Neural networks involving the medial temporal structures in temporal lobe epilepsy

Objectives: In a previous study using the averaged coherence technique to s tudy interactions between medial/limbic and lateral/neocortical regions, we observed that epileptogenic networks in temporal lobe epilepsy seizures (T LES) could be divided into 4 subtypes, i.e. medial (M), medial-lateral (ML) , lateral-medial (LM), and lateral (L). In the ML and LM subtypes, medial s tructures and the anterior temporal neocortex are co-activated at the onset of seizures. However, using this approach, we were unable to determine the direction of coupling and may have overlooked non-linear variations in int erdependency. The purpose of the present study using non-linear regression for analysis of stereoelectroencephalographic (SEEG) signal pairs was to me asure the degree and direction of coupling between medial and neocortical a reas during TLES. in patients with the M, ML, and LM subtypes. methods. Eighteen patients with drug-resistant TLEs who underwent SEEG reco rding were studied. We used a non-linear correlation method as a measure of the degree and the direction of coupling on SEEG signal pairs. Patients wi th pure lateral TLEs were not studied. Wa analyzed the functional coupling between 3 regions of the temporal lobe: the anterior temporal neocortex, th e amygdala, and them anterior hippocampus. A physiological model of EEG gen eration was used to validate the non-linear quantification method and asses s its applicability to real SEEG signals. Results: Results are first based on a physiological model of EEG data in wh ich both degree and direction of coupling are explicitly represented, thus allowing construction of the neural systems inside which causality relation ships are controlled and generation of multichannel EEG signals from these systems. These signals provide an objective way of studying the performance of non-linear regression analysis on real signals. In medial networks (10 patients), the ictal discharge is limited to the med ial limbic structures and may propagate secondarily to the cortex. Quantifi ed results demonstrated no significant coupling between medial and lateral structures at the beginning of the seizures. Conversely, almost constant un idirectional or bidirectional coupling was observed between hippocampus and amygdala. In medial-lateral (5 patients) and lateral-medial (3 patients) networks, th e initial ictal discharge includes both limbic and neocortical. regions. A rapid 'tonic' discharge is observed over the temporal neocortex at the onse t of seizure. Quantitative analysis showed an initial increase in the non-l inear correlation coefficient between neocortex and medial structures. Quan tification of the coupling direction demonstrated influence of medial over lateral structures (medial-lateral) or of the lateral neocortex over medial structures (lateral-medial). Conclusions: These results confirm the existence of several generic and org anized networks involving the medial structures during TLE seizures. (C) 20 01 Elsevier Science Ireland Ltd. All rights reserved.