Monday, 26 September 2011

September (Autumn) illusion

OK..... so these monthly illusions aren't exactly monthly, but we have been very busy with the new grant so they will now be quarterly illusions. Therefore, September's illusion is our autumn illusion, which is pulling the finger off, and stabbing it

 Finger being pulled off by the experimenter (Perception, 2010)

This illusion was developed from the finger stretch illusion that we covered in June and was involved in our paper published last year in Perception. It was found that when stretching the finger participants still feel like it belongs to them, whereas when we pull the finger tip off they no longer feel like it is their hand, even though they still have control over the hand and the detached finger tip. This therefore separates the sense of agency (feeling in control of an action) and ownership (feeling like the hand belongs to you). Ownership is measured in this example by attacking the hand and finger with sharp implements whilst measuring skin conductance (tiny changes in the amount we sweat) from the other hand. Increased ownership is associated with higher skin conductance in response to threat - basically you sweat a little bit more when you perceive a sharp stick to be quickly approaching part of your body opposed to approaching something that isn't part of your body. 

First the finger is stretched before being pulled off and attacked by the experimenter. The trick is to pull hard on the finger until the moment it detaches when pressure is released so it feels as though the tip has come off.

In the video above the threatening stimuli has been slowed down for demonstration purposes but it shows the experimenter's hand attacking the participant's hand with a sharp stick and a syringe. You can also see the experimenter placing a block between the stump of the finger and the detached fingertip. This served as a convincer showing that the tip was detached opposed to part of the finger being just hidden.

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