Abstracts

Rationale: Modern neuroimaging studies have shown the involvement of the human retrosplenial cortex (RSC) in episodic memory processing. However, due to its location deep in the brain and between the two cerebral hemispheres, the electrophysiological correlates of RSC function cannot be understood with scalp EEG. The RSC shares strong anatomical connections with regions involved in episodic memory processing, such as the medial temporal lobe (MTL). These regions both exhibit well characterized theta oscillations (2-6 Hz) that coordinate local neural activity and may mediate a functional interaction between RSC and MTL during memory processing. Methods: Intracranial electrocorticography was recorded from RSC and MTL sites in 4 patients undergoing surgical evaluation for the treatment of refractory epilepsy. Responses were recorded from patients during self-paced true/false judgments of arithmetic and autobiographical stimuli. Results: As predicted by their anatomical relationship, RSC and MTL were found to exhibit significant (p < 0.05) functional connectivity selective to episodic memory processing. Phase locking (PLV) was found to increase with episodic memory stimuli (but not other control stimuli) in the hypothesized theta band (2-6 Hz) that was independently from concomitant theta band power increases. Furthermore, an increase in the RSC broadband power (70-180 Hz) followed the termination of theta band PLV between the RSC and MTL. Conclusions: We have shown a role of slow oscillations in mediating memory selective communication between RSC and MTL. Furthermore, our results suggest RSC's role in memory processing to be initiated by transitory involvement in a larger memory processing network including the MTL and hippocampus. Due to its anatomical and functional connectivity with the MTL, the RSC can be a key structure for propagation of ictal discharges and memory deficits in patients with temporal lobe epilepsy. It is also possible that seizures originating in the RSC may be clinically similar to MTL seizures.