What do YOU see? Mind-boggling optical illusion tricks your brain into seeing black-and-white images in vivid colour

Forget the black and blue dress – this photo of an assuming building is the latest image to demonstrate just how sophisticated our brains are at viewing the colours we see around us.

The strange illusion – called a 'colour afterimage illusion' – works by priming the receptors in our eyes so they recreate colour in a back and white image.

To try it yourself, first stare at the dot in the centre of the first false-colour photo for around 30 seconds. 

Next, turn your attention to the black–and–white photo, which should now appear in vivid colours. 

Scientists have long debated what causes colour afterimages, and how our brains create them. 

Now, Dr Christopher Witzel, Associate Professor of Psychology at the University of Southampton, claims to have settled the debate once at for all. 

According to the expert, it all comes down to photoreceptors in our eyes called cones.

'We've finally got a conclusive answer – colour afterimages are not opposing colours as everybody had thought. Instead, those illusory colours reflect precisely what happens in the cone photoreceptors,' he said. 

In another classic example, you can also use the colour afterimage illusion to see 'true cyan'.

Most TV smartphone screens can't produce true cyan because they only use a mixture of red, green, and blue pixels.

However, if you stare at a red circle on a blue background for around 30 seconds, then look at a blank wall, you will see a pure cyan orb floating before your eyes.

Although these examples have been well known for decades, until now researchers have struggled to explain why afterimages appear.

To get to the bottom of it once and for all, Dr Witzel conducted experiments that precisely measured the colours people see in afterimages.

In one experiment, 50 participants were asked to stare at a specific 'starter' colour before quickly colour–matching the afterimage they saw.

Dr Witzel took the data from these experiments and compared it to computational models of human visual perception.

The models revealed what colours people would see if afterimages were produced by cells in the eye, intermediate structures in the brain like the thalamus, or the visual cortex in the brain.

The results revealed that the data from people's real afterimages matched what we would expect to see if the illusion was produced by cells in the eye.

This suggests these colourful illusions are a product of how our eyes convert light into colour information for the brain.

In our eyes, we have three different types of cone cells, and colour is produced by the balance between them.

Speaking to the Daily Mail, Dr Witzel explained: 'When you look at a white surface without any prior colour adaptation, all three cone types are equally active. But if some cones are more active and others less so, you perceive colours.'

This new evidence finally disproves the theory that afterimages were produced by 'opponent colours'.

In the past, other scientists have suggested that each colour had a complementary counterpart that cancels it out when combined.

The idea was that staring at a coloured image tricks our brains into producing the opponent colour at a later stage in the colour perception process.

However, Dr Witzel's computer simulation disproved this theory once and for all. 

He added: 'When mathematically modelled, cone adaptation produces exactly the three groupings of perceived afterimages that emerged consistently in my experiments.'

The study comes shortly after a scientist from Harvard Medical School discovered a deceptively simple illusion featuring nine dots on a plain dark purple background.

But are the dots blue or purple? 

While viewers on Reddit were left divided, in reality, all of the dots are a bold purple and placed on a blueish background.

But, by holding your phone about 30 cm from your face and looking at each dot in turn, only the dot at the centre of your current focus should appear to be purple.

Dr Hinnerk Schulz-Hildenbrandt, who invented the illusion, explained: 'A pattern of purple objects on a blueish background appears only purple where the viewer looks directly at it. 

'In the periphery, the perception shifts towards blue.'