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Vision: adaptation

Colour adaptation #1

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Stare at the very centre of the moving pattern below, blinking as little as possible. Where does the glowing yellow dot come from? There's an explanation below the image.

Note: if the illusion below isn't moving, it's because your browser is set to prevent animations from running. GIF files here store several images, which are loaded and then shown in sequence: with animation turned off, you'll see only the static first frame. To turn it on in Internet Explorer, select Tools/Internet Options/Advanced, then scroll to Multimedia, and turn on Play animations in web pages. (Animation should be on in Firefox: if not, type "about:config" as URL, double-click to "image.animation_mode", and type "normal".)

What's happening?

Cells called cones in your retina respond to the wavelength (colour) of incoming light. (In fact, it's the combination of responses from three types of cone cell that causes the sensation of colour.) As with other senses, the cones will reduce their response when they're exposed to the same colour for some time. So, in the image above, as you stare at the central cross, two things begin to happen.

First, the blue circles will slowly disappear (especially opposite the yellow dot), as the blue-sensitive cones start to adapt, and reduce the strength of signal being sent to the visual cortex. The blue areas begin to merge with the surroundings.

More dramatically, a yellow circle appears, which – if you keep staring at the centre – will eventually look like it's glowing, as it rotates. In the right colour and lighting conditions, some people see only the yellow dot!

What's happening is that each blue dot vanishes in turn, and – because the blue-sensitive cones are now less active in that region – the complimentary colour to blue (yellow) is seen. The gap is quickly replaced by the next blue circle, so there's never time for the blue cones to return to their normal state.

Flick your eyes around to convince yourself that there is no yellow dot, only a gap in the blue dots...

The next illusion makes use of the same effect, but in a different way. Make sure that your browser allows Javascript to run, otherwise you won't see the roll-over image...

Colour adaptation #2

Start by making sure that the cursor isn't over the image below: it's a black-and-white shot of a beach. The image below should change when you move the mouse over it, into a coloured splodge. (If it doesn't, you'll have to alter your browser settings.)

With the coloured image showing, stare at the central black dot for, say, 30 seconds, then move the mouse away to bring back the black-and-white picture. It should look like a colour photograph!

Colour adaptation image 1

What's happening?

The blurred image is a colour (tonal) negative of the scene. As you stare at it, the cones in your eyes begin to adapt (and reduce their response) to the colours shown. Then, when you flick back to the black-and-white picture, and the sky and sea are white (or light grey), for a few seconds your cones' sensitivity to the orange of the sky and the pink of the sea is reduced. When you take orange away from white, you get its complementary colour, light blue! The same sort of thing happens when pinkish-red is removed from white: you get yellow-green.

There's another principle which we used when making this image (see below for how): the eye is much less sensitive to detail in colours (tones) than it is to differences in contrast (the black and white, or luminance part of the image). We could therefore blur the colour image above, without causing any apparent loss of detail when viewing the black and white picture. Analogue television works in this way: the resolution (detail) of the colour part of the signal is a lot poorer than the black-and-white picture (and the colour signal can be added to the black and white without need for extra bandwidth), but our eyes don't see the slight blurring effect of the colours.

Here is another example: it's Edinburgh Castle.

Move the cursor over the image to bring up the coloured blur, stare at a fixed point for 30 seconds, then move the cursor away to bring back the black-and-white version...

Colour adaptation image 2

And here's one final example: a photograph from the extraordinary outdoor event held all over Europe in 2005, The Sultan's Elephant, by Royal de Luxe.

Move the cursor over the image to bring up the coloured blur, stare at a fixed point for 30 seconds, then move the cursor away to bring back the black-and-white version...

Colour adaptation image 3

How to make an inverted image

Click here for a page that describes how to make images like the ones above.

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