Spatial localization and time-dependant changes of electrographic high frequency oscillations in human temporal lobe epilepsy

High frequency oscillations (HFOs) > 200 Hz are believed to be associated with epileptic processes. The spatial distribution of HFOs and their evolution over time leading up to seizure onset is unknown. Also, recording HFOs through conventional intracranial electrodes is not well established. We therefore wished to determine whether HFOs could be recorded using commercially available depth macroelectrodes. We also examined the spatial distribution and temporal progression of HFOs during the transition to seizure activity. Intracranial electroencephalography (EEG) recordings of 19 seizures were obtained from seven patients with temporal lobe epilepsy using commercial depth or subdural electrodes. EEG recordings were analyzed for frequency content in five spectral bands spanning DC-500 Hz. We examined the spatial distribution of the different spectral bands 5 s before and 5 s after seizure onset. Temporal changes in the spectral bands were studied in the 30-s period leading up to seizure onset. Three main observations were made. First, HFOs (100-500 Hz) can be recorded using commercial depth and subdural grid electrodes. Second, HFOs, but not < 100 Hz oscillations, were localized to channels of ictal onset (100-200, 400-500 Hz, p < 0.05; 300-400 Hz, p < 0.001). Third, temporal analysis showed increased HFO power for approximately 8 s prior to electrographic onset (p < 0.05). These results suggest that HFOs can be recorded by depth macroelectrodes. Also, HFOs are localized to the region of primary ictal onset and can exhibit increased power during the transition to seizure. Thus, HFOs likely represent important precursors to seizure initiation.