This is a Discovery Research Grant awarded to Dr Torsten Marquardt at University College London in 2020.
The human inner ear has an astonishing sensitivity to sound. It’s able to hear sounds from a pin dropping to a jet engine roar, and across a wide range of pitches.
This sensitivity is due, in part, to a group of specialised sound-sensing cells in the inner ear which can change their shape in response to sound, becoming longer (expanding) or shorter (contracting) as a result. Their movement enhances quiet sound signals, acting like an amplifier to make the signal stronger for our hearing brain.
These cells, the outer hair cells, must work properly for us to hear well, but they are very fragile. The inner ear has a number of ways in which it protects these cells and keeps them in good working order, primarily by providing a stable environment for them. But this environment can be disrupted; for example, by listening to loud sounds for too long or as a side effect of some life-saving drugs. This environment also becomes less stable as we age. Such disruptions can cause both temporary and permanent hearing loss.
We can detect the activity of outer hair cells through a simple hearing test. When the inner ear detects a sound, the movement and activity of the outer hair cells generates an echo, known as an ‘otoacoustic emission’, in response. This can be detected by a microphone positioned in the ear canal. Otoacoustic emissions form the basis of the newborn hearing screen and are used to measure whether the outer hair cells are working correctly. If they are not (if they don’t generate an ‘echo’), the baby most likely has hearing loss and can be referred to audiology for further testing and support. Otoacoustic emissions are a common tool in hearing research and the clinic to assess the health of the inner ear.
A specialised kind of hearing test is used to capture these echoes. If 2 simultaneous tones (at different pitches) are played to a listener, the echoes produced by the outer hair cells in response to those sounds are called ‘distortion products’. It’s these distortion products that give an indication of how well the outer hair cells are working.
The researchers will investigate a new way of combining these distortion products as the first step in developing a new and more sensitive test of how damage to outer hair cells affects hearing. It’ll extend the use of otoacoustic emissions in the clinic, allowing researchers and audiologists to diagnose hearing loss more accurately.
An important part of the project will be to relate the new test to actual changes in shape of the outer hair cells in the inner ear. To do this, the researchers will use a new method of measuring slow changes in the length of the outer hair cells using an animal model. These slow changes are thought to underlie how the inner ear maintains its exquisite sensitivity to sound, by ensuring that the inner ear is always in the optimal state to detect the widest range of sounds.
The length changes, and the time over which they change, will be compared with simultaneously recorded otoacoustic emissions. If they match, this will show that the new diagnostic method truly reflects the ability of the inner ear to maintain its sensitivity to sound, even when the environment around it changes.
The measurements of changing outer hair cell length will help us to better understand how the inner ear can control and maintain its own sensitivity, and how problems with this process may underlie hearing loss.
The new otoacoustic emission technique will also provide a tool for researchers studying hearing and hearing loss, but may also form the basis of the first clinical test that can non-invasively detect problems with the inner ear’s ability to control its sensitivity. This’ll allow audiologists to more specifically diagnose the cause of a person’s hearing loss (or hearing problems) and choose the best treatment.