How many spots can you see in this optical illusion?
An optical illusion image has gone viral as people try to work out why they can't see all 12 dots at the same time.
Japanese psychology professor Akiyoshi Kitaoka posted the image on Facebook on Sunday morning and within 24 hours it had been shared 7,000 times.
Game developer Will Kerslake tweeted the image on Sunday night with the caption: "There are twelve black dots at the intersections in this image. Your brain won't let you see them all at once."
By Monday morning he had received more than 27,000 reactions to the tweet.
Called Ninio's Extinction Illusion after the French scientist Jacques Ninio, the image shows a grid of horizontal, vertical and diagonal lines with 12 black dots at various intersections.
Most people are unable to see all 12 at the same time.
So why does this happen?
It is when the eye sees for instance a particularly arranged pattern that the brain may not be able to process accurately.
Ninio explains: "When the white disks in a scintillating grid are reduced in size, and outlined in black, they tend to disappear. One sees only a few of them at a time, in clusters which move erratically on the page.
"Where they are not seen, the grey alleys seem to be continuous, generating grey crossings that are not actually present.
"Some black sparkling can be seen at those crossings where no disk is seen. The illusion also works in reverse contrast."
Twitter user Dan commented: "So its actually your retina that won't let you see them all at once."
The optical illusion has been confusing the internet over the weekend.
"I double-checked on different formats. Couldn't catch 'em all," tweeted one person and another said: "Gives you a headache after a while."
One user doctored the image so that he could see all of the dots:
By Andree Massiah UGC & Social News team