Investigating the relationship between pain indicators and observers' judgements of pain

Social Learning of Placebo Effects in Pain: A Critical Review of the Literature and a Proposed Revised Model

Relatively recently, in 2009, experimental studies were undertaken to determine the role of social observational learning in forming hypoalgesic, analgesic and hyperalgesic responses to a placebo. The research findings obtained in studies published before 2018 were integrated and formed the basis of the theoretical model of social learning of placebo effects in pain proposed by Bajcar and Bąbel. This model considered the involvement of different types of modeling (ie, behavioral modeling, symbolic modeling, and verbal modeling) in shaping placebo hypoalgesia/analgesia and nocebo hyperalgesia. The model assumed that pain expectancies might be involved in observationally induced placebo effects in pain and that the effectiveness of observational learning in shaping placebo effects could be moderated by the observer's dispositions, especially empathy. Based on the latest research data, we propose a modified and significantly extended version of this model. The revised model includes the involvement of particular types of modeling in placebo effects and their role in shaping conscious pain-related expectancies. It explains the role of dispositional empathy in shaping observationally induced placebo effects. Notably, the extended version of the model considers the contribution of the characteristics of the observed person to the magnitude of placebo effects induced by social learning. PERSPECTIVE: The paper proposes a comprehensive theoretical approach to explaining the role of observational learning in shaping placebo effects in pain. The proposed model emphasizes the potential of this form of learning in shaping placebo responses and indicates factors that can modify the effectiveness of observational learning.

Pain Rating is Worth a Thousand Words: Nocebo Hyperalgesia Induced by Verbal Modeling Prevails Over the Effects of Symbolic Modeling and Verbal Suggestion

This study compares the effectiveness of verbal modeling, symbolic modeling, and verbal suggestion in inducing nocebo hyperalgesia. It is the first study to examine the contribution of stress to observationally induced nocebo hyperalgesia. This study's experimental groups represented various sources of social information: a group of people participating in the study (verbal modeling), a single participant (symbolic modeling), and an experimenter (verbal suggestion). During the experiment, participants received electrocutaneous stimuli at the same intensity, some of which were applied with a nocebo (sham device). Participants in the verbal modeling group were acquainted with pain ratings that had allegedly been provided by other participants. The ratings suggested that other participants experienced more pain in the nocebo trials than in the control trials. In the symbolic modeling group, participants observed a videotaped model experiencing more pain in the nocebo than in the control trials. In the verbal suggestion group, participants received a verbal suggestion of hyperalgesia in the nocebo trials and no suggestion in the control trials. No manipulations were used in the control group. To investigate whether nocebo hyperalgesia is stable over time, an additional extinction phase was conducted. Nocebo hyperalgesia was induced by verbal modeling only and was partially mediated by expectancy. Stress was a significant moderator of the induced effect. Nocebo hyperalgesia was extinguished during the extinction phase. The obtained results provide potential implications for minimizing nocebo hyperalgesia in clinical practice by, for instance, controlling patients' expectancies and stress levels. PERSPECTIVE: The study shows the role of pain-related information derived from other people in shaping negative treatment experiences in the individual. Because information from others has a particular impact on individuals experiencing stress, both this information and the stress level of patients should be monitored in the treatment process.

The Relationship Between the Ability to Infer Another's Pain and the Expectations Regarding the Appearance of Pain Facial Expressions: Investigation of the Role of Visual Perception

Although pain is a commonly experienced and observed affective state, it is frequently misinterpreted, which leads to inadequate caregiving. Studies show the ability at estimating pain in others (estimation bias) and detecting its subtle variations (sensitivity) could emerge from independent mechanisms. While estimation bias is modulated by variables such as empathy level, pain catastrophizing tendency, and overexposure to pain, sensitivity remains unimpacted. The present study verifies if these 2 types of inaccuracies are partly explained by perceptual factors. Using reverse correlation, we measured their association with participants' mental representation of pain, or more simply put, with their expectations of what the face of a person in pain should look like. Experiment 1 shows that both parameters are associated with variations in expectations of this expression. More specifically, the estimation bias is linked with expectations characterized by salient changes in the middle face region, whereas sensitivity is associated with salient changes in the eyebrow region. Experiment 2 reveals that bias and sensitivity yield differences in emotional representations. Expectations of individuals with a lower underestimation tendency are qualitatively rated as expressing more pain and sadness, and those of individuals with a higher level of sensitivity as expressing more pain, anger, and disgust. Together, these results provide evidence for a perceptual contribution in pain inferencing that is independent of other psychosocial variables and its link to observers' expectations. PERSPECTIVE: This article reinforces the contribution of perceptual mechanisms in pain assessment. Moreover, strategies aimed to improve the reliability of individuals' expectations regarding the appearance of facial expressions of pain could potentially be developed, and contribute to decrease inaccuracies found in pain assessment and the confusion between pain and other affective states.

Development of a binary classifier model from extended facial codes toward video-based pain recognition in cancer patients

OBJECTIVES

The Automatic Pain Assessment (APA) relies on the exploitation of objective methods to evaluate the severity of pain and other pain-related characteristics. Facial expressions are the most investigated pain behavior features for APA. We constructed a binary classifier model for discriminating between the absence and presence of pain through video analysis.

METHODS

A brief interview lasting approximately two-minute was conducted with cancer patients, and video recordings were taken during the session. The Delaware Pain Database and UNBC-McMaster Shoulder Pain dataset were used for training. A set of 17 Action Units (AUs) was adopted. For each image, the OpenFace toolkit was used to extract the considered AUs. The collected data were grouped and split into train and test sets: 80 % of the data was used as a training set and the remaining 20 % as the validation set. For continuous estimation, the entire patient video with frame prediction values of 0 (no pain) or 1 (pain), was imported into an annotator (ELAN 6.4). The developed Neural Network classifier consists of two dense layers. The first layer contains 17 nodes associated with the facial AUs extracted by OpenFace for each image. The output layer is a classification label of "pain" (1) or "no pain" (0).

RESULTS

The classifier obtained an accuracy of ∼94 % after about 400 training epochs. The Area Under the ROC curve (AUROC) value was approximately 0.98.

CONCLUSIONS

This study demonstrated that the use of a binary classifier model developed from selected AUs can be an effective tool for evaluating cancer pain. The implementation of an APA classifier can be useful for detecting potential pain fluctuations. In the context of APA research, further investigations are necessary to refine the process and particularly to combine this data with multi-parameter analyses such as speech analysis, text analysis, and data obtained from physiological parameters.

Personalized pain assessment: What does 'acceptable pain' mean to you?

BACKGROUND

What 'acceptable pain' means may be different for everyone and dependent on the moment and the context. In this text, we explore the concepts of pain acceptability and acceptance. We explain why we need to better explore (un)acceptable pain, to eventually facilitate pain assessment and management.

METHODS

Using different approaches and perspectives (with examples and application from multiple disciplines, i.e. orthopaedics, psychology, pharmacological therapy), we discussed anecdotal examples and included a systematic, scoping and literature review.

RESULTS

We rejected the idea that in the context of chronic pain, acceptability, disability and manageability overlap neatly. Additionally, we rejected the validity of pain intensity rating scales to sufficiently explore individuals' experience of pain. In the one study that met our criteria, a definition of 'acceptable pain' was dropped as participants deemed it inappropriate because it did not address the significant challenges associated with pain. This is important, however, because the acceptability of pain may precede, follow and/or inform the 'pain acceptance' process, which is an important concept associated with better outcomes.

CONCLUSIONS

Very little is known regarding what 'acceptable pain' may mean to people living with pain. Qualitative studies may improve our understanding of individuals' perceptions, perspectives and expectations as we do not know, for the moment, what 'acceptable pain' may mean to a particular person and, potentially, regarding a specific treatment or other contextual aspects that are not captured with currently used scores and quantitative measures.

SIGNIFICANCE

What does 'acceptable pain' mean may differ between people with painful experiences and may depend on contextual factors. Pain acceptability may be distinct from manageability, and may precede, follow and/or inform the 'pain acceptance' process. This text, rigorously based on a review of the existing literature, defends the idea that acceptable pain should be better studied.