Effects of Fear Conditioning on Pain : Moderation by Mindfulness and the HPA-axis
A vicious cycle through which fear and pain maintain each other explains the development and maintenance of disorders involving fear conditioning. While the behavioral processes and neurobiological circuits of fear conditioning have been extensively studied, the effects of fear learning on pain remain poorly understood. The objectives of this thesis were to examine the effects of fear conditioning on the neuropsychophysiology of pain, and the factors that could moderate these effects. The effects of fear learning on pain were examined in Study 1 in 47 human participants during a delay Pavlovian classical fear conditioning task. Conditioned stimuli were abstract visual cues that co-terminated with a painful electric shock on 50% of trials. Pain ratings and the spinal nociceptive flexion reflex were recorded in response to each US, and anticipatory skin conductance responses were recorded to each CS. A computational model of reinforcement learning was fitted to anticipatory SCRs and used to estimate fear learning parameters of expected shock probabilities and associability (uncertainty) to each CS+ paired. Both fear learning parameters positively predicted pain responses. These effects operated in part directly on pain ratings, and in part indirectly by facilitating ascending spinal nociceptive activity. The results also showed that the mediation of the effects of fear learning on pain by spinal nociception was enhanced for individuals reporting more trait harm vigilance, and decreased for individuals reporting more emotional detachment. In Study 2, we investigated the role of long term mindfulness meditation experience on the effects of fear learning on pain. Eleven experienced meditators (>1000 hours of experience) were tested using the same experimental and analysis protocol as in Study1, and were compared with the meditation-naïve participants from Study1. Compared to controls, experienced meditators showed an overall reduction in pain ratings during fear learning, as well as reduced effects of learning parameters on pain. No effects of fear learning on lower-level spinal or anticipatory learning responses were observed. Finally, Study 3 examined how individual differences in HPA axis reactivity, operationalized by the level of cortisol secreted during the task, affected pain modulation induced by fear-learning (N=23). A similar experimental and analysis protocol as in Studies 1-2 was used with an additional visual CS paired with the US on 100% of trials. Individuals with greater cortisol output during fear conditioning reported a global decrease in pain during the task, and showed a facilitation of defensive spinal responses via fear learning mechanisms. The results of this thesis support the notion of a vicious fear-pain cycle with neuropsychophysiological evidence, and show that this cycle is moderated by certain personality traits, meditation experience, and individual differences in HPA reactivity. Our results also highlight the role of techniques based on acceptation and mindfulness meditation to break the fear-pain cycle and prevent/treat pathological manifestations of repeated threat exposure (eg. anxiety, chronic pain).
fear conditioning reinforcement learning models nociceptive flexion reflex mindfulness meditation cortisol pain