One night when she was trying to fall asleep, a 60-year-old woman suddenly began hearing music, as if a radio were playing at the back of her head.

The songs were popular tunes her husband recognized when she sang or hummed them. But she herself could not identify them.

This is the first known case of a patient hallucinating music that was familiar to people around her, but that she herself did not recognize, according to Dr. Danilo Vitorovic and Dr. José Biller of Loyola University Medical Center. The neurologists describe the unique case in the journal Frontiers in Neurology.

The case raises "intriguing questions regarding memory, forgetting and access to lost memories," the authors write.

Musical hallucinations are a form of auditory hallucinations, in which patients hear songs, instrumental music or tunes, even though no such music is actually playing. Most patients realize they are hallucinating, and find the music intrusive and occasionally unpleasant. There is no cure.

Musical hallucinations usually occur in older people. Several conditions are possible causes or predisposing factors, including hearing impairment, brain damage, epilepsy, intoxications and psychiatric disorders such as depression, schizophrenia and obsessive-compulsive disorder. Hearing impairment is the most common predisposing condition, but is not by itself sufficient to cause hallucinations.

Vitorovic and Biller describe a hearing-impaired patient who initially hallucinated music when she was trying to fall asleep. Within four months, she was hearing music all the time. For example, she would hear one song over and over for three weeks, then another song would begin playing. The volume never changed, and she was able to hear and follow conversations while hallucinating the music.

The patient was treated with carbamazepine, an anti-seizure drug, and experienced some improvement in her symptoms.

The unique feature of the patient was her ability to hum parts of some tunes and recall bits of lyrics from some songs that she did not even recognize. This raises the possibility that the songs were buried in her memory, but she could not access them except when she was hallucinating.

"Further research is necessary on the mechanisms of forgetfulness," Vitorovic and Biller write. "In other words, is forgotten information lost, or just not accessible?"

Vitorovic is a former chief neurology resident and Biller is a professor and chair in the Department of Neurology of Loyola University Chicago Stritch School of Medicine.