People with similar degrees of hearing loss don't all have the same ability to understand and recognise speech. This is a significant challenge for health practitioners when diagnosing and treating hearing loss.
Common forms of hearing loss, such as age-related or noise-induced hearing loss, are caused by a mixture of damage to inner and outer hair cells, the cells in the inner ear which detect and respond to sounds. As inner and outer hair cells contribute to hearing in fundamentally different ways, their differing contributions to hearing loss when they are damaged may underlie the differences we see between people in their ability to understand speech, especially in noise, even when their degree of hearing loss is the same.
Recent studies have suggested that correct processing of slowly changing fluctuations in speech (called the temporal envelope) in the hearing brain may be important for someone to correctly understand speech when listening in background noise – the most difficult conditions for people with hearing loss. Professor Heinz and his team have discovered that a specific measure related to these slow fluctuations in speech, called SNR_ENV (the Signal to Noise ENVelope Power Ratio), may predict overall speech intelligibility in difficult listening conditions, such as background noise, better than many other measures.
The SNR_ENV measure hasn't been investigated in people with hearing loss yet – little is known about how hearing loss affects processing of these slow fluctuations. Michael Heinz and his team are finding out more by using an animal model. They are measuring these effects in the auditory nerve, by measuring the responses of single nerve cells to speech in the presence of different types of background noise (both steady and fluctuating).
They are also looking at how different types of hearing loss (for example, noise-induced hearing loss and ototoxic (‘ear-toxic’) drug-induced hearing loss) affect the SNR_ENV when it is measured during listening to speech in background noise. This will allow them to tease out how inner and outer hair cell damage have different effects on hearing.
The Purdue University team is also trying out a new non-invasive method to measure SNR_ENV, called the frequency following response (FFR). By carrying out both FFR measures and single nerve cell measures in the same animal, the researchers will be able to directly compare the two ways of measuring SNR_ENV.
This work is very important; if the FFR can measure the SNR_ENV, it could be developed further for use in people, to measure their ability to understand speech in background noise.
"This project could lead to improved diagnosis of hearing problems and, ultimately, improved treatment."
Professor Heinz and his team will gain a much better understanding of the different effects of inner and outer hair cell damage on sound perception in the auditory brain. We expect this will lead to the development of better diagnostic tools for measuring someone’s hearing loss and, hopefully, to the design of better hearing aids and cochlear implants.