From the cigar nestled in the brickwork to 'The Dress', many optical illusions have left viewers around the world baffled over the years.
But the latest illusion is arguably one of the most bizarre, and will trick your brain into distorting celebrity faces into terrifying monsters.
The powerful illusion, called the 'flashed face distortion' effect, is an example of how our brains work in strange ways to fill in the gaps in our perception.
In aTikTokvideo showing off the illusion, Dr Karan Raj, an NHS general surgeon in Hampshire, explained: 'It's an example of your brain trying to fill in the gaps based on its memory and experience - similar to how it does with your blindspot.'
He added: 'This is going to terrify you.'
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To try the illusion, keep your eyes on the cross in the middle of the screen, while the celebrity faces flash before you.
By keeping your eyes fixed on the cross in the centre and viewing the faces in your peripheral vision, the faces will begin to appear warped.
'Your favourite celebrities distort into terrifying monsters with grotesque features,' the video explains.
However, upon pausing the video and looking at the faces closely, their features will appear to return to normal.
On TikTok, commenters on the post flocked to share their horror at what they had witnessed.
'I watched it 5 times. Indeed, it's terrifying', wrote one commenter.
In this terrifying TikTok Dr Karan Raj shares an example of a creepy optical illusion called the flashed face distortion effect
One commenter added: 'This scared the hell out of me. I blinked they were 'normal' and then distorted.'
Meanwhile another wrote: 'Mum I'm scared I want to go home.'
Thankfully, this is not any kind of supernatural phenomenon and is actually a well-documented optical illusion called the flashed face distortion effect.
Dr Raj explains that this is an example of how your brain makes assumptions about what it should be seeing when it doesn't have all the information.
He says: 'For example, if your brain thinks it sees a big nose it will magnify that feature into a grotesque shape.'
Interestingly, unlike many optical illusions, this phenomenon was only discovered quite recently.
The flashed face distortion effect was first described in 2011 by a team of researchers from the University of Queensland.
In the video, pairs of celebrity faces are quickly flashed on the screen. As you keep your eye focused on the faint cross in the centre of the image, the celebrity faces begin to warp into grotesque monsters
As Dr Raj points out, this effect is particularly pronounced if the face shown has a distinctive feature like a big nose or forehead
In their paper, published in the journal Perception, the researchers described the effect, saying: 'When cycling through the faces on a computer screen, each face seems to become a caricature of itself and some faces appear highly deformed, even grotesque.'
As Dr Raj noted, the researchers also found that the effect was most pronounced in faces with some sort of feature which made them distinctive.
For instance, the researchers note that if the person in the picture had a large forehead, the resulting illusion would have a particularly large forehead.
But what is so strange about the flashed face distortion effect is that scientists still don't fully understand why it works.
In 2019, researchers from North Dakota State University found that the effect was not dependent on any mechanisms which are unique to our perception of faces.
They noticed that showing faces wearing makeup or inverting the images, which normally has a large effect on face perception, did not change the level of distortion.
However, very unusually for an optical illusion, giving the participants more time to look at the faces actually increased the levels of distortion experienced.
They write: 'It is unclear why prolonged viewing doesn't ultimately weaken the effect, especially if participants could be adapting to something like the average face across all exemplars included in the sequence.'
These researchers suggest that the effect is likely caused by the blurring of objects in our peripheral vision but ultimately conclude that 'despite the popularity of the illusion, there is as yet no clear consensus regarding its basis.'
WHAT IS THE DELBOEUF ILLUSION?
The Delboeuf illusion is one type of visual illusion where a dot surrounded by a large ring is typically perceived to be smaller than the same-sized dot surrounded by a small ring.
This optical trick works because your brain perceives the dot in the context of the outer ring.
It was named after the Belgian philosopher and mathematician Joseph Remi Leopold Delboeuf (1831 – 1896), who created it in 1865.
The Delboeuf illusion is one type of visual illusion where a dot surrounded by a large ring is typically perceived to be smaller than the same-sized dot surrounded by a small ring
In terms of plate size, the theory goes that having a smaller plate tricks people into thinking they have more food.
However, new research suggests that when people are hungry, they are able to identify food portion accurately, no matter how it is served.
According to the researchers, this indicates that hunger stimulates stronger analytic processing that is not as easily fooled by the illusion.
However, the Delboeuf illusion is widely believed to work in other contexts.
