DYNAMIC CHANGES IN HEART RATE VARIABILITY UNDER THREAT: EXPLORING THE EFFECTS OF EMOTION REGULATION ON THE PARASYMPATHETIC NERVOUS SYSTEM
How an organism responds to the fluctuating metabolic demands imposed by the environment – that is, self-regulates – is crucial to its success. Several theorists argue that this self-regulation depends on the connection between brain and heart via cardiac vagal control. The efficiency/integrity of this brain–¬¬¬heart link is reflected in certain measures of heart-rate variability (HRV). Although trait-like HRV measured under resting conditions is often linked to the ability to flexibly regulate emotions, we are yet to fully understand the dynamic changes in cardiac vagal control that occur during emotional challenge (i.e. reactivity and recovery), or the factors that modulate this response. One potential modulating factor is the use of antecedent- and response-focused emotion regulation strategies, i.e. expressive suppression and cognitive reappraisal, respectively. Another is a history of non-suicidal self-injurious behaviour. I conducted three studies analysing continuous HRV recorded before, during and after exposure to a social stressor (Study 1) or a VR plank-walking simulation (Studies 2 and 3). In Study 1, we recorded history of self-injurious behaviour, and in Study 3, we manipulated the use of emotion regulation strategies via explicit instructions. Across studies, findings consistently support a view of vagal withdrawal being a component of the response to emotional challenge. When using individual differences in resting vagal control to predict vagal reactivity, a link was found in Study 1 which points to higher resting levels as a marker for greater vagal withdrawal during social stress (but was not replicated in studies 2 and 3). Self-reported use of emotion regulation strategies during the emotional challenge bore no relationship to vagal reactivity, and neither did a history of self-injurious behaviour. However, relative to the instructed suppression condition, participants in the instructed reappraisal group showed greater vagal withdrawal during the plank-walk. While inconclusive, this thesis points to a role for flexible withdrawal of vagal control in adaptive functioning and provides an important contribution to the scarce literature on dynamic HRV in the context of emotion regulation. The thesis also includes a discussion of theory and the need for more work to explain the complexities of this relationship, as well as crucial methodological factors which future researchers might consider.