ASSESSMENT OF PSYCHOLOGICAL AND PSYCHOPHYSIOLOGICAL CHARACTERISTICS INVOLVED IN THE MODULATION OF ENDOGENOUS PAIN CONTROL PATHWAYS AND IN THE INDUCTION OF PARADOXICAL PAIN

The present doctoral thesis involves three experimental studies on pain processing and its modulation by psychological mechanisms. The first investigation focused on the relationship between associative learning aspects and the endogenous pain control system referred to as diffuse noxious inhibitory controls (DNIC) or conditioned pain modulation (CPM). The aim of the study consisted in uncovering whether descending pain inhibition may depend on specific environmental or circumstantial cues that have been linked to a reduction of pain sensations through associative learning in pain treatment contexts. A heterotopic noxious counter-stimulation (HNCS) was used to trigger the endogenous pain control system in the experimental context. For the study of the potential impact of associative learning on the pain-inhibits-pain phenomenon, a respondent (Pavlovian) conditioning procedure was realized during the HNCS stimulation. It could be shown that the repeated pairing of a phone signal (conditioned stimulus, CS) with the unconditioned tonic pain stimulus (UCS, HNCS) enabled the CS in the post-conditioning phase to generate a DNIC-like effect similar to the one induced by the tonic pain stimulus. The results demonstrated that learning processes are able to influence endogenous pain modulation processes and decreases in pain perceptions and reflex activity. A thermal grill paradigm was used in the second and third study to examine the influence of psychological characteristics on the individual disposition to display the thermal grill illusion (TGI) of pain. First, the impact of pain-related personality traits like trait anxiety, pain catastrophizing, rumination, pessimism, expectancy of pain, suggestibility and interoceptive accuracy on inter-individual differences in paradoxical pain sensitivity was assessed. Second, the potential influence of dispositional self-regulation ability on illusive pain perceptions was measured. Vagally mediated heart rate variability (HRV) at rest was used as an index of individual self-regulatory strength. The results allowed identifying several psychological factors that are substantially affecting thermal grill-related perceptions. Mainly ruminative and interoceptive accuracy features increased the probability of paradoxical pain perceptions. The likelihood of the TGI elicitation was in addition significantly affected by the magnitude of the HRV-indicator respiratory sinus arrhythmia (RSA). Since thermal grill and neuropathic pain-related pain processing share common neural pathways, the identified psychological effects may be relevant in the context of pathological pain conditions