Illusions That Let Your Hands Lie to Your Brain

One of the things your brain excels at is changing itself around to fit new circumstances. People can cut bits out of it, sometimes extending to half of your entire brain, and the mind adapts. You could suffer strokes that, in a completely uncontrolled fashion, kill off parts of your brain, losing function and perception. Eventually, you'll adapt.
You can wire a ferret's optical nerves to its auditory center, and the brain will manage to make the switch, letting the thing see through the part of its brain meant for hearing.

There are a few times, though, when the brain goes overboard. And people have found ways to entertain themselves with their own overeager brains. Since some of the densest nerve clusters in the body are in the hands, they can help you drive your brain over the edge.

The major adaptation that your brain does all the time is filter out whatever information it considers extraneous. This helps you focus in on certain things closely, but it also means that huge parts of your vision, your hearing, and your sense of touch get taken out of the equation. The brain only updates these things if it senses major changes.

The problem? It doesn't differentiate between major changes. Most people have had the sensation of putting cold hands or feet into a warm bath — though not a dangerously hot bath — and experiencing a sensation of them getting scalded as if they were dumped in boiling oil. That's the brain letting us know that there is a major change in temperature. It just hasn't caught up to the fact that it can convey it as a safe change in temperature. The same goes for texture. The brain will register the change, but can't log the absolute amount of texture it feels.

There are a couple of experiments that you can do play with your brain, and hands, that induce an almost queasy feeling of disorientation. One has to do with temperature. Grab a bowl of hot water, a bowl of cold water, and a larger bowl of warm water. Put one of your hands in the hot water and the other in the cold water, and leave them there for about a minute. After that, put them both in the warm water. One hand will report back that the water is cold, while the other one will feel it as hot.

The same thing works with texture. Stroke a piece of carpeting with one hand, and a piece of plastic with the other, then put your hand on something relatively smooth - like a painted wall. One hand will say the wall is rough, while the other will say it's smooth. The feeling of disjointedness when this happens is intense, especially if you allow your hands to touch each other while you're doing this. You get the sense that each hand is in a separate room and you're split down the middle, even though you can see them both next to each other. The brain has adapted itself to the suggestions from each hand, and it takes a while for it to readjust and let you know that you have two hands which are close together again.

In fact, the brain is so willing to give your hands the benefit of the doubt that it will make up entire body parts just on their say-so. A fun, simple trick requires only two hands, two chairs, and someone willing to let you fondle their nose for a while. (If you have found this someone, hold on to them! They are a rare jewel.) Position your chair so that you sit right behind this worthy person. Close your eyes and feel your nose. At the same time, reach forward with your other hand and feel their nose. Try to make the same motion with both hands. Even though you are touching your own nose with your own hand, since your other hand is making the same motion far away, your brain will adapt and tell you that your own nose is either about three feet long, or three feet away from your face. The brain's built to keep track of where your limbs are, and if one of them stretched out fully while seeming to touch your nose, it will just assume that your nose is as far away as your hand.


This even works when your eyes are open, if the other hand is out of sight. A simple experiment that requires you hide your hand behind a barrier while either a rubber hand or another persons hand is placed in front of you. As you stare at the hand, which you can see is not your own, someone with a brush will stroke both your hand and the visible hand simultaneously. After a few minutes, you'll get the feeling that the hand you're looking at is your own. Experimenters got weird results when they would suddenly hit the rubber hand, or have the other person flex their hand. The brain got the idea that this disembodied hand had the same sensations the actual hand did, and switched its proprietary feeling in under five minutes.

We like to think of ourselves as important brains inside incidental bodies. It's true that people can do amazing things by concentrating their brain on the abstract, away from the demands of the body. But sometimes, it's pretty clear that the brain is more the servant than the body is. It's made to serve the needs of the body, to the point where it can't exert itself independently. It just rewires itself, on the fly, to whatever input the most sensitive body parts are giving it at the moment. Even if those sensitive body parts are made of rubber.

Via Naked Scientists, Your Amazing Brain, and Amazing Mind.