title: Peri-ictal correlation dynamics of high-frequency (80-200 Hz) intracranial EEG.
abstract: OBJECTIVE
  To assess (1) how large-scale correlation of intracranial EEG signals in the high-frequency range (80-200Hz) evolves from the pre-ictal, through the ictal into the postictal state and (2) whether the contribution of local neuronal activity to large-scale EEG correlation differentiates epileptogenic from non-epileptogenic brain tissue.
  
  
  METHODS
  Large-scale correlation of intracranial EEG was assessed by the total correlation strength (TCS), a measure derived from the eigenvalue spectra of zero-lag correlation matrices computed in a time-resolved manner by using a moving window approach. The relative change of total correlation strength (Delta(j)) resulting from leaving out EEG channel j ("leave-one-out approach") was used to quantify the contribution of local neuronal activity to large-scale EEG correlation.
  
  
  RESULTS
  19 seizures of 3 patients were analyzed. On average, TCS increased significantly from the pre-ictal to the ictal, and from the ictal to the postictal state. In the pre-ictal state, Delta(j) was significantly more negative when EEG channels that recorded the electrical activity of brain tissue considered to be epileptogenic were left out; the identification of the epileptogenic area, that was subsequently surgically removed in two patients, was based on visual analysis. The spatio-temporal pattern of Delta(j) dramatically changed at seizure onsets and endings, revealing qualitative similarities between the seizures of different patients.
  
  
  CONCLUSIONS
  The evolution of large-scale EEG correlation in the high-frequency range is qualitatively similar to the one previously described for the low-frequency range. Because the two patients who underwent surgery became seizure free, our findings are consistent with the hypothesis that epileptogenic brain tissue may be characterized by its relatively increased contribution to pre-ictal large-scale correlation.
authors.affiliation: qEEG Group, Department of Neurology, Inselspital, Bern University Hospital, University Bern, Switzerland. kaspar.schindler@gmail.com
authors.forename: Kaspar
authors.initials: K
authors.lastname: Schindler
authors.forename: Frédérique
authors.initials: F
authors.lastname: Amor
authors.forename: Heidemarie
authors.initials: H
authors.lastname: Gast
authors.forename: Markus
authors.initials: M
authors.lastname: Müller
authors.forename: Alexander
authors.initials: A
authors.lastname: Stibal
authors.forename: Luigi
authors.initials: L
authors.lastname: Mariani
authors.forename: Christian
authors.initials: C
authors.lastname: Rummel
mesh_terms: D000293:Adolescent; D000328:Adult; D001931:Brain Mapping; D002540:Cerebral Cortex / Q000503:physiopathology*; D004567:Electrodes, Implanted; D004569:Electroencephalography; D004827:Epilepsy / Q000503:physiopathology*; D006801:Humans; D012189:Retrospective Studies; D012815:Signal Processing, Computer-Assisted