Horton hears a WHAT! The consequences of noise-induced hearing loss on the brain
Image by Dany Kurniawan: https://www.pexels.com/@danykurstudio/
Excessive noise exposure throughout one’s life is a common cause for hearing loss, a chronic health condition affecting more that 400 million people worldwide. Although hearing loss can alter daily living by disrupting simple tasks such as communication with others, it has recently gained more attention as a major risk factor for age-related cognitive decline1. To better understand this relationship, a study led by Dr. Brian Allman, PhD, and colleagues at the University of Western Ontario investigated the insult caused by noise-induced hearing loss on different brain regions relevant to cognitive function in a rodent model.
To study noise-induced hearing loss, researchers exposed one group of rats to a 100 dB sound for 4 hours a day for 30 days while another group had no noise exposure as a control. To put this in perspective, a 100 dB sound would be equivalent to a loud lawnmower, a sporting event, or a pair of earbuds on full volume2. Following noise exposures, animals underwent different behavioural assessments over three age periods: young adult (7 months), middle age (10 months), and old age (13 months). The different behavioural assessments were selected based on their relevance to a particular region of interest in the brain.
The first brain region examined was the hippocampus, which plays a crucial role in spatial learning and memory. For this brain region, the Morris water maze task was used. In this task, rats were required to swim in a pool and find a hidden platform based on visual cues within their surroundings (see image on the right for a visual diagram). Finding the platform tests their spatial learning, whereas the ability to recall the location of the platform during future sessions tests their spatial memory. By comparing the performance of the noise-exposed group of rats to the control group, researchers saw no difference in their memory over the three age periods. However, the noise-exposed group did show impaired learning at the old age period in comparison to the control group, as they required more time to find the hidden platform.
Next, the researchers investigated the striatum, a region that is important for visual and motor associative learning. To study this brain region, rats were placed in a behavioural chamber equipped with a nose poke, two feeder troughs, and a light source (see image on the right for a visual diagram). A nose poke results in the presentation of either a steady or a flashing light. Over 18 days, rats learned to associate the steady light with the left feeder trough and the flashing light with the right, and correct trials result in the presentation of a sugar pellet as a reinforcement. Through this task, the researchers were able to examine the rats’ associative learning ability using a visual stimulus. When comparing performance of the noise-exposed rats to the control rats, the researchers observed no difference, suggesting that noise exposure does not change associative learning mediated by the striatum.
Lastly, the neuroinflammatory profile within these brain regions was examined to assess their susceptibility. When analyzed over the three age periods following noise exposure, there did not appear to be a change in the neuroinflammatory profile. Despite that, there was a reduction in the neuroinflammatory response in the hippocampus for the old rats, which corresponded to the age at which rats showcased learning deficits following noise exposure. However, further analysis surprisingly showed no correlation between the two variables.
When considering these results, it appears that not all brain regions are equally susceptible to insult following noise-induced hearing loss. More specifically, the hippocampus appears more susceptible than the striatum, evident by the change in spatial learning but not associative learning following noise-exposure.
Altogether, the current study highlights the effect of excessive noise-exposure on non-auditory brain regions that play vital roles in everyday cognitive functions. While we are left with more questions than answers, the take home message from the study is the importance of protecting your hearing. When discussing how this study provided a different perspective on auditory health with Salonee Patel, an MD/PhD student and the primary researcher behind this study, she said, “The impact of hearing loss goes beyond just our initial idea of the inability to hear auditory information. It can have greater effects on brain function and cognition that can manifest later in life”. Finally, the simple message Salonee wanted to share with all of us was, “Protect your hearing to protect your brain health.”
Article:
Patel SV, DeCarlo CM, Book SA, Schormans AL, Whitehead SN, Allman BL, Hayes SH. Noise exposure in early adulthood causes age-dependent and brain region-specific impairments in cognitive function. Front Neurosci. 2022 Oct 13;16:1001686. doi: 10.3389/fnins.2022.1001686. PMID: 36312027; PMCID: PMC9606802.
Note: both diagrams used were adapted from the original study cited above
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