Abstracts

RATIONALE:_Previous studies have shown that nonlinear analysis of intracranial activities can detect a preictal phase several minutes before the seizure in mesial temporal lobe epilepsy (MTLE). Nevertheless, the underlying neuronal process need to be clarified. Here we utilize a new measure of phase-locking adapted to complex signals with broad-band spectrum in order to examine the synchronization between the epileptogenic zone and remote sites during preictal dynamical changes. METHODS: 19 preseizure recordings, each 1 hour in duration, were analyzed from 10 patients with MTLE during evaluation for epilepsy surgery. The instantaneous phase was extracted for consecutive windows of 5 seconds by the Hilbert transform. The strength of phase-locking between pairs of channels was statistically characterized in 2 Hz steps over the whole part of the spectrum from 0.1 to 100 Hz by the Shannon entropy of the phase differences. In addition, preictal changes were characterized by a nonlinear measure of similarity quantifying the extent to which the brain dynamics differs from a reference state taken from the seizure (NeuroReport 10, 2149). RESULTS: The dynamics of synchrony between the epileptogenic zone and remote areas are clearly associated with changes of the nonlinear measure. The changes began in most cases (17/19) several minutes (mean 546 seconds) before the seizure. In all cases, these changes were mostly observable in the gamma band (30-80 Hz). Furthermore the changes in gamma synchrony do not take place within a narrow frequency band, but present a broad-band spectrum across high frequencies, supporting their role as the underlying nonlinear mechanism. CONCLUSIONS: Persistent changes of the gamma synchrony underlies nonlinear changes detected several minutes prior to seizure and may give important clues about mechanism of ictogenesis in MTLE. These results further support the hypothesis that dynamical entrainment of a critical mass is a necessary condition for seizure occurrence. Finally, because gamma frequency synchronies are demonstrated to be implicated in active cognition, our results might contribute to a new understanding of prodromic symptoms.