The constant whir of a fan. The sensation of the clothes against your skin. The chair pressing against your legs. Chances are that you were not acutely aware of these until I pointed them out. The reason you had somehow forgotten about their existence? A fundamental brain process that we call adaptation.
Our brains are remarkably good at cancelling out all sorts of constants in our everyday lives. The brain is interested in changes that it needs to react or respond to, and so brain cells are charged with looking for any of these differences, no matter how minute. This makes it a waste of time registering things that are not changing, like the sensation of clothes or a chair against your body, so the brain uses adaptation to tune this background out, allowing you to focus on what is new.
If you don’t believe me, try this simple, but startling demonstration. First, hold your eyeball perfectly still. You could use calipers to do this, or a drug that paralyses the eye muscles, but my favourite method is to use my thumb and index finger. Using the sides of your thumb and finger, press on the bone of the eye socket, through your upper and lower eyelids. Do this gently. Try it with one eye first, closing the other eye or covering it with your hand.
With your eye fixed in position, keep your head still and soon you will experience the strangest thing. (You will have to stop reading at this point. I don't mind. We will pick up when you have finished). After a few seconds the world in front of you will fade away. As long as you are holding your eyeball perfectly still, you will very quickly discover that you can see nothing at all. Blink, or move your head, let go of your eye and the world will come back. What's going on?!
Now you see it…
For all of our senses, when a certain input is constant we gradually get used to it. As you are holding your eye still, exactly the same pattern of light is falling on each brain cell that makes up the receptors in the back of your eye. Adaptation cancels out this constant stimulation, fading out the visual world. The receptors in your eye are still processing information. They have not gone to sleep. They simply stop firing as much, reducing the messages they pass on about incoming sensations – in effect the message passed on to the rest of the brain is "nothing new... nothing new... nothing new…". You can make your brain cells spring into action by moving your eye, or by waving your hand in front of your face. Your hand, or anything moving in the visual world, is enough of a change to counteract the adaptation.
This sounds like it could go badly wrong. What if I am watching something, or someone, I am thinking hard about it, and I forget to move my eyes for a few seconds. Will adaptation mean that thing disappears? Well, yes, it could in principle. But the reason it does not happen in practice is due to an ingenious work-around that the evolution has built into the design of the eyes – they constantly jiggle in their sockets. As well as the large rapid eye movements we make several times a second, there is also a constant, almost unnoticeable twitching of the eye muscles that means that your eyes are never absolutely still, even when you are fixing your gaze on one point. This prevents any fading out due to adaptation.