Visual stimulus-dependent changes in interhemispheric EEG coherence in humans

Electrophysiological Evidence for Interhemispheric Connectivity and Communication in Young Human Infants

Little is known empirically about connectivity and communication between the two hemispheres of the brain in the first year of life, and what theoretical opinion exists appears to be at variance with the meager extant anatomical evidence. To shed initial light on the question of interhemispheric connectivity and communication, this study investigated brain correlates of interhemispheric transmission of information in young human infants. We analyzed EEG data from 12 4-month-olds undergoing a face-related oddball ERP protocol. The activity in the contralateral hemisphere differed between odd-same and odd-difference trials, with the odd-different response being weaker than the response during odd-same trials. The infants' contralateral hemisphere "recognized" the odd familiar stimulus and "discriminated" the odd-different one. These findings demonstrate connectivity and communication between the two hemispheres of the brain in the first year of life and lead to a better understanding of the functional integrity of the developing human infant brain.

Aging-Related Differences in Structural and Functional Interhemispheric Connectivity

There is substantial evidence of age-related declines in anatomical connectivity during adulthood, with associated alterations in functional connectivity. But the relation of those functional alterations to the structural reductions is unclear. The complexities of both the structural and the functional connectomes make it difficult to determine such relationships. We pursue this question with methods, based on animal research, that specifically target the interhemispheric connections between the visual cortices. We collect t1- and diffusion-weighted imaging data from which we assess the integrity of the white matter interconnecting the bilateral visual cortices. Functional connectivity between the visual cortices is measured with electroencephalography during the presentation of drifting sinusoidal gratings that agree or conflict across hemifields. Our results show age-related reductions in the integrity of the white matter interconnecting the visual cortices, and age-related increases in the difference in functional interhemispheric lagged coherence between agreeing versus disagreeing visual stimuli. We show that integrity of the white matter in the splenium of the corpus callosum predicts the differences in lagged coherence for the agreeing versus disagreeing stimuli; and that this relationship is mediated by age. These results give new insight into the causal relationship between age and functional connectivity.

The functional characterization of callosal connections

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The functional characterization of callosal connections is informed by anatomical data.

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Callosal connections play a conditional driving role depending on the brain state and behavioral demands.

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Callosal connections play a modulatory function, in addition to a driving role.

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The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.

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The corpus callosum contributes to language processing and cognitive functions.

The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.

A brain connectivity characterization of children with different levels of mathematical achievement based on graph metrics

Recent studies aiming to facilitate mathematical skill development in primary school children have explored the electrophysiological characteristics associated with different levels of arithmetic achievement. The present work introduces an alternative EEG signal characterization using graph metrics and, based on such features, a classification analysis using a decision tree model. This proposal aims to identify group differences in brain connectivity networks with respect to mathematical skills in elementary school children. The methods of analysis utilized were signal-processing (EEG artifact removal, Laplacian filtering, and magnitude square coherence measurement) and the characterization (Graph metrics) and classification (Decision Tree) of EEG signals recorded during performance of a numerical comparison task. Our results suggest that the analysis of quantitative EEG frequency-band parameters can be used successfully to discriminate several levels of arithmetic achievement. Specifically, the most significant results showed an accuracy of 80.00% (α band), 78.33% (δ band), and 76.67% (θ band) in differentiating high-skilled participants from low-skilled ones, averaged-skilled subjects from all others, and averaged-skilled participants from low-skilled ones, respectively. The use of a decision tree tool during the classification stage allows the identification of several brain areas that seem to be more specialized in numerical processing.

Intra-hemispheric integration underlies perception of tilt illusion

The integration of inputs across the entire visual field into a single conscious experience is fundamental to human visual perception. This integrated nature of visual experience is illustrated by contextual illusions such as the tilt illusion, in which the perceived orientation of a central grating appears tilted away from its physical orientation, due to the modulation by a surrounding grating with a different orientation. Here we investigated the relative contribution of local, intra-hemispheric and global, inter-hemispheric integration mechanisms to perception of the tilt illusion. We used Dynamic Causal Modelling of fMRI signals to estimate effective connectivity in human early visual cortices (V1, V2, V3) during bilateral presentation of a tilt illusion stimulus. Our analysis revealed that neural responses associated with the tilt illusion were modulated by intra- rather than inter-hemispheric connectivity. Crucially, across participants, intra-hemispheric connectivity in V1 correlated with the magnitude of the tilt illusion, while no such correlation was observed for V1 inter-hemispheric connectivity, or V2, V3 connectivity. Moreover, when the illusion stimulus was presented unilaterally rather than bilaterally, the illusion magnitude did not change. Together our findings suggest that perception of the tilt illusion reflects an intra-hemispheric integration mechanism. This is in contrast to the existing literature, which suggests inter-hemispheric modulation of neural activity as early as V1. This discrepancy with our findings may reflect the diversity and complexity of integration mechanisms involved in visual processing and visual perception.

Functional connections between activated and deactivated brain regions mediate emotional interference during externally directed cognition

Recent evidence shows that task-deactivations are functionally relevant for cognitive performance. Indeed, higher cognitive engagement has been associated with higher suppression of activity in task-deactivated brain regions - usually ascribed to the Default Mode Network (DMN). Moreover, a negative correlation between these regions and areas actively engaged by the task is associated with better performance. DMN regions show positive modulation during autobiographical, social, and emotional tasks. However, it is not clear how processing of emotional stimuli affects the interplay between the DMN and executive brain regions. We studied this interplay in an fMRI experiment using emotional negative stimuli as distractors. Activity modulations induced by the emotional interference of negative stimuli were found in frontal, parietal, and visual areas, and were associated with modulations of functional connectivity between these task-activated areas and DMN regions. A worse performance was predicted both by lower activity in the superior parietal cortex and higher connectivity between visual areas and frontal DMN regions. Connectivity between right inferior frontal gyrus and several DMN regions in the left hemisphere was related to the behavioral performance. This relation was weaker in the negative than in the neutral condition, likely suggesting less functional inhibitions of DMN regions during emotional processing. These results show that both executive and DMN regions are crucial for the emotional interference process and suggest that DMN connections are related to the interplay between externally-directed and internally-focused processes. Among DMN regions, superior frontal gyrus may be a key node in regulating the interference triggered by emotional stimuli.

No Evidence for Phase-Specific Effects of 40 Hz HD-tACS on Multiple Object Tracking

Phase synchronization drives connectivity between neural oscillators, providing a flexible mechanism through which information can be effectively and selectively routed between task-relevant cortical areas. The ability to keep track of objects moving between the left and right visual hemifields, for example, requires the integration of information between the two cerebral hemispheres. Both animal and human studies have suggested that coherent (or phase-locked) gamma oscillations (30–80 Hz) might underlie this ability. While most human evidence has been strictly correlational, high-density transcranial alternating current stimulation (HD-tACS) has been used to manipulate ongoing interhemispheric gamma phase relationships. Previous research showed that 40 Hz tACS delivered bilaterally over human motion complex could bias the perception of a bistable ambiguous motion stimulus (Helfrich et al., 2014). Specifically, this work showed that in-phase (0° offset) stimulation boosted endogenous interhemispheric gamma coherence and biased perception toward the horizontal (whereby visual tokens moved between visual hemifields—requiring interhemispheric integration). By contrast, anti-phase (180° offset) stimulation decreased interhemispheric gamma coherence and biased perception toward the vertical (whereby tokens moved within separate visual hemifields). Here we devised a multiple object tracking arena comprised of four quadrants whereby discrete objects moved either entirely within the left and right visual hemifields, or could cross freely between visual hemifields, thus requiring interhemispheric integration. Using the same HD-tACS montages as Helfrich et al. (2014), we found no phase-specific effect of 40 Hz stimulation on overall tracking performance. While tracking performance was generally lower during between-hemifield trials (presumably reflecting a cost of integration), this difference was unchanged by in- vs. anti-phase stimulation. Our null results could be due to a failure to reliably modulate coherence in our study, or that our task does not rely as heavily on this network of coherent gamma oscillations as other visual integration paradigms.

Intact hemisphere and corpus callosum compensate for visuomotor functions after early visual cortex damage

Unilateral damage to the primary visual cortex (V1) leads to clinical blindness in the opposite visual hemifield, yet nonconscious ability to transform unseen visual input into motor output can be retained, a condition known as "blindsight." Here we combined psychophysics, functional magnetic resonance imaging, and tractography to investigate the functional and structural properties that enable the developing brain to partly overcome the effects of early V1 lesion in one blindsight patient. Visual stimuli appeared in either the intact or blind hemifield and simple responses were given with either the left or right hand, thereby creating conditions where visual input and motor output involve the same or opposite hemisphere. When the V1-damaged hemisphere was challenged by incoming visual stimuli, or controlled manual responses to these unseen stimuli, the corpus callosum (CC) dynamically recruited areas in the visual dorsal stream and premotor cortex of the intact hemisphere to compensate for altered visuomotor functions. These compensatory changes in functional brain activity were paralleled by increased connections in posterior regions of the CC, where fibers connecting homologous areas of the parietal cortex course.

Interhemispheric Binding of Ambiguous Visual Motion Is Associated with Changes in Beta Oscillatory Activity but Not with Gamma Range Synchrony

In vision, perceptual features are processed in several regions distributed across the brain. Yet, the brain achieves a coherent perception of visual scenes and objects through integration of these features, which are encoded in spatially segregated brain areas. How the brain seamlessly achieves this accurate integration is currently unknown and is referred to as the "binding problem." Among the proposed mechanisms meant to resolve the binding problem, the binding-by-synchrony hypothesis proposes that binding is carried out by the synchronization of distant neuronal assemblies. This study aimed at providing a critical test to the binding-by-synchrony hypothesis by evaluating long-range connectivity using EEG during a motion integration visual task that entails binding across hemispheres. Our results show that large-scale perceptual binding is not associated with long-range interhemispheric gamma synchrony. However, distinct perceptual interpretations were found to correlate with changes in beta power. Increased beta activity was observed during binding under ambiguous conditions and originates mainly from parietal regions. These findings reveal that the visual experience of binding can be identified by distinct signatures of oscillatory activity, regardless of long-range gamma synchrony, suggesting that such type of synchrony does not underlie perceptual binding.

Frontal eye fields control attentional modulation of alpha and gamma oscillations in contralateral occipitoparietal cortex

Covertly directing visuospatial attention produces a frequency-specific modulation of neuronal oscillations in occipital and parietal cortices: anticipatory alpha (8-12 Hz) power decreases contralateral and increases ipsilateral to attention, whereas stimulus-induced gamma (>40 Hz) power is boosted contralaterally and attenuated ipsilaterally. These modulations must be under top-down control; however, the control mechanisms are not yet fully understood. Here we investigated the causal contribution of the human frontal eye field (FEF) by combining repetitive transcranial magnetic stimulation (TMS) with subsequent magnetoencephalography. Following inhibitory theta burst stimulation to the left FEF, right FEF, or vertex, participants performed a visual discrimination task requiring covert attention to either visual hemifield. Both left and right FEF TMS caused marked attenuation of alpha modulation in the occipitoparietal cortex. Notably, alpha modulation was consistently reduced in the hemisphere contralateral to stimulation, leaving the ipsilateral hemisphere relatively unaffected. Additionally, right FEF TMS enhanced gamma modulation in left visual cortex. Behaviorally, TMS caused a relative slowing of response times to targets contralateral to stimulation during the early task period. Our results suggest that left and right FEF are causally involved in the attentional top-down control of anticipatory alpha power in the contralateral visual system, whereas a right-hemispheric dominance seems to exist for control of stimulus-induced gamma power. These findings contrast the assumption of primarily intrahemispheric connectivity between FEF and parietal cortex, emphasizing the relevance of interhemispheric interactions. The contralaterality of effects may result from a transient functional reorganization of the dorsal attention network after inhibition of either FEF.

The fine-scale functional correlation of striate cortex in sighted and blind people

To what extent are spontaneous neural signals within striate cortex organized by vision? We examined the fine-scale pattern of striate cortex correlations within and between hemispheres in rest-state BOLD fMRI data from sighted and blind people. In the sighted, we find that corticocortico correlation is well modeled as a Gaussian point-spread function across millimeters of striate cortical surface, rather than degrees of visual angle. Blindness produces a subtle change in the pattern of fine-scale striate correlations between hemispheres. Across participants blind before the age of 18, the degree of pattern alteration covaries with the strength of long-range correlation between left striate cortex and Broca's area. This suggests that early blindness exchanges local, vision-driven pattern synchrony of the striate cortices for long-range functional correlations potentially related to cross-modal representation.

How does a surgeon's brain buzz? An EEG coherence study on the interaction between humans and robot

Introduction

In humans, both primary and non-primary motor areas are involved in the control of voluntary movements. However, the dynamics of functional coupling among different motor areas have not been fully clarified yet. There is to date no research looking to the functional dynamics in the brain of surgeons working in laparoscopy compared with those trained and working in robotic surgery.

Experimental procedures

We enrolled 16 right-handed trained surgeons and assessed changes in intra- and inter-hemispheric EEG coherence with a 32-channels device during the same motor task with either a robotic or a laparoscopic approach. Estimates of auto and coherence spectra were calculated by a fast Fourier transform algorithm implemented on Matlab 5.3.

Results

We found increase of coherence in surgeons performing laparoscopy, especially in theta and lower alpha activity, in all experimental conditions (M1 vs. SMA, S1 vs. SMA, S1 vs. pre-SMA and M1 vs. S1; p < 0.001). Conversely, an increase in inter-hemispheric coherence in upper alpha and beta band was found in surgeons using the robotic procedure (right vs. left M1, right vs. left S1, right pre-SMA vs. left M1, left pre-SMA vs. right M1; p < 0.001).

Discussion

Our data provide a semi-quantitative evaluation of dynamics in functional coupling among different cortical areas in skilled surgeons performing laparoscopy or robotic surgery. These results suggest that motor and non-motor areas are differently activated and coordinated in surgeons performing the same task with different approaches. To the best of our knowledge, this is the first study that tried to assess semi-quantitative differences during the interaction between normal human brain and robotic devices.

The visual callosal connection: a connection like any other?

Recent work about the role of visual callosal connections in ferrets and cats is reviewed, and morphological and functional homologies between the lateral intrinsic and callosal network in early visual areas are discussed. Both networks selectively link distributed neuronal groups with similar response properties, and the actions exerted by callosal input reflect the functional topography of those networks. This supports the notion that callosal connections perpetuate the function of the lateral intrahemispheric circuit onto the other hemisphere. Reversible deactivation studies indicate that the main action of visual callosal input is a multiplicative shift of responses rather than a changing response selectivity. Both the gain of that action and its excitatory-inhibitory balance seem to be dynamically adapted to the feedforward drive by the visual stimulus onto primary visual cortex. Taken together anatomical and functional evidence from corticocortical and lateral circuits further leads to the conclusion that visual callosal connections share more features with lateral intrahemispheric connections on the same hierarchical level and less with feedback connections. I propose that experimental results about the callosal circuit in early visual areas can be interpreted with respect to lateral connectivity in general.

Splenium of corpus callosum: patterns of interhemispheric interaction in children and adults

The splenium of the corpus callosum connects the posterior cortices with fibers varying in size from thin late-myelinating axons in the anterior part, predominantly connecting parietal and temporal areas, to thick early-myelinating fibers in the posterior part, linking primary and secondary visual areas. In the adult human brain, the function of the splenium in a given area is defined by the specialization of the area and implemented via excitation and/or suppression of the contralateral homotopic and heterotopic areas at the same or different level of visual hierarchy. These mechanisms are facilitated by interhemispheric synchronization of oscillatory activity, also supported by the splenium. In postnatal ontogenesis, structural MRI reveals a protracted formation of the splenium during the first two decades of human life. In doing so, the slow myelination of the splenium correlates with the formation of interhemispheric excitatory influences in the extrastriate areas and the EEG synchronization, while the gradual increase of inhibitory effects in the striate cortex is linked to the local inhibitory circuitry. Reshaping interactions between interhemispherically distributed networks under various perceptual contexts allows sparsification of responses to superfluous information from the visual environment, leading to a reduction of metabolic and structural redundancy in a child's brain.

The corpus callosum and the visual cortex: plasticity is a game for two

Throughout life, experience shapes and selects the most appropriate brain functional connectivity to adapt to a changing environment. An ideal system to study experience-dependent plasticity is the visual cortex, because visual experience can be easily manipulated. In this paper, we focus on the role of interhemispheric, transcallosal projections in experience-dependent plasticity of the visual cortex. We review data showing that deprivation of sensory experience can modify the morphology of callosal fibres, thus altering the communication between the two hemispheres. More importantly, manipulation of callosal input activity during an early critical period alters developmental maturation of functional properties in visual cortex and modifies its ability to remodel in response to experience. We also discuss recent data in rat visual cortex, demonstrating that the corpus callosum plays a role in binocularity of cortical neurons and is involved in the plastic shift of eye preference that follows a period of monocular eyelid suture (monocular deprivation) in early age. Thus, experience can modify the fine connectivity of the corpus callosum, and callosal connections represent a major pathway through which experience can mediate functional maturation and plastic rearrangements in the visual cortex.

High-frequency oscillatory response to illusory contour in typically developing boys and boys with autism spectrum disorders

Illusory contour (IC) perception, a fruitful model for studying the automatic contextual integration of local image features, can be used to investigate the putative impairment of such integration in children with autism spectrum disorders (ASD). We used the illusory Kanizsa square to test how the phase-locked (PL) gamma and beta electroencephalogram (EEG) responses of typically developing (TD) children aged 3-7 years and those with ASD were modulated by the presence of IC in the image. The PL beta and gamma activity strongly differentiated between IC and control figures in both groups of children (IC effect). However, the timing, topography, and direction of the IC effect differed in TD and ASD children. Between 40 msec and 120 msec after stimulus onset, both groups demonstrated lower power of gamma oscillations at occipital areas in response to IC than in response to the control figure. In TD children, this relative gamma suppression was followed by relatively higher parieto-occipital gamma and beta responses to IC within 120-270 msec after stimulus onset. This second stage of IC processing was absent in children with ASD. Instead, their response to IC was characterized by protracted (40-270 msec) relative reduction of gamma and beta oscillations at occipital areas. We hypothesize that children with ASD rely more heavily on lower-order processing in the primary visual areas and have atypical later stage related to higher-order processes of contour integration.

Do Horizontal Saccadic Eye Movements Increase Interhemispheric Coherence? Investigation of a Hypothesized Neural Mechanism Underlying EMDR

Series of horizontal saccadic eye movements (EMs) are known to improve episodic memory retrieval in healthy adults and to facilitate the processing of traumatic memories in eye movement desensitization and reprocessing (EMDR) therapy. Several authors have proposed that EMs achieve these effects by increasing the functional connectivity of the two brain hemispheres, but direct evidence for this proposal is lacking. The aim of this study was to investigate whether memory enhancement following bilateral EMs is associated with increased interhemispheric coherence in the electroencephalogram (EEG). Fourteen healthy young adults were asked to freely recall lists of studied neutral and emotional words after a series of bilateral EMs and a control procedure. Baseline EEG activity was recorded before and after the EM and control procedures. Phase and amplitude coherence between bilaterally homologous brain areas were calculated for six frequency bands and electrode pairs across the entire scalp. Behavioral analyses showed that participants recalled more emotional (but not neutral) words following the EM procedure than following the control procedure. However, the EEG analyses indicated no evidence that the EMs altered participants' interhemispheric coherence or that improvements in recall were correlated with such changes in coherence. These findings cast doubt on the interhemispheric interaction hypothesis, and therefore may have important implications for future research on the neurobiological mechanism underlying EMDR.

Specificity of neuronal responses in primary visual cortex is modulated by interhemispheric corticocortical input

Within the visual cortex, it has been proposed that interhemispheric interactions serve to re-establish the continuity of the visual field across its vertical meridian (VM) by mechanisms similar to those used by intrinsic connections within a hemisphere. However, other specific functions of transcallosal projections have also been proposed, including contributing to disparity tuning and depth perception. Here, we consider whether interhemispheric connections modulate specific response properties, orientation and direction selectivity, of neurons in areas 17 and 18 of the ferret by combining reversible thermal deactivation in one hemisphere with optical imaging of intrinsic signals and single-cell electrophysiology in the other hemisphere. We found interhemispheric influences on both the strength and specificity of the responses to stimulus orientation and direction of motion, predominantly at the VM. However, neurons and domains preferring cardinal contours, in particular vertical contours, seem to receive stronger interhemispheric input than others. This finding is compatible with interhemispheric connections being involved in horizontal disparity tuning. In conclusion, our results support the view that interhemispheric interactions mainly perform integrative functions similar to those of connections intrinsic to one hemisphere.

On the recording reference contribution to EEG correlation, phase synchrony, and coherence

The degree of synchronization in electroencephalography (EEG) signals is commonly characterized by the time-series measures, namely, correlation, phase synchrony, and magnitude squared coherence (MSC). However, it is now well established that the interpretation of the results from these measures are confounded by the recording reference signal and that this problem is not mitigated by the use of other EEG montages, such as bipolar and average reference. In this paper, we analyze the impact of reference signal amplitude and power on EEG signal correlation, phase synchrony, and MSC. We show that, first, when two nonreferential signals have negative correlation, the phase synchrony and the absolute value of the correlation of the two referential signals may have two regions of behavior characterized by a monotonic decrease to zero and then a monotonic increase to one as the amplitude of the reference signal varies in [0, +∞). It is notable that even a small change of the amplitude may lead to significant impact on these two measures. Second, when two nonreferential signals have positive correlation, the correlation and phase-synchrony values of the two referential signals can monotonically increase to one (or monotonically decrease to some positive value and then monotonically increase to one) as the amplitude of the reference signal varies in [0, +∞). Third, when two nonreferential signals have negative cross-power, the MSC of the two referential signals can monotonically decrease to zero and then monotonically increase to one as reference signal power varies in [0, +∞). Fourth, when two nonreferential signals have positive cross-power, the MSC of the two referential signals can monotonically increase to one as the reference signal power varies in [0, +∞). In general, the reference signal with small amplitude or power relative to the signals of interest may decrease or increase the values of correlation, phase synchrony, and MSC. However, the reference signal with high relative amplitude or power will always increase each of the three measures. In our previous paper, we developed a method to identify and extract the reference signal contribution to intracranial EEG (iEEG) recordings. In this paper, we apply this approach to referential iEEG recorded from human subjects and directly investigate the contribution of recording reference on correlation, phase synchrony, and MSC. The experimental results demonstrate the significant impact that the recording reference may have on these bivariate measures.

Duration of coherence intervals in electrical brain activity in perceptual organization

We investigated the relationship between visual experience and temporal intervals of synchronized brain activity. Using high-density scalp electroencephalography, we examined how synchronized activity depends on visual stimulus information and on individual observer sensitivity. In a perceptual grouping task, we varied the ambiguity of visual stimuli and estimated observer sensitivity to this variation. We found that durations of synchronized activity in the beta frequency band were associated with both stimulus ambiguity and sensitivity: the lower the stimulus ambiguity and the higher individual observer sensitivity the longer were the episodes of synchronized activity. Durations of synchronized activity intervals followed an extreme value distribution, indicating that they were limited by the slowest mechanism among the multiple neural mechanisms engaged in the perceptual task. Because the degree of stimulus ambiguity is (inversely) related to the amount of stimulus information, the durations of synchronous episodes reflect the amount of stimulus information processed in the task. We therefore interpreted our results as evidence that the alternating episodes of desynchronized and synchronized electrical brain activity reflect, respectively, the processing of information within local regions and the transfer of information across regions.

Dynamic interactions between the cerebral hemispheres

The cortical areas of the two hemispheres interact via the corpus callosum. This paper reviews recent findings in animals and man, showing that the visual areas of the two hemispheres control each other's dynamics. The interaction is stimulus-dependent and stimulus-specific. It consists of both excitatory and inhibitory inputs controlling the formation of synchronous neuronal assemblies across and within the hemispheres. The findings are consistent with the geometry of callosal axons and their inferred computational properties. These are the first findings to suggest a direct relationship between the geometry of cortical connections, and the formation of stimulus-driven synchronous neuronal assemblies.

A novel interhemispheric interaction: modulation of neuronal cooperativity in the visual areas

Background

The cortical representation of the visual field is split along the vertical midline, with the left and the right hemi-fields projecting to separate hemispheres. Connections between the visual areas of the two hemispheres are abundant near the representation of the visual midline. It was suggested that they re-establish the functional continuity of the visual field by controlling the dynamics of the responses in the two hemispheres.

Methods/Principal Findings

To understand if and how the interactions between the two hemispheres participate in processing visual stimuli, the synchronization of responses to identical or different moving gratings in the two hemi-fields were studied in anesthetized ferrets. The responses were recorded by multiple electrodes in the primary visual areas and the synchronization of local field potentials across the electrodes were analyzed with a recent method derived from dynamical system theory. Inactivating the visual areas of one hemisphere modulated the synchronization of the stimulus-driven activity in the other hemisphere. The modulation was stimulus-specific and was consistent with the fine morphology of callosal axons in particular with the spatio-temporal pattern of activity that axonal geometry can generate.

Conclusions/Significance

These findings describe a new kind of interaction between the cerebral hemispheres and highlight the role of axonal geometry in modulating aspects of cortical dynamics responsible for stimulus detection and/or categorization.

Dysconnection topography in schizophrenia revealed with state-space analysis of EEG

Background

The dysconnection hypothesis has been proposed to account for pathophysiological mechanisms underlying schizophrenia. Widespread structural changes suggesting abnormal connectivity in schizophrenia have been imaged. A functional counterpart of the structural maps would be the EEG synchronization maps. However, due to the limits of currently used bivariate methods, functional correlates of dysconnection are limited to the isolated measurements of synchronization between preselected pairs of EEG signals.

Methods/Results

To reveal a whole-head synchronization topography in schizophrenia, we applied a new method of multivariate synchronization analysis called S-estimator to the resting dense-array (128 channels) EEG obtained from 14 patients and 14 controls. This method determines synchronization from the embedding dimension in a state-space domain based on the theoretical consequence of the cooperative behavior of simultaneous time series—the shrinking of the state-space embedding dimension. The S-estimator imaging revealed a specific synchronization landscape in schizophrenia patients. Its main features included bilaterally increased synchronization over temporal brain regions and decreased synchronization over the postcentral/parietal region neighboring the midline. The synchronization topography was stable over the course of several months and correlated with the severity of schizophrenia symptoms. In particular, direct correlations linked positive, negative, and general psychopathological symptoms to the hyper-synchronized temporal clusters over both hemispheres. Along with these correlations, general psychopathological symptoms inversely correlated within the hypo-synchronized postcentral midline region. While being similar to the structural maps of cortical changes in schizophrenia, the S-maps go beyond the topography limits, demonstrating a novel aspect of the abnormalities of functional cooperation: namely, regionally reduced or enhanced connectivity.

Conclusion/Significance

The new method of multivariate synchronization significantly boosts the potential of EEG as an imaging technique compatible with other imaging modalities. Its application to schizophrenia research shows that schizophrenia can be explained within the concept of neural dysconnection across and within large-scale brain networks.

Functional connectivity of frontal cortex in healthy and ADHD children reflected in EEG coherence

Abnormal functional brain connectivity is a candidate factor in developmental brain disorders associated with cognitive dysfunction. We analyzed a substantial (10 min per subject) record of dense array electroencephalography with spectral power and coherence methods in attention-deficit hyperactivity disorder (ADHD) (n = 42) and control (n = 21) 10- to 13-year-old children. We found topographically distinct narrow band coherence differences between subject groups: ADHD subjects showed elevated coherence in the lower alpha (8 Hz) band and reduced coherence in the upper alpha (10-11 Hz) band. The 8-Hz ADHD elevation and a 2- to 6-Hz control group coherence elevation were independent of stimulus presentation. In response to visual stimulation, the ADHD group exhibited reduced evoked potential power and elevated frontal coherence. Only the upper alpha band control group coherence elevation discriminated according to ADHD group medication status. The findings suggest a static state of deficient connectivity in ADHD and a stimulus-induced state of overconnectivity within and between frontal hemispheres.

Cortical Activity Prior to, and During, Observation and Execution of Sequential Finger Movements

The aim of this study was to provide further evidence for the existence of a mirror neuron system in humans using electroencephalography during the observation and execution of non-object-related movements. Event-related desynchronization and synchronization (ERD/ERS) were used to characterize brain activity prior to, and during, observation and execution of a finger movement in four frequency bands (7-10 Hz, 10-13 Hz, 13-20 Hz, and 20-30 Hz). Electroencephalograms (EEGs) were recorded from 19 electrode sites in eight participants. In all the frequency bands and electrode sites, results revealed that there was no significant differences in EEG cortical activity between the observation condition and the execution conditions. Comparison of the two stages of the movement (i.e., pre-movement and movement) in the observation and execution conditions showed, in most cases, that pre-movement ERD values were less than movement ERD values. Whilst there was not an identical match of EEG cortical indices, this study provides further support for the existence of a mirror neuron system in humans. The incomplete congruence may be explained by the different behaviors, the nature of the task and factors in the observed action coded by the mirror system.

Interhemispheric integration at different spatial scales: the evidence from EEG coherence and FMRI

The early visual system processes different spatial frequencies (SFs) separately. To examine where in the brain the scale-specific information is integrated, we mapped the neural assemblies engaged in interhemispheric coupling with electroencephalographic (EEG) coherence and blood-oxygen-level dependent (BOLD) signal. During similar EEG and functional magnetic resonance imaging (fMRI) experiments, our subjects viewed centrally presented bilateral gratings of different SF (0.25-8.0 cpd), which either obeyed Gestalt grouping rules (iso-oriented, IG) or violated them (orthogonally oriented, OG). The IG stimuli (0.5-4.0 cpd) synchronized EEG at discrete beta frequencies (beta1, beta2) and increased BOLD (0.5 and 2.0 cpd tested) in ventral (around collateral sulcus) and dorsal (parieto-occipital fissure) regions compared with OG. At both SF, the beta1 coherence correlated with the ventral activations, whereas the beta2 coherence correlated with the dorsal ones. Thus distributed neural substrates mediated interhemispheric integration at single SF. The relative impact of the ventral versus dorsal networks was modulated by the SF of the stimulus.

Neural coupling binds visual tokens to moving stimuli

Spatially separated visual objects that appear in alternating sequence can be perceived as a single moving object. This phenomenon of apparent motion enables us to perceive sequentially presented images as a motion picture. How does the visual system bind together single visual tokens to one moving object? Here we report a series of experiments investigating apparent motion with electroencephalographic recordings showing that gamma-band oscillatory coupling in the visual cortex is crucial for this phenomenon. We used an ambiguous stimulus that could be perceived as moving either vertically or horizontally. Because visual information from the right and left visual hemifields is routed to the contralateral early visual cortex, in the case of perceived horizontal movement, information from both hemispheres has to be integrated. In accord with this assumption, our data show stronger oscillatory coupling between right and left visual cortices during perception of horizontal motion compared with vertical motion.

Imaging of a synchronous neuronal assembly in the human visual brain

Perception, motion, and cognition involve the formation of cooperative neuronal assemblies distributed over the cerebral cortex. It remains to explore what characterizes the assemblies, their location, and the structural substrate of assembly formation. In this EEG/fMRI study, we describe the response of the visual areas of the two hemispheres in subjects who viewed bilateral iso-oriented (IG) or orthogonally-oriented (OG) moving gratings projected in the two hemifields. The IG stimulus synchronized activity across the hemispheres, as shown by an increased EEG coherence. The increase was restricted to the occipital electrodes and to the beta band. Compared with OG, IG increased the BOLD signal in a restricted territory corresponding to area VP/V4. Within this territory, a linear relation was found between the increased interhemispheric EEG coherence and BOLD. Thus, the increased BOLD localized a trans-hemispheric, synchronous neuronal assembly probably achieved by a callosal cortico-cortical connection. This assembly might reflect an early stage of perceptual grouping since the IG stimulus conforms to Gestalt psychology principles of collinearity and common fate.

Integration of Local Features to a Global Percept by Neural Coupling

The integration of different visual attributes into the percept of a single global shape is a central aspect of object processing. In hierarchically organized stimuli with local and global levels, the attentional focus largely determines which level is processed. Here we tested the hypothesis that object processing during attention to the global aspect of the stimulus is characterized by an increased neural coupling between visual areas reflecting the integration of local features. In the present experiment, we used global letters that were constructed by smaller local letters, and a cue signaled which spatial level should be identified. On the local level, only 1 relevant letter was presented laterally in 1 visual hemifield. In contrast, the global letter extended into both hemifields, and the integration of information from both hemispheres was necessary to identify the global stimulus. Therefore, we expected an increased functional coupling between hemispheres during global processing. This hypothesis was investigated using electroencephalographic recordings and an analysis of phase locking and coherence. The results show that stimulus-locked neural coupling within the gamma band (30-40 Hz) across hemispheres in visual cortex increased for global processing after stimulus presentation and could therefore reflect the integration of local visual information.

Functional clustering in EEG photic and auditory driving in schizophrenia

The purpose of this study is to investigate the effects of photic and auditory stimuli on brain functions in schizophrenics by investigating the functional cluster (FC) of EEGs. We recorded EEGs using 16 electrodes on 10 schizophrenic patients and on 10 normal controls during photic and auditory stimuli. We estimated FC would characterize the strongly interactive brain regions among many brain regions. FC refers to the brain regions that interact much more strongly among themselves than with the rest of the brain. Brain regions that belong to the same cluster are therefore all functionally involved while, presumably, the regions that belong to separate clusters are functionally unrelated. When photic and auditory stimuli are applied, the schizophrenic patients have a very similar cluster composed of the right temporal and occipital regions for both conditions, whereas the normal controls show the normally driven information stream from the posterior areas to the prefrontal cortex. Our findings may suggest that in schizophrenics the right temporal and occipital regions strongly interact with neuronal activities not only in the resting condition but also during the stimulation condition. In addition, this strong interaction supports the abnormal brain functional connectivity and the dysfunction of the cortical structure during photic and auditory stimuli. Our study shows the existence and different pattern of FCs for normal controls and schizophrenics. Thus, FC analysis would be a potential tool to investigate the simultaneous neuronal activity of human EEGs.

Assessment of EEG synchronization based on state-space analysis

Cortical computation involves the formation of cooperative neuronal assemblies characterized by synchronous oscillatory activity. A traditional method for the identification of synchronous neuronal assemblies has been the coherence analysis of EEG signals. Here, we suggest a new method called S estimator, whereby cortical synchrony is defined from the embedding dimension in a state-space. We first validated the method on clusters of chaotic coupled oscillators and compared its performance to that of other methods for assessing synchronization. Then nine adult subjects were studied with high-density EEG recordings, while they viewed in the two hemifields (hence with separate hemispheres) identical sinusoidal gratings either arranged collinearly and moving together, or orthogonally oriented and moving at 90 degrees . The estimated synchronization increased with the collinear gratings over a cluster of occipital electrodes spanning both hemispheres, whereas over temporo-parietal regions of both hemispheres, it decreased with the same stimulus and it increased with the orthogonal gratings. Separate calculations for different EEG frequencies showed that the occipital clusters involved synchronization in the beta band and the temporal clusters in the alpha band. The gamma band appeared to be insensitive to stimulus diversity. Different stimulus configurations, therefore, appear to cause a complex rearrangement of synchronous neuronal assemblies distributed over the cortex, in particular over the visual cortex.

Left hemisphere specialization for rapid temporal processing: a study with auditory 40Hz steady-state responses

OBJECTIVE

To investigate rapid temporal processing in the auditory cortex by using auditory 40 Hz steady-state responses (SSRs).

METHODS

A 40 Hz tone-burst at 500 Hz spectral frequency was presented monaurally to record SSRs in 10 normal subjects. The recording electrodes were placed over C1, C2, C3, C4, C5, C6, T3, T4, Fz, Cz and Pz, referring to an electrode at the 7th cervical spinous process. For comparison, unstimulated SSRs were recorded. A total of 200 responses of 1s epoch were averaged and subjected to discrete fast Fourier transforms to yield the amplitude and phase of the 40 Hz component. The coherence (Coh) values of the 40 Hz component between homologous electrodes were also calculated.

RESULTS

At the temporal electrodes contralateral to the stimulated ear, the amplitude was significantly larger and its phase was significantly smaller than those of the ipsilateral side. The interhemispheric Coh between T3 and T4 in response to right ear stimulation was significantly greater than those of left ear stimulation or the unstimulated condition.

CONCLUSIONS

Our results suggest that 40 Hz auditory information is predominantly processed in the left auditory cortex, interacting with the right hemisphere. This finding is consistent with the fact that the left auditory cortex plays an important role in rapid temporal processing.

SIGNIFICANCE

Auditory 40 Hz SSRs with Coh analysis are useful for investigating the left hemisphere specialization for rapid temporal processing.

Detection of very high correlation in the alpha band between temporal regions of the human brain using MEG

It is generally believed that alpha band (8-12 Hz) electric and magnetic activity in the area of the left and right temporal regions in the human brain are at best poorly correlated. There are no previous reports of very high alpha band correlation between left and right temporal regions by magnetoencephalography (MEG) or electroencephalography (EEG). We present whole head magnetoencephalography (MEG) results that demonstrate that, for temporal channels in the majority of healthy subjects tested, the alpha band signals are highly to very highly correlated and are antiparallel in direction. A correlation as high as -0.97 was found for a limited time in one subject. We suggest that the correlation found may be the consequence of strong direct or indirect coupling between homologue areas in left and right temporal regions rather than a common source. The correlation may provide a valuable index of loss of connectivity in the brain due to disease as well providing valuable insight to brain function and deserves further investigation.

BCI Competition 2003—Data Set Ia: Combining Gamma-Band Power With Slow Cortical Potentials to Improve Single-Trial Classification of Electroencephalographic Signals

In one type of brain-computer interface (BCI), users self-modulate brain activity as detected by electroencephalography (EEG). To infer user intent, EEG signals are classified by algorithms which typically use only one of the several types of information available in these signals. One such BCI uses slow cortical potential (SCP) measures to classify single trials. We complemented these measures with estimates of high-frequency (gamma-band) activity, which has been associated with attentional and intentional states. Using a simple linear classifier, we obtained significantly greater classification accuracy using both types of information from the same recording epochs compared to using SCPs alone.

Interhemispheric functional synchronization at the first step of visual information processing in humans

OBJECTIVE

We examined the interhemispheric functional synchronization of the visual cortex using coherence (Coh) analysis.

METHODS

Achromatic or isoluminant chromatic sinusoidal grating stimuli were presented to each hemifield at a rate of 8 reversals/s to record steady-state visual-evoked potentials (S-VEPs) in 10 healthy subjects. Four recording electrodes were placed at O1, O2, P3 and P4, referred to an electrode at Cz. A total of 50 responses of 1 s epoch were averaged, and were subjected to discrete fast Fourier transforms to yield the amplitude and phase of the 8 Hz component. Ordinary and partial Coh values were also calculated.

RESULTS

For both achromatic and chromatic stimuli, the 8 Hz amplitudes of O1 and O2 were significantly larger than those of P3 and P4 without any significant difference between O1 and O2. The phase lag between O1 and O2 was approximately 30 degrees (latency shift 10.4 ms). Partial Coh between O1 and O2 at 8 Hz was significantly greater than that of the unstimulated condition, and this was only observed at 8 Hz.

CONCLUSIONS

These results suggest that interhemispheric synchronization in the occipital area occurs despite the nature of the visual stimuli. Therefore, the activation of interhemispheric connection is important for the early stage of the visual information processing.

SIGNIFICANCE

Our results indicate that the first step of the visual information processing requires interhemispheric functional synchronization.

Oscillatory synchrony and human visual cognition

Oscillatory synchrony could be used to establish dynamic links between the various cortical areas participating in the same cognitive process. Is it possible to detect oscillatory synchrony in humans, and is it relevant to behavior? There is now converging evidence for the existence of a transient oscillatory activity in the gamma range (30-60 Hz), obtained in response to static visual objects, and having only a loose temporal relationship to stimulus onset. This so-called "induced" gamma response is much larger in response to coherent static or moving objects. However, functional variations of gamma and/or beta (15-20 Hz) oscillations are not restricted to perceptive, bottom-up mechanisms, but are also observed during visual imagery or short-term memory maintenance. Oscillations at the scalp level thus seem to reflect large-scale neural cooperativity in a variety of task-dependent networks. Human intra-cranial recordings in a short-term memory paradigm further reveal the existence and the task-dependency of oscillatory synchrony in the beta range, between focal sites separated by several centimeters and with a few milliseconds time-lag. These findings thus confirm experimentally the hypothesis of a functional role of synchronized oscillatory activity in the coordination of distributed neural activity in humans, and support Hebb's concept of short-term memory maintenance by reentrant activity within the activated network. In addition, the intra-cranial data obtained in humans and monkeys also help to better understand the neural mechanisms generating scalp-recorded oscillations.

Hemispheric laterality and dimensional complexity in schizophrenia under sound and light stimulation

The aim of the present study is to investigate the relationship between linear and non-linear activities in human electroencephalograms (EEGs) by examining the linear lateral asymmetry index and the correlation dimension as a non-linear measure of complexity and to typify the characteristics of EEGs between schizophrenic patients and normal controls. We recorded the EEG from 16 electrodes in 10 schizophrenics (6 males and 4 females) and 10 age-matched normal controls (10 males), and calculated their asymmetry indices. The asymmetry index shows which hemispheric activity is dominant through examination of interhemispheric pairs in the frequency domain with EEGs between two regions. We also estimated correlation dimension. Remarkably, lower dimensional complexities appeared on the brain regions, which had significantly lower brain activity, as determined by a lateral asymmetry analysis, in schizophrenics before sound and light (SL) stimulation. We may suggest the possibility of co-varying of both linear and non-linear properties. This co-varying phenomenon maintained after the SL stimulation. Furthermore, schizophrenic patients revealed opposite asymmetric patterns compared to normal controls, as well as reversal phenomena and abnormalities in the left frontal region when SL stimuli were applied.

Electrophysiological properties and input-output organization of callosal neurons in cat association cortex

Intracellular recordings from association cortical areas 5 and 7 were performed in cats under barbiturate or ketamine-xylazine anesthesia to investigate the activities of different classes of neurons involved in callosal pathways, which were electrophysiologically characterized by depolarizing current steps. Excitatory postsynaptic potentials (EPSPs), inhibitory postsynaptic potentials (IPSPs), and/or antidromic responses were elicited by stimulating homotopic sites in the contralateral cortical areas. Differential features of EPSPs related to latencies, amplitudes, and slopes were detected in closely located (50 microm or less) neurons recorded in succession along the same electrode track. In contrast to synchronous thalamocortical volleys that excited most neurons within a cortical column, stimuli applied to homotopic sites in the contralateral cortex activated neurons at restricted cortical depths. Median latencies of callosally evoked EPSPs were 1.5 to 4 ms in various cortical cell-classes. Fast-rhythmic-bursting neurons displayed EPSPs whose amplitudes were threefold larger, and latencies two- or threefold shorter, than those found in the three other cellular classes. Converging callosal and thalamic inputs were recorded in the same cortical neuron. EPSPs or IPSPs were elicited by stimulating foci spaced by <1 mm in the contralateral cortex. In the overwhelming majority of neurons, latencies of antidromic responses were between 1.2 and 3.1 ms; however, some callosal neurons had much longer latencies, <or=18.5 ms. Some neurons were excited monosynaptically through the callosal pathway and identified antidromically from appropriate thalamic nuclei, thus revealing a callosal-corticothalamic pathway. Data are discussed in relation to the commissural spread of fast and slow normal oscillations as well as paroxysmal activities.

Schizophrenia, neurodevelopment and corpus callosum

The Zeitgeist favors an interpretation of schizophrenia as a condition of abnormal connectivity of cortical neurons, particularly in the prefrontal and temporal cortex. The available evidence points to reduced connectivity, a possible consequence of excessive synaptic pruning in development. A decreased thalamic input to the cerebral cortex appears likely, and developmental studies predict that this decrease should entail a secondary loss of both long- and short-range cortico-cortical connections, including connections between the hemispheres. Indeed, morphological, electrophysiological and neuropsychological studies over the last two decades suggest that the callosal connections are altered in schizophrenics. However, the alterations are subtle and sometimes inconsistent across studies, and need to be investigated further with new methodologies.

Effect of volatile anesthetics on interhemispheric EEG cross-approximate entropy in the rat

Dissimilarity of the electroencephalogram (EEG) between the two hemispheres was characterized by cross-approximate entropy (C-ApEn), an information statistical parameter applicable to nonlinear, aperiodic signals. EEGs were recorded bipolarly with pairs of epidural electrodes in the left and right frontal cortices. The signals were filtered for 1-100 Hz and digitized at 200 Hz. Inhaled anesthetic concentration was varied between 0.3 and 2.1% with 45-min equilibration periods while the rats were breathing spontaneously. Anesthetics produced concentration-dependent changes in C-ApEn. A greater dynamic range of C-ApEn was obtained by reducing the epoch length from 2 s to 100 ms. At a 0.4% inspired agent concentration halothane caused an increase of C-ApEn, whereas isoflurane did not. When the inspired concentrations of both agents were greater than 0.4%, C-ApEn dose-dependently decreased as agent concentrations increased. Isoflurane depressed C-ApEn more than did halothane at all equivalent minimum alveolar concentration levels, but the two agents became equipotent at 1.5% inspired concentration. C-ApEn fell below the awake baseline at 0.8% anesthetic concentration that also abolished the righting reflex. C-ApEn increased after high-pass filtering (>20 Hz) and decreased after-low pass filtering (<20 Hz) of the digitized EEG; the anesthetic dependence of C-ApEn was diminished by both filters. The results suggest that C-ApEn of bihemispheric EEG is a sensitive, agent-specific correlate of anesthetics' central effect.

Functional activation of microgyric visual cortex in a human

We report on the case of a 20-year-old man with bilateral parasagittal parieto-occipital polymicrogyria and epilepsy. Functional magnetic resonance imaging responses to reversing checkerboard and interhemispheric electroencephalogram coherence changes to moving gratings were investigated. Results of both studies indicate that the polymicrogyric cortex was activated by visual stimuli, suggesting preserved function in the dysplastic area.

EEG coherence studies in the normal brain and after early-onset cortical pathologies

Visual corpus callosum (CC) preferentially interconnects neurons selective for similar stimulus orientation near the representations of the vertical meridian. These properties allow studying the CC functionality with EEG coherence analysis. Iso-oriented and orthogonally-oriented gratings were presented to the two hemifields, either close to the vertical meridian or far from it. In animals with intact CC, and in man, interhemispheric coherence (ICoh) increased only with iso-oriented gratings presented near or crossing the vertical meridian. The increase was localized to occipital electrodes and was specific for the beta-gamma frequency band. Visual-stimulus induced changes in ICoh were studied in patients with early pathologies of the visual areas. From a girl with abnormal vision and severe bilateral lesion of the primary visual areas at 3 weeks, after premature birth at 30 weeks, we obtained no ICoh response until 9 years. In control children visual stimulation increased occipital ICoh at 6-7 years. From a young man having suffered similar lesions when he was 9 months older than the girl, no consistent increase in ICoh could be obtained. In a 14-year-old girl with congenital visual agnosia, no visible lesions, but with a temporal-occipital epileptic focus, ICoh responses were evoked both by iso-oriented, and by orthogonally-oriented gratings. In a young man with bilateral parieto-occipital microgyria extending into the calcarine sulcus, visual stimuli increased ICoh as in normal individuals, but the response was weaker. These cases are discussed in terms of development of CC connections and point to a variety of plastic changes in the cortical connectivity of children.