In vivo laminar electrophysiology co-registered with histology in the hippocampus of patients with temporal lobe epilepsy.

Laminar multiple microelectrodes have been developed to sample cortical and hippocampal activity in animals. If these measurements are adequately co-registered with the anatomy of the region, they can yield important information about its function and structure. In vivo laminar electrophysiological recordings from the human epileptic hippocampus are rare. However, histological and immunohistochemical analyses are widely used to determine the structural changes associated with temporal lobe epilepsy (TLE). Here we present data obtained by a combined approach: intraoperative recording of laminar field potentials, single and multiple unit activity under anesthesia, accompanied by histology and immunohistochemistry from the same hippocampal region of epileptic patients undergoing temporal lobectomy for drug-resistant TLE. The stability of the electrophysiology and the accuracy of its co-registration with histology were tested successfully. We have found large field potential spikes associated with bursting single units in CA1. Intracortical and subdural strip recordings from the lateral temporal cortex showed similar field potential activation patterns. A prominent oscillatory activity was present in the dentate gyrus with highly localized field potential gradient and multiple unit activity. This pattern could be used as a landmark defining the position of the electrode in the hippocampus. Our findings indicate that some aspects of the local and network epileptiform activity in the hippocampal formation are likely preserved under anesthesia. Electrophysiological identification of the functional state of the hippocampus together with its local structural correlates could further enhance our understanding of this disease.