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High-Frequency Oscillations (HFO)

High-frequency oscillations (HFO) are new EEG biomarkers for epileptic tissue. These oscillations range in frequencies from 80 to 500 Hz and can be recorded with standard intracranial macroelectrodes. During the presurgical assessment of patients with refractory epilepsy, HFO have been found to occur mainly over seizure onset areas. HFO might co-occur with epileptic spikes, but are more specific to epileptic tissue than epileptic spikes. Several retrospective studies showed a correlation between the removal of brain areas generating HFO and postsurgical seizure freedom. In addition to demonstrating the clinical value of HFO analysis, this chapter provides a detailed introduction to the techniques for analysing HFO, including recording techniques and visual and automatic detection tools, and to interpretation of the results. It also reviews methodological challenges such as the occurrence of physiological HFO and the variability of HFO rates between patients and brain regions.