The relationship in gating effects between short-latency and long-latency somatosensory-evoked potentials

We investigated the relationship between short-latency and long-latency somatosensory-evoked potentials (SEPs) relating to voluntary movement. In general, the amplitudes of short-latency components in SEPs are attenuated during movement, whereas those of long-latency are enhanced, and this phenomenon is termed 'gating effects'. This study aimed to examine the relationship of changes in amplitude between short-latency and long-latency SEPs. SEPs were recorded from 11 participants at Fz, Cz, Pz, and C4' by stimulating the left median nerve. Two tasks were conducted; Control and Movement. In Control, the participant was asked to relax with no specific task. In Movement, the participant was encouraged to continue a rapid drumming motion of all fingers of the left hand at a self-paced rate. The amplitudes of short-latency SEPs, the P25 at C4' and N30 at Fz, were significantly smaller in the Movement than Control condition. By contrast, the amplitudes of long-latency SEPs, the N140 at Fz, Cz, and Pz were significantly larger in Movement than Control condition. Moreover, a significant positive correlation was observed in the rate of amplitude change between the frontal N30 and vertex N140, indicating that for the participants with a frontal N30 of smaller amplitude during Movement, the amplitude of the vertex N140 was smaller. We inferred that the neural activities in movement-related cortices affected the sources for the frontal N30 and vertex N140 in the same neuronal network simultaneously.     

Median and tibial somatosensory evoked potentials. Changes in short- and long-latency components in patients with lesions of the thalamus and thalamo-cortical radiations

Alterations in short- and long-latency components of median and tibial somatosensory evoked potentials (SEPs) were studied in patients with lesions in the thalamus and thalamo-cortical radiations. When the lesions were located primarily in the ventro-posterior thalamus, the SEP changes consisted of the following combination: absence of response; decrease in response amplitude; delay in peak latency; and attenuation of median N20-P25 and tibial P40. The laterally situated ventro-posterior lesions tended to preferentially affect tibial SEPs whereas the medially situated lesions tended to preferentially affect median SEPs. The lateral thalamic lesions affected primarily the long-latency SEP components, whereas the medial thalamic lesions affected primarily the mid-latency or the mid- and long-latency SEP components. Corona radiata infarcts produced SEP changes similar to those with the ventro-posterior thalamic lesions except that absence of evoked responses was not observed. Subcortical infarcts tended to affect the mid- and long-latency SEP components with relative preservation of the short-latency components. The present data indicate that only the lesions involving the primary thalamic relay area affected all SEP components, particularly the short-latency components, and that the lesions in other thalamic areas can also influence the SEPs, particularly the mid- and long-latency components. The present study further demonstrates that a combined use of median and tibial SEPs is useful in delineating the topographic organization of the somatosensory system in the thalamus.     

Short- and long-latency median somatosensory evoked potentials. Findings in patients with localized neurological lesions

Short- and long-latency somatosensory evoked potentials (SEPs) were elicited by stimulation of the median nerve in 43 patients with neurological disorders. Abnormalities of short-latency peaks, P9, N13, and P14, were seen in patients with lesions of the peripheral nerve, cervical spinal cord, and brain stem, respectively. Subsequent component, N18, was affected in patients with thalamic or hemispheric disease. In some patients with parietal lobe lesions, however, abnormalities were limited to later components, N32 or N63. Analysis of SEPs is helpful in localizing a lesion along the somatosensory pathway, although differentiation between thalamic and other subcortical or cortical involvement may not be possible with the present SEP technique. Both short- and long-latency SEPs should be studied for maximal clinical information. The latter can be most reliably evaluated by simultaneous bilateral stimulation.     

Short- and long-latency tibial somatosensory evoked potentials in cerebral lesions affecting rolandic leg areas

Summary Short- and long-latency tibial somatosensory evoked potentials (SEPs) were studied in nine patients with clinical presentation primarily involving one lower extremity. In group 1, with extensive infarcts in the territory of anterior cerebral artery, tibial cortical SEPs were uniformly absent. In group 2, with small infarcts involving Rolandic leg areas, tibial SEPs showed a decrease in overall response amplitude and attenuation of P40. In group 3, with discrete mass lesions compressing Rolandic leg areas, P40 was preserved but might be delayed. Late SEP components (N75, P100 and N135) tended to be preserved in the patients of group 2 and 3. The data suggest that Rolandic leg areas and the neighboring cortex are crucial for short- and long-latency tibial cortical SEPs and that small lesions affecting Rolandic leg areas tend to affect short-and mid-latency SEP components.     

Mapping study of somatosensory evoked potentials during selective spatial attention.

We have investigated the effects of selective spatial attention on early and middle-latency SEPs. Baseline control responses to electrical stimulation of 2 digits of the hand were recorded first in conditions of mental relaxation, in the absence of any cognitive task, to obtain truly 'neutral' responses uncontaminated by cognitive components. Then, during a 'task condition,' identical stimuli were applied to the same two fingers, but the subject's attention was driven towards the stimulated territory by the bias of mechanical taps delivered to the same digits. The earliest effect of directing attention towards the territory stimulated was a positive shift on contralateral somatosensory responses, with onset at 27.4 +/- 4 msec post stimulus. This SEP modification: (a) did not entail any change in the scalp distribution of components, as assessed by topographic mapping, and (b) was not present when attention was directed towards the hand contralateral to that receiving electrical stimuli. A second effect was represented by a parieto-central negativity in the 60-80 msec latency range; this feature could also be observed during contralaterally driven attention and was associated with topographical changes in SEP scalp distribution. Finally, a late centro-frontal negativity beginning at 90-100 msec (N140) appeared during ipsilateral attention, while P100 was not enhanced. Subcortical P14 and primary cortical N20 were not significantly affected by the tasks. We conclude that the 'early positive shift' is linked to the spatial aspects of selective attention and represents in part modulation of obligatory components (P25 through P45) existing in control SEPs; it probably corresponds with the deflections with similar polarity and time-course that have been described by others in response to somatosensory target stimuli. Conversely, 60-80 msec negative enhancement is less spatially selective and may represent non-specific arousal effects. The late negative component (N140) shares several features with the 'processing negativity' described in auditory paradigms and could represent the equivalent of this effect in the somatosensory system.     

Passive enhancement of the somatosensory P100 and N140 in an active attention task using deviant alone condition.

OBJECTIVE: We investigated the changes in the somatosensory P100 and N140 during passive (reading) versus active tasks (counting, button pressing) and oddball (target=20%, standard=80%) versus deviant alone conditions (standards were omitted). METHODS: Nine healthy subjects performed the 3 tasks (reading, counting and button pressing) under two conditions. Standard and target electrical stimuli were presented in a random order to the index or middle fingers of the left hand at a constant 800 ms interstimulus interval in the oddball conditions. In the deviant alone conditions, only target stimuli were presented with the same timing as in the oddball conditions. RESULTS: The N140 amplitude increased for the deviant alone stimuli compared with the oddball standard and target stimuli regardless of whether the task was passive or active, indicating passive shifts of attention related to temporal infrequency. The P100 amplitude also increased for the deviant alone stimuli compared with the oddball standard and target stimuli in both passive and active tasks, but the enhancement seemed to be even smaller than that of the N140 amplitude. CONCLUSIONS: The somatosensory N140 passively increased even if subjects tried to attend actively to the stimulus source when the deviant alone condition was used. This change in N140 amplitude may be related to a strong orienting effect against a 'silent' background. SIGNIFICANCE: The present study provided evidence that the N140 is an indicator of passive attention against a silent background when the deviant alone condition or long interstimulus interval was used.     

Parietal generators of low- and high-frequency MN (median nerve) SEPs: data from intracortical human recordings.

OBJECTIVE: To identify low and high-frequency median nerve (MN) somatosensory evoked potential (SEP) generators by means of chronically implanted electrodes in the parietal lobe (SI and neighbouring areas) of two epileptic patients. METHODS: Wide-pass short-latency and long-latency SEPs to electrical MN stimulation were recorded in two epileptic patients by stereotactically chronically implanted electrodes in the parietal lobe (SI and neighbouring areas). To study high-frequency responses (HFOs) an off-line digital filtering of depth short-latency SEPs was performed (500-800 Hz, 24 dB roll-off). Spectral analysis was performed by fast Fourier transform. RESULTS: In both patients we recorded a N20/P30 potential followed by a biphasic N50/P70 response. A little negative response in the 100 ms latency range was the last detectable wide-pass SEP in both patients. Two HFOs components (called iP1 and iP2) were detected by mere visual analysis and spectral analysis, and were supposed to be originated within the parietal cortex. CONCLUSIONS: This was the very first study that recorded wide bandpass and high frequency SEPs by electrodes, exploring both the lateral and the mesial part of the parietal lobe and particularly that of the post-central gyrus.     

The relationship between human long-latency somatosensory evoked potentials recorded from the cortical surface and from the scalp.

In scalp recordings, stimulation of the median nerve evokes a number of long-latency (40-300 msec) somatosensory evoked potentials (SEPs) whose neural origins are unknown. We attempted to infer the generators of these potentials by comparing them with SEPs recorded from the cortical surface or from within the brain. SEPs recorded from contralateral sensorimotor cortex can be characterized as "precentral," "postcentral," or "pericentral." The scalp-recorded P45, N60 and P100 potentials appear to correspond to the pericentral P50, N90 and P190 potentials and are probably generated mainly in contralateral area 1 of somatosensory cortex. The scalp-recorded N70-P70 appear to correspond to the precentral and postcentral N80-P80 and are generated mainly in contralateral area 3b of somatosensory cortex. The scalp-recorded N120-P120 appear to correspond to the intracranial N100-P100 and are probably generated bilaterally in the second somatosensory areas. N140 and P190 (the "vertex potentials") are probably generated bilaterally in the frontal lobes, including orbito-frontal, lateral and mesial (supplementary motor area) cortex. The supplementary sensory area probably generates long-latency SEPs, but preliminary recordings have yet to confirm this assumption. Most of the proposed correspondences are speculative because the different conditions under which scalp and intracranial recordings are obtained make comparison difficult. Human recordings using chronically implanted cortical surface electrodes, and monkey studies of SEPs which appear to be analogs of the human potentials, should provide better answers regarding the precise generators of human long-latency SEPs.     

How response inhibition modulates nociceptive and non-nociceptive somatosensory brain-evoked potentials.

OBJECTIVE: To examine and compare the modulation of nociceptive somatosensory laser-evoked potentials (LEPs) and non-nociceptive somatosensory electrically-evoked potentials (SEPs) by brain processes related to response inhibition. METHODS: A warning auditory tone was followed by either an electrical or a laser stimulus. Subjects performed a Go/Nogo task in which they were instructed to respond to the laser stimulus and refrain from responding to the electrical stimulus in half of the runs. In the other half, they performed the opposite. The paradigm allowed a direct, within-subject comparison of the electrophysiological correlates of brain processes related to the Go/Nogo task in both somatosensory submodalities. RESULTS: In the Nogo-condition, SEPs displayed an enhanced N120 (early Nogo-response), a reduced vertex P240 and enhanced frontal P3 (late Nogo-responses). In contrast, LEPs only displayed late Nogo-related responses (reduced vertex P350 and enhanced frontal P3). CONCLUSIONS: The early Nogo-related enhancement of SEPs may reflect brain processes specific to the processing of non-nociceptive somatosensory stimuli. Later components of the Nogo-response may reflect cortical activity common to the processing of both nociceptive and non-nociceptive somatosensory stimuli. SIGNIFICANCE: Response inhibition significantly modulates both LEPs and SEPs. Part of these activities may be specific of the eliciting stimulus modality.     

Effects of distraction on pain-related somatosensory evoked magnetic fields and potentials following painful electrical stimulation

We aimed to compare the effects of distraction on pain-related somatosensory evoked magnetic fields (pain SEF) following painful electrical stimulation with simultaneous recordings of evoked potentials (pain SEP). Painful electrical stimuli were applied to the right index finger of eleven healthy subjects. A table with 25 random two-digit numbers was shown to the subjects, who were asked to add 5 numbers of each line in their mind (calculation task) or to memorize the numbers (memorization task) during the recording. In the SEF recording, 3 short-latency components within 50 ms of the stimulation were generated in the primary sensory cortex (SI) of the hemisphere contralateral to the stimulated finger. Middle-latency components between 100 and 250 ms after the stimuli were recorded from the secondary somatosensory cortex (SII) in the bilateral hemispheres or the cingulate cortex. No SEF components were significantly affected by either task. In the SEP recording, the middle-latency components (N140 and P230) were identified as being maximal around the vertex. Amplitudes of the N140 and P230 were not affected by each task, but the peak-to-peak amplitude (N140-P230) was significantly decreased by both the calculation and memorization tasks, particularly by the former. Subjective pain rating was decreased in both the calculation and memorization tasks, particularly in the former. We concluded that distraction tasks reduced activities in the limbic system, in which the middle-latency EEG component probably generated, while neither the short-latency SEF components generated in SI nor the primary pain-related SEF components generated in SII-insula are affected.     

Task-relevant selective modulation of somatosensory afferent paths from the lower limb

Leg movement attenuates initial somatosensory evoked potentials (SEPS) from both cutaneous and muscle afferent origin. To date, as different sensory inputs become relevant for task performance, selective facilitation from such movement-related gating influences has not been shown. We hypothesized that initial SEP amplitudes from cutaneous (sural nerve, SN) and muscle afferent (tibial nerve, TN) sources are dependent on the relevance of the specific afferent information to task performance. SEPs were obtained at rest and during three movement conditions. In each movement condition, the left foot was passively moved episodically and additional cutaneous 'codes' of sensory information were applied to the dorsum of the left foot. Subjects were instructed to: simply relax (passive), or to make a response following the cessation of movement, dependent either on the cutaneous code (cutaneous task), or the passive movement trajectory of the left foot (position task). Passive movement, with no required subsequent response, attenuated initial TN and SN SEPs to approximately 40% of that at rest (p < 0.05). Versus passive movement, when cutaneous inputs provided the relevant cue for the task, mean SN SEPs significantly increased (p < 0.05), and when the proprioceptive inputs provided the relevant cue for the task, mean TN SEPs significantly increased (p < 0.05). We conclude that specific relevancy of sensory information selectively facilitates somatosensory paths from movement-related attenuation.     

Long-latency somatosensory evoked potentials

Theoretically, long-latency somatosensory evoked potentials (SEPs) provide information on the function of somatosensory associative cortical structures. Their potential role in clinical studies and research has been hampered by the lack of standardized methodology in the use of these SEPs. Other factors, such as drugs, simultaneous stimuli, and state of consciousness, also have far-reaching influences on the various parameters of long-latency SEPs. The knowledge of the origin of most SEP components is at best fragmentary; studies on clinical-electrophysiological correlations seem to be hopeful in this respect. As yet, clinical applications of long-latency SEPs are limited; for future research, studies of disturbances of SEPs are most promising, mainly with regard to diseases of the gray matter, the influence of drugs on the cerebral function, and psychopathology.     

Attentional modulation of the human somatosensory evoked potential in a trial-by-trial spatial cueing and sustained spatial attention task measured with high density 128 channels EEG

We investigated the modulation of the somatosensory evoked potential (SEP) elicited by mechanical stimuli in a spatial sustained attention and a spatial trial-by-trial cueing design by means of high density electrode array EEG recordings. Subjects were instructed to detect rare tactile target stimuli at the to-be-attended hand while ignoring stimuli at the other hand. Analysis of the SEP revealed a highly complex pattern of results. The P50 component was significantly increased for attended stimuli in the sustained attention as opposed to the trial-by-trial cueing condition. However, no difference in amplitude was found for attended as opposed to unattended stimuli. High density electrode array recordings revealed a centero-frontal N140 component (N140c), which preceded the parietal N140 (N140p) by about 20 ms. The N140c exhibited an attention effect in particular in the trial-by-trial spatial cueing condition. The N140p was significantly enlarged with attention across both experimental conditions, but a closer inspection demonstrated that this was mainly due to the great attention effect in the trial-by-trial spatial cueing condition. The late positive component (190-380 ms after stimulus onset) exhibited a significant attention effect in both experimental conditions. The present experiment provides evidence that the attentional modulation of the SEP is different when tactile as opposed to electrical stimuli were used and when only somatosensory stimuli are presented with no further sensory stimulation in other modalities. Furthermore, transient as opposed to sustained spatial attention affected various components of the SEP in a different way.     

Conditioned orienting (alpha) and delayed behavioral and evoked neural responses during classical conditioning

A differentiation of short-latency (alpha) and long-latency (delayed) classically conditioned behavioral and evoked neural (hippocampal) responses was attempted. Further, facilitation and retardation of these responses were studied in an experimental design in which 10 paired conditioning sessions either preceded (CC-CO group) or followed (CO-CC group) 10 randomly unpaired presentations of conditioned stimuli (CS) and unconditioned stimuli (UCS). A 2024-ms tone (1000 Hz) was delivered directly through a miniature earphone to the left ear, eliciting an orienting head movement ('alpha' response) to the left. The unconditioned stimulus (UCS) was a direct 1024-ms stimulation of the lateral hypothalamic area overlapping the CS (delayed paradigm) so that both stimuli terminated simultaneously. The UCS elicited approach behavior and a specific head movement in each animal. The latency and the direction of the head movement were used as criteria for a differentiation of the short-latency and long-latency conditioned responses (CR). All cats showed conditioned short-latency responses. Pairing specific long-latency head movements were observed in 10 of 13 cats and 6 of them showed a long-latency CR which was a head movement to the right, while the short-latency CR on the same trials was a head movement to the left. Hippocampal (subiculum, dentate fascia and CA1) evoked responses also showed pairing specific CRs appearing as increased negativity (short-latency CR), or increased positivity (long-latency CR). Additional reversed stimulus order (backward) sessions supported an assumption of the different nature of the short-latency and long-latency CRs: the long-latency CRs showed extinction while the short-latency CRs remained. The unpaired pre-exposure to the CSs and UCSs in the CO-CC group resulted in the retarded acquisition of the behavioral responses during the subsequent paired sessions.     

Abnormal gating of somatosensory inputs in essential tremor.

OBJECTIVE: To study whether sensorimotor cortical areas are involved in Essential Tremor (ET) generation.BACKGROUND: It has been suggested that sensorimotor cortical areas can play a role in ET generation. Therefore, we studied median nerve somatosensory evoked potentials (SEPs) in 10 patients with definite ET.METHODS: To distinguish SEP changes due to hand movements from those specifically related to central mechanisms of tremor, SEPs were recorded at rest, during postural tremor and during active and passive movement of the hand. Moreover, we recorded SEPs from 5 volunteers who mimicked hand tremor. The traces were further submitted to dipolar source analysis.RESULTS: Mimicked tremor in controls as well as active and passive hand movements in ET patients caused a marked attenuation of all scalp SEP components. These SEP changes can be explained by the interference between movement and somatosensory input ('gating' phenomenon). By contrast, SEPs during postural tremor in ET patients showed a reduction of N20, P22, N24 and P24 cortical SEP components, whereas the fronto-central N30 wave remained unaffected.CONCLUSIONS: Our findings suggest that in ET patients the physiological interference between movement and somatosensory input to the cortex is not effective on the N30 response. This finding thus indicates that a dysfunction of the cortical generator of the N30 response may play a role in the pathogenesis of ET.     

Differential modulation of temporal and frontal components of the somatosensory N140 and the effect of interstimulus interval in a selective attention task

The modulation of the somatosensory N140 was examined in a selective attention task where a control condition was applied and the interstimulus interval (ISI) was varied. Electrical stimuli were randomly presented to the left index (p=0.4) and middle fingers (p=0.1), and right index (p=0.4) and middle fingers (p=0.1). In the attend-right condition, subjects were instructed to count silently the number of infrequent target stimuli presented to the right middle finger, and to the left middle finger in the attend-left condition. They had no task in the control condition. Each condition was performed with two different sets of ISI (mean 400 vs. 800 ms). The somatosensory N140 elicited by frequent standard stimuli was analyzed. The N140 amplitude was larger for the attended ERP compared to the control and unattended ERPs. This attention effect was more marked at the frontal electrodes compared to the temporal electrodes contralateral to the stimulation side. Furthermore, the attention effect at the frontal electrode was larger when the ISI was 800 ms than when it was 400 ms. The N140 amplitude did not differ between the control and unattended ERPs, which might show that a small processing negativity (PN) occurred during the control condition or difference in vigilance level between them. In conclusion, the early lateral ("temporal") and late midline ("frontal") components of the N1 (N140) show different behavior, and thus may have different functional significance. Enhancement of the attention effect at the frontal electrode in the longer ISI condition supports the hypothesis that it is related to stronger, voluntary maintenance of the attentional trace.     

Predictability of the target stimulus for sensory-guided movement modulates early somatosensory cortical potentials.

OBJECTIVE: To investigate the role of sensory modulation in the control of sensory-guided behaviour. Specifically, we hypothesized that early somatosensory evoked potentials (SEPs) would be facilitated during performance of continuous sensory-guided movement requiring sustained attention. METHODS: Median nerve SEPs were elicited via electrical stimulation and recorded from scalp electrodes while subjects performed tasks requiring continuous sensory-motor transformations. Subjects received a predictable (rhythmic amplitude modulation) or unpredictable (random amplitude modulation) amplitude varying tactile stimulus (frequency constant at 20 Hz) delivered to the tip of the index finger either alone or with the requirement to track it by modulating the isometric grip force produced by the opposite hand. RESULTS: Early SEP (N20-P27) amplitudes were differentially modulated during unpredictable tracking compared to sensory-motor controls. Specifically, N20 amplitudes were attenuated and P27 amplitudes were enhanced during sensory-guided tracking. CONCLUSIONS: Sustained attention to task-relevant sensory stimuli differentially modulates areas within primary somatosensory cortex (S1) during a continuous sensory-motor transformation. SIGNIFICANCE: These data have implications for understanding the role of attention in regulating somatosensory cortices during sensory-motor behaviour.     

Human auditory and somatosensory event-related potentials: effects of response condition and age

In order to develop an experimental paradigm for clinical application of cognitive event-related potentials we have recorded these potentials in a group of 27 healthy Japanese, aged 20-78 years, using all 4 stimulus/response combinations of auditory or somatosensory stimuli requiring a counting or button-press response. In an oddball paradigm we recorded N1 and P2 components to frequent auditory stimuli and P100, N150 and P200 components to frequent somatosensory stimuli. These components were also observed in the target responses for their respective modalities together with N2, P270, P3 and slow-wave components. P3 latency increased linearly with age for all 4 experimental conditions, although this increase was not statistically significant for the somatosensory stimulus/button-press response combination. The latency of P270 also increased significantly with age for the auditory stimulus/button-press response combination but did not do so in either of the counting response conditions. The principal difference between the latencies of ERPs to auditory compared with somatosensory stimuli was that P3 was significantly longer for somatosensory stimulation, although differences in task difficulty may have influenced this finding. With regard to amplitude, N2, P3 and slow-wave were all significantly more positive for somatosensory compared with auditory stimulation. The topography of P3 evoked by somatosensory stimuli was most predominant at central electrodes, whereas the auditory P3 was larger parietally. The button-press response was associated with potentials which were smaller in amplitude and shorter in latency than those associated with the count response. The button-press response had a marked effect on the amplitude of P3 recorded at the vertex and the central electrode contralateral to the moving finger.     

Effect of voluntary self-paced movements upon auditory and somatosensory evoked potentials in man.

The effect of voluntary self-paced movements upon auditory (AEPs) and somatosensory (SEPs) evoked potentials has been investigated according to the temporal relationship between movement and delivery of test stimuli. EPs were recorded in 7 subjects and averaged in 10 successive epochs extending from 880 msec before to 2500 msec after movement. AEPs were attenuated in all epochs. The decrease was greatest in the 220 msec epoch just following movement and involved components N85 and P170. SEPs were attenuated similarly to AEPs when movements were performed by the hand contralateral to somatosensory stimulation. Of the 5 SEP components, only P40 failed to reflect the attenuation, while P95 showed the greatest amplitude decrease. When stimulation was ipsilateral, SEP amplitude was attenuated only when close to the movement. N65 and P95 decreased while N130 increased. In all subjects the results were consistent for treatments of AEP and SEP (with contralateral movements), whereas large inter-individual differences were observed for the SEP with ipsilateral movements.     

Unmasking of presynaptic and postsynaptic high-frequency oscillations in epidural cervical somatosensory evoked potentials during voluntary movement.

OBJECTIVE: To investigate the effect of the voluntary movement on the amplitude of the somatosensory evoked potentials (SEPs) recorded by an epidural electrode at level of the cervical spinal cord (CSC). METHODS: Fourteen patients underwent an epidural electrode implant at CSC level for pain relief. After the median nerve stimulation, SEPs were recorded from the epidural electrode and from 4 surface electrodes (in frontal and parietal regions contralateral to the stimulated side, over the 6th cervical vertebra, and on the Erb's point). SEPs were recorded at rest and during a voluntary flexo-extension movement of the stimulated wrist. Beyond the low-frequency SEPs, also the high-frequency oscillations (HFOs) were analysed. RESULTS: The epidural electrode contacts recorded a triphasic potential (P1-N1-P2), whose negative peak showed the same latency as the cervical N13 response. The epidural potential amplitude was significantly decreased during the voluntary movement, as compared to the rest. Two main HFOs were identifiable: (1) the 1200 Hz HFO which was significantly lower in amplitude during movement than at rest, and (2) the 500 Hz HFO which was not modified by the voluntary movement. CONCLUSIONS: The low-frequency cervical SEP component is subtended by HFOs probably generated by: (1) postsynaptic potentials in the dorsal horn neurones (1200 Hz), and (2) presynaptic ascending somatosensory inputs (500 Hz). SIGNIFICANCE: Our findings show that the voluntary movement may affect the somatosensory input processing also at CSC level.