Environmental variables and mobility performance in adults with low vision.

BACKGROUND: Surveys and empirical studies of visually impaired (VI) persons suggest that avoidance of objects while walking may depend on type of vision loss and be influenced by light level, object contrast, and object type.
METHODS: Mobility performance in 88 VI adults was assessed on an indoor obstacle course under photopic and mesopic lighting conditions. Subjects were divided into three categories: primarily an acuity loss, primarily a peripheral field restriction, and a combination of the two. Performance measures were time to walk the course and total number of contacts with objects in the course.
RESULTS: Decreasing light level from photopic to mesopic resulted in a significant increase (roughly double) in the time required to complete the course and in the total number of contacts, regardless of type of vision loss. Under photopic illumination, subjects with acuity loss took less time on average to complete the course and contacted fewer objects than the other two groups. At mesopic levels, acuity loss subjects performed better than those in the other categories. Low contrast obstacles were contacted more frequently than high contrast ones at both light levels. Finally, floor level walk-around objects were contacted significantly less often than either step-over or head level objects, regardless of type of vision loss and light level.
CONCLUSIONS: The ability of VI persons to avoid obstacles is significantly impaired under mesopic illumination. Object contrast and location are significant factors in determining the success of VI persons in avoiding obstacles in the travel path.