MEPs is associated with repetitive central motor initiation. We hypothesized that MEP depression is related to repeated central initiation of motor commands in task-related cortex independent of motor fatigue. Twenty healthy adults had MEPs measured from the dominant first dorsal interosseous (FDI) muscle before and after six different tasks: rest (no activity), contralateral fatiguing hand-grip, ipsilateral fatiguing hand-grip, contralateral finger tapping, ipsilateral finger tapping, and imagined hand-grip (motor imagery). Changes in MEPs from baseline were assessed for each task immediately following the task and at 2-min intervals until MEPs returned to a stable baseline. Measures of subjective effort and FDI maximum voluntary contractions (MVC) were also recorded following each task. A statistically significant drop in MEP amplitude was noted only with contralateral finger tapping and imagined grip. Changes in MEP amplitude did not correlate with subjective fatigue or effort. There was no significant change in FDI MVCs following hand-grip or finger-tapping tasks. This study extends our knowledge of the observed decline in MEP amplitude following certain tasks. Our results suggest that central initiation of motor programs may induce a change in MEP amplitude, even in the absence of objective fatigue.     

Transcranial stimulation parameters to elicit motor evoked potentials

PURPOSE: We studied effectiveness of single, paired, and repetitive transcranial electrical stimulation to activate motor cortex in patients during spine surgery. METHODS: In ten consented patients, the latency and amplitude of the muscle action potential from abductor pollicis brevis muscle after cortical stimulation was measured. RESULTS: The number of responses evoked by double (pair) pulse stimulation was significantly higher than that evoked by single pulse (p = 0.0191). A similar finding was obtained between the repetitive and single pulse stimulation (p = 0.0001). The repetitive stimulation was more effective than the double pulse stimulation (p = 0.0253). An increase of interstimulus interval (ISI), between pulses, from 1-3 msec did not significantly alter latency (p > 0.05) or amplitude (p > 0.05) with the double pulse or repetitive stimulations. The latency or amplitude did not significantly change with varying number of electrical pulses from 3-9. CONCLUSIONS: This study shows that the intraoperative repetitive transcranial stimulation is the most effective method to activate descending motor tracts under anesthesia.     

Depressed mood, index finger force and motor cortex stimulation: a transcranial magnetic stimulation (TMS) study

The present study utilized transcranial magnetic stimulation (TMS) of the motor cortex to understand basic motor processes associated with depressive symptoms independent of cognitive requirements or diagnostic category. To assess the integrity of the basic cortical-spinal-motor circuit associated with depressed mood, TMS to the motor cortex was used to initiate motor evoked potentials (MEPs) in forearm EMG and force production measured in the right (dominant) index finger. While at rest, a group with more depressive symptoms showed less force response in the index finger following stimulations compared with a group endorsing less depressive symptoms. A negative correlation between force response in the index finger at baseline (rest) following stimulation and the Beck depression inventory indicated that depressive mood symptom elevations were associated with less response to stimulations. The results argue for a greater importance placed on the relationship between depressive mood symptoms and basic motor processes.     

Optical mapping reveals the functional organization of the trigeminal nuclei in the chick embryo.

The functional organization of the trigeminal nuclei during embryogenesis was investigated using multiple-site optical recording with a fast voltage-sensitive dye. Brainstem preparations with three classified trigeminal nerve afferents, the ophthalmic, maxillary and mandibular nerves, together with motor nerve fibers, were dissected from five- to eight-day-old chick embryos. Electrical responses evoked by trigeminal nerve stimulations were optically recorded simultaneously from many loci of the stained preparations. We identified three response areas related to the trigeminal nerve: area I, located cephalic to the level of the trigeminal ganglion; area II, located caudal to the level of the trigeminal ganglion; and area III, located at the level of the trigeminal root. The neural responses in areas I and II were evoked by ophthalmic, maxillary or mandibular nerve stimulation, while the responses in area III were detected when the stimulation was applied to the trigeminal motor nerve. In comparison with the morphology indicated by DiI labeling, the results suggest that areas I, II and III correspond to the principal sensory nucleus of the trigeminal nerve, the spinal sensory nucleus of the trigeminal nerve and the trigeminal motor nucleus, respectively. We identified two components of the optical response: a fast and a slow signal. In five-day-old preparations, fast spike-like signals related to action potentials were recorded from the three response areas. In six-day-old preparations, slow optical signals which reflect glutamate-mediated excitatory postsynaptic potentials were detected from area II only when the ophthalmic nerve was stimulated: no slow signal was evoked by maxillary or mandibular nerve stimulation. In seven- and eight-day-old preparations, slow signals were detected from both areas I and II with every nerve stimulation. These results suggest that synaptic function is first generated in the spinal trigeminal nucleus by the six-day embryonic stage, and the developmental organization of synaptic function is not the same in the three trigeminal nerves or in the two sensory nuclei. Contour line maps of the signal amplitude revealed that the size and the area of the neural responses within the trigeminal nuclei changed dramatically with development. We compared the spatial distribution and temporal dynamics of the optical signals between the ophthalmic, maxillary and mandibular nerve stimulations, and we found that somatotopic organization is less clear in a rostrocaudal/mediolateral X-Y plane, although the areas of the maxillary and mandibular nerves appeared to separate in the lateral direction.     

Motor cortex inhibition induced by acoustic stimulation

The influence of the brainstem motor system on cerebral motor areas may play an important role in motor control in health and disease. A new approach to investigate this interaction in man is combining acoustic stimulation activating the startle system with transcranial magnetic stimulation (TMS) over the motor cortex. However, it is unclear whether the inhibition of TMS responses following acoustic stimulation occurs at the level of the motor cortex through reticulo-cortical projections or subcortically, perhaps through reticulo-spinal projections. We compared the influence of acoustic stimulation on motor effects elicited by TMS over motor cortical areas to those evoked with subcortical electrical stimulation (SES) through depth electrodes in five patients treated with deep brain stimulation for Parkinsons disease. SES bypasses the motor cortex, demonstrating any interaction with acoustic stimuli at the subcortical level. EMG was recorded from the contralateral biceps brachii muscle. Acoustic stimulation was delivered binaurally through headphones and used as a conditioning stimulus at an interstimulus interval of 50 ms. When TMS was used as the test stimulus, the area and amplitude of the conditioned motor response was significantly inhibited (area: 57.5±12.9%, amplitude: 47.9±7.4%, as percentage of unconditioned response) whereas facilitation occurred with SES (area: 110.1±4.3%, amplitude: 116.9±6.9%). We conclude that a startle-evoked activation of reticulo-cortical projections transiently inhibits the motor cortex.     

Inhibition of biceps brachii muscle motor area by painful heat stimulation of the skin

This study aimed to evaluate whether painful cutaneous stimuli can affect specifically the excitability of the arm proximal muscle motor area. The motor evoked potentials (MEPs), recorded from the right biceps brachii muscle after either transcranial magnetic or electrical anodal stimulation of the left primary motor (MI) cortex, were conditioned by painful CO₂ laser stimuli delivered either on the right hand dorsum or on the lateral surface of the right arm. Painful CO₂ laser stimuli delivered on the hand skin reduced significantly the amplitude of MEPs evoked by the transcranial magnetic stimulation of the contralateral MI area, while the MEP amplitude was not significantly modified by CO₂ laser pulses delivered on the arm skin. The inhibitory effect followed the arrival of the nociceptive inputs to the cerebral cortex. The amplitude of MEPs evoked by anodal electrical stimulation of the motor cortex was not decreased by conditioning painful stimuli delivered on the hand dorsum. Since the magnetic stimulation led to transynaptic activation of pyramidal neurons, while the anodal stimulation activated directly corticospinal axons, our findings suggest that CO₂ laser pulses delivered on the hand are able to inhibit the arm proximal muscle motor area. Electronic Publication     

Electrically evoked myoelectric signals.

Electrical stimulation of the nervous system is attracting increasing attention because of the possibilities it offers for physiological investigations, clinical diagnosis, muscle function assessment, noninvasive muscle characterization, and functional control of paralyzed extremities. Parameters of the myoelectric signal evoked by surface stimulation of a muscle motor point or by stimulation of a nerve trunk by means of implanted electrodes provide information about muscle performance and properties if the stimulation artifact is properly removed or suppressed. Comparison of these parameters with those obtained during voluntary contractions provides additional insight into muscle physiology. The relationships between myoelectric signal amplitude parameters, spectral parameters, and conduction velocity are discussed with special reference to muscle fatigue. This review focuses on a few methodological aspects concerning electrical stimulation of the peripheral nervous system, detection, and processing of the electrically evoked myoelectric signals in skeletal muscles. The state of the art of the following issues is discussed: (1) properties of voluntary and electrically evoked myoelectric signals; (2) techniques for evoking and detecting myoelectric signals; (3) techniques for suppression of stimulation artifacts; (4) effect of stimulation waveforms and electrode properties; (5) signal processing techniques for electrically evoked myoelectric signals; (6) physiological significance of myoelectric signal variables; (7) order of recruitment of motor units during electrical stimulation; (8) myoelectric manifestations of fatigue in electrically stimulated muscles; (9) assessment of crosstalk by electrical stimulation; and (10) applications in sport, rehabilitation, and geriatric medicine.     

Centrifugal regulation of a task-relevant somatosensory signal triggering voluntary movement without a preceding warning signal

A warning signal followed by an imperative signal generates anticipatory and preparatory activities, which regulate sensory evoked neuronal activities through a top-down centrifugal mechanism. The present study investigated the centrifugal regulation of neuronal responses evoked by a task-relevant somatosensory signal, which triggers a voluntary movement without a warning signal. Eleven healthy adults participated in this study. Electrical stimulation was delivered to the right median nerve at a random interstimulus interval (1.75–2.25 s). The participants were instructed to extend the second digit of the right hand as fast as possible when the electrical stimulus was presented (ipsilateral reaction condition), or extend that of the left hand (contralateral reaction condition). They also executed repetitively extension of the right second digit at a rate of about 0.5 Hz, irrespective of electrical stimulation (movement condition), to count silently the number of stimuli (counting condition). In the control condition, they had no task to perform. The amplitude of short-latency somatosensory evoked potentials, the central P25, frontal N30, and parietal P30, was significantly reduced in both movement and ipsilateral reaction conditions compared to the control condition. The amplitude of long-latency P80 was significantly enhanced only in the ipsilateral reaction condition compared to the control, movement, contralateral reaction, and counting conditions. The long-latency N140 was significantly enhanced in both movement and ipsilateral reaction conditions compared to the control condition. In conclusion, short- and long-latency neuronal activities evoked by task-relevant somatosensory signals were regulated differently through a centrifugal mechanism even when the signal triggered a voluntary movement without a warning signal. The facilitation of activities at a latency of around 80 ms is associated with gain enhancement of the task-relevant signals from the body part involved in the action, whereas that at a latency of around 140 ms is associated with unspecific gain regulation generally induced by voluntary movement. These may be dissociated from the simple effect of directing attention to the stimulation.     

Spatiotemporal characteristics of cerebral blood volume changes in rat somatosensory cortex evoked by sciatic nerve stimulation and obtained by optical imaging

The spatiotemporal characteristics of changes in cerebral blood volume associated with neuronal activity were investigated in the hindlimb somatosensory cortex of alpha-chloralose-urethane anesthetized rats (n=10) with optical imaging at 570 nm through a thinned skull. Activation of the cortex was carried out by electrical stimulation of the contralateral sciatic nerve with 5-Hz, 0.3-V pulses (0.5 ms) for 2 s. The stimulation evoked a monophasic decrease in optical reflectance at the cortical parenchyma and arterial sites soon after the onset of stimulation, whereas no similar response was observed at vein compartments. The optical signal changes reached 10% of the peak response 0.70 +/- 0.32 s after the start of stimulation, and no significant time lag in this 10% start latency time was observed between the response at the cortical parenchyma and artery compartments. The decrease in optical reflectance reached a peak (0.25 +/- 0.047%) 2.66 +/- 0.61 s after stimulus onset at parenchymal sites, which is 0.40 +/- 0.20 s earlier (P<0.05) than that at arterial sites (0.50 +/- 0.068% 3.06 +/- 0.70 s). Varying the locations within the cortical parenchyma and arterial compartments did not significantly affect the temporal characteristics of the evoked signal. These results suggest that stimulation of the sciatic nerve evokes an increase in local blood volume in both capillaries (cortical parenchyma) and arterioles soon after the onset of a stimulus, but the blood volume increase evoked in capillaries could not be entirely accounted for by the dilation of arterioles.     

Responses of contralateral SI and SII in cat to same-site cutaneous flutter versus vibration.

The methods of (14)C-2-deoxyglucose ((14)C-2DG) metabolic mapping and optical intrinsic signal (OIS) imaging were used to evaluate the response evoked in the contralateral primary somatosensory receiving areas (SI and SII) of anesthetized cats by either 25 Hz ("flutter") or 200 Hz ("vibration") sinusoidal vertical skin displacement stimulation of the central pad on the distal forepaw. Unilateral 25-Hz stimulation consistently evoked a localized region of elevated (14)C-2DG uptake in both SI and SII in the contralateral hemisphere. In contrast, 200-Hz stimulation did not evoke elevated (14)C-2DG uptake in the contralateral SI but evoked a prominent, localized region of increased (14)C-2DG uptake in the contralateral SII. Experiments in which the OIS was recorded yielded results that complemented and extended the findings obtained with the 2DG method. First, 25-Hz central-pad stimulation evoked an increase in absorbance in a region in the contralateral SI and SII that corresponded closely to the region in which a similar stimulus evoked increased (14)C-2DG uptake. Second, 200-Hz stimulation of the central pad consistently evoked a substantial increase in absorbance in the contralateral SII but very little or no increase in absorbance in the contralateral SI. And third, 200-Hz central-pad stimulation usually evoked a decrease in absorbance in the same contralateral SI region that underwent an increase in absorbance during same-site 25-Hz stimulation. Experiments in which the OIS responses of both SI and SII were recorded simultaneously demonstrated that continuous (>1 s) 25-Hz central-pad stimulation evokes a prominent increase in absorbance in both SI and SII in the contralateral hemisphere, whereas only SII undergoes a sustained prominent increase in absorbance in response to 200-Hz stimulation to the same central-pad site. SI exhibits an initial, transient increase in absorbance in response to 200-Hz stimulation and at durations of stimulation >1 s, undergoes a decrease in absorbance. It was found that the stimulus-evoked absorbance changes in the contralateral SI and SII are correlated significantly during vibrotactile stimulation of the central pad-positively with 25-Hz stimulation and negatively with 200-Hz stimulation. The findings are interpreted to indicate that 25-Hz central-pad stimulation of the central pad evokes spatially localized and vigorous neuronal activation within both SI and SII in the contralateral hemisphere and that although 200-Hz stimulation evokes vigorous and well maintained neuronal activation within the contralateral SII, the principal effect on the contralateral SI of a 200-Hz stimulus lasting >1 s is inhibitory.     

Investigation of human brain hemodynamics by simultaneous near-infrared spectroscopy and functional magnetic resonance imaging

The aim of this study was to compare functional cerebral hemodynamic signals obtained simultaneously by near infrared spectroscopy (NIRS) and by functional magnetic resonance imaging (fMRI). The contribution of superficial layers (skin and skull) to the NIRS signal was also assessed. Both methods were used to generate functional maps of the motor cortex area during a periodic sequence of stimulation by finger motion and rest. In all subjects we found a good collocation of the brain activity centers revealed by both methods. We also found a high temporal correlation between the BOLD signal (fMRI) and the deoxy-hemoglobin concentration (NIRS) in the subjects who exhibited low fluctuations in superficial head tissues.     

Role of the pulvinar-lateral posterior nucleus complex in turning behavior

The behavioral response to electrical stimulation of the pulvinar-lateral posterior complex (P-LP) was studied in 11 adult cats with chronically implanted electrodes. The EEG of P-LP, caudate nucleus, superior colliculus, dorsal hippocampus and cerebral cortex was recorded during the stimulation sessions. Three cats had a cannula implanted in one P-LP through which drugs were microinjected. Threshold currents evoked contralateral head turning and conjugate eye deviation, and suprathreshold currents induced contralateral circling. Ninety-four percent of 456 stimulations in P-LP elicited contralateral head movements. P-LP EEG did not differ from controls during head turning induced by threshold currents, but it was desynchronized with slightly suprathreshold currents and occasionally showed after-discharges, concomitant with circling, with higher currents. Microinjections of ACh, KCl (25%), and penicillin into P-LP also induced contralateral circling, which was usually followed by a generalized epileptic seizure. This would support the postulate that the behavioral responses observed through electrical stimulation were mediated by synaptic activities within the P-LP. These results indicate that P-LP is involved in turning and circling behavior.     

The time-course of preparatory spinal and cortico-spinal inhibition: an H-reflex and transcranial magnetic stimulation study in man

 In a previous study where reaction-time methods were combined with transcranial magnetic stimulation (TMS) of the motor cortex, cortico-spinal excitability was shown to reflect time preparation. Provided that subjects can accurately estimate time, the amplitude of motor-evoked potentials (MEPs) diminish progressively during the interval separating the warning signal from the response signal (i.e., the foreperiod). On the other hand, several experiments have demonstrated that the amplitude of the Hoffman (H) reflex elicited in prime movers diminishes during the foreperiod of reaction-time tasks. The aim of the present study was to compare the time course of the respective decrements of H-reflex and MEP amplitude during a constant 500-ms foreperiod. The subjects (n=8) participated in two experimental sessions. In one session, H-reflexes were induced in a tonically activated, responding hand muscle, the flexor pollicis brevis, at different times during the foreperiod of a visual-choice reaction-time task. In the other session, motor potentials were evoked in the same muscle by TMS of the motor cortex delivered in the same behavioral conditions and at the same times as in the first session. The results show that both H-reflexes and MEPs diminish in amplitude during the foreperiod, which replicates and extends previous findings. Interestingly, the time constants of the two decrements differed. There was a facilitatory effect of both electrical and magnetic stimulations on the subject’s performance: reaction time was shorter for the trials during which a stimulation was delivered than for the no-stimulation trials. This facilitation was maximal when the stimulations were delivered simultaneously with the warning signal and vanished progressively with stimulation time. Received: 6 November 1997 / Accepted: 2 June 1998     

Cortical control of human soleus muscle during volitional and postural activities studied using focal magnetic stimulation

The surface-recorded electromyographic (EMG) responses evoked in the ankle musculature by focal, transcranial, magnetic stimulation of the motor cortex were studied in healthy human subjects. Such soleus evoked motor responses (EMRs) were characterised over a wide range of background levels of motor activity and using different stimulus intensities. EMRs were recorded during predominantly (1) volitional and (2) postural tasks. In the former task subjects were seated and voluntarily produced prescribed levels of soleus activation by reference to a visual monitor of EMG. In the latter task subjects assumed standing postures without EMG feedback. Comparison of the EMRs of soleus, traditionally considered a slow anti-gravity extensor muscle, during these tasks was used to evaluate its cortical control in primarily volitional versus primarily postural activities. The form of soleus EMRs produced by single magnetic cortical stimuli comprised an initial (approx. 30 ms) increase and subsequent (approx. 50 ms) depression of EMG. Cortical stimulation could elicit substantial excitatory soleus EMG responses; for example, responses evoked by mild, magnetic stimuli (125% threshold for inducing a response in the relaxed muscle) as subjects exerted full voluntary plantarflexor effort averaged almost 20% of the maximum M-wave which could be elicited by an electrical stimulus to the posterior tibial nerve. Excitatory EMRs could be elicited in the voluntarily relaxed soleus muscle of the majority of subjects during sitting. The amplitude of soleus responses, induced by threshold stimuli for the relaxed state or approximately 125% threshold intensity, increased approximately linearly with background EMG over a wide range of volitional contraction levels. By contrast, there was no systematic change in the latency of excitatory soleus EMRs with increasing voluntary effort. The excitatory responses evoked in the voluntarily relaxed soleus of seated subjects by magnetic stimulation were regularly facilitated by incremental, voluntary contraction of the contralateral ankle extensors in a graded manner. However, such facilitation of responses was not observed when subjects voluntarily activated the muscle in which EMRs were elicited. The pattern of the responses elicited in soleus by magnetic stimulation during the postural task generally resembled that found during the volitional task. The amplitudes of excitatory soleus EMRs at a given stimulus intensity, obtained when subjects stood quietly, leaned forwards or stood on their toes to produce differing levels of ankle extensor contraction, increased with background EMG. Overall, the relationship between the size of cortically evoked soleus responses and the tonic level of motor activity, observed in individual subjects at matched stimulus intensities, did not consistently differ between postural and volitional tasks. The present results suggest that the motor cortex is potentially capable of exerting rapid regulation of the soleus muscle, and presumably other ankle extensors, not only when the muscle participates in volitional tasks but also when it is engaged in postural maintenance.     

Characteristics of sensori-motor interaction in the primary and secondary somatosensory cortices in humans: a magnetoencephalography study

We studied sensori-motor interaction in the primary (SI) and secondary somatosensory cortex (SII) using magnetoencephalography. Since SII in both hemispheres was activated following unilateral stimulation, we analyzed SIIc (contralateral to stimulation) as well as SIIi (ipsilateral to stimulation). Four tasks were performed in human subjects in which a voluntary thumb movement of the left or right hand was combined with electrical stimulation applied to the index finger of the left or right hand: L(M)-L(S) (movement of the left thumb triggered stimulation to the left finger), L(M)-R(S) (movement of the left thumb triggered electrical stimulation to the right finger), R(M)-R(S) (movement of the right thumb triggered electrical stimulation to the right finger), and R(M)-L(S) (movement of the right thumb triggered electrical stimulation to the left finger). Stimulation to the index finger only (S condition) was also recorded. In SI, the amplitude of N20m and P35m was significantly attenuated in the R(M)-R(S) and L(M)-L(S) tasks compared with the S condition, but that for other tasks showed no change, corresponding to a conventional gating phenomenon. In SII, the R(M)-L(S) task significantly enhanced the amplitude of SIIc but reduced that of SIIi compared with the S condition. The L(M)-L(S) and R(M)-R(S) tasks caused a significant enhancement only in SIIi. The L(M)-R(S) task enhanced the amplitude only in SIIc. The laterality index showed that SII modulation with voluntary movement was more dominant in the hemisphere ipsilateral to movement but was not affected by the side of stimulation. These results provided the characteristics of activities in somatosensory cortices, a simple inhibition in SI but complicated changes in SII depending on the side of movement and stimulation, which may indicate the higher cognitive processing in SII.     

Direct demonstration of reduction of the output of the human motor cortex induced by a fatiguing muscle contraction

A brief period of strong muscle contraction suppresses the amplitude of EMG responses evoked in relaxed muscle by transcranial magnetic stimulation (TMS) of the contralateral motor cortex. Here we investigate this phenomenon in more detail by recording the descending motor volleys evoked by TMS from electrodes in the cervical epidural space of three conscious patients implanted with chronic electrical stimulators for control of pain. We confirm that fatigue suppresses I waves evoked by TMS. In addition, D waves were suppressed in two of the patients, suggesting that axonal excitability might also be compromised by a period of intense muscle contraction. Electronic Publication     

Changes in segmental and motor cortical output with contralateral muscle contractions and altered sensory inputs in humans

Motor or sensory activity in one arm can affect the other arm. We tested the hypothesis that a voluntary contraction can affect the motor pathway to the contralateral homologous muscle and investigated whether alterations in sensory input might mediate such effects. Responses to transcranial magnetic stimulation [motor-evoked potentials (MEPs)], stimulation of the descending tracts [cervicomedullary MEPs (CMEPs)], and peripheral nerve stimulation (H-reflex) were recorded from the relaxed right flexor carpi radialis (FCR), while the left arm underwent unilateral interventions (5 s duration) that included voluntary contraction, muscle contraction evoked through percutaneous stimulation, tendon vibration, and cutaneous and mixed nerve stimulation. During moderate to strong voluntary wrist flexion on the left, MEPs in the right FCR increased, CMEPs were unaffected, and the H-reflex was depressed. These results are consistent with an increase in excitability of the motor cortex, no effect on the motoneuron pool, and presynaptic inhibition of Ia afferents. In contrast, percutaneous muscle stimulation facilitated both MEPs and the H-reflex. However, muscle contraction produced by a combination of voluntary effort and electrical stimulation also reduced the contralateral H-reflex. After voluntary contractions, the H-reflex remained depressed for 35 s, but after stimulation-evoked contractions, it rapidly returned to baseline. Under both conditions, MEPs recovered rapidly. After voluntary contractions, CMEPs were also depressed for approximately 10 s despite their lack of change during contractions. Wrist tendon vibration (100 Hz) did not affect, and 20-Hz median nerve stimulation or forearm medial cutaneous nerve stimulation mildly facilitated, the H-reflex without affecting MEPs. Voluntary wrist extension, similarly to wrist flexion, increased MEPs and depressed H-reflexes. However, ankle dorsiflexion facilitated the H-reflex akin to the Jendrassik maneuver. These data suggest that a unilateral voluntary muscle contraction has contralateral effects at both cortical and segmental levels and that the segmental effects are not replicated by stimulated muscle contraction or by input from muscle spindles or non-nociceptive cutaneous afferents.