Nick Chater explains viral 'cat on the stairs' illusion

15 April 2015

First there was ‘the dress’ now there is the ‘cat on the stairs’ that is confounding the internet and providing an optical illusion that has divided opinion.

A picture first posted on of a cat has gone viral, confusing the internet as to whether it is going up or down the stairs.

The dress caused a similar frenzy with social media arguments breaking out whether it was blue and black or white and gold. Buzzfeed’s article highlighting the online debate was shared by 20 million people around the globe.

The cat picture has sparked a similar debate, with nearly 4,000 comments on and more than 750,000 views on image sharing site Imgur.

Just like the dress image the picture of the cat highlights how our vision is a top-down process, with our brains having to interpret everything we see.

Nick Chater, Professor of Behavioural Science, reveals just what is going on and why our eyes are so confused when we look at the cat picture.

"We see in 3D, even though our eyes only receive 2D images,” said Professor Chater. “This seems puzzling, because it can be shown mathematically that there is an implied number of 3D scenes that will create the same 2D images. Most of the time, our brain is spectacularly good at solving this problem. It usually turns out that precisely one 3D interpretation is 'sensible' and all the others are bizarre, in one way or another.

It's just like an illusion: This picture has caused a huge internet debate because our brain has to interpret 2D images into 3D ones


"Our brain uses the most sensible 3D interpretation; and mostly - except when we are being subjected to cunning visual illusions - this works just fine.

“But sometimes there are two equally plausible 3D interpretations of the same 2D image. Famous examples include the Necker cube and Rubin's face-vase illusion. Then, the brain flips between one interpretation and the other."

This is what is going on with the cat picture and Professor Chater says the way the image has been cunningly put together has confused our normally reliable brains.

Professor Chater said: “Why does the 'cat on the stairs' picture have two interpretations? The key is the amorphous light grey square at the top of the picture. Consider the angle between the plane of the staircase and the surface represented by this grey patch.

“When we see the cat as coming downstairs, the light grey patch is interpreted as a ceiling - and this makes an acute, roughly 45 degrees, angle with the staircase.

“But when we see the cat as going upstairs, the light grey square is now interpreted as the floor - and this makes an obtuse, roughly 135 degrees, angle with the staircase.

“If we could only tell the slant of the mysterious square patch in relation to the staircase, we would then know if it was floor or ceiling, and so there would no ambiguity. But the image cleverly leaves the grey square bereft of any clues, so we flip from one interpretation to the other.”

Listen to more from Professor Nick Chater on BBC Radio 4's The Human Zoo and take part in behavioural science experiments at the weblab.

Professor Nick Chater teaches Behavioural Sciences for the Manager on the Warwick Executive MBA and Principles of Cognition on the suite of MSc Business courses. He also teaches Emotions in Business on Warwick Business School's Undergraduate courses.

Join the conversation

WBS on social media