Brain imaging provides insights into auditory disorder, tinnitus

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If a healthy noise cancellation system is restored within the brain, it may be possible to treat those who suffer from tinnitus, an irritating and sometimes debilitating auditory disorder in which a person "hears" sounds, such as ringing, that don't actually exist, according to a perspective published in the June 24 issue of Neuron.

"Neurons, trying to compensate for loss of an external signal, fire to produce sound that doesn't exist in tinnitus patients, just like neurons send pain signals to someone who has lost a limb," said Josef P. Rauschecker, PhD, from the department of physiology and biophysics at Georgetown University Medical Center in Washington, D.C. "What both people have in common is that they have lost the feedback loops that stop these signals from reaching consciousness."

Tinnitus starts with damage to hair cells in the cochlea of the inner ear and it has been believed that if those hair cells are restored, tinnitus goes away. But the latest research suggests that while tinnitus may initially arise from such peripheral damage, it becomes a problem in the brain's central auditory pathways, which reorganizes itself in response to that damage, Rauschecker said.

Neuroscientists have employed whole-brain imaging approach, using high-resolution MRI and voxel-based morphometry, to visualize various regions of hyperactivity in the auditory pathways of tinnitus patients. The model that Rauschecker and his colleagues propose is that receptors in the auditory region of the brain that no  longer perceive sensory input from damaged hair cells compensate by firing spontaneously and frequently producing the initial tinnitus signals.

"If we can find a way to turn that feedback system back on to eliminate phantom sound, it might be possible one day to take a pill and make tinnitus go away," noted Rauschecker.

It remains unclear, however, why some individuals who have hearing loss do not develop tinnitus. Given that some people with tinnitus seem to be more susceptible to other disorders like chronic pain and depression, it could be that they have an independent, systemic vulnerability in one or more neurotransmitter systems in the limbic region, added Rauschecker.

"That could explain why drugs that modulate neurotransmitters like serotonin appear to help some people out," he said. In addition insomnia may cause tinnitus, and both may be related to serotonin depletion.

Therefore, identification of the transmitter systems involved in the brain's intrinsic noise cancellation system could open avenues for drug treatment of tinnitus, the authors concluded.