Episodic memory encoding in middle age: effects of ageing and cognitive fatigue on brain activation

Healthy cognitive aging is thought to impact most heavily on episodic memory [1]. However, changes in episodic memory prior to the age of 60 are more controversial than changes in older adults [2]. Furthermore, cognitive decline already present in middle age may not yet manifest in behavior due to the action of neural compensation processes that preserve performance at the behavioral level. Therefore, fMRI can provide valuable insights into age-related changes present in middle age [3]. It is also important to determine the extent to which middle-aged adults must compensate for the effects of cognitive aging in order to maintain performance not just in the short-term, but following sustained, fatigue inducing task performance likely, for example, to be commonly encountered during the workday. Therefore, in the current study, each participant was tested twice: once following a fatiguing condition involving the sustained performance of cognitively demanding tasks and once following a less demanding baseline condition.At baseline, activation was greater, primarily in PFC regions, in middle-aged compared to young adults. This suggests increased exertion of top-down cognitive control in middle-aged adults during successful encoding. In the fatigue condition, activation differences between the two age groups were no longer apparent. Activation in both age groups, but particularly the middle-aged group, decreased in comparison to baseline activation. Therefore, in a state of induced fatigue, middle-aged adults no longer showed greater exertion of cognitive control than young adults, and instead showed activation changes suggestive of an exhaustion of cognitive resources.