Mind Hacks

Hack #63. Keep Hold of Yourself

How do we keep the sensations on our skin up to date as we move our bodies around in space?

When an insect lands on your skin, receptors in that area of skin fire and a signal travels up to your brain. The identity of the receptor indicates which part of your skin has been touched. But how do you know exactly where that bit of your body is so you can swat the fly? As we move our bodies around in space we have to remap and take account of our changes in posture to understand the sensations arriving at our skin; very different movements are required to scratch your knee depending on whether you’re sitting down or standing up. This might seem like a trivial problem, but it is more complex than it seems at first. We have to integrate information from our joints and muscles about the current position of our body—proprioceptive information—as well as touch and vision, for example, to gauge that the sight of a fly landing and the sensation of it contacting your finger are coming from the same place.

In Action

Try closing your eyes and feeling an object on a table in front of you with the fingers of both hands. Now, cross your hands and return your fingers to the object. Despite swapping the point of contact between your two hands, you do not feel that the object has flipped around. The next two illusions attempt to make this remapping fail.

First, try crossing your index finger and middle finger and run the gap between them along the ridge and around the tip ...