My column for BBC Future from last week. The original is here. Thanks to Martin Thirkettle for telling me about the demo that leads the column.
Our brains are programmed to cancel out all manner of constants in our everyday lives. If you don’t believe it, try a simple, but startling experiment.
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.
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.
Mind Hacks 
I think you are wrong and it has nothing to do with adaption. I can press one finger in between my eye and the bone underneath, wait a second or 2, then everything turns gray. I can then move my eye and my head and my sight does not return, not until I release my finger.
I think that’s because when you make the eye movements or the head movements, your brain compensates by means of the efference copy. The brain “knows” how much you have moved and it adjusts the image by that amount, so that you can maintain the registration of the scene. This, along with persistence of vision and sampling is what prevents the world from being a huge blur when you move your head or your eyes normally.
Have you tried it pressing a little less hard? It works for me (vision fades, unless I spin the world by rotating my swivel chair). It might be there’s an additional thing that can happen, but there’s definitely an adaptation story here as well
loved it. in my work I use the example of the fridge going off in the middle of the night.
Can this possibly be contributed in part to a restriction of blood to the eye? I am also able to press my eye till it fades, and remains faded while moving it with pressure applied.
Is this why men adapt to things differently (20 year marriage ends and they marry again in 6 months) and they don’t seem to realize that they take a relationship for granted? They just “tune” it out?
No, it’s just that they’re over you years before the marriage ended.
No, it’s just that they’re over you years before the divorce.
Try staring directly into your own eyes in a mirror for several minutes and watch as your brain starts rearranging your facial features or completely erasing the image in front of you.
hmmm. this is interesting. as a meditator i am taught to ‘stay with’ and also to observe and let go. so the whole of experience slows down. stay with the breath, allow thoughts to arise and drop away while staying with a still point of experience…
after some time either during a meditation or after a period of regular practice the stillness becomes very profound. it is not a tuning out because you have trained yourself to stay with. daily life becomes more present in your experience.
scientists have noticed various neurological patterns when observing meditators and non meditators, and over time a thickening of the insula. this has to be connected to a trained lack of distraction.
Another way of doing this that I often do with groups for them to experience ‘reality blindness’ is to put two halves of table tennis balls over your eyes. This makes the input a single colour. At first you can see ‘white’ (if you are using white balls) but after a few moments everything goes black. I found holding my eye still enough and keeping my body still enough was difficult and using table tennis balls easier. It also shows it then isn’t about the pressure on the eye, or restricting blood etc…
That’s called the Ganzfeld experiment, which introduces sensory deprivation, which is not the same thing as described above.
A number of more sophisticated experiments reveal that there definitely is an adaptation story of some sort going on when our visual field fades out and other oddities of perception, this is not just a matter of blood flow by any means.
Part of what is happening I think is that visual perception is mostly constructed, rather than being a simple reflection of stimuli in front of us. David Eagleman accessibly discusses a number of lines of evidence for this surprising conclusion in Incognito. This probably has a lot to do with the oddities of perception and imagination that happen when we engage in sensory deprivation of various kinds.
The “adaptation” effects seem to be related to that as well. The perceptual mechanisms seem optimized to make us aware of changes and things that the brain predicts might require action, rather than just giving an unbiased capture of everything in front of us.
This is a very big deal conceptually, and I suspect most people probably reject the idea when they first consider it because it seems so different than they way we naturally think of perception. There are a lot of different lines of evidence leading to the same odd counter-intuitive conclusion though.
Is what you are doing preventing saccading? I get a tiny point of light left in the centre–but with the eye not whizzing about building its picture–as it usually does, is all we are left with a (singular) saccade?
I can achieve this gray out effect without holding my eye by staring at a fixed point for a moment. I discovered this in elementary school while sitting bored in the office waiting to see the principle (I was a trouble maker).
works… I used to have to push my eyeballs in to get the golden starry visuals but now it’s as swift as it gets. Thanks bro