Abstract
Recent evidence suggests that some seizures are preceded by preictal changes that start from minutes to hours before an ictal event. Nevertheless an adequate statistical evaluation in a large database of continuous multiday recordings is still missing. Here, we investigated the existence of preictal changes in long-term intracranial recordings from 53 patients with intractable partial epilepsy (in total 531 days and 558 clinical seizures). We describe a measure of brain excitability based on the slow modulation of high-frequency gamma activities (40–140 Hz) in ensembles of intracranial contacts. In prospective tests, we found that this index identified preictal changes at levels above chance in 13.2% of the patients (7/53), suggesting that results may be significant for the whole group (p < 0.05). These results provide a demonstration that preictal states can be detected prospectively from EEG data. They advance understanding of the network dynamics leading to seizure and may help develop novel seizure prediction algorithms.
Highlights
Our study evaluated the feasibility of long-term seizure forecasting using intracranial EEG in a multicenter group of epilepsy patients with continuous long-term recordings
Bias introduced by selecting epochs for analysis was removed by using continuous multi-day EEG
We found that a measure of brain excitability based on the coupling between lowfrequency phase and high-frequency amplitude was able to identify preictal states for a significant number of patients (13.2%)
Summary
Slow modulations of high-frequency activity (40–140 Hz) discriminate preictal changes in human focal epilepsy. These cross-frequency couplings have been termed phase-amplitude coupling or ‘‘nested’’ oscillations and they have been suggested to represent a signature of cortical activation[31,32,33,34,35] In this context, human intracranial EEG studies have identified a spatially distributed modulation of cortical high frequency oscillations in the gamma band (40–120 Hz) by theta oscillations Our analysis focused on interactions between the phase of low frequency rhythms (slow waves and theta) and the amplitude of different sub-bands of gamma rhythms This analysis of spatial fluctuations in the coupling phase of ensembles of intracranial contacts revealed preictal changes, occurring at levels greater than chance in a small but significant number of patients. We found that spectral powers did not lead to similar performances, suggesting that the coupling of different frequency bands is specific to the reported preictal changes
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