The impact of loud noise on cognitive function
Whether it be a concert, in traffic, or the workplace, most of us have been exposed to noisy environments. In these cases, there is an important distinction between non-hazardous and hazardous levels of noise, the latter often leading to noise-induced hearing loss if frequently experienced. Hazardous levels of noise can emerge from listening to loud music via earphones, attending concerts, and within certain occupations such as the military, construction, and manufacturing. Currently, the official rules surrounding maximum noise levels in workplace settings vary between countries and even between provinces and states. This is due to the lack of information regarding the consequences of noise-induced hearing loss. Of interest, recent studies have shown hearing loss to be associated with cognitive impairment. Given this daunting association and that ~466 million individuals suffer from hearing loss worldwide, further studies are needed to investigate the consequences of hearing loss and inform the public on safe levels of noise.
Toward this goal, Dr. Wieczerzak and colleagues at Western University used a rodent model to test the effect of noise-induced hearing loss on cognitive function. First, they wanted to test spatial learning and spatial memory, which are cognitive abilities that help us orient ourselves in space. For an individual that drives to work every morning, their spatial skills are crucial for navigating the route.
When using a rodent model, researchers assess spatial ability by using a task called the Morris water maze. The goal of this task is for the rats to swim in a small pool and use spatial cues, such as symbols on the walls of the room, to find a specific location in the pool. As the rats continue to do this multiple times with the help of the cues, their spatial learning and memory skills allowed them to find this location more quickly each time. Interestingly, Dr. Wieczerzak and colleagues found that rodents with hearing loss were significantly slower in finding the target location when compared to normal hearing rodents, indicative of impairments in spatial learning and memory.
In addition to spatial learning, the authors also evaluated stimulus-response habit learning. This cognitive ability is important for learning associations between a stimulus and a response. For example, when driving, we may come across a red light (the stimulus) that prompts us to hit the brakes (the response).
The researchers tested stimulus-response habit learning in rodents using a chamber that had two levers, each with one light above it. Similar to humans hitting the brakes, rodents had to press the lever below the illuminated light to receive a reward. If they pressed a lever where its corresponding light was off, they did not get a reward. Over time, they were able to learn the task and get a sugar reward consistently. However, the rats with hearing loss made significantly more mistakes than normal hearing rats, indicative of impairments in stimulus-response habit learning.
Lastly, the authors evaluated behavioural flexibility, which is a cognitive ability that allows us to adapt to novel changes in the environment. While driving, some of us may have encountered intersections where traffic guards are present. Although there may be a red light, we must listen to the traffic guard telling us to drive through the intersection. In this scenario, we have abandoned the rules of the traffic lights and adapted to novel external instructions.
In the study, the researchers tested behavioural flexibility in the rodents who had previously already learned to press the lever below an illuminated light. Now, the rodents were only able to receive their reward if they kept pressing one specific lever, regardless of which light was illuminated. To learn that the light no longer mattered is a change in the environment to which they need to adapt to continue receiving their food reward. Interestingly, all the rodents were able to adapt to this new rule, indicating that there were no differences in behavioural flexibility between rodents with hearing loss and normal hearing.
In the journal article, Dr. Wieczerzak explains that these three cognitive abilities are distinct because different brain regions are responsible for them. The hippocampus, striatum, and prefrontal cortex are responsible for spatial learning and memory, stimulus-response habit learning, and behavioural flexibility, respectively. Overall, the authors conclude that while the cognitive abilities primarily performed by hippocampus and striatum may be impaired following hearing loss, prefrontal cortex-dependent behavioural flexibility remains intact. These findings will guide future studies that aim to uncover the precise changes within the hippocampus and striatum that lead to cognitive deficits. Ultimately, understanding the impact of hearing loss on cognitive abilities will aid in enhancing safety guidelines in and outside of the workplace regarding acceptable levels of noise.