The role of tread fixations in the visual control of stair walking.

Although it is likely that foveal information on treads provides important sensory cues for stair walking, it is unclear how gaze stabilization on treads contribute to gait control on stairs. The aim of this study was to determine the extent to which (i) stair walking depends on foveal information on stepped treads, (ii) fixated treads correspond to future foot landing locations, and (iii) the distance looked ahead varies with stepping distance. Gaze and foot position was monitored from six healthy young adults when they ascended and descended a 10 tread long staircase, taking the stairs one or two treads at a time. The results showed that 55-68% of the total fixation time was aimed at treads, and that tread edges were fixated more intensively during stair descent (69% of the total time spent fixating treads) than during stair ascent (48%). A substantial 28-34% of the stepped treads was never fixated and, when the staircase was taken two treads a time, approximately 35% of the fixated treads was never stepped on. Subjects fixated 3.5-4.5 treads ahead in both stepping conditions, but when the staircase was taken 2 treads a time, stepped treads were fixated shorter ahead (2.7-2.9 treads) than treads that were not stepped (3.4-4.1 treads). These results provide new insights into the visual control of stair walking, and suggest that the stabilization of gaze on treads is not used solely to guide foot placement, but may serve other purposes as well, e.g., to facilitate postural control on the staircase.