What happens in the brain when we lose our hearing?
To a large extent, this depends on the age at which hearing is lost and the severity of the loss, including whether one or both ears are affected. If the sensory hair cells in the ear die, they don’t send any signals to the auditory nerve any more. As a result, many of the nerve fibres that send signals from the ear to the brain degenerate. If this form of profound hearing loss happens during a child’s early years of development, it results in a loss and shrinkage of the nerve cells in the cochlear nucleus inside the brainstem. This means it can only send weaker signals to the brain cortex. Even if hearing is subsequently restored, the physiologically changed nerve cells may no longer respond normally to sound, especially to speech.
Profound hearing loss
This form of hearing loss is the most significant degree of hearing loss. People who are affected by profound hearing loss are not able to hear sounds softer than 91-120 dB or more. This means, they may not even be able to hear very loud sounds such as airplane enginges, fire alarms, roadworks or traffic on the street.
What happens to the hearing areas of the brain?
They will slowly be used for other purposes. Most levels of the auditory pathway normally receive weak inputs from other senses such as vision or touch. Following profound hearing loss, particularly in early life, the auditory cortex will increasingly receive inputs from other non-auditory areas. This is thought to be why other senses such as vision or touch are sometimes more highly developed in deaf people – it’s a sort of compensation for the loss of hearing. However, this reorganisation of the auditory brain centres may also limit the effectiveness of strategies for restoring hearing, for instance, with cochlear implants.
So is restoring hearing a race against time?
The evidence clearly shows that the time between the onset of hearing loss and implantation should be as short as possible. Having said this, there is growing evidence that appropriate behavioural training can improve CI outcome much later in life, even if deafness occurs at a very early age.