Purpose: To analyze and compare spectral properties and interdependencies of intracerebral EEG signals recorded during interictal periods from mesial temporal lobe structures in two groups of epileptic patients defined according to the involvement of these structures in the epileptogenic zone (EZ).
Methods: Interictal EEG activity in mesial temporal lobe (MTL) structures (hippocampus, entorhinal cortex and amygdala) was obtained from intracerebral recordings performed in 21 patients with drug-resistant mesial temporal lobe epilepsy (MTLE group). This group was compared with a "control" group of patients (non-MTLE group) in whom depth-EEG recordings of MTL show that seizures did not start from the MTL. Comparison criteria were based on spectral properties and statistical coupling (nonlinear correlation coefficient h(2)) of MTL signals.
Results: Power spectral density analysis showed a significant decrease in the theta frequency sub-band (p=0.01) in the MTLE group. Nonlinear correlation (h(2)) values were found to be higher in the MTLE group than in the NMTLE group (p=0.0014). This effect was significant for theta, alpha, beta and gamma frequencies. Correlation values were not correlated with the frequency of interictal spikes (IS) and significant differences between groups were still measureable even when spikes were suppressed from analyzed EEG periods.
Discussion: This study shows that, during the interictal state, the EZ in MTLE is characterized by a decrease of oscillations in the theta sub-band and by a general increase of signal interdependencies. This last finding suggests that the EZ is characterized by network of neuronal assemblies with a reinforced functional connectivity.