EEG coherence, lateral preference and schizophrenia

The EEG synchrony between hemispheres during cognitive activity differs from that during rest. With common reference recording and a visual imagery task it increases in healthy right-handers and neurotic patients, and it decreases in healthy left-handers and schizophrenic patients. It is suggested that this implies a less lateralized brain organization in schizophrenia which may account for the often reported association with left-handedness. When associated with impaired corpus callosum transmission, it may contribute to the disturbed behaviour and thought processes in this condition.     

Ocular artifacts in EEG and event-related potentials I: Scalp topography

Summary The ocular artifacts that contaminate the EEG derive from the potential difference between the cornea and the fundus of the eye. This corneofundal or corneoretinal potential can be considered as an equivalent dipole with its positive pole directed toward the cornea. The cornea shows a steady DC potential of approximately +13 mV relative to the forehead. Blink potentials are caused by the eyelids sliding down over the positively charged cornea. The artifacts from eye-movements result from changes in orientation of the corneo-fundal potential. The scalp-distribution of the ocular artifacts can be described in terms of propagation factors — the fraction of the EOG signal at periocular electrodes that is recorded at a particular scalp location. These factors vary with the location of the scalp electrode. Propagation factors for blinks and upward eye-movements are significantly different.This research was supported by a grant from the Medical Research Council of Canada (MA5465). Adrian Kellett helped with technical aspects of the recording.     

Cortical networks generating movement-related EEG rhythms in Alzheimer's disease: an EEG coherence study

Patients with mild Alzheimer's disease (AD) present with abnormally strong values of frontal and ipsilateral central sensorimotor rhythms. The authors tested 2 working hypotheses of the related electroencephalographic (EEG) coherence: disconnection, defined as a sign of a reduced coordination within the frontoparietal and interhemispheric networks, and cooperation, defined as a reflection of the reorganization of the brain sensorimotor networks. Results showed that, compared with healthy controls, patients with mild AD had an unreactive and abnormally low interhemispheric EEG coherence and an unreactive and abnormally high frontoparietal EEG coherence. These findings support the hypothesis of an impaired mechanism of sensorimotor cortical coupling (disconnection) in mild AD.     

Correlation and coherence analysis of the EEG: a selective tutorial review

The measurement of EEG-brain function relationship using measures of EEG amplitude are well known. The application of signal correlation techniques to this problem are less well understood. Selective aspects of EEG signal correlation are described with examples, and their relevance to EEG measurement is discussed.     

EEG correlates of hypnotic susceptibility and hypnotic trance: spectral analysis and coherence

EEG was recorded monopolarly at frontal (F3, F4), central (C3, C4) and occipital (O1, O2) derivations during A-B-A conditions of waking rest, hypnosis (rest, arm immobilization, mosquito hallucination, hypnotic dream), and waking rest. Stringently screened on several measures of hypnotic susceptibility, 12 very low hypnotizable and 12 very highly hypnotizable, right-handed undergraduate, subjects participated in one session. Evaluations were Fast-Fourier spectral analysis, EEG coherence between selected derivations and maximum spectral power within EEG bands. In eyes open and closed conditions in waking and hypnosis, highly hypnotizable subjects generated substantially more mean theta power than did low hypnotizable subjects at all occipital, central and frontal locations in almost all conditions of waking and hypnosis, with a larger difference in frontal locations. Both low and high hypnotizables showed increased mean theta power in hypnosis, suggesting an intensification of attentional processes and imagery enhancement. Mean alpha power was never a predictor of hypnotic susceptibility. Interactions with hypnotic susceptibility showed that highly susceptible subjects had more beta activity in the left than right hemispheres, while low susceptible subjects showed only weak asymmetry. No main effects for or interactions between waking/hypnosis and hypnotic level were found for coherence between derivations or maximum spectral power within theta, alpha and beta EEG bands.     

Dependence of coherence measurements on EEG derivation type

The impact is reported of different EEG derivation types on short-term changes in the inter-hemispheric coherence between the left and right sensorimotor areas, during the planning and execution of right index finger movements. Data are recorded during an event-related paradigm in which cued index finger movements are made. Event-related coherence analysis is then applied to the monopolar (nose reference) data, as well as source derivation. The results show that inter-hemispheric coherence between sensorimotor areas is dependent on the EEG derivation type. An increase in coherence during movement is found with nose reference and bipolar data, whereas for local average reference and source derivations, low inter-hemispheric coherence is observed, with no change in the coherence during movement. It is concluded that the coherence increase seen with nose, reference data is due to an indirecteeffect of mu rhythm desynchronisation, rather than any increase in synchrony of the mu rhythms themselves. Local average reference and source derivations better reflect the activity of the underlying cortical structures (the mu generating networks), and coherence analysis using these derivations shows that the mu rhythms of left and right hemispheres are not coherent.     

EEG coherence in early-blind humans during sound localization

Human blind individuals have demonstrated cross-modal plasticity in research over the past decade. In one such study, we showed that early-blind subjects were able to localize sound sources accurately despite the lack of visual input for the calibration of their auditory space. A further ERP study with these subjects also revealed N1 and P3 components during a sound localization task to be more posteriorly distributed than for sighted controls, indicating an involvement of posterior regions in sound localization for blind subjects not present for sighted subjects. In the current study, we analyzed these data for EEG power and coherence in theta, alpha, beta, and gamma frequency bands to see whether blind individuals would show increased coherence reflecting increased connectivity between the central and posterior cortical regions. Blind and sighted subjects did not differ with respect to overall EEG power in any frequency range. However, EEG coherence was significantly increased in blind subjects compared to sighted in the theta, alpha, and beta frequency bands. These results have implications for cortical plasticity affected by sensory deprivation in humans.     

Music increases frontal EEG coherence during verbal learning

Anecdotal and some empirical evidence suggests that music can enhance learning and memory. However, the mechanisms by which music modulates the neural activity associated with learning and memory remain largely unexplored. We evaluated coherent frontal oscillations in the electroencephalogram (EEG) while subjects were engaged in a modified version of Rey's Auditory Verbal Learning Test (AVLT). Subjects heard either a spoken version of the AVLT or the conventional AVLT word list sung. Learning-related changes in coherence (LRCC) were measured by comparing the EEG during word encoding on correctly recalled trials to the immediately preceding trial on which the same word was not recalled. There were no significant changes in coherence associated with conventional verbal learning. However, musical verbal learning was associated with increased coherence within and between left and right frontal areas in theta, alpha, and gamma frequency bands. It is unlikely that the different patterns of LRCC reflect general performance differences; the groups exhibited similar learning performance. The results suggest that verbal learning with a musical template strengthens coherent oscillations in frontal cortical networks involved in verbal encoding.     

Investigation of bilateral synchronous spike-wave discharge by EEG topography

Summary To investigate the generator mechanism of the generalized discharge in childhood epilepsy, we studied its voltage field distribution using linked ear reference (L+R), common averaged reference (CA), and source derivation methods (SD). Two cases of epilepsy with clinical lapse of consciousness, and diffuse EEG spike-wave burst were divided into primary bilateral synchrony (PBS) and secondary bilateral synchrony (SBS), using coherence-phase analysis. Spike topograph images were shown by three different references. The difference in spike voltage between CA and SD was also investigated (CA-SD). By all three different references, PBS spike topography showed a broad symmetrical voltage gradient. In SBS, a left frontal focal negativity was shown with a steep voltage gradient in the topographic images of spikes by L+R, CA and SD. This asymmetry disappeared gradually at the end of the spike-wave burst. Both patients showed almost identical broad symmetrical voltage gradients on CA-SD topogram. EEG topography using various references suggested that the potential fields produced by deep generators were identical for PBS and SBS, although those generated by surface generators were different. These findings support the hypothesis that SBS may be projected from the subcortical system, and activated by cortical firing.     

EEG coherence and power changes during a continuous movement task

EEG coherence (COH) is a mathematically derived measure of the time- and frequency-related similarities between a pair of EEG channels. In this report, COH was measured during an externally verified motor task in which the areas of cortical involvement are known, with special consideration given to procedural and artifactual issues. Fourteen right-handed women (ages 18-39, means = 26.7 years) were instructed to alternate continuously between fist-clenching and finger extension of the right hand, left hand, both hands, or neither hand (rest condition) in a counter-balanced sequence (4 one-minute trials for each condition; 16 total minutes). One minute each of intentional eye-movement (EOG) and intentional facial muscle tension (EMG) was recorded for artifact assessment. Eight channels of eyes-closed EEG were recorded from Fp1, Fp2, F3, F4, C3, C4, P3 and P4, each referenced to the ipsilateral earlobe. FFT spectral power analyses were conducted on 8 EEG channels and COH analyses (percentage of seconds/minute in which COH greater than or equal to 0.80) were performed on 16 pairs of leads: 4 interhemispheric, 6 intrahemispheric (left) and 6 intrahemispheric (right). COH measures increased during hand movement conditions, especially in the 9-12 Hz range, and were most apparent from prefrontal, premotor and motor areas. Parietal sources were essentially unchanged. Power measures were unchanged for virtually all leads and conditions. Increases in COH were not due to EOG or EMG artifact contamination. Evidence for lateralized increases was equivocal; significant bilateral increases were observed more often regardless of the hand clenched. Implications and suggested areas for future research are discussed.     

Similar sleep EEG topography in middle-aged depressed patients and healthy controls

OBJECTIVES: One of the early hypotheses relating sleep disturbances in depression to a model of sleep regulation is the S-deficiency hypothesis. It is postulated that, in depressed patients, sleep propensity during wakefulness does not rise to the level attained by nondepressed subjects, resulting in altered sleep structure or changes in the electroencephalogram during sleep. We aimed to test this hypothesis by assessing topographic changes in the sleep electroencephalogram associated with depression. DESIGN: Cross-sectional clinical study. SETTING: Mental Health Clinical Research Center. PARTICIPANTS: Sixteen unmedicated depressed outpatients (mean age: 41.2 years) and 16 pair-matched healthy controls (mean age: 41.1 years). Interventions: None. MEASUREMENTS: Baseline sleep electroencephalogram recordings were obtained from a central referential electrode and from 3 bipolar derivations (frontocentral, centroparietal, parietooccipital) along the anteroposterior axis. RESULTS: Symptoms of depression at the time of sleep recordings were moderate (24-item Hamilton Rating Scale of Depression range: 16-31). No differences between patients and controls were found in sleep variables and all-night electroencephalogram spectra in non-rapid-eye-movement and rapid-eye-movement sleep. The ultradian modulation of slow-wave activity (power within 0.75-4.5 Hz), as well as the exponential decline of slow-wave activity, during sleep did not differ between the groups. The statistical analyses of electroencephalogram power gradients between adjacent derivations revealed no Group x Derivation interactions. An anterior dominance in non-rapid-eye-movement sleep power was present in the 0.75- to 2-Hz range, which diminished throughout the night. CONCLUSIONS: These findings in moderately depressed patients do not support the existence of an S-deficiency during sleep. Because the build up of sleep propensity during waking can be dissociated from its decline, future studies need to investigate the waking electroencephalogram spectra in depression.     

Ambiguous figures and binding: EEG frequency modulations during multistable perception

Ambiguous figures induce sudden transitions between rivaling percepts. We investigated electroencephalogram frequency modulations of accompanying change‐related de‐ and rebinding processes. Presenting the stimuli discontinously, we synchronized perceptual reversals with stimulus onset, which served as a time reference for averaging. The resultant gain in temporal resolution revealed a sequence of time–frequency correlates of the reversal process. Most conspicuous was a transient right‐hemispheric gamma modulation preceding endogenous reversals by at least 200 ms. No such modulation occurred with exogenously induced reversals of unambiguous stimulus variants. Post‐onset components were delayed for ambiguous compared to unambiguous stimuli. The time course of oscillatory activity differed in several respects from predictions based on binding‐related hypotheses. The gamma modulation preceding endogenous reversals may indicate an unstable brain state, ready to switch.     

MEG and EEG topography of frontal midline theta rhythm and source localization

Summary In this paper, we report on our study of frontal midline theta (Fm) activity in human subjects, recorded during mental processes such as arithmetic calculation. The Fm is a 6–7 Hz rhythmic wave with a duration of few seconds. The Fm activity is observed in the central region at the front of the head. EEGs and MEGs of Fm were measured simultaneously during mental calculation, and we analyzed these waveforms based on both topographic EEG maps and magnetic fields measurements. A single dipole simulated the EEG topography adequately, but there are many other dipole models which can generate a similar EEG pattern. It is difficult to estimate the source location of the Fm from the EEG topography alone because the EEG technique has a certain ambiguity associated with source estimation. Therefore, we considered the spatial relationships between the sources and the patterns of EEG and MEG that were simulated. Although it is not possible to obtain a unique solution for the source location of Fm from the EEG data alone, the simultaneous recording of MEGs from a large scalp area may result in an unambiguous solution. We therefore conclude that the simultaneous recording of both MEG and EEG data is more useful for accurate localization, than the EEG alone.     

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.     

Longitudinal genetic analysis of EEG coherence in young twins

During middle childhood, continuous changes occur in electroencephalogram (EEG) coherence, an index of cortico-cortical connectivity of the brain. In the gradual development of EEG coherence, occasional growth spurts are observed which coincide with periods of discontinuous development in cognition. Discontinuous development may reflect changes in the genetic architecture of a trait over time, for instance, by the emergence of new genetic factors. To examine stability and change in genetic and environmental influences on EEG coherence from ages 5 to 7 years, intrahemispheric EEG coherences from 14 connections were collected twice in 209 twin pairs. Overall, heritabilities (h ²) were moderate to high for all EEG coherences at both ages (average: 58%). For occipito-cortical connections in the right hemisphere, h ² increased with age due to a decrease in environmental variance. For prefronto-cortical connections in the left hemisphere, h ² decreased with age due to a decrease in genetic variance. New genetic factors at age 7 were found for prefronto-parietal coherence, and centro-occipital and parieto-occipital EEG coherences in both hemispheres and, in the left hemisphere, for prefronto-frontal EEG coherences. Mean genetic correlation for these cortico-cortical connections over time was 0.72, indicating that at least part of the genetic influences is age-specific. We argue that this is convincing evidence for the existence of stage-wise brain maturation from years 5 to 7, and that growth spurts in EEG coherence may be part of the biological basis for discontinuous cognitive development at that age range.     

Genetic determination of EEG frequency spectra

Simultaneous EEG samples were collected under four conditions in one session from 39 pairs of monozygotic and 27 pairs of dizygotic young adult twins of the same sex. The 3 min EEG samples were spectrum analyzed and root mean square differences between pairs of spectra were computed for the two twin samples and also for a group of unrelated pairs obtained from the same samples. Six parameters were defined to characterize the salient features of the individual spectra: delta, theta, alpha, beta, phi (alpha mid-frequency) and kappa (stability of alpha frequency). All six parameters Showed intra-class correlations of about 0.8 for the MZ twins and correlations near zero for the DZ pairs. Comparisons across conditions indicated that, with respect to EEG spectra, the MZ twins resembled their co-twins about 96% as much as they resembled themselves. Holding time and situation constant, most of the variance in the frequency characteristics of the EEG appears genetically determined.     

Visual stimulus-dependent changes in interhemispheric EEG coherence in ferrets

In recent years, the analysis of the coherence between signals recorded from the scalp [electroencephalographic (EEG) coherence] has been used to assess the functional properties of cortico-cortical connections, both in animal models and in humans. However, the experimental validation of this technique is still scarce. Therefore we applied it to the study of the callosal connections between the visual areas of the two hemispheres, because this particular set of cortico-cortical connections can be activated in a selective way by visual stimuli. Indeed, in primary and in low-order secondary visual areas, callosal axons interconnect selectively regions, which represent a narrow portion of the visual field straddling the vertical meridian and, within these regions, neurons that prefer the same stimulus orientation. Thus only isooriented stimuli located near the vertical meridian are expected to change interhemispheric coherence by activating callosal connections. Finally, if such changes are found and are indeed mediated by callosal connections, they should disappear after transection of the corpus callosum. We perfomed experiments on seven paralyzed and anesthetized ferrets, recording their cortical activity with epidural electrodes on areas 17/18, 19, and lateral suprasylvian, during different forms of visual stimulation. As expected, we found that bilateral iso-oriented stimuli near the vertical meridian, or extending across it, caused a significant increase in interhemispheric coherence in the EEG beta-gamma band. Stimuli with different orientations, stimuli located far from the vertical meridian, as well as unilateral stimuli failed to affect interhemispheric EEG coherence. The stimulus-induced increase in coherence disappeared after surgical transection of the corpus callosum. The results suggest that the activation of cortico-cortical connections can indeed be revealed as a change in EEG coherence. The latter can therefore be validly used to investigate the functionality of cortico-cortical connections.     

Visual stimulus-dependent changes in interhemispheric EEG coherence in humans

We analyzed the coherence of electroencephalographic (EEG) signals recorded symmetrically from the two hemispheres, while subjects (n = 9) were viewing visual stimuli. Considering the many common features of the callosal connectivity in mammals, we expected that, as in our animal studies, interhemispheric coherence (ICoh) would increase only with bilateral iso-oriented gratings located close to the vertical meridian of the visual field, or extending across it. Indeed, a single grating that extended across the vertical meridian significantly increased the EEG ICoh in normal adult subjects. These ICoh responses were obtained from occipital and parietal derivations and were restricted to the gamma frequency band. They were detectable with different EEG references and were robust across and within subjects. Other unilateral and bilateral stimuli, including identical gratings that were effective in anesthetized animals, did not affect ICoh in humans. This fact suggests the existence of regulatory influences, possibly of a top-down kind, on the pattern of callosal activation in conscious human subjects. In addition to establishing the validity of EEG coherence analysis for assaying cortico-cortical connectivity, this study extends to the human brain the finding that visual stimuli cause interhemispheric synchronization, particularly in frequencies of the gamma band. It also indicates that the synchronization is carried out by cortico-cortical connection and suggests similarities in the organization of visual callosal connections in animals and in man.