In this study we analyzed intracranial electroencephalographic (EEG) recordings from patients with pharmaco-refractory unilateral epilepsy of the MTL who were undergoing invasive diagnostics prior to resective surgery of the seizure-generating area. During this procedure patients were implanted with intracranial depth electrodes to localize the focus of seizure onset. We take advantage of the fact that in our patient group these electrodes were positioned along the longitudinal axis of the MTL (Spencer, 1987) with contacts both in the hippocampus and in the entorhinal cortex, thereby allowing us to determine the level of synchrony within and between these structures from electrode contacts that are spaced apart at a constant distance, which would not be possible for depth electrodes implanted orthogonally to this axis (Olivier et al., 1987). In addition we apply strictest criteria to ascertain unilaterality of the epileptic process based on histopathological evaluation of the resected tissue and assessment of post-operative seizure control. Within this framework, we address the question whether the interplay between the human hippocampus and entorhinal cortex is governed by a common (global) theta rhythm or whether both regions exhibit essentially independent theta rhythms.
