The Neurophysiology of Age-Related Memory Decline
The mechanisms of memory are energetically investigated, but remain incompletely understood. The better the understanding of the physical underpinnings of memory developed by the research community, the more likely it is to find effective ways to intervene in order to reverse the well-known age-related decline of memory function. The work here is one example of many lines of research aimed at better mapping the way in which memory is stored and maintained in the brain.
Working memory function is a critical cognitive ability that deteriorates with age following adulthood. Beyond its well-studied role in sensorimotor control, rhythmic neural activity in the beta band (15 to 25 Hz) has been suggested to regulate the status of working memory contents. Dynamics in beta-band activity reflect working memory processing. There is a decrease in beta activity when information needs to be maintained and an increase when information needs to be deleted. Maintenance-related beta decrease is primarily observed in the prefrontal cortex. By contrast, post-response beta increase is observed among task-related networks involving frontal and centroparietal regions, facilitating removal of both memory contents and associated representations such as motor plans after responses. Specifically, neurophysiological evidence from nonhuman primates demonstrated localized post-response beta increase at sites containing memory information during the time course of working memory clear-out. Whether such dynamics can be observed in human electrophysiology and how these neural dynamics change with age is unknown.
We adopted a novel approach to assess between- and within-group differences across ages. We combined cross-trial variability, which has largely been studied with broad-band EEG signal and fMRI hemodynamic responses, with rhythmic dynamics in the beta range, and examined them during both working memory maintenance and post-response deletion phases. Our novel analytical approach suggests that, when considering cross-trial fluctuations of beta power, variability explains individual differences in working memory performance during distinct phases for each age group.
Whereas individual memory performance of younger adults was explained by frontal beta variability during maintenance, memory performance of older adults was primarily explained by post-response beta variability. Thus, task-related cross-trial variability augments individual state-dependent characteristics and predicts behavioral differences within and across age groups. With the age-related dissociations between maintenance and post-response phases, beta variability may serve as an age-related, task-sensitive signature of individual differences in distinctive working memory computations.