by Deric Bownds
Kumar et al. report their findings from an unusual opportunity that presented itself when a retired London schoolteacher, Sylvia, reported to her doctors that she increasingly was hearing music, as if it were completely real, in the absence of a source for the music. (People with musical hallucinations usually are psychologically normal — except for the music they are sure someone is playing. ) Sylvia volunteered for a study by Kumar et al. that made use of the fact that real music can sometimes quiet the imaginary music, in effect masking music hallucination. Playing Bach for 30 seconds was used to damp down the hallucinations while the teacher’s brain activity was being monitored by MEG (magnetic recordings), and when the real music stopped the teacher reported the strength of hallucinations as they returned. The brain regions becoming more active as hallucinations returned were the same as those activated by listening to real music. From Zimmer’s review of this work, a suggested model for what is happening:
Our brains… generate predictions about what is going to happen next, using past experiences as a guide. When we hear a sound, for example — particularly music — our brains guess at what it is and predict what it will sound like in the next instant. If the prediction is wrong — if we mistook a teakettle for an opera singer — our brains quickly recognize that we are hearing something else and make a new prediction to minimize the error….people with musical hallucinations often have at least some hearing loss. Sylvia, for example, needed hearing aids after getting a viral infection two decades ago.
The model of our brain as a prediction-generating machine
…could explain why some people with hearing loss develop musical hallucinations. With fewer auditory signals entering the brain, their error detection becomes weaker. If the music-processing brain regions make faulty predictions, those predictions only grow stronger until they feel like reality.
…could explain why real music provides temporary relief for musical hallucinations: the incoming sounds reveal the brain’s prediction errors. And it may also explain why people are prone to hallucinate music, and not other familiar sounds.
Here is the Kumar et al. abstract:
The physiological basis for musical hallucinations (MH) is not understood. One obstacle to understanding has been the lack of a method to manipulate the intensity of hallucination during the course of experiment. Residual inhibition, transient suppression of a phantom percept after the offset of a masking stimulus, has been used in the study of tinnitus. We report here a human subject whose MH were residually inhibited by short periods of music. Magnetoencephalography (MEG) allowed us to examine variation in the underlying oscillatory brain activity in different states. Source-space analysis capable of single-subject inference defined left-lateralised power increases, associated with stronger hallucinations, in the gamma band in left anterior superior temporal gyrus, and in the beta band in motor cortex and posteromedial cortex. The data indicate that these areas form a crucial network in the generation of MH, and are consistent with a model in which MH are generated by persistent reciprocal communication in a predictive coding hierarchy.