Individual Patterns of Heart Rate Variability During Prolonged Cognitive Load with Mental Fatigue Induction

We investigated individual dynamics of cognitive performance and heart rate variability (HRV) measures during prolonged mental load. The aim of this study was to identify characteristic phases of cognitive performance and individual patterns of physiological responses induced by prolonged mentally fatiguing tasks. In prolonged experiments with two cognitive tasks of different structure, cognitive performance, HRV measures, and subjective fatigue were analyzed. The proposed integrated index of relative performance made it possible to identify individual phases of cognitive performance: the adaptation phase (AP), the optimal performance phase (OP), and the fatigue phase (FP). The OP was observed in all subjects, the FP in most subjects, whereas the AP was observed in fewer than half of the subjects, indicating pronounced inter-individual variability in cognitive performance dynamics. Two patterns of the initial heart rate (HR) response were identified, ΔHR ≥ 0 and ΔHR < 0, which were associated with differences in the subsequent dynamics of HR and individual HRV measures. The nature of these differences depended on the phase of cognitive performance and the type of task. Among subjects who performed both tasks, approximately half showed the same direction of the initial HR response in the Serial Subtraction Task (SST) and the Working Memory Task (WMT), whereas the remaining subjects showed different directions, indicating limited reproducibility across tasks. The obtained results demonstrate pronounced inter-individual variability in cognitive performance and physiological responses during prolonged mental load and support the need to account for individual dynamics when developing personalized approaches to monitoring mental fatigue.