Healthy aging, processing speed, and mnemonic brain state engagement

Healthy older adults exhibit both selective impairments in episodic memory - memory for events situated within a specific time and place - and deficits in executive function, reflected by difficulty switching between different tasks and inhibiting task-irrelevant information. Prior work has shown that older adults show diminished mnemonic brain state engagement - recruitment of whole brain activity patterns that selectively support memory encoding and memory retrieval. Our hypothesis is that older adults are biased toward the retrieval state and, due to executive function deficits, cannot easily switch out of this state when task-irrelevant. Our goal was to determine the extent to which stimulus processing time impacts older adult mnemonic state engagement, with the expectation that longer processing times would enable older adults to switch out of a task-irrelevant retrieval state. We recorded scalp electroencephalography (EEG) while younger and older adult participants explicitly encoded and retrieved object stimuli under variable stimulus durations. Using a combination of multivariate decoding approaches, we find that under time constraints, older adults both under-recruit a young-adult like retrieval state when task-relevant, but over-recruit a participant-specific retrieval state when task-irrelevant. Older adults may thus recruit idiosyncratic activity patterns to compensate for difficulties engaging young-adult like mnemonic brain states. Taken together, these findings suggest that although older adults retain the ability to engage encoding and retrieval brain states, they require more processing time to both initiate and maintain goal-relevant mnemonic states.