Enhanced Cardiac Vagal Tone in Mental Fatigue: Analysis of Heart Rate Variability in Time-on-Task, Recovery, and Reactivity

Heart Rate Variability (HRV) has been suggested as a useful tool to assess fatigue-sensitive psychological operations. The present study uses between and within-subject design to examine the temporal profile of HRV including the changes related to reactivity, Time-on-Task (ToT), and recovery on a cognitively demanding task. In the fatigue group, participants worked on a bimodal 2-back task with a game-like character (the Gatekeeper task) for about 1.5 hours, followed by a 12-minute break, and a post-break block of performance (about 18 min). In the control group, participants watched documentaries. We hypothesized an increasing vagal-mediated HRV as a function of Time-on-Task. In contrast, we expected to find no change in vagal mediated HRV in the documentary viewing group. We also analysed the trial-based post-response cardiac activity as a physiological associate of task-related motivation. Relative to the control, ToT was associated with an elevated level of subjective fatigue, decreased heart rate, and increased HRV most robustly in the vagal-mediated components. Based on fatigued participants’ post-error cardiac slowing, and post-error reaction time analyses, we found no evidence for motivation deficit in association with increasing HRV and ToT. The present findings suggest that the parasympathetic branch of the autonomous nervous system functioning as a relaxation system tends to be activated under increasing mental fatigue. In addition, the study provides evidence that many HRV indices might be changed also if individuals are engaged in a prolonged less fatiguing activity (e.g. documentary viewing). This finding emphasizes the relevance of control conditions in ToT experiments.