Abstracts

RATIONALE: In a previous study we compared different techniques from linear and nonlinear time series analysis in an application to intracranial EEG recordings of the seizure free interval of patients with mesial temporal lobe epilepsy (MTLE) (Epilepsia 2001, 42, Suppl. 7, 98). It was demonstrated that particularly nonlinear time series analysis measures in combination with the method of surrogates (termed NST techniques) allowed a correct lateralization of the focal hemisphere in a high percentage of cases. However, NST techniques implicitly assume stationarity of the investigated dynamics, a pre-requisite not fulfilled for neuronal dynamics. Consequently, many periods of EEG recordings exhibit nonstationary features, while other periods appear to be stationary. The aim of the present study was therefore to elucidate the influence of nonstationarity on the discriminative power of NST techniques for the focal hemisphere in MTLE patients.
METHODS: Our retrospective out-of-sample study was based on intracranial EEG recordings of the seizure-free interval of 38 patients with unilateral MTLE. Using a moving window technique on average 116 minutes per patient were analyzed. In a first step a criterion for weak stationarity was applied in order to separate the EEG into stationary and nonstationary segments. For all segments a set of surrogate time series was generated using an iterative amplitude adjusted technique. Three measures from nonlinear time series analysis (coarse grained flow average, prediction error, and an estimate of the correlation dimension) were calculated from both the original EEG time series and the corresponding set of surrogate time series. The differences between these values (EEG and the surrogates[ssquote] mean values) were used as NST measures. The subsequent evaluation was carried for all segments and solely based on stationary segments.
RESULTS: Based on all segments a correct localization could be established for 35 out of 38 cases for the NST measures based on the coarse grained flow average and the correlation dimension, and in 33 cases for the prediction error. For the individual patients the portion of segments that were classified as nonstationary ranged from 15% to 40%. Nonetheless, based solely on the remaining stationary segments still 34 cases were correctly lateralized for the coarse grained flow average and the correlation dimension, while the performance of the NST measure based on the prediction error did not change at all.
CONCLUSIONS: In agreement with recent studies our results indicate that NST measures can contribute valuable information to the lateralization of the focal hemisphere without the necessity of observing actual seizure activity. Furthermore, the discriminative power of NST techniques for the focal hemisphere is not related to the influence of nonstationarity of the EEG as tested here.
[Supported by: Deutsche Forschungsgemeinschaft]