We are what we hear: Hearing loss and brain change

Life is all about change. The body and mind naturally adapt to the world around us. An unhealthy diet tends to lead to obesity. Neglect exercise, and the body gets weak.The mind is likewise adaptable: we all know that role models shape us from young and, we were constantly reminded by our parents to “stay away from bad influence.” Even isolation transforms: solitary confinement drives one mad. And so we are what we eat, what we experience, what we see — and indeed what we hear.

Most of us have heard that hearing loss is a big problem, recognizing too that with ageing populations and an increasingly noisy world it is only getting worse. Of course there are statistics: 5.3% of the world’s population, according to the World Health Organization (WHO), has disabling hearing loss (DHL) with about one-third of persons over 65 years of age being affected. But for those with hearing loss (and their families) statistics don’t tell the whole picture as each becomes afflicted in their own unique, personal way to the gradual, often imperceptible, loss of that auditory connection to the world. It is an insidious form of creeping solitary confinement and while we know quite a bit about hearing loss, it is only until recently that we are beginning to understand the impact of hearing loss on the brain and mind.

Of course, most may take this connection between hearing and the brain for granted. The auditory system, after all, has two parts: the ear, which perceives sound, and the brain, which comprehends that sound, so they certainly must influence each other. But while science has made great progress in elucidating the mechanisms of sound perception, for example in the laboratory or with animal models, deciphering the brain’s functions, particularly with respect to that quintessentially human trait of language, has been more challenging. But new techniques such as sophisticated brain scans are now making that possible. One could say we are not just becoming acutely aware of hearing loss, but now understand more deeply its impact.

First, a bit of basic biology. As mentioned, sound is perceived in the ear, specifically by the hair cells in the Organ of Corti within the inner ear. These delicate hair cells transduce these sound vibrations into electrical signals that eventually make their way via nerve pathways to the brain, specifically the auditory cortex in the temporal lobe. How these hair cells (and all the other associated mechanisms) accomplish this is extraordinarily complex but it suffices to highlight two points: one, that different hair cells are tuned to different frequencies of sound with the high-frequency cells being the most delicate and thus the most susceptible to noise-induced hearing loss (NIHL) and, two, that the brain actually feeds back on these cells to fine-tune their response, in away that is believed to focus attention on particular sounds. In other words, the ear is not a one-way conduit of sound to the brain but a two-way relationship. The ear influences the brain and, in turn, the brain influences the ear. In other words, the auditory system doesn’t just take in information from the outside world, it changes and adapts, dynamically and in real-time, to that world.

But the influence of hearing on the brain goes deeper and more long-term than that, as there is increasing evidence that hearing loss actually changes one’s cognitive ability, changing not just the perception of sound (or the ability to focus on sounds, as described above) but also the comprehension of speech. The extensive network of brain regions involved in interpreting sounds and, in particular, to understanding language can now be mapped out using structural and functional MRI scans. Using such techniques researchers at Brandeis University have shown that decreased hearing ability does indeed change brain structure and function. The brain regions related to hearing (“auditory cortex”) were actually smaller in people with hearing loss. And on the functional side, the increased brain activity typically associated with linguistically complex sentences was less evident in those with hearing loss. Although not unexpected, hearing loss apparently does change those parts of the brain related to comprehending sounds. We are what we hear.

But it may surprise some to learn that hearing loss changes the brain even beyond that as there have been many studies connecting diminished hearing acuity with overall cognitive decline—in other words, hearing loss is strongly related to dementia. Although the exact mechanisms of this link between hearing disability and dementia are still being unraveled (is it the extra cognitive demands of “effortful listening”, or a result of the depression and social isolation associated with disabling hearing loss?), the correlation is clear. And even personality can be affected, as recent research from the University of Gothenburg recently demonstrated. In that study, 400 elderly persons were followed over six years in order to discern the influence of various physical, mental and social measures on personality. With respect to decline in extroversion—or an “outgoing” personality—hearing loss turned out to be the strongest predictor of such a change, more so than other variables such as cognitive ability or social circumstances. Yes, we may have heard about hearing loss but when the brain shrivels and the mind turns away from the world then we can truly understand the tragedy that ensues. Life is all about change. Indeed, a hallmark of intelligent life is the ability to positively adapt to such change. If we are near-sighted, we don’t just blindly stumble along—but get glasses. If we have chest pain, we don’t just meekly wait for the heart attack but seek a doctor, and strive to improve our diet and lifestyle. And likewise when it comes to healthy hearing, we avoid excessive noise, and, should it be necessary, use a properly fitted hearing aid that will continue to fill and inspire our minds with beautiful sounds.