Corticomuscular and bilateral EMG coherence reflect distinct aspects of neural synchronization

Using electroencephalography (EEG) and electromyography (EMG), corticomuscular and bilateral motor unit synchronization have been found in different frequency bands and under different task conditions. These different types of long-range synchrony are hypothesized to originate from distinct mechanisms. We tested this by comparing time-resolved EEG–EMG and EMG–EMG coherence in a bilateral precision-grip task. Bilateral EMG activity was synchronized between 7 and 13 Hz for about 1 s when force output from both hands changed from an increasing to a stable force production. In contrast, EEG–EMG coherence was statistically significant between 15 and 30 Hz during stable force production. The disparities in their time–frequency profiles accord with the existence of distinct underlying processes for corticomuscular and bilateral motor unit synchronization. In addition, the absence of synchronization between cortical activity and common spinal input at 10 Hz renders a cortical source unlikely.     

利用相位同步聚类方法研究大脑功能连通性

功能连通性是指大脑不同区域的神经细胞群活动之间的瞬时相干性，最常用的方法是估计成对电极信号之间的相关或相干。本文基于相位同步聚类方法，利用头表脑电信号，估计全局功能连通性。应用到空间选择性注意实验的视觉诱发脑电数据中，结果表明，该方法能检测到三个连通源，并且在注意和非注意状态下，功能连通性具有明显的差异。     

Brain wave synchronization and entrainment to periodic acoustic stimuli

As known, different brainwave frequencies show synchronies related to different perceptual, motor or cognitive states. Brainwaves have also been shown to synchronize with external stimuli with repetition rates of ca. 10–40 Hz. However, not much is known about responses to periodic auditory stimuli with periodicities found in human rhythmic behavior (i.e. 0.5–5 Hz). In an EEG study we compared responses to periodic stimulations (drum sounds and clicks with repetition rates of 1–8 Hz), silence, and random noise. Here we report inter-trial coherence measures taken at the Cz-electrode that show a significant increase in brainwave synchronization following periodic stimulation. Specifically, we found (1) a tonic synchronization response in the delta range with a maximum response at 2 Hz, (2) a phasic response covering the theta range, and (3) an augmented phase synchronization throughout the beta/gamma range (13–44 Hz) produced through increased activity in the lower gamma range and modulated by the stimulus periodicity. Periodic auditory stimulation produces a mixture of evoked and induced, rate-specific and rate-independent increases in stimulus related brainwave synchronization that are likely to affect various cognitive functions. The synchronization responses in the delta range may form part of the neurophysiological processes underlying time coupling between rhythmic sensory input and motor output; the tonic 2 Hz maximum corresponds to the optimal tempo identified in listening, tapping synchronization, and event-interval discrimination experiments. In addition, synchronization effects in the beta and gamma range may contribute to the reported influences of rhythmic entrainment on cognitive functions involved in learning and memory tasks.     

Reduced degree of long-range phase synchrony in pathological human brain

In this paper multivariate spontaneous EEG signals from three broad groups of human subjects--control, seizure, and mania--were studied with the aim of investigating the possible effect of these pathologies on the degree of phase synchronization between cortical areas. The degree of phase synchrony was measured by two recently developed measures which are more suitable than classical indices like correlation or coherence when dealing with nonlinear and non-stationary signals like the EEG. Signals were reduced to seven frequency bands (delta, theta, alpha-1, alpha-2, beta-1, beta-2 and gamma) which were statistically compared between the normal and the other two groups. It was found that the degree of long-range synchrony was significantly reduced for both pathological groups as compared with the control group. No clear differences were found in the degrees of short-range synchrony.     

Visually evoked phase synchronization changes of alpha rhythm in migraine: correlations with clinical features.

OBJECTIVE: This study aimed to compute phase synchronization of the alpha band from a multichannel electroencephalogram (EEG) recorded under repetitive flash stimulation from migraine patients without aura. This allowed examination of ongoing EEG activity during visual stimulation in the pain-free phase of migraine. METHODS: Flash stimuli at frequencies of 3, 6, 9, 12, 15, 18, 21, 24, and 27 Hz were delivered to 15 migraine patients without aura and 15 controls, with the EEG recorded from 18 scalp electrodes, referred to the linked earlobes. The EEG signals were filtered in the alpha (7.5-13 Hz) band. For all stimulus frequencies that we evaluated, the phase synchronization index was based on the Hilbert transformation. RESULTS: Phase synchronization separated the patients and controls for the 9, 24 and 27 Hz stimulus frequencies; hyper phase synchronization was observed in patients, whereas healthy subjects were characterized by a reduced phase synchronization. These differences were found in all regions of the scalp. CONCLUSIONS: During migraine, the brain synchronizes to the idling rhythm of the visual areas under certain photic stimulations; in normal subjects however, brain regions involved in the processing of sensory information demonstrate desynchronized activity. Hypersynchronization of the alpha rhythm may suggest a state of cortical hypoexcitability during the interictal phase of migraine. SIGNIFICANCE: The employment of non-linear EEG analysis may identify subtle functional changes in the migraine brain.     

Influences of the Loading on Working Memory on the Spatial Synchronization of Prestimulus Cortical Electrical Activity during Recognition of an Emotional Facial Expression

Experiments using a set to an emotionally negative facial expression were performed in healthy adult subjects (n = 35, 16 males, 19 females) to study the effects of increased loading on working memory (addition of an additional cognitive task to the experimental context) on the spatial synchronization of cortical electrical activity. The EEG theta (4–7 Hz) and alpha (8–13 Hz) frequency ranges were studied, along with the alpha1 (8–10 Hz) and alpha2 (11–13 Hz) ranges. Analysis of coherence functions of cortical potentials was compared with data obtained in experiments without loading. Additional loading on working memory led to increases in the inertia of the set to the facial expression and significantly altered the maps of the spatial synchronization of cortical potentials in the theta and alpha ranges: there were increases in the coherence of theta potentials both between individual frontal areas of the left and right hemispheres and between the frontal and temporal areas. It is hypothesized that the inertia of the set to an emotionally negative facial expression is associated with increased activity in the corticohippocampal system, as indicated by the significant increase in coherence relationships between theta potentials in cortical areas which have key roles in organizing cognitive sets. The increase in the loading on working memory significantly weakened the spatial synchronization of the low-frequency alpha rhythm, evidently reflecting decreases in the role of the frontothalamic selective attention system in the recognition process due to redistribution of attention resources resulting from the additional cognitive task in the context of the subject’s ongoing activity.     

Loss of phase synchrony in an animal model of partial status epilepticus

Interrupting a focal, chronic infusion of GABA to the rat motor cortex initiates the progressive emergence of a sustained spiking electroencephalographic (EEG) activity, associated with myoclonic jerks of the corresponding body territory. This activity is maintained over several hours, has an average frequency of 1.5 Hz, is localized to the infusion site and never generalizes. The GABA withdrawal syndrome (GWS) has therefore features of partial status epilepticus. Changes in EEG signals associated with the GWS were studied in freely moving rats by measuring the phase synchrony between bilateral epidural records from the neocortex. Our results showed (i) epileptic activity was associated with a striking decrease in phase synchrony between all pairs of electrodes including the focus, predominantly in the 1-6 Hz frequency range. There was a mean decrease of 75.34+/-5.26% in phase synchrony levels between the period before GABA interruption and the period after epileptic activity appeared. (ii) This reduction in synchrony contrasted with an increase of power spectral density in the corresponding EEG channels over the same 1-6 Hz frequency range, (iii) neither changes in synchrony nor in nonlinear dynamics were detected before the first EEG spikes, (iv) systemic injection of ketamine, an antagonist of N-methyl-d-aspartic acid (NMDA) receptors, modified transiently both epileptic activity and the synchrony profile. (v) Spiking activity and synchrony changes were suppressed by reperfusion of GABA. Our data suggest that, during a partial status epilepticus, interactions between the epileptic focus and connected neocortical neuronal populations are dramatically decreased in low frequencies.     

Self-regulation of brain oscillations as a treatment for aberrant brain connections in children with autism

Autism is a highly varied developmental disorder typically characterized by deficits in reciprocal social interaction, difficulties with verbal and nonverbal communication, and restricted interests and repetitive behaviors. Although a wide range of behavioral, pharmacological, and alternative medicine strategies have been reported to ameliorate specific symptoms for some individuals, there is at present no cure for the condition. Nonetheless, among the many incompatible observations about aspects of the development, anatomy, and functionality of the autistic brain, it is widely agreed that it is characterized by widespread aberrant connectivity. Such disordered connectivity, be it increased, decreased, or otherwise compromised, may complicate healthy synchronization and communication among and within different neural circuits, thereby producing abnormal processing of sensory inputs necessary for normal social life. It is widely accepted that the innate properties of brain electrical activity produce pacemaker elements and linked networks that oscillate synchronously or asynchronously, likely reflecting a type of functional connectivity. Using phase coherence in multiple frequency EEG bands as a measure of functional connectivity, studies have shown evidence for both global hypoconnectivity and local hyperconnectivity in individuals with ASD. However, the nature of the brain’s experience-dependent structural plasticity suggests that these abnormal patterns may be reversed with the proper type of treatment. Indeed, neurofeedback (NF) training, an intervention based on operant conditioning that results in self-regulation of brain electrical oscillations, has shown promise in addressing marked abnormalities in functional and structural connectivity. It is hypothesized that neurofeedback produces positive behavioral changes in ASD children by normalizing the aberrant connections within and between neural circuits. NF exploits the brain’s plasticity to normalize aberrant connectivity patterns apparent in the autistic brain. By grounding this training in known anatomical (e.g., mirror neuron system) and functional markers (e.g., mu rhythms) of autism, NF training holds promise to support current treatments for this complex disorder. The proposed hypothesis specifically states that neurofeedback-induced alpha mu (8–12 Hz) rhythm suppression or desynchronization, a marker of cortical activation, should induce neuroplastic changes and lead to normalization in relevant mirroring networks that have been associated with higher-order social cognition.     

Alcoholism-related alterations in spectrum, coherence, and phase synchrony of topical electroencephalogram

The objective of present work was to assess differences in spectrum, coherence, and phase synchrony of topical electroencephalogram (EEG) between alcohol-dependent individuals and healthy participants. Surface currents were mitigated by a common average spatial filter. Parametric spectral and coherence estimates obtained for consecutive 0.5s-long EEG fragments were generally lower for alcoholics than for controls while evaluated for low EEG rhythms. Phase synchrony computed for 2.34s-long overlapping EEG fragments was lower for alcoholics than for controls while evaluated in α(2) and β(1) rhythms and for specific electrode pairs. Kruskal-Wallis one-way analysis of variance evaluated these alterations as statistically significant.     

Cortico-cortical phase synchrony in auditory mismatch processing

Previous studies have shown a reduced MMN in patients with lesions in the temporal or frontal lobes, suggesting a temporal-frontal involvement in change detection. However, how the temporal lobe interacts with other brain areas in responding to unexpected deviant stimuli remains unclear. This study aimed to evaluate the functional connectivity between cerebral regions by measuring the phase synchrony of magnetoencephalographic (MEG) signals elicited by regular simple tones and their duration-deviants in an oddball paradigm. We measured MEG responses to deviant (1000-Hz frequency, 50-ms duration, probability of 15%) and standard (1000-Hz frequency, 100-ms duration) sounds in 10 healthy adults. By using the Morlet wavelet-based analysis, relative phase synchronization values of 4-40 Hz MEG responses at 150-300 ms after stimulus onset were calculated with respect to a reference channel from the temporal region. Phase synchronization was clearly identified between the temporal and ipsilateral frontal region in the auditory evoked responses. This temporal-frontal synchronization was significantly larger in deviants-elicited than standards-elicited activation at 4-25 Hz in the left hemisphere (p < 0.05), and at 4-8 Hz in the right hemisphere (p < 0.01). Also, temporal-temporal and temporal-parietal phase synchronies were found in deviants-evoked 4-8 Hz responses.The present results suggest an involvement of temporal-temporal, temporal-frontal, and temporal-parietal neuronal network in detecting auditory change. Phase synchronization analysis may provide a useful window to further understanding of the cerebral reactivity during the processing of auditory deviants.     

Comparison of coupling of impulse activity of cerebral cortical neurons and spatial synchronicity of the EEG

The coherence of the EEG and the coupling of the impulse activity of neurons of the visual and sensorimotor areas of the neocortex of rabbits, recorded simultaneously from the same electrodes, were compared under chronic experimental conditions. An association was found between the presence and properties of the conjugated functioning of neurons and the coherence of the EEG at various frequencies. Greater coherence of the EEG was observed during the correlated functioning of neurons at frequencies of 3–4.5 Hz than during independent functioning. The neurons discharged in pairs, with a smaller delay between them at the highest level of EEG coherence, and a common source participating more often in their synchronization than at the lowest level of EEG coherence.Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 39, No. 6, pp. 1087–1095, November–December, 1989.     

Attenuated asymmetry of functional connectivity in schizophrenia: A high‐resolution EEG study

The interhemispheric asymmetries that originate from connectivity‐related structuring of the cortex are compromised in schizophrenia (SZ). Under the assumption that such abnormalities affect functional connectivity, we analyzed its correlate—EEG synchronization—in SZ patients and matched controls. We applied multivariate synchronization measures based on Laplacian EEG and tuned to various spatial scales. Compared to the controls who had rightward asymmetry at a local level (EEG power), rightward anterior and leftward posterior asymmetries at an intraregional level (1st and 2nd order S‐estimator), and rightward global asymmetry (hemispheric S‐estimator), SZ patients showed generally attenuated asymmetry, the effect being strongest for intraregional synchronization in the alpha and beta bands. The abnormalities of asymmetry increased with the duration of the disease and correlated with the negative symptoms. We discuss the tentative links between these findings and gross anatomical asymmetries, including the cerebral torque and gyrification pattern, in normal subjects and SZ patients.     

Evidence that specific executive functions predict symptom variance among schizophrenia patients with a predominantly negative symptom profile

Introduction. Although deficits in executive functioning in schizophrenia have been consistently reported, their precise relationship to symptomatology remains unclear. Recent approaches to executive functioning in nonschizophrenia studies have aimed to “fractionate” the individual cognitive processes involved. In this study, we hypothesised that if these processes are fractionable, then particular symptom syndromes may be selectively related to executive deficits. In particular, it was hoped that this approach could clarify whether negative and positive symptoms of schizophrenia are differentially related to particular aspects of executive/attentional functions. Methods. A total of 32 patients with schizophrenia and 16 matched controls were assessed on a series of tasks designed to tap the theoretically derived executive functions of Inhibition, Shifting set, Working memory, and Sustained attention. Results. Negative symptoms were significantly predicted by performance on an “Inhibition” task (Stroop), and not by performance on any other task. Furthermore, for a subgroup of patients with predominantly negative symptoms variance in positive symptoms was only significantly predicted by performance on a set-shifting task (Visual Elevator), and not by performance on other tasks, including inhibition. Conclusions. Our results support the contention that negative symptoms can, at least partly, be conceived of as cognitive behaviours expressing specific executive deficits. Specifically, we discuss the possibility that negative symptoms may, in part, express a failure in response monitoring. We further suggest that the disordered metacognition resulting in positive symptoms may be mediated by cognitive flexibility in patients with a predominantly negative symptom profile.     

Functional connectivity analysis of steady-state visual evoked potentials

In this paper, functional connectivity of steady-state visual evoked potential (SSVEP) was investigated. Directed transfer function (DTF) was applied to cortical signals recorded from electroencephalography (EEG) in order to obtain connectivity patterns. Flow gain was proposed to assess the role of the specific brain region involved in the information transmission process. We found network connections exist in many regions beyond occipital region. Flow gain mapping both in 8–12 Hz and 13–30 Hz showed that parietal region seemed to serve as the sole hub of information transmission. Further studies of flow gain obtained from channel Pz showed two distinct peaks centered at about 12 Hz low frequency and 20 Hz medium frequency respectively. The low frequency region had a larger value of flow gain. The present study introduced functional connectivity into SSVEP. Furthermore, we put forward the concept of flow gain for the first time to explore the exchange and processing of brain information during SSVEP.     

Self-regulation of brain oscillations as a treatment for aberrant brain connections in children with autism

Autism is a highly varied developmental disorder typically characterized by deficits in reciprocal social interaction, difficulties with verbal and nonverbal communication, and restricted interests and repetitive behaviors. Although a wide range of behavioral, pharmacological, and alternative medicine strategies have been reported to ameliorate specific symptoms for some individuals, there is at present no cure for the condition. Nonetheless, among the many incompatible observations about aspects of the development, anatomy, and functionality of the autistic brain, it is widely agreed that it is characterized by widespread aberrant connectivity. Such disordered connectivity, be it increased, decreased, or otherwise compromised, may complicate healthy synchronization and communication among and within different neural circuits, thereby producing abnormal processing of sensory inputs necessary for normal social life. It is widely accepted that the innate properties of brain electrical activity produce pacemaker elements and linked networks that oscillate synchronously or asynchronously, likely reflecting a type of functional connectivity. Using phase coherence in multiple frequency EEG bands as a measure of functional connectivity, studies have shown evidence for both global hypoconnectivity and local hyperconnectivity in individuals with ASD. However, the nature of the brain’s experience-dependent structural plasticity suggests that these abnormal patterns may be reversed with the proper type of treatment. Indeed, neurofeedback (NF) training, an intervention based on operant conditioning that results in self-regulation of brain electrical oscillations, has shown promise in addressing marked abnormalities in functional and structural connectivity. It is hypothesized that neurofeedback produces positive behavioral changes in ASD children by normalizing the aberrant connections within and between neural circuits. NF exploits the brain’s plasticity to normalize aberrant connectivity patterns apparent in the autistic brain. By grounding this training in known anatomical (e.g., mirror neuron system) and functional markers (e.g., mu rhythms) of autism, NF training holds promise to support current treatments for this complex disorder. The proposed hypothesis specifically states that neurofeedback-induced alpha mu (8–12 Hz) rhythm suppression or desynchronization, a marker of cortical activation, should induce neuroplastic changes and lead to normalization in relevant mirroring networks that have been associated with higher-order social cognition.     

Digital nerve anaesthesia decreases EMG-EMG coherence in a human precision grip task

There is increasing evidence that the primary motor cortex is involved in the generation of electromyographic (EMG) oscillations at frequencies in the range of 15-30 Hz that are observed during performance of a precision grip task. Since the level of the corticomuscular coherence varies according to the nature of the object that is gripped, it seemed possible that somatosensory inputs from the hand might affect this coherence. The aim of this study was to investigate whether interrupting cutaneous inputs from the digits would affect the coherence between hand muscles during precision grip of a compliant object. Subjects performed a precision grip hold-ramp-hold task before, during and after digital nerve anaesthesia of the index finger and thumb. There were marked deficits in the performance of the task, particularly during the initial formation of the grip and first hold period. Local digital nerve anaesthesia reduced but did not abolish 14-31 Hz coherence between EMG activity recorded from different hand and forearm muscles. Coherence was measured during the second hold phase of the task. Digital nerve anaesthesia did not affect the predominant frequencies in the EMG power spectra compiled from the same phase of the task. We conclude that during a precision grip task, cutaneous input enhances oscillatory synchrony between pairs of hand muscles.     

EEG coherence: topography and frequency structure

Topographical patterns of bipolar EEG coherence are frequency specific, indicating the presence of diverse neuroanatomical and neurophysiological factors in EEG production. Bipolar EEG coherence values were calculated at 50 frequency bins ranging from 3 to 28 Hz for 39 coherence pairs. Data were derived from 4.25 min of resting EEG obtained from 106 healthy adult male subjects and analyzed in 0.5 Hz bins by Fourier transform methods. Frequency bands were clearly separated at 8.5 and 13 Hz, with a less distinct separations at 6 and 20 Hz. Within pair (non-topographic) and across pair (topographic), measures gave similar patterns of separation. Significant pathways were primarily anterior–posterior interhemispheric or perpendicular to the anterior–posterior axis. There was little difference between left and right for comparable pairs. Theta band coherent activity involves distinct midline and temporal sources, with temporal sources showing anterior/posterior differentiation. In contrast, alpha activity has a distinct posterior focus, while beta activity shows no clear global structure. A spatially homogeneous model based on characteristics of thalamocortical connectivity accounts for much of the data, but departures from the model indicate the contribution of other neural factors to coherence.     

Functional connectivity and infant spatial working memory: a frequency band analysis

The limited research on the functional meaning of infant EEG frequency bands has used measures of EEG power. The purpose of this study was to examine task‐related changes in frontal EEG coherence measures for three infant EEG frequency bands (2–5 Hz, 6–9 Hz, 10–13 Hz) during a spatial working memory task. Eight‐month‐olds exhibited baseline‐to‐task changes in frontal EEG coherence for all infant frequency bands. Both the 2–5 Hz and the 10–13 Hz bands differentiated frontal functional connectivity during the distinct processing stages, but each band provided unique information. The 10–13 Hz band, however, was the only frequency band to distinguish frontal EEG coherence values during correct and incorrect responses. These data reveal valuable information concerning frontal functional connectivity and the functional meaning of three different infant EEG frequency bands during working memory processing.     

The Influence of Apolipoprotein E Epsilon4 Polymorphism on qEEG Profiles in Healthy Young Females: A Resting EEG Study

The epsilon4 allele of the Apolipoprotein E (ApoE) gene has been linked to various neurological conditions and the aging process in the elderly. However, evidence has suggested that the influence of ApoE epsilon4 may commence in early life. This study examined the modulatory effects of ApoE epsilon4 on regional neural activity as well as inter-regional neural interactions in a young population aged 19-21. Blood samples and resting state eyes-closed EEG signals were collected from 265 healthy females, and stratified into two groups: epsilon4 carriers and non-carriers. The values of the log-transformed mean power of 18 electrodes and the mutual information of 20 channel pairs across delta, theta, alpha and beta frequencies were analyzed. Our connectivity analysis was based on information theory, which combined Morlet wavelet transform and mutual information calculation. Between-group statistics were performed by independent t-test. We notice a consistent trend across the brain, in which ApoE epsilon4 carriers possess lower regional power at the alpha band. The epsilon4 allele is also associated with lower regional power at the theta frequency in the left frontal and posterior brain regions. Functional connectivity analyses reveal a right-lateralized network that differentiates epsilon4 carriers and non-carriers, with lower connectivity strengths for the former. Our tonic EEG analyses complement those of previous reports in that the ApoE epsilon4 allele has a negative impact on regional neural synchronization and inter-regional neural interaction.     

Mobile phone emission increases inter-hemispheric functional coupling of electroencephalographic alpha rhythms in epileptic patients

It has been reported that GSM electromagnetic fields (GSM-EMFs) of mobile phones modulate - after a prolonged exposure - inter-hemispheric synchronization of temporal and frontal resting electroencephalographic (EEG) rhythms in normal young and elderly subjects (Vecchio et al., 2007, 2010). Here we tested the hypothesis that this can be even more evident in epileptic patients, who typically suffer from abnormal mechanisms governing synchronization of rhythmic firing of cortical neurons. Eyes-closed resting EEG data were recorded in ten patients affected by focal epilepsy in real and sham exposure conditions. These data were compared with those obtained from 15 age-matched normal subjects of the previous reference studies. The GSM device was turned on (45 min) in the "GSM" condition and was turned off (45 min) in the other condition ("sham"). The mobile phone was always positioned on the left side in both patients and control subjects. Spectral coherence evaluated the inter-hemispheric synchronization of EEG rhythms at the following frequency bands: delta (about 2-4 Hz), theta (about 4-6 Hz), alpha1 (about 6-8 Hz), alpha2 (about 8-10 Hz), and alpha3 (about 10-12 Hz). The effects on the patients were investigated comparing the inter-hemispheric EEG coherence in the epileptic patients with the control group of subjects evaluated in the previous reference studies. Compared with the control subjects, epileptic patients showed a statistically significant higher inter-hemispheric coherence of temporal and frontal alpha rhythms (about 8-12 Hz) in the GSM than "Sham" condition. These results suggest that GSM-EMFs of mobile phone may affect inter-hemispheric synchronization of the dominant (alpha) EEG rhythms in epileptic patients. If confirmed by future studies on a larger group of epilepsy patients, the modulation of the inter-hemispheric alpha coherence due to the GSM-EMFs could have clinical implications and be related to changes in cognitive-motor function.