The effect of brain function on coherence patterns in the bipolar EEG

Studies of the functional organization of the brain based on measurements of coherence in the EEG have, in the past, suffered from a methodological defect which has made interpretation of the results difficult. The effect involved is the use of an active common reference for the recording of the EEG. As a consequence, inferences related to the functional connectivity of brain between the non-referential sites using coherence have probably been wrong. To avoid the problem with the common reference, we have used bipolar derivations of the EEG and used measurements of coherence to reflect synchrony not between individual sites but between regions of the brain. The EEGs in a population of normal volunteers were examined with respect to coherence. Changes in the patterning of coherence were induced by utilizing EEGs from the volunteers during 3 different functional brain states. The first of these was the resting state, the second the verbal motor state and the third, the spatial motor state. The stepwise discriminant analysis method was used to study differences in the patterning of coherences in the 3 states. The results show that the spatial motor state was the most distinct in this regard amongst the 3 states. They are also interpreted as indicating that changes in the patterning of coherence from that in the resting state consisted of both functionally specific and functionally non-specific components. Amongst the functionally specific components, the one most strongly distinguishing between the verbal and spatial states was a left posterior-right frontal component. The argument is made that the patterning of coherence induced by the verbal state is to some extent obscured by the functionally non-specific components present in the overall pattern and the concept of partial coherences is suggested as a means of testing this hypothesis. Finally, to determine if psychometric data suggesting sex differences in the ability to perform verbally and spatially based cognitive tasks have correlates in the EEG, the components of coherence showing discriminatory properties with respect to the 3 functional states were examined with respect to sex differences. The results show that while the mean change in the pattern from resting is indeed stronger in the men than in the women for the spatial state and stronger for the women than in the men in the verbal state, the difference is only statistically significant in the spatial state.