Peripheral and Central Auditory Processing in People With Absolute Pitch
Mcketton, Larissa A.
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Absolute pitch (AP) is a rare ability that is defined by being able to name musical pitches without a reference standard. This ability has been of interest to researchers studying music cognition and the processing of pitch information because it is very rarely expressed and raises questions about developmental interactions between biological predispositions and musical training. This dissertation focuses mainly on the peripheral and central neural substrates and is divided into seven chapters. The first chapter reviews the anatomy, function, and frequency resolution of the auditory peripheral and central nervous system. It includes background information pertaining to the origins of AP and describes inconsistencies reported throughout a number of studies that characterize AP emergence. Chapter two details a series of peripheral experiments on twenty AP and thirty-three control subjects recruited for testing at two locations. The goal was to test whether frequency resolution differences could be resolved at the level of the cochlea within both groups as a potential correlate for the genesis of AP. Chapter three details two behavioural tests that were administered to assess the smallest frequency difference that AP musicians could resolve and to test how well they could detect melodic mistuning excerpts compared to non-AP musicians and controls without musical experience. Both AP musicians and non-AP musicians did significantly better in both tests compared to non-musicians. However, there were no differences between the AP and non-AP musician groups. Chapter four details a functional MRI study that measured frequency tuning in the cortex using a population receptive field (pRF) model that estimates preferred frequency bandwidth in each voxel. This method was also tested in auditory subcortical nuclei such as the inferior colliculus and medial geniculate nucleus. Chapter five reports the neuro-anatomical correlates of musicianship and AP using structural MRI. Here we investigated cortical thickness and volume differences among the three groups and found a number of regions differed significantly. Cortical thickness was significantly greater in the left Heschls gyrus (an area that acts as a central hub for auditory processing) in AP musicians compared to non-AP musicians and non-musicians. AP and non-AP musicians also exhibited increased cortical thickness and volume throughout their cortex and subcortex. In line with previous studies, AP musicians showed decreased cortical thickness and volume in frontal regions such as the pars opercularis part of the inferior frontal gyrus. Chapter six reports the neuro-anatomical correlates of musicianship and AP using diffusion tensor imaging (DTI) to measure connectivity and white matter structural integrity in regions associated with audition and language processing. Tracts connecting language processing regions were reduced in volume in AP musicians compared to their non-AP counterparts. Chapter seven includes the general discussion, which integrates the findings and results from the five experiments. Our findings indicate that the sharpness of frequency tuning did not differ in either peripheral or central auditory processing stages among AP and non-AP groups. This implies that AP possessors do not encode or represent auditory frequency any differently than other groups, from the periphery through auditory cortex; instead, the neural substrate of their abilities must lie elsewhere. The automatic and working memory independent categorization abilities in AP may reflect more refined efficiency in local but not global functional connectivity.