Vision impairment affects the visual strategy persons use to walk toward a goal

Abstract

Abstract: : Purpose: Fully sighted persons use both visual motion patterns that result from self–motion (optic flow) and perceived–direction cues to guide their paths. The purpose of this study was to determine whether persons with vision impairment use both types of information and to the same degree as fully sighted persons when walking toward a goal. Methods: A total of 44 subjects participated in the study: 23 fully sighted and 21 with vision impairment: 12 with central visual field loss due to Stargardt’s disease and 9 with peripheral visual field loss due to glaucoma. An immersive virtual environment was used to present a vertical pole as a target in two scenes: a forest and an empty space. Subjects were instructed to walk to the pole 8 m away. Head position, measured with an optical tracker, determined the subject’s point of view and was used to indicate the subject’s walking path. To differentiate walking strategies based on optic flow or perceived direction, the optic–flow pattern was offset 10° to the right or left from the actual direction of walking (Warren et al, 2001). The amount of optic–flow influence on walking can be determined by comparing the heading error in the visually rich forest scene, which is capable of generating optic–flow cues, with the heading error in the empty–space scene, which contains no optic–flow information. An analysis of covariance was performed on heading error, controlling for age. Linear regressions by group were also performed to identify factors associated with the use of optic flow. Results: Scene interacted with group in determining heading error, p<.001. For the fully sighted group, heading error decreased by 37% in the forest scene compared to the empty–space scene. For the visually impaired group, the forest scene decreased heading error by only 12%. Of the factors: age, walking speed, log MAR, and log contrast sensitivity, log MAR was the only factor associated with the magnitude of optic–flow influence and only in the fully sighted group, accounting for 21% of the variance. Conclusions: Vision impairment affects the use of optic flow to guide walking in a visually rich environment. Diseases that compromise the central or peripheral visual field, as well as normal age–related decreases in visual acuity, reduce a person’s use of optic flow in goal–directed walking.

Keywords: heading • vision and action • low vision