This illusion shows that you can't keep track of where your hands go

If you have recently put your hand in something disgusting, take comfort. You haven't been slimed as thoroughly as you had previously assumed. Your brain is making things worse (and longer) than they truly are.

To be fair, the brain has a tough job. Every part of you is taking in information, and that information often changes suddenly. Our eyes jump around. Our hands and bodies move quickly from one thing to another. Even our focus of attention changes suddenly. It takes time to take in these sudden changes in input and assemble them into a coherent perception. If the brain were to show us this time, these brief spaces of nothingness or absolute confusion during which our attention jumps, it would confuse us still more.

What the brain subs in is chronostasis. Our brain takes the latest input and uses it to fill in the gaps in perception while we are refocusing our attention. What results is a sense of time lag. The most famous is the hesitation of the second hand that happens when we stare at a ticking clock. It's called the Stopped Clock Illusion, and has dogged us since we had clocks with second hands. We have no time-keeping equivalent to a clock for our sense of touch, and so it took a while for people to understand that chronostasis happened when people touched an object as well as looking at it. One experiment shows that we touch things for less time than we think.

A group of volunteers were asked to make an arm movement that ended with them touching an object, then give an estimation of when they had touched that object. A second group of volunteers were asked to make an arm movement that ended with a change in motion in an object which they didn't touch. They were asked to participate in a kind of virtual game of catch - the simulated object would move, they'd put their arm up and the motion would stop or change. They could see the object throughout all of this, but no contact occurred. These people were also asked to estimate when the "virtual contact" happened. Despite having input only from their eyes, the second group of volunteers were spot-on in their time sense. The people who had touched the object had a kind of lag in time. They believe they had been in contact with the object longer than they actually had. That change in sensation sent the brain into a scramble to get its data together, and it added an extra second of time to make up for it. We touch things for less time than we imagine we do.

[Via Manual Chronostasis.]