This is a three-year project, led by Dr Roland Schaette, at the UCL Ear Institute. It started in March 2016 and will end in February 2019.
Hearing loss affects 11 million people in the UK, and 360 million people worldwide.
To assess someone’s hearing, audiologists measure their 'hearing thresholds' (the quietest sound they can hear in the absence of background noise). If they are higher than what's considered normal, the audiologist will diagnose hearing loss.
But there are many people who, although they are found to have normal hearing thresholds, nonetheless have considerable difficulty in understanding speech when there is a lot of background noise. This is especially true for older people or those who've been exposed to lots of loud noise (for example, factory workers). Until very recently, we've been baffled as to what causes this.
Research is beginning to show that normal hearing thresholds do not necessarily indicate the absence of damage to the inner ear or wider auditory system. There is now evidence to suggest that there could be a considerable degree of ‘hidden hearing loss’ that is not picked up by standard tests, but which substantially affects how well someone can hear.
Studies in mice have shown that a significant proportion of auditory nerve cells may lose their connections to the sensory hair cells in the cochlea. This can result from exposure to noise or because of ageing; and if these connections are lost, the nerve cells stop sending information to the brain. Hearing thresholds may remain normal (as some connections are still present and can pass on some information), but the loss of information from the damaged nerve cells makes it harder for the brain to ignore background noise.
Dr Schaette and his group have recently shown that mice with this hidden hearing loss are much less able to ignore a noisy background and detect a specific signal. Nerve cells in the auditory part of their brain are also spontaneously active, which is thought to be a sign of tinnitus. But these measurements were taken four weeks after the mice were exposed to noise, so they can give us only a brief snapshot of the potential long-term effects of this kind of hearing damage.
In this project, Roland and his team are investigating the long-term effects of noise-induced hidden hearing loss, and comparing them to the normal decline of hearing function that happens as we age. They are studying what happens when the ageing brain has to deal with a poor-quality signal from a noise-damaged cochlea, a situation which is, potentially, further aggravated through age-related hearing damage. To do this, they are studying mice that were exposed to noise when young and then allowed to age, and comparing them to mice of the same age, that were not exposed to noise.
They are measuring the mice’s hearing ability by recording the activity of nerves in the auditory brain at time points up to two years after the initial noise exposure, in response to different auditory situations. By comparing across the groups of mice, they hope to be able to disentangle the effects of noise-induced and age-related cochlear damage on information processing in the auditory brain.
‘This work will lead to a better understanding of the differences in how noise and ageing affect our hearing, important for developing better treatments.’
Dr Schaette's team's research could be a major step towards understanding the effects of noise- and age-related hearing loss on 'functional hearing' – the method of information processing in the brain that allows us to listen in complex, noisy environments. We also hope it will provide further insights into the underlying causes of tinnitus.