Difference in somatosensory evoked fields elicited by mechanical and electrical stimulations: Elucidation of the human homunculus by a noninvasive method

We recently recorded somatosensory evoked fields (SEFs) elicited by compressing the glabrous skin of the finger and decompressing it by using a photosensor trigger. In that study, the equivalent current dipoles (ECDs) for these evoked fields appeared to be physiologically similar to the ECDs of P30m in median nerve stimulation. We sought to determine the relations of evoked fields elicited by mechanically stimulating the glabrous skin of the great toe and those of electrically produced P40m. We studied SEFs elicited by mechanical and electrical stimulations from the median and tibial nerves. The orientations of dipoles from the mechanical stimulations were from anterior-to-posterior, similar to the orientations of dipoles for P30m. The direction of the dipole around the peak of N20m from median nerve electrical stimulation was opposite to these directions. The orientations of dipoles around the peak of P40m by tibial nerve stimulation were transverse, whereas those by the compression and decompression stimulation of the toe were directed from anterior-to-posterior. The concordance of the orientations in ECDs for evoked fields elicited by mechanical and electrical stimulations suggests that the ECDs of P40m are physiologically similar to those of P30m but not to those of N20m. The discrepancy in orientations in ECDs for evoked field elicited by these stimulations in the lower extremity suggests that electrical and compression stimulations elicit evoked fields responding to fast surface rubbing stimuli and/or stimuli to the muscle and joint.     

Posterior insular activity contributes to the late laser-evoked potential component in EEG recordings

ObjectiveNociceptive activity in some brain areas has concordantly been reported in EEG source models, such as the anterior/mid-cingulate cortex and the parasylvian area. Whereas the posterior insula has been constantly reported to be active in intracortical and fMRI studies, non-invasive EEG and MEG recordings mostly failed to detect activity in this region. This study aimed to determine an appropriate inverse modeling approach in EEG recordings to model posterior insular activity, assuming the late LEP (laser evoked potential) time window to yield a better separation from other ongoing cortical activity.MethodsIn 12 healthy volunteers, nociceptive stimuli of three intensities were applied. LEP were recorded using 32-channel EEG recordings. Source analysis was performed in specific time windows defined in the grand-average dataset. Two distinct dipole-pairs located close to the operculo-insular area were compared.ResultsOur results show that posterior insular activity yields a substantial contribution to the latest part (positive component) of the LEP.ConclusionsEven though the initial insular activity onset is in the early LEP time window, modeling the insular activity in the late LEP time window might result in better separation from other ongoing cortical activity.SignificanceModeling the late LEP activity might enable to distinguish posterior insular activity.     

Spectral and temporal electroencephalography measures reveal distinct neural networks for the acquisition,consolidation, and interlimb transfer of motor skills in healthy young adults

Objective

Plasticity of the central nervous system likely underlies motor learning. It is however unclear, whether plasticity in cortical motor networks is motor learning stage-, activity-, or connectivity-dependent.

Cognitive deficits and rehabilitation mechanisms in mild traumatic brain injury patients revealed by EEG connectivity markers

ObjectiveTo explore the multiple specific biomarkers and cognitive compensatory mechanisms of mild traumatic brain injury (mTBI) patients at recovery stage.MethodsThe experiment was performed in two sections. In Section I, using event-related potential, event-related oscillation and spatial phase-synchronization, we explored neural dynamics in 24 volunteered healthy controls (HC) and 38 patients at least 6 months post-mTBI (19 with epidural hematoma, EDH; 19 with subdural hematoma, SDH) during a Go/NoGo task. In Section II, according to the neuropsychological scales, patients were divided into sub-groups to assess these electroencephalography (EEG) indicators in identifying different rehabilitation outcomes of mTBI.ResultsIn Section I, mean amplitudes of NoGo-P3 and P3d were decreased in mTBI patients relative to HC, and NoGo-theta power in the non-injured hemisphere was decreased in SDH patients only. In Section II, patients with chronic neuropsychological defects exhibited more serious impairments of intra-hemispheric connectivity, whereas inter-hemispheric centro-parietal and frontal connectivity were enhanced in response to lesions.ConclusionsEEG distinguished mTBI patients from healthy controls, and estimated different rehabilitation outcomes of mTBI. The centro-parietal and frontal connectivity are the main compensatory mechanism for the recovery of mTBI patients.SignificanceEEG measurements and network connectivity can track recovery process and mechanism of mTBI.     

Impairment of brain functions in Parkinson’s disease reflected by alterations in neural connectivity in EEG studies: A viewpoint

Clinical symptoms of Parkinson’s disease (PD) are accompanied by pathological phenomena detected locally in the basal ganglia (BG) as changes in local field potentials (LFPs) and also in cortical regions by electroencephalography (EEG). The literature published mainly between 2000 and 2017 was reviewed with an emphasis on approaches emerging after 2000, in particular on oscillatory dynamics, connectivity studies, and deep brain stimulation. Eighty-five articles were reviewed. The main observations were a general slowing of background activity, excessive synchronization of beta activity, and disturbed movement-related gamma oscillations in the BG and in the cortico-subcortical and cortico-cortical motor loops, suppressible by dopaminergic medication as well as by high-frequency deep brain stimulation (DBS). Non-motor symptoms are related mainly to changes in the alpha frequency range. EEG parameters can be useful in defining the risk of dementia in PD. Further progress was reported recently using advanced analytical technologies and high-performance computing (graph theory). Detailed knowledge of LFPs in PD enabled progress particularly in DBS therapy, which requires optimizing the clinical effect and minimizing adverse side effects. The neurocognitive networks and their dysfunction in PD and DBS therapy are promising targets for future research.     

Enlarged high frequency oscillations of the median nerve somatosensory evoked potential and survival in amyotrophic lateral sclerosis

ObjectiveA large N20 and P25 of the median nerve somatosensory evoked potential (SEP) predicts short survival in amyotrophic lateral sclerosis (ALS). We investigated whether high frequency oscillations (HFOs) over N20 are enlarged and associated with survival in ALS.MethodsA total of 145 patients with ALS and 57 healthy subjects were studied. We recorded the median nerve SEP and measured the onset-to-peak amplitude of N20 (N20o-p), and peak-to-peak amplitude between N20 and P25 (N20p-P25p). We obtained early and late HFO potentials by filtering SEP between 500 and 1 kHz, and measured the peak-to-peak amplitude. We followed up patients until endpoints (death or tracheostomy) and analyzed the relationship between SEP or HFO amplitudes and survival using a Cox analysis.ResultsPatients showed larger N20o-p, N20p-P25p, and early and late HFO amplitudes than the control values. N20p-P25p was associated with survival periods (p = 0.0004), while early and late HFO amplitudes showed no significant association with survival (p = 0.4307, and p = 0.6858, respectively).ConclusionsThe HFO amplitude in ALS is increased, but does not predict survival.SignificanceThe enlarged HFOs in ALS might be a compensatory phenomenon to the hyperexcitability of the sensory cortex pyramidal neurons.     

Clinical and advanced neurophysiology in the prognostic and diagnostic evaluation of disorders of consciousness: review of an IFCN-endorsed expert group

The analysis of spontaneous EEG activity and evoked potentials is a cornerstone of the instrumental evaluation of patients with disorders of consciousness (DoC). The past few years have witnessed an unprecedented surge in EEG-related research applied to the prediction and detection of recovery of consciousness after severe brain injury, opening up the prospect that new concepts and tools may be available at the bedside. This paper provides a comprehensive, critical overview of both consolidated and investigational electrophysiological techniques for the prognostic and diagnostic assessment of DoC. We describe conventional clinical EEG approaches, then focus on evoked and event-related potentials, and finally we analyze the potential of novel research findings. In doing so, we (i) draw a distinction between acute, prolonged and chronic phases of DoC, (ii) attempt to relate both clinical and research findings to the underlying neuronal processes and (iii) discuss technical and conceptual caveats. The primary aim of this narrative review is to bridge the gap between standard and emerging electrophysiological measures for the detection and prediction of recovery of consciousness. The ultimate scope is to provide a reference and common ground for academic researchers active in the field of neurophysiology and clinicians engaged in intensive care unit and rehabilitation.