title: Detectability of the somatosensory evoked high frequency oscillation (HFO) co-recorded by scalp EEG and ECoG under propofol.
abstract: OBJECTIVE
  The somatosensory evoked potential (SEP) elicited by median nerve stimulation consists of the N20 peak together with the concurrent high frequency oscillation (HFO, > 500 Hz). We describe the conditions for HFO detection in ECoG and scalp EEG in intraoperative recordings.
  
  
  METHODS
  During neurosurgical interventions in six patients under propofol anesthesia, the SEP was recorded from subdural electrode strips (15 recordings) and from scalp electrodes (10/15 recordings). We quantified the spatial attenuation of the Signal-to-Noise Ratio (SNR) of N20 and HFO along the contacts of the electrode strip. We then compared the SNR of ECoG and simultaneous scalp EEG in a biophysical framework.
  
  
  RESULTS
  HFO detection under propofol anesthesia was demonstrated. Visual inspection of strip cortical recordings revealed phase reversal for N20 in 14/15 recordings and for HFO in 10/15 recordings. N20 had higher maximal SNR (median 33.5 dB) than HFO (median 23 dB). The SNR of N20 attenuated with a larger spatial extent (median 7.2 dB/cm) than the SNR of HFO (median 12.3 dB/cm). We found significant correlations between the maximum SNR (rho = 0.58, p = 0.025) and the spatial attenuation (rho = 0.86, p < 0.001) of N20 and HFO. In 3/10 recordings we found HFO in scalp EEG. Based on the spatial attenuation and SNR in the ECoG, we estimated the scalp EEG amplitude ratio N20/HFO and found significant correlation with recorded values (rho = 0.65, p = 0.049).
  
  
  CONCLUSIONS
  We proved possible the intraoperative SEP HFO detection under propofol anesthesia. The spatial attenuation along ECoG contacts represents a good estimator of the area contributing to scalp EEG. The SNR and the spatial attenuation in ECoG recordings provide further insights for the prediction of HFO detectability in scalp EEG. The results obtained in this context may not be limited to SEP HFO, but could be generalized to biological signatures lying in the same SNR and frequency range.
authors.affiliation: Neurosurgery Department, University Hospital Zurich, Zurich, Switzerland; University of Zurich, Zurich, Switzerland; Neuroscience Center Zurich, ETH Zurich, Zurich, Switzerland.
authors.forename: Sergey
authors.initials: S
authors.lastname: Burnos
authors.affiliation: Neurosurgery Department, University Hospital Zurich, Zurich, Switzerland; University of Zurich, Zurich, Switzerland.
authors.forename: Tommaso
authors.initials: T
authors.lastname: Fedele
authors.affiliation: Neurosurgery Department, University Hospital Zurich, Zurich, Switzerland.
authors.forename: Olivier
authors.initials: O
authors.lastname: Schmid
authors.affiliation: Neurosurgery Department, University Hospital Zurich, Zurich, Switzerland; University of Zurich, Zurich, Switzerland.
authors.forename: Niklaus
authors.initials: N
authors.lastname: Krayenbühl
authors.affiliation: Neurosurgery Department, University Hospital Zurich, Zurich, Switzerland; University of Zurich, Zurich, Switzerland; Neuroscience Center Zurich, ETH Zurich, Zurich, Switzerland.
authors.forename: Johannes
authors.initials: J
authors.lastname: Sarnthein
mesh_terms: D000328:Adult; D000368:Aged; D018686:Anesthetics, Intravenous / Q000008:administration & dosage*; D058256:Brain Waves / Q000187:drug effects*; D002540:Cerebral Cortex / Q000187:drug effects* / Q000503:physiopathology*; D004558:Electric Stimulation; D000069280:Electrocorticography; D004569:Electroencephalography; D005073:Evoked Potentials, Somatosensory / Q000187:drug effects*; D005260:Female; D006801:Humans; D008297:Male; D008475:Median Nerve / Q000503:physiopathology; D008875:Middle Aged; D015742:Propofol / Q000008:administration & dosage*; D059629:Signal-To-Noise Ratio
chemical_list: D018686:Anesthetics, Intravenous; D015742:Propofol
keywords: ECoG; EEG; HFO burst; Neurosurgery; Propofol; SEP; Sigma burst