The image shows a number partly concealed within a striped black and grey circle. Viewers are asked if they see a number and, if so, what number do they see?
Thousands of people from around the world have viewed the visual puzzle after it went viral on Twitter, and have come to very different conclusions on the hidden figures.
One person tweeted: ’45 283…and what’s the catch? Should I book an appointment with my GP?’
Another posted: ‘I can only see 528. Does that mean anything about my eyesight?’
However the actual answer is seven digits: 3452839.
It is understood that the illusion works by playing on contrast sensitivity, an important measure of visual function, which allows us to perceive the difference between objects and their backgrounds.
This comes into play in real-life situations such as driving in low light or fog, where a higher level of contrast sensitivity is required because the contrast between objects and their backgrounds is reduced.
In the case of the puzzle, the numbers are on one level and the swirls are on the other.
People with higher levels of contrast sensitivity are more likely to be able to see all seven digits straight away.