Background: Motion of a ship at sea often challenges crew performance. In previous studies, the influence of rough weather on stance at sea has been evaluated in terms of the likelihood of staggers or falls. Few studies have evaluated the influence of sea state on visual performance. Effects of rough seas on visual vigilance performance, subjective mental workload, and the kinematics of postural control have not been demonstrated.
Method: Crewmembers of the R/V Thomas G. Thompson stood on a force plate, from which we obtained data on the center of pressure (COP). We varied stance width (the distance between the feet in side-by-side stance; 5 cm, 17 cm, and 30 cm) and the difficulty of visual vigilance tasks (Easy vs. Hard). Separately, we evaluated subjects' self-selected foot positioning.
Results: Visual performance was better on the Easy task (mean d' = 4.20) than on the Hard task (mean d' = 3.57). Overall vigilance performance (mean d' = 3.88) was worse than when the same subjects were tested under mild sea states (mean d' = 4.11). Subjective mental workload (mean = 28.0) was greater than under mild sea states (mean = 19.9). Relative to mild sea states the variability of postural activity was greater and its predictability was reduced. In addition, postural dynamics were influenced by controlled variations in stance width.
Conclusions: Rough seas affect visual vigilance performance and postural activity, but do not eliminate the effects of vigilance task difficulty or stance width that have been found in mild seas.