"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," Rauschecker says. "What both people have in common is that they have lost the feedback loops that stop these signals from reaching consciousness."
Tinnitus can be caused by damage to hair cells from a loud noise or from neurotoxicity from medications, he says, but more often than not, it is associated with hearing loss in some frequencies that commonly occurs as people grow older. And given that the world is becoming noisier and the population is aging in the U.S., incidence of tinnitus, which is already the most common auditory disorder in humans, is expected to increase even more, researchers say.
Adding to that increase are the rapidly mounting cases of tinnitus in soldiers due to loud explosions, Rauschecker says. "According to the Veterans Administration, tinnitus and post-traumatic stress disorder are the leading medical complaints," he says.
"This circuit serves as an active noise-cancelation mechanism -- a feedback loop that subtracts sounds that should not be there," says Rauschecker. "But in cases where the limbic regions become dysfunctional, this noise-cancelation breaks down and the tinnitus signal permeates to the auditory cortex, where it enters consciousness."
Researchers have also found evidence that this inhibiting gating mechanism can be switched on and off, which explains why some tinnitus patients have a ringing sensation intermittently
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