Spectral modulation of cortical connections measured by EEG coherence in humans

Objective: Description of coherence patterns of cortical EEG. Methods: EEG recordings were collected from 9 subdural electrode grids implanted in 6 patients undergoing EEG monitoring for refractory epilepsy. Results: Coherence decreased with increasing inter-electrode distance and exhibited considerable variability at the same inter-electrode distances. Analysis of variance demonstrated that both spatial and temporal factors contributed significantly (P<0.01) to this variability. The spatial factor contributed the largest portion (up to 90%) of the variability, and was modulated by frequency and inter-electrode distance. Conclusion: The finding that the mean frequency modulation was consistent over time for the same pair of electrodes and was different between different pairs of electrodes indicated a spatial-spectral pattern of cortical synchrony. The connections (pairs of electrodes) could be accordingly predicted from their spatial-spectral pattern, which suggested that the spatial heterogeneity of neuronal synchrony was expressed not only by the degree of synchrony, but also by distinct spectral channels of synchrony. A model based on neuronal connection and activation is proposed to account for the observations.