Identification of the epileptogenic zone using phase-amplitude coupling on ictal stereoelectroencephalography

Phase-Amplitude Coupling (PAC) has been proposed to characterize electrophysiological changes at seizure onset. However, PAC can arise from actual relations between oscillations at different frequencies (authentic PAC) or from harmonic/transient effects (spurious PAC). To assess the importance of these PAC mechanisms, we investigated the Modulation Index (MI) and Time Locked Index (TLI) between Epileptogenic Zone (EZ) and non-EZ channels during seizures.

We analyzed 34 patients with drug-resistant epilepsy who underwent stereoelectroencephalography (SEEG) implantation. SEEG data was classified into three periods: "preictal," "fast," and "rhythmic." The amplitude of ripple bands and the phase of three slow-wave bands were used for MI and TLI. The capacity of MI to detect the EZ was tested with precision and recall measures, summarized by the F1 score.

The MI was significantly greater in EZ channels than that in non-EZ channels for all periods. The best F1 results were obtained for the "fast" period and spurious PAC. Both authentic and spurious PAC were present during all periods and bands.

Different PAC values could be observed during different seizure periods. Care should be taken when interpreting MI as a measure of authentic PAC.

MI and TLI can help the interpretation of ictal cross-frequency observations.