Detection of seizure-associated high-frequency oscillations above 500Hz.

High-frequency oscillations (HFOs) of up to 500Hz in EEG are considered to have close relation with ictogenesis. We had the unique opportunity to record a seizure in EEG with intracerebral macroelectrodes and a sampling frequency of 10kHz. Considering the notion that faster HFOs are likely more ictogenic, we investigated this ictal EEG data to find if even faster HFOs were present. HFOs were investigated in interictal spikes and seizure activity using time-frequency spectra: t values corresponding to frequencies from 100 to 1000Hz were obtained by comparison to the background and controlled by the false discovery rate (FDR). The seizure had a right hippocampal onset. HFOs up to 800Hz as well as HFOs below 500Hz built up in the hippocampal discharges more at the beginning of the seizure and during the preictal period than in the interictal period. These HFOs were visually confirmed in temporally expanded EEG traces. We demonstrated for the first time the existence of HFOs above 500Hz and up to 800Hz with intracerebral macroelectrodes in an epileptic patient; they occurred primarily in association with the seizure discharge. HFOs above 500Hz possibly reflect facilitation of ictogenic neuronal hypersynchronization.