Traditional clinical hearing tests often fail to diagnose patients with a common form of inner ear damage that might otherwise be detected by more challenging behavioural tests, a new study has found. This type of "hidden hearing loss" paradoxically presents itself as essentially normal hearing in the clinic, where audiograms - the gold-standard for measuring hearing thresholds - are typically conducted in a quiet room. The reason some forms of hearing loss may go unrecognised in the clinic is that hearing involves a complex partnership between the ear and the brain. It turns out that the central auditory system can compensate for significant damage to the inner ear by turning up its volume control, partially overcoming the deficiency, said Richard Salvi, from the University at Buffalo in the US. "You can have tremendous damage to inner hair cells in the ear that transmit information to the brain and still have a normal audiogram," said Salvi. "But people with this type of damage have difficulty hearing in certain situations, like hearing speech in a noisy room. Their thresholds appear normal.
So they're sent home," he said. To understand why a hearing test is not identifying a hearing problem it is necessary to follow the auditory pathway as sound-evoked neural signals travel from the ear to the brain, researchers said. About 95 per cent of sound input to the brain comes from the ear's inner hair cells. Ear damage reduces the signal that goes the brain. That results in trouble hearing, but that is not what is happening here, because the brain "has a central gain control, like a radio, the listener can turn up the volume control to better hear a distant station," Salvi said. However, this weakened sound-evoked activity is progressively amplified as it travels along the central auditory pathway to the inferior colliculus and onward. By the time it reaches the auditory cortex, things are hyperactive because the brain has recognised a problem. "Once the signal gets high enough to activate a few neurons it's like your brain has a hearing aid that turns up the volume," said Salvi. It is not clear how many people might have this type of hearing loss, but Salvi said it is a common complaint to have difficulty hearing in noisy environments as people get older. The perceptual consequences include apparently normal hearing for tests administered in quiet settings, but adding background noise often results in deficits in detecting and recognising sounds. The study was published in the journal Frontiers in Neuroscience.
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