An optical illusion has sparked debate on social media, after it caused each person to see a different number.

The image, shared by Twitter user @benownie, features a black-and-white circle with a zig-zag pattern inside that contains a set of hidden numbers.

However, viewers can’t seem to agree on what those numbers are.

“DO you see a number? If so, what number?”, the user captioned the post.

Since the pattern aims to trick the eye into thinking the image is moving, it can be quite tricky to distinguish which numbers lie behind it, with most seeing a jumble of different numbers.

“845283,” one person guessed.

“45 283… and what’s the catch? Should I book an appointment with my GP?” another said.

Another user and fan of Douglas Adams’ novel The Hitchhiker’s Guide to Galaxy pointed out the correct answer could be a sneaky Easter egg for fellow fans.

“3452939. Strangely, this is also the telephone number of an Islington flat where Arthur Dent went to a fancy dress party, and met a very nice young woman whom he totally blew it with,” they wrote.

“A few hitchhikers [sic] guide diehard fans peeking out of the closet here. I love you all x.”

The image even prompted some to suggest others who struggled with the photo to get their eyes tested for astigmatism.

“I can only see 528. Does that mean anything about my eyesight?” one concerned person wrote.

“When was the last time you had a sight test?” a helpful commenter replied. “Sounds like you may have an astigmatism or difficulty with contrast.”

Another user explained that the numbers a person can see could be proof of how good someone’s eyes were.

“I’m guessing the numbers you see depends on your ‘contrast sensitivity’ (different from what a standard eye measures),” they explained. “It can be tested by opticians. Worth doing if you’re struggling, as it can affect your ability to see at night, or rain, fog, etc.”

According to ScienceDirect, a repository for scientific research, contrast sensitivity describes “the ability to detect subtle differences in shading and patterns”, which is important for “detecting objects without clear outlines” such as “steps covered with a patterned carpet”.

An example contrast sensitivity test. Image: Vision Center

It’s likely that your contrast sensitivity has been tested if you wear glasses, which involves reading out black letters from a white chart which become less distinct across each row and from the top to the bottom of the chart.

This test is different from the one usually used during eye exams, such as those you take to receive or renew your driver’s licence.

Image: Twitter

This article first appeared on OverSixty.