Do you think that you perceive your body and the world around you as they really are? If your answer to that question is “yes”, then think again. Our perceptions are little more than the brain’s best guess of the nature of reality, constructed from fragments of information it receives through the senses. This is demonstrated by visual illusions, which produce discrepancies between physical reality and what we see of it, and by illusions of bodily awareness, which distort the way we perceive our bodies.
Psychologists and neuroscientists have long used illusions to investigate the mechanisms underlying perception. This is usually done in a laboratory setting, using specially designed equipment and, in the case of visual illusions, sensory stimuli. Some illusions of bodily awareness can, however, be easily induced without an elaborate experimental set-up or complex images. You too can dazzle your friends by perturbing their bodily awareness, using nothing more than the items that are commonly found in pubs and bars.
The first illusion that I have adapted for the pub is the rubber hand illusion. As shown in the video above, this involves seating a subject at a table, with one hand placed in front of them on the table surface and the other placed out of sight on their thigh. A fake hand is then placed onto the table, where the hidden hand might realistically be positioned. The experimenter then uses two pens or paintbrushes to synchronously stroke the subject’s hidden hand and the fake hand. The subject therefore feels their real hand being stroked and sees the fake hand being stroked in the same way and at the same time.
Simply manipulating the sensory information about the hand in this way causes the subject to perceive the felt sensations as originating in the fake hand and not their real one. It occurs because the brain has been tricked into incorporating the fake hand into its representation of the body, effectively treating it as if it were a part of the body. (In other words, the brain takesownership of the rubber hand.) How do we know that this happens? If the experimenter lunges at the rubber hand with a knife, the subject reacts as if it were their real hand being attacked, and various physiological reactions – such as sweating and increased heart rate – can be measured. Lorimer Moseley and his colleagues found that the temperature of the real hand drops, showing that the brain neglects – or disowns – it during the illusion. I also predict that the real hand becomes insensitive to pain, and this is now being tested.
I have been practising the rubber hand illusion, and so far have induced it with varying degrees of success in about half a dozen willing participants, using a Star Nail Practice Hand purchased from Amazon, which is designed for trainee beauticians to practice their manicure skills. Recently I described the illusion to a group of people in a pub, and wanted to demonstrate it to them, but of course I didn’t have my rubber hand with me. Then it occurred to me that I could probably induce it without a rubber hand, by using my own hand instead.
The photograph above shows it was done. In it, you can see my experimental subject’s right hand on the left and my left hand on the right. Out of view to the right was my assistant, who used two knives to synchronously stroke my experimental subject’s left hand and mine. My subject reported that the illusion worked, or at least that she felt confused about where the sensations she felt were coming from. I could have verified this by asking Jo to stab my hand, but for obvious reasons I did not. Nevertheless, my attempt to induce what might hereafter be referred to as the “hand-swap illusion” – basically, a scaled-down version of the body swap illusion – was apparently successful.
I have also “discovered” a novel variation of Aristotle’s illusion, also known as the crossed fingers illusion. As its name suggests, this involves first crossing your fingers, then touching the fingertips simultaneously with a small spherical object such as a pea. This induces the sensation that you are touching two objects. It can also be done by touching the tip of your nose with the crossed fingers and, I have found, using the tip of a pen. My pub variation involves standing with your legs crossed and placing a pool cue so that it touches both feet simultaneously (as shown in the photo above). This should induce the illusion that your feet are being touched by two objects and not one.
Like the rubber hand illusion, this one arises because crossing your fingers alters the sensory information entering the brain. Specifically, it is thought to occur because two regions of the skin, which are usually further apart and would not normally be touched simultaneously by the same object, are brought closer together. Conversely, touching two areas of the skin surface that are ordinarily close together with two objects gives the impression of touching one object. Try it for yourself – cross your fingers, then lightly squeeze their outer sides with the thumb and forefinger of the other hand. This will feel like you are holding a single object between your crossed fingers.
- The body swap illusion
- Interpreting hybrid images
- How to morph into another person
- Rubber hand feels real for amputees
- The virtual body illusion and immersive Second Life avatars
Sekinea, T. & Mogia, K. (2009). Distinct neural processes of bodily awareness in crossed fingers illusion. NeuroReport 20: 467-472 [PDF]
Botvinick, M. & Cohen, J. (1998). Rubber hands ‘feel’ touch that eyes see. Nature 391: 756 [PDF]