title: Ictal time-irreversible intracranial EEG signals as markers of the epileptogenic zone.
abstract: To show that time-irreversible EEG signals recorded with intracranial electrodes during seizures can serve as markers of the epileptogenic zone.
  
  
  
  We use the recently developed method of mapping time series into directed horizontal graphs (dHVG). Each node of the dHVG represents a time point in the original intracranial EEG (iEEG) signal. Statistically significant differences between the distributions of the nodes' number of input and output connections are used to detect time-irreversible iEEG signals.
  
  
  
  In 31 of 32 seizure recordings we found time-irreversible iEEG signals. The maximally time-irreversible signals always occurred during seizures, with highest probability in the middle of the first seizure half. These signals spanned a large range of frequencies and amplitudes but were all characterized by saw-tooth like shaped components. Brain regions removed from patients who became post-surgically seizure-free generated significantly larger time-irreversibilities than regions removed from patients who still had seizures after surgery.
  
  
  
  Our results corroborate that ictal time-irreversible iEEG signals can indeed serve as markers of the epileptogenic zone and can be efficiently detected and quantified in a time-resolved manner by dHVG based methods.
  
  
  
  Ictal time-irreversible EEG signals can help to improve pre-surgical evaluation in patients suffering from pharmaco-resistant epilepsies.
authors.affiliation: Sleep-Wake-Epilepsy-Center, Department of Neurology, Inselspital, Bern University Hospital, University Bern, Switzerland. Electronic address: kaspar.schindler@insel.ch.
authors.forename: Kaspar
authors.initials: K
authors.lastname: Schindler
authors.affiliation: Support Center for Advanced Neuroimaging (SCAN), Institute of Diagnostic and Interventional Neuroradiology, University Hospital, Inselspital, University of Bern, Switzerland.
authors.forename: Christian
authors.initials: C
authors.lastname: Rummel
authors.affiliation: Department of Information and Communication Technologies, Universitat Pompeu Fabra, Barcelona, Spain.
authors.forename: Ralph G
authors.initials: RG
authors.lastname: Andrzejak
authors.affiliation: College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK; Centre for Biomedical Modelling and Analysis, University of Exeter, Exeter, UK; EPSRC Centre for Predictive Modelling in Healthcare, University of Exeter, Exeter, UK.
authors.forename: Marc
authors.initials: M
authors.lastname: Goodfellow
authors.affiliation: Sleep-Wake-Epilepsy-Center, Department of Neurology, Inselspital, Bern University Hospital, University Bern, Switzerland.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Zubler
authors.affiliation: Support Center for Advanced Neuroimaging (SCAN), Institute of Diagnostic and Interventional Neuroradiology, University Hospital, Inselspital, University of Bern, Switzerland.
authors.forename: Eugenio
authors.initials: E
authors.lastname: Abela
authors.affiliation: Support Center for Advanced Neuroimaging (SCAN), Institute of Diagnostic and Interventional Neuroradiology, University Hospital, Inselspital, University of Bern, Switzerland.
authors.forename: Roland
authors.initials: R
authors.lastname: Wiest
authors.affiliation: Department of Neurosurgery, Inselspital, Bern University Hospital, University Bern, Switzerland.
authors.forename: Claudio
authors.initials: C
authors.lastname: Pollo
authors.affiliation: Sleep-Wake-Epilepsy-Center, Department of Neurology, Inselspital, Bern University Hospital, University Bern, Switzerland.
authors.forename: Andreas
authors.initials: A
authors.lastname: Steimer
authors.affiliation: Sleep-Wake-Epilepsy-Center, Department of Neurology, Inselspital, Bern University Hospital, University Bern, Switzerland.
authors.forename: Heidemarie
authors.initials: H
authors.lastname: Gast
mesh_terms: D000328:Adult; D000069280:Electrocorticography / Q000379:methods; D004567:Electrodes, Implanted; D004569:Electroencephalography / Q000295:instrumentation / Q000379:methods*; D004827:Epilepsy / Q000175:diagnosis* / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D055815:Young Adult
keywords: Complex networks; Pre-surgical evaluation; Quantitative EEG; Seizure dynamics; Symbolic analysis