Confusing optical illusion has people divided over what number is shown

No matter what Elon Musk does with it, a core part of Twitter (sorry, X) remains the same: it’s a great place for disagreeing over things with strangers.

Sometimes it’s about something pretty serious, sometimes it’s about the latest celebrity story and sometimes it’s really just about pointless sh**e.

On that more innocent end of the spectrum is how people perceive optical illusions - I mean, we all see things like this differently, right?

And a viral one in particular really gets people going as they just won’t agree on which number is concealed.

The image shows a black and white circle with a number hidden within it - the question is, what number?

At first glance, you're probably seeing 45283, right? Or maybe you just see the number 2 to begin with.

But on closer inspection it appears to be 3452839 - at least to some people anyway.

The tweet has amassed more than 20,000 replies, and naturally, people can't understand why everyone doesn't see the same number they do.

One person commented: "45 283...and what's the catch? Should I book an appointment with my GP?"

Another said: "I can only see 528. Does that mean anything about my eyesight?"

A third added: "I can see 45283, since looking at the other replies I've others are seeing 2 other numbers. I can see there are numbers there but can't make them out."

It seems that the illusion is a measure of contrast sensitivity, as much as anything.

Contrast sensitivity measures the amount of contrast required to detect an object, such as the numbers in the image.

According to Vision Center, contrast sensitivity is an important aspect of visual function.

Explaining what it all means, its website reads: "Contrast sensitivity is the ability to distinguish between an object and the background behind it.

"Contrast sensitivity is different from visual acuity, which measures how clear your vision is at a given distance.

"High spatial frequencies make up detailed features such as sharp edges, facial features, and similar. Low spatial frequencies are more like coarse images, where you can see the overall shape of something but not detailed features.

"A contrast sensitivity test measures how well you can tell the difference between light and dark. For this, your doctor will use a different type of chart where the characters fade from black to grey gradually.

"Visual acuity is measured when you read the eye chart during an exam. This is considered a high contrast test (black letters on a white background).

"You can have excellent visual acuity, but reduced contrast sensitivity and vice versa."

Contrast sensitivity is important in low contrast situations, and if yours is poor you may be prone to accidents when driving in the fog or stepping off a kerb, for example.

It can also be a symptom of other eye conditions, such as cataracts or glaucoma.

So, if you didn't see any numbers in the image above, you may want to look into that - no pun intended.