Blue or black? 12 dots or 3? The science of optical illusions - and why they matter

In early February 2015, a viral meme posted on the social media site Tumblr took the internet by storm. It posed the question: What colour is this dress? Those viewing the picture were sent into a spin: was it blue and black? Or was it white and gold? Everybody had an opinion, and everybody was right.

It was an optical illusion; a trick of the light that fooled the brain into misunderstanding the real colour. These tricks are not just trivial fodder for online spats: they help us to understand the complex and sometimes deceptive ways in which our brains work.

A more recent example of a hugely popular optical illusion is perhaps one that people are more used to seeing. There are 12 dots on this page, but the brain can’t see them all at once.

There’s even a competition – the Illusion of the Year Contest – to see who can come up with the most baffling trick. It’s organized by the Neural Correlate Society, a non-profit organization that promotes scientific research into the neural correlates of perception and cognition. The winner shows how we are tricked into thinking something is moving – when it’s not.

The illusions beg the questions: why did people have such differing opinions about the dress? Why can’t we see all the dots? How are we tricked into thinking that an object is moving? The answer is that human perception is not always about what we see – what the eye sees is not always what the brain interprets.

Light enters the eye, and different wavelengths of that light correspond to different colours, Adam Rogers explains in an article for Wired Magazine. When those wavelengths of light hit the retina, this fires up the neural connections to the part of the brain that turns those signals into an image. The reason people see different colours is that the first wavelengths that reach the eye are reflected from the object we are looking at.

“Without you having to worry about it, your brain figures out what colour light is bouncing off the thing your eyes are looking at, and essentially subtracts that colour from the ‘real’ colour of the object,” he writes.

According to two professors at the SUNY Downstate Medical Center in Brooklyn, New York, Susana Martinez-Conde and Stephen L. Macknik, the mystery of optical illusions helps to explain the inner workings of the brain.

“One of the most important tools used by neuroscientists to understand how the brain creates its sense of reality is the visual illusion,” they say.

“Visual illusions are defined by the dissociation between the physical reality and the subjective perception of an object or event.

“Because of this disconnect between perception and reality, visual illusions demonstrate the ways in which the brain can fail to re-create the physical world. By studying these failings, we can learn about the computational methods used by the brain to construct visual experience.”

They explain that the part of the brain that interprets what we see is the same part of the brain that creates dreams and delusions. “In other words, the real and the imagined share a physical source in the brain.”

Scientist Donald Hoffman says the difference between what we see and what we perceive is simply a matter of staying alive:

"Every bit of information that you process costs calories, meaning that's more food you need to kill and eat. So an organism that sees all of reality would never be more fit than one tuned only to see what it needs to survive.

"The standard view of vision is that we're akin to cameras, taking an image from light reflected off an object," he explains. "But billions of neurons and trillions of synapses are involved between light hitting the retina and the construction of the 3D objects that we perceive."

“We're all living in a 'conceptual prison': our brains perceive a fraction of reality to keep us alive.”

Beau Lotto, a neuroscientist who specialises in visual perception, takes up this theme. He explains that as humans we've evolved to see the world usefully - rather than accurately.

"What illusions tell us is that we did not evolve to see the world accurately. Instead, we evolved to see it usefully. But what was once useful, may no longer be useful. Which is why the brain also evolved to continually redefine normality.”

Lotto is director of the Lab of Misfits - the world’s first public perception research space.

Consider the following two images from the Lab of Misfits' website. The two large surfaces in the foreground appear very different in brightness. In reality, they are the same.

You can see all of the Lab of Misfits' illusions on their website, here.

The dress, by the way, was black and blue.