Nonlinear dynamic properties of low calcium-induced epileptiform activity

The analysis of the dynamic properties of epileptiform activity in vitro ha s led to a better understanding of the time course of neural synchronizatio n and seizure states. Nonlinear analysis is thus potentially useful for the prediction of seizure onset. We have used nonlinear analysis methods to in vestigate the development of activity in the low calcium model of epilepsy in brain slices. This model is particularly interesting since neurons synch ronize in the absence of synaptic transmission. The dynamic properties calc ulated from extracellular recordings of activity were used to analyze the t ransition to synchronous firing and their relation to neuronal excitability . The global embedding dimension, local dimension and the Lyapunov exponent were calculated from time segments corresponding to the onset, transition and fully developed stages of activity. The analysis was repeated for recor dings made in the presence of various levels of DC electric fields to modul ate neuronal excitability. The global and local dimensions did not change o nce activity was first initiated, even in the presence of the electric fiel d. The maximum Lyapunov exponents increased during the onset of activity bu t decreased when the applied hyperpolarizing electric field was large enoug h to partially suppress the activity. These findings establish a relationsh ip between neuronal excitability and the maximum Lyapunov exponent, and sug gest that the Lyapunov exponent may be used to distinguish between various states of the neural network and might be important in seizure prediction a nd control. (C) 2001 Elsevier Science B.V. All rights reserved.