Visualization of epileptogenic phenomena using cross-correlation analysis:Localization of epileptic foci and propagation of epileptiform discharges

The main objectives of the preoperative evaluation of a patient with medica lly intractable epileptic seizures are localization of the foci and propaga tion of the epileptiform discharges. Electrocorticographic (ECoG) data of i ntractable focal epilepsy were analyzed using an AR model, wavelet analysis , and cross-correlation analysis. In order to derive the time-shifts, the c ross correlations of the epileptiform discharges were calculated beta een e lectrodes for every unit of time. Further analyses were made by means of a set of contour maps of the time-shifts and sequential two- and three-dimens ional visualizations of the time-shift maps in order to localize the epilep tic foci and study their propagation process. Two types of foci and propaga tion were revealed in the results. In the first type, epileptiform discharg es were generated at localized focal sites and spread quickly to other site s. In the second type, the foci of epileptiform discharges, which appeared soon after the former bursts, were localized at more than one site, and the discharges tended to spread more slowly. The findings suggest that epilept ic phenomena can be caused by at least two kinds of mechanisms in one patie nt: in the former, the propagation might be mediated through synaptic proje ctions, while in the latter, the extracellular diffusion of an excitatory f actor might play an important role. In addition, our newly developed visual ization technique for the localization of epileptic foci and the propagatio n of epileptiform discharges should prove useful in the study of epileptoge nesis etiology.