Abstracts

Rationale: High frequency oscillations (HFOs) > 200 Hz are believed to be associated with epileptic processes. The spatial distribution and evolution over time of HFOs leading up to seizure onset is unknown. Also, recording of HFOs through conventional intracranial electrodes is not well established. We therefore wished to determine: whether HFOs could be recorded using commercially available depth macroelectrodes, and to study the spatial distribution and temporal progression of HFOs during the pre-ictal-to-ictal transition.Methods: Intracranial EEG recordings from seven patients (19 seizures) with temporal lobe epilepsy were obtained using commercial depth or subdural electrodes. EEG recordings were analyzed for frequency content in five spectral bands spanning DC-500Hz in two ways: 5 s before and after seizure onset (spatial analysis), and for 30 s leading up to seizure onset (temporal analysis).Results: Three main observations were made: HFOs (100-500Hz) can be recorded using commercial depth and subdural grid electrodes. HFOs, but not < 100Hz oscillations, were localized with maximal power to channels of ictal onset (100-200,400-500Hz p<0.05; 300-400Hz p<0.001). Temporal analysis showed increased HFO power for ≈ 8 s prior to electrographic onset, with the greatest difference observed in the 100-200Hz band when the ictal onset channel was compared with a distant contact (p<0.05).Conclusions: 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.