When you look at the picture below, do you see straight or curved lines? 

That’s the debate triggered by the viral optical illusion that’s now raging on social media. 

The image is an eight-by-eight grid of grey squares separated by narrow green lines – almost like a bird’s-eye view of a city. 

Bizarrely, when we look at the image, the green lines appear to be constantly moving and curling in all directions.

But the moment we focus on any one particular line, it looks perfectly straight. 

The seemingly shape-shifting image is perplexing users on Reddit, who described it as ‘irritating’, ‘neat’ and ‘fantastic’. 

One user replied: ‘This is genuinely irritating. Well done.’ 

Another posted: ‘Brain now broken thanks.’ 

The image posted on Reddit shows grey bricks met with green lines that intersect to create a grid – but the lines appear to be moving even though they’re not

The stunning image was created by Florida-based illustrator Lesha Porche, who shared it on her Facebook page in December 2021. 

Dr Jolyon Troscianko, a visual ecologist at the University of Exeter who studies animal perception systems, called it a ‘very interesting illusion’.

He pointed out that it only seems to affect the ‘peripheral vision’ – what we see beyond our direct line of sight, or out of the corner of our eye. 

‘The bits I’m looking straight at have straight lines, but the lines further from where I’m directly fixated go all curved,’ Dr Troscianko told the Daily Mail. 

The answer to why this illusion works appears to be hidden inside the grey squares, of which there are 64 in total. 

Looking closely inside the grey squares, we can see tiny grey shapes of various shades and sizes, like assortments of little pebbles.   

It is these little shapes that trick the brain into thinking the straight green lines are curved rather than straight, the academic suggested. 

‘Hidden in those little grey squares are actually a series of lines/stripes at different angles,’ Dr Troscianko said.

To illustrate the hidden power of the image, Dr Troscianko passed it through a computer model he developed as part of a 2023 study. Note the hidden curved lines in the boxes that play havoc with our peripheral vision 

‘When we look directly at these grey squares they appear random, but to our peripheral vision these lines become much more apparent.’ 

To illustrate the hidden power of the image, Dr Troscianko passed it through a computer model he developed as part of a 2023 study. 

The computer model replicates how human vision can see ‘contrasts’, which in this context just means the difference between shades of grey. 

And it reveals patterns in the original image that are otherwise difficult to spot. Namely, there are hidden curved lines formed by the ‘pebbles’ that play havoc with our peripheral vision. 

‘There are actually hidden patterns in those grey squares that are obvious to our peripheral vision,’ Dr Troscianko said.

Our peripheral vision helps us to see things next to us without turning our head and makes up the biggest portion of our visual field. In comparison, our central vision, made up of a small eye region called the fovea, makes up a very small portion of our visual field. 

‘Our eyes have a ‘high resolution’ (high acuity) region called the fovea, and when we look directly at something this region is used,’ Dr Troscianko said.

‘When we look straight at them (and use our high-powered foveal vision) we can’t see them.’ 

Peripheral vision helps us to see things next to us without turning our head – and makes up the biggest portion of our visual field In comparison, our central vision, made up of a small eye region called the fovea, makes up a very small portion of our visual field. Not the fovea in this diagram of the human eye 

In any case, just like many other optical illusions, the image is not moving before our very eyes at all. 

Optical illusions work because our eyes and brain ‘speak to each other in a very simple language, like a child who doesn’t know many words’, according to experts at the University of Queensland’s Brain Institute.

‘Most of the time that’s not a problem and our brain is able to understand what the eyes tell it,’ they say.

‘But your brain also has to “fill in the blanks” meaning it has to make some guesses based on the simple clues from the eyes.

‘Mostly those guesses are right… sometimes, however, the brain guesses wrong.’