Plastic interactions between hand and face cortical representations in patients with trigeminal neuralgia: a somatosensory-evoked potentials study

The effect of local vs remote experimental pain on motor learning and sensorimotor integration using a complex typing task

Recent work demonstrated that capsaicin-induced acute pain improved motor learning performance; however, baseline accuracy was very high, making it impossible to discern the impact of acute pain on motor learning and retention. In addition, the effects of the spatial location of capsaicin application were not explored. Two experiments were conducted to determine the interactive effects of acute pain vs control (experiment 1) and local vs remote acute pain (experiment 2) on motor learning and sensorimotor processing. For both experiments, somatosensory evoked potential (SEP) amplitudes and motor learning acquisition and retention (accuracy and response time) data were collected at baseline, after application, and after motor learning. Experiment 1: N11 (P < 0.05), N13 (P < 0.05), and N30 (P < 0.05) SEP peak amplitudes increased after motor learning in both groups, whereas the N20 SEP peak increased in the control group (P < 0.05). At baseline, the intervention group outperformed the control group in accuracy (P < 0.001). Response time improved after motor learning (P < 0.001) and at retention (P < 0.001). Experiment 2: The P25 SEP peak decreased in the local group after application of capsaicin cream (P < 0.01), whereas the N30 SEP peaks increased after motor learning in both groups (P < 0.05). Accuracy improved in the local group at retention (P < 0.005), and response time improved after motor learning (P < 0.005) and at retention (P < 0.001). This study suggests that acute pain may increase focal attention to the body part used in motor learning, contributing to our understanding of how the location of pain impacts somatosensory processing and the associated motor learning.

Clinical Significance of Invasive Motor Cortex Stimulation for Trigeminal Facial Neuropathic Pain Syndromes

BACKGROUND:

Invasive neuromodulation of the cortical surface for various chronic pain syndromes has been performed for &gt;20 years. The significance of motor cortex stimulation (MCS) in chronic trigeminal neuropathic pain (TNP) syndromes remains unclear. Different techniques are performed worldwide in regard to operative procedure, stimulation parameters, test trials, and implanted materials.

OBJECTIVE:

To present the clinical experiences of a single center with MCS, surgical approach, complications, and follow-up as a prospective, noncontrolled clinical trial.

METHODS:

The implantation of epidural leads over the motor cortex was performed via a burr hole technique with neuronavigation and intraoperative neurostimulation. Special focus was placed on a standardized test trial with an external stimulation device and the implementation of a double-blinded or placebo test phase to identify false-positive responders.

RESULTS:

A total of 36 patients with TNP were operated on, and MCS was performed. In 26 of the 36 patients (72%), a significant pain reduction from a mean of 8.11 to 4.58 (on the visual analog scale) during the test trial was achieved (P &lt;.05). Six patients were identified as false-positive responders (17%). At the last available follow-up of 26 patients (mean, 5.6 years), active MCS led to a significant pain reduction compared with the preoperative pain ratings (mean visual analog scale score, 5.01; P &lt;.05).

CONCLUSION:

MCS is an additional therapeutic option for patients with refractory chronic TNP, and significant long-term pain suppression can be achieved. Placebo or double-blinded testing is mandatory.

The effect of experimental pain on motor training performance and sensorimotor integration

Experimental pain is known to affect neuroplasticity of the motor cortex as well as motor performance, but less is known about neuroplasticity of somatosensory processing in the presence of pain. Early somatosensory evoked potentials (SEPs) provide a mechanism for investigating alterations in sensory processing and sensorimotor integration (SMI). The overall aim of this study was to investigate the interactive effects of acute pain, motor training, and sensorimotor processing. Two groups of twelve participants (N = 24) were randomly assigned to either an intervention (capsaicin cream) or placebo (inert lotion) group. SEP amplitudes were collected by stimulation of the median nerve at baseline, post-application and post-motor training. Participants performed a motor sequence task while reaction time and accuracy data were recorded. The amplitude of the P22-N24 complex was significantly increased following motor training for both groups F(2,23) = 3.533, p < 0.05, while Friedman's test for the P22-N30 complex showed a significant increase in the intervention group [χ(2) (df = 2, p = 0.016) = 8.2], with no significant change in the placebo group. Following motor training, reaction time was significantly decreased for both groups F(1,23) = 59.575, p < 0.01 and overall accuracy differed by group [χ(2) (df = 3, p < 0.001) = 19.86], with post hoc testing indicating that the intervention group improved in accuracy following motor training [χ(2) (df = 1, p = 0.001) = 11.77] while the placebo group had worse accuracy [χ(2) (df = 1, p = 0.006) = 7.67]. The improved performance in the presence of capsaicin provides support for the enhancement of knowledge acquisition with the presence of nontarget stimuli. In addition, the increase in SEP peak amplitudes suggests that early SEP changes are markers of SMI changes accompanying motor training and acute pain.

Altered cortical activation from the hand after facial botulinum toxin treatment

Plastic interactions between face and hand cortical tactile circuits occur after severe injuries that affect the hand such as in amputation or spinal cord injury. However, whether loss of facial movements alters the cortical circuits involved in processing tactile inputs from the hand remains unknown. In this prospective observational study we used electroencephalography (EEG) to measure cortical activity evoked by tactile stimulation of the hands before and after botulinum toxin-A-induced facial paralysis. We found a reduction in the tactile event-related potentials (ERPs) 6 weeks after the treatment. This suggests that the limited paralysis of facial muscles induced during cosmetic interventions designed to smooth lines and wrinkles on the face is sufficient to alter the cortical processing of tactile inputs from the hand.

Spatio-temporal mapping cortical neuroplasticity in carpal tunnel syndrome

Neuroimaging data demonstrate that carpal tunnel syndrome, a peripheral neuropathy, is accompanied by maladaptive central neuroplasticity. To further investigate this phenomenon, we collected magnetoencephalography data from 12 patients with carpal tunnel syndrome and 12 healthy control subjects undergoing somatosensory stimulation of the median nerve-innervated Digits 2 and 3, as well as Digit 5, which is innervated by the ulnar nerve. Nerve conduction velocity and psychophysical data were acquired to determine whether standard clinical measures correlated with brain response. In subjects with carpal tunnel syndrome, but not healthy controls, sensory nerve conduction velocity for Digits 2 and 3 was slower than Digit 5. However, somatosensory M20 latencies for Digits 2 and 3 were significantly longer than those of Digit 5. The extent of the M20 delay for median nerve-innervated Digit 2 was positively correlated with decreasing nerve conduction velocity and increasing pain severity. Thus, slower peripheral nerve conduction in carpal tunnel syndrome corresponds to greater delays in the first somatosensory cortical response. Furthermore, spectral analysis demonstrated weaker post-stimulus beta event-related desynchronization and earlier and shorter event-related synchronization in subjects with carpal tunnel syndrome. The extent of the decreased event-related desynchronization for median nerve-innervated digits was positively correlated with paraesthesia severity. We propose that ongoing paraesthesias in median nerve-innervated digits render their corresponding sensorimotor cortical areas 'busy', thus reducing their capacity to process external stimulation. Finally, subjects with carpal tunnel syndrome demonstrated a smaller cortical source separation for Digits 2 and 3 compared with healthy controls. This supports our hypothesis that ongoing paraesthesias promote blurring of median nerve-innervated digit representations through Hebbian plasticity mechanisms. In summary, this study reveals significant correlation between the clinical severity of carpal tunnel syndrome and the latency of the early M20, as well as the strength of long latency beta oscillations. These temporal magnetoencephalography measures are novel markers of neuroplasticity in carpal tunnel syndrome and could be used to study central changes that may occur following clinical intervention.

A wireless system for monitoring transcranial motor evoked potentials

Intraoperative neurophysiological monitoring (IONM) is commonly used as an attempt to minimize neurological morbidity from operative manipulations. The goal of IONM is to identify changes in the central and peripheral nervous system function prior to irreversible damage. Intraoperative monitoring also has been effective in localizing anatomical structures, including peripheral nerves and sensorimotor cortex, which helps guide the surgeon during dissection. As part of IONM, transcranial motor evoked potentials (TcMEPs), and somatosensory evoked potentials (SSEPs) are routinely monitored. However, current wired systems are cumbersome as the wires contribute to the crowded conditions in the operating room and in doing so not only it limits the maneuverability of the surgeon and assistants, but also places certain demand in the total anesthesia required during surgery, due to setup preoperative time needed for proper electrode placement, due to the number and length of the wires, and critical identification of the lead wires needed for stimulation and recording. To address these limitations, we have developed a wireless TcMEP IONM system as a first step toward a multimodality IONM system. Bench-top and animal experiments in rodents demonstrated that the wireless method reproduced with high fidelity, and even increased the frequency bandwidth of the TcMEP signals, compared to wired systems. This wireless system will reduce the preoperative time required for IONM setup, add convenience for surgical staff, and reduce wire-related risks for patients during the operation.

Altered central integration of dual somatosensory input after cervical spine manipulation

OBJECTIVE

The aim of the current study was to investigate changes in the intrinsic inhibitory interactions within the somatosensory system subsequent to a session of spinal manipulation of dysfunctional cervical joints.

METHOD

Dual peripheral nerve stimulation somatosensory evoked potential (SEP) ratio technique was used in 13 subjects with a history of reoccurring neck stiffness and/or neck pain but no acute symptoms at the time of the study. Somatosensory evoked potentials were recorded after median and ulnar nerve stimulation at the wrist (1 millisecond square wave pulse, 2.47 Hz, 1 x motor threshold). The 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.

RESULTS

There was a significant decrease in the MU/M + U ratio for the cortical P22-N30 SEP component after chiropractic manipulation of the cervical spine. The P22-N30 cortical ratio change appears to be due to an increased ability to suppress the dual input as there was also a significant decrease in the amplitude of the MU recordings for the same cortical SEP peak (P22-N30) after the manipulations. No changes were observed after a control intervention.

CONCLUSION

This study suggests that cervical spine manipulation may alter cortical integration of dual somatosensory input. These findings may help to elucidate the mechanisms responsible for the effective relief of pain and restoration of functional ability documented after spinal manipulation treatment.

Movement gating of beta/gamma oscillations involved in the N30 somatosensory evoked potential

Evoked potential modulation allows the study of dynamic brain processing. The mechanism of movement gating of the frontal N30 component of somatosensory evoked potentials (SEP) produced by the stimulation of the median nerve at wrist remains to be elucidated. At rest, a power enhancement and a significant phase-locking of the electroencephalographic (EEG) oscillation in the beta/gamma range (25-35 Hz) are related to the emergence of the N30. The latter was also perfectly identified in presence of pure phase-locking situation. Here, we investigated the contribution of these rhythmic activities to the specific gating of the N30 component during movement. We demonstrated that concomitant execution of finger movement of the stimulated hand impinges such temporal concentration of the ongoing beta/gamma EEG oscillations and abolishes the N30 component throughout their large topographical extent on the scalp. This also proves that the phase-locking phenomenon is one of the main actors for the N30 generation. These findings could be explained by the involvement of neuronal populations of the sensorimotor cortex and other related areas, which are unable to respond to the phasic sensory activation and to phase-lock their firing discharges to the external sensory input during the movement. This new insight into the contribution of phase-locked oscillation in the emergence of the N30 and in its gating behavior calls for a reappraisal of fundamental and clinical interpretation of the frontal N30 component.

Area-specific representation of mechanical nociceptive stimuli within SI cortex of squirrel monkeys

While functional imaging studies in humans have consistently reported activation of primary somatosensory cortex (SI) with painful stimuli, the specific roles of subdivisions of areas 3a, 3b, and 1 within SI during pain perception are largely unknown, particularly in the representation of mechanical evoked pain. In this study, we investigated how modality, location, and intensity of nociceptive stimuli are represented within SI by using high-spatial resolution optical imaging of intrinsic signals in Pentothal-anesthetized squirrel monkeys. Perceptually comparable mechanical nociceptive and innocuous tactile stimuli were delivered by indenting the glabrous skin of the distal finger pads with 0.2 and 2mm diameter probes, respectively. Within each of areas 3a, 3b, and 1, activations to mechanical nociceptive stimulation of individual distal finger pads were spatially distinct and somatotopically organized. We observed differential cortical activation patterns. Areas 3a, 3b, and 1 were all activated during mechanical nociceptive stimulation and were modulated by nociceptive stimulus intensity. However, with innocuous tactile stimulation, mainly areas 3b and 1 exhibited response modulation with different levels of stimulation. In summary, mechanical nociceptive inputs are area-specific and topographically represented within SI. We propose that all areas of SI are implicated in encoding the features of mechanical nociception, where areas 3a and 3b are distinctively involved in coding nociceptive and pressure sensation components of stimulation.

Nursing aspects of phantom limb pain following amputation

Phantoms (painless and painful) occur following the removal of virtually all body parts. Phantoms of the limbs, including phantom limb pain (PLP), are the most studied. As yet there is no agreed theory to explain phantom limb pain but the neuromatrix and cortical reorganization theories have come to prominence over recent years. Multiple treatment strategies have been applied to PLP; however, none of these strategies have been proven to be effective for the majority of amputees. As a result of knowledge acquired through the cortical reorganization theory, new avenues for treatment have opened up. These include pre-emption and normalization strategies which have significant nursing aspects. This article explores all of these issues and identifies the implications that they have for the nursing treatment of patients with PLP and those that are expected to develop it. This involves the care of people pre-, peri- and post-amputation. All aspects of phantoms and phantom pain need to be taken into account by nurses and other healthcare workers when planning rehabilitation packages for this group.

Altered sensorimotor integration with cervical spine manipulation

OBJECTIVE

This study investigates changes in the intrinsic inhibitory and facilitatory interactions within the sensorimotor cortex subsequent to a single session of cervical spine manipulation using single- and paired-pulse transcranial magnetic stimulation protocols.

METHOD

Twelve subjects with a history of reoccurring neck pain participated in this study. Short interval intracortical inhibition, short interval intracortical facilitation (SICF), motor evoked potentials, and cortical silent periods (CSPs) were recorded from the abductor pollicis brevis and the extensor indices proprios muscles of the dominant limb after single- and paired-pulse transcranial magnetic stimulation of the contralateral motor cortex. The experimental measures were recorded before and after spinal manipulation of dysfunctional cervical joints, and on a different day after passive head movement. To assess spinal excitability, F wave persistence and amplitudes were recorded after median nerve stimulation at the wrist.

RESULTS

After cervical manipulations, there was an increase in SICF, a decrease in short interval intracortical inhibition, and a shortening of the CSP in abductor pollicis brevis. The opposite effect was observed in extensor indices proprios, with a decrease in SICF and a lengthening of the CSP. No motor evoked potentials or F wave response alterations were observed, and no changes were observed after the control condition.

CONCLUSION

Spinal manipulation of dysfunctional cervical joints may alter specific central corticomotor facilitatory and inhibitory neural processing and cortical motor control of 2 upper limb muscles in a muscle-specific manner. This suggests that spinal manipulation may alter sensorimotor integration. These findings may help elucidate mechanisms responsible for the effective relief of pain and restoration of functional ability documented after spinal manipulation.

GAMMA KNIFE RADIOSURGERY FOR TRIGEMINAL NEURALGIA

OBJECTIVE

Gamma knife radiosurgery (GKR) is an increasingly used, minimally invasive treatment option for patients with trigeminal neuralgia (TN) refractory to medical therapy. This retrospective study evaluates the long-term results and side effects of GKR in the treatment of TN focusing on potentially predictive factors.

METHODS

One hundred sixty patients with TN were included in this study (minimum follow-up, &gt;6 mo; mean, 37.4 mo; range, 6–144 mo). In 92 patients, GKR represented the first nonmedical option (“primary GKR”). In 68 patients, invasive treatments had been previously attempted. All patients were treated using a single 4-mm collimator shot targeting the pontine trigeminal root entry zone with a maximal dose of 75 to 95 Gy. Brainstem dose exposure never exceeded 15 Gy. Treatment outcome results were classified as Grade I (pain-free with no pharmacological treatment), Grade II (pain-free with pharmacological treatment), and Grade III (no result). Data were analyzed using the log-rank test for univariate analysis and the ordered logit model for multivariate analysis.

RESULTS

In the overall series, 98 (61%) out of 160 patients reached a Grade I outcome, 45 (29%) reached a Grade II outcome, and 17 (10%) patients had no results from GKR. These results were encouraging for patients with typical facial pain features and for patients treated by a “primary” gamma knife. Considering the global outcome, the most effective and safest dose was found to be in the 80 to 90 Gy range.

CONCLUSION

According to our experience, GKR represents a reliable second-line therapeutic approach for TN after pharmacological failure. Favorable prognostic factors include “primary GKR” and maximal GKR dose ranging between 80 and 90 Gy.

Pure phase-locking of beta/gamma oscillation contributes to the N30 frontal component of somatosensory evoked potentials

Background

Evoked potentials have been proposed to result from phase-locking of electroencephalographic (EEG) activities within specific frequency bands. However, the respective contribution of phasic activity and phase resetting of ongoing EEG oscillation remains largely debated. We here applied the EEGlab procedure in order to quantify the contribution of electroencephalographic oscillation in the generation of the frontal N30 component of the somatosensory evoked potentials (SEP) triggered by median nerve electrical stimulation at the wrist. Power spectrum and intertrial coherence analysis were performed on EEG recordings in relation to median nerve stimulation.

Results

The frontal N30 component was accompanied by a significant phase-locking of beta/gamma oscillation (25–35 Hz) and to a lesser extent of 80 Hz oscillation.

After the selection in each subject of the trials for which the power spectrum amplitude remained unchanged, we found pure phase-locking of beta/gamma oscillation (25–35 Hz) peaking about 30 ms after the stimulation. Transition across trials from uniform to normal phase distribution revealed temporal phase reorganization of ongoing 30 Hz EEG oscillations in relation to stimulation. In a proportion of trials, this phase-locking was accompanied by a spectral power increase peaking in the 30 Hz frequency band. This corresponds to the complex situation of 'phase-locking with enhancement' in which the distinction between the contribution of phasic neural event versus EEG phase resetting is hazardous.

Conclusion

The identification of a pure phase-locking in a large proportion of the SEP trials reinforces the contribution of the oscillatory model for the physiological correlates of the frontal N30. This may imply that ongoing EEG rhythms, such as beta/gamma oscillation, are involved in somatosensory information processing.

Pain in chronic pancreatitis: the role of reorganization in the central nervous system

BACKGROUND & AIMS

In various chronic pain conditions cortical reorganization seems to play a role in the manifestations. The aim of this study was to investigate cortical reorganization in patients with pain caused by chronic pancreatitis.

METHODS

Twelve healthy subjects and 10 patients with chronic pancreatitis were included. The esophagus, stomach, and duodenum were stimulated electrically at the pain threshold using a nasal endoscope. The electroencephalogram was recorded from 64 surface electrodes and event-related brain potentials (EPs) were obtained.

RESULTS

As compared with healthy subjects, the patient group showed decreased latencies of the early EP components (N1, P < .001; P1, P = .02), which is thought to reflect the exogenous brain pain processing specifically. Source analysis showed that the dipolar activities corresponding to the early EPs were located consistently in the bilateral insula, in the anterior cingulate gyrus, and in the bilateral secondary somatosensory area. The bilateral insular dipoles were localized more medial in the patient group than in the healthy subjects after stimulation of all 3 gut segments (P < .01). There also were changes in the cingulate cortex where the neuronal source was more posterior in patients than in controls to stimulation of the esophagus (P < .05).

CONCLUSIONS

The findings indicate that pain in chronic pancreatitis leads to changes in cortical projections of the nociceptive system. Such findings also have been described in somatic pain disorders, among them neuropathic pain. Taken together with the clinical data this suggests a neuropathic component in pancreatic pain, which may influence the approach to treatment.

Cervical spine manipulation alters sensorimotor integration: A somatosensory evoked potential study

OBJECTIVE

To study the immediate sensorimotor neurophysiological effects of cervical spine manipulation using somatosensory evoked potentials (SEPs).

METHODS

Twelve subjects with a history of reoccurring neck stiffness and/or neck pain, but no acute symptoms at the time of the study were invited to participate in the study. An additional twelve subjects participated in a passive head movement control experiment. Spinal (N11, N13) brainstem (P14) and cortical (N20, N30) SEPs to median nerve stimulation were recorded before and for 30min after a single session of cervical spine manipulation, or passive head movement.

RESULTS

There was a significant decrease in the amplitude of parietal N20 and frontal N30 SEP components following the single session of cervical spine manipulation compared to pre-manipulation baseline values. These changes lasted on average 20min following the manipulation intervention. No changes were observed in the passive head movement control condition.

CONCLUSIONS

Spinal manipulation of dysfunctional cervical joints can lead to transient cortical plastic changes, as demonstrated by attenuation of cortical somatosensory evoked responses.

SIGNIFICANCE

This study suggests that cervical spine manipulation may alter cortical somatosensory processing and sensorimotor integration. These findings may help to elucidate the mechanisms responsible for the effective relief of pain and restoration of functional ability documented following spinal manipulation treatment.

Cortical changes to experimental sensitization of the human esophagus

Topographical organization in the neocortex shows experience-dependent plasticity. We hypothesized that experimental sensitization of the esophagus results in changes of the topographical distribution of the evoked potentials and the corresponding dipole source activities to painful stimulation. An endoscopic method was used to deliver 35 electrical stimuli at the pain threshold to a fixed area of the mucosa in 10 healthy volunteer men and women. The stimulations were repeated after 30 min (reproducibility experiment), and after 60 min following perfusion of 200 ml 0.1 N hydrochloric acid (sensitization experiment). During stimulation the electroencephalogram was recorded from 64 surface electrodes. The sensitization resulted in a decrease in the pain threshold (F=6.2; P=0.004). The topographic distribution of the evoked potentials showed reproducible negative (N1, N2) and positive (P1, P2) components. After acid perfusion a reduced latency and a change in localization was seen for the P1 subdivided into frontal and occipital components (F=29.5, P<0.001; F=53.7, P<0.001). Furthermore the sensitization resulted in a reduction of the latency for P2 (F=6.2, P=0.009). The source analysis showed consistent dipolar activity in the bilateral opercular-insular cortex before and after acid perfusion. For the anterior cingulate dipole there was a reduction in latency (P=0.03) and a posterior shift (P=0.0002) following acid perfusion. The findings indicate that short-term sensitization of the esophagus results in central neuroplastic changes involving the cingulate gyrus, which also showed pathological activation in functional diseases of the gut, thus reflecting the importance of this region in visceral pain and hyperalgesia.

Motor cortex stimulation for long-term relief of chronic neuropathic pain: A 10 year experience

Chronic subthreshold stimulation of the contralateral precentral gyrus is used in patients with intractable neuropathic pain for more than 15 years. The aim of this study was to analyse retrospectively our own patient group with long term follow-up of 10 years. Seventeen patients with chronic neuropathic pain were treated with contralateral epidural stimulation electrodes. In 10 cases, trigeminal neuropathic pain (TNP) and in seven cases post-stroke pain (PSP) were diagnosed. The placement of the electrodes was performed in local anaesthesia using neuronavigation and intraoperative neuromonitoring. A test trial of minimum one week including double-blind testing was conducted and pain intensity was measured using a visual analogue scale (VAS). Correct placement of the electrode was achieved in all patients using intraoperative neurophysiological monitoring. Double-blind testing was able to identify 6 (35%) non-responders. In 5 of 10 (50%) with TNP and 3 of 7 (43%) with PSP a positive effect with pain reduction > or = 50% was observed. The mean follow-up period was 3.6 years (range 1-10 years) and includes a patient with 10 years of positive stimulation effect. Stimulation of the motor cortex is a treatment option for patients with chronic neuropathic pain localized in the face or upper extremity. Double-blind testing can identify non-responders. Patients with TNP profit more than patients with PSP. The positive effect can last for ten years in long-term follow-up.

Modulation of laser-evoked potentials by experimental cutaneous tonic pain

The present study aimed to investigate whether tonic cutaneous pain exerts any effect on the cortical processing of nociceptive input and if this effect may involve only body parts in pain. Tonic cutaneous pain was obtained in nine healthy human subjects by infusion of a hypertonic saline (5%) in the s.c. tissue over the hypothenar muscles (10 ml/h for 20 min). Nociceptive cutaneous CO2 laser-evoked potentials were recorded after stimulation of the right hand dorsum, which was adjacent to the painful area, and the right perioral region, corresponding to the adjacent cortical sensory area. Laser-evoked potentials were obtained before saline injection, at the peak pain and 20 min after pain disappeared. During saline infusion, the laser-evoked pain to right hand stimulation was reduced and the vertex laser-evoked potentials (N2a-P2, mean latency 181 ms and 319 ms for the N2a and the P2 potentials, respectively), which are generated in the anterior cingulate cortex, were significantly decreased in amplitude compared with the baseline. Moreover, the topography of these potentials was modified by cutaneous pain, shifting from the central toward the parietal region. Dipolar modeling showed that the dipolar source in the anterior cingulate cortex moved backward during saline infusion. This result suggests that cutaneous pain may modify the relative activities of the anterior and posterior anterior cingulate cortex parts, which are thought to be devoted to encode different aspects of pain sensation. No laser-evoked potential change was observed after stimulation of the right perioral region, suggesting that functional changes in the nociceptive system are selective for the painful regions and not for areas with cortical proximity.

Timing-dependent plasticity in human primary somatosensory cortex

Animal experiments suggest that cortical sensory representations may be remodelled as a consequence of changing synaptic efficacy by timing-dependent associative neuronal activity. Here we describe a timing-based associative form of plasticity in human somatosensory cortex. Paired associative stimulation (PAS) was performed by combining repetitive median nerve stimulation with transcranial magnetic stimulation (TMS) over the contralateral postcentral region. PAS increased exclusively the amplitude of the P25 component of the median nerve-evoked somatosensory-evoked potential (MN-SSEP), which is probably generated in the superficial cortical layers of area 3b. SSEP components reflecting neuronal activity in deeper cortical layers (N20 component) or subcortical regions (P14 component) remained constant. PAS-induced enhancement of P25 amplitude displayed topographical specificity both for the recording (MN-SSEP versus tibial nerve-SSEP) and the stimulation (magnetic stimulation targeting somatosensory versus motor cortex) arrangements. Modulation of P25 amplitude was confined to a narrow range of interstimulus intervals (ISIs) between the MN pulse and the TMS pulse, and the sign of the modulation changed with ISIs differing by only 15 ms. The function describing the ISI dependence of PAS effects on somatosensory cortex resembled one previously observed in motor cortex, shifted by approximately 7 ms. The findings suggest a simple model of modulation of excitability in human primary somatosensory cortex, possibly by mechanisms related to the spike-timing-dependent plasticity of neuronal synapses located in upper cortical layers.