Our brains instinctively view our shadows as an extension of our bodies, a new research has shown.

Subjects in the study reacted to stimuli near the shadow of one hand as if the stimuli were affecting the hand itself, found Francesco Pavani, at Royal Holloway University of London, UK, and Umberto Castiello, at the Università degli Studi di Trento, Italy.

The results confirm an intuitive bond people feel with their shady outlines, says Margaret Livingstone, a vision researcher at Harvard Medical School in Boston, Massachusetts.

"We all have, as children, experienced a reluctance to have others step on our shadows," she told New Scientist. "I have a graduate student in my lab right now who still feels that way."

Mental connection
It was known that people form a mental connection with a false limb or tools if they are trained to associate manipulation of that object with a sensation, for example if their own hidden hand is stroked whenever a visible false hand is touched.

But it was not clear if this connection could occur with something as immaterial as a shadow.

So Pavani and Castiello placed stimulators on the thumbs and forefingers of 10 volunteers and asked them to indicate via foot levers when a particular digit was being touched.

Previous work has shown that if a distracting flash of light occurs near a touched body part, the reaction time in this test increases, because the subject is busy processing two separate inputs from the same region of the brain's body map.

Dummy hand
However, the researchers observed the same effect if they flashed a light which was equal distance from both hands of a subject, but very close to one hand's shadow. The subjects identified flashes near their shadow hand in an average of 72 milliseconds, one third slower than in the control experiment.

In contrast, there was no significant change in reaction time if the flash occurred near a silhouette drawing identical to the shape of the shadow, but in a different position. That suggests our brains identify the shadow itself - and not just any hand-shaped outline  as an extension of self.

The work implies that the brain uses visual clues from not only our appendages, but also their shadows, to map the body in space and to interact with the world. "Cast shadows could provide additional cues about body position in relation to objects," they write in Nature Neuroscience.

Journal reference: Nature Neuroscience (DOI: 10.1038/nn1167)