‘Supernumerary’ means more than the usual number and in this simple illusion we decided to give people an extra arm. We have used our supernumerary limb illusion in several of our MIRAGE experiments (Newport et al. 2010, Newport and Preston 2011, Preston and Newport 2011a, Preston and Newport 2011b) and use it to instantly alter perception of the body.
Participants put their right or left hand into the illusion box and instead of seeing just one arm they see two. The two images of the hand can be positioned either with both images slightly offset to either side of the actual hand (as in Newport et al. 2010, Preston and Newport 2011a and b), or one image can be in the same location as the real hand with the other offset to one side (Newport and Preston 2010).
We usually then ask people to tap their finger up and down or to stroke a brush tip. When they see their hands move, we can either have both hands move together at the same time, or add a slight delay to one of them. When asked to identify which of the seen hands feels like their real hand, participants will nearly always go for the one moving in time with their real hand. The match between the planned movement and the timing of the seen movement creates a strong and immediate sense of ownership over the hand that moves in time (what we call the synchronous condition). We can then control feelings of ownership by manipulating which image moves in time with the real limb by swapping the delay from one hand to the other. The sense of movement (and hand) ownership instantly switches from the hand that used to be moving in time to the hand that is now moving in time. This strong feeling of ownership can then be instantly transferred to the other hand by swapping which of the images is synchronous... and back again…and again….and again! We could do this all day if we wanted to because the feeling of ownership does not seem to reduce regardless of the number of times it is transferred from one hand to the other, although this is a weird experience for the participant. Another weird experience occurs if we make both hands synchronous with the real hand simultaneously. Then participants claim ownership over both hands at the same time claiming that they have two right hands!
The transfer of ownership (and owning an extra limb) is a good illustration of how flexible our brains are in updating the representation of the body. This can help to inform us about the healthy brain, but also in understanding clinical populations when body ownership is disturbed.
Video of a participant experiencing The supernumerary limb illusion