Before reading any further, listen to the video above.

That was Mariah Carey’s Christmas classic All I Want For Christmas Is You, but as you have probably never heard it before. Earlier this week, someone converted the song to MIDI – a digital way to record sound – and posted it on Tumblr.

Another blogger shared the song in frustration: “I’m driving myself up the wall because I swear I can hear the vocal line but I don’t know how that could be if it was truly converted to MIDI.”

Carey’s voice seemed to be there, even though it shouldn’t be – the MIDI music standard is simple, and can’t capture the full richness of the human voice (or any other real instrument).


The post has generated thousands of notes and intense curiosity. Here at New Scientist we agree: we swear we can hear a ghostly Mariah belting along in the background. What is going on?

James Devaney/Getty

Diana Deutsch, a cognitive psychologist at the University of California, San Diego, thinks she knows. Deutsch specialises in auditory illusions: tricks of sound that make people hear words or melodies that aren’t there.

I waited on the phone as Deutsch opened up the MIDI Mariah file and listened to the song for the first time. And then, I was crushed to hear her say: “Actually, I don’t hear it.”

In fact, Deutsch hadn’t ever heard the original. That’s why the illusion worked for me and failed for her, she says. It’s an example of what psychologists call top-down processing, when the brain uses information it already has to try to understand the world around it.

I already knew the song: my brain had a template saved that it could match the new sounds to, filling in the non-existent vocals. My expectations shaped my perceptions.

“This must be a very strong example of having the template and then invoking it,” she says. “It seems like the brain abhors ambiguity and wants to make sense out of things, so we create for ourselves precepts of things that aren’t really there.”

MIDI Mariah

For an example that predates the Carey clip, check out this video of the Bee Gees disco hit Stayin’ Alive, also converted to MIDI. People who hadn’t heard the song before say they didn’t hear lyrics on their first listen. But when they listened to the real song and then went back to the MIDI, the voices suddenly seemed to appear.

New Scientist also conducted our own internal test. While most of us could pick out every word of MIDI Mariah, far fewer could hear the words to MIDIfied Itchycoo Park by the Small Faces, because they had never heard the song.

Top-down processing is to our advantage, says Petr Janata, a cognitive neuroscientist at the University of California, Davis. It makes humans and animals more sensitive to potential threats or benefits in the world around them.

“There’s a lot of value to predicting what might happen based on prior knowledge,” he says. “It’s the role of nervous systems in general to learn to perceive and interact with the environments in which they find themselves.”

Primed for words

It may also help that most people who hear the clip are primed to hear Carey’s voice, by reading her name and the title of the song before they listen.

He compares the Carey illusion to the phenomenon of people claiming to hear hidden messages in songs played backwards. In one of his classes, he plays Led Zeppelin’s Stairway to Heaven backwards and asks students if they caught any words. Then, he gives them a paragraph of text supposedly encoded in the recording and plays it for a second time.

“When I play it again, they’re all just blown away that all of the sudden they can hear all that in the music,” he says. “But it’s really just our brain imposing that interpretation, picking out the viable evidence in the signal to conform and support our expectations.”

In other words: we hear what we want to hear.

Click here for more cool auditory illusions.

Image credit: James Devaney/Getty