Frequency band coupling with high-frequency activities in tonic-clonic seizures shifts from θ to δ band.

• The θ band (4–8 Hz) was coupled with high-frequency activity (HFA) in the tonic phase of focal to bilateral tonic-clonic seizures. • The δ band (2–4 Hz) was coupled with HFA in the clonic phase of focal to bilateral tonic-clonic seizures. • The δ-HFA phase-amplitude coupling discriminated well the clonic from the tonic phase. To clarify variations in the relationship between high-frequency activities (HFAs) and low-frequency bands from the tonic to the clonic phase in focal to bilateral tonic-clonic seizures (FBTCS), using phase-amplitude coupling. This retrospective study enrolled six patients with drug-resistant focal epilepsy who underwent intracranial electrode placement at Osaka University Hospital (July 2018–July 2019). We recorded 11 FBTCS. The synchronization index (SI) and receiver-operating characteristic (ROC) analysis were used to analyze the coupling between HFA amplitude (80–250 Hz) and lower frequencies phase. In the tonic phase, the θ (4–8 Hz)-HFA coupling peaked, and the HFA power occurred at baseline (0 μV) of θ oscillations. In contrast, in the clonic phase, the δ (2–4 Hz)-HFA coupling peaked, and the HFA power occurred at the trough of δ oscillations. ROC analysis indicated that the δ-HFA SI discriminated well the clonic from the tonic phase. The main low-frequency band modulating the HFA shifted from the θ band in the tonic phase to the δ band in the clonic phase. Neurophysiological key frequency bands were implied to be the θ band and δ band in tonic and clonic seizures, respectively, which improves our understanding of FBTCS. [ABSTRACT FROM AUTHOR]