Abstracts

Rationale: Previous studies have shown that bursts of high-frequency activity are localized to seizure onset zones. The frequency of their occurrence and their durations are greater in the seizure onset zone than outside it, suggesting that they are important for seizure generation. We hypothesize that the patterns by which they propagate in cortical networks will provide useful information for the planning of epilepsy surgery. Methods: Intracranial EEG of several patients, recorded during multiple seizures (5-86), was parameterized using multivariate autoregressive models (MVAR), and analyzed with short-time direct directed transfer function (SdDTF), which estimates the direction, intensity and spectral content of activity propagation among cortical sites, as well as its evolution in time. Segments of epileptic activity were aligned at their electrographic onset, and the propagation of interictal activity (intervals of 500 sec, recorded 1 hour before seizure onset) and pre-ictal, activity (intervals of 60-240 sec, recorded immediately before seizure onset defined by routine EEG interpretation) were analyzed, for the frequency range 70-170 Hz. Results: One hour before seizure onset, as well as immediately before seizure onset, SdDTF analyses showed bidirectional propagation of high-frequency activity between sites within ictal onset zones identified by epileptologists, and between these sites and all other analyzed recording sites. In contrast, relatively little propagation of this activity was observed among the other recording sites. This pattern was observed for depth electrodes, as well as for subdural electrodes. For a patient diagnosed with a non-focal, regional seizure onset, however, this pattern was not observed. Conclusions: Analysis of the propagation of high-frequency activity may be a useful tool for delineating the epileptogenic zone during invasive monitoring for epilepsy surgery. The seizure onset zone, at least in some patients, may be characterized by a large divergence and convergence of propagated high frequency activity that is present not only in the immediate pre-ictal period, but also during interictal intervals. Supported by: NINDS R01 NS40596 and NS48222