Task difficulty influences heart rate variability: evidence from a pilot study using a multi-level mental arithmetic task

Mental arithmetic task is a classic paradigm for inducing psychological stress and is widely used in heart rate variability research. However, findings across task difficulty are inconsistent, partly due to a lack of standardized difficulty gradation, uncontrolled task order effects, and unconsidered response delays in heart rate variability. We developed a multi-level arithmetic system with low, medium, and high level, combining a serial subtraction task and a Unity-based programmed task. Fifteen healthy graduate students completed the experiment. Electrocardiogram was recorded before and during tasks, and heart rate variability frequency-domain and nonlinear metrics were analyzed as baseline-relative changes. Subjective workload was assessed with NASA-TLX. NASA-TLX ratings and error rates indicated that the difficulty manipulation was effective. Frequency-domain HRV metrics showed higher values under medium- and high-difficulty conditions compared with the low-difficulty condition, while exhibiting no further proportional increases between the medium- and high-difficulty levels, suggesting a possible saturation pattern. In contrast, nonlinear HRV metrics differentiated task difficulty levels more consistently and exhibited response patterns that were more closely aligned with subjective workload ratings. Within the current experimental context, frequency-domain HRV metrics appeared to show limited sensitivity under low and high workload conditions, potentially due to disengagement and saturation effects. By comparison, nonlinear HRV metrics demonstrated greater sensitivity to graded task difficulty and may provide complementary information for characterizing psychological stress.