The brain identifies individual musical notes differently from how it decides whether the pitch is high or low, according to a report published online this week by the Proceedings of the National Academy of Sciences. Researchers believe the brain may process sound in this way in order to help distinguish between multiple sources of noise, such as different voices in a crowded room.
All music (with the exception of the most avant-garde compositions) is made up of just twelve notes: A, A-sharp, B, C, C-sharp, D, D-sharp, E, F, F-sharp, G and G-sharp. This sequence, called the chromatic scale, repeats from the lowest to the highest pitch. For example, an 88-key piano represents seven cycles of the chromatic scale, plus four extra notes. The interval between one note and its counterpart in the next cycle is called an octave. Thus, when a C-sharp and a C-sharp one octave higher are heard at the same time, the two notes are virtually indistinguishable. If the notes are alternated, however, it becomes easier to tell that one pitch is higher.
To test how the brain can understand the two C-sharps as the same note but also different pitches, Tim Griffiths of Newcastle University Medical School in Britain and his colleagues developed a new technique to isolate the height of a pitch from its place on the chromatic scale. By tweaking the harmonics and overtones of a musical note, the researchers could manufacture a tone that sounds up to an octave higher than the original pitch did. "What we¿ve done in the experiment is actually do something you can¿t do on the piano," says Griffiths.
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In the experiment, 10 subjects listened to both the manufactured tones and a series of normal musical notes while undergoing a functional magnetic resonance imaging (fMRI) brain scan. The researchers found that the height-manipulated tones stimulated brain activity near the front of an area known as the primary auditory cortex, whereas notes from the chromatic scale activated areas to the rear of this region. Griffiths suggests that this specialization in the brain may help to distinguish between male and female voices, while still listening to what the person is saying. "There might be two ¿what¿ pathways," he explains. " ¿What is the object [the voice]?¿ and ¿What is the information transmitted by the object?¿"