1. Interactions between conscious and unconscious memories.

We assume that there is one memory space for both conscious and unconscious forms of relational memory, and that the hippocampus is its neural substrate. Therefore, we expected that unconscious and conscious retrieval of relational memory traces could interact with one another.

In an fMRI experiment, we found that this was the case: an unconscious retrieval of a relational memory trace facilitated the subsequent retrieval of a similar conscious memory trace, and this facilitation was associated with reduced hippocampal activation. This reduced engagement of the hippocampus suggests an efficiency effect: if you just unconsciously activated a certain memory trace, like "Obama = politician", it becomes easier for you to name the occupation of, say, Angela Merkel. We assume that the unconscious reactivation has pre-activated overlapping relational representations in the hippocampus reducing the neural effort for conscious retrieval.

2. Long-term effects of subliminal messages

Using subliminal stimulus presentation techniques, we could show that relational memories can be encoded and later retrieved without conscious access - but how long do these unconsciously formed memory traces last? In this study, we found that unconscious episodic memory traces endured over a period of 15 to 25 minutes and could still influence decision making thereafter.

3. Memory encoding during sleep

In an ongoing project, we investigate verbal memory encoding during sleep with EEG and fMRI. Slow wave sleep is of principal importance for the consolidation and abstraction of memories acquired during the previous waking phase. The peaks of the eponymous electrophysiological slow waves, measured in the EEG during slow wave sleep, are markers of the neuronal plasticity required for memory functions of sleep. We presented translation pairs of a fake foreign language to participants while they were in slow wave sleep. After waking, participants were able to unconsciously identify semantic aspects (object size) of these foreign words if we managed to play the translations into ongoing slow wave peaks multiple times. These results show that the sleeping brain can react to its environs, and that it is possible to encode new semantic information even while in the deepest sleep.

Züst, M. A., Colella, P., Reber, T. P., Vuilleumier, P., Hauf, M., Ruch, S., & Henke, K. (2015). Hippocampus Is Place of Interaction between Unconscious and Conscious Memories. PLoS ONE, 10(3), e0122459. http://doi.org/10.1371/journal.pone.0122459

Ruch, S., Züst, M. A., & Henke, K. (2016). Subliminal messages exert long-term effects on decision-making. Neuroscience of Consciousness, 2016(1), niw013. https://doi.org/10.1093/nc/niw013