Abstract
Rationale: High-frequency oscillations (HFOs) in intracranial EEG (iEEG) are used to delineate the epileptogenic zone during presurgical diagnostic assessment in patients with epilepsy. HFOs are historically divided into ripples (80–250 Hz), fast ripples (FR, >250 Hz), and their co-occurrence (FRandR). In a previous study, we had validated the rate of FRandRs during deep sleep to predict seizure outcome. Here, we ask whether epileptic FRandRs might be confounded by physiological FRandRs that are unrelated to epilepsy.Methods: We recorded iEEG in the medial temporal lobe MTL (hippocampus, entorhinal cortex, and amygdala) in 17 patients while they performed cognitive tasks. The three cognitive tasks addressed verbal working memory, visual working memory, and emotional processing. In our previous studies, these tasks activated the MTL. We re-analyzed the data of these studies with the automated detector that focuses on the co-occurrence of ripples and FRs (FRandR).Results: For each task, we identified those channels in which the HFO rate was modulated during the task condition compared to the control condition. However, the number of these channels did not exceed the chance level. Interestingly, even during wakefulness, the HFO rate was higher for channels within the seizure onset zone (SOZ) than for channels outside the SOZ.Conclusion: Our prospective definition of an epileptic HFO, the FRandR, is not confounded by physiological HFOs that might be elicited by our cognitive tasks. This is reassuring for the clinical use of FRandR as a biomarker of the EZ.
Highlights
When considering epilepsy surgery, the recording of intracranial EEG is a standard procedure to identify the seizure onset zone (SOZ; Jobst et al, 2020)
For each task, we identified those channels in which the High-frequency oscillations (HFOs) rate was modulated during the task condition compared to the control condition
Even during wakefulness, the HFO rate was higher for channels within the seizure onset zone (SOZ) than for channels outside the SOZ
Summary
The recording of intracranial EEG (iEEG) is a standard procedure to identify the seizure onset zone (SOZ; Jobst et al, 2020). HFOs have been shown to indicate the EZ both in intraoperative ECoG (Fedele et al, 2016, 2017b; van ’t Klooster et al, 2017; Weiss et al, 2018; Boran et al, 2019c) and in presurgical iEEG recordings (Akiyama et al, 2011; Fedele et al, 2017a) while the results of a clinical trial are still pending (van ’t Klooster et al, 2015). The HFO rate in surface EEG mirrors epilepsy severity (Boran et al, 2019d; Fan et al, 2020; Klotz et al, 2021)
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