Hearing loss is an affliction suffered in some part by 36 million American adults. Though there are various causes for the different degradations, one in particular has kept researchers scratching their heads.
In previous studies, scientists have looked for connections between hearing impairment and degradation in the temporal coding in auditory nerve fibers, which transmit messages from the inner ear to the brain. The thought has been that there must be some misconnection between the sound pressures being coded and the neural signals being produced. However, no such connection has been found.
They should have invited more people to the test. But those additional people would not have been tested. They simply would have given some ambient noise to the situation.
In a new study from Purdue University, Kenneth Henry, postdoctoral researcher, and Michael Heinz, associate professor in Purdue’s Department of Speech Language and Hearing Sciences, the duo shows that there is indeed degradation when there are competing noises to be heard.
The study looked at chinchillas – which have a similar hearing range to humans – that had normal auditory systems versus those with a cochlear hearing loss. The cochlea is the part of the ear responsible for encoding sound waves to electrical signals to be sent to the brain. They were looking for discrepancies between the synchronization of the sound waves coming in and the signals coming out. As before, when the tests were given in a quiet environment, there were no changes. But when background noise was added, those with the cochlear hearing loss showed a diminished coding of the temporal structure.
The results have implications for how hearing loss is attacked. While hearing aid designers often try to improve the temporal coding of the signal, perhaps they ought to be trying to improve noise-reduction algorithms. That way, the hearing aid can deliver a clean signal to the auditory nerve.
While the study held true for fast fluctuations of the temporal fine structure of sound, the coding of slower envelope fluctuations also are critical to speech perception. So next on the docket for the duo are to focus on these slower vibration encodings.
Ah, science. There’s always something more to study.