Coleman, Sebastian C.
(2024)
Investigating Post-Task Responses using MEG and 7T fMRI.
PhD thesis, University of Nottingham.
Abstract
Electrophysiology and haemodynamics related to human brain function can be measured using several modern imaging techniques. Magnetoencephalography (MEG) enables direct measurement of neuromagnetic fields created by signal transmission in neuronal ensembles, with high temporal resolution but limited spatial resolution and depth sensitivity. Functional magnetic resonance imaging (fMRI) enables the indirect measurement of neuronal activity through changes in blood oxygenation, with lower temporal resolution but high spatial specificity throughout the entire brain. These techniques can be used to investigate the spatial origin and dynamics of brain activity, in response to tasks or during resting state. This thesis uses MEG and fMRI to study responses that occur in the transition period between task cessation and rest, which we term post-task responses (PTRs).
The most well-known PTR in electrophysiology is the beta rebound, a rise in motor cortex oscillatory power following movement cessation, lasting up to 10 seconds. The haemodynamic PTR often manifests as a post-stimulus undershoot in the blood oxygenation level dependent (BOLD) signal, lasting up to a minute. Both of these responses have displayed characteristics that warrant neuroscientific and clinical investigation, including reliable amplitude modulations with task conditions and reduction in clinical populations. Despite this, current research into electrophysiological PTRs outside of the motor cortex, following non-motor activities, is severely lacking. In addition, the relationship between electrophysiological and haemodynamic PTRs is poorly understood. This thesis details three experiments aiming to establish PTRs as an important brain-wide phenomenon that enables regulation of functional network activity during the transition to rest.
The first experimental chapter, “Post-Task Responses in Oscillatory Activity follow Cessation of Working Memory Processes”, used MEG to measure PTRs following an n-back working memory task. PTRs were measured across the brain in regions associated with higher cognition, increasing in amplitude with working memory load and showing promising relationships with reaction times. This chapter establishes PTRs in electrophysiology as a more general property of the cortex than previously thought, induced by non-motor activity and spanning multiple frequency bands and regions.
The next experimental chapter, “Characterising Post-Task Responses using Hidden Markov Models”, compares PTRs in electrophysiology following working memory and movement by examining burst characteristics of these responses, using a hidden Markov model (HMM) to infer burst timing in a data- driven manner. The analysis revealed stark similarities between region-wise burst properties across two different tasks and showed that PTRs are supported by coincidence of these bursts in widespread cortical networks. This chapter provides further evidence that PTRs are a fundamental property of the cortex, underpinned by long-range cortical interactions.
The final experiment, “Spatial and Dynamic Dissociation of Primary and Post-Stimulus BOLD Responses”, used ultra-high field fMRI to study the post-stimulus BOLD response following working memory. Results showed similarities between poststimulus BOLD responses and the electrophysiological PTR, with modulations that are spatially and dynamically decoupled from responses that occurred during the preceding task. Using the high spatial resolution and depth sensitivity of fMRI, this chapter reveals the influence of the PTR on several distinct functional networks, including interactions with deep grey matter regions that are difficult to study with MEG.
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