Scalp Somatosensory Evoked Potentials Research Articles

O-12 Different contribution of muscular and cutaneous afferents to the scalp SEPs

Background Although somatosensory evoked potentials (SEPs) represent a largely diffused neurophysiological technique, the pathways generating the short-latency scalp components are not fully known. We aimed to investigate the SEPs recorded from the scalp and deep brain stimulation (DBS) leads to selective electrical stimulation of either muscular or cutaneous afferents. Material and methods SEPs were recorded in 6 patients suffering from advanced Parkinson’s disease who underwent electrode implantation in the pedunculopontine (PPTg) nucleus area. We compared SEPs recorded from the scalp and DBS electrode contacts to electrical stimulation of: (a) the median nerve at the wrist (mixed stimulation), (b) the abductor pollicis brevis motor point (pure stimulation of the muscle afferents), and (c) the distal phalanx of the thumb (pure cutaneous stimulation). High-frequency oscillations (HFOs) subtending the low-frequency SEPs were also analysed. Results The macroelectrode contacts recorded a triphasic (P1-N1-P2) potential and the scalp electrodes recorded all the classical SEP components, including the widespread N18 response, after both median nerve and pure cutaneous stimulation. After motor point stimulation, the scalp N18 component and the PPTg N1 response were not recorded. No difference in the HFOs between the stimulation modalities was found. The scalp N20 component was recorded to stimulation of any modality, even if with different latency and amplitude. Conclusions Considering that both the scalp N18 response and PPTg N1component reflect the activity of a pre-postsynaptic neural network in the cuneatus nucleus, our data suggest that the cuneate response is specifically evoked by cutaneous inputs, while muscle afferents are processed elsewhere.

T88. Optimizing lower limb Somatosensory Evoked Potentials (SEPs) with additional derivations

SEPs recorded over the scalp serve as surrogate markers of dorsal column proprioceptive pathway integrity during surgery. Changes in SEP amplitude and/or latency meeting “alarm criteria” can indicate neural injury and prompt the surgical team to take action. Alarm criteria (e.g. 50% drop in amplitude) lose specificity when SEP measurement variability is high (e.g. ⩾25%). Lower limb scalp SEPs has vary dipole vectors between individuals. Standard recording electrode derivations of SEPs have been shown to be suboptimal when compared to a derivation individually optimized for each patient. Scalp lower limb SEPs were monitored from a consecutive sample of 51 patients during non-emergent procedures at Mayo Clinic Rochester. Two additional scalp electrodes at Cz and Pz were setup beside the standard scalp electrodes (CPz, Fz, CPi and CPc). Two standard derivations (CPz-Fz and CPi-CPc) and five expanded derivations (CPz-CPc, Cz-CPc, Pz-CPc, CPz-CPi and Pz-Fz) were recorded. P38 amplitudes were measured for all derivations; the derivation with the lowest coefficient of variation (CV) and highest amplitude from each group (standard vs. expanded montage) were determined. The P38 with highest amplitude and lowest CV were found in the expanded montage in 78 (76%) and 71 (70%) out of 102 limbs from 51 patients. The expanded montage significantly lowered the minimum CV by 1.75% (P < 0.0001, linear regression) and increased the maximal amplitude by 0.13 μV (P < 0.0001, linear regression) as compared to the standard montage. The CPz-CPc was the most frequent derivation with lowest CV and highest amplitude among all seven derivations. The effort to implement an expanded scalp montage should be balanced with the potential improvements, which by our data, are only incremental. An expanded scalp montage may be highest yield in patients where recordings from a conventional montage are of poor quality.

27. Sural and tibial nerve SEPs are mediated by the same spinal pathway

Scalp somatosensory evoked potentials (SEPs) are later in latency after sural than after tibial nerve stimulation. This led to hypothesize that different spinal pathways could be involved in the scalp SEP generation (Rossini et al., 1986). Our aim was to investigate the SEP latencies at different subcortical sites after sural and tibial nerve stimulation. We studied 7 patients. Four patients with Parkinson’s disease (PD) were implanted with intracerebral electrodes for DBS: one in the thalamus bilaterally (Vim nucleus), and 3 in the Pedunculopontine nucleus (PPTg). The remaining 3 patients were implanted with cervical epidural electrodes for intractable pain. SEPs were recorded from the scalp and the deep recording electrodes after both tibial and sural nerve stimulation. In all our patients, the scalp SEP latencies were later after sural than after tibial nerve stimulation. On the contrary, SEPs recorded at all subcortical sites showed similar latencies for both sural and tibial nerve stimulation. In conclusion, sural and tibial nerve scalp SEPs are mediated by the same spinal pathway. The difference in latency observed between the scalp SEPs after sural and tibial nerve stimulation is probably due to the orientation of the dipolar sources generating these components.

Inhibitory effects of heterotopic noxious counter-stimulation on perception and brain activity related to Aβ-fibre activation.

Heterotopic noxious counter-stimulation (HNCS) inhibits pain and pain processes through cerebral and cerebrospinal mechanisms. However, it is unclear whether HNCS inhibits non-nociceptive processes, which needs to be clarified for a better understanding of HNCS analgesia. The aim of this study was to examine the effects of HNCS on perception and scalp somatosensory evoked potentials (SEPs). Seventeen healthy volunteers participated in two counter-balanced sessions, including non-nociceptive (selective Aβ-fibre activation) or nociceptive electrical stimulation, combined with HNCS. HNCS was produced by a 20-min cold pressor test (left hand) adjusted individually to produce moderate pain (mean±SEM: 42.5±5.3 on a 0-100 scale, where 0 is no pain and 100 the worst pain imaginable). Non-nociceptive electrical stimulation was adjusted individually at 80% of pain threshold and produced a tactile sensation in every subject. Nociceptive electrical stimulation was adjusted individually at 120% of RIII-reflex threshold and produced moderate pain (45.3±4.5). Shock sensation was significantly decreased by HNCS compared with baseline for non-nociceptive (P<0.001) and nociceptive (P<0.001) stimulation. SEP peak-to-peak amplitude at Cz was significantly decreased by HNCS for non-nociceptive (P<0.01) and nociceptive (P<0.05) stimulation. These results indicate that perception and brain activity related to Aβ-fibre activation are inhibited by HNCS. The mechanisms of this effect remain to be investigated to clarify whether it involves inhibition of spinal wide-dynamic-range neurons by diffuse noxious inhibitory controls, supraspinal processes or both.

Basal μ-opioid receptor availability in the amygdala predicts the inhibition of pain-related brain activity during heterotopic noxious counter-stimulation

The aim of this study was to investigate the association between the magnitude of anti-nociceptive effects induced by heterotopic noxious counter-stimulation (HNCS) and the basal μ-opioid receptor availability in the amygdala. In 8 healthy volunteers (4 females and 4 males), transcutaneous electrical stimulation was applied to the right sural nerve to produce the nociceptive flexion reflex (RIII-reflex), moderate pain, and scalp somatosensory evoked potentials (SEPs). Immersion of the left hand in cold water for 20min was used as HNCS. In a separate session, basal μ-opioid receptor availability was measured using positron emission tomography with the radiotracer [11C]carfentanil. HNCS produced a reduction of the P260 amplitude (p<0.05), a late component of SEP that reflects activity in the anterior cingulate cortex. This reduction was associated with higher basal μ-opioid receptor availability in the amygdala on the right (R2=0.55, p=0.03) with a similar trend on the left (R2=0.24, p=0.22). Besides, HNCS did not induce significant changes in pain and RIII-reflex amplitude (p>0.05). These results suggest that activation of μ-opioid receptors in the amygdala may contribute to the anti-nociceptive effects of HNCS. The lack of RIII-reflex modulation further suggests that μ-opioid receptor activation in the amygdala contributes to decrease pain-related brain activity through a cerebral mechanism independent of descending modulation.

31. Low- and high-frequency subcortical SEP amplitude reduction during pure passive movement

We aimed to investigate the effect of a pure passive movement on the amplitude of the somatosensory evoked potentials (SEPs) recorded by intracerebral leads implanted for deep brain stimulation (DBS). Eleven patients, suffering from Parkinson disease resistent to pharmacological treatment, underwent electrode implantation in the ventral intermediate nucleus (VIM) of the thalamus (2 patients), in the subthalamic (STN) nucleus (3 patients) and pedunculopontine (PPTg) nucleus (6 patients). The study was performed in the operating room with the patient under total intravenous anesthesia. Median nerve SEPs were recorded at rest and during a passive opposition movement of the ipsilateral thumb. Passive movement was performed at 1, 0.5, 0.33, 0.25, and 0.2 Hz. Beyond the low-frequency SEPs, also the high-frequency responses were analysed. Both DBS lead potentials and scalp components reduced their amplitudes during passive movement at 1 Hz, as compared to the rest. Also the high-frequency SEP amplitudes were significantly lower during passive movement than at rest. Our study suggests that passive movement reduces the scalp SEP amplitude (centripetal gating) through a pure subcortical mechanism.

Non-invasive detection of cortical population spikes: Functional discrimination of pre- vs. postsynaptic components in SEP at 1 kHz

Introduction: Ultrafast EEG signals, featuring frequencies above 500 Hz, can be observed in scalp somatosensory evoked potentials (SEP). Usually, these recordings have a low Signal-to-Noise Ratio (SNR) because weak signals are overlaid by intrinsic noise of much higher amplitude as generated by biological sources and the amplifier. Scalp recordings of SEP show bursts above 400 Hz with submicro-volt amplitudes, which could be extracted from noise up to now only by the use of massive averaging (n > 1000) and digital signal processing. Methods: Here, we investigate SEP up to 1 kHz elicited by median nerve stimulation with rates of either 0.99 Hz or 8.1 Hz. We exploit the increased SNR provided by a custom-made low noise amplifier technology together with an adapted implementation of Canonical Correlation Analyis algorithm, based on the assumption of high level stimulus phase locking across trials. Results: It proved possible in 3/3 subjects to discriminate hf-SEP generators at subcortical and intracortical levels at the single trial level, relying on the different neuronal refractory properties of these components which are phase-locked to the stimulus onset. Discussion: A custom-made low-noise EEG system enables the noninvasive detection of pre- and postsynaptic evoked activity at 1 kHz, i.e., in the frequency range housing spike-related multi-unit activity in intracortical recordings, providing the opportunity of non-invasive monitoring of multi-unit spike activity in man.

Pain modulation induced by respiration: Phase and frequency effects

The voluntary control of respiration is used as a common means to regulate pain and emotions and is fundamental to various relaxation and meditation techniques. The aim of the present study was to examine how breathing frequency and phase affect pain perception, spinal nociceptive activity (RIII-reflex) and brain activity (scalp somatosensory-evoked potentials – SEP’s). In 20 healthy volunteers, painful electric shocks individually adjusted to 120% of the RIII-reflex threshold were delivered to the sural nerve near the end of inspiration or expiration phases, during three cued-breathing conditions: (1) slow breathing (0.1Hz) with slow (4s) inspiration (0.1Hz-SlowIns), (2) slow breathing (0.1Hz) with fast (2s) inspiration (0.1Hz-FastIns), and (3) normal breathing (0.2Hz) with fast (2s) inspiration (0.2Hz). Pain ratings were not affected by breathing patterns (p=0.3), but were significantly lower during inspiration compared with expiration (p=0.02). This phase effect was also observed on the N100 component of SEP’s, but only in the 0.1-Hz-FastIns condition (p=0.03). In contrast, RIII-reflex amplitude was greater during inspiration compared with expiration (p=0.02). It was also decreased in the 0.1-Hz-SlowIns compared with the 0.2-Hz condition (p=0.01). Slow breathing also increased the amplitude of respiratory sinus arrhythmia (RSA), although these changes were not significantly associated with changes in pain responses. In conclusion, this study shows that pain and pain-related brain activity may be reduced during inspiration but these changes are dissociated from spinal nociceptive transmission. The small amplitude of these effects suggests that factors other than respiration contribute to the analgesic effects of relaxation and meditation techniques.

Non-invasive detection of cortical population spikes: Functional discrimination of pre- vs. postsynaptic components in SEP at 1 kHz

Introduction: Ultrafast EEG signals, featuring frequencies above 500 Hz, can be observed in scalp somatosensory evoked potentials (SEP). Usually, these recordings have a low Signal-to-Noise Ratio (SNR) because weak signals are overlaid by intrinsic noise of much higher amplitude as generated by biological sources and the amplifier. Scalp recordings of SEP show bursts above 400 Hz with submicro-volt amplitudes, which could be extracted from noise up to now only by the use of massive averaging (n > 1000) and digital signal processing. Methods: Here, we investigate SEP up to 1 kHz elicited by median nerve stimulation with rates of either 0.99 Hz or 8.1 Hz. We exploit the increased SNR provided by a custom-made low noise amplifier technology together with an adapted implementation of Canonical Correlation Analyis algorithm, based on the assumption of high level stimulus phase locking across trials. Results: It proved possible in 3/3 subjects to discriminate hf-SEP generators at subcortical and intracortical levels at the single trial level, relying on the different neuronal refractory properties of these components which are phase-locked to the stimulus onset. Discussion: A custom-made low-noise EEG system enables the noninvasive detection of pre- and postsynaptic evoked activity at 1 kHz, i.e., in the frequency range housing spike-related multi-unit activity in intracortical recordings, providing the opportunity of non-invasive monitoring of multi-unit spike activity in man.

Are high-frequency (600 Hz) oscillations in human somatosensory evoked potentials due to phase-resetting phenomena?

ObjectiveMedian nerve somatosensory evoked potentials (SEP) contain a brief oscillatory wavelet burst at about 600Hz (σ-burst) superimposed on the initial cortical component (N20). While invasive single-cell recordings suggested that this burst is generated by increased neuronal spiking activity in area 3b, recent non-invasive scalp recordings could not reveal concomitant single-trial added-activity, suggesting that the SEP burst might instead be generated by phase-reset of ongoing high-frequency EEG. Here, a statistical model and exemplary data are presented reconciling these seemingly contradictory results. MethodsA statistical model defined the conditions required to detect added-activity in a set of single-trial SEP. Its predictions were tested by analyzing human single-trial scalp SEP recorded with custom-made low-noise amplifiers. ResultsThe noise level in previous studies did not allow to detect single-trial added-activity in the period concomitant with the trial-averaged σ-burst. In contrast, optimized low-noise recordings do reveal added-activity in a set of single-trials. ConclusionsThe experimental noise level is the decisive factor determining the detectability of added-activity in single-trials. A low-noise experiment provided direct evidence that the SEP σ-burst is at least partly generated by added-activity matching earlier invasive single-cell recordings. SignificanceQuantitative criteria are provided for the feasibility of single-trial detectability of band-limited added-activity.

Cortical Potential Imaging of Somatosensory Evoked Potentials by Means of the Boundary Element Method in Pediatric Epilepsy Patients

The aim of the present study was to assess the feasibility of identifying the primary hand sensory area and central sulcus in pediatric patients using the cortical potential imaging (CPI) method from the scalp recorded somatosensory evoked potentials (SEPs). The CPI method was used to reconstruct the cortical potential distribution from the scalp potentials with the boundary element (3-layer: scalp, skull and brain) head model based on MR images of individual subjects. The cortical potentials estimated from the pre-operative scalp SEPs of four pediatric patients, were compared with the post-op subdural SEP recordings made in the same subjects. Estimated and directly recorded cortical SEP maps showed comparable spatial patterns on the cortical surface in four patients (spatial correlation coefficient >0.7 in the SEP spikes). For two of four patients, the estimated waveforms correlated significantly to the waveforms obtained by direct cortical recordings. The present results demonstrated the feasibility of the cortical potential imaging approach in noninvasive imaging spatial distribution and temporal waveforms of cortical potentials for pediatric patients. These also suggest that the CPI method may provide a promising means of estimating the cortical potential and noninvasive localizing the central sulcus to aid surgical planning for pediatric patients.

Thalamo‐cortical processing of near‐threshold somatosensory stimuli in humans

Somatosensory stimuli elicit complex cortical responses that are discernible as somatosensory evoked potentials (SEPs) in scalp electroencephalographic recordings. Whereas earlier SEP components, occurring up to 100 ms after stimulus delivery, have been labeled 'preconscious', later responses have been associated with stimulus awareness. To date, how far these processes are primarily cortical or comprise additional subcortical operations remains open. Therefore, we recorded thalamic and scalp SEPs evoked by perceived as well as unperceived median nerve stimulation in neurosurgical patients with electrodes implanted into the ventral intermediate nucleus of the thalamus for deep brain stimulation. At stimulation intensities below perceptual threshold, only thalamic SEP components appeared consistently during the first 75 ms after stimulus delivery. Stimulation that was perceived by the patients elicited cortical as well as thalamic SEPs that lasted longer than 75 ms. These results indicate that the thalamus remains active after the primary propagation of a sensory signal to the cortex, and suggest that the transition from elementary to higher-order somatosensory processing is based on thalamo-cortical interactions.

Somatosensory and Spinal Evoked Potentials in Patients With Upper Cervical Neurinoma

Scalp somatosensory evoked potentials (SEPs) and spinal evoked potentials (SpEP) were simultaneously recorded from the exposed surface of the upper cervical cord after median nerve stimulation in five patients undergoing surgery for upper cervical neurinomas. Two of the neurinomas were localized at C1 nerve root, two at C2, and one at C3. All patients showed good postsurgical recovery, suggesting that the tumors had not progressed to the stage where most of the nerve fibers were irreparably damaged. In patients with unaffected superficial and deep skin sensation, both SEP and SpEP were normal. In patients with more advanced tumor, the superficial sensation was abnormal but the deep skin sensation was intact. In these patients, the action potential propagation slowed down but continued partially through the tumor site on the relatively less affected side contralateral to the tumor; however, it stopped at the site of the tumor on the ipsilateral side. It is possible that full functional recovery becomes more difficult during the next stage of tumor development when the propagation of action potentials ceases bilaterally. The intraoperative monitoring of both SEP and SpEP thus appears useful for inferring details of functional integrity and prognosis of the spinal cord near a space-occupying tumor during the critical first two stages of neoplasm in which the spinal function is normal, or a sufficiently large fraction of ascending and descending nerve fibers are functionally suppressed, but are capable of recovery after a surgical intervention.

Evaluation of cortical current density imaging methods using intracranial electrocorticograms and functional MRI

Objective:EEG source imaging provides important information regarding the underlying neural activity from noninvasive electrophysiological measurements. The aim of the present study was to evaluate source reconstruction techniques by means of the intracranial electrocorticograms (ECoGs) and functional MRI. Methods:Five source imaging algorithms, including the minimum norm least square (MNLS), LORETA with Lp-norm (p equal to 1, 1.5 and 2), sLORETA, the minimum Lp-norm (p equal to 1 and 1.5; when p=2, the MNLS method is mathematically equivalent to the minimum Lp-norm) and L1-norm (the linear programming) methods, were evaluated in a group of 10 human subjects, in a paradigm with somatosensory stimulation. Cortical current density (CCD) distributions were estimated from the scalp somatosensory evoked potentials (SEPs), at approximately 30 ms following electrical stimulation of median nerve at the wrist. Realistic geometry boundary element head models were constructed from the MRIs of each subject and used in the CCD analysis. Functional MRI results obtained from a motor task and sensory stimulation in all subjects were used to identify the central sulcus, motor and sensory areas. In three patients undergoing neurosurgical evaluation, ECoGs were recorded in response to the somatosensory stimulation, and were used to help determine the central sulcus and the sensory cortex. Results:The CCD distributions estimated by the Lp-norm and LORETA-Lp methods were smoother when the p values were high. The LORETA based on the L1-norm performed better than the LORETA-L2 method for imaging well localized sources such as the P30 component of the SEP. The mean and standard deviation of the distance between the location of maximum CCD value and the central sulcus, estimated by the minimum Lp-norm (with p equal to 1), L1-norm (the Linear programming) and LORETA-Lp (with p equal to 1) methods, were 4, 7, 7 mm and 3, 4, 2 mm, respectively (after converting into Talairach coordinates). The mean and standard deviation of the aforementioned distance, estimated by the MNLS, LORETA with Lp-norm (p equal to 1.5 and 2.0), sLORETA and the minimum Lp-norm (p equal to 1.5) methods, were over 11 mm and 6 mm, respectively. Conclusions:The present experimental study suggests that L1-norm-based algorithms provide better performance than L2 and L1.5-norm-based algorithms, in the context of CCD imaging of well localized sources induced by somatosensory electrical stimulation of median nerve at the wrist.

Human intracranially-recorded cortical responses evoked by painful electrical stimulation of the sural nerve

Intracranial recordings were obtained from 5 epilepsy patients to help identify the generators of the scalp somatosensory evoked potential (SEP) components that appear to be involved in orienting attention towards a potentially threatening, painful sural nerve electrical stimulus. The intracranial recording data support, for the most part, the generators suggested by our scalp SEP studies. The generators of the central negativity at 70–110 ms post-stimulus and the contralateral temporal negativity at 100–180 ms are located in the somatosensory association areas in the medial wall of the parietal cortex and in the parietal operculum and insula, respectively. The negative potential at 130–200 ms recorded from over the fronto-central scalp appears to be generated in the medial prefrontal cortex and primary somatosensory cortex foot area. The intracranial recording data suggest that the positive scalp potential at 280–320 ms, which corresponds to the pain-related P2, has multiple generators, including the anterior cingulate cortex, inferior parietal cortex, and possibly the somatosensory association areas in the medial wall of the parietal cortex. Finally, the positive scalp potential at 320–400 ms has generators in brain areas that others have shown to generate the P3a, including the dorsolateral and medial prefrontal cortices, temporal parietal junction, and the posterior hippocampus, which supports our hypothesis that this potential is a pain-evoked P3a. The putative functional roles of the brain areas generating these components and the response properties of the intracranial peaks recorded from these brain areas are in most cases consistent with the attention- and pain-related properties of their corresponding scalp SEP components. The intracranial recordings also demonstrate that the source configuration underlying the SEP components are often more complex than was suggested from the scalp studies. This complexity implies that the dipole source localization analysis of these components will at best provide only a very crude estimate of source location and activity, and that caution must be used when interpreting a change in the scalp component amplitude.

Assessment of somatosensory evoked potentials during resuscitation of a 15-year-old boy with Duchenne muscular dystrophy

Patients with Duchenne muscular dystrophy (DMD) are likely to suffer from cardiac insufficiency. Subclinical cardiac insufficiency may decompensate intraoperatively. During spinal surgery, recording of somatosensory evoked potentials (SSEP) is the standard method of spinal cord monitoring. Assessment of SSEP has proven to be a highly prognostic measure of neurological outcome after cardiopulmonary resuscitation (CPR). In the case presented, scalp SSEP as response to stimulation of both median and tibial nerves were recorded during spinal surgery in a 15-year-old boy with DMD. The patient developed severe hypotension and circulatory collapse intraoperatively. SSEP were measured before, during and up to 3h after circulatory collapse. He was successfully resuscitated and fully recovered. Latencies of SSEP remained stable from all extremities whereas amplitudes significantly decreased during CPR, but recovered completely within 3h. The amplitudes of SSEP serve as a more sensitive marker for brain ischaemia than latencies. Stability of latencies and full recovery of amplitudes within 3h indicated sufficient CPR and predicted a good neurological recovery.

Effects of intrathecal anandamide on somatosensory evoked responses in rats

Anandamide, endogenous ligand of cannabinoid receptors produces similar effects of cannabinoids via CB1 receptors in the central nervous system. Its effect on ascending pathways of somatosensory conduction and somatosensory cortex is not known. The aim of this study was to determine the effects of anandamide on somatosensory evoked potentials (SEP). In this study, 24 Wistar male rats were used. The rats were divided into 4 groups. At the beginning, sciatic nerve stimulated scalp SEP traces were obtained from all of the rats. Later, 0.02 cm 3 anhydrous ethanol, 100 μg/kg, 200 μg/kg and 400 μg/kg anandamide dissolved in anhydrous ethanol were injected intrathecally to the first (control), second, third and fourth groups, respectively. Five minutes later, second SEP traces were started. In every SEP trace, two negative waves (N1, N2) following positive deflections were obtained. The latency and amplitudes of these waves assessed were compared in each group. In control and second groups, the parameters of these waves before and after the injections were not significantly different. However, in the third and fourth groups, latencies of N1 and N2 after injections were found significantly longer. This effect was dose dependent. In any of the groups, no significant changes were detected in the amplitudes after injections. In conclusion, anandamide, when injected intrathecally in pharmacological doses caused an induction of moderate conduction delay in SEP systems.

Evaluation of different cortical source localization methods using simulated and experimental EEG data

Different cortical source localization methods have been developed to directly link the scalp potentials with the cortical activities. Up to now, these methods are the only possible solution to noninvasively investigate cortical activities with both high spatial and time resolutions. However, the application of these methods is hindered by the fact that they have not been rigorously evaluated nor compared. In this paper, the performances of several source localization methods (moving dipoles, minimum Lp norm, and low resolution tomography (LRT) with Lp norm, p equal to 1, 1.5, and 2) were evaluated by using simulated scalp EEG data, scalp somatosensory evoked potentials (SEPs), and upper limb motor-related potentials (MRPs) obtained on human subjects (all with 163 scalp electrodes). By using simulated EEG data, we first evaluated the source localization ability of the above methods quantitatively. Subsequently, the performance of the various methods was evaluated qualitatively by using experimental SEPs and MRPs. Our results show that the overall LRT Lp norm method with p equal to 1 has a better source localization ability than any of the other investigated methods and provides physiologically meaningful reconstruction results. Our evaluation results provide useful information for choosing cortical source localization approaches for future EEG/MEG studies.

Reduction in amplitude of the subcortical low- and high-frequency somatosensory evoked potentials during voluntary movement: an intracerebral recording study

To investigate whether the reduction of amplitude of the scalp somatosensory evoked potentials (SEPs) during movement (gating) is due to an attenuation of the afferent volley at subcortical level. Median nerve SEPs were recorded from 9 patients suffering from Parkinson's disease, who underwent implant of intracerebral (IC) electrodes in the subthalamic nucleus or in the globus pallidum. SEPs were recorded from Erb's point ipsilateral to stimulation, from the scalp surface and from the IC leads, at rest and during a voluntary flexo-extension movement of the stimulated wrist. The recorded IC traces were submitted to an off-line filtering by a 300-1500 bandpass to obtain the high-frequency SEP bursts. IC leads recorded a triphasic component (P1-N1-P2) from 14 to 22 ms of latency. The amplitudes of the scalp N20, P20 and N30 potentials and of the IC triphasic component were significantly decreased during movement, while the peripheral N9 amplitude remained unchanged. Also the IC bursts, whose frequency was around 1000 Hz, were reduced in amplitude by the voluntary movement. Since the IC triphasic component is probably generated by neurons of the thalamic ventro-postero-lateral nucleus, which receive the somatosensory afferent volley, the P1-N1 amplitude reduction during movement suggests that the gating phenomenon involves also the subcortical structures.

Abnormal gating of somatosensory inputs in essential tremor.

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