Previously, we looked at the way the brain constantly balances and prioritises the amount of energy dedicated to processing either sound or sight inputs. And that refocusing of the brain’s energy means that at any given moment either vision or hearing is compromised. The brain simply cannot give 100% attention to both hearing and vision at the same time.
We also saw (or is it heard?) how the brain, thanks to its amazing plasticity, can adapt to certain types of injury to re-establish function. Let’s now look a little more closely at how the brain can respond to inner ear damage.
The brain is a natural amplifier
Research conducted by the Harvard Medical School has demonstrated the brain’s ability to compensate almost completely for even severe damage to the auditory nerve fibres.[1] The findings suggest that the brain is able to form a mental image of the sounds despite the lack of input from the hearing nerve cells.
According Dr Polley, Director of the Amelia Peabody Neural Plasticity Laboratory at Massachusetts Eye and Ear and an Associate Professor of Otolaryngology at Harvard Medical School, as little as 3% of normal hearing input is enough for the brain to process the sounds and restore hearing threshold levels to near normal.
In fact, this function is somewhat similar to that of a hearing aids. The brain is acting as an amplifier but as with most brain functionality this adaptation is a compromise and such remarkable plasticity comes at a cost.
Speak up; I’m not deaf
Despite the brain’s intervention and a return to almost normal hearing levels the brain’s attempt to re-establish hearing thresholds for general listening comes at the cost of the more subtle hearing required for speech recognition.
It seems that the brain can make up to a large extent for the loss of sound input for general sound recognition. However, the sounds involved in speech are simply too complex and involve such rapid variations that either the brain cannot devote energy to decoding them or there is simply not enough data for the brain to translate.
It’s perhaps a little like our inability to identify clearly images immediately in front of our eyes while focussing on the horizon: a physical and mental impossibility. Try it; focus on a distant object and while retaining that focus try to focus on an object near at hand. This gives a real insight into the challenges facing the brain as it tries to make sense of limited sound inputs to build up the larger aural picture.
Many people with hearing loss have experienced the frustration of listening to a conversation and being able to hear the sounds but having difficulties understanding the words. And the death of hearing nerve cells that are essential to processing conversation is a natural part of the aging process.
But it seems that the amplification process actually increases these difficulties.
And the brain’s role as an amplifier carries another cost too.
With a hiss and a roar
The brain has a wonderful innate ability to act as an amplifier but this can create a whole new set of problems for those with hearing loss. When all sounds are amplified equally all the time some sounds will be painfully loud. It appears that the brain‘s attempts to restore hearing are inextricably linked with the development of hypersensitivity to noise and those phantom noises known as tinnitus.
Dr. Polley likened it to “feedback from a microphone, having too much gain in the system can push neural circuits toward becoming pathologically hyperactive and hypersensitive”. He offered hope for a future where, by identifying and controlling the brain’s amplifier at a cellular level, “that one day we might be able to turn the volume knob up and down to find that ‘sweet spot’ where people can reconnect to the auditory world without hearing phantom ringing or cringing at a loud noise that most people would shrug off as ‘tolerable.’”
This research highlights once again the brain’s amazing ability to adapt to new challenges. The challenge is now for science to find ways to work with the brain to restore hearing more fully. There a huge number of questions about brain function that need to be explored and one of those questions is whether or not the brain can actually get in the way of the body’s own healing process.
Join the https://www.houseofhearing.ca/ blog as we explore these issues further.
[1] The source for this article: https://www.sciencedaily.com/releases/2016/01/160128133035.htm
