Once there was the dress; now there’s Yanny or Laurel.

This audio illusion, which went viral after first appearing on Reddit, has the internet torn. After all, what kind of monster doesn’t hear Yanny?

When Verge Science listened to it this morning, fighting broke out between the Yanny and Laurel factions. (Ed. note: I briefly lost my mind, as I first heard Yanny, then heard Laurel for about two hours, and now hear Yanny again. Same device, same speakers. Please send help.) Obviously something was going on — so we called up some scientists to help us figure it out. According to Lars Riecke, an assistant professor of audition and cognitive neuroscience at Maastricht University, it’s not actually an illusion. In fact, it’s an ambiguous figure, the auditory equivalent of two figures in profile that also form a vase, called Rubin’s vase. “The input can be organized in two alternative ways,” he says.

What do you hear?! Yanny or Laurel pic.twitter.com/jvHhCbMc8I — Cloe Feldman (@CloeCouture) May 15, 2018

The secret is frequency. The acoustic information that makes us hear Yanny is higher frequency than the acoustic information that makes us hear Laurel. Some of the variation may be due to the audio system playing the sound, Reicke says. But some of it is also the mechanics of your ears, and what you’re expecting to hear.

Older adults tend to start losing their hearing at the higher frequency ranges, which could explain why Riecke could only hear Laurel, but his eight-year-old daughter could hear Yanny. It’s a phenomenon you can mimic on a computer, he says: if you remove all the low frequencies, you hear Yanny. If you remove the high frequencies, you hear Laurel.

Most sounds — including L and Y, which are among the ones at issue here — are made up of several frequencies at once. So the problems with perception might have something to do with that. But Riecke suspects that these overlap more in the real world than in the audio recording that’s driving everyone up the wall. He thinks that the frequencies of the Y might have been made artificially higher, and the frequencies that make the L sound might have been dropped, Riecke says, although he notes this is speculation. Without knowing where this recording came from, he can’t be sure.

So if your sound card — or your ears — emphasize both the higher and the lower frequencies, you can toggle between the two sounds. And changing the sound mix to emphasize higher or lower frequencies might tip you toward Laurel or Yanny. That’s what it took for Riecke — changing his headset wasn’t enough.

We also called up Bharath Chandrasekaran, a professor in the department of communications sciences and disorders at the University of Texas at Austin. He told us that half his lab hears Yanny, and half his lab hears Laurel. But he also blames the file’s noise for the confusion. “It’s a little bit noisy, so that itself causes perception to be a little more ambiguous,” he says. “Because it’s noisy, your brain is filling in with what it thinks it should be.”

He also points to something else: the visual prompt that comes with the audio, Yanny or Laurel. That might help shape what people hear. Here’s another example of how prompts shape what we hear: the same word can sound like “bill,” “pail,” or “mayo” depending on what’s on-screen.

So it’s not just your ears or your speakers — it’s also your brain, Chandrasekaran says. Not only is it filling in what it thinks the sound should be, based on the prompt, it’s also quirky. What you hear — everything you hear — is shaped in some way by your previous experiences. This is most obvious with music, where training makes it easier to identify component parts of a symphony. So just like in a noisy cocktail party, your brain is filling in what it doesn’t quite hear, based in part on what you expect to hear and what you’ve heard before.

So what makes someone a Yanny hearer instead of a Laurel hearer? Ultimately, Chandrasekaran is curious, more than anything. “We’re going to collect a bunch of Laurel people and Yanny people,” he says. They’ll listen to the recording and his lab can look at their brain waves. Maybe we’ll find out in a few years...