Plasticity And Signal Representation In The Auditory System PDF Download

"The human auditory system is made up of a network of processing centres in the brainstem, thalamus, and cortex, which in turn interact with higher-level functions and the sensory and motor systems. Although the coordinated activity of the entire ensemble is responsible for human auditory perception and behaviour, it has been suggested that the fidelity with which important features of sound are encoded and processed in early auditory areas may place limitations on system performance on auditory tasks. In this thesis, we address a set of research questions within the theme of relationships between early sound encoding and higher-level cognitive function, and their respective neural correlates. Throughout these studies, our primary focus is on temporal encoding of periodic sound, as measured using the frequency following response (FFR), an evoked response that has typically been studied using electroencephalography (EEG) and has been related to individual differences in perception and pathology of the auditory system, is malleable to musical and linguistic training, and can be modulated by top-down factors like attention. This dissertation comprises four studies. In the first study, we recorded FFR using magnetoencephalography (MEG) for the first time and used source modelling to clarify its generators. In addition to confirming sources in brainstem nuclei and thalamus, we found a right-lateralized contribution to the FFR from the auditory cortex, which proved to be behaviourally relevant as it was significantly related to musicianship and fine pitch discrimination skills. In the second study, we used functional magnetic resonance imaging (fMRI) to validate the neural correlates of FFR encoding strength in the cortex and dissociate the right-lateralized FFR-sensitive area from a left-lateralized area of auditory cortex that is sensitive to onset latency. These findings corroborate theories of hemispheric specialization in auditory signal processing. In the third study, we turned our attention to individual differences in periodic sound representation as measured with EEG-FFR and examined their relation to pitch perception and pitch computation. We found that FFR-f0 strength was related to a bias towards perceiving the missing fundamental, which was in turn related to measures of musicianship, and showed that pitch perception mode can be brought under voluntary control, which also affects the FFR-f0 strength in a top-down fashion. In the fourth study, we examined individual differences in periodicity encoding as they relate to speech-in-noise perception abilities, a task for which pitch cues are important and that is thought to be enhanced by music training. We presented further evidence of a musician advantage to a current debate, and added spatial information available via MEG distributed source modelling to show that speech-in-noise performance is correlated with FFR strength in both subcortical and cortical structures. In the experimental work presented in this thesis, we made several contributions to fundamental auditory neuroscience and its methods by clarifying the neural origins of a commonly studied measure of fine periodic encoding, its behavioural meaning, and sources of individual variability. We explored its relationship to long-term training, and to cortical function and structure, using EEG, MEG, fMRI, and diffusion-weighted imaging. We also took several steps towards elucidating if and how better quality periodic sound encoding might result in better behavioural performance on complex tasks, particularly speech-in-noise perception. Together, this work improves our understanding of individual differences in periodic sound representation and how it influences complex behaviour. The conclusions in turn may inform strategies for optimizing and remediating faulty auditory system components, via training." --