The role of the pons in processing affective information

Abstract

Emotions are essential for human beings to motivate behaviours crucial for reproduction and survival, signified by a multitude of theories and models aimed to understand how affective information is processed. Despite these attempts to tease apart the roles of various brain structures involved in affective processing, the mechanisms underlying the fast and unconscious processing of affective information remains unclear. Recent findings in both animals and humans have suggested the involvement of the pons and other brain stem structures in affective processing, given the capacity of these regions in eliciting rapid responses essential for natural survival. However, information regarding the functional roles of the pons remains scarce. This thesis utilised structural and functional magnetic resonance imaging (fMRI) to probe involvement of the pons in affective processing and how it contributes towards major depressive disorder (MDD). Recent animal studies have identified a direct pathway from the retina to the dorsal raphe nucleus capable of regulating affective behaviours. The raphe nucleus is a structure spanning across the brain stem partially encapsulated by the pons in the human brain. Given this background, Chapter Two reported a diffusion weighted imaging study hypothesising that a fibre tract between the optic chiasm and the brain stem may contribute to affective processing in humans. Deterministic tractography revealed that fractional anisotropy of the fibre bundle identified were correlated with ratings of fear and sad stimuli. Probabilistic tractography provided evidence that this fibre tract corresponds to the abducens, a cranial nerve connecting the eyes and the pons. Using task-based fMRI and dynamic causal modelling, Chapter Three extended this work by exploring whether the pons functions together with other cortico-limbic affective structures when processing visual affective stimuli. Results indicated that the pons was involved at an earlier stage, and projects to other cortico-limbic affective regions in a feedforward manner. The optimal model also demonstrates specificity towards negative stimuli of fear and sadness. To further investigate the negative specificity observed in the model, Chapter Four employed high-resolution fMRI data and multivariate pattern analysis to investigate whether the network involving the pons and other cortico-limbic affective brain regions process fear and sad visual affective information differently. Findings revealed that the brain regions involved in processing fear and sadness were largely overlapped. Granger causality mapping results provided evidence that the pons receives directed influence and acts towards delivering behavioural outcomes. Considering patients suffering from MDD exhibit biases towards sadness, Chapter Five examines the previously identified pons-cortico-limbic network for sadness processing in a clinically depressed sample. Elevated coupling in resting-state functional connectivity between the pons and the amygdala was observed in depressed patients compared to healthy controls. Overall, findings of these four studies affirm the involvement of the pons in affective processing, and its relevance towards the manifestation of depression. The pons may have an initial role in relaying negative visual affective information to the amygdala and other cortico-limbic affective regions, followed by receiving feedback to deliver behavioural outcomes. This neural mechanism may underlie the negative biases precipitating in clinically depressed patients.