A role for the hippocampus in encoding simulations of future events

The role of the hippocampus in imagining the future has been of considerable interest. Preferential right hippocampal engagement is observed for imagined future events relative to remembered past events, and patients with hippocampal damage are impaired when imagining detailed future events. However, some patients with hippocampal damage are not impaired at imagining, suggesting that there are conditions in which the hippocampus may not be necessary for episodic simulation. Given the known hippocampal role in memory encoding, the hippocampal activity associated with imagining may reflect the encoding of simulations rather than event construction per se. The present functional (f)MRI study investigated this possibility. Participants imagined future events in response to person, place, and object cues. A postscan cued-recall test probing memory for detail sets classified future events as either successfully encoded or not. A contrast of successfully versus unsuccessfully encoded events revealed anterior and posterior right hippocampal clusters. When imagined events were successfully encoded, both anterior and posterior hippocampus showed common functional connectivity to a network including parahippocampal gyrus, medial parietal and cingulate cortex, and medial prefrontal cortex. However, when encoding was unsuccessful, only the anterior hippocampus, and not the posterior, exhibited this pattern of connectivity. These findings demonstrate that right hippocampal activity observed during future simulation may reflect the encoding of the simulations into memory. This function is not essential for constructing coherent scenarios and may explain why some patients with hippocampal damage are still able to imagine the future.