Rhinal–hippocampal connectivity determines memory formation during sleep

Compared with waking state attention, volition and semantic processing play a minor role during sleep. Thus, investigating declarative memory formation during sleep may allow us to isolate mnemonic core processes. The most feasible approach to memory formation during sleep is the analysis of dream memories. Lesion and imaging studies have demonstrated that encoding of declarative memories, i.e. consciously accessible events and facts, depends on operations within the rhinal cortex and the hippocampus, two substructures of the medial temporal lobe. Successful memory formation is accompanied by a transient rhinal-hippocampal interaction. Consequently, the ability to memorize dreams may be related to mediotemporal connectivity. Therefore, we recorded EEG during sleep from rhinal and hippocampal depth electrodes implanted in 12 epilepsy patients (eight women, mean age 41.1 +/- 6.4 years). They were awakened during rapid eye movement sleep (REM) and asked to recall their dream. Via coherence analyses we show that rhinal-hippocampal connectivity values are approximately twice as large for patients with good dream recall versus those patients with poor recall. This suggests that rhinal-hippocampal connectivity is a key factor in determining declarative memory formation.