Remember 2015? Back when the only thing we had to worry about was what colour a dress was?

Those were the days. The Dress was and is probably still the biggest viral event to strike the internet. In fact, it was bigger than the internet: it burst out of computers and mobile screens and into offices, cafés, pubs - virtually every public space where people could argue about it. Was it blue and black or was it white and gold? Before Brexit, The Dress was the biggest debate of the decade. There was, however, only one right answer: the dress was definitively blue and black. So why did we all see it as one colour or the other? What was actually happening there? Pascal Wallisch is a neuroscientist and Psychology professor at NYU. He has conducted an extensive piece of research into why people saw The Dress in different ways, which may sound frivolous, but as Wallisch says: "The very existence of The Dress challenged our entire understanding of colour vision." "We thought we basically knew how colour vision worked, more or less," he says. "The Dress upended that idea." In an article published in Slate, Wallisch explains that the reason people saw the dress differently is because they were unable to work out what the source of the lighting in the picture was. "People’s perceived colour is also informed by their perception of lighting," Wallisch writes.

"And the image of the dress, taken on a cellphone, contained a lot of uncertainty in terms of lighting conditions. Was it taken inside or outside? This matters because it implies artificial or natural light. Was the dress illuminated from the front or the back? This matters because if it was back-lit, it would be in a shadow, otherwise not."

Our brains are very good at filling in the gaps when they don't have enough information, but sometimes - and in the case of The Dress - it can cause confusion. Our brains couldn't tell where the light in the photograph was coming from, so it made an assumption, which influenced the colour of the dress. Wallisch continues:

"My research showed that if you assumed the dress was in a shadow, you were much more likely to see it as white and gold. Why? Because shadows overrepresent blue light. Mentally subtracting short-wavelength light (which would appear blue-ish) from an image will make it look yellow-ish. "Natural light has a similar effect—people who thought it was illuminated by natural light were also more likely to see it as white and gold. Why? Because the sky is blue, daylight also overrepresents short wavelengths, compared with relatively long-wavelength artificial (until recently, usually incandescent) light."

This is science and as is always the case with science, there's a lot more to the story, so we recommend reading the full thing over on Slate, but if the questions of how and why have been eating you up for the last two years, now you know. For more information on why this dress messed with your head, read the full article on Slate.