Determinants of mental fatigue: The roles of cognitive control and real-time task difficulty adjustments

INTRODUCTION: Mental fatigue impairs performance and safety. This research aims to uncover the underlying causes of mental fatigue by identifying factors that intensify and accelerate its onset. We developed computer-based protocols manipulating (a) cognitive-control demands and (b) real-time task difficulty adjustments designed to promote sustained engagement with the fatiguing activity. METHODS: In a within-subject single-task study (n = 36) and a between-subject multi-task study (n = 72), we manipulated task parameters from three executive function domains (inhibition, switching, updating) across conditions of (a) lower vs higher cognitive-control demands without difficulty adjustments and (b) high cognitive control with vs without difficulty adjustments, where speed increased following correct responses and decreased otherwise. Subjective, behavioural, and physiological data were analysed for changes in cognitive load, task engagement, and fatigue. Statistical significance was evaluated using ANOVA planned contrasts and reported as percentage changes. RESULTS: The within-subject study showed that higher (vs lower) cognitive control increased perceived workload by 16.25% while reducing boredom by 23.80% and sleepiness by 6.40%. It also slowed responses by 9.75%, impaired accuracy by 2.65%, and decreased blink rate by 11.50%. Difficulty adjustments (vs none) heightened workload by 34.33% and led to 7.08% less accurate but 32.58% faster responses, with a further 15.89% reduction in blink rate. The between-subject study extended these findings to mental fatigue. Higher (vs lower) cognitive control did not consistently elevate perceived workload but led to 11.01% lower accuracy and 58.98% slower response times. Difficulty adjustments (vs none) increased workload by 37.22% across all executive function domains and impaired accuracy by 23.85% while speeding up responses by 20.24%. Mental fatigue was 60.87% larger at the end than the beginning of the study on average across all conditions but emerged 28.69% faster under higher cognitive-control demands with (vs without) difficulty speed adjustments. A 15.22% reduction in parasympathetic activity and 78.98% more blink flurries were observed immediately after (vs before) completing the high cognitive-control condition with difficulty adjustments, possibly reflecting compensatory responses to elevated cognitive workload. CONCLUSION: These findings reveal how prolonged exertion of cognitive control and task difficulty adjustments can intensify cognitive load and fatigue. While the former can create additional cognitive load, the latter promotes task engagement and accelerates fatigue onset. Beyond identifying contributors to mental fatigue, this research introduces an innovative, time-efficient approach that addresses limitations of previous one-size fits-all fatiguing protocols, promising to reduce opportunity for disengagement and therefore variability in individual responses in mental fatigue.