Look at this GIF. There’s no sound, but most people who see it hear a “thudding” in their head each time the bouncing structure hits the ground. Why?

Does anyone in visual perception know why you can hear this gif? pic.twitter.com/mcT22Lzfkp — Lisa DeBruine ️‍ (@lisadebruine) December 2, 2017

The answer has to do with how our senses work, says Christopher Fassnidge, a doctoral candidate in psychology at City, University of London. This illusion is an example of synesthesia, or when the senses — like hearing and sight — get crossed in the brain, he explained in an email to The Verge.

Up to 20 percent of people could experience this phenomena

Going through the world, at every moment we are surrounded by movements that are logically associated with sound: we see a ball bouncing, and we expect to hear the bouncing. Because evidence suggests that so-called synesthetic pairings can be learned when we’re small, Fassnidge says, it makes sense that many people can develop synesthesia for very common things. Other examples are a bit more unusual. Take, for instance, the pianist Nikolai Rimsky-Korsakov, who experienced different musical notes as colors. The two are rarely linked in everyday life, and so his ability is notable for that very reason.

Synesthesia is likely what’s happening with the “noisy GIF” phenomenon, which Fassnidge’s lab calls the “visually-evoked auditory response,” or vEAR for short. (His lab is running a survey on vEAR, if you’d like to take part.)

Though movement-hearing synesthesia isn’t well-researched, one of their lab’s recent studies suggests that up to 20 percent of people experience vEAR — which is much higher than the 2 to 4 percent number for other forms of synesthesia, says Fassnidge. And, as with all forms, some people are more susceptible to the illusion than others, depending on how our brains are wired.

It’s possible that many of us experience vEAR without ever noticing, Fassnidge says. Maybe we think a sound is real when it isn’t, simply because the sound makes sense. The very commonness of vEAR could mean that we don’t notice until it’s brought to our attention by these noisy GIFs. The GIF above isn’t the only example; there are plenty more in this subreddit.

The bouncing powerline GIF itself has an interesting history. Twitter user HappyToast first made it in 2008 as part of a weekly Photoshop challenge, he told The Verge in an email, and it was included in the BBC3 TV series The Wrong Door, a sketch show set in a parallel universe. There are over a thousand different hosted copies on the internet, he says, but last weekend was the first time people actually paid attention to the audio illusion — and discovered that the senses aren’t quite as separate as they maybe believed.