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Brain Advance Access published online on November 1, 2006
Brain, doi:10.1093/brain/awl304
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© The Author (2006). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Received May 17, 2006
Revised August 10, 2006
Accepted September 29, 2006

Article

Assessing seizure dynamics by analysing the correlation structure of multichannel intracranial EEG

Kaspar Schindler 1 *, Howan Leung 1, Christian E. Elger 1, and Klaus Lehnertz 2

1 Klinik für Epileptologie, University of Bonn, Bonn, Germany
2 Klinik für Epileptologie, University of Bonn, Bonn, Germany; Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn, Bonn, Germany

* To whom correspondence should be addressed.
Kaspar Schindler, E-mail: kschindler{at}smile.ch


  Abstract

Epileptic seizures are commonly characterized as ‘hypersynchronous states’. This habit is doubly misleading, because seizures are not necessarily synchronous and are not unchanging ‘states’ but dynamic processes. Here the temporal evolution of the correlation structure in the course of 100 focal onset seizures of 60 patients recorded by intracranial multichannel EEG was assessed. To this end a multivariate method was applied that at its core consists of computing the eigenvalue spectrum of the zero-lag correlation matrix of a short sliding window. Our results show that there are clearly observable and statistically significant changes of the correlation structure of focal onset seizures. Specifically, these changes indicate that the zero-lag correlation of multi-channel EEG either remains approximately unchanged or--especially in the case of secondary generalization--decreases during the first half of the seizures. Then correlation gradually increases again before the seizures terminate. This development was qualitatively independent of the anatomical location of the seizure onset zone and therefore seems to be a generic property of focal onset seizures. We suggest that the decorrelation of EEG activity is due to the different propagation times of locally synchronous ictal discharges from the seizure onset zone to other brain areas. Furthermore we speculate that the increase of correlation during the second half of the seizures may be causally related to seizure termination.

Keywords: correlation structure; eigenvalue spectrum; intracranial EEG; multivariate time series analysis; seizure termination.
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