Validation of a dual 2-back task as an effective method for inducing acute mental fatigue

This study proposes a novel dual 2-back task involving updating of visuospatial working memory, which was designed to induce acute mental fatigue. The dual 2-back task was itself embedded in a protocol designed to overcome common limitations of time on task studies, such as learning effects, confounding effect of boredom, and an absence of psychophysiological index of mental effort. Seventy-three healthy young adults were required to continuously update visual information for 30 min without any rest intervals, ensuring sustained high cognitive demand. During the dual 2-back task, multiple measures were collected, including self-reported fatigue and sleepiness, signal detection performance (d'), EEG markers (P300 amplitude, frontal theta and posterior alpha power density) and cardiac pre-ejection period (PEP) reactivity. Participants reported a significant increase in fatigue and sleepiness, and exhibited reduced ability to detect a match, indicating a decline in performance. Concurrently, EEG indices reflected fatigue-related changes, with a decrease in P300 amplitude and an increase in alpha power. In contrast, frontal theta and PEP reactivity, two markers of mental effort, did not vary significantly over time. Interestingly, participants did not report increased boredom, and the changes in performance and EEG markers persisted even when controlling for boredom. Overall, these results confirm that the dual 2-back task is an effective method to induce mental fatigue. However, given the considerable interindividual variability observed, future studies should adopt analytical approaches that consider the strategies participants use to cope with mental fatigue (e.g., effort disengagement versus compensatory effort).