Visual vigilance performance and standing posture at sea

Background: Research on land has demonstrated interactions between visual tasks and the control of stance. Variations in visual tasks can influence postural activity; conversely, changes in stance can influence quantitative measures of visual performance. At sea, crewmembers may alter their bodily movements to compensate for ship motion while simultaneously adjusting movements to optimize visual performance. Mutual influence of visual tasks on postural movements and of postural movements on visual tasks has not been demonstrated at sea. Methods: Crewmembers stood on a force plate, from which we obtained data on the positional variability of the center of pressure (COP). We varied stance width (5 cm, 17 cm, and 30 cm) and the difficulty of visual vigilance tasks (Easy vs. Hard). We collected data on visual performance and we assessed subjective mental workload. Results: Visual performance was better on the Easy task (mean ď = 4.28) than on the Hard task (mean ď = 3.55). Consistent with this result, subjective mental workload was greater for the Hard task than for the Easy task. The variability of postural activity was greater during the Easy task (mean = 2.0 cm • s^-1) than during the Hard task (mean = 1.9 cm • s^-1). Stance width influenced postural activity, but also (in 2-way and 3-way interactions) influenced visual performance. Both the magnitude and dynamics of postural activity changed over days at sea. Conclusions: In mild seas, variations in stance width modulate standing posture and influence the performance of computer-based visual tasks.