For the first time, scientists have pinpointed neurons in the human brain that respond to singing, but not to other types of music. The neurons, found in the auditory cortex, appear to respond to the specific combination of voice and music, but not to regular speech or instrumental music.
The new study, published in Current Biology, builds on previous research by the same team that found a population of neurons in the brain that responds to music in general using functional magnetic resonance imaging (fMRI), a technique that measures blood flow in the brain as a representation of neural activity.
“With most of the methods in human cognitive neuroscience, you can’t see the neural representations,” says senior author Nancy Kanwisher, professor of cognitive neuroscience at MIT. “Most of the kind of data we can collect can tell us that here’s a piece of brain that does something, but that’s pretty limited. We want to know what’s represented in there.”
Now, taking it a step further, the team has collected recordings of electrical activity taken at the surface of the brain, which is much more precise but has one problem: it must be carried out intracranially.
The higher-resolution data was obtained using electrocorticography (ECoG), where the electrical activity of the brain is recorded by electrodes placed inside the skull. ECoG cannot typically be performed on humans because it is such an invasive procedure, though it is often used to monitor patients with epilepsy who are about to undergo surgery to treat their seizures.
Some patients, already monitored for several days to determine the origin of their seizures before operating, also agree to have their brain activity measured while performing certain tasks for scientific research. In this case, data from 15 participants listening to a collection of 165 sounds – the same sounds used in the earlier fMRI study – were collected over the course of several years.
“There’s one population of neurons that responds to singing, and then very nearby is another population of neurons that responds broadly to lots of music,” explains Sam Norman-Haignere, a former MIT researcher and now assistant professor of neuroscience at the University of Rochester Medical Center in the U.S.
“At the scale of fMRI, they’re so close that you can’t disentangle them, but with intracranial recordings, we get additional resolution, and that’s what we believe allowed us to pick them apart.”
Using a new statistical analysis that they developed, the researchers were able to infer the types of neural populations that produced the data recorded by each electrode.
“When we applied this method to this data set, this neural response pattern popped out that only responded to singing,” says Norman-Haignere. “This was a finding we really didn’t expect, so it very much justifies the whole point of the approach, which is to reveal potentially novel things you might not think to look for.”
In the second part of the study, the researchers developed a new mathematical method for combining the data from ECoG and fMRI. Because fMRI can cover a much larger area of the brain, this allowed the researchers to determine precisely where the neural populations that respond to singing are located.
This spot is at the top of the temporal lobe – near language and music regions of the brain –suggesting they might be responding to song features such as the perceived pitch, or the interaction between words and perceived pitch, before sending information to other parts of the brain for further processing.
In the future, the team hopes to learn more about which aspects of singing cause the response of these neurons.