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

Abstract

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.

Figure 1. The two configurations of the step.

On the left is the H configuration with Horizontal gratings on the step riser and on the right is the V configuration with Vertical gratings on the step riser.

Figure 2. Perceived step height under different experimental conditions.

Mean (±SE) perceived step height (mm) for H and V target configurations and monocular and binocular vision conditions.

Figure 3. Maximum toe elevation under different experimental conditions.

Mean (±SE) maximum lead toe elevation (mm) for H and V target configurations and monocular and binocular vision conditions.

Figure 4. Perceived step height vs. maximum toe elevation.

Scatterplot of perceived step height (mm) and maximum lead toe elevation (mm) in the V configuration and binocular vision condition.