For long scientists have wondered if there is some neural mechanism in the human brain that is specific to music perception. Finally, the neuroscientists at the MIT have identified a neural population in the human auditory cortex that responds selectively to sounds that is typically categorized as music, but not to speech or other environmental sounds.
Josh McDermott, the Frederick A. and Carole J. Middleton Assistant Professor of Neuroscience in the Department of Brain and Cognitive Sciences at MIT said that this particular subject has received quite widespread speculation. There has been a lot of curiosity to know to what extent music has dedicated mechanisms in the brain and to what extent it piggybacks off of mechanisms that primarily serve other functions.
This finding has been possible due to the development of a novel method that can identify neural populations from functional magnetic resonance imaging (fMRI) data. Six neural populations with different functions have been identified by the researchers using this method these include the music-selective population and another one that responds selectively to speech. The findings were published recently in an online edition of Neuron.
Sam Norman-Haignere, a postdoc at MIT’s McGovern Institute for Brain Research and lead author of the paper opined that this finding is notable because highly selective responses to music were not clearly seen before.
Nancy Kanwisher, the Walter A. Rosenblith Professor of Cognitive Neuroscience at MIT, a member of MIT’s McGovern Institute for Brain Research and study's senior author remarked that the findings of the study are hard to reconcile with the idea that music piggybacks entirely on neural machinery that is optimized for other functions, because the neural responses that have been seen in the study are highly specific to music.
The brains of 10 human subjects listening to 165 natural sounds which include various types of speech and music along with everyday sounds like phone ringing, footsteps, etc., were scanned.
It is difficult to map brain's auditory system as the spatial resolution of fMRI is coarse it measures blood flow as an index of neural activity. “Voxels”the smallest unit of measurement in fMRIreflect the response of hundreds of thousands of neurons. Norman-Haignere explains that when one goes by the raw voxel responses, it is reflection of a mixture of underlying neural responses. In order to overcome this shortcoming, the researchers made use of a technique that models each voxel as a mixture of multiple underlying neural responses. With the help of this method six neural populations each with a unique response pattern to sounds were identified. One population responded most to music, another to speech, and the other four to different acoustic properties such as pitch and frequency.
Josef Rauschecker, a professor of physiology and biophysics at Georgetown University remarked that the key to this advance is the researchers’ new approach to analyzing fMRI data. He added that this is a study of the highest caliber of data analysis.
The four acoustically responsive neural populations overlap with regions of “primary” auditory cortex, which performs the first stage of cortical processing of sound. Speech and music-selective neural populations lie beyond this primary region. Norman-Haignere opined that this provides evidence that there’s a hierarchy of processing where there are responses to relatively simple acoustic dimensions in this primary auditory area. It is followed by a second stage of processing that represents more abstract properties of sound related to speech and music.
McDermott said that it is not right to conclude that auditory cortex is the seat of music in the brain as there may be other brain regions involved in processing music, including its emotional components.
Kanwisher also notes that an important question for the future will be how this system arises in development like how early it is found in infancy or childhood, and how dependent it is on experience? The next step for the researchers is to investigate if the music-selective population identified in this study contains subpopulations of neurons that respond to different aspects of music, like rhythm, melody, and beat.