The Auditory Deprivation Effect: How Hearing Loss is Bad for your Brain
Most people think of hearing as something that you do with your ears. But your ears are only the sensors; until your brain receives and processes the incoming signals from your ears, you don’t perceive the sound. It’s more accurate, then, to say that you hear with your brain.
When you have hearing loss, the parts of your brain that are responsible for processing and interpreting sound signals aren’t receiving adequate stimulation. Think of it this way: your brain is programmed to receive a certain amount of sound at every frequency, and if your ears aren’t delivering that amount because of damage from aging, noise exposure, or other types of hearing loss, then your brain is being deprived of something it needs. And yet, people wait an average of seven to ten years after being diagnosed with hearing loss before they seek treatment.
If you’re among those people who have hearing loss but haven’t yet done anything about it, read on. Years of inadequate auditory stimulation can be detrimental to your brain health and even lead to permanent structural changes in the brain – even an accelerated loss of brain matter.
How your brain is involved in hearing
When the sensory cells in your inner ears are stimulated by sound, they send electrical impulses up the auditory nerve to your brain. The part of the brain which is responsible for perceiving sound is the auditory cortex; if a person were missing the auditory cortex on both sides of their brain, that person wouldn’t even be aware of sounds despite their ears sending the signals. Partial damage to the auditory cortex results in difficulty differentiating between sounds and pitches, and can lead to significant trouble with speech comprehension.
The brain is responsible for identifying where sounds are coming from, as well as what they are. It helps to filter important sounds from unimportant ones. Most importantly, it translates sounds into language and, in the parts of the brain responsible for higher-level processing, makes sense of the words you hear.
What happens with inadequate auditory stimulation?
If hearing loss is left untreated for too long, your brain will begin to lose the ability to process sound. This is called auditory deprivation and the clinical effects have been well known for decades: patients who waited a long time to get help continue having difficulty understanding what people are saying, even after being fit with hearing aids. This is because some of those connections in the brain between sound and its meaning have degraded. We now know that those connections start degrading even with extremely mild hearing loss, even though it may take a long time for the symptoms to become obvious.
When you experience this loss of clarity, you’ll find yourself working extra hard to make out what people are saying to you, particularly if there is background noise present. Over the course of a day, this can result in increased fatigue. For some people, this fatigue is specific to listening, and they may find that it becomes harder and harder to understand what people are saying as the day goes on. For others, it can be more generalized fatigue.
Increased listening effort can also impact other cognitive processes, such as memory formation. Because listening and understanding are taking up such a high percentage of your cognitive energy, you have less left over for storing information or coming up with intelligent responses to what you’re being told.
Real changes in your brain
- Cortical reorganization
When the auditory cortex doesn’t receive input in the form of sound, the brain will begin to reallocate its resources. It has been well-documented for decades that in cases of complete deafness, the fibers in the auditory cortex will be used for visual processing instead. This ability of the brain to reorganize itself to better process the types of stimulus that it does receive is called neural plasticity; you may have heard of it in the context of language learning, where small children are much better able to learn languages than adults due to the gradual decrease in neural plasticity as we age. However, the brain never loses plasticity entirely – otherwise, we wouldn’t be able to learn new things!
Research in the last ten years has shown that the brain will begin this process of reassigning auditory areas of the brain to visual processing even in cases of mild hearing loss. People with mild hearing loss usually aren’t aware that they’re becoming increasingly reliant on visual cues such as mouth shapes (lip reading) to compensate for their declining hearing sensitivity, but the evidence shows that these changes are occurring in your brain long before you become consciously aware of them. In addition, the frontal cortex, which is responsible for higher-level processing, is more active in response to auditory stimuli than it is in normal-hearing people.
- Loss/atrophy of brain tissue
As you age, your brain will gradually shrink slightly. This is normal. However, untreated hearing loss is associated with an acceleration of this shrinkage. Research out of Johns Hopkins showed that subjects with hearing loss lost over a cubic centimeter of additional brain tissue each year compared with normal hearing subjects. This atrophy was most pronounced in areas of the brain responsible for processing speech and other auditory information.
Those same Johns Hopkins researchers have also found that untreated hearing loss carries a risk of accelerated cognitive decline and even a higher risk of dementia. The risk increases linearly with the severity of hearing loss, such that a person with severe hearing loss is at a much greater risk of dementia than someone with mild loss, who is nonetheless at significantly greater risk than someone with no hearing loss at all.
Can hearing aids help prevent or reverse these changes?
While research into whether hearing aids can prevent or even reverse these brain changes is in the early stages, the evidence so far suggests that they can! A recent study out of the University of Colorado demonstrated that subjects with mild to moderate hearing loss, who showed evidence of cross-modal cortical reorganization as discussed earlier in this article (auditory brain fibers being reassigned to visual processing), experienced a reversal of this cross-modal reorganization after six months of hearing aid use. In addition, the same subjects experienced an increase in cognitive function, specifically in the domains of global cognitive function, executive function, processing speed, and visual working memory.
Further studies are needed to investigate just how extensive this cortical reorganization can be and still be substantially improved by hearing aid use. Clinical experience suggests that there is indeed such a thing as waiting too long: patients who wait decades before seeking treatment for their hearing loss never perform well on tasks of speech understanding, particularly in noise, and often give up on their hearing aids. However, knowing that mild to moderate hearing loss a) does result in brain changes that may lead to atrophy and dementia down the line, and b) can be treated successfully with hearing aids, leading to improved cognition and speech perception measures and even reversing early cortical reorganization, certainly supports the early treatment of hearing loss. If you can prevent these brain changes by getting hearing aids, why wouldn’t you?