It has long been thought that after someone is exposed to loud noise, such as at a rock concert, any hearing loss they experience is temporary, and after a couple of days, their hearing will return to normal. However, more recently, studies have suggested that while their hearing ‘thresholds’ (the quietest sound they can hear against a quiet background) may return to ‘normal’, there may nonetheless be permanent damage in the inner ear. This damage occurs to structures called ‘synapses’, connections between hair cells and the auditory nerve cells which carry sound information to the brain. This damage may affect how well a person can hear when there’s a lot of background noise, but as it isn’t picked up by standard hearing tests, it is known as "hidden hearing loss".
It is likely that problems understanding, for example, speech against a noisy background are caused by damage to one type of nerve cell in the cochlea. These nerve cells are involved in signalling to the brain about louder sounds – they are particularly important for picking out speech sounds against background noise. These 'high-level' responders are more vulnerable to loud noise damage than the nerve cells that carry signals about softer sounds. This explains why someone can still hear quiet sounds after they’ve been exposed to loud noise, as those nerve cells haven’t been damaged.
The high-level cochlear nerve cells connect with a specific structure in the brain called the small cell cap. This is part of the cochlear nucleus, the first part of the hearing brain that sound information reaches after it leaves the ear. The cells in the small cell cap are especially good at processing sounds across a large range of intensities (from quiet to loud). They can also distinguish between different frequencies (pitch) in these sounds, and this ability is essential for understanding speech in noisy environments. The small cell cap takes up a large proportion of the cochlear nucleus in people and could therefore play a major role in the brain’s contribution to hidden hearing loss.
The main aim of this project is to understand how these special cells detect the signals that are important for understanding speech in both normal-hearing animals and those that have been exposed to noise loud enough to cause hidden hearing loss. They will also investigate whether a chemical called neurotrophin-3, which can restore damaged synapses in the cochlea, is able to repair these cells after they are damaged by loud noise.
This study will provide a detailed understanding of the neural (brain) processes that may underlie hidden hearing loss. If the small cell cap is shown to be crucial for receiving information from the high-level nerve cells that are needed to understand speech against a noisy background, this will be important in helping us to understand the causes of and processes involved in hidden hearing loss. If neurotrophin-3 can restore the connections between the inner ear and the small cell cap, and repair the damaged nerve cells, this could lead to the development of the first specific treatment for hidden hearing loss, which could help millions of people around the world.