You can see this twitching when you look at a single point of light against a dark background (such as a single star in the sky, or a glowing cigarette end in a totally dark room). Without a frame of reference your brain will be unable to infer a stable position of the point of light. Every twitch of your eye muscles will seem like a movement of the point of light (a phenomenon called the autokinetic effect).
Adaptation is so useful for the brain's processing of information that it has been kept by evolution, even in basic visual processing, and this extra muscle twitching has been added in to prevent too much adaptation causing problems for us. But the basic mechanism is still there, as my eye experiment revealed.
Once you understand adaptation, you discover that it is all around us. It is the reason people shout when they come out of nightclubs (they have got used to the constant high volume, so it does not seem as loud to them as it does to the people they wake up on the way home). It is why a smell that might have hit you as overpowering when you first enter a room can actually be ignored after you've got used to it. And it is related to the phenomenon of word alienation, whereby you repeat a word so often it loses its meaning. But most of the time it operates quietly, in the background, helping to filtering out the things that do not change, so that we can concentrate on the more important tasks of those that do.
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