Abstracts

Nonlinear dynamical analysis of EEG has provided useful insights into the progressive preictal entrainment, and subsequent postictal disentrainment of the epileptic brains spatio-temporal EEG activity (IEEE TBME 2003; :616-627). This transition at seizures is described as dynamical resetting of the epileptic brain (IEEE TBME 2004; :493-506). We used nonlinear dynamical analysis of EEG data in chronically epileptic rats which were exposed to deep brain stimulation (DBS) paradigms to test the hypothesis of dynamical resetting by DBS. Five male Sprague-Dawley rats (300-350 g) were implanted with an array of 8 Teflon coated Tungsten wire depth electrodes (175 [mu]m diameter): 4 monopolar implanted in cortex, 2 monopolar in the dentate area of hippocampus, and 2 bipolar in the centromedial nucleus of thalamus. Two rats were used as controls while the other three were established with chronic epilepsy following an episode of status epilepticus using the lithium-pilocarpine model. EEG was continuously recorded and all rats were intermittently stimulated in the thalamus for a duration of one minute with either 130, 10, or 1 Hz constant current bipolar stimulation (100 [mu]sec pulse-width, 250-750 [mu]A intensity). All rat EEG data were analyzed using nonlinear dynamical techniques that use the convergence or divergence of the largest short-term Lyapunov exponent over time at each electrode site to statistically quantify the brain[apos]s dynamical entrainment or disentrainment respectively. In all stimulation cases, 130 Hz DBS resulted in disentrainment or no change of brain dynamics depending on the electrode pairs considered, while 1 and 10 Hz stimulation resulted in a mix of entrainment and disentrainment depending on the electrode pair considered. The disentraining effect of 130 Hz DBS lasted [sim]40 minutes in control rats, while in the chronically epileptic rats the effect was shorter, [sim]20 minutes. Dynamical changes due to DBS were usually larger and encompassed more brain areas in the control rats than in epileptic rats. The results of the study showed that effects of DBS can be monitored utilizing nonlinear techniques of brain dynamics in a spatio-temporal manner. The method was able to distinguish both entrainment and disentrainment across a range of stimulation frequencies. Higher frequency (130 Hz) stimulation was the most successful in disentraining multiple brain areas and resulted to a more global resetting of the brain. This is the first time that nonlinear techniques have been used to assess the effectiveness of DBS. This method of analysis is expected to be useful for the evaluation of electrical stimulation paradigms for control of epileptic seizures. (Supported by Barrow Neurological Foundation.)