Shadows are hardwired into the brain
18:00 14 December 03

NewScientist.com news service

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 Universitą 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 with the shadow's identical
shape and position, but in lighting conditions where no shadows were
visible. 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)


Philip Cohen

http://www.newscientist.com/news/news.jsp?id=ns99994478