Abstracts

Rationale: The amplitude of gamma oscillations has been shown to be coupled to the phase of theta oscillations in humans and in animals. The strength of the coupling is modulated by the various states of the brain, including performing mental and/or memory tasks. We investigate here the strength of the coupling between theta and the high-frequency activity during the onset and ictal period of partial complex seizures in humans to assess if the mechanisms responsible for the coupling are maintained during ictal events, and to evaluate the potential of the coupling strength to be used as an indicator of upcoming ictal events. Methods: Data include 331 partial onset seizures recorded from 45 consecutive patients with either mesial temporal, neocortical temporal or extra-temporal onset seizures. Data for each channel closest to the focus was first filtered using a zero phase-lag filter to prevent phase distortion. Instant phase for the theta band (4-8Hz) was calculated using Hilbert transform and local minima in the instantaneous phase were used to determined theta troughs. Higher frequency bands were filtered, and instantaneous amplitudes were calculated using Hilbert transform. Average preictal and ictal periods were calculated over a 20-second window taken 40 seconds before and respectively 20 seconds after the onset. Using a +/- 500ms window centered on each trough, the average theta wave and spectral analysis were computed over all troughs. Results: Due to the occurrence of the ictal event, the average theta wave is significantly larger during the ictal period (Figure 1 - lower). Although there is indication of high-frequency modulation during the preictal periods, during the ictal period the high amplitude theta waves clearly modulated the high-frequency activity in a large range of frequency (Figure 1 - upper). Evidence of the coupling is not apparent in all seizures as not all seizure onsets have a theta component. Conclusions: There is strong indication that the high-frequency theta coupling observed during cognitive tasks is also present during ictal events. Ictal events with high amplitude theta waves, especially during the early phase of the seizures show that the high-frequency activity in the cortical areas is very strongly modulated by the pattern of the theta waves. That modulation is stronger during ictal events than during preictal periods. Previous reports of gamma-theta coupling often highlight the possibility of memory consolidation and learning being linked to that modulation, but the mechanisms behind the coupling observed here may be different. Funded by NIH NS075020