Brain Advance Access originally published online on September 10, 2009
Brain 2009 132(11):3047-3059; doi:10.1093/brain/awp222
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Spatial characterization of interictal high frequency oscillations in epileptic neocortex
1 Department of Neurology, Columbia University, New York, NY, USA 2 School of Neurology, Neurobiology and Psychiatry, University of Newcastle, Newcastle Upon Tyne, UK 3 Department of Psychiatry, Columbia University, New York, NY, USA 4 Department of Neurological Surgery, Columbia University, New York, NY, USA 5 Department of Pediatrics, Columbia University, New York, NY, USA
Correspondence to: Catherine A. Schevon, Neurological Institute, 710 West, 168th Street, NY 10032, USA E-mail: cas2044{at}columbia.edu
Interictal high frequency oscillations (HFOs), in particular those with frequency components in excess of 200 Hz, have been proposed as important biomarkers of epileptic cortex as well as the genesis of seizures. We investigated the spatial extent, classification and distribution of HFOs using a dense 4 x 4 mm2 two dimensional microelectrode array implanted in the neocortex of four patients undergoing epilepsy surgery. The majority (97%) of oscillations detected included fast ripples and were concentrated in relatively few recording sites. While most HFOs were limited to single channels, 
10% occurred on a larger spatial scale with simultaneous but morphologically distinct detections in multiple channels. Eighty per cent of these large-scale events were associated with interictal epileptiform discharges. We propose that large-scale HFOs, rather than the more frequent highly focal events, are the substrates of the HFOs detected by clinical depth electrodes. This feature was prominent in three patients but rarely seen in only one patient recorded outside epileptogenic cortex. Additionally, we found that HFOs were commonly associated with widespread interictal epileptiform discharges but not with locally generated ‘microdischarges’. Our observations raise the possibility that, rather than being initiators of epileptiform activity, fast ripples may be markers of a secondary local response.
Key Words: multichannel extracellular recording; epilepsy; intracranial EEG; interictal epileptiform activity; fast ripples; high frequency oscillations
Abbreviations: HFOE, high frequency oscillation event; HFOs, high frequency oscillations; iEEG, intracranial electroencephalography; MEA, microelectrode array; MUA, multiunit activity; NREM, non-rapid eye movement; µEEG, Electroencephalography derived from microelectrode recording
Received May 4, 2009. Revised June 12, 2009. Accepted July 11, 2009.