We’ve all gotten songs stuck in our heads. “Earworms,” as they’re called, can drive a person crazy. But they’re almost always of a song you’ve heard recently or conjured from memory. For some people, however, the music in their head isn’t quite like that. Instead, it’s a hallucination. The scores and tunes can stay with them constantly—music that only one person will ever hear.

But now researchers have managed to record the internal melody of one patient. Helen Thompson at New Scientist reports on the case of Sylvia, a woman who lost her hearing eleven years ago and then began to hear internal music. Luckily, as Thompson reports, Sylvia was a musician with perfect pitch. So she began to write down the notes that appeared in her head. This is, naturally, fascinating to neuroscientists. Thompson writes:

She discovered that playing real music suppressed her musical hallucinations. This enabled Timothy Griffiths at Newcastle University, UK, and his colleagues to study what was going on in her brain while switching her hallucinations on and off.

Passages from J. S. Bach worked as the “masker” to switch the hallucinations off. Sylvia rated the intensity of her hallucinations every 15 seconds throughout the study, which lasted about 45 minutes. At the time of the experiment, her musical hallucinations happened to consist of sequences from Gilbert and Sullivan’s musical HMS Pinafore. Immediately following the masker, her hallucinations were at their lowest, gradually increasing until the start of the next excerpt.

By watching Syvlia’s brain while she turned on and off these musical hallucinations, researchers were able to pinpoint where they might be coming from. And it looked like her internal music was coming from a handful of brain regions—the ones that process melodies as well as those implicated in memory and processing images.

In people with normal hearing, Thompson explains, as a sound comes in, the brain tries to predict what might come next. If it’s right, you can process immediately. If you’re wrong, the signal goes through the lower brain regions and corrects the high level prediction. This allows you to process sounds quickly. But since Sylvia doesn’t hear anything, her brain never corrects the incorrect predications, and thus hallucinates.

If this all sounds totally irritating, it is. “Sometimes a tune will play itself 1000 times in my head and become really annoying,” Sylvia told Thompson. “I have learned to live with it but I can see how one could be driven mad.”