Lay summary
Established models of human long-term memory do not envision a role for the human hippocampus - a brain structure necessary for conscious learning - in nonconscious forms of learning and memory. These models posit a role for the human hippocampus in the conscious acquisition and retention of experienced episodes. In order to remember an episode, one has to encode the constituent parts of the episode (people, furniture, spoken words, etc.) and their conceptual, temporal and spatial relationships from moment to moment while experiencing the episode. Hence, the on-line encoding of a complete episode requires rapid learning of multiple conceptual, temporal, and spatial associations between the constituents of an episode. Therefore, rapid learning of new associations can be considered the precondition for episodic memory. Recent brain imaging experiments and patient studies have suggested that it is humanly possible to rapidly form and later retrieve new associations even outside consciousness and that the hippocampus is involved in this form of nonconscious learning. Consequently, the existence of a nonconscious form of episodic-like memory can be hypothesized. With this SNF project, we try to draw parallels between conscious and nonconscious forms of episodic learning and memory. We attempt to show that conscious and nonconsicous episodic memories invoke the same storage mechanisms in the brain and that these storage mechanisms are distinct from those storage mechanisms that support cognitively simpler forms of nonconscious learning. We hypothesize that the storage of both consciously and nonconsciously encoded episodic memories depends on the interaction between hippocampus and neocortex during slow wave sleep. To this end, we trigger a reactivation of the formed memories to induce the fixation of consciously and nonconsciously acquired memories by displaying cues over headphones to the sleeping subjects. We collect electroencephalographic and functional magnetic resonance imaging data to compare brain activity associated with conscious and nonconscious learning, memory reactivation during sleep, and retrieval. The results might potentially speak in favor of a new memory model that allots a role to the hippocampus in both conscious and nonconscious forms of episodic memory.