The unique pleasure that humans experience from music is a result of the way the brain's auditory and reward circuits communicate, according to a new study that proved this for the first time by stimulating regions of the brain with magnetic currents while people listened and reacted to music.
In a paper published March 29 in the Journal of Neuroscience, a group of researchers used the noninvasive transcranial magnetic stimulation technique to excite and inhibit the striatum, located in the brain's reward circuit, before running a behavioral music-listening task. This is the second paper from a larger project at McGill University in Montreal, which succeeded in proving that the interaction between the reward system and the auditory system of the brain is what explains our pleasure responses to music.
Structures in the reward circuit such as the striatum are most well known for being associated with primary rewards such as food. Brain imaging in prior studies has also demonstrated that the striatum is active and releases dopamine when people listen to music that they like, compared to music that they do not like.
But even though certain regions of the reward circuit have been shown to be active during music-related pleasure, this correlation cannot definitively prove that they are responsible for the pleasure, Robert Zatorre, a professor at McGill University and co-author of the paper, explained to The Academic Times. Without causal evidence, it could be the case that some other brain system or function, which was not being measured, is actually responsible for the emotion response.
"We needed a technique that would allow us to draw the conclusion that the striatum really is causally related to the pleasure response, it's not just a correlation," Zatorre said. "And so that's [why] we use brain stimulation techniques."
Transcranial magnetic stimulation uses magnetic fields to stimulate nerve cells in the brain by positioning a large coil on top of a person's head. A current is passed through the coil, which generates a magnetic field that goes through the skull to reach targeted neurons. Presenting the current in intermittent bursts of stimulation activates, or excites, the neurons, while a continuous current of stimulation inhibits them.
For the current study, a sample of 18 healthy adults received three different conditions of stimulation on their brain's striatum on three separate days — excitation, inhibition and sham, which involved no stimulation and served as a baseline. Immediately following this, they listened to pop songs and reported how much they enjoyed them while having their brain activity measured with functional magnetic resonance imaging. The participants did not know which condition they received on which day.
The researchers hypothesized that exciting the striatum would cause people to feel more pleasure in response to the music, and that inhibiting it would give them less pleasure, proving that the brain's reward system and auditory regions are responsible for music-driven emotion. And indeed, this is what the experiment demonstrated.
In each session, participants listened to five of their favorite songs that they pre-selected and 10 songs chosen by the experimenters. They were instructed to indicate their degree of pleasure while listening to the music by pressing buttons on a response pad that indicated they were feeling neutral, low pleasure, high pleasure or chills.
The researchers found that with excitatory stimulation, participants reported feeling no pleasure in 22.18% of the trials, low pleasure in 37.3%, high pleasure in 33.14%, and chills in 7.38%. With inhibitory stimulation, they reported no pleasure in 25.73% of the trials, low pleasure in 36.77%, high pleasure in 32.62% and chills in 4.87%.
And in the sham condition, the participants felt no pleasure in 21.96% of the trials, low pleasure in 39.10 %, high pleasure in 33.79%, and chills in 5.17%. Compared to the sham condition, excitatory stimulation overall led to a positive increase in self-reports of pleasure, while inhibitory stimulation decreased how much participants liked both their selected music and the music selected by the experimenters.
The researchers also gave the participants the opportunity to "purchase" music they enjoyed to study their motivation to increase their own pleasure. The listeners showed that they were willing to spend more money following excitatory stimulation than inhibitory, relative to the sham condition.
From the captured brain imaging during the music-listening task, the researchers confirmed that when the stimulation was exciting, activity in the reward system was enhanced, and when it was inhibiting, the activity went down.
"The degree to which it goes up or down is directly related to the degree to which people report how much they like music," Zatorre said. "So this is the key missing link. The whole point was to look for this causal evidence that the striatum, in particular, was really responsible for this pleasure that we get from music, and that's what we can show."
The degree to which the stimulation enhanced or did not enhance how much people enjoyed the music was a function of the connectivity between the striatum and the auditory regions, the researchers said, showing that stimulation enhances the way that these two regions interact. And the more they interact, the more people will experience pleasure.
This was the second time the team ran the behavioral music-listening task with transcranial stimulation. The two experiments used different samples of adults and different music stimuli, but achieved the same level of results. It was significant that the researchers were able to replicate their original findings and build on them, Zatorre said, proving that the behavioral responses were impacted by the stimulation in both directions.
"The reason this paper is very important is because we show what happens 'under the hood' when you stimulate with this technique. It influences the way that the reward system interacts with the auditory system, " Zatorre said. "And it's that interconnection and interaction that is what drives musical pleasure. The more those two systems are interacting, the greater the pleasure that you experience."
The study, "Unraveling the Temporal Dynamics of Reward Signals in Music-Induced Pleasure with TMS," published March 29 in the Journal of Neuroscience, was authored by Ernest Mas-Herrero, Alain Dagher, Marcel Farrés-Franch and Robert J. Zatorre, McGill University.