Both mild and severe inner ear damage can lead to tinnitus

Results from a new study of tinnitus could go some way to explaining why a wide range of potential causes can all end up producing the perception of sound when in fact there is none.

The study, which is published in The Journal of Neuroscience Research, found that different patterns of damage to the hair cells of the inner ear all lead to abnormal activation in one small population of cells, further along the auditory pathway.

Chinchillas were first trained to press a lever in order to gain a food reward, but only in the presence of sound. The louder the test sound, the more lever presses were recorded, with the fewest presses recorded when there was silence. Then the chinchillas were categorised by four different types of hair cells loss: a control group that had normal hearing and no loss, a group that had mild loss of both inner and outer hair cells, a third group that had heavy loss of outer hair cells and mild loss of inner hair cells, and a final group that had heavy loss of inner hair cells but the outer hair cells still intact.

Despite the different pattern of hair cell loss in each of the experimental groups of chinchillas, all three groups continued to lever-press when there was no test sound. In other words, they behaved as though they could hear something, suggesting that all groups experienced tinnitus. The control group, however, treated silence as silence.

Professor Thomas J Brozoski and colleagues at the Southern Illinois University School of Medicine found there was a common pattern of brain cell activity that was particularly evident in the chinchillas with tinnitus. They traced the activity to a small set of neurons in a part of the hearing pathway called the inferior colliculus, which is an area that combines many different features of a sound signal before passing it on to the brain.

“This is very interesting because it shows that even minor damage to the cochlea can result in the perception of tinnitus. Currently audiograms test a range of sound frequencies that are widely spaced and tests usually stop at 8 kHz. The results could explain why many people who suffer from tinnitus appear not to have a hearing loss. They might have some damage, for example to the higher frequencies, that hasn't been measured,” said Professor David McAlpine, Director of the UCL Ear Institute and Professor of Auditory Neuroscience.