Psychiatry - Theses
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ItemThe Brain-Behavioural Basis of Human Safety Learning: An investigation of Pavlovian conditioned inhibition( 2022)Safety learning allows individuals to associate stimuli with the absence of threat, thus conferring the ability to suppress fear and anxiety in safe situations, and by consequence, maintain psychological and physiological well-being. Disrupted safety learning is thought to be a key component of anxiety-related disorders, but as yet, the basic mechanisms of safety learning remain incompletely understood and lack a formal theoretical definition. Across two studies of healthy adults, this thesis sought to examine the behavioural (Study 1) and neural (Study 2) basis of safety learning in humans. Based on long-standing principles of associative learning theory, a novel iteration of the Pavlovian conditioned inhibition paradigm was developed and implemented in lab-based and 7-Tesla functional magnetic resonance imaging (fMRI) settings. Study 1 (N = 73) was an investigation of the behavioural aspects of safety learning, and moreover, sought to validate the utility of the Pavlovian conditioned inhibition paradigm as an experimental model for safety learning, as well as examining individual differences in trait measures of anxiety. This paradigm trained a robust safety signal (the conditioned inhibitor, X) which was conditioned by delivering threat (loud noise) to a conditioned stimulus on its own (A+), but omitting threat when that stimulus was presented in conjunction with the inhibitor (AX-). As a control cue, two stimuli were similarly unreinforced in compound, but neither was presented alone on other trials (BC-). The paradigm also controlled for several possible confounds, including the use of a safety signal as a control cue, rather than using a novel or neutral cue, among other factors. Both the control safety signal and the conditioned inhibitor were shown to inhibit physiological and cognitive threat responses at test, when paired with aversive conditioned stimuli. However, the inhibitor conferred a significantly greater degree of inhibition for cognitive threat responses, as measured by threat-expectancy ratings during a summation test. Further, trait anxiety was positively correlated with threat expectancy towards the inhibitor during learning, indicative of threat responses to safety signals, which are thought be a feature of maladaptive anxiety. Study 2 (N = 49) investigated the functional neural correlates of safety learning via conditioned inhibition. The same paradigm from Study 1 was adapted for use in neuroimaging, using ultra-high field fMRI. Activations were compared between the safety signals directly (AX vs BC), and learning-specific activation was assessed via contrasts between early and late conditioning trials, and conditioning phase activity versus test phase, under the hypothesis that this should identify regions recruited to form stimulus-safety associations when these contingencies are new and unfamiliar. It was found that conditioned inhibition involved activity across a distinct set of cortico-striatal regions, which aligned with known subcortical circuits of the basal ganglia. Further, though showing similar behavioural responses to the inhibitor, the standard safety signal evoked no subcortical engagement, and instead was associated with an expanse of cortical activity, consistent with regions observed in differential fear-safety processing. In total, these studies indicate that the framework of Pavlovian conditioned inhibition can serve as an experimental model for characterising safety learning in humans, with implications for clinical translational work. It suggests that robust safety learning occurs by way of expectancy violation, or in other words, that a stimulus acquires safety value by predicting the unexpected omission of threat, in line with the principles of formal learning theory. Further, though current human studies often emphasise the safety-learning roles of various higher prefrontal regions, Study 2 demonstrates that safety learn- ing engages several subcortical brain regions that are well-known for their involvement in other domains of reinforcement learning. I discuss the theoretical implications that this research has for defining safety through the lens of associative learning, the neurobiology of safety acquisition, and a basis for separating processes of safety acquisition and safety expression between associative subcortical systems and higher cortical brain regions respectively. Several future directions are proposed for the ongoing study of safety learning, including characterisation of safety prediction errors and testing hypotheses of deficient safety learning in psychiatric disorders.
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ItemNeuroimaging Biomarkers in Generalised Anxiety Disorder, and Associated Modulations Following an Anxiolytic Intervention( 2020)Introduction and Aims Generalised anxiety disorder (GAD) comprises a debilitating cluster of psychological and physiological symptoms that markedly impairs quality of life. GAD is characterised by hallmark cognitions of persistent worry and anticipatory anxiety. Evidence exists for dysregulation in excitatory/inhibitory neurobiological pathways in prefrontal and limbic brain regions, with the dorsal anterior cingulate cortex an area of particular interest. However, limited research exists assessing regional activations and the role of metabolites such as gamma-aminobutyric acid in these regions, nor modulations as a function of treatment. The aim of the thesis was to investigate the functional and metabolic features of this region, and to assess the role of neuroimaging biomarkers of anxiolytic treatment response. Methods Two investigations were conducted utilising structural features of the region of interest: task-based functional magnetic resonance blood oxygen level-dependant signal activation and GABA levels via magnetic resonance spectroscopy together with relevant psychometric and psychiatric measures. The first study was a cross-sectional investigation undertaken to compare neuroimaging biomarkers in 41 participants with GAD with 35 healthy control participants. The second study was an 8-week RCT sub-study involving 41 participants randomised to either daily 240mg of kavalactones Piper methysticum (Kava) extract or a matching placebo. This proof-of-concept study assessed the aforementioned outcomes and whether these markers signal the plant’s anxiolytic activity. Results The results of the first investigation did not reveal group differences in GABA level (p = .302). The relationship between GABA and anxiety severity was different for each group; a significant positive correlation in GAD (e.g., HAM-A, p = .018) and a negative correlation in healthy controls (e.g., trait anxiety, p = .019). The functional task was successful in eliciting regional BOLD signal differences between valent congruency conditions. Two regions exhibited significant group differences (at p < .05), showing hyperactivation in GAD and reduced activation in healthy controls. In both groups BOLD signal significantly predicted severity of state anxiety (GAD p = .027; HC p = .041). Gender, age, and comorbidity in the GAD group also influenced the biomarker-anxiety relationships. The results of the second study showed that Kava treatment was associated with a reduction to GABA levels at eight weeks (p = .049). The treatment was not associated with anxiety symptom, nor fMRI signal change, measured at eight weeks. Discussion This research investigated regional brain properties in GAD for biomarker utility, before testing them in a ‘proof of concept’ study using the purported anxiolytic agent, Kava. Metabolic and functional data were successful in producing differences in the dorsal ACC that could be (if replicated in a larger study) be utilised as biomarkers to aid in the management of GAD symptoms. Limitations of the studies were small sample sizes, GABA signal quality and equivocal toolbox results. The neurobiological effects of Kava have not been directly studied using MRI imaging in humans. The findings of a reduction to GABA levels after treatment may potentially reflect a normalising of the GABA system similar to healthy control data observed in the first study. GAD is a prevalent psychiatric disorder that is under-diagnosed and under-treated. While a great deal of work is inherent in establishing biomarkers for clinical benefit, this research contributes MRI evidence of biological differences, and insight into the mechanisms of Kava, together with a translational rationale for the study of novel anxiolytics as potential GAD treatments. The outcomes and findings of this research fit well with the current affective disorder literature and exceed contemporary work in the field of GAD biomarker and treatment research.