Effects of Age and Resource Depletion on Post-retrieval Monitoring and Individual Differences in Memory Performance

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Episodic memory declines substantially and, relative to other forms of memory, disproportionately, with age. Understanding the cognitive and neural bases of age-related episodic decline in healthy subjects is important because even the modest impairment (by clinical standards) typical of healthy individuals entering their 70’s is sufficient to have a detrimental impact on quality of life. Identifying the specific cognitive processes, and their neural substrates, which are most responsible for age-related memory decline is a crucial precursor to the development of potential rehabilitative interventions. Equally important, a full understanding of the much more severe memory impairments characteristic of age-related neurodegenerative diseases such as Alzheimer’s Disease will be difficult to achieve without knowledge of how memory and its neural substrates vary over the course of the healthy lifespan. The aim of the present research program is to investigate the possible role in age-related differences in episodic memory performance of the ‘post-retrieval monitoring’ processes that support evaluation of the products of an episodic retrieval attempt with respect to their relevance for current behavioral goals. The program takes as its starting point highly consistent findings from two experiments conducted as part of an ongoing research program. In both experiments fMRI contrasts between test items imposing high versus low demands on post-retrieval monitoring identified differential activity in regions of the prefrontal cortex previously implicated in monitoring and other cognitive control processes. The magnitude of these putative ‘fMRI monitoring effects’ did not differ between young adults in their 20s and older adults in their mid-60s to mid-70s, and correlated significantly with episodic memory performance. The findings are intriguing given the widely held view that cognitive functions supported by the PFC are especially vulnerable to increasing age. It is proposed that the findings can be understood from the assumption that older adults require more neural resources than young adults to achieve equivalent levels of performance, and thus that older adults will suffer resource depletion at lower levels of cognitive demand than the young. It is therefore predicted that the seeming age invariance in the neural correlates of post-retrieval monitoring and their relationship with memory performance will break down when the demands placed upon monitoring are especially high, or when the cognitive resources available to support monitoring are depleted. Two experiments, one involving fMRI, and one involving TMS, are proposed. If the predictions are fulfilled, the findings would support the proposal that, with increasing age, post-retrieval monitoring becomes more vulnerable to disruption when cognitive demands are high or neural resources are depleted. These findings would suggest that the processes supporting monitoring would be worth targeting in interventions aimed at ameliorating age-related memory decline. Equally important, failure of the predictions would strongly suggest that, although an important determinant of memory performance, monitoring is unlikely to be a significant cause of age-related memory decline.