It’s in the numbers
Anne Trafton | MIT
News
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However, the favored ratios can vary greatly between different societies, according to a large-scale study led by researchers at MIT and the Max Planck Institute for Empirical Aesthetics and carried out in 15 countries.
The study included 39 groups of participants, many
of whom came from societies whose traditional music contains distinctive
patterns of rhythm not found in Western music.
“Our study provides the clearest evidence yet for some degree of universality in music perception and cognition, in the sense that every single group of participants that was tested exhibits biases for integer ratios. It also provides a glimpse of the variation that can occur across cultures, which can be quite substantial,” says Nori Jacoby, the study’s lead author and a former MIT postdoc, who is now a research group leader at the Max Planck Institute for Empirical Aesthetics in Frankfurt, Germany.
The brain’s bias toward simple integer
ratios may have evolved as a natural error-correction system that makes it
easier to maintain a consistent body of music, which human societies often use
to transmit information.
“When people produce music, they often make
small mistakes. Our results are consistent with the idea that our mental
representation is somewhat robust to those mistakes, but it is robust in a way
that pushes us toward our preexisting ideas of the structures that should be
found in music,” says Josh McDermott, an associate professor of brain and
cognitive sciences at MIT and a member of MIT’s McGovern Institute for Brain
Research and Center for Brains, Minds, and Machines.
McDermott is the senior author of the
study, which appears in Nature
Human Behaviour. The research team also included scientists from more
than two dozen institutions around the world.
A global approach
The new study grew out of a smaller
analysis that Jacoby and McDermott published in 2017. In that paper, the researchers compared rhythm
perception in groups of listeners from the United States and the Tsimane’, an
Indigenous society located in the Bolivian Amazon rainforest.
To measure how people perceive rhythm, the
researchers devised a task in which they play a randomly generated series of
four beats and then ask the listener to tap back what they heard. The rhythm
produced by the listener is then played back to the listener, and they tap it
back again. Over several iterations, the tapped sequences became dominated by
the listener’s internal biases, also known as priors.
“The initial stimulus pattern is random,
but at each iteration the pattern is pushed by the listener’s biases, such that
it tends to converge to a particular point in the space of possible rhythms,”
McDermott says. “That can give you a picture of what we call the prior, which
is the set of internal implicit expectations for rhythms that people have in
their heads.”
When the researchers first did this
experiment, with American college students as the test subjects, they found
that people tended to produce time intervals that are related by simple integer
ratios. Furthermore, most of the rhythms they produced, such as those with
ratios of 1:1:2 and 2:3:3, are commonly found in Western music.
The researchers then went to Bolivia and
asked members of the Tsimane’ society to perform the same task. They found that
Tsimane’ also produced rhythms with simple integer ratios, but their preferred
ratios were different and appeared to be consistent with those that have been
documented in the few existing records of Tsimane’ music.
“At that point, it provided some evidence
that there might be very widespread tendencies to favor these small integer
ratios, and that there might be some degree of cross-cultural variation. But
because we had just looked at this one other culture, it really wasn’t clear
how this was going to look at a broader scale,” Jacoby says.
To try to get that broader picture, the MIT
team began seeking collaborators around the world who could help them gather
data on a more diverse set of populations. They ended up studying listeners
from 39 groups, representing 15 countries on five continents — North America,
South America, Europe, Africa, and Asia.
“This is really the first study of its kind
in the sense that we did the same experiment in all these different places,
with people who are on the ground in those locations,” McDermott says. “That
hasn’t really been done before at anything close to this scale, and it gave us
an opportunity to see the degree of variation that might exist around the
world.”
Cultural comparisons
Just as they had in their original 2017 study, the researchers found that in every group they tested, people tended to be biased toward simple integer ratios of rhythm. However, not every group showed the same biases.
People from North America and Western Europe, who have
likely been exposed to the same kinds of music, were more likely to generate
rhythms with the same ratios. However, many groups, for example those in
Turkey, Mali, Bulgaria, and Botswana showed a bias for other rhythms.
“There are certain cultures where there are
particular rhythms that are prominent in their music, and those end up showing
up in the mental representation of rhythm,” Jacoby says.
The researchers believe their findings
reveal a mechanism that the brain uses to aid in the perception and production
of music.
“When you hear somebody playing something and they have errors in their performance, you’re going to mentally correct for those by mapping them onto where you implicitly think they ought to be,” McDermott says.
“If you didn’t have something like this, and you just
faithfully represented what you heard, these errors might propagate and make it
much harder to maintain a musical system.”
Among the groups that they studied, the
researchers took care to include not only college students, who are easy to
study in large numbers, but also people living in traditional societies, who
are more difficult to reach. Participants from those more traditional groups
showed significant differences from college students living in the same
countries, and from people who live in those countries but performed the test
online.
“What’s very clear from the paper is that
if you just look at the results from undergraduate students around the world,
you vastly underestimate the diversity that you see otherwise,” Jacoby says.
“And the same was true of experiments where we tested groups of people online
in Brazil and India, because you’re dealing with people who have internet
access and presumably have more exposure to Western music.”
The researchers now hope to run additional
studies of different aspects of music perception, taking this global approach.
“If you’re just testing college students
around the world or people online, things look a lot more homogenous. I think
it’s very important for the field to realize that you actually need to go out
into communities and run experiments there, as opposed to taking the
low-hanging fruit of running studies with people in a university or on the
internet,” McDermott says.
The research was funded by the James S.
McDonnell Foundation, the Canadian National Science and Engineering Research
Council, the South African National Research Foundation, the United States
National Science Foundation, the Chilean National Research and Development
Agency, the Austrian Academy of Sciences, the Japan Society for the Promotion
of Science, the Keio Global Research Institute, the United Kingdom Arts and
Humanities Research Council, the Swedish Research Council, and the John Fell
Fund.
