With people claiming they see different sequences of numbers at its centre, a mind-boggling optical illusion of a black and grey striped spiral has left thousands of netitizens desperate for an explanation.
The visual puzzle was shared by @Benowine on his Twitter handle on February 17 last year, attracting thousands of responses.
Acclaimed French football writer Philippe Auclair, retired Olympic swimmer Sharron Davies, and radio comedian and author Mitch Benn were among those who offered their answers, The Independent reported.
Some claimed to see only “528” in the middle of the spiral while others saw a longer sequence of “45283”.
In both instances, users expressed anxiety about what this might reveal about the state of their eyesight.
Others joked that they saw only the word “KILL”.
In fact, the real answer is even longer at seven digits – 3452839.
The illusion, somewhere between a psychedelic magic eye poster and the notorious “what colour is the dress?” controversy of 2015, reportedly works by playing on contrast sensitivity, an aspect of ocular function that allows us to distinguish objects from their backgrounds.
A higher degree of contrast sensitivity is beneficial in low light conditions or in mist and fog when it is more difficult to perceive the difference between an object in the foreground and what lies behind it.
The puzzle plays a similar trick on the human eye by placing the swirls and numbers on different levels, meaning that those with strong contrast sensitivity are better able to pick out all seven digits.
Those in whom the quality is less pronounced may only be able to identify a shorter sequence.