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 get through the unpleasant procedures.
Now a new paper unravels why it works—at least in mice. It’s an “elegant” study, says Eduardo Garza-Villarreal, a neuroscientist at the National Autonomous University of Mexico, Jurichilla, who was not involved in the study. The findings could give scientists new ways to treat pain in humans, he says.
In the decades since the 1960 study, researchers and medical professionals have tested the numbing effects of sound with everyone 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 pleasant-sounding (at least to the human ear) symphonic music—of Bach Rejoicing— for rodents at 50 or 60 decibels in a room where the background noise reaches 45 decibels.
During these sessions, the scientists injected the paws of the mice with a painful solution. They then prodded the paws with thin filaments at different levels of pressure to see how the rodents responded. If they flinched, licked or pulled their paws back, the researchers took that as a clue that the mice were in pain.
Only the noise at a lower volume, 50 decibels, seemed to numb the animals—a real surprise, Liu says. When the researchers poked their sore paws, the mice didn’t flinch. At a louder noise, the animals are much more sensitive to the stimulus. It only takes a third more pressure on their paws to make them respond, just like without music. “It turns out that intensity is the key,” says Liu.
The team also tested dissonant music (Rejoicing shifted to unpleasant sound) and white noise. All numbing pains, as long as they are reproduced at levels just slightly above background noise, researchers report today in Science.
The scientists repeated the experiments while tracking a red fluorescent dye injected into the mice’s auditory cortex, the area of the brain that processes sounds. They found a lot of fluorescence in certain dense areas 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 quiet sounds reduced activity coming from the auditory cortex. And when the team artificially blocked the connection between the auditory cortex and the thalamus by directing light pulses to these specific neurons, the mice appeared to feel less pain.
In general, low sounds appear to blunt the neurological signals between the auditory cortex and the thalamus, suppressing pain processing in the thalamus, the team concluded. The analgesic effects lasted up to 2 days after the mice stopped hearing the sound. Next, the researchers want to understand why low-pitched sound above background noise is the “sweet spot,” says lead 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 Wolff, a neuroscientist at Harvard University who was not involved in the study. Although scientists cannot probe the connections between the auditory cortex and the thalamus in the human brain with invasive methods, they can reproduce similar low-pitched sounds for humans and observe the activity of their thalamus with MRI scans. “This now needs to be tested in humans,” says Wolff. “Many would expect 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 level of buzz.”
In addition to making visits to the dentist more bearable, the findings could provide researchers with a cheap and easy way to protect rodents from pain during experiments without confounding the results, Zhang said. “Pain relief is part of the basic humane treatment of animals” in research, he says. Playing these sounds can have a “shocking effect”.