We hope Professor Rosen's work leads to new diagnostic tests that can detect ‘hidden hearing loss’ – and better hearing aid design.
Although speech is the primary way in which we communicate, many of us have trouble understanding it – especially when there's lots of background noise.
Because understanding speech involves a number of complex processes, in the ear and the brain, it's not surprising so many problems arise! One very common cause is hearing loss, which usually means something's wrong in the inner ear (or cochlea). Audiologists diagnose hearing loss by measuring audiometric thresholds – the quietest sound that a person can detect. But many people who have difficulty understanding speech in background noise have normal hearing thresholds, so their problems must lie elsewhere. We call this ‘hidden hearing loss’.
One possibility is that they've experienced changes in the brain related to their cognitive functions, such as memory and attention, important for understanding speech in noise.
And there may be damage specific to the 'hearing brain'. Information about sounds reaches the brain by passing from the cochlea through a network of nerves. The first of these, the auditory nerve, is one of the most important because it carries all the information about sound from the cochlea – as sound travels further up into the brain, information about certain parts or characteristics of it get separated out and carried along different nerves.
Evidence from animals suggests that both ageing, and exposure to noise, can cause deterioration of the auditory, or cochlear, nerve (called cochlear neuropathy, neuropathy literally meaning ‘nerve damage’), without actually changing hearing thresholds. As a result, there may be enough information for the brain to tell that a sound is present, but the finer details of that sound (for example, its pitch and quality) are degraded.
So far, most of the research looking at cochlear neuropathy has been carried out in animals. In this project, Professor Rosen and his team will assess how relevant these animal studies are to humans, by developing tests that reveal the effects of cochlear neuropathy in people.
They will do this by comparing young, normal-hearing listeners to two groups expected to have this neuropathy and also selected to have normal hearing thresholds: people over 55; and young people who have a history of noise exposure, through, for example, frequent clubbing.
Because most complaints about hearing problems focus on not being able to understand speech in noise, many of the tests will address this. Stuart and the team at UCL will also test how well people can detect small changes in simple sounds, the kind expected to be most affected by cochlear neuropathy. Finally, they will measure electrical activity in the brain, to look for evidence of neuropathy.
By the time the project ends in summer 2019, we're confident that we'll have gained a far better understanding of the difficulties people have in understanding speech in the presence of background noise.
Ideally, Professor Rosen and his team will have been able to develop a set of diagnostic tests to assess the extent of cochlear neuropathy. This would be a real boon for people with this form of hearing damage – and for those involved in developing and providing hearing aids. Their research evidence might also convince younger people to protect themselves against damaging their hearing.