An interesting article at the New York Times looks at the phenomemon of musical hallucinations:
Dr. Aziz [a psychiatrist at St. Cadoc’s Hospital in Wales, UK-ed.] believes that people tend to hear songs they have heard repeatedly or that are emotionally significant to them. “There is a meaning behind these things,” he said.
His study also shows that these hallucinations are different from the auditory hallucinations of people with schizophrenia. Such people often hear inner voices. Patients like Mr. King hear only music.
The results support recent work by neuroscientists indicating that our brains use special networks of neurons to perceive music. When sounds first enter the brain, they activate a region near the ears called the primary auditory cortex that starts processing sounds at their most basic level. The auditory cortex then passes on signals of its own to other regions, which can recognize more complex features of music, like rhythm, key changes and melody.
Neuroscientists have been able to identify some of these regions with brain scans, and to compare the way people respond to musical and nonmusical sounds.