Abstracts

Rationale: High-frequency oscillations (HFOs) in EEG are considered to have close relation with epileptogenesis: HFOs with frequencies ranging from 250 to 500 Hz (fast ripples) are thought to be more epileptogenic than HFOs with frequencies of 80 - 250 Hz (ripples). We had the unique opportunity to record a seizure in EEG with intracerebral macroelectrodes and a sampling frequency of 10 kHz. Considering the notion that faster HFOs are likely more epileptogenic, we investigated this ictal EEG data to find if even faster HFOs were present. Methods: The seizure was recorded from depth electrodes (contact surface 3.6 mm2) with a sampling frequency of 10 kHz and a low-pass filter at 3 kHz. Spectral analysis was performed in the ictal EEG data in the bipolar derivations (contact interval 5 mm) recorded from the right amygdala and hippocampus. HFOs were explored 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) (Kobayashi et al., 2009). Declaration of too many pixels in time-frequency spectra as active (type I error) could be avoided by the FDR-control. Results: The seizure had a right hippocampal onset. HFOs up to 800 Hz as well as HFOs below 500 Hz built up especially at the beginning part of the seizure (Figure 1). Changes of HFOs were clear in the hippocampal discharges, but minimal in the amygdala. In the hippocampal electrodes, increase of HFOs above 500 Hz was also noted in the preictal period (no more than 60 s away from the seizure onset) heralding the greatest increase of HFOs at the seizure onset. These HFOs were visually confirmed in the ictal and preictal discharges in temporally expanded and filtered EEG traces. In contrast, HFOs were rare in the interictal period. Conclusions: HFOs in EEG have been so far studied in the frequency range of up to 500 Hz. In the present report, we demonstrated for the first time the existence of HFOs above 500 Hz and up to 800 Hz; they occurred primarily in association with the seizure discharge. Recording HFOs of these very high frequencies using macroelectrodes implies that these HFOs were generated over relatively large brain regions. HFOs above 500 Hz possibly reflects facilitation of epileptogenic neuronal hypersynchronization. They should be investigated to understand better their role. Reference: Kobayashi K, Jacobs J, Gotman J. Detection of changes of high-frequency activity by statistical time-frequency analysis in epileptic spikes. Clin Neurophysiol 2009; 120: 1070-1077.