The Relationship Between Age, Cognitive Performance, and the Neural Correlates of Episodic Memory Encoding and Retrieval
Cognitive aging is associated with a disproportionate decline in episodic memory, the ability to recollect contextual details of previously experienced events. Understanding the mechanisms which underlie age-related episodic memory decline is a critical precursor to developing interventions aimed at ameliorating memory deficits in healthy and pathological aging. Considerable empirical evidence suggests that age-related episodic memory deficits arise from numerous factors which differentially impact multiple neural processes and brain regions. The present work focuses on examining some contributors which have been proposed under this framework. Study 1 investigates age-related neural dedifferentiation, a phenomenon characterized by age-related reductions in the neural selectivity of category-selective cortical regions. Our analyses reveal robust age effects on neural differentiation for scene, but not for face stimuli, adding to prior evidence indicating that age-related neural dedifferentiation is not a ubiquitous phenomenon. Study 1 also reveals that the strength of neural differentiation during encoding is predictive of subsequent memory performance independently of age. The work in Study 1 is complemented by Study 4 in which neural dedifferentiation is operationalized at the level of individual exemplars (as opposed to stimulus categories). To examine item-level neural differentiation, we framed our analyses in terms of age differences in repetition suppression effects, which revealed null effects of age. Collectively, Studies 1 and 4 highlighting the functional significance of age-related neural dedifferentiation and emphasize the urgent need to advance our understanding of the factors that lead to age differences in neural selectivity and specificity. Moving on to Study 2, the work described therein examines age differences in retrieval gating, the ability to regulate the retrieval of mnemonic information according to behavioral goals. Study 2 provides the first evidence that older adults do not engage in retrieval gating, indicating that episodic memory decline may arise as consequence of a decline in the engagement of goal- dependent retrieval strategies. Lastly, Study 3 reveals novel evidence for age differences in the retrieval-related anterior shift, the phenomenon whereby the peak neural activity at retrieval occurs in more anterior portions of single cortical regions relative to encoding. Our analyses show that the shift is greater in older than younger adults, and that greater shift is associated with worse memory performance independently of age. In line with prior empirical work proposing a posterior (perceptual) to anterior (conceptual) gradient in the brain, these findings indicate that the age- related increase in anterior shift may be reflective of an increased reliance on gist-based low- fidelity retrieval in older age. Taken together, the studies comprising this dissertation enhance our understanding of the behavioral and neural correlates of cognitive aging and advance the collective knowledge in the field cognitive neuroscience of age-related episodic memory decline.