Source analysis of median nerve and finger stimulated somatosensory evoked potentials: Multichannel simultaneous recording of electric and magnetic fields combined with 3d-MR tomography

Summary At the current state of technology, multichannel simultaneous recording of combined electric potentials and magnetic fields should constitute the most powerful tool for separation and localization of focal brain activity. We performed an explorative study of multichannel simultaneous electric SEPs and magnetically recorded SEFs. MEG only sees tangentially oriented sources, while EEG signals include the entire activity of the brain. These characteristics were found to be very useful in separating multiple sources with overlap of activity in time. The electrically recorded SEPs were adequately modelled by three equivalent dipoles located: (1) in the region of the brainstem, modelling the P14 peak at the scalp, (2) a tangentially oriented dipole, modelling the N20-P20 and N30-P30 peaks, and part of the P45, and (3) a radially oriented dipole, modelling the P22 peak and part of the P45, both located in the region of the somatosensory cortex. Magnetically recorded SEFs were adequately modelled by a single equivalent dipole, modelling the N20-P20 and N30-P30 peaks, located close to the posterior bank of the central sulcus, in area 3b (mean deviation: 3 mm). The tangential sources in the electrical data were located 6 mm on average from the area 3b. MEG and EEG was able to locate the sources of finger stimulated SEFs in accordance with the somatotopic arrangement along the central fissure. A combined analysis demonstrated that MEG can provide constraints to the orientation and location of sources and helps to stabilize the inverse solution in a multiple-source model of the EEG.     

Synchronized spikes of thalamocortical axonal terminals and cortical neurons are detectable outside the pig brain with MEG.

We show that it is feasible to monitor the synchronized population spikes of the thalamocortical axonal terminals and cortical neurons outside the brain using high-resolution magnetoencephalography (MEG). Electrical stimulation of the snout elicited somatic-evoked magnetic fields (SEFs) above the primary somatosensory cortex (SI) of the piglet. The SEFs contained high-frequency oscillations (HFOs) around 600 Hz similar in many respects to the noninvasively measured HFOs from humans with MEG and electroencephalography (EEG). These HFOs were highly correlated with those in simultaneously measured intracortical somatic-evoked potentials (SEPs) in the snout projection area in SI. Both HFOs in SEFs and SEPs consisted of an initial component insensitive to cortically injected kynurenic acid (Kyna, 20 mM), a nonspecific antagonist of glutamatergic receptors, and a subsequent Kyna-sensitive component. The former was localized in cortical layer IV, indicating that it was due to spikes produced by the specific thalamocortical axonal terminals, whereas the latter was initially localized in layer IV and subsequently in the superficial and deeper layers. These results suggest that it may be possible to study properties of the thalamocortical and cortical spike activities in humans with MEG.     

Cortical somatosensory evoked potentials. II. Effects of excision of somatosensory or motor cortex in humans and monkeys

1. To clarify the generators of human short-latency somatosensory evoked potentials (SEPs) thought to arise in sensorimotor cortex, we studied the effects on SEPs of surgical excision of somatosensory or motor cortex in humans and monkeys. 2. Normal median nerve SEPs (P20-N30, N20-P30, and P25-N35) were recorded from the cortical surface of a patient (G13) undergoing a cortical excision for relief of focal seizures. All SEPs were abolished both acutely and chronically after excision of the hand area of somatosensory cortex. Similarly, excision of the hand area of somatosensory cortex abolished corresponding SEPs (P10-N20, N10-P20, and P12-N25) in monkeys. Excision of the crown of monkey somatosensory cortex abolished P12-N25 while leaving P10-N20 and N10-P20 relatively unaffected. 3. After excision of the hand area of motor cortex, all SEPs were present when recorded from the cortical surface of a patient (W1) undergoing a cortical excision for relief of focal seizures. Similarly, all SEPs were present in monkeys after excision of the hand area of motor cortex. 4. Although all SEPs were present after excision of motor cortex in monkeys, variable changes were observed in SEPs after the excisions. However, these changes were not larger than the changes observed after excision of parietal cortex posterior to somatosensory cortex. We concluded that the changes were not specific to motor cortex excision. 5. These results support two major conclusions. 1) Median nerve SEPs recorded from sensorimotor cortex are produced by generators in two adjacent regions of somatosensory cortex: a tangentially oriented generator in area 3b, which produces P20-N30 (human) and P10-N20 (monkey) [recorded anterior to the central sulcus (CS)] and N20-P30 (human) and N10-P20 (monkey) posterior to the CS; and a radially oriented generator in area 1, which produces P25-N35 (human) and P12-N25 (monkey) recorded from the postcentral gyrus near the CS. 2) Motor cortex makes little or no contribution to these potentials.     

Genetic influence demonstrated for MEG‐recorded somatosensory evoked responses

We tested for a genetic influence on magnetoencephalogram (MEG)‐recorded somatosensory evoked fields (SEFs) in 20 monozygotic (MZ) and 14 dizygotic (DZ) twin pairs. Previous electroencephalogram (EEG) studies that demonstrated a genetic contribution to evoked responses generally focused on characteristics of representative brain potentials. Here we demonstrate significantly smaller amplitude differences within MZ compared to DZ twin pairs for the complete SEF time series (across left and right hand SEFs: 0.37 vs. 0.60 pT² and 0.28 vs. 0.39 pT² for primary [SI] and secondary [SII] sensory cortex activation) and higher MZ than DZ wave shape correlations (.71 vs. .44 and .52 vs. .35 for SI and SII activation). Our findings indicate a genetic influence on MEG‐recorded evoked brain activity and also confirm our recent conclusion ( van 't Ent, van Soelen, Stam, De Geus, & Boomsma, 2009 ) that higher MZ resemblance for EEG amplitudes is not trivially reflecting greater MZ concordance in intervening biological tissues.     

Neuromagnetic investigation of somatotopy of human hand somatosensory cortex

Summary In order to investigate functional topography of human hand somatosensory cortex we recorded somatosensory evoked fields (SEFs) on MEG during the first 40 ms after stimulation of median nerve, ulnar nerve, and the 5 digits. We applied dipole modeling to determine the three-dimensional cortial representations of different peripheral receptive fields. Median nerve and ulnar nerve SEFs exhibited the previously described N20 and P30 components with a magnetic field pattern emerging from the head superior and re-entering the head inferior for the N20 component; the magnetic field pattern of the P30 component was of reversed orientation. Reversals of field direction were oriented along the anterior-posterior axis. SEFs during digit stimulation showed analogous N22 and P32 components and similar magnetic field patterns. Reversals of field direction showed a shift from lateral inferior to medial superior for thumb to little finger. Dipole modeling yielded good fits at these peak latencies accounting for an average of 83% of the data variance. The cortical digit representations were arranged in an orderly somatotopic way from lateral inferior to medial superior in the sequence thumb, index finger, middle finger, ring finger, and little finger. Median nerve cortical representation was lateral inferior to that of ulnar nerve. Isofield maps and dipole locations for these components are consistent with neuronal activity in the posterior bank of central fissure corresponding to area 3b. We conclude that SEFs recorded on MEG in conjunction with source localization techniques are useful to investigate functional topography of human hand somatosensory cortex non-invasively.     

A thalamic component of the cervical evoked potential in man

Somatosensory evoked potentials (SEPs) were recorded simultaneously from scalp and neck locations following median nerve stimulation. By subtracting the latency of the major negative peak of the cervical SEP (N13) from that of the primary cortical response (N20), the central somatosensory conduction time was calculated (5.9 ms). On the descending slope of the cervical SEP was superimposed a positive potential of probable thalamic origin (P17). By subtracting the latency of N13 from that of P17 and P17 from that of N20 respectively, the transit time for the afferent volley both within the brainstem (3.9 ms) and the thalamo-cortical radiation (2.0 ms) was obtained.     

Effects of voluntary hyperventilation on cortical sensory responses Electroencephalographic and magnetoencephalographic studies

 It is well established that voluntary hyperventilation (HV) slows down electroencephalographic (EEG) rhythms. Little information is available, however, on the effects of HV on cortical responses elicited by sensory stimulation. In the present study, we recorded auditory evoked potentials (AEPs) and magnetic fields (AEFs), and somatosensory evoked magnetic fields (SEFs) from healthy subjects before, during, and after a 3- to 5-min period of voluntary HV. The effectiveness of HV was verified by measuring the end-tidal CO₂ levels. Long-latency (100–200 ms) AEPs and long-latency AEFs originating at the supratemporal auditory cortex, as well as long-latency SEFs from the primary somatosensory cortex (SI) and from the opercular somatosensory cortex (OC), were all reduced during HV. The short-latency SEFs from SI were clearly less modified, there being, however, a slight reduction of the earliest cortical excitatory response, the N20m deflection. A middle-latency SEF deflection from SI at about 60 ms (P60 m) was slightly increased. For AEFs and SEFs, the center-of-gravity locations of the activated neuronal populations were not changed during HV. All amplitude changes returned to baseline levels within 10 min after the end of HV. The AEPs were not altered when the subjects breathed 5% CO₂ in air in a hyperventilation-like manner, which prevented the development of hypocapnia. We conclude that moderate HV suppresses long-latency evoked responses from the primary projection cortices, while the early responses are less reduced. The reduction of long-latency responses is probably mediated by hypocapnia rather than by other nonspecific effects of HV. It is suggested that increased neuronal excitability caused by HV-induced hypocapnia leads to spontaneous and/or asynchronous firing of cortical neurones, which in turn reduces stimulus-locked synaptic events. Received: 14 October 1997 / Accepted: 28 October 1998     

Comparative study of evoked potentials in multiple sclerosis and neuro-Beh?et's syndrome

We studied somatosensory evoked potentials (SEPs) and brainstem auditory evoked potentials (BAEPs) in Japanese patients with multiple sclerosis (MS) and those with neuro-Behcet's disease (NB). Abnormal cortical P37 of posterior tibial nerve SEPs or cervical N13 of median nerve SEPs were more frequently found in the MS patients than in the NB patients. On the other hand, prolongation of the central conduction time of median nerve SEPs or abnormal BAEPs were more common in NB than in MS. The present data showed that lesions were mainly present in the spinal cord in MS and in the brainstem in NB. SEPs and BAEPs were considered of great value for detecting the involvement of the central nervous system in MS and NB and distinguishing between these diseases.     

皮层运动诱发电位监测在脑中央区术中的应用

目的:探讨在全凭静脉麻醉下,使用皮层运动诱发电位(MEP)对脑中央区手术进行术中监测的方法。方法:使用皮层电极对12例中央区肿瘤患者进行诱发电位术中监测,在中央后回感觉皮层区相应部位记录皮层体感诱发电位(SEP)的N20-P25波,沿中央后回功能区皮层向前移动电极,直至记录到一个波型相反(位相倒置)、波幅相近的波型P20-N25,将其定为运动中枢刺激点。使用高频串刺激(TS)直接刺激该点,在上肢肌肉记录MEP。结果:12例患者均能成功地记录到MEP。术中注意保护此区,术后患者症状无明显加重,被监测的上肢肌力无明显减退。结论:对于脑中央区手术,在全凭静脉麻醉下找出运动中枢刺激点并作MEP监测(术中注意保护此区)是一种优良的术中监测手段。     

Phasic increases in cortical beta activity are associated with alterations in sensory processing in the human

Oscillatory activity in the beta (β)-frequency band (13–35 Hz) can be recorded over the sensorimotor cortex in humans. It is coherent with electromyographic activity (EMG) during tonic contraction, but whether the cortical β-oscillations are primarily motor or sensorimotor in function remains unclear. We tested the hypothesis that cortical β-activity is associated with an up-regulation of sensory inputs that may be relevant to the organization of the motor response. We recorded cortical somatosensory potentials (SEPs) elicited by electrical stimuli to the median nerve at the wrist triggered by increases of electroencephalographic (EEG) β-activity in the contralateral fronto-central EEG and compared these to SEPs presented at random intervals. The involvement of motor cortex in the triggering EEG activity was confirmed by a simultaneous elevation of cortico-spinal synchrony in the β-band. The negative cortical evoked potential peaking at 20 ms and the positive evoked potential peaking at 30 ms after median nerve shocks were increased in size when elicited after phasic increases in β-activity. The functional coupling of sensory and motor cortices in the β-band was confirmed in recordings of electrocorticographic activity in two patients with chronic pain syndromes, suggesting a means by which β-activity may simultaneously influence cortical sensory processing, motor output and promote sensory-motor interaction. An erratum to this article can be found at     

Human cortical potentials evoked by stimulation of the median nerve. II. Cytoarchitectonic areas generating long-latency activity

1. The anatomic generators of human median nerve somatosensory evoked potentials (SEPs) in the 40 to 250-ms latency range were investigated in 54 patients by means of cortical-surface and transcortical recordings obtained during neurosurgery. 2. Contralateral stimulation evoked three groups of SEPs recorded from the hand representation area of sensorimotor cortex: P45-N80-P180, recorded anterior to the central sulcus (CS) and maximal on the precentral gyrus; N45-P80-N180, recorded posterior to the CS and maximal on the postcentral gyrus; and P50-N90-P190, recorded near and on either side of the CS. 3. P45-N80-P180 inverted in polarity to N45-P80-N180 across the CS but was similar in polarity from the cortical surface and white matter in transcortical recordings. These spatial distributions were similar to those of the short-latency P20-N30 and N20-P30 potentials described in the preceding paper, suggesting that these long-latency potentials are generated in area 3b of somatosensory cortex. 4. P50-N90-P190 was largest over the anterior one-half of somatosensory cortex and did not show polarity inversion across the CS. This spatial distribution was similar to that of the short-latency P25-N35 potentials described in the preceding paper and, together with our and Goldring et al. 1970; Stohr and Goldring 1969 transcortical recordings, suggest that these long-latency potentials are generated in area 1 of somatosensory cortex. 5. SEPs of apparently local origin were recorded from several regions of sensorimotor cortex to stimulation of the ipsilateral median nerve. Surface and transcortical recordings suggest that the ipsilateral potentials are generated not in area 3b, but rather in other regions of sensorimotor cortex perhaps including areas 4, 1, 2, and 7. This spatial distribution suggests that the ipsilateral potentials are generated by transcallosal input from the contralateral hemisphere. 6. Recordings from the periSylvian region were characterized by P100 and N100, recorded above and below the Sylvian sulcus (SS) respectively. This distribution suggests a tangential generator located in the upper wall of the SS in the second somatosensory area (SII). In addition, N125 and P200, recorded near and on either side of the SS, suggest a radial generator in a portion of SII located in surface cortex above the SS. 7. In comparison with the short-latency SEPs described in the preceding paper, the long-latency potentials were more variable and were more affected by intraoperative conditions.     

High frequency vibration induced gating of subcortical and cortical median nerve somatosensory evoked potentials: different effects on the cervical N13 and on the P13 and P14 far-field SEP components.

Subcortical and cortical somatosensory evoked potentials (SEP) to median nerve stimulation were recorded before, during and after high frequency (270 Hz) vibration of the fingers 1-3 in 8 healthy subjects. A marked decrease of the amplitude of all potentials was observed. The attenuation of the sensory nerve action potential (SNAP) of the median nerve and the attenuation of SEP components N9, N11 and N13 showed no differences, while the attenuation of the subcortical P14 component was significantly higher. This is in accordance with a generator of the cervical N13 in the interneurons beside the lemniscal pathway. The cortical N20 (post-rolandic) was significantly more decreased in amplitude than P14 while P22 (pre-rolandic) remained reduced in amplitude like P14. An increased latency of the far-field subcortical P14 was observed, while P13 recorded in the same montage remained unchanged in latency. These findings suggest different generators of these peaks. A generator of P14 above the nucleus cuneatus is confirmed. A presynaptic generator of P13 is suspected.     

Human cortical potentials evoked by stimulation of the median nerve. II. Cytoarchitectonic areas generating short-latency activity

1. The anatomic generators of human median nerve somatosensory evoked potentials (SEPs) in the 40 to 250-ms latency range were investigated in 54 patients by means of cortical-surface and transcortical recordings obtained during neurosurgery. 2. Contralateral stimulation evoked three groups of SEPs recorded from the hand representation area of sensorimotor cortex: P45-N80-P180, recorded anterior to the central sulcus (CS) and maximal on the precentral gyrus; N45-P80-N180, recorded posterior to the CS and maximal on the postcentral gyrus; and P50-N90-P190, recorded near and on either side of the CS. 3. P45-N80-P180 inverted in polarity to N45-P80-N180 across the CS but was similar in polarity from the cortical surface and white matter in transcortical recordings. These spatial distributions were similar to those of the short-latency P20-N30 and N20-P30 potentials described in the preceding paper, suggesting that these long-latency potentials are generated in area 3b of somatosensory cortex. 4. P50-N90-P190 was largest over the anterior one-half of somatosensory cortex and did not show polarity inversion across the CS. This spatial distribution was similar to that of the short-latency P25-N35 potentials described in the preceding paper and, together with our and Goldring et al. 1970; Stohr and Goldring 1969 transcortical recordings, suggest that these long-latency potentials are generated in area 1 of somatosensory cortex. 5. SEPs of apparently local origin were recorded from several regions of sensorimotor cortex to stimulation of the ipsilateral median nerve. Surface and transcortical recordings suggest that the ipsilateral potentials are generated not in area 3b, but rather in other regions of sensorimotor cortex perhaps including areas 4, 1, 2, and 7. This spatial distribution suggests that the ipsilateral potentials are generated by transcallosal input from the contralateral hemisphere. 6. Recordings from the periSylvian region were characterized by P100 and N100, recorded above and below the Sylvian sulcus (SS) respectively. This distribution suggests a tangential generator located in the upper wall of the SS in the second somatosensory area (SII). In addition, N125 and P200, recorded near and on either side of the SS, suggest a radial generator in a portion of SII located in surface cortex above the SS. 7. In comparison with the short-latency SEPs described in the preceding paper, the long-latency potentials were more variable and were more affected by intraoperative conditions.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interference of vibrations with input transmission in dorsal horn and cuneate nucleus in man: a study of somatosensory evoked potentials (SEPs) to electrical stimulation of median nerve and fingers

Summary The effects of 50 Hz palm vibrations on somatosensory potentials (SEPs) evoked by electrical stimulation of the median nerve at the wrist and of the 2nd and 3rd fingers were studied in 10 normal subjects. Vibrations were found to produce attenuation of the N13 spinal and P14 brainstem potentials and of the N20 contralateral parietal response. Brachial plexus (N9, P9) and dorsal column (P11) responses were not modified by vibrations. These SEP findings show: 1) that vibrations do not interfere at the periphery with the processing of brief ascending volleys triggered by an electrical stimulus and 2) that such an interference does occur in spinal dorsal horn and cuneate nucleus. Reduced input transmission in the cuneate nucleus is likely to be responsible for perceptual alterations induced by vibrations.     

Sensitivity of EEG and MEG to the N1 and P2 Auditory Evoked Responses Modulated by Spectral Complexity of Sounds

Acoustic complexity of a stimulus has been shown to modulate the electromagnetic N1 (latency ∼110 ms) and P2 (latency 190 ms) auditory evoked responses. We compared the relative sensitivity of electroencephalography (EEG) and magnetoencephalography (MEG) to these neural correlates of sensation. Simultaneous EEG and MEG were recorded while participants listened to three variants of a piano tone. The piano stimuli differed in their number of harmonics: the fundamental frequency (f ₀ ), only, or f ₀ and the first two or eight harmonics. The root mean square (RMS) of the amplitude of P2 but not N1 increased with spectral complexity of the piano tones in EEG and MEG. The RMS increase for P2 was more prominent in EEG than MEG, suggesting important radial sources contributing to the P2 only in EEG. Source analysis revealing contributions from radial and tangential sources was conducted to test this hypothesis. Source waveforms revealed a significant increase in the P2 radial source amplitude in EEG with increased spectral complexity of piano tones. The P2 of the tangential source waveforms also increased in amplitude with increased spectral complexity in EEG and MEG. The P2␣auditory evoked response is thus represented by both tangential (gyri) and radial (sulci) activities. The radial contribution is expressed preferentially in EEG, highlighting the importance of combining EEG with MEG where complex source configurations are suspected.     

Somatosensory evoked potentials in the diagnosis of cervical spondylotic myelopathy.

Median, ulnar, radial and common peroneal nerve somatosensory evoked potentials (SEPs) were studied in 17 patients suffering from cervical spondylotic myelopathy. Median, ulnar and common peroneal nerve SEPs were abnormal in 41%, 71% and 100% of cases respectively. Abnormalities of the scalp far-field P14 evoked by upper limb stimulation correlated with joint and touch sensation impairment, but not with radiological findings. Therefore, P14 may be a reliable marker of dorsal column impairment in cervical spondylotic myelopathy. The analysis of the cervical N13 response, which was recorded using a cephalic reference electrode, did not give any further information. Common peroneal nerve SEP abnormalities were found in all our patients, but they were obviously of no value in identifying the cervical spine as the site of lesion.     

Cortical somatosensory evoked potentials. I. Recordings in the monkey Macaca fascicularis

1. The anatomic generators of somatosensory evoked potentials (SEPs) to median nerve stimulation in the 10- to 30-ms latency range were investigated in monkeys (Macaca fascicularis) by means of cortical-surface and laminar recordings. 2. Three groups of SEPs evoked by stimulation of the contralateral median nerve were recorded from the hand representation area of sensorimotor cortex: P10-N20, recorded anterior to the central sulcus (CS); N10-P20, recorded posterior to the CS; and P12-N25, recorded near the CS. These potentials were similar in morphology and surface distribution whether the animal was awake or anesthetized. 3. P10-N20 exhibited a polarity inversion to N10-P20 across the CS, both in cortical-surface recordings and in laminar recordings within cortex and white matter of motor and somatosensory cortex. In contrast, P10-N20 and N10-P20 did not exhibit polarity inversion in recordings from the surface and white matter of the crowns of motor and somatosensory cortex, respectively. These results strongly suggest that these potentials are produced by a tangential generator located in the posterior wall of the CS, primarily in area 3b of somatosensory cortex. 4. P12-N25 was largest over the hand area of somatosensory cortex and showed polarity inversion across the crown of somatosensory cortex but not across the crown of motor cortex or across the walls of the CS, suggesting that P12-N25 is due to a radially oriented generator located in areas 1 and 2 of somatosensory cortex. 5. P10-N20 and P12-N25 are thought to be equivalent to the "primary evoked response" recorded from somatosensory cortex of other mammals. 6. These results are very similar to those obtained in human cortical-surface recordings and demonstrate that the monkey P10-N20, N10-P20, and P12-N25 potentials correspond to the human P20-N30, N20-P30, and P25-N35 potentials, respectively. The only appreciable difference in human and monkey SEPs is that the monkey P12-N25 appears to be generated in areas 1 and 2, whereas the human P25-N35 appears to be generated only in area 1. 7. There was no evidence of locally generated activity in areas 3a and 4.     

糖尿病患者的体感诱发电位

应用电子计算机迭加平均技术,记录了23名正常人和25例糖尿病患者的电刺激正中神经和胫后神经的体感诱发电位(somatosensory evoked potentials,SEPs)。结果发现,大部分患者均有SEPs成份峰值潜伏期的延长,以及正中神经和胫后神经传入纤维传导速度的减慢;部分患者有中枢传导时间(N13-N20传导时间和P40-N80传导时间)的延长。此外,外周神经三相电位的波形也出现异常变化。这些结果提示,糖尿病患者不仅可出现外周神经传导功能障碍,而且也可出现中枢神经传导功能异常。     

An explorative evaluation of the local anaesthetic activity of dimethindene

The study investigated the local anaesthetic efficacy and its time course of dimethindene. The pilot-study was carried out with 8 healthy volunteers in a placebo-controlled, randomized, crossover design with four gel formulations [Dimethindene, non-occlusive (DNO) and occlusive (DO); Lidocaine (L); Placebo (P)]. Selective thermo-nociceptive stimulation (A-and C-fibers) was induced by a CO₂-laser. Somatosensory evoked vertex potentials (SEPs) from an EEG record were simultaneously recorded. The effect of the active treatments was concentrated on the peripheral N1-component of the somatosensory evoked potentials. The overall means of the N1-amplitude were reduced to an extent between 12.6% (DNO) and 23.5% (DO) compared to placebo treatment. The N1-latencies were increased between 4% (DNO) and 9.4% (DO). Thus all treatments showed a local anaesthetic effect compared to placebo at all assessment times.     

Altered cortical integration of dual somatosensory input following the cessation of a 20 min period of repetitive muscle activity

The adult human central nervous system (CNS) retains its ability to reorganize itself in response to altered afferent input. Intracortical inhibition is thought to play an important role in central motor reorganization. However, the mechanisms responsible for altered cortical sensory maps remain more elusive. The aim of the current study was to investigate changes in the intrinsic inhibitory interactions within the somatosensory system subsequent to a period of repetitive contractions. To achieve this, the dual peripheral nerve stimulation somatosensory evoked potential (SEP) ratio technique was utilized in 14 subjects. SEPs were recorded following median and ulnar nerve stimulation at the wrist (1 ms square wave pulse, 2.47 Hz, 1× motor threshold). SEP ratios were calculated for the N9, N11, N13, P14–18, N20–P25 and P22–N30 peak complexes from SEP amplitudes obtained from simultaneous median and ulnar (MU) stimulation divided by the arithmetic sum of SEPs obtained from individual stimulation of the median (M) and ulnar (U) nerves. There was a significant increase in the MU/M + U ratio for both cortical SEP components following the 20 min repetitive contraction task, i.e. the N20–P25 complex, and the P22–N30 SEP complex. These cortical ratio changes appear to be due to a reduced ability to suppress the dual input, as there was also a significant increase in the amplitude of the MU recordings for the same two cortical SEP peaks (N20–P25 and P22–N30) following the typing task. No changes were observed following a control intervention. The N20 (S1) changes may reflect the mechanism responsible for altering the boundaries of cortical sensory maps, changing the way the CNS perceives and processes information from adjacent body parts. The N30 changes may be related to the intracortical inhibitory changes shown previously with both single and paired pulse TMS. These findings may have implications for understanding the role of the cortex in the initiation of overuse injuries.