Short and middle-latency Median Nerve (MN) SEPs recorded by depth electrodes in human pre-SMA and SMA-proper

A Novel Approach to Screen for Somatosensory Evoked Potentials in Critical Care

BACKGROUND

Somatosensory evoked potentials (SSEPs) help prognostication, particularly in patients with diffuse brain injury. However, use of SSEP is limited in critical care. We propose a novel, low-cost approach allowing acquisition of screening SSEP using widely available intensive care unit (ICU) equipment, specifically a peripheral "train-of-four" stimulator and standard electroencephalograph.

METHODS

The median nerve was stimulated using a train-of-four stimulator, and a standard 21-channel electroencephalograph was recorded to generate the screening SSEP. Generation of the SSEP was supported by visual inspection, univariate event-related potentials statistics, and a multivariate support vector machine (SVM) decoding algorithm. This approach was validated in 15 healthy volunteers and validated against standard SSEPs in 10 ICU patients. The ability of this approach to predict poor neurological outcome, defined as death, vegetative state, or severe disability at 6 months, was tested in an additional set of 39 ICU patients.

RESULTS

In each of the healthy volunteers, both the univariate and the SVM methods reliably detected SSEP responses. In patients, when compared against the standard SSEP method, the univariate event-related potentials method matched in nine of ten patients (sensitivity = 94%, specificity = 100%), and the SVM had 100% sensitivity and specificity when compared with the standard method. For the 49 ICU patients, we performed both the univariate and the SVM methods: a bilateral absence of short latency responses (n = 8) predicted poor neurological outcome with 0% FPR (sensitivity = 21%, specificity = 100%).

CONCLUSIONS

Somatosensory evoked potentials can reliably be recorded using the proposed approach. Given the very good but slightly lower sensitivity of absent SSEPs in the proposed screening approach, confirmation of absent SSEP responses using standard SSEP recordings is advised.

Sensory Gating during Voluntary Finger Movement in Amyotrophic Lateral Sclerosis with Sensory Cortex Hyperexcitability

Cortical responses in somatosensory evoked potentials (SEP) are enhanced in patients with amyotrophic lateral sclerosis (ALS). This study investigated whether sensory gating is involved in the pathophysiology of sensory cortical hyperactivity in ALS patients. The median nerve SEP was recorded at rest and during voluntary finger movements in 14 ALS patients and 13 healthy control subjects. The parietal N20, P25, and frontal N30 were analyzed, and sensory gating was assessed by measuring the amplitude of each component during finger movement. The amplitudes of the N20 onset-peak, N20 peak-P25 peak, and N30 onset-peak were higher in ALS patients than in controls. Nonetheless, there were no significant differences in the amplitude reduction ratio of SEPs between patients and controls. There was a significant correlation between the baseline amplitudes of the N20 onset-peak or N20 peak-P25 peak and their gating ratios in patients with ALS. Our findings indicate that the excitability of the primary sensory cortex and secondary motor cortex is enhanced in ALS, while sensory gating is preserved in the early stages of ALS. This result suggests that enhanced SEP is caused by the hyperexcitability of the primary sensory and secondary motor cortices but not by the dysfunction of inhibitory mechanisms during voluntary movements.

Pre-movement gating of somatosensory evoked potentials in tourette syndrome

OBJECTIVE

Tourette syndrome (TS) is a neurobehavioral disorder characterized by motor and vocal tics. Simple tics are purposeless involuntary movements that spontaneously resolve during middle adolescence. Complex tics appear to be semi-voluntary movements that may become intractable when associated with obsessive-compulsive disorder (OCD). Sensory tics or urges preceded by tics suggest sensorimotor processing impairment in TS. We aimed to clarify its pathophysiology by exploring the pre-movement gating (attenuation) of somatosensory evoked potentials (SEPs).

METHODS

We examined 42 patients (aged 9-48 years), 4 of whom underwent follow-up assessment, along with 19 healthy controls. We defined patients with only simple tics as TS-S and patients with complex tics as TS-C. Pre-movement gating of SEPs was assessed using a previously described method. Frontal N30 (FrN30) amplitudes were compared between pre-movement and resting states. The gating ratio of pre-movement/resting amplitude of the FrN30 component was assessed: the larger the ratio, the less the gating.

RESULTS

The gating ratio for TS-C patients was larger than that of TS-S patients and healthy controls, but a statistical difference between TS-S and TS-C appeared after 15 years and over (p < 0.001). There were no significant differences in the gating ratio between TS-S patients and healthy controls. The gating ratio was related to the severity of OCD (p < 0.05).

CONCLUSION

Sensorimotor processing was preserved for simple tics but impaired in complex tics, specifically after middle adolescence. Our study supports an age-dependent dysfunction of both motor and non-motor cortico-striato-thalamo-cortical circuits in complex tics. SEP gating seems promising as a tool for assessing age-dependent sensorimotor disintegration in TS.

Long-term effects of concussion on relevancy-based modulation of somatosensory-evoked potentials

OBJECTIVE

The purpose of this investigation was to better understand the effects of concussions on the ability to selectively up or down-regulate incoming somatosensory information based on relevance.

METHODS

Median nerve somatosensory-evoked potentials (SEPs) were elicited from electrical stimulation and recorded from scalp electrodes while participants completed tasks that altered the relevance of specific somatosensory information being conveyed along the stimulated nerve.

RESULTS

Within the control group, SEP amplitudes for task-relevant somatosensory information were significantly greater than for non-relevant somatosensory information at the earliest cortical processing potentials (N20-P27). Alternatively, the concussion history group showed similar SEP amplitudes for all conditions at early processing potentials, however a pattern similar to controls emerged later in the processing stream (P100) where both movement-related gating and facilitation of task-relevant information were present.

CONCLUSIONS

Previously concussed participants demonstrated impairments in the ability to up-regulate relevant somatosensory information at early processing stages. These effects appear to be chronic, as this pattern was observed on average several years after participants' most recent concussion.

SIGNIFICANCE

Given the role of the prefrontal cortex in relevancy-based facilitation during movement-related gating, these findings lend support to the notion that this brain area may be particularly vulnerable to concussive forces.

Multiple time courses of somatosensory responses in human cortex

Here we show how anatomical and functional data recorded from patients undergoing stereo-EEG can be used to decompose the cortical processing following nerve stimulation in different stages characterized by specific topography and time course. Tibial, median and trigeminal nerves were stimulated in 96 patients, and the increase in gamma power was evaluated over 11878 cortical sites. All three nerve datasets exhibited similar clusters of time courses: phasic, delayed/prolonged and tonic, which differed in topography, temporal organization and degree of spatial overlap. Strong phasic responses of the three nerves followed the classical somatotopic organization of SI, with no overlap in either time or space. Delayed responses presented overlaps between pairs of body parts in both time and space, and were confined to the dorsal motor cortices. Finally, tonic responses occurred in the perisylvian region including posterior insular cortex and were evoked by the stimulation of all three nerves, lacking any spatial and temporal specificity. These data indicate that the somatosensory processing following nerve stimulation is a multi-stage hierarchical process common to all three nerves, with the different stages likely subserving different functions. While phasic responses represent the neural basis of tactile perception, multi-nerve tonic responses may represent the neural signature of processes sustaining the capacity to become aware of tactile stimuli.

Comparison of the spatial resolution of source imaging techniques in high-density EEG and MEG

BACKGROUND

The present study aims at evaluating and comparing electrical and magnetic distributed source imaging methods applied to high-density Electroencephalography (hdEEG) and Magnetoencephalography (MEG) data. We used resolution matrices to characterize spatial resolution properties of Minimum Norm Estimate (MNE), dynamic Statistical Parametric Mapping (dSPM), standardized Low-Resolution Electromagnetic Tomography (sLORETA) and coherent Maximum Entropy on the Mean (cMEM, an entropy-based technique). The resolution matrix provides information of the Point Spread Functions (PSF) and of the Crosstalk functions (CT), this latter being also called source leakage, as it reflects the influence of a source on its neighbors.

METHODS

The spatial resolution of the inverse operators was first evaluated theoretically and then with real data acquired using electrical median nerve stimulation on five healthy participants. We evaluated the Dipole Localization Error (DLE) and the Spatial Dispersion (SD) of each PSF and CT map.

RESULTS

cMEM showed the smallest spatial spread (SD) for both PSF and CT maps, whereas localization errors (DLE) were similar for all methods. Whereas cMEM SD values were lower in MEG compared to hdEEG, the other methods slightly favored hdEEG over MEG. In real data, cMEM provided similar localization error and significantly less spatial spread than other methods for both MEG and hdEEG. Whereas both MEG and hdEEG provided very accurate localizations, all the source imaging methods actually performed better in MEG compared to hdEEG according to all evaluation metrics, probably due to the higher signal-to-noise ratio of the data in MEG.

CONCLUSION

Our overall results show that all investigated methods provide similar localization errors, suggesting very accurate localization for both MEG and hdEEG when similar number of sensors are considered for both modalities. Intrinsic properties of source imaging methods as well as their behavior for well-controlled tasks, suggest an overall better performance of cMEM in regards to spatial resolution and spatial leakage for both hdEEG and MEG. This indicates that cMEM would be a good candidate for studying source localization of focal and extended generators as well as functional connectivity studies.

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.

Long-Latency Somatosensory Evoked Potentials of the Subthalamic Nucleus in Patients with Parkinson’s Disease

Somatosensory evoked potentials (SSEPs) are a viable way to measure processing of somatosensory information. SSEPs have been described at the scalp and the cortical level by electroencephalographic, magnetoencephalographic and intracranial cortical recordings focusing on short-latency (SL; latency<40 ms) and long-latency (LL; latency>40 ms) SSEPs as well as by deep brain stimulation (DBS) electrode studies targeting SL-SSEPs. Unfortunately, LL-SSEPs have not been addressed at the subcortical level aside from the fact that studies targeting the characteristics and generators of SSEPs have been neglected for the last ten years. To cope with these issues, we investigated LL-SSEPs of the subthalamic nucleus (STN) in twelve patients with Parkinson’s disease (PD) that underwent deep brain stimulation (DBS) treatment. In a postoperative setting, LL-SSEPs were elicited by median nerve stimulation (MNS) to the patient’s wrists. Ipsilateral or contralateral MNS was applied with a 3 s inter-stimulus interval. Here, we report about four distinctive LL-SSEPs (“LL–complex” consisting of P80, N100, P140 and N200 component), which were recorded by using monopolar/bipolar reference and ipsi/contralateral MNS. Phase reversal and/or maximum amplitude provided support for the generation of such LL-SSEPs within the STN, which also underscores a role of this subcortical structure in sensory processing.

High-order motor cortex in rats receives somatosensory inputs from the primary motor cortex via cortico-cortical pathways

The motor cortex of rats contains two forelimb motor areas; the caudal forelimb area (CFA) and the rostral forelimb area (RFA). Although the RFA is thought to correspond to the premotor and/or supplementary motor cortices of primates, which are higher-order motor areas that receive somatosensory inputs, it is unknown whether the RFA of rats receives somatosensory inputs in the same manner. To investigate this issue, voltage-sensitive dye (VSD) imaging was used to assess the motor cortex in rats following a brief electrical stimulation of the forelimb. This procedure was followed by intracortical microstimulation (ICMS) mapping to identify the motor representations in the imaged cortex. The combined use of VSD imaging and ICMS revealed that both the CFA and RFA received excitatory synaptic inputs after forelimb stimulation. Further evaluation of the sensory input pathway to the RFA revealed that the forelimb-evoked RFA response was abolished either by the pharmacological inactivation of the CFA or a cortical transection between the CFA and RFA. These results suggest that forelimb-related sensory inputs would be transmitted to the RFA from the CFA via the cortico-cortical pathway. Thus, the present findings imply that sensory information processed in the RFA may be used for the generation of coordinated forelimb movements, which would be similar to the function of the higher-order motor cortex in primates.

Interactive effect of acute pain and motor learning acquisition on sensorimotor integration and motor learning outcomes

Previous work has demonstrated differential changes in early somatosensory evoked potentials (SEPs) when motor learning acquisition occurred in the presence of acute pain; however, the learning task was insufficiently complex to determine how these underlying neurophysiological differences impacted learning acquisition and retention. To address this limitation, we have utilized a complex motor task in conjunction with SEPs. Two groups of 12 participants (n = 24) were randomly assigned to either a capsaicin (capsaicin cream) or a control (inert lotion) group. SEP amplitudes were collected at baseline, after application, and after motor learning acquisition. Participants performed a motor acquisition task followed by a pain-free retention task within 24-48 h. After motor learning acquisition, the amplitude of the N20 SEP peak significantly increased (P < 0.05) and the N24 SEP peak significantly decreased (P < 0.001) for the control group while the N18 SEP peak significantly decreased (P < 0.01) for the capsaicin group. The N30 SEP peak was significantly increased (P < 0.001) after motor learning acquisition for both groups. The P25 SEP peak decreased significantly (P < 0.05) after the application of capsaicin cream. Both groups improved in accuracy after motor learning acquisition (P < 0.001). The capsaicin group outperformed the control group before motor learning acquisition (P < 0.05) and after motor learning acquisition (P < 0.05) and approached significance at retention (P = 0.06). Improved motor learning in the presence of capsaicin provides support for the enhancement of motor learning while in acute pain. In addition, the changes in SEP peak amplitudes suggest that early SEP changes reflect neurophysiological alterations accompanying both motor learning and mild acute pain.

The influence of an acute bout of aerobic exercise on cortical contributions to motor preparation and execution

Increasing evidence supports the use of physical activity for modifying brain activity and overall neurological health. Specifically, aerobic exercise appears to have a positive effect on cognitive function, which some have suggested to be a result of increasing levels of arousal. However, the role of aerobic exercise on movement-related cortical activity is less clear. We tested the hypothesis that (1) an acute bout of exercise modulates excitability within motor areas and (2) transient effects would be sustained as long as sympathetic drive remained elevated (indicated by heart rate). In experiment 1, participants performed unimanual self-paced wrist extension movements before and after a 20-min, moderate intensity aerobic exercise intervention on a recumbent cycle ergometer. After the cessation of exercise, Bereitschaftspotentials (BP), representative cortical markers for motor preparation, were recorded immediately postexercise (Post) and following a return to baseline heart rate (Post[Rest]). Electroencephalography (EEG) was used to measure the BP time-locked to onset of muscle activity and separated into three main components: early, late and reafferent potentials. In experiment 2, two additional time points postexercise were added to the original protocol following the Post[Rest] condition. Early BP but not late BP was influenced by aerobic exercise, evidenced by an earlier onset, indicative of a regionally selective effect across BP generators. Moreover, this effect was sustained for up to an hour following exercise cessation and this effect was following a return to baseline heart rate. These data demonstrate that acute aerobic exercise may alter and possibly enhance the cortical substrates required for the preparation of movement.

Neuromagnetic activation following active and passive finger movements

The detailed time courses of cortical activities and source localizations following passive finger movement were studied using whole-head magnetoencephalography (MEG). We recorded motor-related cortical magnetic fields following voluntary movement and somatosensory-evoked magnetic fields following passive movement (PM) in 13 volunteers. The most prominent movement-evoked magnetic field (MEF1) following active movement was obtained approximately 35.3 ± 8.4 msec after movement onset, and the equivalent current dipole (ECD) was estimated to be in the primary motor cortex (Brodmann area 4). Two peaks of MEG response associated with PM were recorded from 30 to 100 msec after movement onset. The earliest component (PM1) peaked at 36.2 ± 8.2 msec, and the second component (PM2) peaked at 86.1 ± 12.1 msec after movement onset. The peak latency and ECD localization of PM1, estimated to be in area 4, were the same as those of the most prominent MEF following active movement. ECDs of PM2 were estimated to be not only in area 4 but also in the supplementary motor area (SMA) and the posterior parietal cortex (PPC) over the hemisphere contralateral to the movement, and in the secondary somatosensory cortex (S2) of both hemispheres. The peak latency of each source activity was obtained at 54-109 msec in SMA, 64-114 msec in PPC, and 84-184 msec in the S2. Our results suggest that the magnetic waveforms at middle latency (50-100 msec) after PM are different from those after active movement and that these waveforms are generated by the activities of several cortical areas, that is, area 4 and SMA, PPC, and S2. In this study, the time courses of the activities in SMA, PPC, and S2 accompanying PM in humans were successfully recorded using MEG with a multiple dipole analysis system.

Dipole source analyses of early median nerve SEP components obtained from subdural grid recordings

The median nerve N20 and P22 SEP components constitute the initial response of the primary somatosensory cortex to somatosensory stimulation of the upper extremity. Knowledge of the underlying generators is important both for basic understanding of the initial sequence of cortical activation and to identify landmarks for eloquent areas to spare in resection planning of cortex in epilepsy surgery. We now set out to localize the N20 and P22 using subdural grid recording with special emphasis on the question of the origin of P22: Brodmann area 4 versus area 1. Electroencephalographic dipole source analysis of the N20 and P22 responses obtained from subdural grids over the primary somatosensory cortex after median nerve stimulation was performed in four patients undergoing epilepsy surgery. Based on anatomical landmarks, equivalent current dipoles of N20 and P22 were localized posterior to (n = 2) or on the central sulcus (n = 2). In three patients, the P22 dipole was located posterior to the N20 dipole, whereas in one patient, the P22 dipole was located on the same coordinate in anterior-posterior direction. On average, P22 sources were found to be 6.6 mm posterior [and 1 mm more superficial] compared with the N20 sources. These data strongly suggest a postcentral origin of the P22 SEP component in Brodmann area 1 and render a major precentral contribution to the earliest stages of processing from the primary motor cortex less likely.

Frontal phasic and oscillatory generators of the N30 somatosensory evoked potential

The N30 component of somatosensory evoked potentials has been recognized as a crucial index of brain sensorimotor processing and has been increasingly used clinically. Previously, we have shown that the N30 is accompanied by both an increase of the power spectrum of the ongoing beta-gamma EEG (event related synchronization, ERS) and by a reorganization (phase-locking) of the spontaneous phase of this rhythm (inter-trials coherency, ITC). In order to localize its sources taking into account both the phasic and oscillatory aspects of the phenomenon, we here apply swLORETA methods on averaged signals of the event-related potential (ERP) from a 128 scalp-electrodes array in time domain and also on raw EEG signals in frequency domain at the N30 peak latency. We demonstrate that the two different mechanisms that generate the N30 component power increase (ERS) and phase locking (ITC) across EEG trials are spatially localized in overlapping areas in the precentral cortex, namely the motor cortex (BA4) and the premotor cortex (BA6). From this common region, the generator of the N30 event-related potential expands toward the posterior part of BA4, the anterior part of BA6 and the prefrontal cortex (BA9). These latter areas also present significant ITC sources in the beta-gamma frequency range, but without significant power increase of this rhythm. This demonstrates that N30 results from network activity that depends on distinct oscillating and phasic generators localized in the frontal cortex.

An integrated fMRI, SEPs and MEPs approach for assessing functional organization in the malformed sensorimotor cortex

PURPOSE

Malformations of cortical development are often accompanied by an abnormal cortical pattern. Due to its propensity to involve discrete cortical areas, polymicrogyria represents an interesting model for assessing the reorganization of cortical function in relation to the disrupted anatomy. Functional MRI, TMS and SEPs can provide a highly complementary, multimodal approach to map noninvasively the functional rearrangement of sensorimotor functions in the polymicrogyric cortex, and to obtain a coherent modelling. We report here an illustrative case which is included in a patients series under study using a block design 3T fMRI, short-latency SEPs as identified on the basis of their latency, polarity, and scalp distribution and an assessment of the area and volume of the motor maps and the relative position of the center of gravity and hot spot.

RESULTS

A 15 years old girl, with drug-resistant epilepsy and left perisylvian polymicrogyria that was part of a large epileptogenic network including also the mesial aspect of the left frontal lobe, exhibited a normal distribution of somatomotor responses in the expected anatomic sites, with a dissociation between motor functions, which were slightly impaired in the malformed hemisphere, and bilaterally normal sensory responses. In this patient, a large resection of epileptogenic zone, sparing eloquent areas as previously identified, should be planned in order to improve seizure outcome.

CONCLUSIONS

An integrated fMRI, TMS and SEP mapping approach helps defining the relationship between epileptogenic zones and somatomotor areas. Studies of greater number of patients will be necessary in order to identify the general rules that determine the functional representation in the malformed cortex.

Inverse modeling on decomposed electroencephalographic data: a way forward?

Inverse modeling is typically applied to instantaneous electroencephalogram signals. However, this approach has several shortcomings including its instability to model multiple and deep located dipole sources and the interference of background noise may hamper the sensitivity, stability, and precision of the estimated dipoles. This article validates different dipole estimation techniques to find the most optimal combination of different analysis principles using both simulations and recordings. Electroencephalogram data were simulated with six known source locations. First, a dataset was simulated with sources chosen to mimic somatosensory-evoked potentials to electrical stimuli. Additionally, 20 further datasets were simulated each containing six randomly located and oriented sources. The simulated sources included superficial, deep, and simultaneously active sources. Furthermore, somatosensory-evoked potentials to median nerve stimuli were recorded from one subject. On both simulated and recorded evoked potential data, three different methods of signal decomposition were compared: independent component analysis (ICA), second-order blind identification (SOBI), and multichannel matching pursuit (MMP). For inverse modeling of the brain sources, the DIPFIT function of the EEGLAB software was used on raw and decomposed data. MMP was able to separate all simulated components that corresponded to superficial, deep, and simultaneously active sources. ICA and SOBI were only able to find components that corresponded to superficial sources. For the 20 randomized simulations, the results from the evoked potential simulation were reproduced. Inverse modeling on MMP components (atoms) was better than on ICA or SOBI components (P < 0.001). DIPFIT on MMP atoms localized 99.2% of the simulated dipoles in correct areas with their correct time/frequency distribution. DIPFIT on ICA and SOBI components localized 35% and 39.6%, respectively of the simulated dipoles in correct areas. As for the real-evoked potentials recorded in one subject, DIPFIT on MMP atoms allowed us to build a dipole model closer to the current physiological knowledge than dipole modeling of ICA and SOBI components. The results show that using MMP before inverse modeling is a reliable way to noninvasively estimate cortical activation.

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.

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.

Cortical oscillatory changes occurring during somatosensory and thermal stimulation

Brief somatosensory stimuli are followed by amplitude decreases (event-related desynchronization, ERD) of the 10 and 20 Hz oscillations over the bilateral primary sensorimotor cortices, and by post-stimulus synchronization (event-related synchronization, ERS) of the 20 Hz oscillations in the contralateral primary sensorimotor cortex and in the supplementary motor area (SMA). The 10 and 20 Hz ERD differentiate weak and strong somatosensory stimuli but not fine intensity gradations, and the ipsilateral ERD is especially sensitive to habituation. Stimulus anticipation, motor imagery, action viewing as well as voluntary movements modulate the stimulus-related changes of cortical oscillations. Noxious laser stimuli, selectively activating Adelta and/or C fibers, and innocuous warm and cold stimuli are associated with 10 and 20 Hz ERD but not with the post-stimulus 20 Hz ERS suggesting that the post-stimulus ERS is only related to neuronal transmission in the lemniscal system. It is proposed that phase-unlocked cortical oscillations modulate the preparedness of a particular sensory channel for upcoming somatosensory processing.

Intracerebral study of gamma oscillations in the human sensorimotor cortex

Since few years, gamma oscillations have given rise to an increasing interest. They have been successively described as being involved in cognitive function and various sensory systems. However, their role remains the subject of much debate. Gamma rhythms are difficult to study in scalp recordings due to low amplitudes and because the skull filters out high-frequency signals. Hence, their study makes necessary intracerebral recordings. Here, we report our intracerebral data issuing from study of gamma oscillations in the human sensorimotor cortex during the preparation and execution of voluntary movements. These studies have been performed in epileptic patients explored by stereoelectroencephalography (SEEG). Whereas mu and beta rhythms reactivity was diffused, the gamma rhythm reactivity to the movement was very focused and was observed predominantly in the primary sensorimotor areas that were involved in the movement, as assessed by the electrical cortical stimulations. Gamma oscillations seemed to be related to the movement execution rather than to the movement preparation. We have compared the temporo-spatial relationships between movement-related cortical potentials (MRCPs) and sensorimotor rhythms. We show that (i) the late components of MRCPs (motor potential--MP and post-movement complex--PMc) and the gamma event-related synchronization (ERS) within the 40-60-Hz band always occurred in the same contacts (located in the primary sensorimotor areas) and (ii) the PMc peaked during the gamma ERS, whereas the MP began before it. The PMc, so-called 'Reafferent Potential', is supposed to reflect the somesthetic reafferentation of the sensorimotor cortex. Hence, it seems that the PMc and the gamma ERS represent two electrophysiological facets of the reafferentation of the cortex during the movement. We suggest that gamma oscillations within the 40-60-Hz band serve to facilitate kinesthesic afferences from the muscles and joints involved in the movement to the primary sensorimotor cortex, which would be necessary for controlling the ongoing movement.