Why does music numb physical pain? Scientists find clues.

In 1960, a Boston-based dentist named Wallace J. Gardner reported using an unusual technique to control his patients’ pain. Instead of using nitrous oxide or injecting a local anesthetic, Gardner handed his patients a pair of headphones and a small volume control box before proceeding to pluck out their rotten teeth. Gardner claimed that he, like other dentists across the country, has performed 5,000 dental procedures using music and noise to provide pain-relieving effects, and 90% of those procedures did not require supplemental anesthesia.

Gardner hypothesized that the brain’s auditory system acts on the pain system when listening to pleasant music. However, how this happens was unclear. More than 60 years after Gardner’s report, neuroscientists have uncovered two clues to how sound blocks pain: the volume of music and a surprising circuit between the sound- and pain-processing regions of the brain.

Soft music relieves pain in mice

Since Gardner’s report, scientists and physicians have discovered that music and noise have a wide range of pain-relieving properties. For example, they can help relieve acute pain, such as that experienced during surgery and childbirth, and chronic pain from long-term conditions, such as cancer. Although it’s clear that sounds can relieve pain, a team of Chinese and American neuroscientists set out to determine how Sound reduces pain because it could reveal new strategies for pain management. However, this requires the manipulation of neurocircuitry, which is generally frowned upon in humans.

Therefore, the team decided to use mice in their study. While it seems like an obvious solution, studying how music relieves pain in rodents is challenging, especially because we don’t know how animals perceive music. Therefore, the researchers first had to determine whether music would produce any analgesic effects in mice at all.

They played three types of sounds to mice with inflammatory pain: a symphonic piece of music (Bachs rejouissance), an awkward remix of Bach’s symphony, and white noise. The researchers found that all three sounds reduced pain sensitivity, but only when the sounds were played at 50 dB (the volume of a quiet conversation in a library). This finding was unexpected.

To induce pain, the researchers injected the mice’s paws with complete Freund’s adjuvant (CFA), a solution that causes painful inflammation. Then they exposed the mouse to a piece of symphonic music, an unpleasant remix of Bach’s symphony, or white noise, and poked the inflamed paw. When mice were exposed to sounds of 50 decibels (dB), they had to be pushed hard before withdrawing their foot, indicating suppression of pain sensitivity. (Image credit: Zhou et al. Science. 2022)

Dental treatments are noisy. Music played at 50dB was drowned out by the hum of drills, the clatter of metal tools on a metal tray, and the sloppy sucking of the saliva ejector. The researchers conducted their study in a relatively quiet lab (ambient noise was about 45 dB). They suggested that the loudness of the music is less important than the difference between the music and the loudness of the surrounding noise.

So they increased the ambient noise of the room to 57 dB and found that when music was played at 62 dB, sensitivity to pain decreased. They lowered the ambient noise level to 30dB, and only music played at 35dB produced the pain-relieving effect. Sound seemed to relieve pain only when played slightly louder than the ambient noise.

An unusual connection between the auditory and pain regions of the brain

After showing that sounds can relieve pain in mice, the researchers began searching for the elusive painkilling neurocircuitry. By injecting a tracking dye into the mice’s auditory cortex (the brain region that receives and processes information about sound), the research team unveiled a route that connected the auditory cortex to the thalamus, a relay station for processing sensory signals such as sound, taste and pain . All sense organs have a direct connection to the thalamus. However, this connection was unusual.

One would expect that listening to music increases neural communication between the auditory cortex and the thalamus. However, the newly discovered neural connection stopped transmitting information when soft music was played. To confirm that this neurocircuit was involved in pain suppression, the team blocked its activation. As a result, the mice seemed to feel less pain even without music. The researchers concluded that low-volume sounds blunted direct communication between the auditory cortex and thalamus and suppressed pain processing in the thalamus.

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The researchers acknowledged that neural mechanisms underlying music-induced analgesia in humans are undoubtedly more complicated than those in mice. However, identifying the connections between the auditory cortex and pain-processing regions could accelerate the study of music-induced analgesia. In the future, these findings could drive the development of alternative interventions to treat pain.

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