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Thesis Format

Monograph

Degree

Master of Science

Program

Neuroscience

Supervisor

Lomber, Stephen G.

Abstract

In the absence of hearing, the brain must adapt and repurpose the former auditory cortex. In this study we scanned normal hearing (n=29) and deaf (n=26) cats to identify cortical areas of differing thickness using the auditory regions from a 3D cortical atlas. Compared to hearing controls, differential thickening and thinning was observed in specific regions of the deaf auditory cortex. More dorsal auditory regions tended to be bilaterally thicker in the deaf group, while more ventral regions in the left hemisphere were thinner. The location and nature of these changes creates a gradient along the dorsoventral axis wherein dorsal auditory cortical fields are thickened while more ventral fields are thinner in deaf animals compared to hearing controls. Whether this reflects the spatial separation between the functionally distinct “where” and "what" pathways, or a more general property of distance along the dorsoventral axis of cortex remains to be examined.

Summary for Lay Audience

The brain is a highly complex organ, primarily composed of grey matter and white matter. Grey matter consists of neurons which process information, as opposed to white matter which consists of axons that carry information between these neurons throughout the brain. Therefore, neurons communicate with one another through a dense network of white matter. However, this grey-white matter network is subject to adaptations in response to changing environmental conditions- a phenomenon known as plasticity. While undergoing plasticity the relative amounts of the different components of grey matter can change. These compositional changes affect the space requirements for the grey matter and can be observed as the change in thickness of cortical grey matter locally or throughout the brain. In human studies researchers have found regional differences in grey matter thickness in certain disease states as well as thickness differences in experimental groups with large amounts of experience in specific tasks, such as playing music or video games. Grey matter thickness can also be used as an indicator of Alzheimer’s disease. Changes in human auditory cortex thickness have been found in different deaf groups in the past with mixed results. To take a different approach to the question of human thickness after deafness, this work used the deaf cat as an animal model of human deafness. Cats have very similar auditory cortices to humans and have been used for many years in auditory and visual neuroscience. In this work structural MRI scans of mature hearing and deaf cats were used to find any regional differences in cortical grey matter thickness of auditory cortex. It was discovered that five auditory cortex regions were thicker on both sides of the brains of the deaf cats relative to the hearing controls, and two regions on the left side were thinner. A pattern of thickness change emerged between the two groups, such that physically higher-up auditory regions in the brain were thicker in the deaf groups, and lower regions were thinner. To discover the nature of these changes further work will need to be conducted.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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