Spatio-temporal dynamics of the primary epileptogenic area in temporal lobe epilepsy characterized by neuronal complexity loss.

Neurons involved in the epileptic processes exhibit high frequency discharges scarcely modulated by physiological brain activity. This behaviour should be accompanied by a loss of complexity in the corresponding electrographic signal. From the theory of non-linear dynamics it is known that the correlation dimension allows a quantitative description of complexity in terms of the number of degrees of freedom. To test whether a relationship exists between spatio-temporal alterations of neuronal complexity and spatial extent and temporal dynamics of the epileptogenic area, a moving-window correlation dimension analysis was applied to intracranially recorded electrocorticograms of 20 patients with unilateral temporal lobe epilepsy. Dimension as a function of time was calculated for interictal activity (n = 98) and seizure activity including the pre- and postictal phase (n = 28) from recording locations within the epileptogenic area, in adjacent areas and in homologous contralateral sites. Pronounced changes of the dimension in time were found, gradually decreasing with increasing distance from the focal area. Extraction of a single value quantifying the dimension variance of interictal activity allowed the primary epileptogenic area to be laterized in exact agreement with the results of the presurgical work-up and the confirmation of the postoperative outcome, without the necessity of observing actual seizure activity.