Age-Related Reduced Somatosensory Gating Is Associated with Altered Alpha Frequency Desynchronization

Beta resting-state functional connectivity predicts tactile spatial acuity

Tactile perception is a complex phenomenon that is processed by multiple cortical regions via the primary somatosensory cortex (S1). Although somatosensory gating in the S1 using paired-pulse stimulation can predict tactile performance, the functional relevance of cortico-cortical connections to tactile perception remains unclear. We investigated the mechanisms by which corticocortical and local networks predict tactile spatial acuity in 42 adults using magnetoencephalography (MEG). Resting-state MEG was recorded with the eyes open, whereas evoked responses were assessed using single- and paired-pulse electrical stimulation. Source data were used to estimate the S1-seed resting-state functional connectivity (rs-FC) in the whole brain and the evoked response in the S1. Two-point discrimination threshold was assessed using a custom-made device. The beta rs-FC revealed a negative correlation between the discrimination threshold and S1-superior parietal lobule, S1-inferior parietal lobule, and S1-superior temporal gyrus connection (all P < 0.049); strong connectivity was associated with better performance. Somatosensory gating of N20m was also negatively correlated with the discrimination threshold (P = 0.015), with weak gating associated with better performance. This is the first study to demonstrate that specific beta corticocortical networks functionally support tactile spatial acuity as well as the local inhibitory network.

Age-related changes of deep-brain neurophysiological activity

Cognitive decline with age is associated with brain atrophy and reduced brain activations, but the underlying neurophysiological mechanisms are unclear, especially in deeper brain structures primarily affected by healthy aging or neurodegenerative processes. Here, we characterize time-resolved, resting-state magnetoencephalography activity of the hippocampus and subcortical brain regions in a large cohort of healthy young (20-30 years) and older (70-80 years) volunteers from the Cam-CAN (Cambridge Centre for Ageing and Neuroscience) open repository. The data show age-related changes in both rhythmic and arrhythmic signal strength in multiple deeper brain regions, including the hippocampus, striatum, and thalamus. We observe a slowing of neural activity across deeper brain regions, with increased delta and reduced gamma activity, which echoes previous reports of cortical slowing. We also report reduced occipito-parietal alpha peak associated with increased theta-band activity in the hippocampus, an effect that may reflect compensatory processes as theta activity, and slope of arrhythmic activity were more strongly expressed when short-term memory performances were preserved. Overall, this study advances the understanding of the biological nature of inter-individual variability in aging. The data provide new insight into how hippocampus and subcortical neurophysiological activity evolve with biological age, and highlight frequency-specific effects associated with cognitive decline versus cognitive maintenance.

Dystonia in Childhood: How Insights from Paediatric Research Enrich the Network Theory of Dystonia

Dystonia is now widely accepted as a network disorder, with multiple brain regions and their interconnections playing a potential role in the pathophysiology. This model reconciles what could previously have been viewed as conflicting findings regarding the neuroanatomical and neurophysiological characteristics of the disorder, but there are still significant gaps in scientific understanding of the underlying pathophysiology. One of the greatest unmet challenges is to understand the network model of dystonia in the context of the developing brain. This article outlines how research in childhood dystonia supports and contributes to the network theory and highlights aspects where data from paediatric studies has revealed novel and unique physiological insights, with important implications for understanding dystonia across the lifespan.

Stress-induced aberrations in sensory processing predict worse cognitive outcomes in healthy aging adults

It is well recognized that not all individuals age equivalently, with functional dependence attributable, at least in part, to stress accumulated across the lifespan. Amongst these dependencies are age-related declines in cognitive function, which may be the result of impaired inhibitory processing (e.g., sensory gating). Herein, we examined the unique roles of life and biological stress on somatosensory gating dynamics in 74 adults (22-72 years old). Participants completed a sensory gating paired-pulse electrical stimulation paradigm of the right median nerve during magnetoencephalography (MEG) and data were subjected to advanced oscillatory and time-domain analysis methods. We observed separable mechanisms by which increasing levels of life and biological stress predicted higher oscillatory gating ratios, indicative of age-related impairments in inhibitory function. Specifically, elevations in life stress significantly modulated the neural response to the first stimulation in the pair, while elevations in biological stress significantly modulated the neural response to the second stimulation in the pair. In contrast, neither elevations in life nor biological stress significantly predicted the gating of time-domain neural activity in the somatosensory cortex. Finally, our study is the first to link stress-induced decline in sensory gating to cognitive dysfunction, suggesting that gating paradigms may hold promise for detecting discrepant functional trajectories in age-related pathologies in the future.

Sensorimotor Integration in Childhood Dystonia and Dystonic Cerebral Palsy-A Developmental Perspective

Dystonia is a disorder of sensorimotor integration, involving dysfunction within the basal ganglia, cortex, cerebellum, or their inter-connections as part of the sensorimotor network. Some forms of dystonia are also characterized by maladaptive or exaggerated plasticity. Development of the neuronal processes underlying sensorimotor integration is incompletely understood but involves activity-dependent modeling and refining of sensorimotor circuits through processes that are already taking place in utero and which continue through infancy, childhood, and into adolescence. Several genetic dystonias have clinical onset in early childhood, but there is evidence that sensorimotor circuit development may already be disrupted prenatally in these conditions. Dystonic cerebral palsy (DCP) is a form of acquired dystonia with perinatal onset during a period of rapid neurodevelopment and activity-dependent refinement of sensorimotor networks. However, physiological studies of children with dystonia are sparse. This discussion paper addresses the role of neuroplasticity in the development of sensorimotor integration with particular focus on the relevance of these mechanisms for understanding childhood dystonia, DCP, and implications for therapy selection, including neuromodulation and timing of intervention.

Reduced coupling of somatosensory gating and gamma oscillation in panic disorder

Previous studies have reported that patients with panic disorder (PD) exhibited an aberrant level of GABA concentration, an inhibitory neurotransmitter in the human brain. However, it remains substantially unclear whether the inhibitory function regarding the neurophysiological characteristics is altered in this disease. Sensory gating (SG) is considered as an automatic inhibitory function in the sensory cortex. In addition, brain's gamma oscillation within the sensory cortex is another index to reflect inhibitory function. Here we aimed to investigate whether the patients with PD showed altered inhibitory function in the somatosensory system, including the primary (SI) and secondary (SII) somatosensory cortices. A total of 20 healthy controls and 21 patients with PD underwent magnetoencephalographic recordings. Paired-pulse and single-pulse paradigms were used to study SG and gamma oscillations, respectively. There were no significant between-group differences in the SG function in the SI and SII. However, patients with PD demonstrated a reduced gamma power in the SI. Among the healthy individuals, strong associations between SG ratios and gamma frequency values were observed in the SI. However, such a functional relationship disappeared among the patients with PD. We suggested the reduced coupling of SG and gamma oscillation as one of the neural signatures in PD.

EEG measures of sensorimotor processing and their development are abnormal in children with isolated dystonia and dystonic cerebral palsy

Dystonia is a disorder of sensorimotor integration associated with abnormal oscillatory activity within the basal ganglia-thalamo-cortical networks. Event-related changes in spectral EEG activity reflect cortical processing but are sparsely investigated in relation to sensorimotor processing in dystonia. This study investigates modulation of sensorimotor cortex EEG activity in response to a proprioceptive stimulus in children with dystonia and dystonic cerebral palsy (CP). Proprioceptive stimuli, comprising brief stretches of the wrist flexors, were delivered via a robotic wrist interface to 30 young people with dystonia (20 isolated genetic/idiopathic and 10 dystonic CP) and 22 controls (mean age 12.7 years). Scalp EEG was recorded using the 10-20 international system and the relative change in post-stimulus power with respect to baseline was calculated for the alpha (8-12 Hz) and beta (14-30 Hz) frequency bands. A clear developmental profile in event-related spectral changes was seen in controls. Controls showed a prominent early alpha/mu band event-related desynchronisation (ERD) followed by an event-related synchronisation (ERS) over the contralateral sensorimotor cortex following movement of either hand. The alpha ERD was significantly smaller in the dystonia groups for both dominant and non-dominant hand movement (ANCOVA across the 3 groups with age as covariate: dominant hand F(2,47) = 4.45 p = 0.017; non-dominant hand F(2,42) = 9.397 p < 0.001. Alpha ERS was significantly smaller in dystonia for the dominant hand (ANCOVA F(2,47) = 7.786 p = 0.001). There was no significant difference in ERD or ERS between genetic/idiopathic dystonia and dystonic CP. CONCLUSION: Modulation of alpha/mu activity by a proprioceptive stimulus is reduced in dystonia, demonstrating a developmental abnormality of sensorimotor processing which is common to isolated genetic/idiopathic and acquired dystonia/dystonic CP.

I'm a believer: Illusory self-generated touch elicits sensory attenuation and somatosensory evoked potentials similar to the real self-touch

Sensory attenuation (i.e., the phenomenon whereby self-produced sensations are perceived as less intense compared to externally occurring ones) is among the neurocognitive processes that help distinguishing ourselves from others. It is thought to be rooted in the motor system (e.g., related to motor intention and prediction), while the role of body awareness, which necessarily accompanies any voluntary movement, in this phenomenon is largely unknown. To fill this gap, here we compared the perceived intensity, somatosensory evoked potentials, and alpha-band desynchronization for self-generated, other-generated, and embodied-fake-hand-generated somatosensory stimuli. We showed that sensory attenuation triggered by the own hand and by the embodied fake hand had the same behavioral and neurophysiological signatures (reduced subjective intensity, reduced of N140 and P200 SEP components and post-stimulus alpha-band desynchronization). Therefore, signals subserving body ownership influenced attenuation of somatosensory stimuli, possibly in a postdictive manner. This indicates that body ownership is crucial for distinguishing the source of the perceived sensations.

Cortical neural activity evoked by bilateral and unilateral mirror therapy after stroke

OBJECTIVE

This study aimed to investigate the differential effects of bilateral and unilateral mirror therapy (MT) on motor cortical activations in stroke patients by magnetoencephalography (MEG).

METHODS

Sixteen stroke patients and 16 right-handed healthy volunteers were recruited. All participants were required to perform 4 conditions: resting, no mirror with bilateral hand movements (Bilateral-No mirror), mirror with bilateral hand movements (Bilateral-Mirror) and mirror with unilateral hand movements (Unilateral-Mirror). Beta oscillatory activities in the primary motor cortex (M1) were collected during each condition using MEG. The percentage change of beta oscillatory activity was calculated for each condition to correct the baseline differences.

RESULTS

In the stroke group, the percentage change of M1 beta oscillatory activity significantly decreased more in the Bilateral-Mirror condition than in the Bilateral-No mirror and Unilateral-Mirror conditions. In the healthy group, no significant differences in the percentage change of beta oscillatory activity were found among the 3 conditions. Further, a significant difference in the percentage change of beta oscillatory activity only in the Bilateral-Mirror condition was found between the 2 groups.

CONCLUSIONS

This study provides new information on the differential cortical activations modulated by bilateral and unilateral MT.

SIGNIFICANCE

Bilateral MT led to greater M1 neural activities than unilateral MT and bilateral movements without a mirror in stroke patients.

Prefrontal gating of sensory input differentiates cognitively impaired and unimpaired aging adults with HIV

Despite effective therapies that have extended the life expectancy of persons living with HIV, 35-70% of these adults still develop some form of cognitive impairment, and with a growing population of aging adults with HIV, the prevalence of these cognitive deficits is likely to increase. The mechanisms underlying these HIV-associated neurocognitive disorders remain poorly understood but are often accelerated by the aging process and accompanied by disturbances in sensory processing, which may contribute to the observed cognitive decline. The goal of the current study was to identify the impact of aging on HIV-related alterations in inhibitory processing and determine whether such alterations are related to cognitive impairment in neuroHIV. We used magnetoencephalographic imaging, advanced time series analysis methods, and a paired-pulse stimulation paradigm to interrogate inhibitory processing in 87 HIV-infected aging adults and 92 demographically matched uninfected controls (22-72 years old). Whole-brain maps linking age and neural indices were computed for each group and compared via Fisher's Z transformations. Peak voxel time-series data were also extracted from the resulting images to quantify the dynamics of spontaneous neural activity preceding stimulation onset in each group. Whole-brain analyses using the somatosensory gating index, a metric of inhibitory processing and age distinguished impaired adults with HIV from unimpaired HIV-infected adults and controls. Briefly, younger cognitively impaired adults with HIV strongly utilized the prefrontal cortices to gate somatosensory input, and the role of this region in gating was uniquely and significantly modulated by aging only in impaired adults with HIV. Spontaneous neural activity preceding stimulus onset was also significantly elevated in the prefrontal cortices of those with HIV-associated neurocognitive disorder, and this elevation was significantly related to the CD4 nadir across both HIV-infected groups. This is the first study to examine the impact of aging on inhibitory processing in HIV-infected adults with and without cognitive impairment. Our findings suggest that young adults with HIV-associated neurocognitive disorder utilize the prefrontal cortices to gate (i.e. suppress) redundant somatosensory input, and that this capacity uniquely diminishes with advancing age in impaired adults with HIV.

Methodological considerations for a better somatosensory gating paradigm: The impact of the inter-stimulus interval

Sensory gating (SG) is a neurophysiological phenomenon whereby the response to the second stimulus in a repetitive pair is attenuated. This filtering of irrelevant or redundant information is thought to preserve neural resources for more behaviorally-relevant stimuli and thereby reflect the functional inhibition of sensory input. Developing a SG paradigm in which optimal suppression of sensory input is achieved requires investigators to consider numerous parameters such as stimulus intensity, time between stimulus pairs, and the inter-stimulus interval (ISI) within each pair. While these factors have been well defined for the interrogation of auditory gating, the precise parameters for eliciting optimal gating in the somatosensory domain are far less understood. To address this, we investigated the impact of varying the ISI within each identical pair of stimuli on gating using magnetoencephalography (MEG). Specifically, 25 healthy young adults underwent paired-pulse electrical stimulation of the median nerve with increasing ISIs between 100 and 1000 ​ms (in 100 ​ms increments). Importantly, for correspondence with previous studies of somatosensory gating, both time-domain and oscillatory neural responses to somatosensory stimulation were evaluated. Our results indicated that gating of somatosensory input was optimal (i.e., best suppression) for trials with an ISI of 200-220 ​ms, as evidenced by the smallest gating ratios and through statistical modeling estimations of optimal suppression. Importantly, this was true irrespective of whether oscillatory or evoked neural activity was used to calculate SG. Interestingly, oscillatory metrics of gating calculated using peak gamma (30-75 ​Hz) power and frequency revealed more robust gating (i.e., smaller ratios) than those calculated using time-domain neural responses, suggesting that high frequency oscillations may provide a more sensitive measure of SG. These findings have important implications for the development of optimal protocols and analysis pipelines to interrogate SG and inhibitory processing with a higher degree of sensitivity and accuracy.

Non-pharmacological treatment changes brain activity in patients with dementia

Non-pharmacological treatment (NPT) improves cognitive functions and behavioural disturbances in patients with dementia, but the underlying neural mechanisms are unclear. In this observational study, 21 patients with dementia received NPTs for several months. Patients were scanned using magnetoencephalography twice during the NPT period to evaluate NPT effects on resting-state brain activity. Additionally, cognitive functions and behavioural disturbances were measured using the Mini-Mental State Examination (MMSE-J) and a short version of the Dementia Behaviour Disturbance Scale (DBD-13) at the beginning and the end of the NPT period. In contrast to the average DBD-13 score, the average MMSE-J score improved after the NPT period. Magnetoencephalography data revealed a reduced alpha activity in the right temporal lobe and fusiform gyrus, as well as an increased low-gamma activity in the right angular gyrus. DBD-13 score changes were correlated with beta activity in the sensorimotor area. These findings corroborate previous studies confirming NPT effects on brain activity in healthy participants and people at risk of dementia. Our results provide additional evidence that brains of patients with dementia have the capacity for plasticity, which may be responsible for the observed NPT effects. In dementia, NPT might lead to improvements in the quality of life.

Rhythmic Spontaneous Activity Mediates the Age-Related Decline in Somatosensory Function

Sensory gating is a neurophysiological process whereby the response to a second stimulus in a pair of identical stimuli is attenuated, and it is thought to reflect the capacity of the CNS to preserve neural resources for behaviorally relevant stimuli. Such gating is observed across multiple sensory modalities and is modulated by age, but the mechanisms involved are not understood. In this study, we examined somatosensory gating in 68 healthy adults using magnetoencephalography (MEG) and advanced oscillatory and time-domain analysis methods. MEG data underwent source reconstruction and peak voxel time series data were extracted to evaluate the dynamics of somatosensory gating, and the impact of spontaneous neural activity immediately preceding the stimulation. We found that gating declined with increasing age and that older adults had significantly reduced gating relative to younger adults, suggesting impaired local inhibitory function. Most importantly, older adults had significantly elevated spontaneous activity preceding the stimulation, and this effect fully mediated the impact of aging on sensory gating. In conclusion, gating in the somatosensory system declines with advancing age and this effect is directly tied to increased spontaneous neural activity in the primary somatosensory cortices, which is likely secondary to age-related declines in local GABA inhibitory function.

Local cortical thickness predicts somatosensory gamma oscillations and sensory gating: A multimodal approach

Two largely distinct bodies of research have demonstrated age-related alterations and disease-specific aberrations in both local gamma oscillations and patterns of cortical thickness. However, seldom has the relationship between gamma activity and cortical thickness been investigated. Herein, we combine the spatiotemporal precision of magnetoencephalography (MEG) with high-resolution magnetic resonance imaging and surface-based morphometry to characterize the relationships between somatosensory gamma oscillations and the thickness of the cortical tissue generating the oscillations in 94 healthy adults (age range: 22-72). Specifically, a series of regressions were computed to assess the relationships between thickness of the primary somatosensory cortex (S1), S1 gamma response power, peak gamma frequency, and somatosensory gating of identical stimuli. Our results indicated that increased S1 thickness significantly predicted greater S1 gamma response power, reduced peak gamma frequency, and improved somatosensory gating. Furthermore, peak gamma frequency significantly and partially mediated the relationship between S1 thickness and the magnitude of the S1 gamma response. Finally, advancing age significantly predicted reduced S1 thickness and decreased gating of redundant somatosensory stimuli. Notably, this is the first study to directly link somatosensory gamma oscillations to local cortical thickness. Our results demonstrate a multi-faceted relationship between structure and function, and have important implications for understanding age- and disease-related deficits in basic sensory processing and higher-order inhibitory function.

Attention modulates the gating of primary somatosensory oscillations

Sensory gating (SG) is a well-studied phenomenon in which neural responses are reduced to identical stimuli presented in succession, and is thought to represent the functional inhibition of primary sensory information that is redundant in nature. SG is traditionally considered pre-attentive, but little is known about the effects of attentional state on this process. In this study, we investigate the impact of directed attention on somatosensory SG using magnetoencephalography. Healthy young adults (n ​= ​26) performed a novel somato-visual paired-pulse oddball paradigm, in which attention was directed towards or away from paired-pulse stimulation of the left median nerve. We observed a robust evoked (i.e., phase-locked) somatosensory response in the time domain, and three stereotyped oscillatory responses in the time-frequency domain including an early theta response (4-8 ​Hz), and later alpha (8-14 ​Hz) and beta (20-26 ​Hz) responses across attentional states. The amplitudes of the evoked response and the theta and beta oscillations were gated for the second stimulus, however, only the gating of the oscillatory responses was altered by attention. Specifically, directing attention to the somatosensory domain enhanced SG of the early theta response, while reducing SG of the later alpha and beta responses. Further, prefrontal alpha-band coherence with the primary somatosensory cortex was greater when attention was directed towards the somatosensory domain, supporting a frontal modulatory effect on the alpha response in primary somatosensory regions. These findings highlight the dynamic effects of attentional modulation on somatosensory processing, and the importance of considering attentional state in studies of SG.

Altered functional connectivity between primary and secondary somatosensory areas in panic disorder

Disturbance in the interpretation of bodily sensation has been widely reported in patients with panic disorder (PD). However, it remains substantially unknown whether patients with PD exhibit any defect in cortical somatosensory processing of non-threatening stimuli. Thus, the present study aimed to examine the functional integrity of the cortical somatosensory system in patients with PD using neurophysiological recordings. A total of 20 patients with PD and 20 healthy controls (HC) were recruited to investigate the cortical responses to median nerve stimulation through whole-head magnetoencephalographic (MEG) imaging. To comprehensively investigate all somatosensory functioning, we studied the regional activation of the primary somatosensory cortex (SI), contralateral (SIIc), and ipsilateral (SIIi) secondary somatosensory cortices, as well as functional connectivity among the SI, SIIc, and SIIi in alpha, beta, and gamma frequency bands. We found that patients with PD demonstrated a reduction in SI activity compared with those in the HC group. Furthermore, a significantly weaker gamma-band functional connectivity between SI and SIIc was found in the PD group relative to the HC group. Our data suggest that patients with PD exhibit abnormal responses to non-threatening (i.e., pain-free) stimuli in the cortical somatosensory system.

Modulation of Motor Cortical Activities by Action Observation and Execution in Patients with Stroke: An MEG Study

Action observation therapy has recently attracted increasing attention; however, the mechanisms through which action observation and execution (AOE) modulate neural activity in stroke patients remain unclear. This study was aimed at investigating the effects of action observation and two types of AOE on motor cortical activations after stroke using magnetoencephalography. Twenty patients with stroke and 20 healthy controls were recruited for the collection of data on the beta oscillatory activity in the primary motor cortex (M1). All participants performed the conditions of resting, observation only, and video observation combined with execution (video AOE). Stroke patients performed one additional condition of affected hand observation combined with execution (affected hand AOE). The relative change index of beta oscillations was calculated, and nonparametric tests were used to examine the differences in conditions. In stroke patients, the relative change index of M1 beta oscillatory activity under the video AOE condition was significantly lower than that under the observation only and affected hand AOE conditions. Moreover, M1 cortical activity did not significantly differ under the observation only and affected hand AOE conditions. For healthy controls, the relative change index under the video AOE condition was significantly lower than that under the observation only condition. In addition, no significant differences in relative change indices were found under the observation only and video AOE conditions between the 2 groups. This study provides new insight into the neural mechanisms underlying AOE, which supports the use of observing videos of normal movements during action observation therapy in stroke rehabilitation.

Children with Cerebral Palsy Hyper-Gate Somatosensory Stimulations of the Foot

We currently have a substantial knowledge gap in our understanding of the neurophysiological underpinnings of the sensory perception deficits often reported in the clinic for children with cerebral palsy (CP). In this investigation, we have begun to address this knowledge gap by using magnetoencephalography (MEG) brain imaging to evaluate the sensory gating of neural oscillations in the somatosensory cortices. A cohort of children with CP (Gross Motor Function Classification System II-III) and typically developing children underwent paired-pulse electrical stimulation of the tibial nerve during MEG. Advanced beamforming methods were used to image significant oscillatory responses, and subsequently the time series of neural activity was extracted from peak voxels. Our experimental results showed that somatosensory cortical oscillations (10-75 Hz) were weaker in the children with CP for both stimulations. Despite this reduction, the children with CP actually exhibited a hyper-gating response to the second, redundant peripheral stimulation applied to the foot. These results have further established the nexus of the cortical somatosensory processing deficits that are likely responsible for the degraded sensory perceptions reported in the clinic for children with CP.

Abnormal frontal generator during auditory sensory gating in panic disorder: An MEG study

Patients with panic disorder (PD) exhibit abnormalities in early-stage information processing, even for the nonthreatening stimuli. A previous event-related potential study reported that PD patients show a deficit in sensory gating (SG), a protective mechanism of the brain to filter out irrelevant sensory inputs. However, there is no clear understanding about the neural correlates of SG deficits in PD. Moreover, whether SG deficits, if any, are associated with clinical manifestations remain unknown. In this study, 18 patients with PD and 20 age- and gender-matched healthy controls were recruited to perform auditory paired-stimulus paradigm using magnetoencephalographic (MEG) recordings. Results showed that PD patients demonstrated significantly higher M50 SG ratios in the right inferior frontal gyrus (RIFG) and higher M100 SG ratios in both RIFG and right superior temporal gyrus (RSTG) than those of the control group. It was important to note that in the RIFG, the M50 SG ratios correlated significantly with the scores of Body Sensation Questionnaire (BSQ) and Distractibility scale of Sensory Gating Inventory among patients with PD. In conclusion, this study suggests that PD patients exhibited a deficient ability to filter out irrelevant information, and such a defect might lead to cognitive misinterpretation of somatic sensations and distractibility.

A study for the mechanism of sensory disorder in restless legs syndrome based on magnetoencephalography

In spite of the relatively high incidence rate, the etiology and pathogenesis of restless legs syndrome (RLS) are still unclear. Long-term drug treatments fail to achieve satisfying curative effects, which is reflected by rebound and augmentation of related symptoms. An electrophysiological endophenotype experiment was done to investigate the mechanism of somatosensory disorder among RLS patients. Together with 15 normal subjects as the control group, with comparable ages and genders to the RLS patients, 15 primitive RLS patients were scanned by Magnetoencephalography (MEG) under natural conditions; furthermore, the somatosensory evoked magnetic field (SEF) with single and paired stimuli, was also measured. Compared to the control group, the SEF intensities of RLS patients' lower limbs were higher, and the paired-pulse depression (PPD) for SEF in RLS patients was attenuated. It was also revealed by time-frequency analysis of somatosensory induced oscillation (SIO) in RLS patients, that 93.3% of somatosensory induced Alpha (8-12 Hz) oscillations were successfully elicited, while 0% somatosensory induced Gamma (30-55 Hz) oscillations were elicited; which was significantly different from the control group. Additionally, in RLS patients exhibit increased excitability of the sensorimotor cortex, a remarkable abnormality existing in early somatosensory gating control (GC) and an attenuated inhibitory interneuron network, which consequently results in a compensatory mechanism through which RLS patients increase their attention-driven lower limb sensory gating control via somatosensory-induced Alpha (8-12 Hz) oscillation. This hyperexcitability, partially due to an electrocortical disinhibition, may have an important therapeutical implication, and become an important target of neuromodulatory interventions.

Inhibitory Control Impairment on Somatosensory Gating Due to Aging: An Event-Related Potential Study

The capacity to suppress irrelevant incoming input, termed sensory gating, is one of the most investigated inhibitory processes associated with cognitive impairments due to aging. The aim of this study was to examine the influence of aging on sensory gating by using somatosensory event-related potentials (ERPs) elicited by repetitive non-painful tactile stimulation (paired-pulsed task). Somatosensory ERPs were recorded in 20 healthy young adults and 20 healthy older adults while they received two identical pneumatic stimuli (S1 and S2) of 100 ms duration with an inter-stimulus interval of 550 ± 50 ms on both forefingers. The difference between the somatosensory ERPs amplitude elicited by S1 and S2 was computed as a sensory gating measure. The amplitude and the latency of P50, N100 and late positive complex (LPC) were analyzed as well as the source generators of the gating effect. Reduced sensory gating was found in older individuals for N100 at frontal and centro-parietal electrodes and for LPC at fronto-central electrodes. Source localization analyses also revealed a reduced current density during gating effect in the older group in frontal areas in N100 and LPC. Moreover, older individuals showed delayed latencies in N100. No significant gating effect differences were found between groups in P50. These findings suggest an age-related slowing of processing speed and a reduced efficiency of inhibitory mechanisms in response to repetitive somatosensory information during stimulus evaluation, and a preservation of processing speed and inhibitory control during early stimulus coding in aging.

Variability and Reliability of Paired-Pulse Depression and Cortical Oscillation Induced by Median Nerve Stimulation

Paired-pulse depression (PPD) has been widely used to investigate the functional profiles of somatosensory cortical inhibition. However, PPD induced by somatosensory stimulation is variable, and the reasons for between- and within-subject PPD variability remains unclear. Therefore, the purpose of this study was to clarify the factors influencing PPD variability induced by somatosensory stimulation. The study participants were 19 healthy volunteers. First, we investigated the relationship between the PPD ratio of each component (N20m, P35m, and P60m) of the somatosensory magnetic field, and the alpha, beta, and gamma band changes in power [event-related desynchronization (ERD) and event-related synchronization (ERS)] induced by median nerve stimulation. Second, because brain-derived neurotrophic factor (BDNF) gene polymorphisms reportedly influence the PPD ratio, we assessed whether BDNF genotype influences PPD ratio variability. Finally, we evaluated the test-retest reliability of PPD and the alpha, beta, and gamma ERD/ERS induced by somatosensory stimulation. Significant positive correlations were observed between the P60m_PPD ratio and beta power change, and the P60m_PPD ratio was significantly smaller for the beta ERD group than for the beta ERS group. P35m_PPD was found to be robust and highly reproducible; however, P60m_PPD reproducibility was poor. In addition, the ICC values for alpha, beta, and gamma ERD/ERS were 0.680, 0.760, and 0.552 respectively. These results suggest that the variability of PPD for the P60m deflection may be influenced by the ERD/ERS magnitude, which is induced by median nerve stimulation.

Age Effect on Automatic Inhibitory Function of the Somatosensory and Motor Cortex: An MEG Study

Age-related deficiency in the top-down modulation of cognitive inhibition has been extensively documented, whereas the effects of age on a bottom-up or automatic operation of inhibitory function were less investigated. It is unknown that whether the older adults (OA)’ reduced behavioral performance and neural responses are due to the insufficient bottom-up processes. Compared to behavioral assessments which have been widely used to examine the top-down control of response inhibition, electrophysiological recordings are more suitable to probe the early-stage processes of automatic inhibitory function. Sensory gating (SG), a phenomenon of attenuated neural response to the second identical stimulus in a paired-pulse paradigm, is an indicator to assess automatic inhibitory function of the sensory cortex. On the other hand, electricity-induced beta rebound oscillation in a single-pulse paradigm reflects cortical inhibition of the motor cortex. From the neurophysiological perspective, SG and beta rebound oscillation are replicable indicators to examine the automatic inhibitory function of human sensorimotor cortices. Thus, the present study aimed to use a whole-head magnetoencephalography (MEG) to investigate the age-related alterations of SG function in the primary somatosensory cortex (SI) and of beta rebound oscillation in the primary motor cortex (MI) in 17 healthy younger and 15 older adults. The Stimulus 2/Stimulus 1 (S2/S1) amplitude ratio in response to the paired-pulse electrical stimulation to the left median nerve was used to evaluate the automatic inhibitory function of SI, and the beta rebound response in the single-pulse paradigm was used to evaluate the automatic inhibitory function of MI. Although there were no significant age-related differences found in the SI SG ratios, the MI beta rebound power was reduced and peak latency was prolonged in the OA. Furthermore, significant association between the SI SG ratio and the MI beta rebound power, which was seen in the younger adults (YA), was absent in the OA. In conclusion, our data suggested an age-related defect of association between sensorimotor cortices regarding automatic inhibitory function.

Effects of observing normal and abnormal goal-directed hand movements on somatosensory cortical activation

Existing evidence indicates the importance of observing correct, normal actions on the motor cortical activities. However, the exact neurophysiological mechanisms, particularly in the somatosensory system, remain unclear. This study aimed to elucidate the effects of observing normal and abnormal hand movements on the contralateral primary somatosensory (cSI), contralateral (cSII) and ipsilateral (iSII) secondary somatosensory activities. Experiment I was designed to investigate the effects of motor outputs on the somatosensory processing, in which subjects were instructed to relax or manipulate a small cube. Experiment II was tailored to examine the somatosensory responses to the observation of normal (Normal) and abnormal (Abnormal) hand movements. The subjects received electrical stimulation to right median nerve and magnetoencephalography (MEG) recordings during the whole experimental period. Regional cortical activation and functional connectivity were analyzed. Compared to the resting condition, a reduction in cSI and an enhancement of SII activation was found when subjects manipulated a cube, suggesting the motor outputs have an influence on the somatosensory responses. Further investigation of the effects of observing different hand movements showed that cSII activity was significantly stronger in the Normal than Abnormal condition. Moreover, compared with Abnormal condition, a higher cortical coherence of cSI-iSII at theta bands and cSII-iSII at beta bands was found in Normal condition. Conclusively, the present results suggest stronger activation and enhanced functional connectivity within the somatosensory system during the observation of normal than abnormal hand movements. These findings also highlight the importance of viewing normal, correct hands movements in the stroke rehabilitation.

Effects of action observation therapy and mirror therapy after stroke on rehabilitation outcomes and neural mechanisms by MEG: study protocol for a randomized controlled trial

Background

Loss of upper-extremity motor function is one of the most debilitating deficits following stroke. Two promising treatment approaches, action observation therapy (AOT) and mirror therapy (MT), aim to enhance motor learning and promote neural reorganization in patients through different afferent inputs and patterns of visual feedback. Both approaches involve different patterns of motor observation, imitation, and execution but share some similar neural bases of the mirror neuron system. AOT and MT used in stroke rehabilitation may confer differential benefits and neural activities that remain to be determined. This clinical trial aims to investigate and compare treatment effects and neural activity changes of AOT and MT with those of the control intervention in patients with subacute stroke.

Methods/design

An estimated total of 90 patients with subacute stroke will be recruited for this study. All participants will be randomly assigned to receive AOT, MT, or control intervention for a 3-week training period (15 sessions). Outcome measurements will be taken at baseline, immediately after treatment, and at the 3-month follow-up. For the magnetoencephalography (MEG) study, we anticipate that we will recruit 12 to 15 patients per group. The primary outcome will be the Fugl-Meyer Assessment score. Secondary outcomes will include the modified Rankin Scale, the Box and Block Test, the ABILHAND questionnaire, the Questionnaire Upon Mental Imagery, the Functional Independence Measure, activity monitors, the Stroke Impact Scale version 3.0, and MEG signals.

Discussion

This clinical trial will provide scientific evidence of treatment effects on motor, functional outcomes, and neural activity mechanisms after AOT and MT in patients with subacute stroke. Further application and use of AOT and MT may include telerehabilitation or home-based rehabilitation through web-based or video teaching.

Trial registration

ClinicalTrials.gov, ID: NCT02871700. Registered on 1 August 2016.

Electronic supplementary material

The online version of this article (doi:10.1186/s13063-017-2205-z) contains supplementary material, which is available to authorized users.

Automatic inhibitory function in the human somatosensory and motor cortices: An MEG-MRS study

While the automatic inhibitory function of the human cerebral cortex has been extensively investigated by means of electrophysiological recordings, the corresponding modulating neurochemical mechanisms remain unclear. We aimed to examine whether the primary somatosensory (SI) and primary motor cortical (MI) inhibitory function is associated with endogenous GABA levels. Eighteen young participants received paired-pulse and single-pulse electrical stimulation to the median nerve during magnetoencephalographic recordings. The SI sensory gating (SG), considered as an automatic inhibitory ability, was measured as the amplitude ratio of Stimulus 2 over Stimulus 1, in the paired-pulse paradigm. In addition, stimulus-induced beta activity, considered to originate from MI and also to be related to inhibitory function, was estimated using the single-pulse paradigm. The GABA+ concentration of the sensorimotor cortex was acquired from each subject by using magnetic resonance spectroscopy (MRS). A lower SG ratio in SI was significantly associated with an increased beta power in MI. More importantly, the beta rebound power, but not SI SG ratio, was positively correlated with GABA+ concentration. Our findings show a tight functional relationship between SI and MI during processing of automatic inhibition. GABA+ levels appear to be more closely related to the automatic inhibitory function of MI than SI.

The temporal stability and variability across frequency bands in neural synchrony between primary and secondary somatosensory areas following somatosensory stimulation

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The theta PLV in SI and contralateral SII showed variability within 2 s after the stimulus onset.

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The alpha PLV showed temporal stability within 2 s after the stimulus onset.

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The neural synchrony between SI and SII varied depending on the frequency band and the inter-regions.

Objectives

To examine the temporal stability and variability of neuronal synchronization among the contralateral primary somatosensory cortex (cSI) and contralateral (cSII) and ipsilateral secondary somatosensory cortex (iSII) in response to median nerve stimulation.

Methods

Both the spontaneous magnetoencephalography (MEG) signals as the pre-stimulus condition and somatosensory evoked magnetic-fields (SEF) were recorded in eleven healthy subjects. We calculated a phase-locking value (PLV) between two areas among cSI, cSII, and iSII in five frequency bands (theta: 5–7 Hz, alpha: 8–12 Hz, beta: 15–29 Hz, gamma-1: 30–59 Hz, and gamma-2: 60–90 Hz), and compared the PLV among in pre-stimulus and stimulus conditions.

Results

The PLV between cSI and cSII for the theta band activity varied within 2 s from the stimulus onset. On the other hand, the PLV between cSI and iSII for the alpha band did not vary within 2 s.

Conclusion

The fluctuation of neuronal synchrony among sensory-related cortices in response to median nerve stimulation depends on the induced frequency band and inter-region.

Significance

This study is the first to report the temporal characteristic of stimulus-driven neural synchrony following somatosensory stimulation.

Oscillatory dynamics and functional connectivity during gating of primary somatosensory responses

KEY POINTS

Sensory gating is important for preventing excessive environmental stimulation from overloading neural resources. Gating in the human somatosensory cortices is a critically understudied topic, particularly in the lower extremities. We utilize the unique capabilities of magnetoencephalographic neuroimaging to quantify the normative neural population responses and dynamic functional connectivity of somatosensory gating in the lower extremities of healthy human participants. We show that somatosensory processing is subserved by a robust gating effect in the oscillatory domain, as well as a dynamic effect on interhemispheric functional connectivity between primary sensory cortices. These results provide novel insight into the dynamic neural mechanisms that underlie the processing of somatosensory information in the human brain, and will be vital in better understanding the neural responses that are aberrant in gait-related neurological disorders (e.g. cerebral palsy).

ABSTRACT

Sensory gating (SG) is a phenomenon in which neuronal responses to subsequent similar stimuli are weaker, and is considered to be an important mechanism for preventing excessive environmental stimulation from overloading shared neural resources. Although gating has been demonstrated in multiple sensory systems, the neural dynamics and developmental trajectory underlying SG remain poorly understood. In the present study, we adopt a data-driven approach to map the spectrotemporal amplitude and functional connectivity (FC) dynamics that support gating in the somatosensory system (somato-SG) in healthy children and adolescents using magnetoencephalography (MEG). These data underwent time-frequency decomposition and the significant signal changes were imaged using a beamformer. Voxel time series were then extracted from the peak voxels and these signals were examined in the time and time-frequency domains, and then subjected to dynamic FC analysis. The results obtained indicate a significant decrease in the amplitude of the neural response following the second stimulation relative to the first in the primary somatosensory cortex (SI). A significant decrease in response latency was also found between stimulations, and each stimulation induced a sharp decrease in FC between somatosensory cortical areas. Furthermore, there were no significant correlations between somato-SG metrics and age. We conclude that somato-SG can be observed in SI in both the time and oscillatory domains, with rich dynamics and alterations in inter-hemispheric FC, and that this phenomenon has already matured by early childhood. A better understanding of these dynamics may provide insight to the numerous psychiatric and neurologic conditions that have been associated with aberrant SG across multiple modalities.

Neuroimaging with magnetoencephalography: A dynamic view of brain pathophysiology

Magnetoencephalography (MEG) is a noninvasive, silent, and totally passive neurophysiological imaging method with excellent temporal resolution (∼1 ms) and good spatial precision (∼3-5 mm). In a typical experiment, MEG data are acquired as healthy controls or patients with neurologic or psychiatric disorders perform a specific cognitive task, or receive sensory stimulation. The resulting data are generally analyzed using standard electrophysiological methods, coupled with advanced image reconstruction algorithms. To date, the total number of MEG instruments and associated users is significantly smaller than comparable human neuroimaging techniques, although this is likely to change in the near future with advances in the technology. Despite this small base, MEG research has made a significant impact on several areas of translational neuroscience, largely through its unique capacity to quantify the oscillatory dynamics of activated brain circuits in humans. This review focuses on the clinical areas where MEG imaging has arguably had the greatest impact in regard to the identification of aberrant neural dynamics at the regional and network level, monitoring of disease progression, determining how efficacious pharmacologic and behavioral interventions modulate neural systems, and the development of neural markers of disease. Specifically, this review covers recent advances in understanding the abnormal neural oscillatory dynamics that underlie Parkinson's disease, autism spectrum disorders, human immunodeficiency virus (HIV)-associated neurocognitive disorders, cerebral palsy, attention-deficit hyperactivity disorder, cognitive aging, and post-traumatic stress disorder. MEG imaging has had a major impact on how clinical neuroscientists understand the brain basis of these disorders, and its translational influence is rapidly expanding with new discoveries and applications emerging continuously.

Abnormal resting-state brain activity in headache-free migraine patients: A magnetoencephalography study

OBJECTIVE

The aim of this study is to quantitatively assess the resting-state brain activity in migraine patients during the headache-free phase with magnetoencephalography (MEG).

METHODS

A total of 25 migraine patients during the headache-free phase and 25 gender- and age-matched control patients were studied with a whole-head MEG system at eyes-closed resting-state. MEG data were analyzed in multifrequency range of 4-200Hz.

RESULTS

In a regional cortex analysis, the spectral power of gamma oscillations in left frontal and left temporal regions was significantly increased in migraine patients as compared to controls (all p<0.001), but no significant difference was found between the two groups for the global channels. Analyses of source location showed that there were significant differences in the distribution of gamma oscillation between migraine subjects and controls (p<0.025).

CONCLUSIONS

Migraine patients in resting-state had altered brain activities in spectral power value and source distribution that can be detected and analyzed by MEG.

SIGNIFICANCE

Abnormal brain activities in the left frontal and temporal regions may be involved in pain modulation and processing of migraine. These findings provide new insights into the possible mechanisms of migraine attacks.

Reduced functional connectivity of somatosensory network in writer's cramp patients

BACKGROUND

The involvement of motor cortex and sensorimotor integration in patients with writer's cramp (WC) has been well documented. However, the exact neurophysiological profile within the somatosensory system, including primary somatosensory cortex (SI), contralateral (SIIc), and ipsilateral (SIIi) secondary somatosensory areas remains less understood.

METHODS

This study investigated the neuromagnetic cortical activities of median nerve stimulation in 10 patients with WC and 10 healthy controls (HC). To comprehensively explore all the aspects of somatosensory functioning, we analyzed our data with the minimum norm estimate (MNE), the time-frequency approach with evoked and induced activities, and functional connectivity between SI and SIIc (SI-SIIc), SI and SIIi (SI-SIIi), and SIIc and SIIi (SIIc-SIIi) from theta to gamma oscillations.

RESULTS

No significant between-group differences were found in the MNE cortical amplitudes of SI, SIIc, and SIIi. Power strengths of evoked gamma oscillation and induced beta synchronization were also equivalent between WC and HC groups. However, we found significantly reduced theta coherence of SI-SIIi, alpha coherence of SI-SIIi and SIIc-SIIi, as well as beta coherence of SIIc-SIIi in patients with WC.

CONCLUSION

Our results suggest the involvement of somatosensory abnormalities, primarily with the form of functional connectivity, in patients with WC.

Sensory gating, inhibition control and gamma oscillations in the human somatosensory cortex

Inhibiting the responses to irrelevant stimuli is an essential component of human cognitive function. Pre-attentive auditory sensory gating (SG), an attenuated neural activation to the second identical stimulus, has been found to be related to the performance of higher-hierarchical brain function. However, it remains unclear whether other cortical regions, such as somatosensory cortex, also possess similar characteristics, or if such a relationship is modality-specific. This study used magnetoencephalography to record neuromagnetic responses to paired-pulse electrical stimulation to median nerve in 22 healthy participants. Somatosensory SG ratio and cortical brain oscillations were obtained and compared with the behavioral performance of inhibition control, as evaluated by somatosensory and auditory Go-Nogo tasks. The results showed that somatosensory P35m SG ratio correlated with behavioral performance of inhibition control. Such relationship was also established in relation to the auditory Go-Nogo task. Finally, a higher frequency value of evoked gamma oscillations was found to relate to a better somatosensory SG ability. In conclusion, our data provided an empirical link between automatic cortical inhibition and behavioral performance of attentive inhibition control. This study invites further research on the relationships among gamma oscillations, neurophysiological indices, and behavioral performance in clinical populations in terms of SG or cortical inhibition.