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Previous research has suggested that heart rate variability (HRV; the variability in the interval between successive heartbeats) can predict the perception of experimentally-induced pain (i.e., pain sensitivity). However, little research has sought to investigate sex-specific associations between HRV and pain sensitivity. This is an important consideration, given that sex differences in resting HRV have been observed, and there has been extensive debate about sex differences in sensitivity to experimentally-induced pain. We examined whether the association between resting HRV and sensitivity to experimentally-induced pain differed in men and women. Fifty-one pain free individuals (26 women, mean age = 21.9 years) participated. Resting electrocardiography (ECG) was collected during a paced breathing task (15 cycles per minute), and measures of HRV were extracted via Fast Fourier Transformation. Thermal heat pain threshold (i.e., the point at which the sensation was first perceived as painful, rather than warm) was measured with a Medoc Pathway Pain and Sensory Evaluation System. There were no sex differences in resting HRV or thermal heat pain threshold, nor did sex moderate the relationship between HRV and thermal heat pain threshold. However, there were significant positive relationships between thermal heat pain threshold and LF-HRV (r = 0.47), and HF-HRV (r = 0.43) in men, but not in women. The results suggest that higher pain threshold appears to be related to greater engagement of the inhibitory parasympathetic nervous system in men only, and that other biopsychosocial mechanisms may contribute to experimental pain experience in women. Future research is needed to study these mechanisms further, accounting for other factors known to influence nociceptive and cardiovascular regulatory processes (e.g., ethnicity, hormones)., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Many factors can influence the way in which we perceive painful events and noxious stimuli, but less is known about how pain perception is altered by explicit knowledge about the impending sensation. This study aimed to investigate the impact of explicit cueing on anxiety, arousal, and pain experience during the anticipation and delivery of noxious thermal heat stimulations. Fifty-two healthy volunteers were randomised to receive explicit instructions about visual cue-stimulus temperature pairings, or no explicit instructions about the cue-stimulus pairs. A pain anxiety task was used to investigate the effects of explicit cueing on anticipatory anxiety, pain experience and electrophysiological responses. Participants who received explicit instructions about the cue-stimulus pairs (i.e., the relationship between the colour of the cue and the temperature of the associated stimuli) reported significantly higher subjective anxiety prior to the delivery of the thermal heat stimuli (p = .025, partial eta squared = .10). There were no effects of explicit cueing on subsequent pain intensity, unpleasantness, or the electrophysiological response to stimulus delivery. The perceived intensity and unpleasantness of the stimuli decreased across the blocks of the paradigm. In both groups anticipating the ambiguous cue elicited the largest change in electrophysiological arousal, indicating that not knowing the impending stimulus temperature led to increased arousal, compared to being certain of receiving a high temperature thermal stimulus (both p < .001). Perceived stimulus intensity varied between ambiguous and non-ambiguous cues, depending on the temperature of the stimulus. Together these findings highlight the impact and importance of explicit cueing and uncertainty in experimental pain studies, and how these factors influence the way healthy individuals perceive and react to noxious and innocuous thermal stimuli.

Background: Traumatic injury can lead to loss, suffering and feelings of injustice. Previous research has shown that perceived injustice is associated with poorer physical and mental wellbeing in persons with chronic pain. This study aimed to identify the relative association between injury, compensation and pain-related characteristics and perceived injustice 12-months after traumatic injury., Methods: 433 participants were recruited from the Victorian Orthopedic Trauma Outcomes Registry and Victorian State Trauma Registry, and completed questionnaires at 12-14 months after injury as part of an observational cohort study. Using hierarchical linear regression we examined the relationships between baseline demographics (sex, age, education, comorbidities), injury (injury severity, hospital length of stay), compensation (compensation status, fault, lawyer involvement), and health outcomes (SF-12) and perceived injustice. We then examined how much additional variance in perceived injustice was related to worse pain severity, interference, self-efficacy, catastrophizing, kinesiophobia or disability., Results: Only a small portion of variance in perceived injustice was related to baseline demographics (especially education level), and injury severity. Attribution of fault to another, consulting a lawyer, health-related quality of life, disability and the severity of pain-related cognitions explained the majority of variance in perceived injustice. While univariate analyses showed that compensable injury led to higher perceptions of injustice, this did not remain significant when adjusting for all other factors, including fault attribution and consulting a lawyer., Conclusions: In addition to the "justice" aspects of traumatic injury, the health impacts of injury, emotional distress related to pain (catastrophizing), and the perceived impact of pain on activity (pain self-efficacy), had stronger associations with perceptions of injustice than either injury or pain severity. To attenuate the likelihood of poor recovery from injury, clinical interventions that support restoration of health-related quality of life, and adjustment to the impacts of trauma are needed.

Background: There is evidence that sensitivity to noxious stimuli differs between the sexes and across the body, but few studies have investigated differences in the perception and experience of acute pain stimuli across the body in healthy individuals., Methods: We recruited 52 healthy participants, aged 18-36 (50% men) and administered 39, 42 and 45 °C stimuli at four body sites bilaterally to examine differences in the experience of pain intensity and unpleasantness between body sites via an 11-point numerical rating scale., Results: Noxious and innocuous thermal heat stimuli were perceived as significantly more intense when delivered to the wrist (M = 3.98, SD = 1.93) and back (M = 4.07, SD = 1.98) compared to the shoulder (M = 3.45, SD = 1.91) and leg (M = 3.46, SD = 1.87). Pain unpleasantness ratings yielded similar findings; stimuli were perceived as more unpleasant when administered to the wrist (M = 2.83, SD = 1.93) and lower back (M = 3.04, SD = 2.11) compared to the shoulder (M = 2.63, SD = 1.85) and leg (M = 2.26, SD = 1.82)., Conclusions: These findings suggest that painful thermal stimuli delivered to the wrist and back are perceived as more intense and unpleasant compared with other body sites in healthy persons. These differences may be due to variations in receptor density, or the relative importance of these sites for daily living and survival., Significance: Moreover, these insights are helpful for the design of studies investigating pain experience in healthy persons in experimental or clinical settings. WHAT DOES THIS STUDY ADD?: We tested sensitivity to acute suprathreshold thermal stimulations across a range of body sites to investigate for potential variability. We found significant differences in the perceived intensity and unpleasantness of noxious and innocuous thermal stimuli at the wrist and lower back, compared with the shoulder and leg. These results suggest that pain experience is driven by receptor density or the relative functional importance of these sites., (© 2016 European Pain Federation - EFIC®.)

Although the experience of vicarious sensations when observing another in pain have been described postamputation, the underlying mechanisms are unknown. We investigated whether vicarious sensations are related to posttraumatic stress disorder (PTSD) symptoms and chronic pain. In Study 1, 236 amputees completed questionnaires about phantom limb phenomena and vicarious sensations to both innocuous and painful sensory experiences of others. There was a 10.2% incidence of vicarious sensations, which was significantly more prevalent in amputees reporting PTSD-like experiences, particularly increased arousal and reexperiencing the event that led to amputation (φ = .16). In Study 2, 63 amputees completed the Empathy for Pain Scale and PTSD Checklist-Civilian Version. Cluster analyses revealed 3 groups: 1 group did not experience vicarious pain or PTSD symptoms, and 2 groups were vicarious pain responders, but only 1 had increased PTSD symptoms. Only the latter group showed increased chronic pain severity compared with the nonresponder group (p = .025) with a moderate effect size (r = .35). The findings from both studies implicated an overlap, but also divergence, between PTSD symptoms and vicarious pain reactivity postamputation. Maladaptive mechanisms implicated in severe chronic pain and physical reactivity posttrauma may increase the incidence of vicarious reactivity to the pain of others., (Copyright © 2015 International Society for Traumatic Stress Studies.)

Background: Through two studies, we introduce and validate the Empathy for Pain Scale (EPS), which characterizes the phenomenology of empathy for pain, including the vicarious experience of pain when seeing others in pain., Methods: In study 1, 406 individuals completed the EPS and Interpersonal Reactivity Index (IRI). In the EPS, four painful scenarios (witnessing surgery, patient recovering from surgery, assault and accidental injury) were rated for 12 emotional, empathic and sensory responses. In study 2, 59 participants completed the same questionnaires and then watched and rated videos of sporting injuries., Results: In study 1, we identified three factors of the EPS with principal component analysis, which were validated with confirmatory factor analysis: affective distress; vicarious pain; and empathic concern. The EPS demonstrated good psychometric properties, re-test reliability (n = 105) and concurrent validity. In study 2, we validated the EPS against empathic reactions to the pain of others as displayed in video clips depicting sporting injuries and showed that the scale has unique utility to characterize empathic reactions to pain above general trait empathy measures. Both studies showed that the affective distress and empathic concern subscales of the EPS correlated with measures of cognitive and affective empathy from the IRI, whereas the vicarious pain subscale was only correlated with the personal distress IRI subscale., Conclusions: The EPS is a psychometrically sound new scale that characterizes empathy for pain and vicarious pain. The EPS offers valuable insight to the phenomenological profile of the affective, empathic and sensory dimensions of empathy for pain., (© 2014 European Pain Federation - EFIC®)

'Mirror pain' describes when the observation of another's pain experience induces a personal experience of pain. It has been suggested that mirror pain could result from changes in neural excitability or inhibition. In this study we used transcranial magnetic stimulation (TMS) to investigate motor cortical excitability in lower-limb amputees who experience mirror pain. Using paired-pulse TMS to assess motor cortical inhibition (CI) and cortical facilitation (CF), recordings were taken from the right first dorsal interosseus in lower-limb amputees who experience mirror pain (MP+), lower-limb amputees who do not experience mirror pain (MP-), and non-amputee controls. No differences in CI or CF were observed between the MP+ and both control groups. Thus, when not paired with a pain-related stimulus, changes in motor cortical excitability do not appear to contribute to the experience of mirror pain in lower-limb amputees., (Copyright © 2012 Elsevier Ireland Ltd. All rights reserved.)

Observing noxious injury to another's hand is known to induce corticospinal inhibition that can be measured in the observer's corresponding muscle. Here, we investigated whether acquired pain synesthetes, individuals who experience actual pain when observing injury to another, demonstrate less corticospinal inhibition than do controls during pain observation, as a potential mechanism for the experience of vicarious pain. We recorded motor-evoked potentials (MEPs) induced at two time points through transcranial magnetic stimulation while participants observed videos of a hand at rest, a hypodermic needle penetrating the skin, a Q-tip touching the skin, and a hypodermic needle penetrating an apple. We compared MEPs in three groups: 7 amputees who experience pain synesthesia, 11 nonsynesthete amputees who experience phantom limb pain, and 10 healthy controls. Results indicated that the pain synesthete group demonstrated significantly enhanced MEP response to the needle penetrating the hand, relative to the needle not having yet penetrated the hand, as compared with controls. This effect was not observed exclusively in the same muscle where noxious stimulation was applied. We speculate that our findings reflect a generalized response to pain observation arising from hyperactivity of motor mirror neurons not involved in direct one-to-one simulation but, rather, in the representation of another's experience.

There are increasing reports of people experiencing pain when observing pain in another. This describes the phenomenon of synaesthetic pain which, until recently, had been primarily reported in amputees with phantom pain. In the current study, we used electroencephalography (EEG) to investigate how amputees who experience synaesthetic pain process pain observed in another. Participants were grouped according to amputees who experience phantom and synaesthetic pain (n=8), amputees who experience phantom pain but not synaesthetic pain (n=10) and healthy controls (n=10). Participants underwent EEG as they observed still images of hands and feet in potentially painful and non-painful situations. We found that pain synaesthetes showed some reduced event-related potential (ERP) components at certain electrode sites, and reduced theta- and alpha band power amplitude at a central electrode. The finding of reduced ERP amplitude and theta band power may reflect inhibition of the processing of observed pain (e.g. avoidance/guarding as a protective strategy), and reduced alpha band power may indicate a disinhibition in control processes that may result in synaesthetic pain. These results provide the first documentation of atypical neurophysiological activity in amputees who experience synaesthetic pain when processing pain in another., (© The Author (2011). Published by Oxford University Press.)

Phantom phenomena are frequent following amputation, but how this often painful experience is modified or triggered by spontaneous events or sensations often puzzles amputees and clinicians alike. We explored triggers of phantom phenomena in a heterogeneous sample of 264 upper and lower limb adult amputees with phantom sensations. Participants completed a structured questionnaire to determine the prevalence and nature of the triggers of phantom phenomena. The four categories of triggers identified include: (a) a quarter of participants experiencing psychological, emotional or autonomic triggers; (b) half experiencing behavioral triggers, "forgetting" the limb's absence and attempting to use the phantom; (c) one-fifth experiencing weather-induced triggers; and (d) one-third experiencing sensations referred from parts of the body. Upper limb amputees; and were more likely to experience weather-induced phantom phenomena than lower limb amputees; and upper and lower limb amputees were equally likely to experience referred sensations from the genitals, contradicting the homuncular remapping hypothesis. Traumatic amputees were more likely to report emotional triggers. Further, while those with emotional triggers exhibited poorer acceptance of the limitations of amputation, they were more likely to employ adaptive coping mechanisms. Finally, habitual "forgetting" behaviors were most common soon after amputation, whereas other more adaptive schemata (e.g., self-defense) were equally likely to be performed at any time following amputation. Various likely inter-related mechanisms are discussed in relation to phantom triggers. Ultimately, optimizing stump and neuroma management, as well as restoring function of central networks for pain, limb movement, and amputation-related memories, should help manage spontaneously triggered phantom phenomena., (Copyright © 2011 European Federation of International Association for the Study of Pain Chapters. All rights reserved.)

Synaesthesia for pain is a phenomenon where a person experiences pain when observing or imagining another in pain. Anecdotal reports of this type of experience have most commonly occurred in individuals who have lost a limb. Distinct from phantom pain, synaesthesia for pain is triggered specifically in response to pain in another. Here, we provide the first preliminary investigation into synaesthesia for pain in amputees to determine the incidence and characteristics of this intriguing phenomenon. Self-referring amputees (n=74) answered questions on synaesthesia for pain within a broader survey of phantom pain. Of the participants, 16.2% reported that observing or imagining pain in another person triggers their phantom pain. Further understanding of synaesthesia for pain may provide a greater insight to abnormal empathic function in clinical populations as well as therapeutic intervention for at risk groups., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

This paper reviews the current literature on "empathy for pain", the ability to understand pain observed in another person, in the context of a newly documented form of pain empathy "synaesthesia for pain". In synaesthesia for pain a person not only empathises with another's pain but experiences the observed or imagined pain as if it was their own. Neural mechanisms potentially involved in synaesthesia for pain include "mirror systems": neural systems active both when observing an action, or experiencing an emotion or sensation and when executing the same action, or personally experiencing the same emotion or sensation. For example, we may know that someone is in pain in part because observation activates similar neural networks as if we were experiencing that pain ourselves. We propose that synaesthesia for pain may be the result of painful and/or traumatic experiences causing disinhibition in the mirror system underlying empathy for pain. We will discuss this theory in the context of a documented group of amputees who experience synaesthesia for pain in phantom limbs., ((c) 2009 Elsevier Ltd. All rights reserved.)

Pain synaesthetes experience pain in a presensitised region when observing or imagining another person in pain. We conducted an upper-limb embodiment study using a modified rubber-hand illusion in which lower-limb amputees originally participated as control subjects for the upper-limb amputees. While we found all subjects experienced topographic illusory sensations, we also serendipitously found that lower-limb amputee pain synaesthetes experienced pain or a motor response in their phantom leg when the embodied hand was threatened (e.g., with a retractable knife, mousetrap, or syringe) or submitted to high-frequency stimulation (e.g., vibration). Embodiment illusions were brought about by touching, manipulating, or threatening a rubber or real hand which was observed through a mirror so that it was superimposed upon the target hand (phantom hand for upper-limb amputees, or real hand in others). Participants included eight pain synaesthetes (six lower-limb amputees, one upper-limb amputee, and one nonamputee), and thirty-one controls (eight lower-limb amputees, twelve upper-limb amputees, and eleven nonamputees). We documented participant's subjective reports, together with quantitative measures including the Questionnaire Measure of Emotional Empathy. We found no association between pain synaesthesia and empathy scores. On the basis of related literature we suggest that pain synaesthetes likely experienced phantom-leg pain because (a) the motor system was already engaged during visual capture; (b) threatening stimuli, to which they are hyper-vigilant, triggered avoidance or 'escape' motor schemata; and (c) there could be no feedback confirming that initiated motor schemata for the phantom limb were successfully performed. Ultimately, we have further defined this new condition, synaesthesia for pain, as not only having a sensory pain component, but also a key motor component, manifesting itself in avoidance, contraction, and withdrawal 'actions'.