Visual illusion could help prevent falls

Visual illusions are not only fun, they also help show how the brain works by exploiting its shortcomings. But what about using visual illusions for practical benefit? By making a step look taller than it really is, David Elliott and colleagues have demonstrated a way of doing just that.

Trips on steps are nasty for anyone, but for the elderly they can be fatal. Two thousand elderly people die in the UK every year following a fall, with the majority of these falls happening on stairs.

Elliott's team asked twenty-one students to judge the height of two steps, one of which was decorated with horizontal bars on its leading face, thus making it look shorter; the other was decorated with vertical bars, thus making it look taller (the right-hand step on the image above). Both steps were actually the same height. Asked to estimate the height of the steps, the students guessed the height of the vertically decorated step to be just over 5 mm higher than the other step.

Most importantly, an eight-camera motion capture system showed that when the students stepped onto the steps, they lifted their foot higher for the vertically decorated step compared with the horizontally decorated step by a distance of about 5 mm. This was true whether the students looked with both eyes, or just one.

Most people trip on stairs because their toes clip the edge of the step, so an illusion that leads people to exaggerate the clearance they give to a step, even by only a small amount, could have significant benefits in terms of reducing falls. Ideally the researchers ought to have included a 'control' step that didn't feature any decoration. However, this was a preliminary study and the researchers anticipate the illusion will be enhanced through future tests, increasing foot clearance still further.

As well as having practical implications, this study also has theoretical importance. An influential account of visual processing posits that there are two pathways in the brain: the dorsal "where" pathway and the ventral "what" pathway, with only the latter being prone to visual illusions. In support of this account, some experiments have shown that people's perception can be tricked by an illusion (such as the size of an object) while their motor system is unaffected, as demonstrated by the person using an appropriate grip size. The current observation that both perception and action were tricked by the design of the steps, challenges this dual pathway account.

"The most parsimonious explanation of our results is that visuomotor actions are directed by the visual system without the need to invoke two wholly separate pathways for action and perception in the dorsal and ventral streams respectively," the researchers said.
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David B. Elliott, Anna Vale, David Whitaker, John G. Buckley (2009). Does My Step Look Big In This? A Visual Illusion Leads To Safer Stepping Behaviour. PLoS ONE, 4 (2) DOI: 10.1371/journal.pone.0004577

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