Objectives: In a previous study using the averaged coherence technique to study interactions between medial/limbic and lateral/neocortical regions, we observed that epileptogenic networks in temporal lobe epilepsy seizures (TLES) 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 structures 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 interdependency. The purpose of the present study using non-linear regression for analysis of stereoelectroencephalographic (SEEG) signal pairs was to measure the degree and direction of coupling between medial and neocortical areas during TLES in patients with the M, ML, and LM subtypes.
Methods: Eighteen patients with drug-resistant TLEs who underwent SEEG recording 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 with pure lateral TLEs were not studied. We analyzed the functional coupling between 3 regions of the temporal lobe: the anterior temporal neocortex, the amygdala, and the anterior hippocampus. A physiological model of EEG generation was used to validate the non-linear quantification method and assess its applicability to real SEEG signals.
Results: Results are first based on a physiological model of EEG data in which both degree and direction of coupling are explicitly represented, thus allowing construction of the neural systems inside which causality relationships 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 medial limbic structures and may propagate secondarily to the cortex. Quantified results demonstrated no significant coupling between medial and lateral structures at the beginning of the seizures. Conversely, almost constant unidirectional or bidirectional coupling was observed between hippocampus and amygdala. In medial-lateral (5 patients) and lateral-medial (3 patients) networks, the initial ictal discharge includes both limbic and neocortical regions. A rapid "tonic" discharge is observed over the temporal neocortex at the onset of seizure. Quantitative analysis showed an initial increase in the non-linear correlation coefficient between neocortex and medial structures. Quantification 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 organized networks involving the medial structures during TLE seizures.