Tuesday, 31 January 2012

Winter Illusion - The Disappearing Hand Trick

This month’s illusion, the disappearing hand trick, is exactly what it says it is - an illusion that makes your hand disappear. It works on the principle that if we can be made to feel like a fake hand is belongs to our body or that our finger is really stretching using multisensory integration (matching of vision and touch) then by using multisensory disintegration we can make our real limb seem to disappear.

Participants place both hands in MIRAGE so that they are hovering just above the tabletop. Blue bars then appear that gradually close in on the hands over about 25 seconds. The participant's only task at this point is to keep their hands within the blue bars.

Participant trying to avoid the blue bars.

What is actually happening here is that the images of the hands move very slowly inwards so that to keep their hands still the participant must move their hands ever so slowly outwards - so slowly that the don't know they are doing it (for full details on the experimental procedure see the supplementary material from the published article - Multisensory disintegration and the disappearing hand trick, Current Biology). After 25 seconds the hands are placed down on the tabletop and then the image of the right hand disappears along with the blue bars.

At this point the participant does not feel like their hand has actually disappeared, although all that they can see is the table where their hand used to be, but then they are asked to reach over with the left hand and touch their disappeared right hand.

We believe vision more than our body position sense (proprioception) so the the participant thinks their hand is where they last saw it, Because their hands are actually further away than they realise, when they reach to where they think their other hand is - it is no longer there and seems to have disappeared. It is this failure to touch the right hand with their left hand that is key to multisensory disintegration and the experience that the hand has actually gone.

The illusion is so effective that most people show some kind of stunned or amused reaction and some even pull their hands out to check they still have them both!
This video shows what the volunteer sees during the illusion.

This video shows a volunteer reaction to her disappeared hand. Note that her reaction is elicited at the moment that she reaches for her right hand but does not find it.

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