How does our brain compensate for hearing loss?

sensorineural hearing loss is caused by abnormalities in the hair cells

sensorineural hearing loss is caused by abnormalities in the hair cells

The brain can do some amazing things and recent research into the effects of intermittent hearing loss has highlighted its impressive powers of adaptability, as well as the mechanisms it uses to locate sound.

Tracey Pollard from our Biomedical Research team tells us more about the research, carried out at Oxford University, and how it could lead to new approaches to treating persistent glue ear, whilst also having implications for hearing aid design.

Plasticity

The neurons, or nerve cells, in the brain are remarkable – they can change what they do depending on the information they receive from the outside world via the senses. This ability is known as plasticity – it means that the function of a neuron is not necessarily hardwired into it but can change depending on circumstances.

An example of this plasticity can be seen in deaf individuals, where the neurons that would normally be involved in processing signals and sounds from the ears are ‘diverted’ to the visual system. Here, they become involved in processing information from the eyes – so deaf people really do see more! This plasticity doesn’t just occur between sensory systems though; it can happen within one system. For example, if sound coming into the ears is distorted or degraded, the auditory nerves can adapt to make the best of the information they do have. This might happen when there is a lot of echo, or because sound is missing to some degree, as in partial deafness or deafness in one ear.

This adaptation isn’t always helpful, particularly if it’s persistent, as it may become permanent. In adults who receive a cochlear implant after being deaf for a long time, the neurons that have been diverted to other systems may no longer be available for the implant to stimulate which can make the implant less effective. Also, children who suffer from prolonged deafness in one ear during childhood can develop a ‘lazy ear’ when their hearing is subsequently restored (e.g. through surgery) – the brain is now too used to only receiving useful information from the unaffected ear. As a result, even though the brain is now getting information from both ears, it isn’t taking as much notice of the second ear as it should. This can cause problems later on, such as difficulties in picking out one particular sound from a noisy environment or locating where a sound is coming from.

Locating sound

The brain usually works out where a particular sound is coming from by combining the information it receives from both ears and using the differences between them to localise the sound. These differences are mostly ones of time (a sound from one side of a person will reach the ear on that side ever so slightly sooner than it reaches the ear on the other side) and volume (the sound will be ever so slightly louder in the ear on the side that it’s coming from than the ear on the other side). It also uses the information received from the external part of the ear (the pinna), which filters sounds coming from different directions in a particular way, to fine-tune the process.

However, if one ear is blocked e.g. by a persistent ear infection, or if a person is deaf in one ear only, the brain can still localise sounds by putting much more emphasis on the information from the unblocked ear. It can’t compare information between the ears, so instead it focuses more on the information from the pinna of the unaffected ear. This allows the brain to continue to localise sounds – it’s not quite as effective, but it’s good enough. However, as mentioned above, this strategy for localising sound may become hard-wired into the brain, so even if the hearing loss is corrected at a later time, the brain can’t go back to the old way of doing things, leading to the ‘lazy ear’ described above. This is a potential problem, as a significant number of children suffer from recurrent bouts of glue ear and thus lose their hearing in one ear until the infection is cleared. These children could therefore be at risk of developing more serious hearing problems later in life.

Glue ear and intermittent hearing loss

The hearing loss experienced in glue ear is not usually persistent – even in recurrent glue ear, once the infection is cleared, hearing in that ear returns to normal, at least until the next infection strikes! It might be that this intermittent hearing loss has a different effect upon the brain, and that’s exactly what Professor Andrew King and his team at Oxford University investigated. They modelled this intermittent hearing loss by plugging the ears of young ferrets and measuring how well they could localise sounds and what was happening in their brains whilst they were doing so. These ear plugs were routinely changed, meaning that the ferrets experienced occasional short bursts of normal ‘two-eared’ hearing. The researchers found that when one ear was plugged, the ferrets became dependent on the information from the unaffected ear for locating where sounds were coming from – they became very good at using this alternative strategy. However, when the ear was then unplugged, they immediately reverted to combining the information from both ears to locate sounds.

The neurons involved could adapt to the context i.e. whether information was coming from one ear or two and switched between the two strategies as appropriate. This meant that one process did not become hardwired over the other, and the ferrets could use whichever one was best for their situation; this is different to what happens when deafness is persistent with no periodic experience of normal hearing.

These findings could lead to new ways of rehabilitating children with glue ear that is serious enough to cause more permanent deafness – it suggests that every attempt should be made to restore their normal hearing whenever possible, so that they are less likely to develop more serious hearing problems later in life. The realisation that the information from the pinna is so important in this alternative strategy for locating sounds could have implications for the design of hearing aids.

If you’re worried about you or your hearing contact your local Hidden Hearing branch.  Hidden Hearing offers free hearing tests at its 60 branches nationwide. To book a test Freephone 1800 370 000 or visit http://www.hiddenhearing.ie.

Read More: From Action on Hearing Loss. Click Here.

We have a handy information booklet on how we hear if you are interested apply below and we will send you a FREE copy.howdowehear2

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