Symbolic Dynamics: Reducing the information content of the EEG for interictal focus localization in mesial temporal lobe epilepsy.

RATIONALE: There is increasing evidence that nonlinear time series analyses of the intracranially recorded EEG provide additional information about the spatio-temporal dynamics of the epileptogenic process. We have evaluated the capability of complexity measures derived from the theory of symbolic dynamics for interictal focus localization in mesial temporal lobe epilepsy. METHODS: Using a moving-window technique, the information content of long-lasting stereoelectroencephalographic recordings was reduced by partitioning the amplitude range. Varying the degree of reduction, three different measures were used to quantify the spatio-temporal variations of determinism in the resulting symbol sequences. These comprise the conditional Shannon entropy, the mutual information, and the algorithmic complexity. The latter was normalized to the mean value of an ensemble of surrogate sequences in order to focus on possible nonlinearity in the EEG. RESULTS: Even with a massively reduced information content in the symbol sequences of the EEG the different measures revealed maximum determinism at recording sites ipsilateral to the side of the primary epileptogenic area in the majority of the investigated cases. CONCLUSIONS: EEG symbol sequences with a massively reduced information content are able to provide additional information for the interictal localization of the primary epileptogenic area in mesial temporal lobe epilepsy. This technique might thus contribute to an improvement of the presurgical evaluation.