Sharing mental simulations and stories: hippocampal contributions to discourse integration

Accumulating evidence suggests that mental simulation of the future and past relies on common processes supported by the hippocampus. However, it is currently unknown whether the hippocampus also supports the ability to share these mental simulations with others. Recently, it has been proposed that language and language-related structures in the brain are particularly important for communicating information not tied to the immediate environment, and indeed specifically evolved so that humans could share their mental time travels into the future and the past with others. The current study investigated whether processes supported by the hippocampus are necessary for effectively communicating the contents of one's mental simulations by examining the discourse of amnesic patients with medial temporal lobe damage. In Experiment 1 we tested whether patients can produce integrated discourse about future and past events by measuring lower-level discourse cohesion and higher-level discourse coherence. Striking reductions in both measures were observed in amnesic patients' narratives about novel future events and experienced past events. To investigate whether these deficits simply reflected concurrent reductions in narrative content, in Experiment 2 we examined the status of discourse integration in patients' verbal narratives about pictures, which contained an equivalent amount of narrative content as controls'. Discourse cohesion and coherence deficits were also present when patients generated narratives based on pictures, and these deficits did not depend on the presence of neural damage outside the hippocampus. Together, these results reveal a pervasive linguistic integration deficit in amnesia that is not limited to discourse about the past or the future and is not simply secondary to reductions in narrative content. More broadly, this study demonstrates that the hippocampus supports the integration of individual narrative elements into coherent and cohesive discourse when constructing complex verbal accounts, and plays a critical role in the effective communication of information to others.