By Erin Hare at Scientific American

As many as two thirds of stroke victims find themselves suddenly unable to comprehend music. Beyond bad karaoke, these people fundamentally cannot differentiate pitches or rhythms. The medical term is acquired amusia, and recently scientists have aggregated data from of a large group of people with this condition to pinpoint the critical brain regions involved.

A study published in August in The Journal of Neuroscience found that the brain is incredibly modular when it comes to the perception of music and speech. Whereas lesions giving rise to deficits in language perception—a condition called aphasia—reside in the brain’s left hemisphere, damage associated with amusia are restricted to the right. Aleksi Sihvonen, a neurologist at the University of Turku in Finland and lead author of the study, says he was shocked to see such a clean split. “I think the lateralization was very surprising because the amusic pattern was so clearly in the right,” Sihvonen says. “Based on [functional neuroimaging] and left-hemisphere lesion patients who were amusic, we thought there must be [critical music perception circuitry] in the left, too.”

According to Steven Sparr, a clinical neurologist at Albert Einstein College of Medicine who was not involved in this study, trying to determine the location of music in the brain has been a long process. It goes back to 1861 when French physician Paul Broca discovered that speech resides entirely within a small circuit in the brain’s left hemisphere. So far, there has been no musical lesion case so clean as Broca’s, which has led to more than a century of frustration, Sparr says. Yet there were clues that music and language are dissociable. “A little-known fact is that Broca’s original patient who was unable to speak was able to sing La Marseillaise [the French national anthem] fluently,” Sparr says. “So from the inception of aphasia research [scientists] began to realize that singing and speaking are wired differently.”

Why has the neurological evidence to support this notion proved so elusive? According to Sihvonen, it has to do with the case study approach. Given the idiosyncrasy of brain damage patterns as well as the fact that strokes rarely target tissue related to a single function, it can be difficult to draw strong conclusions from small samples…. read more here