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.
Such complexity may explain why so many different tinnitus treatments work, but only modestly: Each attacks just one part of the tinnitus network. Christo Pantev of the University of Münster in Germany and his colleagues, for example, have brought some relief to people with tinnitus by rewiring their tone map. To do so, they edited recordings of music, filtering out the frequencies of the ringing in the ears of their patients, who then listened to the filtered music an average of 12 hours per week. Pantev and his collaborators found that their patients’ tinnitus significantly eased. They also found that the neurons tuned to the tinnitus frequency in the auditory cortex became less active.
The scientists cannot say for sure how the filtered music soothed their patients, but they speculate that the incoming signals encouraged the tone map to change its structure. The overactive, eavesdropping neurons became stifled by their neighbors.
Clearly the auditory cortex is just an early stop on the journey that sound takes from the outside world to our awareness. Some neurons in the auditory cortex extend branches down to the brain stem, where they link to a pair of regions called the caudate nucleus and putamen. Those regions may be important for processing the signals in several ways, such as categorizing sounds. In 2004 Louis Lowry, an ear-nose-and-throat doctor at Thomas Jefferson University in Philadelphia, discovered that the caudate and the putamen play an important role in tinnitus by studying an unusual patient—himself.
As a young man, Lowry spent a summer working on a farm with a noisy tractor. The experience left him with partial hearing loss and a high-pitched ringing in his ears that plagued him for 40 years. Then at age 63, Lowry suffered a mild stroke. A CT scan and an MRI revealed that the stroke had damaged his caudate and putamen. But the stroke also brought a pleasant surprise. Lowry was completely cured of his tinnitus, without any further hearing loss.
Steven Cheung and Paul Larson, two doctors at the University of California, San Francisco, set out to reproduce Lowry’s experience. They took advantage of the fact that some people with Parkinson’s disease get electrodes surgically implanted in their brain stem to control their symptoms. The electrodes typically have to pass the caudate and putamen to reach their target. Cheung and Larson engaged five patients preparing to receive an implant who also suffered from tinnitus. The patients agreed to undergo several minutes of deep brain stimulation to these regions during surgery as the electrode was being implanted. Cheung and Larson reported that the tinnitus became much fainter in four of the five patients.
Once signals travel from the ear to the auditory cortex, caudate, and putamen, they eventually make their way to regions of the brain that carry out more sophisticated sound information processing: connecting the sounds with memories, interpreting their meaning, giving them emotional significance. It is precisely these regions that Schlee and his colleagues noted were behaving strangely in people with tinnitus. He argues that it is only when signals reach this large-scale network that we become conscious of sounds, and it is only at this stage that tinnitus starts to cause people real torment. Schlee’s results suggest that the higher regions of the brain send their own feedback to the auditory cortex, amplifying its false signals. Schlee’s model of tinnitus and consciousness could explain some curious observations. Even in bad cases of tinnitus, people can become unaware of the phantom sound if they are distracted. It may be that distractions deprive the errant signals from the auditory cortex of the attention they need to cause real distress. What’s more, some of the most effective treatments for tinnitus appear to work by altering the behavior of the front of the brain. Counseling, for example, can make people better aware of the sounds they experience by explaining the brain process that may underlie the disorder, so they can consciously reduce their distress.
Solving the mystery of tinnitus will probably get even more urgent in years to come. Traffic, iPods, and other features of modern life may cause more hearing damage, hence more tinnitus. But if a real cure ever comes, it will probably not be a single silver bullet. It will instead attack the tinnitus network from top-down and bottom-up. For now, though, you should probably skip the hot bread on the ears.