One way to think about a body ownership illusion is that it arises when the brain attempts to solve a contradiction between different sensory modes, and chooses as a resolution the simplest hypothesis that appears to explain what's happening. For example, in the rubber hand illusion
your real out-of-sight hand feels tactile stimulation, while a visible rubber hand on the table in front of you, and in a plausible position and orientation, is seen to be touched. The felt touch and seen touch are synchronous. So there is a contradiction between the felt touch in one place and the seen touch in another. The contradiction is resolved by going along with the hypothesis that the rubber hand is indeed your own hand.
There is a more severe contradiction discussed in the literature: you see your body some metres in front of you being tapped
on the back, but you feel the tapping (of course) on your back (synchronously with the seen tapping). Here the contradiction produces a strange effect - somehow your body is over 'there' (the sensation of touch can shift to the body in front) but of course your visual ego-centre is 'here' (where you are really located). There can be a reported sensation of body ownership over that distant virtual body. But is this 'body ownership' in the sense of somehow feeling your body to be at the distant location, or is it just a question that you recognise the body as your body, and so in answer to a questionnaire would give a high score to a question about body ownership, but qualitatively this is not the same as when your virtual body is spatially coincident with your real body
and seen from first person perspective?
We examined this issue with immersive virtual reality, and partially reproduced the experiment described above
. The conclusion we reached is that the way the brain attempts to resolve the contradiction between you being 'here' but feeling stimulation on the body in front over 'there', is to produce illusions of drift. One way to resolve the contradiction between the two locations is to make them coincide. So if the virtual body in front is illusorily perceived as drifting backwards towards your position, or if you have the illusion of drifting towards the virtual body in front, then the two bodies will become united into one.
In our experimental setup the virtual body in front was subjected to a threat (a spinning fan was lowered towards its head). So if you drift towards the body in front this would put you in danger, but if the body in front drifts back towards you that would put it out of danger. We found a strong positive correlation between the illusion of drifting forward and heart rate deceleration (increasing danger) but a negative correlation with the virtual body drifting backwards (decreasing danger).