The Fragility of Temporal Memory in Alzheimer's Disease

Abstract

Background: Although episodic memory impairment is one of the hallmarks of Alzheimer's disease (AD), the relative decline in the components of episodic memory (What, Where, and When) and the effects of cognitive training on each of them are still unknown. Objective: We aimed to independently assess the impairment in each component of episodic memory in early to moderate AD and address whether it can be enhanced through active, spatiotemporal episodic training. Methods: A non-verbal scene-based episodic memory task was developed to assess the ability to remember What, Where, and When information. Experiment 1 tested whether this task can differentiate AD subjects (N = 16) from healthy controls (N = 16). In Experiment 2, 13 AD subjects underwent 16 training sessions, followed by a re-administration of the scene-based memory task. Experiment 3 tested 42 healthy older adults and 51 younger adults on the same task to investigate the effects of normal aging. Results: Of the three components, When memory had the highest predictive power in distinguishing AD from normal aging. Following training of AD subjects, only Where memory improved. Only What memory revealed a significant decline in healthy subjects from 65-85 years of age. Conclusion: These findings shed new light on the importance of the temporal component of episodic memory as a behavioral marker of AD. The selective improvement of spatial but not temporal memory through training further demonstrates the fragility of temporal memory even in early AD. Neuroscientific research is needed to distinguish whether the Where component was enhanced by improvements in hippocampal spatial representation or by other compensatory mechanisms.