title: Subdural to subgaleal EEG signal transmission: the role of distance, leakage and insulating affectors.
abstract: OBJECTIVE
  To estimate the area of cortex affecting the extracranial EEG signal.
  
  
  METHODS
  The coherence between intra- and extracranial EEG channels were evaluated on at least 10 min of spontaneous, awake data from seven patients admitted for epilepsy surgery work up.
  
  
  RESULTS
  Cortical electrodes showed significant extracranial coherent signals in an area of approximately 150 cm(2) although the field of vision was probably only 31 cm(2) based on spatial averaging of intracranial channels taking into account the influence of the craniotomy and the silastic membrane of intracranial grids. Selecting the best cortical channels, it was possible to increase the coherence values compared to the single intracranial channel with highest coherence. The coherence seemed to increase linearly with an accumulation area up to 31 cm(2), where 50% of the maximal coherence was obtained accumulating from only 2 cm(2) (corresponding to one channel), and 75% when accumulating from 16 cm(2).
  
  
  CONCLUSIONS
  The skull is an all frequency spatial averager but dominantly high frequency signal attenuator.
  
  
  CONCLUSIONS
  An empirical assessment of the actual area of cerebral sources generating the extracranial EEG provides better opportunities for clinical electroencephalographers to determine the location of origin of particular patterns in the EEG.
authors.affiliation: Technical University of Denmark, Department of Electrical Engineering, Building 349, Oersteds Plads, 2800 Kgs. Lyngby, Denmark. jhe@elektro.dtu.dk
authors.forename: Jonas
authors.initials: J
authors.lastname: Duun-Henriksen
authors.forename: Troels Wesenberg
authors.initials: TW
authors.lastname: Kjaer
authors.forename: Rasmus Elsborg
authors.initials: RE
authors.lastname: Madsen
authors.forename: Bo
authors.initials: B
authors.lastname: Jespersen
authors.forename: Anne Katrine
authors.initials: AK
authors.lastname: Duun-Henriksen
authors.forename: Line Sofie
authors.initials: LS
authors.lastname: Remvig
authors.forename: Carsten Eckhart
authors.initials: CE
authors.lastname: Thomsen
authors.forename: Helge Bjarup Dissing
authors.initials: HB
authors.lastname: Sorensen
mesh_terms: D000293:Adolescent; D000368:Aged; D001931:Brain Mapping; D002540:Cerebral Cortex / Q000503:physiopathology*; D004566:Electrodes; D004569:Electroencephalography; D004827:Epilepsy / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008297:Male; D013355:Subdural Space / Q000503:physiopathology*
keywords: Breach rhythm; Coherence; Intra- and extracranial electroencephalography; Skull; Transfer function