Expansion of perceived passable width in older adults.

Humans can perceive whether an aperture is passable; this ability is known as passable width perception. Previous studies have shown that passable width in older adults is larger when walking and suggested the effect of larger body sway in older adults while walking on passable width expansion. However, no studies have directly investigated this. Thus, the present study conducted two experiments to investigate the effect of body instability on passable width perception in young and older adults. In Experiment 1, young and older adult participants kept standing upright with their feet together (i.e., Romberg stance) or with their feet in line (i.e., Tandem stance) while observing various aperture widths presented on a large display at a distance of 3 m. The participants were required to judge whether the aperture was passable without turning their shoulders. During the task, body sway was measured using a motion capture device. The results showed larger sway in the Tandem stance than in the Romberg stance, irrespective of age group; however, a change in perceived passable width was observed only in older adults: it expanded in the unstable condition compared to the stable condition. In Experiment 2, the young adults performed the same task with a more unstable stance (i.e., one-leg standing) than that adopted in Experiment 1. Nevertheless, it did not affect their passable width perception. These results suggest that only older adults exploit the online status of their body sway to estimate their safety margins even during a non-walking situation. Future studies on passable width perception are expected to measure body sway.