Conundrums of High-Frequency Oscillations (80–800 Hz) in the Epileptic Brain

Abstract

Pathological HFOs (80-800 Hz) are considered biomarkers of epileptogenic tissue, but the underlying complex neuronal events are not well understood. Here, we identify and discuss several outstanding issues or conundrums in regards to the recording, analysis and interpretation of HFOs in the epileptic brain to critically highlight what is known and what is not about these enigmatic events. HFOs reflect a range of neuronal processes contributing to overlapping frequencies from the lower 80 Hz to the very fast spectral frequency bands. Given their complex neuronal nature, HFOs are extremely sensitive to recording conditions and analytical approaches. We provide a list of recommendations that could help obtain comparable HFO signals in clinical and basic epilepsy research. Adopting basic standards will facilitate data sharing and interpretation that collectively will aid in understanding the role of HFOs in health and disease for translational purpose.

Progress and remaining challenges in the application of high frequency oscillations as biomarkers of epileptic brain

“…Further improvements of HFOs signal quality rely on improvements in EEG acquisition [41,42]. Recording in a shielded environment and utilization of specialized hardware amplifiers can improve signal-to-noise ratio and improve the performance of HFOs detectors [33].…”

“…While early studies were focused on the frequency band of the HFOs oscillations to classify them as pathologic or physiologic, considering the wide range of overlapping pHFOs and nHFOs frequencies [3,4,11,18,42–44], more recent studies have explored other frequency-independent characteristics of HFOs: rate, amplitude, and duration [45–52]. Regions showing a high rate of interictal HFOs have been correlated with pathological epileptic tissue in different studies [2,15,48,52–54].…”

“…To investigate physiological HFOs and pathological HFOs, multiple studies have used the task-induced HFOs [15,55,67]. These task-induced HFOs cover a wide range of frequency bands [42,51,53], for instance, task-induced high gamma oscillations recorded in visual cortex [68], auditory cortex [69], motor cortex [70], and language cortex [71]. Physiologic ripples can also be evoked by visual stimuli in the occipital cortex [55], by somatosensory stimuli in the somatosensory cortex [72], and during memory processing in the hippocampus, parahippocampal and specific neocortical areas [3,36,44,49,50,73].…”

Utilization of independent component analysis for accurate pathological ripple detection in intracranial EEG recordings recorded extra- and intra-operatively

“…First, referential montage increases the spatial resolution of the iEEG as compared with bipolar montage by increasing the total number of recordings. Second, referential montage could improve HFO detection by increasing the signal to noise ratio, since the dipole generators of the HFO could be distributed across multiple macro-electrode sites, which could obscure the HFO due to in phase cancellation (de la Prida et al, 2016). …”

Avaliação de métodos de análises não lineares em sinais eletroencefalográficos na presença de oscilações de alta frequência em pacientes portadores de epilepsia refratária

“…A distinção entre HFO fisiológicas normais e HFO epileptiformes patológicas permanece um desafio para epileptologia clínica [77]- [79]. Uma abordagem simples é classificar HFO associadas com sharp waves epileptiformes como HFO patológicas e HFO evento-relacionadas a tarefas motoras e de memória como normais [80].…”