Study uncovers mechanism behind hearing condition

Research continues into the causes of tinnitus. The latest research is not focused on the ear, but in the brain where it’s now thought the condition really originates.

A study by a team of UC Berkeley neuroscientists has uncovered new cerebral mechanisms behind tinnitus, a currently incurable condition that produces a constant ringing or buzzing sound in the ear in the absence of other noise.

The study, published Sept. 6 in the journal Proceedings of the National Academy of Sciences, provides an alternative hypothesis to the prevailing view that tinnitus is associated with neurons in the region of the cortex that is affected by hearing loss. The research, which was conducted by the UC Berkeley Helen Wills Neuroscience Institute, shows that the higher-frequency neurons in the area of the cortex affected by hearing loss are responsible for the high-pitched noise characteristic of tinnitus.

According to Shaowen Bao, co-author of the study and adjunct assistant professor of neuroscience at UC Berkeley, this study is the first to attribute tinnitus to the sensory-deprived region — which is affected by hearing loss — of the cortex.

“Researchers have come up with the prevailing theory based on numerous studies and findings,” he said. “However, these findings are somewhat inconsistent. We hope we can get a more coherent idea of what’s happening.”

According to the American Tinnitus Association, tinnitus affects more than 50 million Americans with varying degrees of severity ranging from barely noticeable to debilitating. Although scientists have yet to identify a cure for the condition, popular treatment options include masking the tinnitus noise with music, which provides temporary relief, and training the neurons in the auditory cortex to enhance their response to lost frequencies, which helps to gradually reconnect the ear to the sensory-deprived neurons.

Bao and his team treated hearing-impaired rats with two drugs in order to increase their brain’s production of the GABA neurotransmitter, which inhibits the spontaneous firing of the sensory-deprived neurons — the cause of the ringing associated with tinnitus. But although these drugs were effective in treating the symptoms of tinnitus in rats, they cannot be used on humans because of the long-term side effects associated with them.

Although this is his first study on tinnitus, Bao said he hopes to build off this research to provide further insight into the field.

“We are very interested in using these molecular and cellular tools to pinpoint the mechanisms behind tinnitus and hearing loss more precisely,” he said.

Michael DeWeese, UC Berkeley assistant professor of physics and member of the institute, who researches the inability to focus attention on important sounds in the presence of distractors, said that researchers are focusing on this condition.

“Tinnitus is actually a very serious problem, and I think that many people are very surprised to hear that,” he said. “In the very worst cases, sufferers are suicidal.”

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