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 evoked potential measures of interhemispheric transfer time in humans

Four experiments testing right-handed adult males examined interhemispheric transfer time (IHTT) estimation with visual evoked potentials (EPs) elicited in response to hemiretinal presentations of checkerboard-flash stimuli. Experiment 1 was a study of the relation between reaction time (RT) and EP measures of IHTT. EP measures provided more valid estimates than RT measures because more subjects showed IHTT in the direction of anatomical prediction. Experiment 2 showed that EPs derived from lateral occipital sites provided more valid and longer estimates of IHTT compared with EPs from medial occipital sites. Experiment 3 showed no difference between random versus blocked hemiretinal stimuli. Experiment 4 showed that IHTT derived with a linked-ears reference provided more valid estimates than IHTT derived with a mid-frontal reference and that small changes in stimulus eccentricity did not influence IHTT. The findings of these experiments indicate that noninvasive estimates of visual IHTT can be obtained in humans.     

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.     

Directed coherence as a measure of interhemispheric correlation of EEG.

Based on a bidirectional model and the temporal relation of the signals, directed coherence is defined to describe the EEG correlation according to the direction of information transmission in the frequency domain. The interhemispheric directed coherences of EEG pairs in the prefrontal, frontal, central, parietal and occipital cortices were investigated. Statistically, significantly greater right-left directed coherences were found in the parietal and occipital regions than that of the left-right direction for alpha activity. The results indicate different information processing for different frequency bands between the left and right hemispheres, and this can not be derived from the examination of original coherence. This may suggest that the measure of directed coherence can provide more information than that of original coherence.     

Psychoactive drug-induced changes of interhemispheric EEG amplitude relationships

Analyses of the EEG amplitude levels (integrative method) in the right (R) and left (L) occipital areas of human subjects reveal that there exists in most subjects a R/L difference, and that certain psychoactive drugs can change the relationships which prevail under normal conditions (lateralization). Stimulants tend to decrease interhemispheric differences while, on the contrary, following drug-induced euphoria, these differences are increased. Hallucinogens can reverse lateralization; neuroleptics produce a decrease in the variability of amplitude ratios. Sedatives, minor tranquillizers and placebo are without effect. With hallucinogenic drugs, changes similar to those occurring in man have also been obtained in rabbits.A preliminary communication of these results was presented at the Fifth International Congress of Pharmacology (San Francisco, USA, July 1972).     

Alexithymia and the split brain. V. EEG alpha-band interhemispheric coherence analysis

Electroencephalographic (EEG) recordings were obtained while 6 completely commissurotomized patients, 2 partially commissurotomized patients, and 8 precision-matched normal control subjects watched 4 times a film symbolically depicting death. Our hypothesis was that alexithymia--a diminished capability to verbally express moods, symbols, and feelings--would be greater for patients because loss of cerebral commissures reduces interhemispheric communication, separating right-hemisphere-dependent affective understanding and left-hemisphere-dependent verbalization. Path and covariance structure analyses confirmed that callosotomy decreased alpha-band EEG coherences (after adjustment for mean intrahemispheric coherence) between pairs of scalp electrode channels placed homologously over frontal, parietal, and temporal lobes of the brain. This result occurred both for an index of interhemispheric coherence and for a latent variable indicated by the 3 adjusted coherences. Reduced levels of interhemispheric coherence in turn increased alexithymia, for an overall index and for a latent variable indicated by lexical-level content-analytic measures of verbal responses to the filmic stimulus.     

EEG coherence in humans: relationship with success in recognizing emotions in the voice

EEG recordings from two groups of subjects – with high and low levels of recognition of emotions from voices were made. Comparisons were performed of the numbers of pairs of leads with different levels of coherence in baseline conditions and on recognition of emotions in six standard frequency ranges and in individual bands with 1-Hz steps. Significant differences were seen between groups 1 and 2 both in baseline conditions and during recognition of emotions: in most cases, coherence was greater in subjects with poor recognition of emotions from voices. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 94, No. 6, pp. 650–660, June, 2008.     

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.     

Regional changes in EEG power and coherence during cognition: intensive study of two individuals

Two men underwent weekly electroencephalogram (EEG) recordings while living for several months in a controlled laboratory environment. Data collected from an eight-channel EEG during a resting period and during performance of two cognitive tasks (word fluency and mental imagery) were subjected to spectral analysis. Statistical analyses on power and coherence were conducted for each subject separately, to determine whether that individual showed a characteristic pattern of EEG activity for a given cognitive task which was stable over time. Although substantial individual differences were observed, particularly for the theta band, both subjects showed changes in the spectral information over the anterior left hemisphere during the word fluency task.     

Handedness leads to interhemispheric EEG asymmetry during sleep in the rat

Sleep electroencephalographic (EEG) slow-wave activity is increased after wakefulness and decreases during sleep. Regional sleep EEG differences are thought to be a consequence of activation of specific cortical neuronal circuits during waking. We investigated the relationship between handedness and interhemispheric brain asymmetry. Bilateral EEG recordings were obtained from the frontal and occipital cortex in rats with a clear paw preference in a food-reaching task (right, n = 5; left, n = 5). While still na?ve to the task, no waking or sleep EEG asymmetry was present. During the food-reaching task, the waking EEG showed significant, substantial power increases in the frontal hemisphere contralateral to the dominant paw in the low theta range (4.5-6.0 Hz). Moreover, the non-REM sleep EEG following feeding bouts was markedly asymmetric, with significantly higher power in the hemisphere contralateral to the preferred paw in frequencies >1.5 Hz. No asymmetry was evident in the occipital EEG. Correlation analyses revealed a positive association between the hemispheric asymmetry during sleep and the degree of preferred use of the contralateral paw during waking in frequencies <9.0 Hz. Our findings show that handedness is reflected in specific, regional EEG asymmetry during sleep. Neuronal activity induced by preferential use of a particular forelimb led to a local enhancement of EEG power in frequencies within the delta and sigma ranges, supporting the hypothesis of use-dependent local sleep regulation. We conclude that inherent laterality is manifested when animals are exposed to complex behavioral tasks, and sleep plays a role in consolidating the hemispheric dominance of the brain.     

Crawling experience is related to changes in cortical organization during infancy: Evidence from EEG coherence

Greenough's model of experience-expectant plasticity was used to examine EEG coherence among four groups of 8-month-old infants that varied in hands-and-knees crawling experience. Groups included prelocomotor infants, novice crawlers with 1–4 weeks experience, infants with 5–8 weeks, and long-term crawlers with 9+ weeks experience. Resting EEG was recorded from frontal, parietal, and occipital sites of both hemispheres. EEG coherence between intrahemispheric sites was computed. Novice crawlers (1–4 weeks) displayed greater coherence than either prelocomotor infants or experienced crawlers. These data suggest that the anticipation and onset of locomotion was related to an overproduction of cortico-cortical connections. Pruning of these overabundant connections may be a source of the decrease in coherence as crawling becomes more routine. © 1996 John Wiley & Sons, Inc.     

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.     

EEG coherence: topography and frequency structure

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

Regional differences in cortical electroencephalogram (EEG) slow wave activity and interhemispheric EEG asymmetry in the fur seal

Slow wave sleep (SWS) in the northern fur seal (Callorhinus ursinus) is characterized by a highly expressed interhemispheric electroencephalogram (EEG) asymmetry, called ‘unihemispheric’ or ‘asymmetrical’ SWS. The aim of this study was to examine the regional differences in slow wave activity (SWA; power in the range of 1.2–4.0 Hz) within one hemisphere and differences in the degree of interhemispheric EEG asymmetry within this species. Three seals were implanted with 10 EEG electrodes, positioned bilaterally (five in each hemisphere) over the frontal, occipital and parietal cortex. The expression of interhemispheric SWA asymmetry between symmetrical monopolar recordings was estimated based on the asymmetry index [AI = (L?R)/(L+R), where L and R are the power in the left and right hemispheres, respectively]. Our findings indicate an anterior–posterior gradient in SWA during asymmetrical SWS in fur seals, which is opposite to that described for other mammals, including humans, with a larger SWA recorded in the parietal and occipital cortex. Interhemispheric EEG asymmetry in fur seals was recorded across the entire dorsal cerebral cortex, including sensory (visual and somatosensory), motor and associative (parietal or suprasylvian) cortical areas. The expression of asymmetry was greatest in occipital–lateral and parietal derivations and smallest in frontal–medial derivations. Regardless of regional differences in SWA, the majority (90%) of SWS episodes with interhemispheric EEG asymmetry meet the criteria for ‘unihemispheric SWS’ (one hemisphere is asleep while the other is awake). The remaining episodes can be described as episodes of bilateral SWS with a local activation in one cerebral hemisphere.     

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.     

Interaction between changes in local and temporospatial spectral EEG characteristics during exposure of humans to hypoxia

The temporospatial and local characteristics of the EEG were studied in healthy subjects during the respiration of a hypoxic oxygen-nitrogen gas mixture containing 8% oxygen. Analysis of the spectral power density, coherence, phase shift, similarity of dominant frequencies in the EEG in different leads was performed separately for epochs containing and not containing visually apparent patterns of EEG spatial synchronization. In addition, assessment of spectral measures took account of the fact of the dominance of the frequency being analyzed in the EEG spectrum of the corresponding lead. The results of these studies showed that overall, hypoxia was accompanied by a decrease in EEG coherence in the alpha range and, for connections in the left mid-temporal lead, the delta range. The beta range showed an increase in the values of this measure, while the theta range showed different changes in coherence: increases in epochs containing spatial synchronization patterns and decreases from control levels in other epochs. Hypoxia was also associated with increases in the EEG phase shift in the frontal and temporal leads (in relation to the EEG recordings in the other leads) in the delta and theta ranges. In the beta range, conversely, there was a decrease in the mean phase shift. Consideration of the fact of dominance of one or another frequency in the spectra of local EEG traces is needed for correct interpretation of analyses of temporospatial measures. A mathematical model of the interaction between processes at different frequencies is presented to explain some of the observations in the current work. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 91, No. 11, pp. 1260–1280, November, 2005.     

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.