Episodic memory decline is one of the hallmarks of human cognitive ageing, but our understanding of the neurocognitive mechanisms underlying this decline remains limited. In particular, it is unclear whether healthy ageing differentially affects distinct components of episodic memory retrieval; specifically, the probability of successfully retrieving information from memory, and the quality, or precision, of the retrieved memory representations. The research reported in this PhD thesis used continuous measures of memory retrieval to dissociate these two alternative sources of age-related memory deficits and their cognitive and neural underpinnings, providing more detailed insight into the nature of age-related episodic memory decline.

Two behavioural experiments reported in Chapter 2 provided initial evidence for differential effects of healthy ageing on the success and precision of episodic memory retrieval, suggesting greater sensitivity of mnemonic precision to age-related declines. Chapter 3 assessed whether these decreases in memory precision are specific to long-term memory or may be explained by age-related decreases in the fidelity of perceptual or working memory processes. The results from this experiment demonstrated that age-related reductions in the precision of episodic memory retrieval persisted after controlling for decreases in the fidelity of perception and working memory, suggesting a predominantly long-term memory basis for this deficit.

The functional and structural magnetic resonance imaging experiments reported in Chapters 4 and 5 sought to elucidate the neural basis of age-related changes in the success and precision of episodic memory retrieval. Results from Chapter 4 revealed distinct encoding and retrieval contributions to the decreases in these two aspects of memory retrieval exhibited by older adults. At retrieval, age-related reductions in activity associated with successful memory retrieval were observed in the hippocampus, while decreases in activity underlying the precision of memory retrieval were evident in the angular gyrus. Furthermore, at encoding, age-related decreases in activity predicting both later success and precision of memory retrieval were evident in the fusiform gyrus, while prefrontal reductions were observed in the encoding activity predicting the subsequent success of memory retrieval only. In addition to these functional changes, Chapter 5 provided evidence for the role of structural integrity of the lateral parietal cortex in individual differences in mnemonic precision across older adults.

Together, the results reported in this thesis highlight the sensitivity of memory precision to age-related cognitive decline, and suggest both distinct and common factors underlying age-related decreases in the success and precision of episodic memory retrieval.