Cast your mind back to this post in our series on The illusion of testing. Here we looked at how the Stroop Effect put your brain and eyes in conflict: your brain read the word while your eyes processed the colour. If that was confusing what should we make of this #TheDress? And if you've been beyond the reach of the internet for the last few days, we're not talking about this dress

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made famous by Monica Lewinsky.

Or that dress
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made famous by Liz Hurley.

Or even a dress modelled by Mrs Doubtfire.
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No, it’s the furore surrounding #TheDress which has captured the attention of the online world.
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Is this dress blue and black or gold and white (or even some other combination)? And what is it about this optical illusion that is causing people to see it so differently? As usual in this series on illusions and testing we begin by trying to understand what causes the illusion and then consider if there is anything we can do in testing to combat the effect of the illusion.

So could this be explained by colour blindness or people giving a different name to the same colour (as in the turquoise v blue v green debate). Extremely unlikely. Less than 5% of the population is colour blind and neither of the colour camps had less than 5% of the total. And how likely is it that a large group of people who call black "gold" has gone unnoticed until now?

What about the wide variety of different devices (with different resolutions, brightness and contrast) the image is being viewed on? No. People looking at the same image on the same device were reporting the #whiteandgold and #blackandblue split. Some people who stared at the image reported that it shifted from white and gold to blue and black. And some people just weren't prepared to share what they saw.

We know we don’t see the world as it is but rather as it is useful to see it. So what could be happening here? The most compelling theory centres on how we see colour. When you see something you see both its colour and the colour of the light source illuminating it. Your brain separates out these two colours by making an adjustment for the illuminating light. The remaining colour is what you report you are seeing. As light changes through the day and night, the brain changes what it filters out to determine the colour of an object.

This image seems to be on some visual boundary where the brain is confused about what colours it should be filtering out. (“What is important is to spread confusion, not eliminate it.” said Salvador Dali, who would probably have enjoyed this hullabaloo). If the brain thinks that the image is lit by blue light, it filters out the blue and you end up with a white and gold dress. If the brain thinks it is lit by yellow light, it filters out yellow and you see a blue and black dress.

This doesn't explain why different people’s brains are making different assumptions about the lighting of this picture, but it does explain what's happening once an assumption has been made. Back to the dress itself, can we get a conclusive answer to the colour-of-the-dress test? We can be sure that it’s not possible to determine this from the image alone. The last few days have shown that. But are there other images of this dress available that would help us. Yes there are. And it is without a doubt blue and black. No uncertainty. No ambiguity. Which is how we really like things in testing.

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