Research on tinnitus has shown that it’s rooted in the very way we process and understand sound.
In some of the world’s oldest medical texts — papyrus scrolls from ancient Egypt, clay tablets from Assyria — people complain about noise in their ears. Some of them call it a buzzing. Others describe it as whispering or even singing. Today we call such conditions tinnitus. In the distant past, doctors offered all sorts of strange cures for it. The Assyrians poured rose extract into the ear through a bronze tube. The Roman writer Pliny the Elder suggested that earthworms boiled in goose grease be put in the ear. Medieval Welsh physicians in the town of Myddfai recommended that their patients take a freshly baked loaf of bread out of the oven, cut it in two, “and apply to both ears as hot as can be borne, bind and thus produce perspiration, and by the help of god you will be cured.”
Early physicians based these prescriptions on what they believed tinnitus to be. Some were convinced it was caused by wind that got trapped inside the ear and swirled around endlessly, so they tried to liberate the wind by drilling a hole into the bones around the ear or using a silver tube to suck air out of the ear canal. The treatments didn’t work, but they did have an internal logic.
Today tinnitus continues to resist medicine’s best efforts, despite being one of the more common medical disorders. Surveys show that between 5 and 15 percent of people say they have heard some kind of phantom noise for six months or more; some 1 to 3 percent say tinnitus lowers their quality of life. Tinnitus can force people to withdraw from their social life, make them depressed, and give them insomnia.
Some modern doctors prescribe drugs like lidocaine. Others offer patients cognitive therapy. Some have people listen to certain sounds, others apply magnetic pulses to the brain and even implant electrodes in the brain stem. Although many treatments have shown some promise, none is consistently effective. Recent research suggests why: Tinnitus is a lot more complicated than just a ringing in the ears. It is more like a ringing across the brain.
Normally, we hear sounds only when they make our eardrums vibrate. The vibrations cause nerve hairs in the inner ear to shiver, and that triggers electric signals that travel along the auditory nerve into the brain. One of their first stops is a patch of gray matter called the auditory cortex. Each nerve hair is tuned to a particular frequency of sound and excites only certain neurons in the auditory cortex. As a result, the neurons in the auditory cortex form what is known as a tone map. The neurons at one end of the auditory cortex are tuned to low frequencies; the farther you go toward the other end, the higher the tuning of the neurons.
This sound system comes with an elaborate feedback mechanism. Neurons do more than just relay signals forward into the brain. They also signal back down the line, reaching out to neighboring neurons tuned to nearby frequencies, exciting some and muzzling others. These feedback controls allow us to sift through incoming sounds for the most important information, so that we are not overwhelmed by meaningless noise. In young brains, the neurons and their feedback controls grow and link up to each other. Even in adulthood, experiencing new sounds can rewire the auditory cortex. If a rat is trained to recognize sounds at a particular frequency, the corresponding region of the tone map will get bigger.
Tinnitus arises when this flexibility goes bad. Things may start to go awry when toxic drugs, loud noises, or even whiplash cause damage to the nerve hairs in the ears. The injured nerve hairs can no longer send signals from the ear to the tone map. Bereft of incoming signals, the neurons undergo a peculiar transformation: They start to eavesdrop on their neighbors, firing in response to other frequencies. They even start to fire sometimes without any incoming signals. As the brain’s feedback controls get rewired, the neurons end up in a self-sustaining loop, producing a constant ringing. That is why tinnitus often doesn’t go away when people get their auditory nerve surgically cut.
It’s not just the auditory cortex that is affected when people get tinnitus. Neuroscientists, using increasingly sophisticated brain scans, are finding that changes ripple out across the entire brain. Winfried Schlee of the University of Konstanz in Germany and his colleagues have been making some of the most detailed studies of tinnitus ever, using a method called magnetoencephalography (MEG, for short). They take advantage of the fact that every time neurons send each other signals, their electric current creates a tiny magnetic field. MEG allows scientists to detect such changing patterns of activity in the brain 100 times per second.
Schlee and his colleagues find widespread differences in the brains of people with tinnitus and those without it. A network of regions in the brains of people with tinnitus tend to fire their neurons in sync. Schlee has determined that his tinnitus-stricken subjects have a more synchronized pattern of signals coming out of regions in the front and the back of the brain. (For brain anatomy junkies, they are the dorsolateral prefrontal cortex, orbitofrontal cortex, and anterior cingulate cortex in the front; in the back, they are the precuneus and posterior cingulate cortex.) Schlee and his colleagues also discovered a more strongly synchronized flow of signals coming into the temporal cortex — a region that includes the auditory cortex — in people with tinnitus.
When Schlee compared people who suffer a lot of distress from tinnitus with those who are not much bothered by it, he found that the more distress people felt, the stronger the flow of signals out of the front and back of the brain and into the temporal cortex. This pattern suggests that the network Schlee discovered is important for the full experience of tinnitus. Tinnitus, in other words, extends beyond the ear, beyond a hearing-specialized part of the brain, beyond even any single piece of neural real estate. It is a disease of networks that span the brain.