For the first time, MIT neuroscientists have identified a population of neurons in the human brain that lights up when we hear singing, but not other types of music.
These neurons, found in the auditory cortex, appear to
respond to the specific combination of voice and music, but not to either
regular speech or instrumental music. Exactly what they are doing is unknown
and will require more work to uncover, the researchers say.
“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,” says Sam Norman-Haignere, a
former MIT postdoc who is now an assistant professor of neuroscience at the
University of Rochester Medical Center.
The work builds on a 2015 study in which the same
research team used functional magnetic resonance imaging (fMRI) to identify a
population of neurons in the brain’s auditory cortex that responds specifically
to music. In the new work, the researchers used recordings of electrical
activity taken at the surface of the brain, which gave them much more precise
information than fMRI.
“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,” says
Norman-Haignere.
Norman-Haignere is the lead author of the study, which
appears today in the journal Current Biology. Josh McDermott,
an associate professor of brain and cognitive sciences, and Nancy Kanwisher,
the Walter A. Rosenblith Professor of Cognitive Neuroscience, both members of
MIT’s McGovern Institute for Brain Research and Center for Brains, Minds and
Machines (CBMM), are the senior authors of the study.
Neural recordings
In their 2015 study, the researchers used fMRI to scan
the brains of participants as they listened to a collection of 165 sounds,
including different types of speech and music, as well as everyday sounds such
as finger tapping or a dog barking. For that study, the researchers devised a
novel method of analyzing the fMRI data, which allowed them to identify six
neural populations with different response patterns, including the
music-selective population and another population that responds selectively to
speech.
In the new study, the researchers hoped to obtain
higher-resolution data using a technique known as electrocorticography (ECoG),
which allows electrical activity to be recorded by electrodes placed inside the
skull. This offers a much more precise picture of electrical activity in the
brain compared to fMRI, which measures blood flow in the brain as a proxy of
neuron activity.
“With most of the methods in human cognitive
neuroscience, you can’t see the neural representations,” Kanwisher says. “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.”
Electrocorticography cannot be typically be performed
in humans because it is an invasive procedure, but it is often used to monitor
patients with epilepsy who are about to undergo surgery to treat their
seizures. Patients are monitored over several days so that doctors can
determine where their seizures are originating before operating. During that
time, if patients agree, they can participate in studies that involve measuring
their brain activity while performing certain tasks. For this study, the MIT
team was able to gather data from 15 participants over several years.
For those participants, the researchers played the
same set of 165 sounds that they used in the earlier fMRI study. The location
of each patient’s electrodes was determined by their surgeons, so some did not
pick up any responses to auditory input, but many did. Using a novel statistical
analysis that they developed, the researchers were able to infer the types of
neural populations that produced the data that were 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.”
That song-specific population of neurons had very weak
responses to either speech or instrumental music, and therefore is distinct
from the music- and speech-selective populations identified in their 2015
study.
Music in the brain
In the second part of their study, the researchers
devised a mathematical method to combine the data from the intracranial
recordings with the fMRI data from their 2015 study. Because fMRI can cover a
much larger portion of the brain, this allowed them to determine more precisely
the locations of the neural populations that respond to singing.
“This way of combining ECoG and fMRI is a significant
methodological advance,” McDermott says. “A lot of people have been doing ECoG
over the past 10 or 15 years, but it’s always been limited by this issue of the
sparsity of the recordings. Sam is really the first person who figured out how
to combine the improved resolution of the electrode recordings with fMRI data
to get better localization of the overall responses.”
The song-specific hotspot that they found is located
at the top of the temporal lobe, near regions that are selective for language
and music. That location suggests that the song-specific population may be
responding to 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, the researchers say.
The researchers now hope to learn more about what
aspects of singing drive the responses of these neurons. They are also working
with MIT Professor Rebecca Saxe’s lab to study whether infants have
music-selective areas, in hopes of learning more about when and how these brain
regions develop.
Source: https://news.mit.edu/2022/singing-neurons-0222
Source: Singing in the
brain – Scents of Science (myfusimotors.com)
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