Did a butterfly ever settle on your hand?
What did you feel?
The picture below.
You see a central cross set against a blurry background pattern of pastel colours.
Focus on the cross. Do not let your eyes wander, however tempting it is to do so. It may take up to 30 seconds but then you will probably see the background dissolve and only the central cross stay visible.
If you let your eyes move, the background springs back into view.
If you don’t notice any fading, focus a bit longer on the cross, and also try closing one eye, or unfocussing, while still fixating on the cross. Varying the distance from the screen, or enlarging the image, may also help.
Here’s where it touches the butterfly. A butterfly alighting the back of your hand is felt at first. But if it doesn’t scrabble around the feeling fades quickly.
To simulate this, tear off a scrap of paper and put it on to the back of your hand. You’ll feel it for a second, but then it’s imperceptible. This is adaptation. An unchanging stimulus stops being perceived. This happens with all our senses: think of smells that fade for example.
So why doesn’t the whole world fade and disappear before our eyes?
The fovea – the part of the retinal responsible for central vision – has the highest density of light-sensitive cells, and the density of these photoreceptors lessens and lessens as we go toward the periphery.
When you fixate your eyes still make small movements (technical term tremors). Because foveal vision uses many closely-packed photoreceptors, the smallest tremor brings a fresh one to bear in place of a photoreceptor that has adapted. That is why the central cross in the picture is always perceptible – a new neuron is always there to take over.
Further from the centre things are very different. Photoreceptors thin out more and more. Each cell is responsible for a bigger and bigger catchment, and each tremor has a relatively smaller effect. The tremors are not wide enough to overcome the adaptations.