Imaging the subthalamic nucleus in Parkinson’s disease

This thesis is comprised of a set of work that aims to visualize and quantify the anatomy, structural variability, and connectivity of the subthalamic nucleus (STN) with optimized neuroimaging methods. The study populations include both healthy cohorts and individuals living with Parkinson's disease (PD). PD was chosen specifically due to the involvement of the STN in the pathophysiology of the disease. Optimized neuroimaging methods were primarily obtained using ultra-high field (UHF) magnetic resonance imaging (MRI). An additional component of this thesis was to determine to what extent UHF-MRI can be used in a clinical setting, specifically for pre-operative planning of deep brain stimulation (DBS) of the STN for patients with advanced PD. The thesis collectively demonstrates that i, MRI research, and clinical applications must account for the different anatomical and structural changes that occur in the STN with both age and PD. ii, Anatomical connections involved in preparatory motor control, response inhibition, and decision-making may be compromised in PD. iii. The accuracy of visualizing and quantifying the STN strongly depends on the type of MR contrast and voxel size. iv, MRI at a field strength of 3 Tesla (T) can under certain circumstances be optimized to produce results similar to that of 7 T at the expense of increased acquisition time

Top-down amplification of predicted visual input behind a frosted occluder

This thesis is comprised of five chapters. It includes two experimental chapters in which I detail both psychophysical and fMRI studies carried out at the University of Glasgow as part of this PhD project. These are followed by a literature review which outlines the implementation of ultra-high-resolution fMRI, both generally within the field and within a specific project proposal. Chapter 1 is a general introduction. I outline the broad organisation and basic functions of the visual system at the pre-cortical and cortical stages, in turn. I then discuss the concept of feedback within the visual system, outlining what feedback is, what it does and how it is implemented before outlining the rationale for the thesis. Chapter 2 is an experimental chapter detailing a series of psychophysical experiments. These experiments employ a partial occlusion paradigm to explore how top-down predicted information can influence the processing of degraded feedforward input. Throughout the experimental series, different aspects of this question are addressed in order to investigate whether the consistency of contextual information influences the detection and/or recognition of low-contrast visual scenes. Chapter 3 is another experimental chapter which details two 3T fMRI experiments. These projects also employed a partial occlusion paradigm to investigate contextual modulation on the processing of degraded feedforward input at the neuronal level in early visual cortex. Both univariate and multivariate analysis techniques were used to reveal the impact of consistency within top-down information. Chapter 4 contains a literature review which looks into ultra-high-resolution fMRI. Here, I detail the motivation behind the development of higher resolution imaging as well as potential confounds and limitations. I also outline adaptations required at higher resolution in terms of data acquisition and analysis as well as briefly exploring layer-specific findings within the visual cortex. Finally, I propose a 7T fMRI project that would continue to explore the influence of top-down predictions on the processing of degraded visual input by expanding the investigation to a laminar level. Chapter 5 is a general discussion which summarises the key points from each of the previous chapters and briefly discusses their conceptual relation to the current field and beyond