NONLINEAR-ANALYSIS OF EPILEPTIC ACTIVITY IN RABBIT NEOCORTEX

We report on the nonlinear analysis of electroencephalogram (EEG) reco rdings in the rabbit visual cortex. Epileptic seizures were induced by local penicillin application and triggered by visual stimulation. The analysis procedures for nonlinear signals have been developed over th e past few years and applied primarily to physical systems. This is an early application to biological systems and the first to EEG data. We find that during epileptic activity, both global and local embedding dimensions are reduced with respect to nonepileptic activity. Interest ingly, these values are very low (d(E) approximate to 3) and do not ch ange between preictal and tonic stages of epileptic activity, also the Lyapunov dimension remains constant. However, between these two stage s the manifestations of the local dynamics change quite drastically, a s can be seen, e.g., from the shape of the attractors. Furthermore, th e largest Lyapunov exponent is reduced by a factor of about two in the second stage and characterizes the difference in dynamics. Thus, the occurrence of clinical symptoms associated with the tonic seizure acti vity seems to be mainly related to the local dynamics of the nonlinear system. These results thus seem to give a strong indication that the dynamics remains much the same in these stages of behavior, and change s are due to alterations in model parameters and consequent bifurcatio ns of the observed orbits.