Abstract:

Healthy aging is generally associated with a decline in episodic memory. Usually, older adults show more significant declines in associative memory than in item memory. Most previous cognitive aging studies of episodic memory focus on mean changes in memory performance with age and thus assume that older adults are a homogenous group. However, older adults demonstrate notable individual differences in episodic memory. While most older adults show a normal or pathological decline in episodic memory, some indicate successful episodic memory aging. Therefore, it is essential to investigate the neural mechanisms of individual differences in episodic memory aging to demystify the determinants of successful memory aging. This helps reveal the neural basis of human cognitive function and is insightful for developing effective interventions to improve memory function in older adults and delay cognitive aging. To date, four critical theories have been proposed to explain why some older adults exhibit successful memory aging: brain maintenance, neural dedifferentiation, cognitive reserve, and neural compensation. The brain maintenance account claims that the ability of some older adults to demonstrate successful memory aging may be explained by individual differences in brain preservation. In line with this prediction, individuals who demonstrate a relative lack of senescent brain changes and more youth-like brain activation patterns show higher levels of memory performance. The neural dedifferentiation view states that individuals with higher functional specificity of brain regions and networks may have superior episodic memory. The concept of cognitive reserve states that individual differences in cognitive operations or processes shaped by lifetime exposures allow some individuals to maintain cognitive function in the face of brain aging or pathology. The above concepts may reflect the optimization process of the selective optimization with compensation (SOC) model. Specifically, brain maintenance may reflect the results of the optimization, whereas cognitive reserve may reflect the approach for conducting the optimization process. The neural compensation hypothesis, which reflects the compensation process of the SOC model, states that some older adults can compensate for age-related neural decline or pathology to preserve high levels of cognitive and behavioral output. Based on these theories and the SOC model, we speculate that some older adults display successful memory aging because they have higher cognitive reserve shaped by several lifestyle factors throughout their lifespans. These factors include education, occupation, physical activity, cognitively stimulating activities, and other lifestyle aspects. Older adults with higher cognitive reserve can optimize the function of the brain regions and networks related to episodic memory and more successfully compensate for age-related neural decline. Ultimately, the benefits of the optimization and compensation processes are reflected in maintaining a higher level of brain function (e.g., the fidelity of neural representation or functional segregation of brain networks). Nevertheless, there are still debates regarding how to define the concept of cognitive reserve operationally, how cognitive reserve and compensation relate to one another, and how lifestyle factors affect brain maintenance, cognitive reserve, and neural compensation in older adults. Future research should incorporate more longitudinal studies to investigate the relationship between these theories and their impact factors, which would be beneficial for understanding the neural mechanisms of successful memory aging and providing support for improving brain and cognitive health in older adults.

Key words: episodic memory, successful aging, brain maintenance, neural dedifferentiation, cognitive reserve, neural compensation