Abstracts

Rationale: Synchronization of local and distributed neuronal assemblies is thought to underlie fundamental brain processes such as perception, learning, and cognition. In neurological disease, neuronal synchrony can be altered, and in epilepsy it may play an important role in the generation of seizures. Multiple studies have reported that epileptic brain is characterized by increased neuronal synchrony, except possibly prior to seizure onset when synchrony may decrease. Previous studies using intracranial EEG, however, have been limited to patients with epilepsy. Methods: We investigate neuronal synchrony in epileptic and control brain using intracranial EEG recordings from patients with medically resistant partial epilepsy and control subjects with intractable facial pain. Linear cross-correlation and mean phase coherence of local field potentials (LFP) are used to measure neuronal synchrony. Results: For both epilepsy and control patients, average LFP synchrony decreases with interelectrode distance. Interestingly, we find that control patients have greater average LFP synchrony than patients with epilepsy. The difference in average synchrony between control and epileptic brain is shown to be from the functional disconnection of the epileptic brain region generating seizures from surrounding brain. Synchrony between electrode pairs bridging seizure generating brain and other regions was significantly less than between other electrode pairs in the epileptic brain and control brain. With greater activity in a seizure generating region, there is less synchrony with neighboring tissue outside that region. Conclusions: We propose that the region of epileptic brain generating seizures is functionally isolated from surrounding brain regions. We further speculate that this functional isolation may contribute to the spontaneous generation of focal seizures, and it could be a clinically useful electrophysiological signature of epileptic brain.