Sleep increases firing rate modulation during interictal epileptiform discharges in mesial temporal structures

Epileptic seizures and interictal epileptiform discharges are strongly influenced by sleep and circadian rhythms. However, human data on the effect of sleep on neuronal behaviour during interictal activity have been lacking. We analysed EEG from eight epileptic patients implanted with macro and micro electrodes in mesial temporal structures. Sleep staging was performed on polysomnography and video-EEG. Automated detection identified thousands of interictal epileptiform discharges per patient. Both their rate and amplitude increased with deeper stages of non-rapid eye movement sleep. Single- and multi-unit firing rates were often temporally coupled with local field potentials, exhibiting increased firing during the spike and decreased activity during the following slow wave. These time-locked firing rate modulations were shown to increase during deeper stages of non-rapid eye movement sleep. Furthermore, neuronal background activity showed a decrease in firing rate, bursting and regularity with deeper stages of non-rapid eye movement sleep.