Cross-frequency coupling in mesiotemporal EEG recordings of epileptic patients

Semi-invasive foramen ovale (Fov) electrodes were used to record electrical activity in the vicinity of the inferior mesial temporal region of epileptic patients, in addition to standard scalp EEG. Third order cumulant analysis was used to measure the phase-coupled frequencies corresponding to non-linear coupling of spectral frequency components, somewhat analogous to frequencies of resonance. On the basis of the distribution of these frequencies, an index of resonance (IR) is defined as the ratio between the number of peaks in the gamma-band (40-55Hz) vs. the number of peaks in the beta-band (15-30Hz). The epileptogenic focus was located in the hemisphere with lower resonant frequencies because these frequencies were characteristic of a spread of the seizure over a broader area. In the case of Fov electrodes IR could differentiate a group of patients affected by a tumor compared to patients with mesial temporal sclerosis. The novel index IR appears as an interesting parameter to evaluate the level of interareal functional connectivity in Fov recordings in epileptic patients, but its usage is likely to be extended in electrophysiological studies.