The Missing Link: Validating the Working Memory Adaptive Fatigue with N-Back Difficulty Protocol for Active Cognitive Fatigue Induction

Cognitive fatigue is widely invoked to explain performance breakdowns, yet many laboratory inductions are repurposed tasks that may inadvertently confound fatigue with learning, monotony, or disengagement. We present WAND (Working-memory Adaptive-fatigue with N-back Difficulty), a purpose-built, open-source protocol designed to induce active cognitive fatigue (ACF) under controlled conditions. WAND combines (a) structured practice and a performance-plateau phase to minimise learning effects, (b) tiering of task performance, and (c) adaptive difficulty and time compression in interleaved tasks to sustain high cognitive load of appropriate difficulty while limiting boredom. In a behavioural validation study (N = 27), after completing a comprehensive practice phase, participants completed a 65–70-minute induction comprising fixed “probe” Sequential N-back blocks interleaved with adaptive Spatial and Dual N-back blocks. Confirmatory mixed-effects modelling showed a robust decline in working-memory sensitivity from the first to the final block, representing a medium-to-large effect size (Cohen's d = −0.71). This impairment was accompanied by a modest speeding of responses rather than the compensatory slowing typically expected in passive fatigue contexts. A brief visual mini-distractor embedded within the probe task did not show an increased post-distractor cost with fatigue. Subjective ratings of mental fatigue, effort, mind wandering, and feeling overwhelmed increased monotonically across five assessment points. Together, these results validate WAND as a practical and methodologically precise tool for inducing a behavioural profile consistent with ACF, while providing a transparent framework for reproducible fatigue induction and measurement in behavioural and neurophysiological studies.