In 1960, a group of dentists published a curious study: when they played music for their patients during operations, the people experienced less pain. Some didn’t even need nitrous oxide or local anesthesia to go through uncomfortable procedures.
Now a new paper unravels why this works – at least in mice. It’s an “elegant” study, says Eduardo Garza-Villarreal, a neurobiologist at the National Autonomous University of Mexico, Juriquilla, who was not involved in the research. The findings could give scientists new ways to treat pain in humans, he says.
In the decades since the 1960 study, researchers and medical providers have tested the numbing effect of sound with everything from Mozart to Michael Bolton. Both seem to work: in one study, fibromyalgia patients had less pain when they listened to their favorite music, including Mozart and Bolton.
To better understand why music helps with pain, Yuanyuan Liu, a neurobiologist at the US National Institute of Dental and Craniofacial Research, and his colleagues turned to mice. For 20 minutes a day, they played symphonic music that sounded pleasant (at least to human ears).–of Bach Having fun– for rodents at 50 or 60 decibels in a room where the background noise was 45 decibels.
During these sessions, the scientists injected the rats’ paws with a painful solution. Then, they prodded the paws with thin filaments at different levels of pressure to see how the rodents reacted. If they twitched, licked or pulled their paws back, the researchers took this as a clue that the mice were feeling pain.
Only noise at the lowest volume, 50 decibels, seemed to numb the animals—a real surprise, Liu says. When the researchers tapped their inflamed paws, the mice did not flinch. With louder noise, the animals were much more sensitive to the stimulus. It only took a third of the pressure on their paws to make them respond, just like without the music. “It turns out that intensity is the key,” says Liu.
The team also tested dissonant music (Having fun pitch shifted to unpleasant sound) and white noise. All dulled pain, as long as it was played at levels just above background noise, researchers report today in science.
The scientists repeated the experiments while tracking a fluorescent red dye injected into the mice’s auditory cortex, the region of the brain that processes sound. They found a lot of fluorescence in several dense regions of the thalamus, the center of sensory processing, suggesting that connections between this region and the auditory cortex are involved in pain suppression. Tiny electrodes implanted in the animals’ brains further revealed that relatively soft sounds decreased activity emanating from the auditory cortex. And when the team artificially blocked the connection between the auditory cortex and the thalamus by targeting light pulses to these specific neurons, the mice appeared to feel less pain.
Overall, low sounds appear to dampen neurological signals between the auditory cortex and thalamus, reducing pain processing in the thalamus, the team concluded. The analgesic effects lasted up to 2 days after the mice stopped hearing the sound. The researchers then want to understand why a low-pitched sound above background noise is the “sweet spot,” says author Zhi Zhang, a neurobiologist at the University of Science and Technology of China.
The ultimate goal, however, is to manage pain in humans—and there are many differences between mice and humans, notes Clifford Woolf, a neurobiologist at Harvard University who was not involved in the study. Although scientists cannot probe auditory cortex-thalamus connections in the human brain with invasive methods, they can play similar low-pitched sounds to humans and monitor their thalamus activity with MRI scans. “This now needs to be tested in humans,” says Woolf. “Many would have predicted that you have to listen to Mozart to get pain relief,” he says. “But maybe all we need to do is give patients a little noise level.”
Beyond making visits to the dentist more bearable, the findings could offer researchers a cheap and easy way to protect rodents from pain during experiments without confounding the results, Zhang says. “Pain relief is part of basic animal well-being” in research, he says. Playing these sounds can have “a wonderful effect”.