High-frequency EEG activity from different perspectives

Abstract

Electroencephalography (EEG) records electrical activity generated by brain tissue. An EEG can be recorded on the scalp, or directly from the surface or from within the brain (invasive EEG). We studied high-frequency activity (HFA), which is activity of ± 70 Hz and higher. HFA can be physiological, reflecting normal brain function, or pathological, in which case it reflects abnormal brain function, as can occur in people with epilepsy. In patients with epilepsy who are candidates for epilepsy surgery, HFA can probably assist in determining which brain areas need to be spared, and which part of the brain needs to be removed. We studied the potential of physiological HFA, recorded directly on the brain, for localizing language areas that need to be spared during surgery. Pathological HFA, called high frequency oscillations (HFOs), is considered a biomarker for epilepsy, but identifying pathological HFOs (ripples, 80-250 Hz and fast ripples, 250-500 Hz) in EEG recordings can be challenging. We developed a method to identify channels from invasive EEG that contain HFOs, without the need to detect the individual events. Pathological HFOs can also be recorded on the scalp, but physiological HFOs had not yet been observed in scalp EEG recordings. We found physiological ripples in scalp EEG, and report that they often co-occur with EEG phenomena specific for sleep. Future research is needed to establish if these physiological ripples are involved in memory consolidation, as is the case for physiological ripples that have been recorded in animal studies.