When the music is rousing, we often involuntarily bob along. The innate ability to move to the beat of music was previously thought to be uniquely human. A new study now shows that rats also have an innate sense of rhythm: when they hear music, tiny movements of their head show that they are picking up the beat. The effect is most pronounced in music in a speed range that also has a particularly rousing effect on people. This suggests that this does not depend on the species-specific size and speed of a creature, but is determined by universal properties of the brain.
We humans typically prefer music at a speed of 120 to 140 beats per minute. This is about the same as our walking speed when we are moving and is about twice as fast as our resting heart rate. But what about animals and what role does it play whether a creature is significantly larger or smaller, faster or slower than humans? For a long time, science assumed that animals have no sense of rhythm anyway. While some individuals can be trained to respond to music, unlike humans, this ability did not appear to be innate.
Mozart in the laboratory
A team led by Yoshiki Ito from the University of Tokyo has now shown that rats not only have an innate sense of music, but also prefer the same speed range as humans when it comes to rhythm. "Without any training and without prior exposure to music, rats exhibit innate clock synchronization, which, like humans, is most evident at 120 to 140 beats per minute," says Ito's colleague Hirokazu Takahashi.
For their experiment, the researchers played excerpts from Mozart's Sonata for Two Pianos in D major to 20 human subjects and ten rats. They chose four different tempos: the original tempo at 132 beats per minute, as well as two faster versions at double and quadruple the tempo, and a slow version at 75 percent of the tempo. They measured the head movements of the people with an accelerometer on the headphones. They also fitted the rats with an accelerometer on their heads, which recorded even the smallest of head movements that were imperceptible to the naked eye. The researchers also measured the brain waves in the auditory cortex of the music-listening rats.
Same preferences in rats and humans
With the different tempos, the researchers wanted to find out which tempo range of the music has the greatest effect on rats. They tested two hypotheses: "According to the first hypothesis, the optimal speed is determined by the constant of the body and the speed of movement," they explain. According to this hypothesis, the optimal speed for small animals like rats should be significantly faster than for humans. "The second hypothesis states that the preferred speed depends on the constant of the brain," say the researchers. "Since all species have similar short-term plasticity time constants in the auditory cortex, this hypothesis predicts that the optimal tempo for clock synchronization is the same in all species."
In fact, the second hypothesis was confirmed: both rats and humans bobbled their heads most clearly to the beat of the music when the music was playing at the original tempo of 132 beats per minute. With the faster versions, rocking decreased not only in humans, but also in rats. This was also reflected in the rats' auditory cortex, which was also set to a speed range of 120 to 140 beats per minute. "This suggests that the optimal tempo for clock synchronization depends on the time constant in the brain," says Takahashi. With the help of a mathematical model, the researchers also confirmed that short-term neural adaptation processes are involved in clock synchronization in the auditory cortex.
musical brain
According to the researchers, the findings not only offer new insights into the animal mind, but also into the origins and impact of our own music. “Music exerts a powerful attraction on the brain and has profound effects on emotions and cognition. In order to use music effectively, we need to uncover the neural mechanism that underlies this empirical fact," Takahashi said. "Next, we want to study how other musical properties, such as melody and harmony, are related to brain dynamics."
Source: Yoshiki Ito (University of Tokyo, Japan) et al., Science Advances, doi: 10.1126/sciadv.abo7019