Attention effects on vicarious modulation of nociception and pain

Vicarious facilitation of facial responses to pain

INTRODUCTION

Observing facial expressions of pain has been shown to lead to increased subjective, neural and autonomic pain responses. Surprisingly, these vicarious facilitation effects on its corresponding response channel, namely facial responses to pain have mostly been neglected. We aim to examine whether the prior exposure to facial expressions of pain leads to a facilitation of facial responses to experimental pain; and whether this facilitation is linked to the valence (pain vs. neutral expression) or also linked to specific motor-features of the facial pain expressions (different facial muscle movements).

METHOD

Subjective (intensity and unpleasantness ratings) and facial responses (Facial Action Coding System) of 64 participants (34 female) to painful and non-painful heat stimuli were assessed. Before each heat stimulus, video clips of computer-generated facial expressions (three different pain expressions and a neutral expression) were presented.

RESULTS

The prior exposure to facial expressions of pain led to increased subjective and facial responses to pain. Further, vicarious pain facilitation of facial responses was significantly correlated with facilitation of unpleasantness ratings. We also found evidence that this vicarious facilitation of facial responses was not only linked to the presentation of pain versus neutral expressions but also to specific motor-features of the pain cue (increase in congruent facial muscle movements).

DISCUSSION

Vicarious pain facilitation was found for subjective and facial responses to pain. The results are discussed with reference to the motivational priming hypothesis as well as with reference to motor priming.

SIGNIFICANCE

Our study uncovers evidence that facial pain responses are not only influenced by motivational priming (similar to other types of pain responses), but also by motor-priming. These findings shed light on the complexity - ranging from social, affective and motor mechanisms - underling vicarious facilitation of pain.

Brain responses to the vicarious facilitation of pain by facial expressions of pain and fear

Observing pain in others facilitates self-pain in the observer. Vicarious pain facilitation mechanisms are poorly understood. We scanned 21 subjects while they observed pain, fear and neutral dynamic facial expressions. In 33% of the trials, a noxious electrical stimulus was delivered. The nociceptive flexion reflex (NFR) and pain ratings were recorded. Both pain and fear expressions increased self-pain ratings (fear > pain) and the NFR amplitude. Enhanced response to self-pain following pain and fear observation involves brain regions including the insula (INS) (pain > fear in anterior part), amygdala, mid-cingulate cortex (MCC), paracentral lobule, precuneus, supplementary motor area and pre-central gyrus. These results are consistent with the motivational priming account where vicarious pain facilitation involves a global enhancement of pain-related responses by negatively valenced stimuli. However, a psychophysiological interaction analysis centered on the left INS revealed increased functional connectivity with the aMCC in response to the painful stimulus following pain observation compared to fear. The opposite connectivity pattern (fear > pain) was observed in the fusiform gyrus, cerebellum (I-IV), lingual gyrus and thalamus, suggesting that pain and fear expressions influence pain-evoked brain responses differentially. Distinctive connectivity patterns demonstrate a stronger effect of pain observation in the cingulo-insular network, which may reflect partly overlapping networks underlying the representation of pain in self and others.

Different priming effects of empathy on neural processing associated with firsthand pain and nonpain perception

The shared-representation model of empathy is still debated. One of the major questions is whether empathy-eliciting stimuli depicting others' pain selectively activate the representations of self-pain. To address this issue, we assessed the priming effects of empathy-eliciting pictures on firsthand pain and nonpain perception, as well as its associated neural processing. In Experiment 1, when compared with nonpainful pictures depicting individuals' body parts with no injury, participants primed by painful pictures showing individuals' body parts with injury reported higher ratings for perceived intensity, unpleasantness, and salience of nociceptive and auditory stimuli, but they only exhibited increased N2 amplitude in response to nociceptive stimuli. In Experiment 2, the results from another group of participants replicated the observations of Experiment 1 and validated the findings in the non-nociceptive somatosensory modality. Importantly, participants' concern ratings for priming pictures predicted their unpleasantness ratings for subsequent nociceptive stimuli, while participants' attention ratings predicted their unpleasantness ratings for subsequent auditory and tactile stimuli. This finding implies that empathy for pain might influence pain and nonpain perception via different psychological mechanisms. In summary, our findings highlight the existence of pain-selective representations in empathy for pain and contribute to a better understanding of the shared-representation model of empathy.

Pharmacological fMRI provides evidence for opioidergic modulation of discrimination of facial pain expressions

The endogenous opioid system is strongly involved in the modulation of pain. However, the potential role of this system in perceiving painful facial expressions from others has not been sufficiently explored as of yet. To elucidate the contribution of the opioid system to the perception of painful facial expressions, we conducted a double‐blind, within‐subjects pharmacological functional magnetic resonance imaging (fMRI) study, in which 42 participants engaged in an emotion discrimination task (pain vs. disgust expressions) in two experimental sessions, receiving either the opioid receptor antagonist naltrexone or an inert substance (placebo). On the behavioral level, participants less frequently judged an expression as pain under naltrexone as compared to placebo. On the neural level, parametric modulation of activation in the (putative) right fusiform face area (FFA), which was correlated with increased pain intensity, was higher under naltrexone than placebo. Regression analyses revealed that brain activity in the right FFA significantly predicted behavioral performance in disambiguating pain from disgust, both under naltrexone and placebo. These findings suggest that reducing opioid system activity decreased participants' sensitivity for facial expressions of pain, and that this was linked to possibly compensatory engagement of processes related to visual perception, rather than to higher level affective processes, and pain regulation.

The behavioral and neural findings of this psychopharmacological fMRI study shed light on a causal role of the opioid system in the discrimination of painful facial expressions, paving the way for further exploration of clinical implications in the domains of pain diagnosis and treatment, on the one hand, and future research on the relationship between basic socio‐perceptual processing and empathy, on the other.

Images depicting human pain increase exercise-induced pain and impair endurance cycling performance

The current study investigated whether viewing images of others in pain influences exercise-induced pain (EIP) and cycling performance. Twenty-one recreational cyclists attended five laboratory visits. The first two visits involved measuring participants' maximal aerobic capacity and familiarized participants to the fixed power (FP) and 16.1 km cycling time trial (TT) tasks. The FP task required participants to cycle at 70% of their maximal aerobic power for 10-minutes. In the subsequent three visits, participants performed the FP and TT tasks after viewing pleasant, ‎painful or neutral images. Participants rated the subset of painful images as more painful than the pleasant and neutral images; with no difference in the pain ratings of the pleasant and neutral images. In the FP task, EIP ratings were higher following painful compared to pleasant images, while no differences in EIP were observed between any other condition . In the TT, performance did not differ between the pleasant and neutral conditions. However, TT performance was reduced after viewing painful images compared to neutral or pleasant images. HR, B[La], perceived exertion and EIP did not differ between the three conditions. These results suggest that viewing painful images decreases TT performance and increases pain during fixed intensity cycling.‎ Abbreviations: EIP: Exercise Induced Pain; FP: Fixed Power; TT: Time Trial; HR:Heart Rate; B[La]: Blood Lactate; RPE: Rating of Perceived Exertion; IAPS: International Affective Picture System; PO: Power Output.

Attentional control moderates the relationship between pain catastrophizing and selective attention to pain faces on the antisaccade task

Cognitive models of chronic pain emphasize the critical role of pain catastrophizing in attentional bias to pain-related stimuli. The aim of this study was (a) to investigate the relationship between pain catastrophizing and the ability to inhibit selective attention to pain-related faces (attentional bias); and (b) to determine whether attentional control moderated this relationship. One hundred and ten pain-free participants completed the anti-saccade task with dynamic facial expressions, specifically painful, angry, happy, and neutral facial expressions and questionnaires including a measure of pain catastrophizing. As predicted, participants with high pain catastrophizing had significantly higher error rates for antisaccade trials with pain faces relative to other facial expressions, indicating a difficulty disinhibiting attention towards painful faces. In moderation analyses, data showed that attentional control moderated the relationship between attentional bias to pain faces and pain catastrophizing. Post-hoc analyses demonstrated that it was shifting attention (not focusing) that accounted for this effect. Only for those with high self-reported ability to shift attention was there a significant relationship between catastrophizing and attentional bias to pain. These findings confirm that attentional control is necessary for an association between attentional bias and catastrophizing to be observed, which may explain the lack of relationships between attentional bias and individual characteristics, such as catastrophizing, in prior research.

Observers' impression of the person in pain influences their pain estimation and tendency to help

BACKGROUND

Estimation of a patient's pain may have a considerable impact on the level of care that patient receives. Many studies have shown that contextual factors may influence an observer's pain estimation. Here, we investigate the effect of an observer's impression of a person in pain and justification of his/her pain on the observer's pain estimation, tendency to help and perceived empathy.

METHODS

Thirty healthy individuals (half females) read scenarios aimed to manipulate the reader's impression of characters who ultimately were fired from their work (four positive characters and four negative; half females). Then they observed 1-s videos of four levels of pain expression (neutral, mild, moderate, strong) in those characters during an examination. Subsequently, they rated pain estimation, tendency to help and perceived empathy. Afterwards, they rated their likability of characters and how just they find the end of story.

RESULTS

People rated pain in positive characters higher than the pain in negative characters. They also expressed more tendency to help and a higher level of perceived empathy towards positive characters than negative characters. For the highest level of pain in positive characters, perceived injustice towards that person was the best predictor of the observer's pain estimation, tendency to help and perceived empathy. For negative characters, dislikeability was the best predictor of tendency to help and perceived empathy. Justification of their pain was a predictor of pain estimation and tendency to help.

CONCLUSION

Observers used different information to evaluate pain in positive and negative individuals.

SIGNIFICANCE

Observers' estimation of pain, perceived empathy and tendency to help biases by their understanding of the characteristics of the person in pain. In clinical settings, these biases may influence the quality of care and well-being of patients. Understanding the underlying mechanisms of these biases can help us improve the quality of care and control the effect of prejudices on pain assessment.

Development, validation and utility of a simulation model of the nociceptive flexion reflex threshold

OBJECTIVE

A variety of algorithms is used for nociceptive flexion reflex threshold (NFRT) estimation, but their estimation accuracy is unknown. We developed a computer based simulation model of the NFRT to quantify and compare the accuracy of available estimation algorithms.

METHODS

This simulation model is based on basic characteristics of the NFRT and specified by data collected from 60 healthy volunteers. We validated the model by comparing simulated data with data obtained independently in another volunteer population. The model was used to quantify the accuracy of previously published NFRT estimation algorithm for three NFRT variabilities representing sensory deprivation, distraction and general anaesthesia.

RESULTS

The dynamic staircase algorithm obtained most accurate NFRT estimates during all NFRT variabilities. The number of stimuli applied can be chosen higher to increase estimate precision or lower to reduce measurement time.

CONCLUSIONS

Our simulation model is a valid tool to measure the accuracy of NFRT estimation algorithms. It can be applied to analyse and develop algorithms. The dynamic staircase algorithm shows the highest precision in NFRT estimation and is recommended for NFRT studies.

SIGNIFICANCE

Using optimized NFRT estimation algorithms increases precision in clinical and experimental NFRT studies and might therefore reduce the measurement effort necessary.

On the Origin of Interoception

Over the course of a century, the meaning of interoception has changed from the restrictive to the inclusive. In its inclusive sense, it bears relevance to every individual via its link to emotion, decision making, time-perception, health, pain, and various other areas of life. While the label for the perception of the body state changes over time, the need for an overarching concept remains. Many aspects can make any particular interoceptive sensation unique and distinct from any other interoceptive sensation. This can range from the sense of agency, to the physical cause of a sensation, the ontogenetic origin, the efferent innervation, and afferent pathways of the tissue involved amongst others. In its overarching meaning, interoception primarily is a product of the central nervous system, a construct based on an integration of various sources, not per se including afferent information. This paper proposes a definition of interoception as based on subjective experience, and pleas for the use of specific vocabulary in addressing the many aspects that contribute to it.

Sub-optimal presentation of painful facial expressions enhances readiness for action and pain perception following electrocutaneous stimulation

Observation of others’ painful facial expressions has been shown to facilitate behavioral response tendencies and to increase pain perception in the observer. However, in previous studies, expressions were clearly visible to the observer and none of those studies investigated the effect of presence of peripheral stimulation on response tendencies. This study focuses on the effect of sub-optimal presentation of painful facial expressions in the presence and absence of an electrocutaneous stimulus. Twenty-two healthy individuals categorized arrow targets which were preceded by a sub-optimally presented facial expression (painful, happy, or neutral in different blocks). On half of the trials, aversive electrocutaneous stimulation was delivered to the wrist of the non-dominant hand between the presentation of facial expression and target (an arrow directing to right or left). Participants’ task was to indicate direction of the arrow as soon as it appears on the screen by pressing the corresponding key on the keyboard and to rate their pain at the end of block. Analysis showed that responses were faster to targets preceded by aversive stimulation than to targets not preceded by stimulation, especially following painful expressions. Painfulness ratings were higher following painful expressions than following happy expressions. These findings suggest that sub-optimally presented painful expressions can enhance readiness to act to neutral, non-pain-related targets after aversive stimulation and can increase pain perception.