In this series we have explored how the brain constantly monitors and adjusts the amount of energy and attention dedicated to processing either sound or sight inputs. We have also touched on the extraordinary ability of the brain to adapt to injury or loss of function by borrowing neural capacity from other sensory processing areas of the brain.
This brain plasticity, its ability to adapt, would seem to be a perfect response but this adaptation can come at a surprisingly high cost. Let’s look more closely at how the brain responds to injury.
Is my brain multitasking?
The brain is not hard-wired as once thought; the neurons in the brain are able to do various tasks. For example we have long known that blind people frequently develop increased hearing perception and deaf people may acquire sharper visual processing skills and modern research confirms this.
In this case the brain isn’t multi-tasking in the sense of doing two things at the same time but is simply seconding parts of the brain previously dedicated to one task to process a completely different task. Often this adaptation results in a compromise on performance but is it possible that this brain plasticity can actually hinder an effective response to injury or sensory loss?
Vision and hearing neurons aren’t the same
One of the deficiencies of brain plasticity arises because a 100% transfer of, for example, visual processing capacity to replace the damaged hearing facility does not necessarily occur. A study by the scientists at Georgia State University has shown that sometimes the brain’s rewiring process may be less than effective particularly when the auditory cortex is trying to process both visual and auditory information
In some cases it seems that the visual neurons called on to assist hearing may be too isolated or too few in number to give an adequate sound picture. The brain is not getting enough input to translate the sound inputs through the visual neurons. The brain’s struggle to interpret sound using visual processing centres further compromises the auditory response by reducing the sensitivity and speed of response in the auditory neurons so it’s a lose, lose scenario.
In these situations, calling on the assistance of the visual nerves actually hinders hearing. As one of the research team stated, “Usually we think of plasticity as a good thing, but in this case, it’s a bad thing. We would like to limit the plasticity so that we can keep the function that’s supposed to be there.”
Help! My brain is on overdrive
Another negative aspect of brain plasticity is more well-known: the development of hyper sensitivity. This occurs when, in response to lack of sensory input, the brain turns up its sensitivity levels (a little like turning up your hearing aid). In ordinary circumstances this inbuilt safety mechanism can protect us by making our senses more alert to danger.
However, this adaptation becomes unhelpful when it won’t turn off. In its attempts to adapt to the loss of sense input the brain switches to constant overdrive mode. Then the heightened sensitivity can manifest as pain in absent limbs (in the case of limb amputees), phantom noises in tinnitus sufferers, and noise or light sensitivity.
It would seem logical that such an advanced and complex structure as the human brain would be able to monitor and adjust sensitivity levels to suit the context. And in most people with fully functional normal responses it does this very successfully.
We do not yet fully understand why the switch mechanism doesn’t work properly in some people.
You mean my brain isn’t perfect?
The brain is an extraordinarily complex structure and its responses are not always predictable and may not even appear logical at times. But the more we understand how the brain works the more hope science can offer those suffering the effects of hearing loss. The brain’s plastic nature offers two particularly exciting areas of hope.
One exciting avenue of research into brain plasticity indicates that more highly developed visual processing abilities enable a person with hearing loss to be more successful in transferring those abilities to processing sounds. This has important implications for the successful rehabilitation of hearing after receiving a cochlear implant.
The second area of exciting research centres on the same plastic nature of the brain that causes the initial problems. If the brain can adapt in the first place then it also means that those painful adaptations such as tinnitus or hyperacusis are potentially reversible.
And that is exciting news for those who suffer the effects of hearing loss in all its forms.
Join the https://www.houseofhearing.ca/ blog as we explore these issues further.