Effects of constant and incremental modes of music volume and odor concentration on vigilance

Introduction: Vigilance is critical in safety-critical domains, yet sustained attention often declines over time due to sensory adaptation. This study aimed to investigate whether dynamic incremental adjustments of music volume and odor concentration could counteract vigilance decrement more effectively than constant stimulation. Methods: A within-subjects design was adopted, in which 24 participants completed the Psychomotor Vigilance Task (PVT) under five experimental conditions. Subjective vigilance ratings (Karolinska Sleepiness Scale, KSS), behavioral performance indicators (reaction time and number of lapses), and physiological indicators (high-frequency heart rate variability, HF-HRV) were collected simultaneously. Results: Compared to the no-stimulus control condition, any condition involving music or odor was associated with significantly enhanced vigilance. For both auditory and olfactory modalities, the dynamic incremental mode was significantly superior to the constant mode in improving subjective alertness (lower KSS scores), shortening reaction time, reducing lapse count, and decreasing HF-HRV (indicating enhanced physiological arousal). When both modalities were presented incrementally, the optimal vigilance maintenance effect was observed, with most metrics showing significant improvements over the dual-constant condition and numerical trends consistently favoring the dual-incremental condition across all measures. Discussion: The core mechanism underlying this advantage is that the dynamic incremental mode, by increasing stimulus intensity in stages, effectively breaks the cycle of sensory adaptation induced by constant stimulation while dynamically maintaining arousal levels within the optimal range. The additive benefit observed under cross-modal dynamic input suggests enhanced regulation at the level of the autonomic nervous system. This study provides empirical evidence supporting the mechanisms by which dynamic multisensory stimulation counteracts vigilance decrement and offers theoretical support for designing adaptive, multimodal vigilance maintenance strategies in safety-critical domains such as driving and monitoring.