Monday, 20 June 2011

June Monthly Illusion - Finger Stretching

A student at the University of Nottingham having her finger stretched

This month’s (and our first) monthly illusion is finger stretching. As well as being a popular trick for our public engagement events, this illusion has been used in two of our most recent publications. The first was published in Perception, in which we stretched the finger before detaching the fingertip whilst investigating agency and ownership. The second appears in Rheumatology in which we found that finger stretching (along with finger shrinking) could reduce pain in osteoarthritis. 

Figure 1: SCR (Skin Conductance Response) from a 9 year old boy when the finger is gently pulled without stretching (white) and then with stretching (red). When we are shocked or scared then we sweat a little more than normal and these tiny changes can be measured by electrophysiological equipment that we have in the lab. The graph above captures a peak in skin conductance (increase in sweat) on the index and middle fingers of the left hand when the right hand is being stretched.

The illusion is simple but effective; the experimenter gently pulls on the fingertip whilst the image is stretched giving a very strong illusion that the finger is really elongating. Reactions to the illusion are often quite striking; we have even had grown adults shriek and flinch despite knowing that nothing is actually happening to their finger. The graph in Figure 1 shows skin conductance response to the illusion. Many participants who take part also report that their finger is too short when it is then ‘un-stretched’ and is in fact normal size.

An important part of the illusion is that it is multisensory. The experimenter gently pulls on the tip of the finger (Figure 2) when the image is stretched so that what you feel from your finger matches what you see happening to it. This makes the illusion very effective.

Figure 2: This shows a finger of an arthritis patient being stretched. The experimenter is gently pulling on the finger tip whilst the image is stretched

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