Interspike intervals during interictal periods in human temporal lobe epilepsy

We recorded 259 single neurons from mesial temporal lobe structures of 21 patients with complex partial seizures. Interspike intervals within clusters of action potentials (clustered interspike intervals) recorded from cells in mesial temporal structures ipsilateral to seizure initiation were compared to clustered interspike intervals in the contralateral temporal lobe. ‘Clusters’ were defined as any group of three or more spikes separated by intervals of less than a defined maximum, or two spikes separated by less than half that maximum. The maximum interspike interval which defined a cluster was varied from 5 to 40 ms in 5-ms steps. Significantly smaller proportions of clustered spikes were discharged by neurons in the amygdala, hippocampus and entorhinal cortex from the temporal lobe commonly initiating seizures, compared to neurons in contralateral homotopic regions. When data from the same three structures were combined, significantly fewer cluster interspike intervals between 10 and 25 ms were recorded from cells on the side of seizure onset. Because clustered action potential discharge is a normal pattern of firing for cells that discharge endogenous bursts, the relative decrease in proportions of 10–25 ms clustered interspike intervals occurring in the temporal lobe initiating seizures might reflect a reduction in endogenous burst discharges from that side. Reduced endogenous bursting could be due to the loss of burst discharging neurons as a product of seizure-related excitotoxicity. The identification of decreased interictal single neuronal burst discharge in epileptogenic structures stresses the difference between the interictal and ictal states in patients with complex partial seizures, and the importance of the transition between those states.