Document Type

Dissertation

Date of Degree

Spring 2016

Degree Name

PhD (Doctor of Philosophy)

Degree In

Neuroscience

First Advisor

Melissa C. Duff

Abstract

Semantic memory includes vocabulary and word meanings, conceptual information, and general facts about the world (Tulving, 1972). According to the standard view of semantic memory in cognitive neuroscience, the hippocampus is necessary to first acquire new semantic information (Gabrieli, Cohen, & Corkin, 1988), but these representations are then consolidated in the neocortex and become independent of the hippocampus with time (McClelland, McNaughton, & O'Reilly, 1995). Remote semantic memory is considered independent of the hippocampus, and the hippocampus is not thought to play a critical role in the processing and use of such representations.

The current work challenges the notion that previously acquired semantic knowledge, and its use during communication, is independent of the hippocampus. A group of patients with bilateral hippocampal damage and severe impairments in declarative memory were tested. Intact naming and word-definition matching performance in amnesia, has led to the notion that remote semantic memory is intact in patients with hippocampal amnesia. Motivated by perspectives of word learning as a protracted process where additional features and senses of a word are added over time, and by recent discoveries about the time course of hippocampal contributions to on-line relational processing, reconsolidation, and the flexible integration of information, we revisit the notion that remote semantic memory is intact in amnesia. Using measures of semantic richness and vocabulary depth from psycholinguistics and first and second language-learning studies, we examined how much information is associated with previously acquired, highly familiar words in hippocampal amnesic patients. Relative to healthy demographically matched comparison participants and a group of brain-damaged comparison participants, the patients with hippocampal amnesia performed significantly worse on both productive and receptive measures of vocabulary depth and semantic richness. In the healthy brain, semantic memory appears to get richer and deeper with time. Healthy participants of all ages were tested on these measures and strong correlations are seen with age as older healthy adults displayed richer semantic knowledge than the younger adults. The patient data provides a mechanism: hippocampal relational binding supports the deepening and enrichment of knowledge over time. These findings suggest that remote semantic memory is impoverished in patients with hippocampal amnesia and that the hippocampus supports the maintenance and updating of semantic memory beyond its initial acquisition.

The use of lexical and semantic knowledge during discourse was also examined. Amnesic patients displayed significantly lower levels of lexical diversity in the speech they produced, and showed a strong trend toward producing language with reduced levels of semantic detail suggesting that patients cannot use their semantic representations as richly during communication. These results add to a growing body of work detailing a role for the hippocampus in language processing more generally.

By documenting a role for the hippocampus in maintaining, updating, and using semantic knowledge, this work informs theories of semantic memory and it's neural bases, advances knowledge of the role of the hippocampus in supporting human behavior, and brings more sensitive measures to the neuroscientific study of semantic memory.

Public Abstract

This work examines how the brain supports humans’ knowledge about the world. Semantic memory is not memory of specific times and places, but is general knowledge of facts about the world and words’ meaning. It’s been previously shown that a part of the brain called the hippocampus, which is necessary for making new memories, is important for learning new facts about the world. However, it’s thought that with time, our knowledge becomes independent of the hippocampus and is supported by other brain structures. This work challenges that idea and attempts to document a role for the hippocampus in updating and enriching our knowledge over time, and in using our knowledge richly during communication.

By testing a rare group of patients with damage relatively confined to the hippocampus on both sides of their brain, I was able to show that they have much less information associated with the words that they know than healthy people. By borrowing methods from other fields that consider how rich and deep our knowledge can be, I show that these patients have profound impairments that have been missed by the standard tests that neuroscience traditionally uses. Furthermore, their use of this knowledge during communication is impaired. The variety of words that patients use when speaking is significantly less diverse than healthy comparisons and they user fewer semantic details. Together, these studies advance our knowledge of semantic memory and the brains structures that support it, advance our knowledge of the role of the hippocampus in supporting human behavior, and bring more sensitive methods to the neuroscientific study of semantic memory.

Keywords

publicabstract, Hippocampus, Memory, Psycholinguistics, Relational memory, Semantic Memory, Updating knowledge

Pages

xi, 149 pages

Bibliography

Includes bibliographical references (pages 122-146).

Copyright

Copyright 2016 Nathaniel B. Klooster

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