Blame the brain for lack of rhythm.

At the other end of the musical talent scale from those with perfect pitch are people with dysmusia, a condition marked by difficulty learning to play music or recognizing melodies. It's essentially a musical version of dyslexia, the well-recognized reading disability.

On rare occasions, a person suffering a stroke or other forms of brain trauma will develop dysmusia, says Julie Ayotte of the University of Montreal. As she and her colleague Isabelle Peretz studied a few of these brain-damaged individuals, they began to wonder how frequently dysmusia occurs among healthy people. A search of scientific literature yielded just two reports of people born with dysmusia, notes Ayotte.

Through ads in local newspapers, the investigators recruited several dozen men and women who called themselves "tone deaf." After narrowing the group to 12 individuals who had actually tried to pursue musical training as children, Ayotte and Peretz gave them a battery of musical tests. One trial involved identifying wrong notes in a familiar melody. Another asked subjects to recognize melodies, lyrics, or environmental sounds that they had been given an opportunity to learn. A third test centered on identifying familiar melodies, lyrics, or voices.

In contrast to 16 other volunteers not claiming the impairment, all the self-declared tone-deaf people struggled with anything involving music. They would recognize songs from lyrics but not from melodies, for example. Or they would fail to discern a wrong note that the other subjects easily heard. "It's a specific disorder in music," says Ayotte.

The dozen volunteers with dysmusia are otherwise normal. Their ability to discriminate pitch, the frequency of a sound, is poorer than normal, the researchers report. But that doesn't pinpoint the underlying cause of dysmusia, says Ayotte. It remains unclear what percentage of the general population has the condition, she adds.