Placebo and Nocebo Effects as Bayesian-Brain Phenomena: The Overlooked Role of Likelihood and Attention

The prediction of the analgesic placebo response is moderated by outward-focused attention: A sham, randomized clinical trial of chronic back pain patients

The within-subject variability (WSV) of pain-intensity reports has gained attention as a predictor of the placebo response but has demonstrated mixed results. We hypothesized that participants' inward- and outward-directed attention will moderate WSV's prediction of the analgesic placebo response. In this sham randomized clinical trial (protocol number NCT05994118); placebo response was induced in chronic back-pain patients (n=113) through a saline injection plus verbal suggestion. The WSV assessed by the focused analgesia selection test (FAST) served as a predictor. The revised Self-Consciousness Scale (SCS-R), evaluating a person's tendency to focus attention inward or outward was used to moderate the prediction. The placebo response prediction was significantly moderated by 2 SCS-R subscales: public self-consciousness (b = 46.36, SE = 23.08, t = 2.0, p = 0.047) and social anxiety (b = 44.01, SE = 18.02, t = 2.44, p = 0.016). The prediction was significant at low levels of both moderators (p < 0.01), but not at high levels of these traits. Prediction of the placebo response is of value and could promote personalized medical care. Better understanding of factors shaping the placebo response could further contribute to both clinical practice and clinical trials. PERSPECTIVE: The current study demonstrates that the prediction of the analgesic placebo response could be improved if relevant personal characteristics are included as moderators of the prediction. Better predictions of the placebo response could contribute to improve both clinical research and clinical care.

Premovement activity in the corticospinal tract is amplified by the placebo effect: an active inference account

The aim of this study is to investigate whether expectancy, induced through a placebo procedure, favors the activation of the corticospinal tract before movement initiation. By adopting the premovement facilitation paradigm, we applied transcranial magnetic stimulation over the left or right primary motor cortex at rest and 100 ms or 50 ms before movement onset while healthy volunteers performed a reaction time (RT) motor task consisting of abductions of the right or left thumb after a go signal. Participants in the placebo group received an inert electrical device applied on the right forearm along with information on its speed-enhancing properties. A control group received the same device with overt information about its inert nature, while another control group underwent no intervention. Along with RT, we measured the amplitude of the motor evoked potential (MEP) before and after the procedure. Compared to the control groups, the placebo group had faster RT and greater MEP amplitude before movement initiation. This study demonstrates that the placebo effect can boost the activity of the corticospinal tract before movement onset, and this modulation positively impacts motor performance. These results give experimental support to the active inference account.

How the magnitude and precision of pain predictions shape pain experiences

BACKGROUND

In Bayesian models including predictive processing, the magnitude and precision of pain expectancies are key determinants of perception. However, relatively few studies have directly tested whether this holds for pain, and results so far have been inconclusive. Here, we investigated expectancy effects on pain experiences and associated affective responses.

METHODS

In two studies, healthy participants (n = 30 in each) received painful electrical stimuli preceded by explicit pain predictions. In study 1, the magnitude of pain predictions and administered pain intensities were varied. In study 2, the magnitude and precision of pain predictions were varied, while administered pain intensity was kept constant. Experienced pain intensity was the primary outcome in both studies.

RESULTS

Pain experiences assimilated towards both under- and overpredictions of pain. In study 1, however, effects were small, if present at all, for non-painful stimuli and effects were not necessarily larger with predictions of greater magnitude. In study 2, assimilation of pain experiences appeared regardless of precision level, while no significant effects on EMG eyeblink startle responses were observed. Moreover, under- and overpredictions caused disappointment and relief, respectively, with greater disappointment upon precise than imprecise predictions.

CONCLUSIONS

The influence of pain predictions on pain might be more complex than assumed in simple instantiations of current theoretical frameworks, with no systematically stronger assimilation of pain experiences to larger and more precise predictions. Since overpredictions are associated with relief, but underpredictions with disappointment, these findings underline the importance of providing correct predictions when preparing for upcoming painful procedures.

SIGNIFICANCE STATEMENT

Our work supports, challenges, and extends the application of Bayesian and predictive processing frameworks to the influence of pain predictions on pain. Under- and overpredictions of pain yielded assimilation of pain experiences, but assimilation was not systematically stronger with larger prediction errors or greater precision. Moreover, under- and overpredictions resulted in disappointment and relief, respectively. This research signifies the importance of establishing accurate predictions of pain in clinical practice.

Visual placebo and nocebo effects

Placebo and nocebo effects modulate symptom perception through expectations and learning processes in various domains. Predominantly, their impact has been investigated on pain and physical performance. However, the influence of placebos and nocebos on visual system functionality has yet to be explored. The present study aimed to test whether placebo and nocebo effects can intervene in altering participants' performance outcomes during a novel visual accuracy task and to examine the underlying neural mechanisms through EEG. After performing a baseline session, visual accuracy was said to be enhanced or disrupted by a sham transcranial electrical stimulation over the occipital lobe. Behavioural results showed a significant increase in visual accuracy for the placebo group, from the baseline session to the test session, whereas the nocebo group showed a decrease in visual accuracy. EEG analyses on the event-related potential P300 component, conducted on both a centro-parietal electrode patch and a parieto-occipital, one displayed an increase in the amplitude of P300 for the placebo group, and a decrease in the nocebo group. These findings suggest for the first time that placebo and nocebo effects can influence visual perception and attentional processes linked to it. Overall, the present study contributes to understanding how expectations affect sensory perception beyond pain and the motor system, paving the way for investigating these phenomena in other sensory modalities such as auditory or olfactory perception. KEY POINTS: Placebo and nocebo effects have been studied predominantly in pain and motor performance fields. In a novel visual task, the impact of placebo and nocebo effects on the visual system has been evaluated, in both early components (stimuli-related) and late components (attention-related). The placebo group showed an increase in visual accuracy and EEG-evoked potential amplitudes, whereas the nocebo group showed a decrease in both. This study shows how expectations and the related placebo and nocebo effects can shape basic stimuli sensory perception in the visual domain.

The Role of Attention in Placebo and Nocebo Effects

BACKGROUND

Although some existing models propose that attention may be crucially implicated in placebo/nocebo effects, empirical research on this aspect remains limited and scattered.

PURPOSE

This systematic review aims to provide an inclusive overview of studies that have either directly manipulated or assessed attention within the context of placebo and nocebo procedures so to gain a synthetized picture of the role of this variable in placebo/nocebo effects. Importantly, only studies in which attention represented a mechanism or mediator of the placebo/nocebo response, and not a primary outcome, were included.

METHODS

A systematic search was conducted across multiple databases, including PubMed, Scopus, PsycINFO, Web of Science, and Embase, to identify peer-reviewed studies. These studies were subjected to methodological evaluation and eligibility criteria for inclusion.

RESULTS

We identified and classified 12 studies into three categories based on their focus: (i) those that directly assessed attention, (ii) those that directly manipulated participants' attention, and (iii) those that combined both a direct manipulation and assessment of attention. In all selected studies attention acted as a mechanism or mediator of the placebo/nocebo response, and was not considered a primary outcome of the placebo/nocebo manipulation.

CONCLUSIONS

The synthesis of the included studies reveals that the role of attention in placebo and nocebo effects is still a topic of debate, marked by variations in how attention is conceptualized and measured. Results suggest that attention has significant clinical implications, particularly in optimizing therapeutic efficacy by directing patients' focus toward signs of healing and away from indicators of illness or distress. To advance our understanding, future research should explore these attentional mechanisms, in conjunction with neurophysiological correlates.

Autobiographical Memory in Feeding and Eating Disorders: A Systematic Review

INTRODUCTION

Prominent eating disorders (EDs) theories identify a critical relationship between body and self. One of the ways to study this relationship is through autobiographical memories (AMs). The present review aimed to evaluate the studies that investigated AM in patients with EDs.

METHODS

A search of PubMed, ScienceDirect, and Scopus databases was performed to identify relevant articles. Of the 57,113 studies found, 25,016 were not duplicated. After screening, 27 articles were included.

RESULTS

The studies had some methodological flaws: none of the articles was a randomized control trial and the sample sizes were small. Nevertheless, important evidence emerged because all studies showed that patients with EDs have impaired AM function. This is because the way patients with EDs remember and define themselves is through an allocentric perspective associated with the gazes of others whose role has an impact on AM, body shape, and self.

CONCLUSION

To our knowledge, this is the first systematic review to examine AM in patients with EDs. Future research is needed in EDs to expand knowledge about the relationship between the body and the self.

Imagining flying increases jumping performance in volleyball players: A pilot study

OBJECTIVE

The study aimed to examine the effect of mental imagery on sports performance. Specifically, it was tested whether imagining flying (i.e., air travel) increases jumping performance in a group of female volleyball players.

METHODS

The study included 46 female young athletes (mean age = 15.23 years; standard deviation = 2.4) divided into two groups: the experimental group viewed a three-dimensional video that simulated a flying experience, while the control group watched neutral footage. The jump performance of both groups was measured before and after viewing the videos, using the Vertec Like® instrument to assess jump height. Participants were assessed for their flow disposition, mental imagery skills, and image vividness.

RESULTS

Comparing pre-post jump performance scores, the experimental group showed a significant improvement over the control group, with a medium-large effect size (d = 0.634). There was no significant association between flow disposition, mental imagery skills, image vividness, and pre-post jumping performance differences (respectively: β = -0.107, p = .484; β = -0.008, p = .957; β = 0.024, p = .913).

CONCLUSIONS

These findings suggest that the experience of imagining flying, enhanced with an immersive video, has a positive effect on jumping performance in a one-session study with young female volleyball players. This effect does not appear to be associated with pre-existing characteristics or the vividness of the image itself.

Social cognition in Parkinson's disease and functional movement disorders

INTRODUCTION

Functional movement disorders (FMD) can overlap with Parkinson's disease (PD), and distinguishing between the two clinical conditions can be complex. Framing social cognition (theory of mind) (TOM) disorder, attention deficit, and psychodynamic features of FMD and PD may improve diagnosis.

METHODS

Subjects with FMD and PD and healthy controls (HC) were administered tasks assessing TOM abilities and attention. The psychodynamic hypothesis of conversion disorder was explored by a questionnaire assessing dissociative symptoms. A comprehensive battery of neuropsychological tasks was also administered to FMD and PD.

RESULTS

Although both FMD and PD scored lower than HC on all TOM tests, significant correlations between TOM and neuropsychological tasks were found only in PD but not in FMD. Only PD showed a reduction in attentional control. Dissociative symptoms occurred only in FMD.

DISCUSSION

Cognitive-affective disturbances are real in FMD, whereas they are largely dependent on cognitive impairment in PD. Attentional control is preserved in FMD compared to PD, consistent with the hypothesis that overload of voluntary attentional orientation may be at the basis of the onset of functional motor symptoms. On a psychodynamic level, the confirmation of dissociative symptoms in FMD supports the conversion disorder hypothesis.

CONCLUSION

FMD and PD can be distinguished on an affective and cognitive level. At the same time, however, the objective difficulty often encountered in distinguishing between the two pathologies draws attention to how blurred the boundary between 'organic' and 'functional' can be.

The effect of mindfulness-based interventions on biomarkers in cancer patients and survivors: A systematic review

Various reviews and meta-analyses have shown the positive effects of mindfulness-based interventions (MBIs) on the mental health of cancer patients and survivors. Some studies have also investigated the impact of MBIs on physiological markers of health in oncology, but a systematic review has not been conducted in this field. The current paper aims to fill this gap in the literature. Following preferred reporting items for systematic reviews and meta-analyses 2020 guidelines, data were obtained from the databases of Pubmed, Scopus, Web of Science in May 2022. Twenty-five studies were included. Globally, 35 biomarkers were employed in these studies and were categorized 8 groups (cortisol; blood pressure (BP), heart rate, and respiratory rate; C-reactive protein; telomere length and telomerase activity (TA); genetic signature; cytokines and hormones; leucocyte activation; leucocyte count and cell subpopulation analysis). In seven of these categories of biomarkers, positive effects of MBIs were observed. The most promising results were obtained for cortisol, BP, TA and pro-inflammatory gene expression. However, the generally low number of studies per single biomarker limits the possibility to draw reliable conclusions. The present review presents a comprehensive state-of-the-art for MBIs in oncology on biomarkers, confirming MBIs' potential for improving physiological health in cancer patients and survivors besides those already shown in literature on psychological well-being.

Risk factors associated with nocebo effects: A review of reviews

Objective

This meta-review aims to identify and categorize the risk factors that are associated with nocebo effects. The nocebo effect can exert a negative impact on treatment outcomes and have detrimental outcomes on health. Learning more about its potential predictors and risk factors is a crucial step to mitigating it.

Methods

Literature review studies about the risk factors for nocebo effects were searched through five databases (PubMed, Scopus, The Cochrane Library, PsycINFO, and Embase) and through grey literature. Methodological validity and risk of bias were assessed. We conducted a thematic analysis of the results of the forty-three included reviews.

Results

We identified nine categories of risk factors: prior expectations and learning; socio-demographic characteristics; personality and individual differences; neurodegenerative conditions; inflammatory conditions; communication of information and patient-physician relationship; drug characteristics; setting; and self-awareness. We also highlighted the main biochemical and neurophysiological mechanisms underlying nocebo effects.

Conclusions

Nocebo effects arise from expectations of adverse symptoms, particularly when triggered by previous negative experiences. A trusting relationship with the treating physician and clear, tailored treatment instructions can act as protective factors against a nocebo effect. Clinical implications are discussed.

Mindfulness, curiosity, and creativity

Curiosity and creativity are manifestations of novelty-seeking mechanisms, closely intertwined and interdependent. This principle aligns seamlessly with the foundational tenets of Langerian mindfulness, which places novelty seeking as a cornerstone. Creativity, curiosity, openness, and flexibility all harmoniously converge in this framework. Spanning over four decades, research in the realm of mindfulness has diligently delved into the intricate interplay among these constructs.

Resonance tuning of rhythmic movements is disrupted at short time scales: A centrifuge study

The human body exploits its neural mechanisms to optimize actions. Rhythmic movements are optimal when their frequency is close to the natural frequency of the system. In a pendulum, gravity modulates this spontaneous frequency. Participants unconsciously adjust their natural pace when cyclically moving the arm in altered gravity. However, the timescale of this adaptation is unexplored. Participants performed cyclic movements before, during, and after fast transitions between hypergravity levels (1g-3g and 3g-1g) induced by a human centrifuge. Movement periods were modulated with the average value of gravity during transitions. However, while participants increased movement pace on a cycle basis when gravity increased (1g-3g), they did not decrease pace when gravity decreased (3g-1g). We highlight asymmetric effects in the spontaneous adjustment of movement dynamics on short timescales, suggesting the involvement of cognitive factors, beyond standard dynamical models.

Does Active Inference Provide a Comprehensive Theory of Placebo Analgesia?

Placebo interventions generate mismatches between expected pain and sensory signals from which pain states are inferred. Because we lack direct access to bodily states, we can only infer whether nociceptive activity indicates tissue damage or results from noise in sensory channels. Predictive processing models propose to make optimal inferences using prior knowledge given noisy sensory data. However, these models do not provide a satisfactory explanation of how pain relief expectations are translated into physiological manifestations of placebo responses. Furthermore, they do not account for individual differences in the ability to endogenously regulate nociceptive activity in predicting placebo analgesia. The brain not only passively integrates prior pain expectations with nociceptive activity to infer pain states (perceptual inference) but also initiates various types of actions to ensure that sensory data are consistent with prior pain expectations (active inference). We argue that depending on whether the brain interprets conflicting sensory data (prediction errors) as a signal to learn from or noise to be attenuated, the brain initiates opposing types of action to facilitate learning from sensory data or, conversely, to enhance the biasing influence of prior pain expectations on pain perception. Furthermore, we discuss the role of stress, anxiety, and unpredictability of pain in influencing the weighting of prior pain expectations and sensory data and how they relate to the individual ability to regulate nociceptive activity (endogenous pain modulation). Finally, we provide suggestions for future studies to test the implications of the active inference model of placebo analgesia.

Placebo and nocebo effects and mechanisms associated with pharmacological interventions: an umbrella review

OBJECTIVES

This review aimed to summarise the existing knowledge about placebo and nocebo effects associated with pharmacological interventions and their mechanisms.

DESIGN

Umbrella review, adopting the Assessment of Multiple Systematic Reviews 2 tool for critical appraisal.

DATA SOURCES

MEDLINE/PubMed, Scopus, Web of Science, PsycINFO, Cochrane Central Register of Controlled Trial were searched in September 2022, without any time restriction, for systematic reviews, narrative reviews, original articles. Results were summarised through narrative synthesis, tables, 95% CI.

OUTCOME MEASURES

Mechanisms underlying placebo/nocebo effects and/or their effect sizes.

RESULTS

The databases search identified 372 studies, for a total of 158 312 participants, comprising 41 systematic reviews, 312 narrative reviews and 19 original articles. Seventy-three per cent of the examined systematic reviews were of high quality.Our findings revealed that mechanisms underlying placebo and/or nocebo effects have been characterised, at least in part, for: pain, non-noxious somatic sensation, Parkinson's disease, migraine, sleep disorders, intellectual disability, depression, anxiety, dementia, addiction, gynaecological disorders, attention-deficit hyperactivity disorder, immune and endocrine systems, cardiovascular and respiratory systems, gastrointestinal disorders, skin diseases, influenza and related vaccines, oncology, obesity, physical and cognitive performance. Their magnitude ranged from 0.08 to 2.01 (95% CI 0.37 to 0.89) for placebo effects and from 0.32 to 0.90 (95% CI 0.24 to 1.00) for nocebo effects.

CONCLUSIONS

This study provides a valuable tool for clinicians and researchers, identifying both results ready for clinical practice and gaps to address in the near future.

FUNDING

Università Cattolica del Sacro Cuore, Milan, Italy with the 'Finanziamento Ponte 2022' grant.

PROSPERO REGISTRATION NUMBER

CRD42023392281.

Microgravity induces overconfidence in perceptual decision-making

Does gravity affect decision-making? This question comes into sharp focus as plans for interplanetary human space missions solidify. In the framework of Bayesian brain theories, gravity encapsulates a strong prior, anchoring agents to a reference frame via the vestibular system, informing their decisions and possibly their integration of uncertainty. What happens when such a strong prior is altered? We address this question using a self-motion estimation task in a space analog environment under conditions of altered gravity. Two participants were cast as remote drone operators orbiting Mars in a virtual reality environment on board a parabolic flight, where both hyper- and microgravity conditions were induced. From a first-person perspective, participants viewed a drone exiting a cave and had to first predict a collision and then provide a confidence estimate of their response. We evoked uncertainty in the task by manipulating the motion's trajectory angle. Post-decision subjective confidence reports were negatively predicted by stimulus uncertainty, as expected. Uncertainty alone did not impact overt behavioral responses (performance, choice) differentially across gravity conditions. However microgravity predicted higher subjective confidence, especially in interaction with stimulus uncertainty. These results suggest that variables relating to uncertainty affect decision-making distinctly in microgravity, highlighting the possible need for automatized, compensatory mechanisms when considering human factors in space research.

Stimulus-independent and stimulus-dependent neural networks underpin placebo analgesia responsiveness in humans

The neural circuits that regulate placebo analgesia responsivity are unknown, although engagement of brainstem pain modulatory regions is likely critical. Here we show in 47 participants that differences are present in neural circuit connectivity's in placebo responders versus non-responders. We distinguish stimulus-independent and stimulus-dependent neural networks that display altered connections between the hypothalamus, anterior cingulate cortex and midbrain periaqueductal gray matter. This dual regulatory system underpins an individual's ability to mount placebo analgesia.