Foreseeing the future: occurrence probability of imagined future events modulates hippocampal activation

Episodic memory and episodic future thinking are known to share a set of brain regions. Potential differences in activation patterns associated with the two conditions are as yet inconclusive, in particular with respect to hippocampal involvement. Hippocampal activation is modulated by a range of phenomenal qualities during the imagination of both past and future events (Addis et al. (2004) Hippocampus 14:752-762; Addis and Schacter (2008) Hippocampus 18:227-237). A relevant variable in this regard is the occurrence probability of an episode, which varies for future but not past events and thus cannot be equated across conditions. Using parametric modulation analysis, we investigated the effect of occurrence probability of imagined future events on brain activation patterns, while effects of temporal distance, amount of details, and emotionality were controlled for. Activation of right anterior hippocampus increased with decreasing occurrence probability, presumably reflecting higher processing demands during binding of more disparate details for unlikely events. This finding may contribute to the understanding of previously reported inconsistent results concerning hippocampal involvement during the imagination of past and future events.