Music and the human brain seem to be deeply intertwined, a bond that may have first appeared when the first australopithecine ancestor got up on her hind legs 4.4 million years ago and walked. This bipedal rhythm may have made our lineage particularly sensitive to musicality, so much so that we now know that the human brain has dedicated neural circuitry for processing and interpreting musical information.
In 2015, neuroscientists at MIT identified a population of neurons in the auditory cortex that responds specifically to music. In a new study that appeared today in the journal Current Biology, the same team of researchers led by Sam Norman-Haignere have identified specific neurons in the brain that light up only when we hear singing, but not other types of music.
“The work provides evidence for relatively fine-grained segregation of function within the auditory cortex, in a way that aligns with an intuitive distinction within music,” said Norman-Haignere, a former MIT postdoc who is now an assistant professor of neuroscience at the University of Rochester Medical Center.
The singing brain
For their original 2015 work, the scientists used functional magnetic resonance imaging (fMRI) to scan the brains of participants as they listened to a collection of 165 sounds. These included everyday sounds like a dog barking or traffic in a busy city, as well as different types of speech and music.
After analyzing the brain patterns using a novel interpretation technique for fMRI data, the researchers identified a neural population that responded differently to both music and speech.
However, fMRI –which detects the changes in blood oxygenation and flow that occur in response to neural activity while a person lies down inside a machine equipped with very powerful magnets — has its limitations. A much more precise method for recording electrical activity in the brain is electrocorticography (ECoG), which directly measures patterns of activity using electrodes implanted inside the skull. The obvious drawback is that this is highly invasive. Let’s just say there aren’t too many keen volunteers that would gladly have their skulls drilled for science — unless you already don’t have much to lose.
Electrocorticography is becoming relatively widely used to monitor patients with epilepsy who are about to undergo surgery to treat their seizures. This allows doctors to pinpoint the exact location in the brain where a patient’s seizures are originating, which can be different from person to person.
Some of these patients agreed to participate, and MIT researchers were able to gather data from them over several years. Many of the 15 participants involved in the study didn’t have electrodes fitted in their auditory cortex, but some did — and the insight they provided proved valuable. Using a novel statistical approach, the researchers were able to identify neural populations that were responsible for the electrical activity recorded by each electrode.
“When we applied this method to this data set, this neural response pattern popped out that only responded to singing,” Norman-Haignere says. “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.”
“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. 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,” he added.
When ECoG data was combined with fMRI, the researchers were able to determine even more precisely the locations of the neural populations that responded specifically to signing, but not other kinds of music.
“The intracranial recordings in this study replicated our prior findings with fMRI and revealed a novel component of the auditory response that responded nearly exclusively to song,” Norman-Haignere told ZME Science.
These song-specific hotspots were found at the top of the temporal lobe, near regions that are selective for language and music. This suggests that song-specific populations of neurons likely respond to perceived pitch, so they might tell the difference between spoken words and musical vocalization, before sending this information to other parts of the brain for further processing.
These findings enrich our understanding of how the human brain responds to music. For instance, previous research showed music impacts brain function and human behavior, including reducing stress, pain and symptoms of depression, as well as improving cognitive and motor skills, spatial-temporal learning, and neurogenesis, which is the brain’s ability to produce neurons.
But many mysteries still remain, which is why the MIT researchers plan to study infants’ neural response to music, in hopes of learning more about how brain regions tuned to music develop.
“At present, we know very little about song-selective neural populations, in part because we just discovered them and in part because this type of data takes a long time to collect. Those are great questions that future research will hopefully shed some light on,” Norman-Haignere told ZME Science.