Learning potentiates neurophysiological and behavioral placebo analgesic responses

Nature of the placebo and nocebo effect in relation to functional neurologic disorders

Placebos have long been considered a nuisance in clinical research, for they have always been used as comparators for the validation of new treatments. By contrast, today they represent an active field of research, and, due to the involvement of many mechanisms, the study of the placebo effect can actually be viewed as a melting pot of concepts and ideas for neuroscience. There is not a single placebo effect, but many, with different mechanisms across different medical conditions and therapeutic interventions. Expectation, anxiety, and reward are all involved, as well as a variety of learning phenomena and genetic variants. The most productive models to better understand the neurobiology of the placebo effect are pain and Parkinson's disease. In these medical conditions, several neurotransmitters have been identified, such as endogenous opioids, cholecystokinin, dopamine, as well as lipidic mediators, for example, endocannabinoids and prostaglandins. Since the placebo effect is basically a psychosocial context effect, these data indicate that different social stimuli, such as words and therapeutic rituals, may change the chemistry of the patient's brain, and these effects are similar to those induced by drugs.

Nocebo and pain: An overview of the psychoneurobiological mechanisms

INTRODUCTION

Nocebo effects are defined as adverse events related to negative expectations and learning processes that are involved in the modulation of the descending pain pathways. Research over the last couple of decades has illustrated that behavioral, psychoneurobiological and functional changes occur during nocebo-induced pain processing.

OBJECTIVES

We aimed to review published human and non-human research on algesia and hyperalgesia resulting from negative expectations and nocebo effects.

METHODS

Herein, we searched and comprehensively reviewed scientific literature providing informative knowledge about the psychoneurobiological bases of the nocebo effect in the field of pain with an emphasis on how pain processes are shaped by both cognitive and non-cognitive factors.

RESULTS

Negative expectations are formed through verbal suggestions of heightened pain, prior nociceptive and painful experiences and observation of pain in others. Susceptibility to the nocebo effect can be also influenced by genetic variants, conscious and nonconscious learning processes, personality traits and psychological factors. Moreover, providers' behaviors, environmental cues and the appearance of medical devices can induce negative expectations that dramatically influence pain perception and processing in a variety of pain modalities and patient populations.

CONCLUSION

Importantly, we concluded that nocebo studies outline how individual expectations may lead to physiological changes underpinning the central integration and processing of magnified pain signaling. Further research is needed to develop strategies that can identify nocebo-vulnerable pain patients in order to optimize the psychosocial and therapeutic context in which the clinical encounter occurs, with the ultimate purpose of improving clinical outcomes.

Nocebo effects in clinical studies: hints for pain therapy

INTRODUCTION

Nocebo-induced algesic responses occurring within clinical contexts present a challenge for health care practitioners working in the field of pain medicine.

OBJECTIVES

Following the recent research on algesic nocebo effects, the scope of this review is to develop ethically acceptable strategies to help avoid, or at least reduce, nocebo responses within clinical settings.

METHODS

We reviewed relevant clinical studies that depict how patient-practitioner interactions may contribute to the reduction of nocebo responses.

RESULTS

A strong algesic nocebo effect may adversely impact a patient's condition by causing decreases in both the efficacy and effectiveness of interventions, as well as by promoting treatment nonadherence and discontinuation. These effects may be triggered through multiple channels and can lead to significant alterations in a patient's perception of pain, consequently producing a weakening of the specific positive effects of pharmacological, psychological, or physical pain-management interventions.

CONCLUSION

To minimize nocebo effects in clinical settings, we identified and discussed five contextual aspects relevant to the treatment of patients with chronic pain: (1) negative patient-clinician communication and interaction during treatment; (2) emotional burden of patients during treatment with analgesic medication; (3) negative information provided via informational leaflets; (4) cued and contextual conditioning nocebo effects; and (5) patient's lack of positive information. Through an understanding of these elements, many preventive and ethically acceptable clinical actions can be taken to improve multidisciplinary pain treatment outcomes.

Hypnotizability and Placebo Analgesia in Waking and Hypnosis as Modulators of Auditory Startle Responses in Healthy Women: An ERP Study

We evaluated the influence of hypnotizability, pain expectation, placebo analgesia in waking and hypnosis on tonic pain relief. We also investigated how placebo analgesia affects somatic responses (eye blink) and N100 and P200 waves of event-related potentials (ERPs) elicited by auditory startle probes. Although expectation plays an important role in placebo and hypnotic analgesia, the neural mechanisms underlying these treatments are still poorly understood. We used the cold cup test (CCT) to induce tonic pain in 53 healthy women. Placebo analgesia was initially produced by manipulation, in which the intensity of pain induced by the CCT was surreptitiously reduced after the administration of a sham analgesic cream. Participants were then tested in waking and hypnosis under three treatments: (1) resting (Baseline); (2) CCT-alone (Pain); and (3) CCT plus placebo cream for pain relief (Placebo). For each painful treatment, we assessed pain and distress ratings, eye blink responses, N100 and P200 amplitudes. We used LORETA analysis of N100 and P200 waves, as elicited by auditory startle, to identify cortical regions sensitive to pain reduction through placebo and hypnotic analgesia. Higher pain expectation was associated with higher pain reductions. In highly hypnotizable participants placebo treatment produced significant reductions of pain and distress perception in both waking and hypnosis condition. P200 wave, during placebo analgesia, was larger in the frontal left hemisphere while placebo analgesia, during hypnosis, involved the activity of the left hemisphere including the occipital region. These findings demonstrate that hypnosis and placebo analgesia are different processes of top-down regulation. Pain reduction was associated with larger EMG startle amplitudes, N100 and P200 responses, and enhanced activity within the frontal, parietal, and anterior and posterior cingulate gyres. LORETA results showed that placebo analgesia modulated pain-responsive areas known to reflect the ongoing pain experience.

Expectation to feel more pain disrupts the habituation of laser-pain rating and laser-evoked potential amplitudes

Increased pain perception due to the expectation to feel more pain is called nocebo effect. The present study aimed at investigating whether: (1) the mere expectation to feel more pain after the administration of an inert drug can affect the laser-pain rating and the laser-evoked potential (LEP) amplitude, and (2) the learning potentiates the nocebo effect. Eighteen healthy volunteers were told that an inert cream, applied on the right hand, would increase the laser pain and LEP amplitude to right hand stimulation. They were randomly assigned to either "verbal session" or "conditioning session". In the "verbal session", LEPs to both right and left hand stimulation were recorded at the same intensity before (baseline) and after cream application. In the "conditioning session", after an initial cream application the laser stimulus intensity was increased surreptitiously to make the subjects believe that the treatment really increased the pain sensation. Then, the cream was reapplied, and LEPs were recorded at the same stimulus intensity as at the baseline. It was found that the verbal suggestion to feel more pain disrupted the physiological habituation of the laser-pain rating and LEP amplitude to treated (right) hand stimulation. Unlike previously demonstrated for the placebo effect, the learning did not potentiate the nocebo effect.

Informing Patients About Placebo Effects: Using Evidence, Theory, and Qualitative Methods to Develop a New Website

Background

According to established ethical principles and guidelines, patients in clinical trials should be fully informed about the interventions they might receive. However, information about placebo-controlled clinical trials typically focuses on the new intervention being tested and provides limited and at times misleading information about placebos.

Objective

We aimed to create an informative, scientifically accurate, and engaging website that could be used to improve understanding of placebo effects among patients who might be considering taking part in a placebo-controlled clinical trial.

Methods

Our approach drew on evidence-, theory-, and person-based intervention development. We used existing evidence and theory about placebo effects to develop content that was scientifically accurate. We used existing evidence and theory of health behavior to ensure our content would be communicated persuasively, to an audience who might currently be ignorant or misinformed about placebo effects. A qualitative ‘think aloud’ study was conducted in which 10 participants viewed prototypes of the website and spoke their thoughts out loud in the presence of a researcher.

Results

The website provides information about 10 key topics and uses text, evidence summaries, quizzes, audio clips of patients’ stories, and a short film to convey key messages. Comments from participants in the think aloud study highlighted occasional misunderstandings and off-putting/confusing features. These were addressed by modifying elements of content, style, and navigation to improve participants’ experiences of using the website.

Conclusions

We have developed an evidence-based website that incorporates theory-based techniques to inform members of the public about placebos and placebo effects. Qualitative research ensured our website was engaging and convincing for our target audience who might not perceive a need to learn about placebo effects. Before using the website in clinical trials, it is necessary to test its effects on key outcomes including patients’ knowledge and capacity for making informed choices about placebos.

Effects of OnabotulintoxinA on Habituation of Laser Evoked Responses in Chronic Migraine

Onabotulintoxin A (BontA) is an efficacious preventive treatment for chronic migraine, though the specific mechanism of action is still under discussion. The study aims: (1) To evaluate pain processing modifications in chronic migraine patients (CM) under single BontA administration in pericranial muscles, by means of CO² Laser Evoked Potentials (LEPs) obtained by the stimulation of the skin over the right frontal and trapezius injection sites and hand dorsum, in a double blind placebo controlled crossover design. (2) To correlate main LEPs findings with clinical outcome after one year of BontA treatment. Twenty refractory CM patients were included in the analysis. The LEPs were recorded in basal conditions and seven days after BontA (PREEMPT protocol) and saline solution injection. The N1, N2 and P2 amplitude and latencies and N2P2 habituation index were evaluated and correlated with the percent change of headache frequency after one year of toxin treatment. After seven days of BontA treatment, a normalization of the trigeminal habituation index was observed, which was correlated with the clinical outcome after one year of BontA therapy. Patients displaying trigeminal LEPs facilitation at T0 time showed a more efficient therapeutic outcome. Neurotoxin may exert a modulating effect on trigeminal nociception, normalizing central neurotransmission.

Different contexts, different pains, different experiences

Pain is an ambiguous perception: the same pain stimulation can be perceived differently in different contexts, producing different experiences, ranging from mild to unbearable pain. It can be even experienced as a rewarding sensation within the appropriate context. Overall, placebo and nocebo effects appear to be very good models to understand how the psychosocial context modulates the experience of pain. In this review, we examine the effects of different contexts on pain, with a specific focus on the neurobiological mechanisms. Positive and rewarding contexts inform the patients that an effective treatment is being delivered and are capable of producing pain relief through the activation of specific systems such as opioids, cannabinoids and dopamine. Conversely, a negative context can produce pain exacerbation and clinical worsening through the modulation of different systems, such as the activation of cholecystokinin and the deactivation of opioids and dopamine. In addition, when a therapy is delivered unbeknownst to the patient, its effects are reduced. A better understanding of the neurobiological underpinnings of the context-pain interaction is a challenge both for future pain research and for good clinical practice.

Placebo analgesia: understanding the mechanisms

Expectations of pain relief drive placebo analgesia. Understanding how expectations of improvement trigger distinct biological systems to shape therapeutic analgesic outcomes has been the focus of recent pharmacologic and neuroimaging studies in the field of pain. Recent findings indicate that placebo effects can imitate the actions of real painkillers and promote the endogenous release of opioids and nonopioids in humans. Social support and observational learning also contribute to placebo analgesic effects. Distinct psychological traits can modulate expectations of analgesia, which facilitate brain pain control mechanisms involved in pain reduction. Many studies have highlighted the importance and clinical relevance of these responses. Gaining deeper understanding of these pain modulatory mechanisms has important implications for personalizing patient pain management.

Selective REM Sleep Deprivation Improves Expectation-Related Placebo Analgesia

The placebo effect is a neurobiological and psychophysiological process known to influence perceived pain relief. Optimization of placebo analgesia may contribute to the clinical efficacy and effectiveness of medication for acute and chronic pain management. We know that the placebo effect operates through two main mechanisms, expectations and learning, which is also influenced by sleep. Moreover, a recent study suggested that rapid eye movement (REM) sleep is associated with modulation of expectation-mediated placebo analgesia. We examined placebo analgesia following pharmacological REM sleep deprivation and we tested the hypothesis that relief expectations and placebo analgesia would be improved by experimental REM sleep deprivation in healthy volunteers. Following an adaptive night in a sleep laboratory, 26 healthy volunteers underwent classical experimental placebo analgesic conditioning in the evening combined with pharmacological REM sleep deprivation (clonidine: 13 volunteers or inert control pill: 13 volunteers). Medication was administered in a double-blind manner at bedtime, and placebo analgesia was tested in the morning. Results revealed that 1) placebo analgesia improved with REM sleep deprivation; 2) pain relief expectations did not differ between REM sleep deprivation and control groups; and 3) REM sleep moderated the relationship between pain relief expectations and placebo analgesia. These results support the putative role of REM sleep in modulating placebo analgesia. The mechanisms involved in these improvements in placebo analgesia and pain relief following selective REM sleep deprivation should be further investigated.

Differential neurophysiological correlates of bottom-up and top-down modulations of pain

The perception of pain is highly variable. It depends on bottom-up-mediated factors like stimulus intensity and top-down-mediated factors like expectations. In the brain, pain is associated with a complex pattern of neuronal responses including evoked potentials and induced responses at alpha and gamma frequencies. Although they all covary with stimulus intensity and pain perception, responses at gamma frequencies can be particularly closely related to the perception of pain. It is, however, unclear whether this association holds true across all types of pain modulation. Here, we used electroencephalography to directly compare bottom-up- and top-down-mediated modulations of pain, which were implemented by changes in stimulus intensity and placebo analgesia, respectively. The results show that stimulus intensity modulated pain-evoked potentials and pain-induced alpha and gamma responses. In contrast, placebo analgesia was associated with changes of evoked potentials, but not of alpha and gamma responses. These findings reveal that pain-related neuronal responses are differentially sensitive to bottom-up and top-down modulations of pain, indicating that they provide complementary information about pain perception. The results further show that pain-induced gamma oscillations do not invariably encode pain perception but may rather represent a marker of sensory processing whose influence on pain perception varies with behavioral context.

Opposite effects of the same drug: reversal of topical analgesia by nocebo information

Several studies have shown that psychological factors such as learning, expectation, and emotions can affect pharmacological treatment and shape both favorable and adverse effects of drugs. This study investigated whether nocebo information provided during administration of an analgesic cream could reverse topical analgesia to hyperalgesia. Furthermore, we tested whether nocebo effects were mediated by negative emotional activation. A total of 142 healthy volunteers (73 women) were randomized into 6 groups. A topical analgesic cream (Emla) was administered together with suggestions of analgesia in 1 group, whereas another group received Emla with suggestions of hyperalgesia. Two other groups received a placebo cream together with the same information as the groups receiving Emla. A fifth group received Emla with no specific information about the effect, and the sixth group received no treatment but the same pain induction as the other groups. Heat pain stimulation (48°C) was administered during a pretest and 2 posttests. Pain was continuously recorded during stimulation, and measures of subjective stress and blood pressure were obtained before the pretest, after the application of cream, and after the posttests. The results revealed that pain was significantly lower in the group receiving Emla with positive information and highest in the groups receiving suggestions of hyperalgesia, regardless of whether Emla or the placebo was administered. Mediation analyses showed that stress and blood pressure mediated hyperalgesia after nocebo suggestions. These results suggest that nocebo information can reverse topical analgesia and that emotional factors can explain a significant proportion of variance in nocebo hyperalgesia.

Conceptual Conditioning: Mechanisms Mediating Conditioning Effects on Pain

Classical conditioning can profoundly modify subsequent pain responses, but the mechanisms that drive this effect are unresolved. In pain-conditioning studies, cues are typically conditioned to primary aversive reinforcers; hence, subsequent pain modulation could reflect learned precognitive associations (i.e., those involving neural plasticity independent of expectations and other forms of conceptual thought) or conceptual expectancies. We isolated conceptual contributions using a thermal pain-conditioning procedure in which different conditioned stimulus (CS) cues were repeatedly paired with symbolic representations of high and low noxious heat. In a subsequent test phase, identical noxious stimuli evoked larger skin conductance responses (SCRs) and pain ratings when preceded by CS cues associated with high temperature than by those associated with low temperature. These effects were mediated by participants' self-reported expectancies. CS cues associated with high temperature also evoked larger anticipatory SCRs than did CS cues associated with low temperature, but larger anticipatory SCRs predicted smaller subsequent heat-evoked SCRs. These results provide novel evidence that conditioned modulation of pain physiology can be acquired through purely conceptual processes, and that self-reported expectancies and physiological threat responses have opposing effects on pain.

The neuroscience of placebo effects: connecting context, learning and health

Placebo effects are beneficial effects that are attributable to the brain-mind responses to the context in which a treatment is delivered rather than to the specific actions of the drug. They are mediated by diverse processes--including learning, expectations and social cognition--and can influence various clinical and physiological outcomes related to health. Emerging neuroscience evidence implicates multiple brain systems and neurochemical mediators, including opioids and dopamine. We present an empirical review of the brain systems that are involved in placebo effects, focusing on placebo analgesia, and a conceptual framework linking these findings to the mind-brain processes that mediate them. This framework suggests that the neuropsychological processes that mediate placebo effects may be crucial for a wide array of therapeutic approaches, including many drugs.

How Is Pain Influenced by Cognition? Neuroimaging Weighs In

Neuroimaging can inform cognitive theories to the extent that particular patterns of brain activity are sensitively and specifically associated with particular types of cognitive processes. We illustrate the utility of neuroimaging data in one specific case: understanding cognitive influences on pain. We first argue that pain self-reports are often inadequate to fully characterize pain experience and the processes that underlie it. Then, we describe how neuroimaging measures have been used to corroborate the effects of psychological manipulations on pain by focusing on placebo treatments and demonstrating effects on the best available correlates of pain experience. In addition, using placebo analgesia as an example, we argue that brain evidence is useful for building psychological theories likely to yield valid and generalizable predictions, because biologically informed theories are grounded in the constraints inherent in the relevant physiological systems. Finally, we suggest that neuroimaging findings will become increasingly useful for constraining psychological inference as brain patterns diagnostic of particular types of mental events are identified and characterized. In our view, the relationships between biological findings and cognitive theory are empirically based and must develop through an iterative process of synthesis across studies, topics, and methods.

N-acetyl-cysteine, a drug that enhances the endogenous activation of group-II metabotropic glutamate receptors, inhibits nociceptive transmission in humans

Background

Emerging research seeking novel analgesic drugs focuses on agents targeting group-II metabotropic glutamate receptors (mGlu2 and mGlu3 receptors). N-Acetylcysteine (NAC) enhances the endogenous activation of mGlu2/3 receptors by activating the glial glutamate:cystine membrane exchanger. Here, we examined whether NAC inhibits nociceptive responses in humans and animals. We tested the effect of oral NAC (1.2 g) on thermal-pain thresholds and laser-evoked potentials in 10 healthy volunteers, according to a crossover, double-blind, placebo-controlled design, and the effect of NAC (100 mg/kg, i.p.) on the tail-flick response evoked by radiant heat stimulation in mice.

Results

In healthy subjects, NAC treatment left thermal-pain thresholds unchanged, but significantly reduced pain ratings to laser stimuli and amplitudes of laser-evoked potentials. NAC induced significantly greater changes in these measures than placebo. In the tail-flick test, NAC strongly reduced the nocifensive reflex response to radiant heat. The action of NAC was abolished by the preferential mGlu2/3 receptor antagonist, LY341495 (1 mg/kg, i.p.).

Conclusions

Our findings show for the first time that NAC inhibits nociceptive transmission in humans, and does the same in mice by activating mGlu2/3 receptors. These data lay the groundwork for investigating the therapeutic potential of NAC in patients with chronic pain.

A Bayesian perspective on sensory and cognitive integration in pain perception and placebo analgesia

The placebo effect is a component of any response to a treatment (effective or inert), but we still ignore why it exists. We propose that placebo analgesia is a facet of pain perception, others being the modulating effects of emotions, cognition and past experience, and we suggest that a computational understanding of pain may provide a unifying explanation of these phenomena. Here we show how Bayesian decision theory can account for such features and we describe a model of pain that we tested against experimental data. Our model not only agrees with placebo analgesia, but also predicts that learning can affect pain perception in other unexpected ways, which experimental evidence supports. Finally, the model can also reflect the strategies used by pain perception, showing that modulation by disparate factors is intrinsic to the pain process.

Understanding placebo and nocebo responses for pain management

Placebo analgesia makes individuals experience relief of their pain simply by virtue of the anticipation of a benefit. A reduction of pain can occur also when placebos follow the administration of active and effective painkillers. In fact, studies indicate that placebos mimic the action of active treatments and promote the endogenous release of opioids in both humans and animals. Finally, social support and observational learning also lead to analgesic effects. Thus, different psychological factors and situations induce expectations of analgesia facilitating the activation of the top-down systems for pain control along with the release of endogenous mediators crucially involved in placebo-induced benefits. Recent scientific investigation in the field of brain imaging is opening new avenues to understanding the cognitive mechanisms and neurobiological substrates of expectation-induced pain modulation. Gaining deeper knowledge of top-down mechanisms of pain modulation has enormous implications for personalizing and optimizing pain management.

Placebo and nocebo effects: a complex interplay between psychological factors and neurochemical networks

Placebo and nocebo effects have recently emerged as an interesting model to understand some of the intricate underpinnings of the mind-body interaction. A variety of psychological mechanisms, such as expectation, conditioning, anxiety modulation, and reward, have been identified, and a number of neurochemical networks have been characterized across different conditions, such as pain and motor disorders. What has emerged from the recent insights into the neurobiology of placebo and nocebo effects is that the psychosocial context around the patient and the therapy, which represents the ritual of the therapeutic act, may change the biochemistry and the neuronal circuitry of the patient's brain. Furthermore, the mechanisms activated by placebos and nocebos have been found to be the same as those activated by drugs, which suggests a cognitive/affective interference with drug action. Overall, these findings highlight the important role of therapeutic rituals in the overall therapeutic outcome, including hypnosis, which may have profound implications both in routine medical practice and in the clinical trials setting.

Intracortical modulation, and not spinal inhibition, mediates placebo analgesia

Suppression of spinal responses to noxious stimulation has been detected using spinal fMRI during placebo analgesia, which is therefore increasingly considered a phenomenon caused by descending inhibition of spinal activity. However, spinal fMRI is technically challenging and prone to false-positive results. Here we recorded laser-evoked potentials (LEPs) during placebo analgesia in humans. LEPs allow neural activity to be measured directly and with high enough temporal resolution to capture the sequence of cortical areas activated by nociceptive stimuli. If placebo analgesia is mediated by inhibition at spinal level, this would result in a general suppression of LEPs rather than in a selective reduction of their late components. LEPs and subjective pain ratings were obtained in two groups of healthy volunteers - one was conditioned for placebo analgesia while the other served as unconditioned control. Laser stimuli at three suprathreshold energies were delivered to the right hand dorsum. Placebo analgesia was associated with a significant reduction of the amplitude of the late P2 component. In contrast, the early N1 component, reflecting the arrival of the nociceptive input to the primary somatosensory cortex (SI), was only affected by stimulus energy. This selective suppression of late LEPs indicates that placebo analgesia is mediated by direct intracortical modulation rather than inhibition of the nociceptive input at spinal level. The observed cortical modulation occurs after the responses elicited by the nociceptive stimulus in the SI, suggesting that higher order sensory processes are modulated during placebo analgesia.

Pain and placebo in pediatrics: a comprehensive review of laboratory and clinical findings

Pain modulation by placebo mechanisms is one of the most robust and best-studied phenomena, yet almost all research investigating the mechanisms and implications of the placebo analgesia are based on adult research. After highlighting crucial aspects that need to be considered in studying pain modulation in children, this comprehensive review examines studies related to pain modulation with an emphasis on factors such as age, neural development and pain measures. We critically discuss psychological mechanisms underlying placebo effects, including (1) verbally induced expectations, (2) conditioning and learning mechanisms, and (3) child-parent-physician interactions. Taken together, research suggests that placebo mechanisms can affect therapeutic outcomes and potentially be exploited clinically to improve clinical outcomes in pediatric population. Recommendations for further investigating the mechanistic bases and harnessing placebo effects for supportive therapeutic applications are given.

Mind matters: placebo enhances reward learning in Parkinson's disease

Expectations have a powerful influence on how we experience the world. Neurobiological and computational models of learning suggest that dopamine is crucial for shaping expectations of reward and that expectations alone may influence dopamine levels. However, because expectations and reinforcers are typically manipulated together, the role of expectations per se has remained unclear. Here, we separated these two factors using a placebo dopaminergic manipulation in Parkinson’s patients. We combined a reward learning task with fMRI to test how expectations of dopamine release modulate learning-related activity in the brain. We found that the mere expectation of dopamine release enhances reward learning and modulates learning-related signals in the striatum and the ventromedial prefrontal cortex. These effects were selective to learning from reward: neither medication nor placebo had an effect on learning to avoid monetary loss. These findings suggest a neurobiological mechanism by which expectations shape learning and affect.

Role of explicit verbal information in conditioned analgesia

BACKGROUND

The exact role of expectation in conditioned analgesia is still elusive as it is not clear whether conditioning is an automatic process or rather it is cognitively mediated. This study is aimed at understanding the role of explicit verbal information in conditioned analgesia.

METHODS

Two groups of healthy subjects received a conditioning procedure whereby two visual cues were paired with increase and decrease in stimulus intensity. In the 'conditioning/verbal information' group (VER), subjects were informed about the meaning of the cues, whereas no information was given to the second group (noVER). After two conditioning blocks, an evocation session was run in which the stimulus intensity was the same, irrespective of the cues. Pain perception was assessed according to a numerical rating scale from 0 (no pain) to 10 (maximal pain). The N2-P2 component of laser-evoked potentials (LEP) was used as an index of index of brain responses to nociceptive stimuli.

RESULTS

In the evocation session, only the VER group reported a decrease in pain rating and LEP amplitude when the cues were presented, suggesting that the visual-analgesic association does not occur without explicit verbal information.

CONCLUSIONS

In line with the cognitive theory of conditioning, our results indicate that just pairing a cue with different pain stimulus intensities is not sufficient, per se, to produce a learning process. What matters is the informational cognitive content of the cue, i.e. the meaning assigned to the cue itself. These findings may help understand the mechanisms of conditioned analgesia and more in general of learning.

Placebo-induced changes in excitatory and inhibitory corticospinal circuits during motor performance

Despite behavioral evidence showing placebo modulations of motor performance, the neurophysiological underpinnings of these effects are still unknown. By applying transcranial magnetic stimulation (TMS) over the primary motor cortex, we investigated whether a placebo modulation of force could change the excitability of the corticospinal system. Healthy human volunteers performed a motor task by pressing a piston as strongly as possible with the right index finger. Two experimental groups were instructed that treatment with peripheral low-frequency transcutaneous electrical nerve stimulation (TENS) applied on the first dorsal interosseus would induce force enhancement. One experimental group was conditioned about the effects of TENS with a surreptitious amplification of the visual feedback signaling the force level. The other group, instead, was only verbally influenced, without conditioning. At the end of the instructive placebo procedure, the two experimental groups reached higher levels of force, believed that TENS had been effective and expected to perform better compared with two control groups, who were not influenced about TENS. Moreover, the experimental groups presented enhanced excitability of the corticospinal system in the muscle specifically involved in the task (first dorsal interosseus), as shown by increased amplitude of the motor evoked potentials and decreased duration of the cortical silent period (the latter only in the conditioned group). Crucially, the TMS pulse was delivered when all the subjects exerted the same amount of force, ruling out bottom-up influences. These findings hint at a top-down, cognitive enhancement of corticospinal excitability as a neural signature of placebo modulation of motor performance.

Clinical and ethical implications of placebo effects: enhancing patients' benefits from pain treatment

Expectancy and learning are the core psychological mechanisms of placebo analgesia. They interact with further psychological processes such as emotions and motivations (e.g., anxiety, desire for relief), somatic focus, or cognitions (e.g., attitudes toward the treatment). The development of placebo responsiveness and the actual placebo response in a person is the result of the complex interaction between factors traced back to the individual learning history related to analgesic drugs or treatments and factors of the current context referring to the analgesic or placebo treatment. The aim of this chapter is to depict these complex interactions in a new model of analgesic placebo effects. It joins aspects of the learning history (preexisting experiences and preexisting expectations) of a patient with aspects of the current context (current expectation as a result of external and internal situation in which a pain medication/treatment/placebo is taken, e.g., current information about pain medication, current specific context/cues, desire for pain relief, certainty about upcoming pain relief, current expectation about pain reducing course, current selective attention, increased pain experience, or decreased pain experience). In order to exploit placebo efficacy for an analgesic treatment it is worthwhile to assess in which direction each of these factors exerts its influence in order to maximize placebo effects for a specific patient. By applying placebo mechanisms in this differentiated way, the efficacy of pain treatment can be deliberately boosted.

Placebo and nocebo effects: an introduction to psychological and biological mechanisms

Placebo and nocebo effects are essential components of medical practice and efficacy research, and can be regarded as a special case of context learning. A fundamental function of the central nervous system is to configure the way in which previous learned context becomes linked to corresponding responses. These responses could be either automatic procedures with little flexibility or highly adaptive procedures modified by associated contexts and consequences. Placebo and nocebo effects may represent a typical example of the combination of the two: conditioning effect, which is an inflexible, instinctual, and automatic response, and cognitive expectancy effect, which is a flexible adaptive response modified by prevailing conscious context. Given the fact that contextual learning originates in the brain, neuroimaging tools have been widely used to study placebo and nocebo effects. In addition, pretest resting state fMRI may be a valuable biomarker to predict placebo responses.

The effects of placebos and nocebos on physical performance

In this chapter we present and discuss recent studies on the mechanisms underlying placebo and nocebo effects in physical performance, showing how expectations and both pharmacological and nonpharmacological preconditioning procedures can be very effective in inducing placebo responses, with important implications for sport competitions. Furthermore, we place these findings within the biological model of central governor of fatigue, whose main goal is to protect our body from damage. A crucial aspect of this emerging field of placebo studies is related to the limit beyond which these procedures can be called doping in all respects.

The Effects of Manipulating Verbal Suggestions on Physical Performance

Expectation induced by verbal suggestions is known to be a central mechanism in the generation of the placebo response in different physiological and pathological conditions. Here, we analyze the impact of different verbal suggestions in an experimental setting, measuring work performed, and rate of perceived exertion in healthy subjects undergoing strenuous exercise. A double protocol is used, with different probabilities of receiving what is believed to be an ergogenic substance (in fact, a placebo), with or without an associated conditioning procedure. We show that while high verbal suggestions of motor improvement can work alone, low verbal suggestions produce a response only after conditioning, and that in the absence of verbal suggestions (open-label placebo) no response can ever be elicited. These results can be useful for the design of both clinical trial and sports training protocols.

Placebo, nocebo, and learning mechanisms

Recent substantial laboratory and theoretical research hints for different learning mechanisms regulating the formation of placebo and nocebo responses. Moreover, psychological and biological variants may play a role as modulators of learning mechanisms underlying placebo and nocebo responses. In this chapter, we present pioneering and recent human and nonhuman research that has impressively increased our knowledge of learning mechanisms in the context of placebo and nocebo effects across different physiological processes and pathological conditions.

Cognitive factors mediate placebo responses in patients with house dust mite allergy

Background

Placebo effects have been reported in type I allergic reactions. However the neuropsychological mechanisms steering placebo responses in allergies are largely unknown. The study analyzed whether and to what extend a conditioned placebo response is affecting type I allergic reactions and whether this response can be reproduced at multiple occasions.

Methods

62 patients with house dust mite allergy were randomly allocated to either a conditioned (n = 25), sham-conditioned (n = 25) or natural history (n = 12) group. During the learning phase (acquisition), patients in the conditioned group received the H₁-receptor antagonist desloratadine (5mg) (unconditioned stimulus/US) together with a novel tasting gustatory stimulus (conditioned stimulus/CS). Patients in the sham-conditioned control group received the CS together with a placebo pill. After a wash out time of 9 days patients in the conditioned and sham-conditioned group received placebo pills together with the CS during evocation. Allergic responses documented by wheal size after skin prick test and symptom scores after nasal provocation were analyzed at baseline, after last desloratadine treatment and after the 1^st and 5^th CS re-exposure.

Results

Both conditioned and sham-conditioned patients showed significantly decreased wheal sizes after the 1^st CS-evocation and significantly decreased symptom scores after the 1^st as well as after the 5^th evocation compared to the natural history control group.

Conclusions

These results indicate that placebo responses in type I allergy are not primarily mediated by learning processes, but seemed to be induced by cognitive factors such as patients’ expectation, with these effects not restricted to a single evocation.

Are all placebo effects equal? Placebo pills, sham acupuncture, cue conditioning and their association

Placebo treatments and healing rituals have been used to treat pain throughout history. The present within-subject crossover study examines the variability in individual responses to placebo treatment with verbal suggestion and visual cue conditioning by investigating whether responses to different types of placebo treatment, as well as conditioning responses, correlate with one another. Secondarily, this study also examines whether responses to sham acupuncture correlate with responses to genuine acupuncture. Healthy subjects were recruited to participate in two sequential experiments. Experiment one is a five-session crossover study. In each session, subjects received one of four treatments: placebo pills (described as Tylenol), sham acupuncture, genuine acupuncture, or no treatment rest control condition. Before and after each treatment, paired with a verbal suggestion of positive effect, each subject's pain threshold, pain tolerance, and pain ratings to calibrated heat pain were measured. At least 14 days after completing experiment one, all subjects were invited to participate in experiment two, during which their analgesic responses to conditioned visual cues were tested. Forty-eight healthy subjects completed experiment one, and 45 completed experiment two. The results showed significantly different effects of genuine acupuncture, placebo pill and rest control on pain threshold. There was no significant association between placebo pills, sham acupuncture and cue conditioning effects, indicating that individuals may respond to unique healing rituals in different ways. This outcome suggests that placebo response may be a complex behavioral phenomenon that has properties that comprise a state, rather than a trait characteristic. This could explain the difficulty of detecting a signature for "placebo responders." However, a significant association was found between the genuine and sham acupuncture treatments, implying that the non-specific effects of acupuncture may contribute to the analgesic effect observed in genuine acupuncture analgesia.

Placebo and the new physiology of the doctor-patient relationship

Modern medicine has progressed in parallel with the advancement of biochemistry, anatomy, and physiology. By using the tools of modern medicine, the physician today can treat and prevent a number of diseases through pharmacology, genetics, and physical interventions. Besides this materia medica, the patient's mind, cognitions, and emotions play a central part as well in any therapeutic outcome, as investigated by disciplines such as psychoneuroendocrinoimmunology. This review describes recent findings that give scientific evidence to the old tenet that patients must be both cured and cared for. In fact, we are today in a good position to investigate complex psychological factors, like placebo effects and the doctor-patient relationship, by using a physiological and neuroscientific approach. These intricate psychological factors can be approached through biochemistry, anatomy, and physiology, thus eliminating the old dichotomy between biology and psychology. This is both a biomedical and a philosophical enterprise that is changing the way we approach and interpret medicine and human biology. In the first case, curing the disease only is not sufficient, and care of the patient is of tantamount importance. In the second case, the philosophical debate about the mind-body interaction can find some important answers in the study of placebo effects. Therefore, maybe paradoxically, the placebo effect and the doctor-patient relationship can be approached by using the same biochemical, cellular and physiological tools of the materia medica, which represents an epochal transition from general concepts such as suggestibility and power of mind to a true physiology of the doctor-patient interaction.

Modulation of laser-evoked potentials and pain perception by transcutaneous electrical nerve stimulation (TENS): a placebo-controlled study in healthy volunteers

OBJECTIVE

To investigate the effects of transcutaneous electrical nerve stimulation (TENS) on brain nociceptive responses (laser-evoked potentials, LEPs) and pain perception.

METHODS

Twenty healthy subjects were included. Nociceptive CO(2)-laser pulses were sequentially delivered to the dorsum of both feet. The amplitude of LEPs and nociceptive thresholds were collected in three consecutive conditions: T1: "sham" TENS (2 Hz/low-intensity) positioned heterotopically, over the left thigh; T2: "active" TENS (120 Hz/low-intensity) applied homotopically, over the left common peroneal nerve; and T3: "sham" TENS (replication of condition T1).

RESULTS

Compared with "sham" TENS, "active" TENS significantly decreased the LEPs amplitude. This effect was observed exclusively when "active" TENS was applied ipsilaterally to the painful stimulus. Nociceptive thresholds increased with sessions in both limbs, but the increase observed during the "active" condition of TENS (T2) exceeded significantly that observed during the condition T3 only on the foot ipsilateral to TENS.

CONCLUSIONS

Compared with a credible placebo TENS, high-frequency TENS induced a significant attenuation of both the acute pain and LEPs induced by noxious stimuli applied on the same dermatome.

SIGNIFICANCE

This modulation of subjective and objective concomitants of pain processing reflects a real neurophysiological TENS-related effect on nociceptive transmission.

Placebo effects on the immune response in humans: the role of learning and expectation

Placebo responses are primarily mediated via two neuropsychological mechanisms: patients’ expectation towards the benefit of a treatment and associative learning processes. Immune functions, like other physiological responses, can be modulated through behavioral conditioning. However, it is unknown whether learned immune responses are affected by the number of re-expositions to the conditioned stimulus (CS) during evocation. Moreover, it is unclear whether immune functions can also be modulated through mere verbally induced expectation. In the experiments reported here, we investigated in healthy male volunteers with an established model of learned immunosuppression whether a single re-exposition to the CS is able to induce a behaviorally conditioned immunosuppression. This conditioned immunosuppression is reflected through a significantly decreased interleukin (IL)-2 production by anti-CD3 stimulated peripheral blood mononuclear cells. Our data revealed that in contrast to four CS re-expositions (control group n = 15; experimental group n = 17), a single CS re-exposition was not sufficient to significantly suppress IL-2 production (control group n = 9, experimental group n = 10). Furthermore, we could demonstrate that mere expectation of taking an immunosuppressant did not cause an immunosuppressive response (n = 8–9 per expectation condition). Together, these findings extend our knowledge about the kinetics and mechanisms of placebo-induced immunosuppression and provide therewith information for designing conditioning protocols, which might be employed as a supportive therapy in clinical settings.

Shared "core" areas between the pain and other task-related networks

The idea of a ‘pain matrix’ specifically devoted to the processing of nociceptive inputs has been challenged. Alternative views now propose that the activity of the primary and secondary somatosensory cortices (SI, SII), the insula and cingulate cortex may be related to a basic defensive system through which significant potentially dangerous events for the body's integrity are detected. By reviewing the role of the SI, SII, the cingulate and the insular cortices in the perception of nociceptive and tactile stimuli, in attentional, emotional and reward tasks, and in interoception and memory, we found that all these task-related networks overlap in the dorsal anterior cingulate cortex, the anterior insula and the dorsal medial thalamus. A thorough analysis revealed that the ‘pain-related’ network shares important functional similarities with both somatomotor-somatosensory networks and emotional-interoceptive ones. We suggest that these shared areas constitute the central part of an adaptive control system involved in the processing and integration of salient information coming both from external and internal sources. These areas are activated in almost all fMRI tasks and have been indicated to play a pivotal role in switching between externally directed and internally directed brain networks.

Enhancing non-noxious perception: behavioural and neurophysiological correlates of a placebo-like manipulation

Sensory perception can be influenced by cognitive functions like attention and expectation. An emblematic case of this is the placebo effect, where a reduction in pain perception can be obtained by inducing expectation of benefit following a treatment. The current study assessed the behavioural and brain activity correlates of a placebo procedure inducing an enhancement of non-noxious somatic sensation. An experimental group was verbally suggested and surreptitiously conditioned about the effect of an inert cream in enhancing tactile perception, while a control group was informed about the actual inefficacy of the cream. Both groups received non-noxious electric shocks activating A-Beta fibres on the right index finger, before and after application of the cream in the same site. The behavioural and neurophysiological effects of this procedure were measured by a numerical rating scale of subjective perception and by recording cortical and subcortical somatosensory-evoked potentials (SEPs). Although the intensity of stimulation was physically identical in the two sessions, the experimental group reported stronger tactile sensation after cream treatment than before. In parallel, the experimental group showed enhanced somatosensory cortical responses (N140, P200) after treatment, whereas subcortical and early-cortical SEP components did not change. We suggest that these findings reflect top-down modulation on tactile perception probably due to an interplay between expectation and attention and might rely on interactions between prefrontal and parietal brain regions.

Distraction and placebo: two separate routes to pain control

An explosion of recent research has studied whether placebo treatments influence health-related outcomes and their biological markers, but almost no research has examined the psychological processes required for placebo effects to occur. This study tested whether placebo treatment and cognitive distraction reduce pain through shared or independent processes. We tested the joint effects of performance of an executive working memory task and placebo treatment on thermal pain perception. An interactive effect of these two manipulations would constitute evidence for shared mechanisms, whereas additive effects would imply separate mechanisms. Participants (N = 33) reported reduced pain both when they performed the working memory task and when they received the placebo treatment, but the reductions were additive, a result indicating that the executive demands of the working memory task did not interfere with placebo analgesia. Furthermore, placebo analgesia did not impair task performance. Together, these data suggest that placebo analgesia does not depend on active redirection of attention and that expectancy and distraction can be combined to maximize pain relief.

The placebo effect: advances from different methodological approaches

There is accumulating evidence from different methodological approaches that the placebo effect is a neurobiological phenomenon. Behavioral, psychophysiological, and neuroimaging results have largely contributed to accepting the placebo response as real. A major aspect of recent and future advances in placebo research is to demonstrate linkages between behavior, brain, and bodily responses. This article provides an overview of the processes involved in the formation of placebo responses by combining research findings from behavioral, psychophysiological, and neuroimaging methods. The integration of these different methodological approaches is a key objective, motivating our scientific pursuits toward a placebo research that can inform and guide important future scientific knowledge.

Patients' direct experiences as central elements of placebo analgesia

Placebo analgesic effects appear to be related to patients' perception of the therapeutic intervention. In this paper, we review quantitative findings of how the relationship with the physician and the verbal suggestions given for relief may influence patients' perception of a treatment and how patients' expectations and emotional feelings may affect treatment outcome. We also present qualitative data from interviews with patients who have experienced pain relief following a placebo or an active treatment. A special focus is given to the temporal development of placebo analgesia at psychological and neurophysiological levels. Finally, we discuss the extent to which the quantitative and qualitative findings supplement or contrast with each other, and we touch upon possible implications of patients' direct experience as central for placebo analgesia.

Variability in placebo analgesia and the role of fear of pain--an ERP study

Fear of pain (FOP) and its effect on placebo analgesia was investigated. It was hypothesized that FOP should interfere with placebo-mediated pain inhibition and result in weaker placebo responding in pain intensity, pain unpleasantness, stress, and event-related potentials to contact heat pain. Thirty-three subjects participated in a balanced 2 condition (natural history, placebo)×3 test (pretest, posttest 1, posttest 2) within-subject design, tested on 2 separate days. FOP was measured by the Fear of Pain Questionnaire and subjective stress by the Short Adjective Check List. Placebo effects were found on reported pain unpleasantness and N2 and P2 amplitudes. FOP was related to reduced placebo responding in pain unpleasantness, but this was only evident for the subjects who received the placebo condition on day 1. Subjects who received the placebo condition on day 1 experienced more pretest stress than those who received the placebo condition on day 2 (ie, reversed condition order), and this explained the interaction effect on placebo responding. FOP was related to reduced placebo responding on P2 amplitude, whereas placebo responding on N2 amplitude was unaffected by FOP. Higher placebo responses on N2 and P2 amplitudes were both related to higher placebo analgesic magnitude in pain unpleasantness. In conclusion, increased FOP was found to reduce subjective and electrophysiological placebo analgesic responses.