Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2004.Also issued in leaves.Includes bibliographical references.(cont.) increased activity whenever recollection was attempted, independent of the level of recollection success. The results are consistent with the hypothesis that increased left-lateralized retrieval activity in older adults supports recollection attempt. Age deficits in semantic memory are more subtle than in episodic memory. However, older adults are impaired when automatic, data-driven processes are not sufficient to support the retrieval of conceptual knowledge. The fMRI study described in Chapter 3 used semantic repetition priming to test two theories of the role LIPC plays in semantic retrieval. Young adults exhibited repetition-related BOLD response reductions in LIPC that were specific to the particular semantic task engaged, consistent with the hypothesis that LIPC supports controlled semantic retrieval. Older adults, in contrast, exhibited repetition-related signal reductions even when the semantic judgment made about a word differed across the two exposures, consistent with the hypothesis that older adults fail to gate irrelevant semantic information from working memory during initial presentation of the word.Older adults are impaired in episodic and semantic retrieval but the extent of these deficits and their neural correlates is unknown. In episodic memory, older adults appear particularly impaired in retrieving bound information, such as conjunctions of items or of an item and its context. These retrieval deficits, however, may be merely the downstream effects of poor encoding. Chapter 1 presents a series of studies that test the theory that age-related recollection deficits are due to encoding failures. Results revealed that older adults were impaired in associative recognition when self-initiated processes were required at acquisition. Additional encoding support eliminated age differences, however, even when the retrieval task was made more difficult. The results support the hypothesis that recollection deficits are primarily due to poor encoding. Although older adults with encoding support can retrieve information as well as young adults, it is an open question whether brain activity supporting retrieval is identical in the two groups. In past studies, greater left prefrontal activity has been observed in older adults even when their performance does not differ from young adults. However, the circumstances under which this pattern arises and its functional significance are still unknown. Chapter 2 presents a functional magnetic resonance imaging (fMRI) study of associative recognition by young and older adults who performed equally well but who showed different patterns of recollection-related activity. Young adults exhibited greater activity in left inferior prefrontal cortex (LIPC) and inferior temporal/fusiform gyri for retrieval based on recollection relative to retrieval based on familiarity. In the same regions, older adults exhibitby Gail O'Kane.Ph.D.