Hallucination Proneness and Musical Aptitude: Functional and Microstructural Underpinnings

The current thesis aimed to explore links between hallucination proneness and musical aptitude utilising a variety of brain imaging methodologies to characterise associated functional and microstructural individual variabilities. A further aim was to investigate whether a short duration of musical training could be used to modulate functional activity and microstructure in regions associated with hallucinatory experiences. It was hypothesised that hallucination proneness and musical aptitude would be negatively associated with each other and inversely related to underlying functional activity and microstructure within a shared network of brain regions. Moreover, it was hypothesised that musical training would lead to changes in functional activity and microstructure within this shared network of regions. Measures of musical aptitude and hallucination proneness were assessed in conjunction with diffusion imaging models which enabled the characterisation of the microstructural features of the corpus callosum. Results revealed an inverse relationship between musical aptitude and hallucination proneness, with a mediating effect of musical aptitude on hallucination proneness through the microstructure of the corpus callosum. The use of a multi-shell biophysical model, based on neurite orientation dispersion density imaging, further revealed that the relationship between hallucination proneness and musical aptitude was primarily due to callosal neurite orientation dispersion rather than neurite density. With the addition of functional connectivity MRI the degree of callosal neurite orientation dispersion also shown to impact on the functional connectivity during a musical categorisation task, such that higher neurite alignment was associated with increased ROI- ROI fronto-temporal functional connectivity. Hallucination proneness was shown to be negatively associated with performance on a speech perception task and functional connectivity between the left IFG and the superior temporal gyrus (STG) (bilaterally) during task completion. Dendritic complexity within the STG grey matter was also found to be negatively associated with individual variability in propensity to hallucinate. Investigations of the effects of exposure to a short musical training session (learning to tap polyrhythms for one hour) provided evidence of an increase in ROI-ROI function within a bilateral network of fronto-temporal regions following training. Moreover, using three distinct but complimentary diffusion imaging models, polyrhythm training was shown to facilitate a decrease in extra-axonal space diffusion in the central portions of the CC which correlated with performance gains on the polyrhythm discrimination task. The overall results of this thesis therefore support the hypothesis that musical aptitude and hallucination proneness are linked and associated with the underlying microstructure of the CC. Moreover, musical aptitude was shown to be positively associated with task based functional fronto-temporal connectivity whereas hallucination proneness was shown to be negatively associated. Hallucination proneness was further shown to be related to microstructure of the STG with orientation dispersion deemed the most sensitive metric for assessing this relationship. Importantly, results offer evidence that musical training may offer a novel approach for improving fronto-temporal functional connectivity and the microstructure of the corpus callosum, providing an initial foundation for investigation of future novel interventions for hallucinatory experiences.