Interhemispheric EEG coherence during photic stimulation: sex differences in normal young adults

Individual EEG alpha profiles are gender-dependent and indicate subjective experiences in Whole-Body Perceptual Deprivation

We use a unique environment of Whole Body Perceptual Deprivation (WBPD) to induce an altered state of consciousness (ASC) in our participants, and employ online EEG recording. We present individual EEG alpha profiles, and show how these data can be analyzed at the individual level. Our goal is to investigate to what degree subjective experience matches EEG alpha profile, and in particular, the various alpha hemispheric asymmetries observed in the frontal, parietal, and occipital lobes. Specifically, we consider positive (frontal L < R) or negative (frontal L > R) affect; a more verbal (L > R) or a more imagistic (R > L) mode of thinking; and a more trancelike (frontal > parietal) or more reflective (frontal < parietal) state of consciousness. Our results indicate that the individual alpha profiles are reflected in individual differences in subjective experience. However, the alpha profiles are confounded with the gender of the participant. Specifically, there is a predominant R > L asymmetry found for male participants, and a predominant L > R asymmetry found for female participants.

Testing Mean Differences among Groups: Multivariate and Repeated Measures Analysis with Minimal Assumptions

To date, there is a lack of satisfactory inferential techniques for the analysis of multivariate data in factorial designs, when only minimal assumptions on the data can be made. Presently available methods are limited to very particular study designs or assume either multivariate normality or equal covariance matrices across groups, or they do not allow for an assessment of the interaction effects across within-subjects and between-subjects variables. We propose and methodologically validate a parametric bootstrap approach that does not suffer from any of the above limitations, and thus provides a rather general and comprehensive methodological route to inference for multivariate and repeated measures data. As an example application, we consider data from two different Alzheimer's disease (AD) examination modalities that may be used for precise and early diagnosis, namely, single-photon emission computed tomography (SPECT) and electroencephalogram (EEG). These data violate the assumptions of classical multivariate methods, and indeed classical methods would not have yielded the same conclusions with regards to some of the factors involved.

Sex Differences in Simple Reaction Tasks

Many anatomical and brain mapping studies show a higher bilateral symmetry of female brains and a higher asymmetry of male brains so correlations between simple visual and auditory, left- and right-hand reaction times were examined for such sex differences. 20 healthy women and 20 men were tested in two sessions. For women all six response times correlated with each other significantly in Session A, but in Session B there were only two significant interhemispheric correlations. This represented different changes in visual and auditory reaction times between Sessions A and B. Men showed the same pattern in both sessions: a significant correlation between the interhemispheric visual reaction times and one between auditory reaction times. Women showed a total correlation pattern in Session A and an interhemispheric correlation pattern in Session B. This was interpreted as a transition between a holistic information-processing strategy in Session A and an analytic strategy in Session B. Men showed an analytic strategy in both sessions.

Reduced interhemispheric connectivity in childhood autism detected by electroencephalographic photic driving coherence

The EEG coherence among 14 scalp points during intermittent photic stimulation at 11 fixed frequencies of 3-24 Hz was studied in 14 boys with autism, aged 6-14 years, with relatively intact verbal and intellectual functions, and 19 normally developing boys. The number of interhemispheric coherent connections pertaining to the 20 highest connections of each individual was significantly lower in autistic patients than in the control group at all the EEG beta frequencies corresponding to those of stimulation. The coefficient of coherence values between homologous occipital, parietal and central areas at the same frequencies were also lower in the autistic group in both mono- and bipolar montages due to a deficit in reactive photic driving increase. No differences between the groups were observed in the spontaneous EEG.

Autism, lateralisation, and handedness: a review of the literature and meta-analysis

A number of recent investigators have hypothesised a link between autism, left-handedness, and brain laterality. Their findings have varied widely, in part because these studies have relied on different methodologies and definitions. We conducted a systematic review and meta-analysis to assess the literature, with the hypothesis that there would be an association between autism and laterality that would be moderated by handedness, sex, age, brain region studied, and level of autism. From a broad search resulting in 259 papers, 54 were identified for inclusion in the literature review. This list was narrowed further to include only studies reporting results in the inferior frontal gyrus for meta-analysis, resulting in four papers. The meta-analysis found a moderate but non-significant effect size of group on lateralisation, suggesting a decrease in strength of lateralisation in the autistic group, a trend supported by the literature review. A subgroup analysis of sex and a meta-regression of handedness showed that these moderating variables did not have a significant effect on this relationship. Although the results are not conclusive, there appears to be a trend towards a relationship between autism and lateralisation. However, more rigorous studies with better controls and clearer reporting of definitions and results are needed.

Cognition is related to resting-state small-world network topology: an magnetoencephalographic study

Brain networks and cognition have recently begun to attract attention: studies suggest that more efficiently wired resting-state brain networks are indeed correlated with better cognitive performance. "Small-world" brain networks combine local segregation with global integration, hereby subserving information processing. Furthermore, recent studies implicate that gender effects may be present in both network dynamics and its correlations with cognition. This study reports on the relation between resting-state functional brain topology with overall and domain-specific cognitive performance in healthy participants and possible gender differences herein. Healthy participants underwent neuropsychological tests, of which individual scores were converted to z-scores. Network analysis was performed on resting-state, eyes-closed magnetoencephalography (MEG) data, after determining functional connectivity between each pair of sensors. The clustering coefficient (local specialization), average path length (overall integration and efficiency) and "small-world index" (i.e. ratio between clustering and path length) were calculated in six frequency bands. 14 male and 14 female participants were included. Better total cognitive performance was related to increased local connectivity in the theta band, higher clustering coefficient (in delta and theta bands) and higher small-worldness (in theta and lower gamma bands). Women showed less clustering and shorter path length in the delta band. There were no significant correlations between network topology and cognitive functioning in females. In contrast, higher cognitive scores in men were associated with increased theta band clustering and small-worldness. These results provide further evidence for the value of functional brain network topology for cognitive functioning and suggest that gender is an important factor in this respect.

The lesioned brain: still a small-world?

The intra-arterial amobarbital procedure (IAP or Wada test) is used to determine language lateralization and contralateral memory functioning in patients eligible for neurosurgery because of pharmaco-resistant epilepsy. During unilateral sedation, functioning of the contralateral hemisphere is assessed by means of neuropsychological tests. We use the IAP as a reversible model for the effect of lesions on brain network topology. Three artifact-free epochs (4096 samples) were selected from each electroencephalogram record before and after amobarbital injection. Functional connectivity was assessed by means of the synchronization likelihood. The resulting functional connectivity matrices were constructed for all six epochs per patient in four frequency bands, and weighted network analysis was performed. The clustering coefficient, average path length, small-world index, and edge weight correlation were calculated. Recordings of 33 patients were available. Network topology changed significantly after amobarbital injection: clustering decreased in all frequency bands, while path length decreased in the theta and lower alpha band, indicating a shift toward a more random network topology. Likewise, the edge weight correlation decreased after injection of amobarbital in the theta and beta bands. Network characteristics after injection of amobarbital were correlated with memory score: higher theta band small-world index and increased upper alpha path length were related to better memory score. The whole-brain network topology in patients eligible for epilepsy surgery becomes more random and less optimally organized after selective sedation of one hemisphere, as has been reported in studies with brain tumor patients. Furthermore, memory functioning after injection seems related to network topology, indicating that functional performance is related to topological network properties of the brain.

Interhemispheric asymmetry in EEG photic driving coherence in childhood autism

OBJECTIVE

Examination of the EEG photic driving coherence during intermittent photic stimulation in autistic patients with relatively intact verbal and intellectual functions in order to enhance the likely latent interhemispheric asymmetry in neural connectivity.

METHODS

Fourteen autistic boys, aged 6-14years, free of drug treatment, with I.Q. 91.4+/-22.8, and 19 normally developing boys were subject to stimulation of 12 fixed frequencies of 3-27Hz. The number of high coherent connections (HCC) (coherence >0.6-0.8) was estimated among 7 leads in each hemisphere.

RESULTS

In contrast to the spectral characteristics showing the right hemisphere deficit in the photic driving reactivity, the number of HCC differentiated the groups only in the left hemisphere where it was higher in autistics at the EEG frequencies corresponding to those of stimulation at 6-27Hz without asymmetry at other frequencies, the left-side prevalence increasing with frequency. No asymmetry was observed in the resting state.

CONCLUSIONS

Spectral and coherence characteristics of the EEG photic driving show different aspects of latent abnormal interhemispheric asymmetry in autistics: the right hemisphere "hyporeactivity" and potential "hyperconectivity" of likely compensatory nature in the left hemisphere.

SIGNIFICANCE

The EEG photic driving can reveal functional topographic alterations not present in the spontaneous EEG.

Influence of sinusoidally modulated visual stimuli at extremely low frequency range on the human EEG activity

The purpose of this study was to investigate whether sinusoidally modulated visual stimuli at extremely low frequencies (ELF) of 50, 16.66, 13, 10, 8.33 and 4Hz could influence the changes in EEG activity in 33 human subjects. An improved design of visual stimulator system has addressed an issue of electrical interference from electrical signals driven by LED arrays onto simultaneously recorded EEG. A comparison between 1 and 3-Way ANOVA was performed in order to evaluate whether visual stimuli at ELFs could influence the EEG in humans to compliment the currently active medical research in seasonal affective disorder (SAD) and photic driving. The results revealed that under evaluation of 1 and 3-Way repeated-measures ANOVA tests, the Theta, Alpha2 and Gamma EEG bands exhibited a common significant difference between ELF visual stimuli.

Trait affect moderates cortical activation in response to state affect

To examine the influence of trait affect on cortical activation during state affect, 32 healthy female participants viewed video clips to induce positive and negative emotional states. Frontal and parietal electroencephalogram (EEG) of the alpha band was assessed while participants imagined the scene of each video that generated the most intense emotions. The results showed that affect-induced frontal asymmetry was moderated by negative trait affect. The difference in frontal asymmetry between positive and negative state affect increased linearly with negative trait affect. Moreover, positive trait affect was negatively correlated with activation in the right-parietal brain region. These results suggest that trait affect moderates the effects of state affect on brain activation.

Spatiotemporal characteristics of MEG and EEG entrainment with photic stimulation in schizophrenia

We examined the periodic synchronous characteristic response to photic stimulation in schizophrenia using electroencephalography (EEG) and magnetoencephalography (MEG). We tested whether neural synchronization deficits were present in subjects with schizophrenia using photic stimulation to evaluate the frequency entrainment in 18 normal subjects and 19 schizophrenia patients. A conventional vertical-type 160-channel MEG (PQ1160C, Yokogawa Electric Corporation) was used. Photic stimulation was at frequencies from 8 to 10.5 Hz at intervals of 0.5 Hz. There were ten stimuli at each frequency, and each lasted 10 seconds. The power spectrum at each site was based on the international 10/20 derivation. The power spectrum in schizophrenia patients was smaller than that in normal subjects at each site. A gender difference was observed in normal subjects, but not in schizophrenia patients. MEG, like EEG, is an effective method for research on neuropathy of the psyche.

Attention modulates hemispheric differences in functional connectivity: evidence from MEG recordings

The present study examined intrahemispheric functional connectivity during rest and dichotic listening in 8 male and 9 female healthy young adults measured with magnetoencephalography (MEG). Generalized synchronization within the separate hemispheres was estimated by means of the synchronization likelihood that is sensitive to linear as well as non-linear coupling of MEG signals. We found higher functional intrahemispheric connectivity of frontal and temporal areas within the right as compared to the left hemisphere in the lower and higher theta band during rest and in the lower theta band during dichotic listening. In addition, higher synchronization in the lower theta band correlated with better task performance. In the upper alpha band, hemispheric differences in intrahemispheric connectivity of the frontal regions were found to be modulated by focused attention instructions. That is, attention to the right ear exaggerates the pattern of higher synchronization likelihood for the right frontal region, while attention to the left ear has an opposite effect. We found higher intrahemispheric connectivity in males compared to females as shown by higher synchronization in the lower alpha band. Taken together, our results reflect a physiological basis for functional hemispheric laterality and support the general assumption of sex differences in brain organization. Furthermore, in addition to studies that show that controlled attention processes modulate activation of the frontal areas, our study indicates that attention modulates ipsilateral functional connectivity in the frontal areas. This supports the idea of a supervisory role for the frontal cortex in attention processes.

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.

Adjusting EEG coherence for inter-electrode distance effects: an exploration in normal children

Electroencephalographic (EEG) coherence between two points is strongly related to the distance between them, being inflated by volume conduction effects at short distances and reduced by signal phase differences at larger distances. This precludes simple comparison of coherence estimates involving different inter-electrode distances. We investigated adjusting coherence measures to remove such distance effects. After subtracting the estimated effects of random coherence due to volume conduction, exponential regression of the reduced coherence values against measured inter-electrode distance was used to estimate the remaining effects of inter-electrode distance. Residuals from this procedure were taken as coherences corrected for the systematic distance effects. These were adjusted to the mean reduced coherence level to avoid complexities in conceptualising negative residual coherences. It was found that systematic inter-electrode distance effects accounted for more than 50% of the variance remaining after removal of random coherence estimates. After these were also removed, substantial effects of EEG frequency band, different regions of the brain, and interhemispheric versus intrahemispheric values, as well as laterality effects within the latter, were obtained. Regional and frequency differences in adjusted coherence appear to reflect patterns expected from normal cortical development, but detailed understanding of more complex interactive effects is limited by the lack of relevant developmental data. Adjusting coherence values to remove systematic variability due to inter-electrode distances better represents cortico-cortical coupling and allows more efficient statistical analysis. This may contribute towards a better integration of coherence data in the EEG exploration of both normal and atypical brain functioning.

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.

Frequency dependence and gender effects in visual cortical regions involved in temporal frequency dependent pattern processing

Neural response to flickering stimuli has been shown to be frequency dependent in the primary visual cortex. Controversial gender differences in blood oxygen level dependent (BOLD) amplitude upon 6 and 8 Hz visual stimulation have been reported. In order to analyze frequency and gender effects in early visual processing we employed a passive graded task paradigm with a dartboard stimulus combining eight temporal frequencies from 0 to 22 Hz in one run. Activation maps were calculated within Statistical Parametric Mapping, and BOLD amplitudes were estimated for each frequency within the striate and extrastriate visual cortex. The BOLD amplitude was found to steadily rise up to 8 Hz in BA 17 and 18 with an activation plateau at higher frequencies. In addition, we observed a laterality effect in the striate cortex with higher BOLD contrasts in the right hemisphere in men and in women. BOLD response rises similarly in men and women up to 8 Hz but with lower amplitudes in women at 4, 8, and 12 Hz (30% lower). No frequency effect above 1 Hz was found in the extrastriate visual cortex. There was also a regional specific gender difference. Men activated more in the right lingual gyrus (BA 18) and the right cerebellum compared with women, whereas women showed more activation in the right inferior temporal gyrus (BA 17). The study indicates that frequency dependent processing at the cortical level is limited to the striate cortex and may be associated with a more global information processing (right hemisphere dominance), particularly in men. The finding of significantly lower BOLD amplitudes in women despite previously shown larger VEP (visual evoked potential) amplitudes might suggest gender differences in cerebral hemodynamics (baseline rCBV, rCBF, or neurovascular coupling). The regional distinction points at additional differences in psychological processing even when using a simple visual stimulus.

Sex differences in EEG coherence during a verbal memory task in normal adults

Coherence analysis was applied to the EEG of 15 female and 15 male subjects who had to memorise dichotically presented lists of concrete nouns. The EEG was recorded from 16 scalp electrodes placed in accordance with the 10/20 system. The results show significant gender-related differences in total coherence reactivity due to a greater increase of rest to task coherence in female than in male subjects. Women differed by showing higher coherence reactivity in the right hemisphere for all analysed (theta 1, theta 2, alpha 1, and alpha 2) frequency bands. They also had more extensive task-induced increases of interhemispheric coherence in theta bands in comparison with single changes in male subjects. In women these changes were mainly between the frontal electrodes of the left hemisphere paired with posterior electrodes of the other hemisphere. These findings indicate sex-related differences in functional cortical organisation during verbal memory tasks.