Adult age differences in the effects of chronic mental fatigue on task-related fatigue, appraisals, and performance.

Engagement in potentially beneficial activities is assumed to be based partially on perceived costs and benefits. Mental fatigue may be one factor that affects perceived costs. We examined age differences in both chronic and situational mental fatigue, and their relation to task perceptions and engagement levels. Younger (M age = 32.6) and older (M age = 73.1) adults completed questionnaires measuring subjective mental fatigue, physical and mental health, and motivational states, along with several assessments of cognitive ability. In addition, assessments of effort expenditure (systolic blood pressure) and task difficulty were collected during performance of a cognitively demanding memory-scan task. Two components of chronic mental fatigue (CMF) relating to affect and motivation were identified. Although the structure of CMF did not vary with age, CMF was differentially predictive of engagement and appraisal levels across groups. As CMF-Affect levels increased, older adults' task-difficulty appraisals increased more than those of younger adults. In addition, CMF-Affect was positively associated with levels of engagement in the young, but negatively associated in the old as memory load increased. Older adults also exhibited higher levels of situational fatigue than did the young, as evidenced by increased levels of effort expenditure as both time on task and objective demands increased. However, little relationship existed between chronic and situational mental fatigue in either age group. These findings suggest that the affective aspect of CMF is particularly consequential in older adults, with both task appraisals and engagement levels being negatively affected when fatigue is high.