Exploring the effects of cognitive load on mental fatigue across lifespan using behavioral task and eye-metrics data

Although mental fatigue is known to negatively impact performance, it remains unclear how cognitive resources interact with fatigue levels across the adult lifespan. Using a dual-task paradigm, we manipulated cognitive load (low vs. high) to induce varying levels of mental fatigue in young, middle-aged, and older adults. We assessed self-reported fatigue, sleepiness, and motivation pre- and post-task, while exploring task-related fluctuations in performance (accuracy and reaction times) and eye-metrics (pupil size, pupillary response speed, and long blinks). Younger adults reported higher subjective fatigue and had lower task performance than older adults. There was no significant interaction between age and cognitive load condition in performance. Results indicated that pupil size and long blinks are useful indicators of cognitive load in younger and middle-aged adults but may be less reliable in older adults. These findings suggest exploring alternative measures to better capture cognitive and physiological responses to mental fatigue in older populations.