Subcortical Somatosensory Evoked Potentials Research Articles

Standardized Kalman filtering for dynamical source localization of concurrent subcortical and cortical brain activity

ObjectiveWe introduce standardized Kalman filtering (SKF) as a new spatiotemporal method for tracking brain activity. Via the Kalman filtering scheme, the computational workload is low, and by spatiotemporal standardization, we reduce the depth bias of non-standardized Kalman filtering (KF). MethodsWe describe the standardized KF methodology for spatiotemporal tracking from the Bayesian perspective. We construct a realistic simulation setup that resembles activity due to somatosensory evoked potential (SEP) to validate the proposed methodology before we run our tests using real SEP data. ResultsIn the experiments, SKF was compared with standardized low-resolution brain electromagnetic tomography (sLORETA) and the non-standardized KF. SKF localized the cortical and subcortical SEP originators appropriately and tracked P20/N20 originators for investigated signal-to-noise ratios (25, 15, and 5 dB). sLORETA distinguished those for 25 and 15 dB suppressing the subcortical originators. KF tracked only the evolution of cortical activity but mislocalized it. ConclusionsThe numerical results suggest that SKF inherits the estimation accuracy of sLORETA and traceability of KF while producing focal estimates for SEP originators. SignificanceSKF could help study time-evolving brain activities and localize landmarks with a deep contributor or when there is no prior knowledge of evolution.

Somatosensory evoked potentials recorded from DBS electrodes: the origin of subcortical N18.

The different peaks of somatosensory-evoked potentials (SEP) originate from a variety of anatomical sites in the central nervous system. The origin of the median nerve subcortical N18 SEP has been studied under various conditions, but the exact site of its generation is still unclear. While it has been claimed to be located in the thalamic region, other studies indicated its possible origin below the pontomedullary junction. Here, we scrutinized and compared SEP recordings from median nerve stimulation through deep brain stimulation (DBS) electrodes implanted in various subcortical targets. We studied 24 patients with dystonia, Parkinson's disease, and chronic pain who underwent quadripolar electrode implantation for chronic DBS and recorded median nerve SEPs from globus pallidus internus (GPi), subthalamic nucleus (STN), thalamic ventral intermediate nucleus (Vim), and ventral posterolateral nucleus (VPL) and the centromedian-parafascicular complex (CM-Pf). The largest amplitude of the triphasic potential of the N18 complex was recorded in Vim. Bipolar recordings confirmed the origin to be close to Vim electrodes (and VPL/CM-Pf) and less close to STN electrodes. GPi recorded only far-field potentials in unipolar derivation. Recordings from DBS electrodes located in different subcortical areas allow determining the origin of certain subcortical SEP waves more precisely. The subcortical N18 of the median nerve SEP-to its largest extent-is generated ventral to the Vim in the region of the prelemniscal radiation/ zona incerta.

Movement predictability modulates sensorimotor processing.

An important factor for optimal sensorimotor control is how well we are able to predict sensory feedback from internal and external sources during movement. If predictability decreases due to external disturbances, the brain is able to adjust muscle activation and the filtering of incoming sensory inputs. However, little is known about sensorimotor adjustments when predictability is increased by availability of additional internal feedback. In the present study we investigated how modifications of internal and external sensory feedback influence the control of muscle activation and gating of sensory input. Co-activation of forearm muscles, somatosensory evoked potentials (SEP) and short afferent inhibition (SAI) were assessed during three object manipulation tasks designed to differ in the predictability of sensory feedback. These included manipulation of a shared object with both hands (predictable coupling), manipulation of two independent objects without (uncoupled) and with external interference on one of the objects (unpredictable coupling). We found a task-specific reduction in co-activation during the predictable coupling compared to the other tasks. Less sensory gating, reflected in larger subcortical SEP amplitudes, was observed in the unpredictable coupling task. SAI behavior was closely linked to the subcortical SEP component indicating an important function of subcortical sites in predictability related SEP gating and their direct influence on M1 inhibition. Together, these findings suggest that the unpredictable coupling task cannot only rely on predictive forward control and is compensated by enhancing co-activation and increasing the saliency for external stimuli by reducing sensory gating at subcortical level. This behavior might serve as a preparatory step to compensate for external disturbances and to enhance processing and integration of all incoming external stimuli to update the current sensorimotor state. In contrast, predictive forward control is accurate in the predictable coupling task due to the integrated sensory feedback from both hands where sensorimotor resources are economized by reducing muscular co-activation and increasing sensory gating.

A Prediction Model for Normal Variation of Somatosensory Evoked Potential During Scoliosis Surgery

Somatosensory evoked potential (SEP) has been commonly used as intraoperative monitoring to detect the presence of neurological deficits during scoliosis surgery. However, SEP usually presents an enormous variation in response to patient-specific factors such as physiological parameters leading to the false warning. This study proposes a prediction model to quantify SEP amplitude variation due to noninjury-related physiological changes of the patient undergoing scoliosis surgery. Based on a hybrid network of attention-based long-short-term memory (LSTM) and convolutional neural networks (CNNs), we develop a deep learning-based framework for predicting the SEP value in response to variation of physiological variables. The training and selection of model parameters were based on a 5-fold cross-validation scheme using mean square error (MSE) as evaluation metrics. The proposed model obtained MSE of 0.027[Formula: see text][Formula: see text] on left cortical SEP, MSE of 0.024[Formula: see text][Formula: see text] on left subcortical SEP, MSE of 0.031[Formula: see text][Formula: see text] on right cortical SEP, and MSE of 0.025[Formula: see text][Formula: see text] on right subcortical SEP based on the test set. The proposed model could quantify the affection from physiological parameters to the SEP amplitude in response to normal variation of physiology during scoliosis surgery. The prediction of SEP amplitude provides a potential varying reference for intraoperative SEP monitoring.

Contribution of different somatosensory afferent input to subcortical somatosensory evoked potentials in humans

Cortical projection of the somatosensory fibers of different modalities has been studied in humans. The aim of this study was to investigate the subcortical somatosensory evoked potentials (SEPs) to electrical stimulation of either muscle or cutaneous afferents.

Contribution of different somatosensory afferent input to subcortical somatosensory evoked potentials in humans

ObjectivesTo investigate the subcortical somatosensory evoked potentials (SEPs) to electrical stimulation of either muscle or cutaneous afferents. MethodsSEPs were recorded in 6 patients suffering from Parkinson’s disease (PD) who underwent electrode implantation in the pedunculopontine (PPTg) nucleus area. We compared SEPs recorded from the scalp and from the intracranial electrode contacts to electrical stimuli applied to: 1) median nerve at the wrist, 2) abductor pollicis brevis motor point, and 3) distal phalanx of the thumb. Also the high-frequency oscillations (HFOs) were analysed. ResultsAfter median nerve and pure cutaneous (distant phalanx of the thumb) stimulation, a P1-N1 complex was recorded by the intracranial lead, while the scalp electrodes recorded the short-latency far-field responses (P14 and N18). On the contrary, motor point stimulation did not evoke any low-frequency component in the PPTg traces, nor the N18 potential on the scalp. HFOs were recorded to stimulation of all modalities by the PPTg electrode contacts. ConclusionsStimulus processing within the cuneate nucleus depends on modality, since only the cutaneous input activates the complex intranuclear network possibly generating the scalp N18 potential. SignificanceOur results shed light on the subcortical processing of the somatosensory input of different modalities.

Opposite modulations of corticospinal excitability by intermittent and continuous peripheral electrical stimulation in healthy subjects

Peripheral electrical stimulation (PES) modulates the excitability of the corticospinal tract (CST). This modulation of CST excitability depends on the PES intensity, defined by the amplitude and the width of each pulse, the total pulse number, the stimulation frequency, and the intervention duration. Another key PES parameter is the stimulation pattern; little is known about how PES pattern affects CST excitability, as previous studies did not control other PES parameters. Here, we investigated the effect of the net difference in PES pattern on CST excitability. We use three controlled PESs, intermittent PES (30 Hz) (stimulation trains at 30 Hz with pauses), continuous PES (12 Hz) (constant stimulation at 12 Hz without pauses), and continuous PES (30 Hz) with the same stimulation frequency as the intermittent PES (30 Hz), to compare the effect of the stimulation frequency. The motor evoked potentials (MEPs) and somatosensory evoked potentials (SEPs) of healthy subjects were recorded before and after these three types of PESs in separate sessions. We found that intermittent PES (30 Hz) increased MEP amplitudes, whereas continuous PES (12 and 30 Hz) decreased amplitudes. A significant change in subcortical SEP component occurred during continuous PES (12 and 30 Hz), but not intermittent PES (30 Hz), whereas cortical SEP components showed similar behavior in three types of PESs. We conclude that (1) opposing modulations of CST excitability were induced by the differences in the PES pattern, and (2) these modulations appear to be mediated through different processes in the sensorimotor system. Our findings suggest the possibility that it may be preferable to select the PES pattern in therapeutic interventions based on the putative desired effect and the neural structure being targeted.

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

ObjectivesTo investigate the effect of pure passive movement on both cortical and subcortical somatosensory evoked potentials (SEPs). MethodsMedian nerve SEPs were recorded in 8 patients suffering from Parkinson’s disease (PD) and two patients with essential tremor. PD patients underwent electrode implantation in the subthalamic (STN) nucleus (3 patients) and pedunculopontine (PPTg) nucleus (5 patients), while 2 patients with essential tremor were implanted in the ventral intermediate nucleus (VIM) of the thalamus. In anesthetized patients, SEPs were recorded at rest and during a passive movement of the thumb of the stimulated wrist from the intracranial electrode contacts and from the scalp. Also the high-frequency oscillations (HFOs) were analyzed. ResultsAmplitudes of both deep and scalp components were decreased during passive movement, but the reduction was higher at cortical than subcortical level. Also the HFOs were reduced by movement. ConclusionThe different amount of the movement-related decrease suggests that the cortical SEP gating is not only the result of a subcortical somatosensory volley attenuation, but a further mechanism acting at cortical level should be considered. SignificanceOur results are important for understanding the physiological mechanism of the sensory–motor interaction during passive movement.

P834: Nocebo effect dissociates the laser-pain rating from the N2/P2 laser evoked potential amplitude

P834: Nocebo effect dissociates the laser-pain rating from the N2/P2 laser evoked potential amplitude

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.

Enhancing non-noxious perception: Behavioural and neurophysiological correlates of a placebo-like manipulation

Sensory perception can be influenced by cognitive functions like attention and expectation. An emblematic case of this is the placebo effect, where a reduction in pain perception can be obtained by inducing expectation of benefit following a treatment. The current study assessed the behavioural and brain activity correlates of a placebo procedure inducing an enhancement of non-noxious somatic sensation. An experimental group was verbally suggested and surreptitiously conditioned about the effect of an inert cream in enhancing tactile perception, while a control group was informed about the actual inefficacy of the cream. Both groups received non-noxious electric shocks activating A-Beta fibres on the right index finger, before and after application of the cream in the same site. The behavioural and neurophysiological effects of this procedure were measured by a numerical rating scale of subjective perception and by recording cortical and subcortical somatosensory-evoked potentials (SEPs). Although the intensity of stimulation was physically identical in the two sessions, the experimental group reported stronger tactile sensation after cream treatment than before. In parallel, the experimental group showed enhanced somatosensory cortical responses (N140, P200) after treatment, whereas subcortical and early-cortical SEP components did not change. We suggest that these findings reflect top-down modulation on tactile perception probably due to an interplay between expectation and attention and might rely on interactions between prefrontal and parietal brain regions.

Validation of an Adaptive Signal Enhancer in Intraoperative Somatosensory Evoked Potentials Monitoring

The conventional approach of ensemble averaging in intraoperative somatosensory evoked potentials (SEP) monitoring requires more than 500 trials to extract a reliable waveform for neurologic diagnosis. Previous studies showed that an adaptive signal enhancer (ASE) could increase the signal-to-noise ratio of input signals. This study assessed the accuracy and efficiency of the ASE in the extraction of neurologic normal human and abnormal rat SEP. Cortical and subcortical SEP were taken from 16 subjects undergoing scoliosis surgery. SEP extracted by ASE were compared with those obtained with 500-trial averaging in terms of peak latency, amplitude, and waveforms using correlation coefficients. An animal study composed of 18 rats was used to test the ASE in detecting abnormal SEP changes due to spinal cord compression. The results demonstrate the accuracy of ASE by showing very high correlations between ASE-processed SEP and ensemble averaging-processed SEP in waveforms, peak latencies, and amplitudes. The results also show the efficiency of the ASE in extracting SEP waveforms from 50 input trials, which provided waveforms of sufficiently high quality and latency/amplitude measurements equivalent to those obtained in 500 trials of conventional ensemble averaging. Because of its fast extraction ability, adaptive signal enhancement could be an appropriate alternative to conventional ensemble averaging in intraoperative spinal cord monitoring.

Importance of Montage Variation for the Assessment of the Cervical Spinal Cord

ABSTRACT.Median somatosensory evoked potentials (SEPs) were recorded from two patients who sustained a cerx'ical spinal cord injury. In addition to the standard cortical montage, an electrode over the sixth cervical vertebra (CV6) was referenced to Fz and also to the anterior cerx'ical (AC) region. In both patients, cortical and subcortical SEP responses, along with N13 recorded using CV6-Fz. derivation, were present and normal in latency. However, N13 using the CV6-AC derivation was not recorded in either patient, suggesting a selective loss of the cerx'ical cord response with preserved dorsal column conduction. This demonstrates that the Fz reference alone is not only inadequate for assessing the cervical N13 response, but clearly misleading for detecting a dorsal horn lesion at the lower cervical cord. It is concluded that in order to avoid ambiguous findings, additional derivations are necessary to dissociate overlapping SEP components contributing to a unitary potential and to unveil not readily recorded potentials.

Somatosensory evoked potential monitoring in anterior thoracic vertebrectomy

Spine surgeons have used intraoperative cortical and subcortical somatosensory evoked potential (SSEP) monitoring to detect changes in spinal cord function when intraoperative procedures can be performed to prevent neurological deterioration. However, the reliability of SSEP monitoring as applied to anterior thoracic vertebral body resections has not been rigorously assessed. The authors retrospectively reviewed hospital charts and operating room records obtained between August 1993 and December 1998 and found that SSEP monitoring was used in 44 surgical procedures involving an anterior approach for thoracic vertebral body resections. There were no patients in whom SSEP changes did not return to baseline during the surgical procedure. Patients in four cases, despite their stable SSEP recordings throughout the procedure, were noted immediately postoperatively to have experienced significant neurological deterioration. The false-negative rate in SSEP monitoring was 9%. Sensitivity was determined to be 0%. It is important to recognize high false-negative rates and low sensitivity of SSEP monitoring when it is used to record spinal cord function during anterior approaches for thoracic vertebrectomies. The insensitivity of SSEPs for motor deterioration during anterior thoracic vertebrectomies is likely due to the limitation of SSEPs, which monitor only posterior column function whereas motor paths are conveyed in the anterior and anterolateral spinal cord. The authors believe that SSEPs can not be relied on to detect reversible spinal damage during anterior thoracic vertebrectomies.

Abnormal subcortical somatosensory evoked potentials indicate high cervical myelopathy in achondroplasia.

Children with achondroplasia may have high cervical myelopathy due to stenosis of the cranio-cervical junction resulting in neurological disability and an increased rate of sudden death. To detect myelopathy we recorded somatosensory evoked potentials (SEPs) after median nerve stimulation in 30 patients with achondroplasia aged 13 months to 18 years (mean 6 years). In addition to the conventional technique of recording the cortical N20 and the central conduction time (CCT), we employed a noncephalic reference electrode recording the subcortical waveforms N13b and P13. generated near the cranio-cervical junction. The findings were related to the clinical status and MRI results. Eighteen patients had MRI evidence of spinal cord compression with indentation or narrowing of the upper cervical cord, and 13 showed signs of myelomalacia. Seven patients had neurological abnormalities. The sensitivities of the SEPs were 0.89 for cervical cord compression, 0.92 for myelomalacia and 1.0 for the clinically symptomatic patients. There were no false-positive results. The subcortical SEPs were more sensitive than the conventional recordings. However, the conventional SEPs were highly specific in the most severely affected patients; here the specificity was 1.0 for patients with myelomalacia and 0.96 for symptomatic patients. Postoperative SEPs improved after occipital decompression in two children. The analysis of somatosensory evoked potentials, in particular of subcortical tracings, is useful in the detection of early cervical myelopathy in children with achondroplasia. Early neurosurgical decompression may prevent irreversible damage.

Subcortical somatosensory evoked potentials after median nerve stimulation in children

We report our normative data of subcortical somatosensory evoked potentials (SEPs) after median nerve stimulation from a group of 55 children 4-15 years of age and 18 young adults 18-29 years of age. We recorded near-field potentials from the brachial plexus, the cervical cord and the somatosensory cortex. The far-field potentials P13, P14 and N18 from the brainstem were recorded from the scalp electrodes, when a non-cephalic reference at the contralateral Erb's point or an ear reference was used. The N9 (brachial plexus), N13a (dorsal horn), P13 (caudal medulla oblongata), N18 (medulla oblongata) and N20 (somatosensory cortex) were present in all subjects. The N13b (dorsal column near the foramen magnum or cuneate nucleus) was observed in all children and in 16 adults, P14 (medial lemniscus) in 52 children and 17 adults. The median nerve SEPs provide reliable information about the function of the somatosensory pathway from the upper limb. The subcortical median nerve SEPs should be particularly useful to detect lesions of the upper cervical cord and the cervicomedullary junction. The subcortical SEPs remain unchanged during sleep and facilitate reliable SEP recordings, when sedation is necessary in infants and children.

Somatosensory evoked potential monitoring in cervical surgery: identification of pre- and intraoperative risk factors associated with neurological deterioration.

Cortical and subcortical somatosensory evoked potentials (SSEPs) were noninvasively monitored in 191 surgical procedures involving the cervical spine. In nine patients in the poorest neurological condition, SSEPs could not be monitored. Lower limb SSEPs were often too degraded to be useful. Upper limb responses were reliably recorded in 182 procedures, with a sensitivity of 99% and a specificity of 27% in 10 patients who developed neurological signs postsurgery. The aim of monitoring was to detect changes in spinal cord function at a time when neurological deterioration could be prevented or reversed, and these studies alerted the authors to certain clinical and SSEP risk factors associated with deterioration. Clinical and operative risk factors were: 1) poor pre-operative neurological function (one-third of Ranawat Class IIIb patients deteriorated); 2) use of instrumentation (the risk doubled in preoperatively unimpaired patients); 3) upper cervical and clival surgery (the risk tripled); and 4) and multisegmental surgery (increased risk with each additional level). There were SSEP changes in 33 patients. Fifty percent of patients with a complete loss had neurological damage, unlike those who had incomplete loss or whose electrical changes had recovered by the end of surgery. In the authors' view these "false positives" may represent real physiological changes, the effects of which might have been minimized by an alteration in the surgeon's response as a result of the warning. Although these initial studies have made this surgical team more alert to potential problems, the role of intraoperative SSEP monitoring is still being debated.

Effects of hypocapnia on canine spinal, subcortical, and cortical somatosensory-evoked potentials during isoflurane anesthesia.

Although hyperventilation with hypocapnia is frequently used in the management of neurosurgical patients in whom sensory-evoked potentials may be monitored, the effects of hypocapnia on evoked potentials have not been described with precision. In the present experiment, the effects of randomized arterial carbon dioxide tensions of 20, 25, 30, and 35 mm Hg on spinal, subcortical, and cortical somatosensory-evoked potentials (SEPs) were measured in dogs anesthetized with 1.40% isoflurane. Other variables known to affect the SEP (temperature, blood pressure, and arterial oxygen tension) were stable throughout the experiment. Hypocapnia caused reductions in the latencies of the early peaks of the spinal and subcortical SEPs. These differences were small, consisting of a 2% shortening of latency at 20 mm Hg carbon dioxide tension when compared with 35 mm Hg. No changes were detected in the later subcortical and cortical latencies. SEP amplitudes were also unchanged. These results in a controlled animal study corroborate the direction and magnitude of changes due to hypocapnia observed by other investigators in surgical patients. The magnitude of the changes indicates that SEP monitoring sensitivity is not compromised by clinically useful levels of induced hypocapnia during isoflurane anesthesia. Because hypocapnia may produce small SEP changes, baseline recordings should be acquired prior to initiation of hyperventilation. It is not warranted, however, to impute a severe deterioration of the SEP to hypocapnia alone, and causes must be sought elsewhere in a patient's status and management.

SEP testing in deeply comatose and brain dead patients: the role of nasopharyngeal, scalp and earlobe derivations in recording the P14 potential

Median nerve somatosensory evoked potentials (SEPs) were tested in 50 patients (20 brain dead, 18 comatose and in 12 progessing from coma to brain death, i.e., 32 cases with brain death and 30 cases with coma were recorded). Derivations were taken from nasopharynx, earlobes, scalp, and neck using cephalic and non-cephalic references. Cortical and subcortical SEP components were evaluated, focussing on the P14 potential. There is evidence that rostral and caudal parts of the P14 generator (lemniscus medialis) are differently affected in brain death, resulting in an abolition of the rostral part, while occassionally leaving intact for some time the caudal part. Non-cephalic referenced scalp records pick up the whole P14 dipole, whereas nasopharyngeal and earlobe derivations pick up different parts of P14, depending on the reference used. Scalp-to-nasopharynx records derive the most rostral part of P14; this “rostral P14” was bilaterally lost in all brain dead patients, but preserved in all deeply comatose patients with diffuse brain-sttem injuries. Scalp-to-earlobe records in contrast, picked up a P14 dipole segment reaching more caudally, resulting in a P14 potential also in brain dead patients. It is concluded that midfrontal scalp-to-nasopharynx derivations give the moset valuable contribution to the electrophysiological assessment of brain death versus deep coma.

Serial recording of median nerve stimulated subcortical somatosensory evoked potentials (SEPs) in developing brain death

Subcortical somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded serially in 35 patients during the evolution towards brain death and in brain death. Neuropathological alterations of the central nervous system down to the C1/C2 spinal cord segment in brain death are well known. SEP components supposed to be generated above this level should be lost in brain death, while components generated below should not be altered. Erb's point, scalp and neck potentials were recorded at C3/4, or over the spinous process C7, using an Fz reference. In 10 patients additional montages, including spinous process C2-Fz, a non-cephalic reference (Fz-contralateral shoulder) and a posterior to anterior neck montage (spinous process C7-jugulum) were used. The cephalic referenced N9 and N11 peaks remained unchanged until brain death. N9 and N11 decreased in parallel in amplitude and increased in latency after systemic effects like hypoxia or hypothermia occurred. The cephalic referenced ‘N14’ decreased in amplitude and increased in latency after the clinical brain death syndrome was observed, while N13 in the posterior to anterior neck montage remained unchanged. The alteration of ‘N14’ went parallel to the decrease of the P14 amplitude. The subcortical SEPs in the cephalic referenced lead are supposed to be a peak composed by a horizontally orientated dorsal horn generated N13 and a rostrally orientated P14 arising at the level of the foramen magnum. The deterioration of the non-cephalic referenced P14 and of its cephalic referenced reflection ‘N14’ seems to provide an additional objective criterion for the diagnosis of brain death.