  To study body ownership and control, illusions that elicit these feelings in
non-body objects are widely used. Classically introduced with the Rubber Hand
Illusion, these illusions have been replicated more recently in virtual reality
and by using brain-computer interfaces. Traditionally these illusions
investigate the replacement of a body part by an artificial counterpart,
however as brain-computer interface research develops it offers us the
possibility to explore the case where non-body objects are controlled in
addition to movements of our own limbs. Therefore we propose a new illusion
designed to test the feeling of ownership and control of an independent
supernumerary hand. Subjects are under the impression they control a virtual
reality hand via a brain-computer interface, but in reality there is no causal
connection between brain activity and virtual hand movement but correct
movements are observed with 80% probability. These imitation brain-computer
interface trials are interspersed with movements in both the subjects' real
hands, which are in view throughout the experiment. We show that subjects
develop strong feelings of ownership and control over the third hand, despite
only receiving visual feedback with no causal link to the actual brain signals.
Our illusion is crucially different from previously reported studies as we
demonstrate independent ownership and control of the third hand without loss of
ownership in the real hands.
