Brain sees shadow as part of body

The brain thinks the shadow is a body part

Our brains respond to our shadows as if they were another part of the body, according to a scientific study.

When we see something about to come into contact with the edge of our shadow, brain activity suggest it is as if they are about to touch us instead.

Scientists tested volunteers' reaction speeds and accuracy while distracting them with flashing lights.

They found that similar errors happened when lights flashed either next to a hand's shadow or the hand itself.

The brain develops an internal "map" which helps it define exactly where the body is - which helps it navigate around the world outside.

The results, by researchers at Royal Holloway College in London and the Universita degli Studi di Trento in Rovereto, suggest that the body's shadow may form part of that map.

Reaction test

The tests used shadows cast onto a white table in front of a seated volunteer.

The subject was asked to distinguish between touches on the thumb and forefinger of the hand, and the number of errors - and the overall reaction time - were recorded.

When flashing lights placed near the hand are activated, the number of errors rises and reaction time slows as the brain is forced to deal with the distraction.

However, when the flashes happened not alongside the hand itself, but instead alongside the shadow outline of the hand, the effect was exactly the same, suggesting the brain was equally distracted.

Flashes in the same place but without the shadow, or even flashes next to a disguised hand shadow forming a different shape all failed to produce this effect.

'Remarkable'

Professor Umberto Castiello, from Royal Holloway College, who led the research, told BBC News Online: "Our shadows help us reach out for objects and when we move about.

"It's a remarkable effect - when the shadow is a different shape, it doesn't happen."

He said that the brain was equally capable of distinguishing between shadows cast by its body and those cast by other people.

Dr Simon Unger, a psychologist from the University of Guildford in Surrey, said that a similar phenomenon operated in other situations.

He said: "When blind people have to use a white cane, they report that it feels like an extension of their finger."

The study was published in the journal Nature Neuroscience.