Reliable detection of bilateral activation in human primary somatosensory cortex by unilateral median nerve stimulation

Tracking gaze position from EEG: Exploring the possibility of an EEG-based virtual eye-tracker

INTRODUCTION

Ocular artifact has long been viewed as an impediment to the interpretation of electroencephalogram (EEG) signals in basic and applied research. Today, the use of blind source separation (BSS) methods, including independent component analysis (ICA) and second-order blind identification (SOBI), is considered an essential step in improving the quality of neural signals. Recently, we introduced a method consisting of SOBI and a discriminant and similarity (DANS)-based identification method, capable of identifying and extracting eye movement-related components. These recovered components can be localized within ocular structures with a high goodness of fit (>95%). This raised the possibility that such EEG-derived SOBI components may be used to build predictive models for tracking gaze position.

METHODS

As proof of this new concept, we designed an EEG-based virtual eye-tracker (EEG-VET) for tracking eye movement from EEG alone. The EEG-VET is composed of a SOBI algorithm for separating EEG signals into different components, a DANS algorithm for automatically identifying ocular components, and a linear model to transfer ocular components into gaze positions.

RESULTS

The prototype of EEG-VET achieved an accuracy of 0.920° and precision of 1.510° of a visual angle in the best participant, whereas an average accuracy of 1.008° ± 0.357° and a precision of 2.348° ± 0.580° of a visual angle across all participants (N = 18).

CONCLUSION

This work offers a novel approach that readily co-registers eye movement and neural signals from a single-EEG recording, thus increasing the ease of studying neural mechanisms underlying natural cognition in the context of free eye movement.

Suppression of perceptual sensitivity to digital nerve stimulation induced by afferent volley from digital nerve of contralateral homologous finger

The purpose of the present study is to investigate whether perceptual sensitivity to digital nerve stimulation is modulated by the afferent volley from the digital nerve of a contralateral finger. Fifteen healthy humans participated in this study. A test stimulus was given to the right-hand index finger, and a conditioning stimulus was given to one of the five fingers on the left hand 20, 30, or 40 ms before the test stimulus. The perceptual threshold of the finger stimulation was measured. The perceptual threshold of the test stimulus was significantly increased by a conditioning stimulus to the left-hand index finger given 40 ms before the test stimulus. In contrast, the threshold was not significantly changed by a conditioning stimulus to any finger other than the index finger. Perceptual sensitivity to digital nerve stimulation is suppressed by the afferent volley from the digital nerve of the contralateral homologous finger. This means that the afferent volley from the digital nerve suppresses the homologous finger representation in the ipsilateral somatosensory areas. These findings can be explained by the view that the afferent volley from the digital nerve of the index finger projects to the index finger representation in the contralateral primary sensory cortex and that the interhemispheric transcallosal inhibitory drive is provided from the secondary sensory cortex to the homologous finger representation in the contralateral secondary sensory cortex.

Sensory and motor cortical excitability changes induced by rTMS and sensory stimulation in stroke: A randomized clinical trial

Background

The ability to produce coordinated movement is dependent on dynamic interactions through transcallosal fibers between the two cerebral hemispheres of the brain. Although typically unilateral, stroke induces changes in functional and effective connectivity across hemispheres, which are related to sensorimotor impairment and stroke recovery. Previous studies have focused almost exclusively on interhemispheric interactions in the primary motor cortex (M1).

Objective

To identify the presence of interhemispheric asymmetry (ASY) of somatosensory cortex (S1) excitability and to investigate whether S1 repetitive transcranial magnetic stimulation (rTMS) combined with sensory stimulation (SS) changes excitability in S1 and M1, as well as S1 ASY, in individuals with subacute stroke.

Methods

A randomized clinical trial. Participants with a single episode of stroke, in the subacute phase, between 35 and 75 years old, were allocated, randomly and equally balanced, to four groups: rTMS/sham SS, sham rTMS/SS, rTMS/SS, and sham rTMS/Sham SS. Participants underwent 10 sessions of S1 rTMS of the lesioned hemisphere (10 Hz, 1,500 pulses) followed by SS. SS was applied to the paretic upper limb (UL) (active SS) or non-paretic UL (sham SS). TMS-induced motor evoked potentials (MEPs) of the paretic UL and somatosensory evoked potential (SSEP) of both ULs assessed M1 and S1 cortical excitability, respectively. The S1 ASY index was measured before and after intervention. Evaluator, participants and the statistician were blinded.

Results

Thirty-six participants divided equally into groups (nine participants per group). Seven patients were excluded from MEP analysis because of failure to produce consistent MEP. One participant was excluded in the SSEP analysis because no SSEP was detected. All somatosensory stimulation groups had decreased S1 ASY except for the sham rTMS/Sham SS group. When compared with baseline, M1 excitability increased only in the rTMS/SS group.

Conclusion

S1 rTMS and SS alone or in combination changed S1 excitability and decreased ASY, but it was only their combination that increased M1 excitability.

Clinical trial registration

clinicaltrials.gov, identifier (NCT03329807).

Anesthetic modulations dissociate neuroelectric characteristics between sensory-evoked and spontaneous activities across bilateral rat somatosensory cortical laminae

Spontaneous neural activity has been widely adopted to construct functional connectivity (FC) amongst distant brain regions. Although informative, the functional role and signaling mechanism of the resting state FC are not intuitive as those in stimulus/task-evoked activity. In order to bridge the gap, we investigated anesthetic modulation of both resting-state and sensory-evoked activities. We used two well-studied GABAergic anesthetics of varying dose (isoflurane: 0.5–2.0% and α-chloralose: 30 and 60 mg/kg∙h) and recorded changes in electrophysiology using a pair of laminar electrode arrays that encompass the entire depth of the bilateral somatosensory cortices (S1fl) in rats. Specifically, the study focused to describe how varying anesthesia conditions affect the resting state activities and resultant FC between bilateral hemispheres in comparison to those obtained by evoked responses. As results, isoflurane decreased the amplitude of evoked responses in a dose-dependent manner mostly due to the habituation of repetitive responses. However, α-chloralose rather intensified the amplitude without exhibiting habituation. No such diverging trend was observed for the spontaneous activity, in which both anesthetics increased the signal power. For α-chloralose, overall FC was similar to that obtained with the lowest dose of isoflurane at 0.5% while higher doses of isoflurane displayed increased FC. Interestingly, only α-chloralose elicited relatively much greater increases in the ipsi-stimulus evoked response (i.e., in S1fl ipsilateral to the stimulated forelimb) than those associated with the contra-stimulus response, suggesting enhanced neuronal excitability. Taken together, the findings demonstrate modulation of the FC profiles by anesthesia is highly non-linear, possibly with a distinct underlying mechanism that affects either resting state or evoked activities differently. Further, the current study warrants thorough investigation of the basal neuronal states prior to the interpretation of resting state FC and evoked activities for accurate understanding of neural signal processing and circuitry.

A resting-state network for novelty: Similar involvement of a global network under rest and task conditions

Neuroimaging research provides converging evidence in support of functional networks active under rest conditions. While these networks are typically locally-distributed, a globally-distributed resting-state network (gRSN) was recently identified. The gRSN component is characterized by a scalp topography similar to that of the widely-studied P3 component of the event related potential, thought to represent the brain's response to novelty. In this study, we investigate similarities between the neural generators underlying these two networks to test the hypothesis that the gRSN is a resting-state network for novelty. By using the second-order blind identification (SOBI) algorithm, which works with temporal information, we show that (1) a resting-state component resembling the topography of the P3 can be recovered in all participants; (2) this gRSN component can be modeled with a set of ECDs with high goodness of fit; (3) ECD locations of the gRSN correspond to a network of globally-distributed brain structures overlapping heavily with the networking underlying the P3; and, (4) structures underlying these two networks are similarly involved during task and resting-state conditions. We interpret this as evidence in support of a resting-state network for detection and response to novelty.

Functional Significance of Individual Differences in P3 Network Spatial Configuration

Abstract. The amplitude and latency of the P3 component in the electroencephalogram (EEG) event-related potentials (ERPs) are among the most extensively used markers for individual differences in normal and abnormal brain functions. In contrast, individual variations in spatial topography of the temporally-defined P3 component are relatively under-explored. Development in EEG-based source imaging opened up the possibility that individual-specific spatial configuration of the neural network underlying the temporally-defined P3 component bear a novel source of information for marking an individual difference in behavioral and cognitive function. In testing this hypothesis, a hybrid method consisting of blind source separation (BSS), equivalent current dipole (ECD) modeling, and hits-vector-based analysis was applied to continuous un-epoched EEG data collected from 13 healthy human participants performing a visual color oddball task. By analyzing the spatial configuration of the network underlying the temporally-defined P3 component, hereafter referred to as the P3N, we discovered that the contribution of each constituent structure within the P3N is not uniform. Instead, frontal lobe structures have significantly more involvement than other constituent structures, as quantitatively characterized by cross-individual reliability and a within-individual contribution to the P3N. A factor analysis of the hits vector data revealed that although P3 latency and amplitude did not show significant correlations with measures of the behavioral outcomes, scores of two factors derived from the hits vectors selectively predict behavioral reaction time and response correctness. These results support the hypothesis that variations in P3 spatial configuration reflect not merely noise but individual-specific features with functional significance.

Bilateral Representation of Sensorimotor Responses in Benign Adult Familial Myoclonus Epilepsy: An MEG Study

Patients with cortical reflex myoclonus manifest typical neurophysiologic characteristics due to primary sensorimotor cortex (S1/M1) hyperexcitability, namely, contralateral giant somatosensory-evoked potentials/fields and a C-reflex (CR) in the stimulated arm. Some patients show a CR in both arms in response to unilateral stimulation, with about 10-ms delay in the non-stimulated compared with the stimulated arm. This bilateral C-reflex (BCR) may reflect strong involvement of bilateral S1/M1. However, the significance and exact pathophysiology of BCR within 50 ms are yet to be established because it is difficult to identify a true ipsilateral response in the presence of the giant component in the contralateral hemisphere. We hypothesized that in patients with BCR, bilateral S1/M1 activity will be detected using MEG source localization and interhemispheric connectivity will be stronger than in healthy controls (HCs) between S1/M1 cortices. We recruited five patients with cortical reflex myoclonus with BCR and 15 HCs. All patients had benign adult familial myoclonus epilepsy. The median nerve was electrically stimulated unilaterally. Ipsilateral activity was investigated in functional regions of interest that were determined by the N20m response to contralateral stimulation. Functional connectivity was investigated using weighted phase-lag index (wPLI) in the time-frequency window of 30–50 ms and 30–100 Hz. Among seven of the 10 arms of the patients who showed BCR, the average onset-to-onset delay between the stimulated and the non-stimulated arm was 8.4 ms. Ipsilateral S1/M1 activity was prominent in patients. The average time difference between bilateral cortical activities was 9.4 ms. The average wPLI was significantly higher in the patients compared with HCs in specific cortico-cortical connections. These connections included precentral-precentral, postcentral-precentral, inferior parietal (IP)-precentral, and IP-postcentral cortices interhemispherically (contralateral region-ipsilateral region), and precentral-IP and postcentral-IP intrahemispherically (contralateral region-contralateral region). The ipsilateral response in patients with BCR may be a pathologically enhanced motor response homologous to the giant component, which was too weak to be reliably detected in HCs. Bilateral representation of sensorimotor responses is associated with disinhibition of the transcallosal inhibitory pathway within homologous motor cortices, which is mediated by the IP. IP may play a role in suppressing the inappropriate movements seen in cortical myoclonus.

Electrophysiological underpinnings of reward processing: Are we exploiting the full potential of EEG?

Understanding how the brain processes reward is an important and complex endeavor, which has involved the use of a range of complementary neuroimaging tools, including electroencephalography (EEG). EEG has been praised for its high temporal resolution but, because the signal recorded at the scalp is a mixture of brain activities, it is often considered to have poor spatial resolution. Besides, EEG data analysis has most often relied on event-related potentials (ERPs) which cancel out non-phase locked oscillatory activity, thus limiting the functional discriminative power of EEG attainable through spectral analyses. Because these three dimensions -temporal, spatial and spectral- have been unequally leveraged in reward studies, we argue that the full potential of EEG has not been exploited. To back up our claim, we first performed a systematic survey of EEG studies assessing reward processing. Specifically, we report on the nature of the cognitive processes investigated (i.e., reward anticipation or reward outcome processing) and the methods used to collect and process the EEG data (i.e., event-related potential, time-frequency or source analyses). A total of 359 studies involving healthy subjects and the delivery of monetary rewards were surveyed. We show that reward anticipation has been overlooked (88% of studies investigated reward outcome processing, while only 24% investigated reward anticipation), and that time-frequency and source analyses (respectively reported by 19% and 12% of the studies) have not been widely adopted by the field yet, with ERPs still being the dominant methodology (92% of the studies). We argue that this focus on feedback-related ERPs provides a biased perspective on reward processing, by ignoring reward anticipation processes as well as a large part of the information contained in the EEG signal. Finally, we illustrate with selected examples how addressing these issues could benefit the field, relying on approaches combining time-frequency analyses, blind source separation and source localization.

Three-Year Reliability of MEG Visual and Somatosensory Responses

A major goal of many translational neuroimaging studies is the identification of biomarkers of disease. However, a prerequisite for any such biomarker is robust reliability, which for magnetoencephalography (MEG) and many other imaging modalities has not been established. In this study, we examined the reliability of visual (Experiment 1) and somatosensory gating (Experiment 2) responses in 19 healthy adults who repeated these experiments for three visits spaced 18 months apart. Visual oscillatory and somatosensory oscillatory and evoked responses were imaged, and intraclass correlation coefficients (ICC) were computed to examine the long-term reliability of these responses. In Experiment 1, ICCs showed good reliability for visual theta and alpha responses in occipital cortices, but poor reliability for gamma responses. In Experiment 2, the time series of somatosensory gamma and evoked responses in the contralateral somatosensory cortex showed good reliability. Finally, analyses of spontaneous baseline activity indicated excellent reliability for occipital alpha, moderate reliability for occipital theta, and poor reliability for visual/somatosensory gamma activity. Overall, MEG responses to visual and somatosensory stimuli show a high degree of reliability across 3 years and therefore may be stable indicators of sensory processing long term and thereby of potential interest as biomarkers of disease.

Inhibitory control dysfunction in parkinsonian impulse control disorders

Impulse control disorders (ICDs) in Parkinson's disease have been associated with dysfunctions in the control of value- or reward-based responding (choice impulsivity) and abnormalities in mesocorticolimbic circuits. The hypothesis that dysfunctions in the control of response inhibition (action impulsivity) also play a role in Parkinson's disease ICDs has recently been raised, but the underlying neural mechanisms have not been probed directly. We used high-resolution EEG recordings from 41 patients with Parkinson's disease with and without ICDs to track the spectral and dynamical signatures of different mechanisms involved in inhibitory control in a simple visuomotor task involving no selection between competing responses and no reward to avoid potential confounds with reward-based decision. Behaviourally, patients with Parkinson's disease with ICDs proved to be more impulsive than those without ICDs. This was associated with decreased beta activity in the precuneus and in a region of the medial frontal cortex centred on the supplementary motor area. The underlying dynamical patterns pinpointed dysfunction of proactive inhibitory control, an executive mechanism intended to gate motor responses in anticipation of stimulation in uncertain contexts. The alteration of the cortical drive of proactive response inhibition in Parkinson's disease ICDs pinpoints the neglected role the precuneus might play in higher order executive functions in coordination with the supplementary motor area, specifically for switching between executive settings. Clinical perspectives are discussed in the light of the non-dopaminergic basis of this function.

Validation of SOBI-DANS method for automatic identification of horizontal and vertical eye movement components from EEG

Neurophysiological investigation of neural processes are hindered by the presence of large artifacts associated with eye movement. Although blind source separation (BSS)-based hybrid algorithms are useful for separating, identifying, and removing these artifacts from EEG, it remains unexamined to what extent neural signals can remain mixed with these artifact components, potentially resulting in unintended removal of critical neural signals. Here, we present a novel validation approach to quantitatively evaluate to what extent horizontal and vertical saccadic eye movement-related artifact components (H and V Comps) are indeed ocular in origin. To automate the identification of the H and V Comps recovered by the second-order blind identification (SOBI), we introduced a novel Discriminant ANd Similarity (DANS)-based method. Through source localization, we showed that over 95% of variance in the SOBI-DANS identified H and V Comps' scalp projections were ocular in origin. Through the analysis of saccade-related potentials (SRPs), we found that the H and V Comps' SRP amplitudes were finely modulated by eye movement direction and distance jointly. SOBI-DANS' component selection was in 100% agreement with human experts' selection and was 100% successful in component identification across all participants indicating a high cross-individual consistency or robustness. These results set the stage for future work to transform the to-be-thrown-away artifacts into signals indicative of gaze position, thereby providing readily co-registered eye movement and neural signal without using a separate eye tracker.

No Evidence for a Role of Spatially Modulated α-Band Activity in Tactile Remapping and Short-Latency, Overt Orienting Behavior

Oscillatory α-band activity is commonly associated with spatial attention and multisensory prioritization. It has also been suggested to reflect the automatic transformation of tactile stimuli from a skin-based, somatotopic reference frame into an external one. Previous research has not convincingly separated these two possible roles of α-band activity. Previous experimental paradigms have used artificially long delays between tactile stimuli and behavioral responses to aid relating oscillatory activity to these different events. However, this strategy potentially blurs the temporal relationship of α-band activity relative to behavioral indicators of tactile-spatial transformations. Here, we assessed α-band modulation with massive univariate deconvolution, an analysis approach that disentangles brain signals overlapping in time and space. Thirty-one male and female human participants performed a delay-free, visual search task in which saccade behavior was unrestricted. A tactile cue to uncrossed or crossed hands was either informative or uninformative about visual target location. α-Band suppression following tactile stimulation was lateralized relative to the stimulated hand over central-parietal electrodes but relative to its external location over parieto-occipital electrodes. α-Band suppression reflected external touch location only after informative cues, suggesting that posterior α-band lateralization does not index automatic tactile transformation. Moreover, α-band suppression occurred at the time of, or after, the production of the saccades guided by tactile stimulation. These findings challenge the idea that α-band activity is directly involved in tactile-spatial transformation and suggest instead that it reflects delayed, supramodal processes related to attentional reorienting.SIGNIFICANCE STATEMENT Localizing a touch in space requires integrating somatosensory information about skin location and proprioceptive or visual information about posture. The automatic remapping between skin-based tactile information to a location in external space has been proposed to rely on the modulation of oscillatory brain activity in the α-band range, across the multiple cortical areas that are involved in tactile, multisensory, and spatial processing. We report two findings that are inconsistent with this view. First, α-band activity reflected the remapped stimulus location only when touch was task relevant. Second, α-band modulation occurred too late to account for spatially directed behavioral responses and, thus, only after remapping must have taken place. These characteristics contradict the idea that α-band directly reflects automatic tactile remapping processes.

The Causal Role of the Prefrontal Cortex and Somatosensory Cortex in Tactile Working Memory

In the present study, we searched for causal evidence linking activity in the bilateral primary somatosensory cortex (SI), posterior parietal cortex (PPC), and prefrontal cortex (PFC) with behavioral performance in vibrotactile working memory. Participants performed a vibrotactile delayed matching-to-sample task, while single-pulse transcranial magnetic stimulation (sp-TMS) was applied over these cortical areas at 100, 200, 300, 600, 1600, and 1900 ms after the onset of vibrotactile stimulation (200 ms duration). In our experiments, sp-TMS over the contralateral SI at the early delay (100 and 200 ms) deteriorated the accuracy of task performance, and over the ipsilateral SI at the late delay (1600 and 1900 ms) also induced such deteriorating effects. Furthermore, deteriorating effects caused by sp-TMS over the contralateral DLPFC at the same maintenance stage (1600 ms) were correlated with the effects caused by sp-TMS over the ipsilateral SI, indicating that information retained in the ipsilateral SI during the late delay may be associated with the DLPFC. Taken together, these results suggest that both the contralateral and ipsilateral SIs are involved in tactile WM, and the contralateral DLPFC bridges the contralateral SI and ipsilateral SI for goal-directed action.

Study of the activation in sensorimotor cortex and topological properties of functional brain network following focal vibration on healthy subjects and subacute stroke patients: An EEG study

Modulation on cerebral cortex and cerebral networks can induce reorganization of the brain, which contributes to rehabilitation. Previous studies have proved that focal vibration (FV) on limb muscles can modulate the activities of sensorimotor cortex in healthy subjects (HS). The objective of this paper is to study the modulatory effects of FV on the sensorimotor cortex and cerebral network in HS and subacute stroke patients (SP). An experiment was designed and conducted, during which FV of 75 Hz was applied over biceps muscle of right limb of 10 HS and 10 SP with right hemiplegia. Electroencephalography (EEG) was recorded in the following phases: before FV, control condition and three sessions of FV. EEG analysis showed a significant decrease in motor-related power desynchronization (MRPD) of contralesional primary sensorimotor cortex (contralesional S1-M1) in the beta2 band (18-21 Hz) for SP during FV sessions, as well as in MRPD of bilateral S1-M1 in the beta1 (13-18 Hz) and the beta2 band for HS. Moreover, MRPD of contralesional S1-M1 was significantly lower than MRPD of ipsilesional S1-M1 during FV. Besides, a significant increase of global efficiency (E) and decrease of characteristic path length (L) were identified in the beta1 band for SP, whereas a significant increase of L was identified for HS. The results indicated that FV could enhance the excitability of contralesional S1-M1 and alter topological properties of functional brain network for SP, which was different in HS. This indication can contribute to understanding the modulatory effects of FV on cerebral cortex and cerebral network.

Resting state oscillations suggest a motor component of Parkinson's Impulse Control Disorders

OBJECTIVES

Impulse control disorders (ICDs) in Parkinson's disease (PD) have been associated with cognitive impulsivity and dopaminergic dysfunction and treatment. The present study tests the neglected hypothesis that the neurofunctional networks involved in motor impulsivity might also be dysfunctional in PD-ICDs.

METHODS

We performed blind spectral analyses of resting state electroencephalographic (EEG) data in PD patients with and without ICDs to probe the functional integrity of all cortical networks. Analyses were performed directly at the source level after blind source separation. Discrete differences between groups were tested by comparing patients with and without ICDs. Gradual dysfunctions were assessed by means of correlations between power changes and clinical scores reflecting ICD severity (QUIP score).

RESULTS

Spectral signatures of ICDs were found in the medial prefrontal cortex, the dorsal anterior cingulate and the supplementary motor area, in the beta and gamma bands. Beta power changes in the supplementary motor area were found to predict ICDs severity.

CONCLUSION

ICDs are associated with abnormal activity within frequency bands and cortical circuits supporting the control of motor response inhibition.

SIGNIFICANCE

These results bring to the forefront the need to consider, in addition to the classical interpretation based on aberrant mesocorticolimbic reward processing, the issue of motor impulsivity in PD-ICDs and its potential implications for PD therapy.

Blood Oxygen Level-Dependent Response Changes in the Ipsilateral Primary Somatosensory Cortex and Thalamus of Patients With Moyamoya Disease During Median Nerve Electrical Stimulation

OBJECTIVE

The objective of this study was to investigate the changes in the blood oxygen level-dependent (BOLD) response in the ipsilateral primary somatosensory cortex (SI) and thalamus of patients with moyamoya disease (MMD) during sensory stimulation.

METHODS

Sixty-four MMD patients, and 15 healthy volunteers were enrolled. Thirty-three MMD patients exhibited paroxysmal numbness or hypoesthesia in the unilateral limbs. Fifteen patients with acroparesthesia underwent unilateral encephaloduroarteriosynangiosis (EDAS). All volunteers underwent BOLD functional magnetic resonance imaging (BOLD-fMRI) under median nerve electrical stimulation (MNES). Blood oxygen level-dependent fMRI data were processed to obtain time-signal intensity curves in the activation areas of the bilateral SI and thalamus. Processed dynamic susceptibility contrast-enhanced magnetic resonance imaging data were used to measure the time to peak of the BOLD response in the regions of interest, including the bilateral SI, thalamus, and cerebellum. Changes in the time-signal intensity curve-related hemodynamic parameters in the ipsilateral SI and thalamus were examined between healthy controls, nonacroparesthesia patients, and asymptomatic and symptomatic sides of unilateral acroparesthesia patients during MNES. Changes in these parameters in MMD patients before and after EDAS were examined.

RESULTS

Compared with healthy volunteers, 3 groups of MMD patients exhibited an increased peak of the positive BOLD response in the ipsilateral thalamus during MNES (0.65 ± 0.24 vs 0.79 ± 0.35, 0.94 ± 0.57, and 0.89 ± 0.50; P = 0.0335). The positive response peak in the ipsilateral SI markedly increased in MMD patients with acroparesthesia during MNES on the asymptomatic side (0.56 ± 0.37 vs 0.38 ± 0.27, P = 0.0243). The time to peak negative response in the ipsilateral SI was prolonged during MNES on the symptomatic side after EDAS (12.14 ± 8.90 seconds vs 18.86 ± 9.20 seconds, P = 0.0201).

CONCLUSIONS

During sensory stimulation treatment, BOLD response changes occurred in the ipsilateral SI and thalamus of MMD patients. These changes enabled the contralateral hemisphere of the brain to better deal with sensory stimuli.

A systematic review investigating the relationship of electroencephalography and magnetoencephalography measurements with sensorimotor upper limb impairments after stroke

BACKGROUND

Predicting sensorimotor upper limb outcome receives continued attention in stroke. Neurophysiological measures by electroencephalography (EEG) and magnetoencephalography (MEG) could increase the accuracy of predicting sensorimotor upper limb recovery.

NEW METHOD

The aim of this systematic review was to summarize the current evidence for EEG/MEG-based measures to index neural activity after stroke and the relationship between abnormal neural activity and sensorimotor upper limb impairment. Relevant papers from databases EMBASE, CINHAL, MEDLINE and pubMED were identified. Methodological quality of selected studies was assessed with the Modified Downs and Black form. Data collected was reported descriptively.

RESULTS

Seventeen papers were included; 13 used EEG and 4 used MEG applications. Findings showed that: (a) the presence of somatosensory evoked potentials in the acute stage are related to better outcome of upper limb motor impairment from 10 weeks to 6 months post-stroke; (b) an interhemispheric imbalance of cortical oscillatory signals associated with upper limb impairment; and (c) predictive models including beta oscillatory cortical signal factors with corticospinal integrity and clinical measures could enhance upper limb motor prognosis.

COMPARING WITH EXISTING METHOD

The combination of neurological biomarkers with clinical measures results in higher statistical power than using neurological biomarkers alone when predicting motor recovery in stroke.

CONCLUSIONS

Alterations in neural activity by means of EEG and MEG are demonstrated from the early post-stroke stage onwards, and related to sensorimotor upper limb impairment. Future work exploring cortical oscillatory signals in the acute stage could provide further insight about prediction of upper limb sensorimotor recovery.

Functional Activation-Informed Structural Changes during Stroke Recovery: A Longitudinal MRI Study

Objective

Neuroimaging studies revealed the functional reorganization or the structural changes during stroke recovery. However, previous studies did not combine the functional and structural information and the results might be affected by heterogeneous lesion. This study aimed to investigate functional activation-informed structural changes during stroke recovery.

Methods

MRI data of twelve stroke patients were collected at four consecutive time points during the first 3 months after stroke onset. Functional activation during finger-tapping task was used to inform the analysis of structural changes of activated brain regions. Correlation between structural changes in motor-related activated brain regions and motor function recovery was estimated.

Results

The averaged gray matter volume (aGMV) of contralesional activated brain regions and laterality index of gray matter volume (LI_GMV) increased during stroke recovery, and LI_GMV was positively correlated with Fugl-Meyer Index (FMI) at initial stage after stroke. The aGMV of bilateral activated brain regions was negatively correlated with FMI during the stroke recovery.

Conclusion

This study demonstrated that combining the stroke-induced functional reorganization and structural change provided new insights into the underlying innate plasticity process during stroke recovery.

Significance

This study proposed a new approach to integrate functional and structural information for investigating the innate plasticity after stroke.

Spatiotemporal Dynamics of the Cortical Responses Induced by a Prolonged Tactile Stimulation of the Human Fingertips

The sense of touch is fundamental for daily behavior. The aim of this work is to understand the neural network responsible for touch processing during a prolonged tactile stimulation, delivered by means of a mechatronic platform by passively sliding a ridged surface under the subject's fingertip while recording the electroencephalogram (EEG). We then analyzed: (i) the temporal features of the Somatosensory Evoked Potentials and their topographical distribution bilaterally across the cortex; (ii) the associated temporal modulation of the EEG frequency bands. Long-latency SEP were identified with the following physiological sequence P100-N140-P240. P100 and N140 were bilateral potentials with higher amplitude in the contralateral hemisphere and with delayed latency in the ipsilateral side. Moreover, we found a late potential elicited around 200 ms after the stimulation was stopped, which likely encoded the end of tactile input. The analysis of cortical oscillations indicated an initial increase in the power of theta band (4-7 Hz) for 500 ms after the stimulus onset followed a decrease in the power of the alpha band (8-15 Hz) that lasted for the remainder of stimulation. This decrease was prominent in the somatosensory cortex and equally distributed in both contralateral and ipsilateral hemispheres. This study shows that prolonged stimulation of the human fingertip engages the cortex in widespread bilateral processing of tactile information, with different modulations of the theta and alpha bands across time.

The importance of the negative blood-oxygenation-level-dependent (BOLD) response in the somatosensory cortex

In recent years, multiple studies have shown task-induced negative blood-oxygenation-level-dependent responses (NBRs) in multiple brain regions in humans and animals. Converging evidence suggests that task-induced NBRs can be interpreted in terms of decreased neuronal activity. However, the vascular and metabolic dynamics and functional importance of the NBR are highly debated. Here, we review studies investigating the origin and functional importance of the NBR, with special attention to the somatosensory cortex.

Bilateral representations of touch in the primary somatosensory cortex

According to current textbook knowledge, the primary somatosensory cortex (SI) supports unilateral tactile representations, whereas structures beyond SI, in particular the secondary somatosensory cortex (SII), support bilateral tactile representations. However, dexterous and well-coordinated bimanual motor tasks require early integration of bilateral tactile information. Sequential processing, first of unilateral and subsequently of bilateral sensory information, might not be sufficient to accomplish these tasks. This view of sequential processing in the somatosensory system might therefore be questioned, at least for demanding bimanual tasks. Evidence from the last 15 years is forcing a revision of this textbook notion. Studies in animals and humans indicate that SI is more than a simple relay for unilateral sensory information and, together with SII, contributes to the integration of somatosensory inputs from both sides of the body. Here, we review a series of recent works from our own and other laboratories in favour of interactions between tactile stimuli on the two sides of the body at early stages of processing. We focus on tactile processing, although a similar logic may also apply to other aspects of somatosensation. We begin by describing the basic anatomy and physiology of interhemispheric transfer, drawing on neurophysiological studies in animals and behavioural studies in humans that showed tactile interactions between body sides, both in healthy and in brain-damaged individuals. Then we describe the neural substrates of bilateral interactions in somatosensation as revealed by neurophysiological work in animals and neuroimaging studies in humans (i.e., functional magnetic resonance imaging, magnetoencephalography, and transcranial magnetic stimulation). Finally, we conclude with considerations on the dilemma of how efficiently integrating bilateral sensory information at early processing stages can coexist with more lateralized representations of somatosensory input, in the context of motor control.

Tracking markers of response inhibition in electroencephalographic data: why should we and how can we go beyond the N2 component?

Response inhibition is a pivotal component of executive control, which is especially difficult to assess. Indeed, it is a substantial challenge to gauge brain-behavior relationships because this function is precisely intended to suppress overt measurable behaviors. A further complication is that no single neuroimaging method has been found that can disentangle the accurate time-course of concurrent excitatory and inhibitory mechanisms. Here, we argue that this objective can be achieved with electroencephalography (EEG) on some conditions. Based on a systematic review, we emphasize that the standard event-related potential N2 (N200) is not an appropriate marker of prepotent response inhibition. We provide guidelines for assessing the cortical brain dynamics of response inhibition with EEG. This includes the combined use of inseparable data processing steps (source separation, source localization, and single-trial and time-frequency analyses) as well as the amendment of the classical experimental designs to enable the recording of different kinds of electrophysiological activity predicted by different models of response inhibition. We conclude with an illustration based on recent findings of how fruitful this approach can be.

Magnetoencephalographic study of hand and foot sensorimotor organization in 325 consecutive patients evaluated for tumor or epilepsy surgery

Objectives

The presence of intracranial lesions or epilepsy may lead to functional reorganization and hemispheric lateralization. We applied a clinical magnetoencephalography (MEG) protocol for the localization of the contralateral and ipsilateral S1 and M1 of the foot and hand in patients with non-lesional epilepsy, stroke, developmental brain injury, traumatic brain injury and brain tumors. We investigated whether differences in activation patterns could be related to underlying pathology.

Methods

Using dipole fitting, we localized the sources underlying sensory and motor evoked magnetic fields (SEFs and MEFs) of both hands and feet following unilateral stimulation of the median nerve (MN) and posterior tibial nerve (PTN) in 325 consecutive patients. The primary motor cortex was localized using beamforming following a self-paced repetitive motor task for each hand and foot.

Results

The success rate for motor and sensory localization for the feet was significantly lower than for the hands (motor_hand 94.6% versus motor_feet 81.8%, p < 0.001; sensory_hand 95.3% versus sensory_feet 76.0%, p < 0.001). MN and PTN stimulation activated 86.6% in the contralateral S1, with ipsilateral activation < 0.5%. Motor cortex activation localized contralaterally in 76.1% (5.2% ipsilateral, 7.6% bilateral and 11.1% failures) of all motor MEG recordings. The ipsilateral motor responses were found in 43 (14%) out of 308 patients with motor recordings (range: 8.3–50%, depending on the underlying pathology), and had a higher occurrence in the foot than in the hand (motor_foot 44.8% versus motor_hand 29.6%, p = 0.031). Ipsilateral motor responses tended to be more frequent in patients with a history of stroke, traumatic brain injury (TBI) or developmental brain lesions (p = 0.063).

Conclusions

MEG localization of sensorimotor cortex activation was more successful for the hand compared to the foot. In patients with neural lesions, there were signs of brain reorganization as measured by more frequent ipsilateral motor cortical activation of the foot in addition to the traditional sensory and motor activation patterns in the contralateral hemisphere. The presence of ipsilateral neural reorganization, especially around the foot motor area, suggests that careful mapping of the hand and foot in both contralateral and ipsilateral hemispheres prior to surgery might minimize postoperative deficits.

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Using MEG, S1 and M1 responses of the hand and foot were mapped in patients with brain tumors or epilepsy.

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Localization of the hand was more successful than of the foot.

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Ipsilateral S1 responses were rarely seen but ipsilateral M1 responses differed by underlying pathology and limb.

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Results indicate that differential sensorimotor re-organization can occur in the presence of pathology.

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Ipsilateral and contralateral mapping of the hand and foot should be done to minimize postsurgical dysfunction.

Differences in Early Stages of Tactile ERP Temporal Sequence (P100) in Cortical Organization during Passive Tactile Stimulation in Children with Blindness and Controls

Compared to their seeing counterparts, people with blindness have a greater tactile capacity. Differences in the physiology of object recognition between people with blindness and seeing people have been well documented, but not when tactile stimuli require semantic processing. We used a passive vibrotactile device to focus on the differences in spatial brain processing evaluated with event related potentials (ERP) in children with blindness (n = 12) vs. normally seeing children (n = 12), when learning a simple spatial task (lines with different orientations) or a task involving recognition of letters, to describe the early stages of its temporal sequence (from 80 to 220 msec) and to search for evidence of multi-modal cortical organization. We analysed the P100 of the ERP. Children with blindness showed earlier latencies for cognitive (perceptual) event related potentials, shorter reaction times, and (paradoxically) worse ability to identify the spatial direction of the stimulus. On the other hand, they are equally proficient in recognizing stimuli with semantic content (letters). The last observation is consistent with the role of P100 on somatosensory-based recognition of complex forms. The cortical differences between seeing control and blind groups, during spatial tactile discrimination, are associated with activation in visual pathway (occipital) and task-related association (temporal and frontal) areas. The present results show that early processing of tactile stimulation conveying cross modal information differs in children with blindness or with normal vision.

Effects of Increasing Neuromuscular Electrical Stimulation Current Intensity on Cortical Sensorimotor Network Activation: A Time Domain fNIRS Study

Neuroimaging studies have shown neuromuscular electrical stimulation (NMES)-evoked movements activate regions of the cortical sensorimotor network, including the primary sensorimotor cortex (SMC), premotor cortex (PMC), supplementary motor area (SMA), and secondary somatosensory area (S2), as well as regions of the prefrontal cortex (PFC) known to be involved in pain processing. The aim of this study, on nine healthy subjects, was to compare the cortical network activation profile and pain ratings during NMES of the right forearm wrist extensor muscles at increasing current intensities up to and slightly over the individual maximal tolerated intensity (MTI), and with reference to voluntary (VOL) wrist extension movements. By exploiting the capability of the multi-channel time domain functional near-infrared spectroscopy technique to relate depth information to the photon time-of-flight, the cortical and superficial oxygenated (O₂Hb) and deoxygenated (HHb) hemoglobin concentrations were estimated. The O₂Hb and HHb maps obtained using the General Linear Model (NIRS-SPM) analysis method, showed that the VOL and NMES-evoked movements significantly increased activation (i.e., increase in O₂Hb and corresponding decrease in HHb) in the cortical layer of the contralateral sensorimotor network (SMC, PMC/SMA, and S2). However, the level and area of contralateral sensorimotor network (including PFC) activation was significantly greater for NMES than VOL. Furthermore, there was greater bilateral sensorimotor network activation with the high NMES current intensities which corresponded with increased pain ratings. In conclusion, our findings suggest that greater bilateral sensorimotor network activation profile with high NMES current intensities could be in part attributable to increased attentional/pain processing and to increased bilateral sensorimotor integration in these cortical regions.

Granger causal time-dependent source connectivity in the somatosensory network

Exploration of transient Granger causal interactions in neural sources of electrophysiological activities provides deeper insights into brain information processing mechanisms. However, the underlying neural patterns are confounded by time-dependent dynamics, non-stationarity and observational noise contamination. Here we investigate transient Granger causal interactions using source time-series of somatosensory evoked magnetoencephalographic (MEG) elicited by air puff stimulation of right index finger and recorded using 306-channel MEG from 21 healthy subjects. A new time-varying connectivity approach, combining renormalised partial directed coherence with state space modelling, is employed to estimate fast changing information flow among the sources. Source analysis confirmed that somatosensory evoked MEG was mainly generated from the contralateral primary somatosensory cortex (SI) and bilateral secondary somatosensory cortices (SII). Transient Granger causality shows a serial processing of somatosensory information, 1) from contralateral SI to contralateral SII, 2) from contralateral SI to ipsilateral SII, 3) from contralateral SII to contralateral SI, and 4) from contralateral SII to ipsilateral SII. These results are consistent with established anatomical connectivity between somatosensory regions and previous source modeling results, thereby providing empirical validation of the time-varying connectivity analysis. We argue that the suggested approach provides novel information regarding transient cortical dynamic connectivity, which previous approaches could not assess.

Invariance in current dipole moment density across brain structures and species: physiological constraint for neuroimaging

Although anatomical constraints have been shown to be effective for MEG and EEG inverse solutions, there are still no effective physiological constraints. Strength of the current generator is normally described by the moment of an equivalent current dipole Q. This value is quite variable since it depends on size of active tissue. In contrast, the current dipole moment density q, defined as Q per surface area of active cortex, is independent of size of active tissue. Here we studied whether the value of q has a maximum in physiological conditions across brain structures and species. We determined the value due to the primary neuronal current (q primary) alone, correcting for distortions due to measurement conditions and secondary current sources at boundaries separating regions of differing electrical conductivities. The values were in the same range for turtle cerebellum (0.56-1.48 nAm/mm(2)), guinea pig hippocampus (0.30-1.34 nAm/mm(2)), and swine neocortex (0.18-1.63 nAm/mm(2)), rat neocortex (~2.2 nAm/mm(2)), monkey neocortex (~0.40 nAm/mm(2)) and human neocortex (0.16-0.77 nAm/mm(2)). Thus, there appears to be a maximum value across the brain structures and species (1-2 nAm/mm(2)). The empirical values closely matched the theoretical values obtained with our independently validated neural network model (1.6-2.8 nAm/mm(2) for initial spike and 0.7-3.1 nAm/mm(2) for burst), indicating that the apparent invariance is not coincidental. Our model study shows that a single maximum value may exist across a wide range of brain structures and species, varying in neuron density, due to fundamental electrical properties of neurons. The maximum value of q primary may serve as an effective physiological constraint for MEG/EEG inverse solutions.

Early integration of bilateral touch in the primary somatosensory cortex

Animal, as well as behavioural and neuroimaging studies in humans have documented integration of bilateral tactile information at the level of primary somatosensory cortex (SI). However, it is still debated whether integration in SI occurs early or late during tactile processing, and whether it is somatotopically organized. To address both the spatial and temporal aspects of bilateral tactile processing we used magnetoencephalography in a tactile repetition-suppression paradigm. We examined somatosensory evoked-responses produced by probe stimuli preceded by an adaptor, as a function of the relative position of adaptor and probe (probe always at the left index finger; adaptor at the index or middle finger of the left or right hand) and as a function of the delay between adaptor and probe (0, 25, or 125 ms). Percentage of response-amplitude suppression was computed by comparing paired (adaptor + probe) with single stimulations of adaptor and probe. Results show that response suppression varies differentially in SI and SII as a function of both spatial and temporal features of the stimuli. Remarkably, repetition suppression of SI activity emerged early in time, regardless of whether the adaptor stimulus was presented on the same and the opposite body side with respect to the probe. These novel findings support the notion of an early and somatotopically organized inter-hemispheric integration of tactile information in SI.

EMG activity of finger flexor muscles and grip force following low-dose transcutaneous electrical nerve stimulation in healthy adult subjects

Somatosensory stimulation modulates cortical and corticospinal excitability and consequently affects motor output. Therefore, low-amplitude transcutaneous electrical nerve stimulation (TENS) has the potential to elicit favorable motor responses. The purpose of the two presented pilot studies was to shed light on TENS parameters that are relevant for the enhancement of two desirable motor outcomes, namely, electromyographic (EMG) activity and contraction strength of the finger flexors and wrist muscles. In 5 and 10 healthy young adults (in Study I and Study II, respectively) TENS was delivered to the volar aspect of the forearm. We manipulated TENS frequency (150 Hz vs. 5 Hz), length of application (10, 20, and 60 min), and side of application (unilateral, right forearm vs. bilateral forearms). EMG amplitude and grip force were measured before (Pre), immediately after (Post), and following 15 min of no stimulation (Study I only). The results indicated that low-frequency bursts of TENS applied to the skin overlying the finger flexor muscles enhance the EMG activity of the finger flexors and grip force. The increase in EMG activity of the flexor muscles was observed after 20 min of stimulation, while grip force was increased only after 1 h. The effects of uni- and bilateral TENS were comparable. These observations allude to a modulatory effect of TENS on the tested motor responses; however, unequivocal conclusions of the findings are hampered by individual differences that affect motor outcomes, such as in level of attention.

Neural correlates of somatosensory paired-pulse suppression: a MEG study using distributed source modeling and dynamic spectral power analysis

Paired-pulse stimulation has been used previously to evaluate cortical excitability and sensory gating. To help elucidate the neural network involved in paired-pulse suppression of somatosensory cortical processing, magnetoencephalographic (MEG) responses to paired-pulse electrical stimulation of the left median nerve of the wrists of 13 healthy males were recorded using an intra-pair interstimulus interval (ISI) of 500ms and an inter-pair ISI of 8s. Minimum norm estimates showed the presence of cortical activation in the bilateral primary somatosensory cortex, the post-central sulcus and the supplementary motor areas. Compared with the responses to the first stimulation, the responses to the second stimulation were attenuated in these areas with gating ratios (the amplitude ratios of the second response to the first response) of 0.54-0.69. By spectral power dynamic analysis, beta frequency oscillations were found to be associated with an early-latency (30-36ms) gating process in the contralateral primary somatosensory cortex and post-central sulcus, whereas theta and alpha oscillations were correlated with paired-pulse suppression of activations at 98-136ms in the ipsilateral primary somatosensory cortex, the bilateral post-central sulcus and the supplementary motor areas. In summary, it can be concluded that differential oscillatory activities are involved in the pair-pulse suppression in various somatosensory regions in response to repetitive external stimulations.

The cortical and sub-cortical network of sensory evoked response in healthy subjects

The aim of this study was to find the cortical and sub-cortical network responsible for the sensory evoked coherence in healthy subjects during electrical stimulation of right median nerve at wrist. The multitaper method was used to estimate the power and coherence spectrum followed by the source analysis method dynamic imaging of coherent sources (DICS) to find the highest coherent source for the basic frequency 3 Hz and the complete cortical and sub-cortical network responsible for the sensory evoked coherence in healthy subjects. The highest coherent source for the basic frequency was in the posterior parietal cortex for all the subjects. The cortical and sub-cortical network comprised of the primary sensory motor cortex (SI), secondary sensory motor cortex (SII), frontal cortex and medial pulvinar nucleus in the thalamus. The cortical and sub-cortical network responsible for the sensory evoked coherence was found successfully with a 64-channel EEG system. The sensory evoked coherence is involved with a thalamo-cortical network in healthy subjects.

Greater robustness of second order statistics than higher order statistics algorithms to distortions of the mixing matrix in blind source separation of human EEG: implications for single-subject and group analyses

A mandatory assumption in blind source separation (BSS) of the human electroencephalogram (EEG) is that the mixing matrix remains invariant, i.e., that the sources, electrodes and geometry of the head do not change during the experiment. Actually, this is not often the case. For instance, it is common that some electrodes slightly move during EEG recording. This issue is even more critical for group independent component analysis (gICA), a method of growing interest, in which only one mixing matrix is estimated for several subjects. Indeed, because of interindividual anatomo-functional variability, this method violates the mandatory principle of invariance. Here, using simulated (experiments 1 and 2) and real (experiment 3) EEG data, we test how eleven current BSS algorithms undergo distortions of the mixing matrix. We show that this usual kind of perturbation creates non-Gaussian features that are virtually added to all sources, impairing the estimation of real higher order statistics (HOS) features of the actual sources by HOS algorithms (e.g., Ext-INFOMAX, FASTICA). HOS-based methods are likely to identify more components (with similar properties) than actual neurological sources, a problem frequently encountered by BSS users. In practice, the quality of the recovered signal and the efficiency of subsequent source localization are substantially impaired. Performing dimensionality reduction before applying HOS-based BSS does not seem to be a safe strategy to circumvent the problem. Second order statistics (SOS)-based BSS methods belonging to the less popular SOBI family class are much less sensitive to this bias.

Spatiotemporal dynamics of bimanual integration in human somatosensory cortex and their relevance to bimanual object manipulation

Little is known about the spatiotemporal dynamics of cortical responses that integrate slightly asynchronous somatosensory inputs from both hands. This study aimed to clarify the timing and magnitude of interhemispheric interactions during early integration of bimanual somatosensory information in different somatosensory regions and their relevance for bimanual object manipulation and exploration. Using multi-fiber probabilistic diffusion tractography and MEG source analysis of conditioning-test (C-T) median nerve somatosensory evoked fields in healthy human subjects, we sought to extract measures of structural and effective callosal connectivity between different somatosensory cortical regions and correlated them with bimanual tactile task performance. Neuromagnetic responses were found in major somatosensory regions, i.e., primary somatosensory cortex SI, secondary somatosensory cortex SII, posterior parietal cortex, and premotor cortex. Contralateral to the test stimulus, SII activity was maximally suppressed by 51% at C-T intervals of 40 and 60 ms. This interhemispheric inhibition of the contralateral SII source activity correlated directly and topographically specifically with the fractional anisotropy of callosal fibers interconnecting SII. Thus, the putative pathway that mediated inhibitory interhemispheric interactions in SII was a transcallosal route from ipsilateral to contralateral SII. Moreover, interhemispheric inhibition of SII source activity correlated directly with bimanual tactile task performance. These findings were exclusive to SII. Our data suggest that early interhemispheric somatosensory integration primarily occurs in SII, is mediated by callosal fibers that interconnect homologous SII areas, and has behavioral importance for bimanual object manipulation and exploration.

The postcentral gyrus shows sustained fMRI activation during the tactile motion aftereffect

The tactile motion aftereffect (tMAE) is a perceptual illusion in which a stationary stimulus feels as though it is moving when presented following adaptation to a unidirectionally moving tactile stimulus. Using functional magnetic resonance imaging (fMRI), we localized the brain areas responsive to tactile motion and then investigated whether these areas underlie the tMAE. Tactile stimulation was delivered to the glabrous surface of the right hand by means of a plastic cylinder with a square-wave patterned surface. In the tactile motion localizer, we contrasted periods in which the cylinder rotated at 15 rpm with periods of rest (stationary contact). Activation was observed in the contralateral (left) thalamus, postcentral gyrus, and parietal operculum. In the tMAE experiment, the cylinder rotated at 15 or 60 rpm for 2 min. The 60-rpm speed induced reliable tMAEs, whereas the 15-rpm speed did not. Of the areas activated by the tactile motion localizer, only the postcentral gyrus showed a sustained fMRI response following the offset of 60-rpm (but not 15-rpm) stimulation, presumably reflecting the illusory perception of motion.

Time jitter of somatosensory evoked potentials in recovery from hypoxic-ischemic brain injury

Impaired neural conductivity shown by delayed latency and reduced amplitude of characteristic peaks in somatosensory evoked potentials (SSEPs), has been used to monitor hypoxic-ischemic brain injury after cardiac arrest (CA). However, rather than characteristic peak deferral and suppression, the time jitter of the peak in SSEP related with time-variant neurological abnormalities is diminished by the commonly used ensemble average method. This paper utilizes the second order blind identification (SOBI) technique to extract characteristic peak information from one trial of SSEPs. Sixteen male Wistar rats were subjected to 7 or 9 min of asphyxial CA (n=8 per group). The SSEPs from median nerve stimulation were recorded for 4h after CA and then for 15 min periods at 24, 48 and 72 h. Neurological outcomes were evaluated by neurologic deficit score (NDS) at 72 h post-CA. The SSEP signal was analyzed offline with SOBI processing in Matlab. The N10 feature of SSEP was compared between good (NDS≥50) and bad (NDS<50) outcomes. After processed by SOBI, the N10 detection rate was significantly increased (p<0.001) from 90 min post-CA. Statistical difference of the latency variance of the N10 between good and bad outcome groups existed at 24, 48 and 72 h post-CA (p≤0.001). Our study is the first application using SOBI detecting variance in neural signals like SSEP. N10 latency variance, related with neurophysiological dysfunction, increased after hypoxic-ischemic injury. The SOBI technique is an efficient method in the identification of peak detection and offers a favorable alternative to reveal the neural transmission variation.

Spatiotemporal properties of neuron response suppression in owl monkey primary somatosensory cortex when stimuli are presented to both hands

Despite the lack of ipsilateral receptive fields (RFs) for neurons in the hand representation of area 3b of primary somatosensory cortex, interhemispheric interactions have been reported to varying degrees. We investigated spatiotemporal properties of these interactions to determine the following: response types, timing between stimuli to evoke the strongest bimanual interactions, topographical distribution of effects, and their dependence on similarity of stimulus locations on the two hands. We analyzed response magnitudes and latencies of single neurons and multineuron clusters recorded from 100-electrode arrays implanted in one hemisphere of each of two anesthetized owl monkeys. Skin indentations were delivered to the two hands simultaneously and asynchronously at mirror locations (matched sites on each hand) and nonmirror locations. Since multiple neurons were recorded simultaneously, stimuli on the contralateral hand could be within or outside of the classical RFs of any given neuron. For most neurons, stimulation on the ipsilateral hand suppressed responses to stimuli on the contralateral hand. Maximum suppression occurred when the ipsilateral stimulus was presented 100 ms before the contralateral stimulus onset (p < 0.0005). The longest stimulus onset delay tested (500 ms) allowed contralateral responses to recover to control levels (p = 0.428). Stimulation on mirror digits did not differ from stimulation on nonmirror locations (p = 1.000). These results indicate that interhemispheric interactions are common in area 3b, somewhat topographically diffuse, and maximal when the suppressing ipsilateral stimulus precedes the contralateral stimulus. Our findings point to a neurophysiological basis for "interference" effects found in human psychophysical studies of bimanual stimulation.

Functional deactivations: multiple ipsilateral brain areas engaged in the processing of somatosensory information

Somatosensory signals modulate activity throughout a widespread network in both of the brain hemispheres: the contralateral as well as the ipsilateral side of the brain relative to the stimulated limb. To analyze the ipsilateral somatosensory brain areas that are engaged during limb stimulation, we performed functional magnetic resonance imaging (fMRI) in 12 healthy subjects during electrical median nerve stimulation using both a block- and an event-related fMRI design. Data were analyzed through the use of model-dependent (SPM) and model-independent (ICA) approaches. Beyond the well-known positive blood oxygenation level-dependent (BOLD) responses, negative deflections of the BOLD response were found consistently in several ipsilateral brain areas, including the primary somatosensory cortex, the supplementary motor area, the insula, the dorsal part of the posterior cingulate cortex, and the contralateral cerebellum. Compared to their positive counterparts, the negative hemodynamic responses showed a different time course, with an onset time delay of 2.4 s and a peak delay of 0.7 s. This characteristic delay was observed in all investigated areas and verified by a second (purely tactile) event-related paradigm, suggesting a systematic difference for brain areas involved in the processing of somatosensory information. These findings may indicate that the physiological basis of these deactivations differs from that of the positive BOLD responses. Therefore, an altered model for the negative BOLD response may be beneficial to further model-dependent fMRI analyses.

Assessment of brain responses to innocuous and noxious electrical forepaw stimulation in mice using BOLD fMRI

Functional magnetic resonance imaging (fMRI) using the blood oxygen level-dependent (BOLD) contrast was used to study sensory processing in the brain of isoflurane-anesthetized mice. The use of a cryogenic surface coil in a small animal 9.4T system provided the sensitivity required for detection and quantitative analysis of hemodynamic changes caused by neural activity in the mouse brain in response to electrical forepaw stimulation at different amplitudes. A gradient echo-echo planar imaging (GE-EPI) sequence was used to acquire five coronal brain slices of 0.5mm thickness. BOLD signal changes were observed in primary and secondary somatosensory cortices, the thalamus and the insular cortex, important regions involved in sensory and nociceptive processing. Activation was observed consistently bilateral despite unilateral stimulation of the forepaw. The temporal BOLD profile was segregated into two signal components with different temporal characteristics. The maximum BOLD amplitude of both signal components correlated strongly with the stimulation amplitude. Analysis of the dynamic behavior of the somatosensory 'fast' BOLD component revealed a decreasing signal decay rate constant k(off) with increasing maximum BOLD amplitude (and stimulation amplitude). This study demonstrates the feasibility of a robust BOLD fMRI protocol to study nociceptive processing in isoflurane-anesthetized mice. The reliability of the method allows for detailed analysis of the temporal BOLD profile and for investigation of somatosensory and noxious signal processing in the brain, which is attractive for characterizing genetically engineered mouse models.

Age- and time-dependent effects on functional outcome and cortical activation pattern in patients with median nerve injury: a functional magnetic resonance imaging study

Object

The authors conducted a study to determine age- and time-dependent effects on the functional outcome after median nerve injury and repair and how such effects are related to changes in the pattern of cortical activation in response to tactile stimulation of the injured hand.

Methods

The authors studied 11 patients with complete unilateral median nerve injury at the wrist repaired with epineural suture. In addition, 8 patients who were reported on in a previous study were included in the statistical analysis. In the entire study cohort, the mean age at injury was 23.3 ± 13.4 years (range 7–57 years) and the time after injury ranged from 1 to 11 years. Sensory perception was measured with the static 2-point discrimination test and monofilaments. Functional MR imaging was conducted during tactile stimulation (brush strokes) of Digits II–III and IV–V of both hands, respectively.

Results

Tactile sensation was diminished in the median territory in all patients. The strongest predictor of 2-point discrimination was age at injury (p < 0.0048), and when this was accounted for in the regression analysis, the other age- and time-dependent predictors had no effect. The activation ratios (injured/healthy hand) for Digit II–III and Digit IV–V stimulation were positively correlated (rho 0.59, p < 0.011). The activation ratio for Digit II–III stimulation correlated weakly with time after injury (p < 0.041). The activation ratio of Digits IV–V correlated weakly with both age at injury (p < 0.048) and time after injury (p < 0.033), but no predictor reached significance in the regression model. The mean ratio of ipsi- and contralateral hemisphere activation after stimulation of the injured hand was 0.55, which was not significantly different from the corresponding ratio of the healthy hand (0.66).

Conclusions

Following a median nerve injury (1–11 years after injury) there may be an initial increase in the volume of the cortical representation, which subsequently declines during the restoration phase. These dynamic changes may involve both median and ulnar nerve cortical representation, because both showed negative correlation with time after injury. These findings are in agreement with animal studies showing that cortical plasticity is an important mechanism for functional recovery after peripheral nerve injury and repair.

Effects of feedback from active and passive body parts on spatial and temporal parameters in sensorimotor synchronization

Previous research on sensorimotor synchronization has manipulated the somatosensory information received from the tapping finger to investigate how feedback from an active effector affects temporal coordination. The current study explored the role of feedback from passive body parts in the regulation of spatiotemporal motor control parameters by employing a task that required finger tapping on one's own skin at anatomical locations of varying tactile sensitivity. A motion capture system recorded participants' movements as they synchronized with an auditory pacing signal by tapping with the right index finger on either their left index fingertip (Finger/Finger) or forearm (Finger/Forearm). Results indicated that tap timing was more variable, and movement amplitude was larger and more variable, when tapping on the finger than when tapping on the less sensitive forearm. Finger/Finger tapping may be impaired relative to Finger/Forearm tapping due to ambiguity arising through overlap in neural activity associated with tactile feedback from the active and the passive limb in the former. To compensate, the control system may strengthen the assignment of tap-related feedback to the active finger by generating correlated noise in movement kinematics and tap dynamics.

Diffuse optical tomography activation in the somatosensory cortex: specific activation by painful vs. non-painful thermal stimuli

BACKGROUND

Pain is difficult to assess due to the subjective nature of self-reporting. The lack of objective measures of pain has hampered the development of new treatments as well as the evaluation of current ones. Functional MRI studies of pain have begun to delineate potential brain response signatures that could be used as objective read-outs of pain. Using Diffuse Optical Tomography (DOT), we have shown in the past a distinct DOT signal over the somatosensory cortex to a noxious heat stimulus that could be distinguished from the signal elicited by innocuous mechanical stimuli. Here we further our findings by studying the response to thermal innocuous and noxious stimuli.

METHODOLOGY/PRINCIPAL FINDINGS

Innocuous and noxious thermal stimuli were applied to the skin of the face of the first division (ophthalmic) of the trigeminal nerve in healthy volunteers (N = 6). Stimuli temperatures were adjusted for each subject to evoke warm (equivalent to a 3/10) and painful hot (7/10) sensations in a verbal rating scale (0/10 = no/max pain). A set of 26 stimuli (5 sec each) was applied for each temperature with inter-stimulus intervals varied between 8 and 15 sec using a Peltier thermode. A DOT system was used to capture cortical responses on both sides of the head over the primary somatosensory cortical region (S1). For the innocuous stimuli, group results indicated mainly activation on the contralateral side with a weak ipsilateral response. For the noxious stimuli, bilateral activation was observed with comparable amplitudes on both sides. Furthermore, noxious stimuli produced a temporal biphasic response while innocuous stimuli produced a monophasic response.

CONCLUSIONS/SIGNIFICANCE

These results are in accordance with fMRI and our other DOT studies of innocuous mechanical and noxious heat stimuli. The data indicate the differentiation of DOT cortical responses for pain vs. innocuous stimuli that may be useful in assessing objectively acute pain.

Dynamical activities of primary somatosensory cortices studied by magnetoencephalography

A blind identification method of transfer functions in feedback systems is introduced for examination of dynamical activities of cortices by magnetoencephalography study. Somatosensory activities are examined in 5 Hz periodical median nerve stimulus. In the present paper, we will try two careful preprocessing procedures for the identification method to obtain impulse responses between primary somatosensory cortices. Time series data of the somatosensory evoked field are obtained by using a blind source separation of the T/k type (fractional) decorrelation method. Time series data of current dipoles of primary somatosensory cortices are transformed from the time series data of the somatosensory evoked field by the inverse problem. Fluctuations of current dipoles of them are obtained after elimination of deterministic periodical evoked waveforms. An identification method based on feedback system theory is used for estimation of transfer functions in a feedback model from obtained fluctuations of currents dipoles of primary somatosensory cortices. Dynamical activities between them are presented by Bode diagrams of transfer functions and their impulse responses: the time delay of about 30 ms via corpus callosum is found in the impulse response of identified transfer function.

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.

Cortical Activity in Relation to Velocity Dependent Movement Resistance in the Flexor Muscles of the Hand After Stroke

Background. The role of spinal networks in spasticity is well investigated, but little is known about possible cortical contributions to hypertonicity across a joint. Objective. The authors hypothesized that there are cortical activation correlates to spasticity in stroke patients with increased muscle tone of the wrist flexors. Methods. Stroke patients and controls were scanned using event-related functional magnetic resonance imaging (fMRI) during slow and fast passive movements of the hand with simultaneous recording of passive movement resistance (PMR). Results. Control participants had velocity-dependent activity (greater for slow than fast movements) of 2 types, in areas that were also more active in passive movement than rest (eg, relative increase in activation in contralateral S1 and M1 was greater for slow than fast) and in areas that were also more active in rest than passive movement (eg, relative decrease in activation in occipital areas and ipsilateral precentral gyrus was greater for fast than slow). In the patient group, with large interindividual variation of spasticity, we found an association between PMR and the velocity-dependent activity in ipsilateral S1 (area 3b) extending into M1 (area 4a), contralateral cingulate cortex, supplementary motor area (SMA), Brodmann Area 45 (BA 45), and cerebellum. Post hoc testing also revealed a similar correlation in S1 and M1 bilaterally in controls and showed that patients activated ipsilateral S1 and M1 more than controls in the velocity-dependent condition. Conclusions. The findings suggest the possibility of ipsilateral sensory and motor cortical involvement in spasticity after stroke, which warrant further investigation.

Cortical and subcortical correlates of functional electrical stimulation of wrist extensor and flexor muscles revealed by fMRI

The main scope of this study was to test the feasibility and reliability of FES in a MR-environment. Functional Electrical Stimulation (FES) is used in the rehabilitation therapy of patients after stroke or spinal cord injury to improve their motor abilities. Its principle lies in applying repeated electrical stimulation to the relevant nerves or muscles for eliciting either isometric or concentric contractions of the treated muscles. In this study we report cerebral activation patterns in healthy subjects undergoing fMRI during FES stimulation. We stimulated the wrist extensor and flexor muscles in an alternating pattern while BOLD-fMRI was recorded. We used both block and event-related designs to demonstrate their feasibility for recording FES activation in the same cortical and subcortical areas. Six out of fifteen subjects repeated the experiment three times within the same session to control intraindividual variance. In both block and event-related design, the analysis revealed an activation pattern comprising the contralateral primary motor cortex, primary somatosensory cortex and premotor cortex; the ipsilateral cerebellum; bilateral secondary somatosensory cortex, the supplementary motor area and anterior cingulate cortex. Within the same subjects we observed a consistent replication of the activation pattern shown in overlapping regions centered on the peak of activation. Similar time course within these regions were demonstrated in the event-related design. Thus, both techniques demonstrate reliable activation of the sensorimotor network and eventually can be used for assessing plastic changes associated with FES rehabilitation treatment.