Relationship between single-unit activity and the electroencephalogram in a neocortical, cobalt-induced epileptogenic...

Although the importance of neuronal synchrony in epilepsy has not been disputed, few attempts have been made to examine quantitatively the relationship between this parameter and seizure occurrence. The specific objective of the present investigation was to determine how the amount and type of synchrony between EEG and single-unit activity in an experimental model of focal epilepsy are related to the occurrence of seizures. This was accomplished by examining EEG/single unit relationships in two types of cobalt-induced epileptogenic focus: (1) foci that initiated seizures, and (2) foci that exhibited only interictal spike activity. These relationships were examined during slow-wave sleep, a time when synchronous neuronal activity is thought to be augmented. In control rats and rats that had seizures, the majority of units exhibited a non-random relationship between unit discharge and the EEG. In cobalt-treated rats that were not observed to have seizures, however, the percentage of units exhibiting EEG/single unit relationships was significantly less than that in either controls or rats that had seizures. This observation, paired with observations of the details of the EEG/single unit relationships, led to the hypothesis that cobalt treatment produces a shift from an inhibition dominated synchrony (observed in controls) to an excitation dominated synchrony (observed in rats that had seizures). Intermediate between these two types of synchrony is a less synchronized state (observed in seizure-free, cobalt-treated rats), which probably results from a loss of inhibition dominated synchrony without a concomitant increase in excitation dominated synchrony.