When ‘The Dress’ illusion went viral earlier this year, people were divided over whether the body con number was white and gold or black and blue.

But while this colour switch was temporary, another trick can have a much longer-lasting effect.

Called the McCollough effect, it involves staring at images of vertical and horizontal lines, and it can make you see black and white as green and red for up to three and a half months.

WARNING: This trick can affect your vision for a long period of time

The McCollough effect was found by a psychologist in Canada in 1965, and explained by Tom Scott in a video on YouTube (shown). It involves staring at a black and white test image for up to a minute. This is followed by staring at horizontal and vertical red and green lines, causing other lines to appear red and green

The effect was first discovered by American psychologist Celeste McCollough Howard in 1965, and explained by YouTube user Tom Scott on his channel Things You Might Not Know.

To perform the trick, first look at a test grid image of black and white horizontal and vertical lines to confirm they appear black and white.

Next, you must stare alternately at a square containing horizontal red and black stripes, and then a vertical green equivalent for up to three minutes - alternating between staring at the centre of both for several seconds.

Once this is done, stare back at the original grid image of black and white gratings.

Now, instead of appearing black and white, the top right and bottom left vertical gratings will have a pinkish hue, while the top left and bottom right horizontal gratings will appear slightly green.

HOW TO PERFORM THE MCCOLLOUGH EFFECT Step One Look at this test image below to confirm that it appears black and white. Step Two Stare alternately at the centre of these coloured induction images for up to three minutes. You should stare at the centre of one for several seconds, before switching to the other for a few seconds, and back again repeatedly. Step Three Look back at the test image above. You should find that the vertical sections in the bottom left and top right appear very slightly red, while the horizontal sections top left and bottom right will appear slightly green. The effect will be more pronounced if you spent longer on step two. You’ll notice that even after a few minutes, the effect is still noticeable. The effect can actually remain for up to three and a half months, and may be noticeable on other horizontal and straight lines aside from the ones in the grid. Step Four To reverse the effect, you simply need to stare at the original inducion images again, but rotated 90 degrees counter clockwise, for half the time you originally stared at them. The rotated image is shown below. Advertisement

The longer you perform the initial trick, anywhere up to 15 minutes in total, the more pronounced the effect will be.

But unlike many optical illusions, the most bizarre thing about this one is that, even after several minutes, the effect is still noticeable.

In fact, reports by Dr McCollough suggest that it can last up to three and a half months.

Fear not, though; if you want to reverse the effect, you simply need to stare at the original coloured images again for half the time you originally looked at them, but rotate them 90 degrees counter clockwise.

As Mr Scott explains in his video, this trick isn’t taking place in the retinas of your eyes.

Instead, he says, ‘the McCollough effect, for some reason, works in the level of the brain.’

He continued: ‘The McCollough effect is a very rare thing. It’s quite scary. It’s a real image that can lock into your brain, change the way it works, and stick around for a good while.

‘It is a real, [headache-inducing] sensation, and that is something quite scary that you might not have known.'

The McCollough bears some similarities to the infamous dress (shown) that caused a stir on the Internet in February, with some people seeing it as white and gold, while others saw it as black and blue

The exact reason the trick works, however, is not completely understood.

There are three main theories. The first is that staring at the coloured images causes your visual cortex in your brain to see colour next to particular straight edges.

Another theory is that your brain is error correcting after staring at the black and white image for so long, and compensates by adding colour where it might not have been before.

The other theory is that it might be related to withdrawal symptoms from staring at the colours - with the brain now associated vertical lines with green, and horizontal lines as red.

‘The McCollough effect is a clever, if slightly mean trick, to play on the brain because it taps into processes that are usually very important and useful,’ said Dr Georgina Powell, a research associate in the School of Psychology at Cardiff University.

She favours the first theory to explain the effect.

‘Think about going from a dark room into bright sunlight - initially we are blinded by the light but the brain quickly adjusts so we can see things more clearly.

‘In the McCollough effect, the brain thinks that the pairing of the colours and the lines must be some sort of internal error.

‘It makes this mistake because it is very unnatural to be exposed to such patterns for so long and so it attributes them to a problem in visual system not something real in the world.’

The exact reason the trick works, however, is not completely understood. There are three main theories. One is that staring at the coloured images causes your visual cortex (shown in pink on the right in this illustration) in your brain to see colour next to particular straight edges

She said the brain starts to fix the problem by subtracting the colours to get back to black and white again.

But if the brain is shown lines that are really black and white, it will still subtract the colours, creating an illusion where the coloured lines are the opposite way around.

This likely occurs in the visual cortex - at the back of the brain, which combines visual information about colour, shape and orientation.

‘One intriguing property of the McCollough effect is that it can last for a long time - usually minutes or hours, but sometimes days and months,’ Dr Powell added.

‘One possible explanation for this is that there aren’t many good opportunities for the brain to recalibrate again - we don’t see many patterns in the real world that are similar enough to the original pattern.