Hippocampal contributions to language: an examination of referential processing and narrative in amnesia

Language production is characterized by an unlimited expressive capacity and creative flexibility that allows speakers to rapidly generate novel and complex utterances. In turn, listeners interpret language "on-line", incrementally integrating diverse representations to create meaning in real-time. A challenge for theories of language has been to understand how speakers generate, integrate, and maintain representations in service of language use and processing and how this is accomplished in the brain. Much of this work has focused prefrontal cortex mechanisms such as "working memory". The goal of this dissertation is to understand the role of the hippocampal declarative memory system (HDMS) in language use and processing, specifically in referential processing and narrative construction.

To test the role of the hippocampus in referential processing, healthy comparisons, brain damaged comparisons (BDC), individuals with bilateral hippocampal damage participated in an eyetracking experiment in which individuals viewed scenes and listened to short stories. The amount of time participants spent looking at the characters after a pronoun reference was recorded. Healthy comparisons and BDC participants preferentially targeted the first mentioned character while participants with hippocampal damage did not, suggesting that the hippocampus plays a role in maintaining and integrating information, even in short discourse history.

In a second experiment, participants with bilateral hippocampal damage and healthy comparisons told narratives multiple times over the course of a month. The narratives were analyzed for the number of words, the number of episodic details, the number of semantic details, the number of editorials and the consistency of details over the multiple tellings. The patients with hippocampal damage told stories that were significantly shorter, more semanticized and less consistent from telling to telling than healthy comparisons.

The final goal of this study was to understand the effects of unilateral hippocampal damage on language processing. Individuals with unilateral hippocampal damage participated in all of the previous experiments. It was predicted that individuals with left hippocampal damage would perform worse than individuals with right hippocampal damage, and their performance was significantly impaired across measures. This suggests that the left hippocampus may be particularly important for processing linguistic material outside of even verbal memory.