Multimodal Investigation of Neuronal Responses

This thesis describes an investigation of neuronal responses with both magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI). MEG and fMRI are widely used in neuroscience. However, aspects of the MEG and fMRI signal are still not well understood, particularly post-stimulus responses – responses which occur after a stimulus has ended. Post-stimulus responses have been shown to correlate with various illnesses and as a result, MEG and fMRI have yet to reach their full potential clinically. By developing carefully controlled experiments, MEG is used in this thesis to characterise post-stimulus responses to a grip-force task. The results showed that the beta-band post-stimulus response (post-movement beta rebound, PMBR) is modulated by task duration. Functional network analysis, using amplitude envelope correlation and a hidden Markov model, showed that the PMBR re-establishes networks after breaking down during a task, suggesting the PMBR is related to functional connectivity. The results of this thesis provide new information about the nature of the PMBR, demonstrating that it can be systematically controlled by task parameters and provides insight into its generation. It is hoped this research will contribute to a deeper understanding of the PMBR and provide a step forward for its use clinically. In fMRI, the origin of the post-stimulus response is also poorly understood. To investigate fMRI post-stimulus responses, an MR pulse sequence was developed and optimised to measure blood flow, volume and oxygenation changes simultaneously at 7 T. This was implemented with the grip-force task, allowing direct comparison between MEG and fMRI. This study provides new insights into the fMRI post-stimulus undershoot which warrant further investigation. Understanding the link between fMRI and MEG signals will help further understanding of both modalities and how they relate to neuronal activity. Finally, the applications of fMRI were explored by comparing fMRI responses in patients with focal hand dystonia (FHD) with healthy controls. 7 T fMRI was used to map cortical fingertip representations and measures were developed to compare overlap of digit representations between patients and healthy controls. This project provided an important opportunity to advance the understanding of FHD and was the first study to use fMRI to explore the effects of treatment on patients with FHD