• Does my step look big in this? A visual illusion leads to safer stepping behaviour

      Elliott, David B.; Vale, Anna; Whitaker, David J.; Buckley, John G. (2009)
      BACKGROUND: Tripping is a common factor in falls and a typical safety strategy to avoid tripping on steps or stairs is to increase foot clearance over the step edge. In the present study we asked whether the perceived height of a step could be increased using a visual illusion and whether this would lead to the adoption of a safer stepping strategy, in terms of greater foot clearance over the step edge. The study also addressed the controversial question of whether motor actions are dissociated from visual perception. METHODOLOGY/PRINCIPAL FINDINGS: 21 young, healthy subjects perceived the step to be higher in a configuration of the horizontal-vertical illusion compared to a reverse configuration (p = 0.01). During a simple stepping task, maximum toe elevation changed by an amount corresponding to the size of the visual illusion (p<0.001). Linear regression analyses showed highly significant associations between perceived step height and maximum toe elevation for all conditions. CONCLUSIONS/SIGNIFICANCE: The perceived height of a step can be manipulated using a simple visual illusion, leading to the adoption of a safer stepping strategy in terms of greater foot clearance over a step edge. In addition, the strong link found between perception of a visual illusion and visuomotor action provides additional support to the view that the original, controversial proposal by Goodale and Milner (1992) of two separate and distinct visual streams for perception and visuomotor action should be re-evaluated.
    • Gait Alterations Negotiating A Raised Surface Induced by Monocular Blur

      Vale, Anna; Buckley, John G.; Elliott, David B. (2008-12-01)
      Falls in the elderly are a major cause of serious injury and mortality. Impaired and absent stereopsis may be a significant risk factor for falls or hip fracture, although data from epidemiological studies are not consistent. Previous laboratory based studies, however, do suggest that stereoacuity is an important factor in adaptive gait. The present study investigates how acute impairment of stereopsis, through monocular blur of differing levels, ranging from 0.50 diopter (D) to a monovision correction affected gait when negotiating a raised surface in elderly subjects. Eleven elderly subjects (73.3 3.6 years) walked up to and negotiated a raised surface under nine visual conditions, binocular vision, one eye occluded and 0.50 D, 1.00 D and monovision correction (mean 2.50 D 0.20 D) with blur and occlusion either over the dominant or non-dominant eye. Analysis focused on foot positioning and toe clearance parameters. There was no effect of ocular dominance on any parameters. Monocular blur impaired stereopsis (p 0.01), with more minor effects on high and low contrast acuity. Vertical and horizontal lead limb toe clearance both increased under all levels of monocular blur including the lowest level of 0.50 DBlur (p 0.03) and monovision correction led to toe clearance levels similar to that found with occlusion of one eye. Findings demonstrated that even small amounts of monocular blur can lead to a change in gait when negotiating a raised surface, suggesting acute monocular blur affected the ability to accurately judge the height of a step in the travel path. Further work is required to investigate if similar adaptations are used by patients with chronic monocular blur.