EEG coherence and hemisphere asymmetry
Following a brief introduction into the areas of hemisphere asymmetries of function and EEG techniques in general, a detailed review of EEG studies of lateralisation, including evoked potential studies, is presented. The evidence concerning anatomical asymmetries between the hemispheres, and the implications of this for EEG studies, are considered. The nature and uses of the coherence function are also reviewed prior to a description of the first experiment. This experiment involved having subjects perform one nonverbal tone-detection task and three verbal tasks which differed in level of difficulty. EEG was recorded from pairs of electrodes in the region of Broca's and Wernicke's areas and right hemisphere homologues. Analysis of power and coherence effects in the alpha band showed greater power and coherence over the left hemisphere than the right for the anterior electrode pairs. It seemed possible that the higher coherence over the left hemisphere was due to the use of auditory stimuli and this was investigated in the second experiment, when simple clicks and flashes were presented to subjects while EEG was recorded from occipital and temporal sites over each hemisphere. More power was found at the temporal sites than the occipital for the theta band, but the only coherence effect was that in all bands coherence was higher for the occipital electrode pair compared to the more widely separated temporal pair. Experiment III repeated the general procedure of Experiment II (with slight modifications) but examined coherence for EPs; once again, no hemisphere effects were found. Studies of phase angle effects are reviewed, and then an attempt is made in the final three experiments to 'map' coherence, phase angle and power effects topographically during a simple task. Several reliable effects were found and are discussed, but problems of interpretation are noted.