Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Neuroscience

Supervisor

Allman, Brian L.

Abstract

Hearing loss is one of the most prevalent chronic health conditions affecting Canadians, with excessive exposure to loud noise contributing to this burden. Recent epidemiological studies have identified that hearing loss is a major risk factor for cognitive decline, and preclinical studies have identified that the hippocampus, prefrontal cortex, and striatum are sensitive to noise exposure. However, it is unclear if noise-induced changes in these brain regions ultimately affects how animals learn to perform complex cognitive tasks. In this thesis, I investigated the relationship between noise-induced hearing loss and the learning strategies underlying successful performance of cognitive tasks (i.e., assays of spatial working and reference memory, spatial learning, and pattern separation) by noise/sham exposing rats and assessing their hearing sensitivity before and after behavioural testing. Overall, my results suggest that noise exposure conferred an advantage in some rats by causing learning strategies to rely on egocentric (e.g., striatal-dependent) cues.

Summary for Lay Audience

Hearing loss is a prevalent chronic health condition affecting Canadians, with excessive exposure to loud noise contributing to this burden. Recent studies in humans have identified that hearing loss is a major risk factor for cognitive decline, including complex brain functions (e.g., the ability to temporarily remember a phone number). Using various techniques that allow researchers to examine the brain on both microscopic and macroscopic levels, rodent studies have identified that specific regions of the brain are negatively affected by noise exposure. These brain regions are of special interest to us, as they can affect how animals approach a complex task by various strategies. Generally, these strategies start off as more complex processes when navigating to a goal, and they initially require elements of spatial processing (e.g., making your way to a new apartment for the first few times requires you to remember the locations of important environmental features that distinguish your home from other buildings) – but over time, these strategies become more abstracted and habitual (e.g., turning left/right, and navigating for a certain period of time before you arrive home). Based on clinical studies in humans and previous experiments in animal models, it was predicted that a hearing loss, produced by excessively loud noise, would damage the rats’ ability to rely on the initial spatial processing, and cause the abstracted/habitual navigational process to become more prominent. Using brain recordings, the rats’ hearing ability was assessed before and after noise exposure, and several maze-based tests were used to examine how hearing loss affected their cognitive performance. Standard comparisons between the noise-exposed rats and those with normal hearing suggested that hearing loss did not affect cognitive abilities (e.g., memory). However, using a novel approach that included separating the rats’ performance based on whether they used a spatial or a habitual strategy to learn the task ultimately revealed that the rats that relied on a spatial strategy had significantly impaired performance; findings consistent with our original prediction. Collectively, the data included in this thesis suggest that the brain can accommodate noise-induced damage by altering the learning strategies used to navigate towards a goal.

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