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This study tested the hypothesis that early functional alterations in neuronal synchrony in the partially deafferented cortex may lead to spontaneously occurring electrographic seizures. In vivo experiments with partial deafferentation of cat suprasylvian gyrus after extensive undercut of the white matter were conducted using multi-site EEG, extracellular unit and intracellular recordings. The amplitudes of EEG waves were much higher in the areas surrounding deafferented cortical fields as compared with control and with undercut cortex. In 40% of animals with undercut cortex, paroxysmal activity occurred 2–3 h after the undercut and was initiated in the relatively intact cortex, adjacent to the more disconnected one. The seizures that followed the undercut consisted of spike-wave/polyspike-wave complexes and fast runs, resembling the electrographic patterns of some clinical epileptic syndromes. An increased local synchrony in the relatively intact cortex evolved into paroxysmal activity that ultimately spread to the deafferented cortex. The electrographic seizures were found only in animals that showed a propagation of the slow sleep-like oscillation in control conditions. The increase of long-range synchrony within a given seizure was associated with seizure termination. These results indicate that alterations in neuronal synchrony following neuronal trauma can be a critical factor triggering electrographic seizures.