Neuroscientific evidence for pain being a classically conditioned response to trauma- and pain-related cues in humans

Why are threatening experiences remembered so well? Insights into memory strengthening from protocols of gradual aversive learning

Aversive experiences often result in strong and persistent memory traces, which can sometimes lead to conditions such as Post-Traumatic Stress Disorder or phobias. Aversive stimulation tests are key tools in psychology and neuroscience for studying learning and memory. These tests typically use electric shocks as the unconditioned stimulus, allowing for precise control over the aversive content of the learning event. This feature has led to extensive research applying these tests with varying shock intensities to examine differences in learning, behavior, and memory formation between low- and high-aversive experiences. This line of research is particularly valuable for understanding the neurobiology underlying memory strengthening, but, to our knowledge, no review has yet compiled and organized the findings from this specific methodology. In this comprehensive review, we focus primarily on animal studies that have employed the same aversive test (i.e. Fear Conditioning, Passive Avoidance, Active Avoidance or Operant boxes) at different intensities. We will first outline and briefly describe the main aversive learning paradigms used in this field. Next, we will examine the relationship between aversiveness and memory strength. Finally, we will explore the neurobiological insights these studies have revealed over the years. Our aim is to gain a better understanding of how the nervous system gradually strengthens memory, while also addressing the remaining gaps and challenges in this area of research.

Pain distribution can be determined by classical conditioning

ABSTRACT

Chronic widespread pain (CWP)-as in many other clinical presentations-manifests in ongoing pain without identifiable structural cause, with pain that spreads over multiple body areas. The development and maintenance of symptoms may involve learning mechanisms. Ninety-four healthy volunteers participated in this study and were randomly distributed to 4 groups. In the classical conditioning combined with the verbal suggestion group, US- (small pain distribution) and US+ (large pain distribution) were paired with visual stimuli (CS+ and CS-), and participants were told about this association. In the verbal suggestion group, the conditioning was not performed, whereas in the classical conditioning-only group, learning was not combined with suggestion. In the control group, conditioning and suggestion did not take place. Ratings of perceived pain distribution were collected after each trial and ratings of pain intensity after each block of trials. During the testing phase, participants were exposed to electrocutaneous stimuli corresponding to only the small (US-) pain distribution. The results showed significant differences between CS+ and CS- pain distribution ratings across the experimental groups: conditioning + verbal suggestion (P < 0.01), conditioning-only group (P < 0.05), and verbal suggestion-only group (P < 0.05), but not in the control group (P > 0.05). Furthermore, significant differences in the perceived pain distribution were found between the control group and all experimental groups. This result supports our main hypothesis that the perceived pain distribution can be influenced by classical conditioning as well as verbal suggestion, although the effect is stronger when both are combined.

Multisite pain among United States Veterans with posttraumatic stress disorder: Prevalence, predictors, and associations with symptom clusters

Persistent pain in multiple distinct body sites is associated with poorer functional outcomes above and beyond pain intensity and interference. Veterans, and especially those with posttraumatic stress disorder (PTSD), may be at risk for multisite pain. However, the research to date characterizing this presentation is limited. This secondary analysis examined the prevalence of multisite pain in a cross-sectional sample of Veterans and explored demographic, military service-related, and PTSD symptom cluster variables associated with multisite pain among those with clinically significant PTSD symptoms. Participants were 4303 post-9/11 U.S. Veterans (16.55% female gender, 58.45% White/Caucasian, Mage = 35.52), of whom 1375 (31.95%) had clinically significant PTSD symptoms. Multisite pain was defined as endorsing pain that "bothered [me] a lot" in ≥3 body sites out of 5 on the Patient Healthcare Questionnaire-15. A total of 20.03% of all participants, and 40.00% of those with likely PTSD, reported multisite pain. Female gender (OR = 1.55), older age (OR = 1.70), minority race identification (White/Caucasian racial identity OR = 0.75), history of military sexual trauma (OR = 1.99), and spine, abdomen and joint/muscle injuries (ORs = 1.66-3.68) were associated with higher odds of multisite pain. Adjusting for these potential confounders, higher z-scores on the PTSD arousal/reactivity (OR = 1.58, p <.001) subscale was associated with higher multisite pain odds. In summary, multisite pain was common among Veterans with PTSD, especially those who experienced military sexual trauma or certain physical injuries. Multisite pain and PTSD may be associated due to a shared threat reactivity mechanism. PERSPECTIVE: This study investigates the rates and factors associated with having pain in three or more distinct body sites (multisite pain) among United States Veterans. The study findings highlight the unique importance of specific posttraumatic stress symptoms and experiences associated with multisite pain.

Exploration of stress reactivity and fear conditioning on intrusive memory frequency in a conditioned-intrusion paradigm

BACKGROUND AND OBJECTIVES

The conditioned-intrusion paradigm was designed to provide insight into the relationship between fear conditioning and intrusive memory formation, which is relevant to understanding posttraumatic stress disorder symptoms and treatment. However, boundary conditions of this new paradigm have not been explored and it is currently not known whether findings from this work are valid in a clinical context.

METHODS

In the current study, we explored the relationship between stress reactivity to trauma film clips, usual exposure to violent media, renewal of fear conditioning using skin conductance as well as subjective ratings, and the effect of shock versus film clip during conditioning on the frequency of intrusive memories. An adapted fear conditioning paradigm using trauma clips as unconditional stimuli was used, and participants subsequently reported intrusive memories of the trauma clips.

RESULTS

Skin conductance responses to conditioned stimuli paired with shocks and film clips were significantly higher than conditioned stimuli paired with film clips alone. Subjective stress reactivity, previous exposure to violent media, and film valence rating were associated with the frequency of intrusive memories. No aspects of fear conditioning were associated with intrusive memories, and factor analysis suggested the fear conditioning and stress related to film clip viewing were mostly separate constructs. Similarly, content and triggers of intrusive memories were usually film-clip related rather than conditional stimulus related.

LIMITATIONS

We did not observe strong conditioning effects of the unconditional stimuli to conditional stimuli, which were shapes rather than high frequency stimuli such as faces.

CONCLUSIONS

These findings provide potential boundary conditions for this paradigm and suggest multiple ways in which the validity of the paradigm can be tested in the future.

Manual Therapy Techniques Versus Occlusal Splint Therapy for Temporomandibular Disorders: A Systematic Review with Meta-Analysis

Background: Manual therapy (MT) and occlusal splint therapy (OST) are the most conservative therapies applied on patients with temporomandibular disorders (TMDs). The aim was to compare the efficacy of MT vs. OST in improving pain, maximal mouth opening (MMO), disability, and health related-quality of life (hr-QoL) in these patients. Methods: According to PRISMA guidelines, a meta-analysis (CRD42022343915) was conducted including randomized controlled trials comparing the effectiveness of MT vs. OST in TMD patients, after searching in PubMed, PEDro, SCOPUS, and WOS up to March 2024. Methodological quality and risk of bias were assessed using the PEDro Scale. Cohen's standardized mean difference (SMD) and its 95% confidence interval (95% CI) were the pooled effect measures calculated. Results: Nine studies, providing data from 426 patients, were included. Meta-analyses revealed that MT is more effective than OST in reducing disability (SMD = -0.81; 95% CI -1.1 to -0.54) and increasing MMO (SMD = 0.52; 95% CI 0.27 to 0.76) without differences for improving pain intensity and hr-QoL. Subgroup analyses revealed the major efficacy of OST in reducing pain in myogenic patients (SMD = 0.65; 95% CI 0.02 to 1.28). Conclusions: With caution, due to the low number of studies included, MT may be more effective than OST for improving disability and MMO in patients with TMDs.

Hitting the Rewind Button: Imagining Analogue Trauma Memories in Reverse Reduces Distressing Intrusions

Background

Intrusive re-experiencing of trauma is a core symptom of posttraumatic stress disorder. Intrusive re-experiencing could potentially be reduced by 'rewinding', a new treatment approach assumed to take advantage of reconsolidation-updating by mentally replaying trauma fast-backward.

Methods

The present analogue study was the first to investigate 'rewinding' in a controlled laboratory setting. First, 115 healthy women watched a highly aversive film and were instructed to report film-related intrusions during the following week. Twenty-four hours after film-viewing, participants reporting at least one intrusion (N = 81) were randomly allocated to an intervention (fast-backward, or fast-forward as active control condition) or a passive control condition. Intervention groups reactivated their trauma memory, followed by mentally replaying the aversive film either fast-backward or fast-forward repeatedly.

Results

Results indicate that replaying trauma fast-backward reduced intrusion load (intrusion frequency weighted for intrusion distress) compared to the passive group, whereas replaying fast-forward did not. No above-threshold differences between fast-backward and fast-forward emerged.

Conclusion

Present findings strengthen the view that 'rewinding' could be a promising intervention to reduce intrusions.

In search of conditioned pain: an experimental analysis

ABSTRACT

There is an ongoing debate about whether pain can be classically conditioned, but surprisingly, evidence is scarce. Here, we report 3 experiments investigating this idea. In a virtual reality task, healthy participants were approached and touched near or on their hand with a coloured pen (blue or yellow). During acquisition, participants learned that one of the colours of the pen (CS+) was predictive of a painful electrocutaneous stimulus (ECS) whereas the other coloured pen (CS-) was not. During the test phase, more frequent reports of experiencing an US when none was delivered ("false alarm") for the CS+ vs CS- qualified as evidence of conditioned pain. Notable differences between experiments were that the US was delivered when the pen touched a spot between the thumb and index finger (experiment 1; n = 23), when it virtually touched the hand (experiment 2; n = 28) and when participants were informed that the pen caused pain rather than simply predicting something (experiment 3; n = 21). The conditioning procedure proved successful in all 3 experiments: Self-reported fear, attention, pain, fear, and US expectancy were higher ( P < 0.0005) for the CS+ than the CS-. There was no evidence for conditioned pain in experiment 1, but there was some evidence in experiments 2 and 3. Our findings indicate that conditioned pain may exist, albeit most likely in rare cases or under specific situations. More research is needed to understand the specific conditions under which conditioned pain exists and the underlying processes (eg, response bias).

Learning pain in context: Response-conditioned placebo analgesia and nocebo hyperalgesia in male rats with chronic neuropathic pain

BACKGROUND

Animal models of placebo analgesia and nocebo hyperalgesia have great potential to assist in the development of novel treatments for chronic pain that exploit or inhibit these phenomena. This study sought to elicit both conditioned placebo analgesia and conditioned nocebo hyperalgesia in rats with chronic neuropathic pain using non-pharmacological, contextual conditioning approaches, similar to those most often used in humans.

METHODS

Sciatic nerve-injured male Sprague-Dawley rats (n = 80), and sham controls (n = 16), underwent a conditioning procedure in which three different thermal stimulus intensities (4 °C, 20 °C or 30 °C) were paired with contextual cues. Injured hind paw withdrawal behaviours were used to determine pain sensitivity, and either conditioned analgesia or conditioned hyperalgesia was evoked by re-exposing the rats to the same context with either an increased or decreased thermal stimulus, respectively.

RESULTS

Stronger conditioned analgesia and conditioned hyperalgesia were seen when rats were conditioned in a more complex environment, highlighting the importance of context in these processes. Rats that did not undergo conditioning procedures showed fewer hind paw withdrawals, indicating a learned component to these pain behaviours.

CONCLUSIONS

Our data call attention to context and learning as two critical factors in the development of placebo and nocebo effects in male rodents with a neuropathic injury. Additionally, the response-conditioning model we present in this study affords better comparisons between human and animal studies, in particular for those seeking to identify commonalities in the neurobiological mechanisms of placebo and nocebo responses.

Estradiol during (analogue-)trauma: Risk- or protective factor for intrusive re-experiencing?

Intrusions, a key symptom of posttraumatic stress disorder (PTSD), can occur in the form of images but also as pain sensations. Similar to audiovisual intrusions, the frequency and persistence of pain intrusions varies greatly between individuals. In the current study, we examined whether peritraumatic circulating 17β-estradiol (E2) levels are a biologic factor associated with subsequent audiovisual (i.e., film) and pain intrusion development, and whether peritraumatic stress levels modulate this relationship. Forty-one free-cycling women participated in an ecologically informed trauma-pain-conditioning (TPC) paradigm, using trauma-films and pain as unconditioned stimuli. Independent variables were salivary peritraumatic E2 levels and stress indexed by salivary cortisol and self-reported state-anxiety during TPC. Outcomes were film- and pain-intrusions occurring during daily-life in the week following TPC and a Memory-Triggering-Task in response to conditioned stimuli 24 h after TPC. In the week after analogue-trauma, higher peritraumatic E2 levels were associated with a greater probability of experiencing film-intrusions in the beginning of the week, which switched to a lower probability toward the end of the week. This time-dependent relationship between E2 and film-intrusions only held for higher state-anxious women. In contrast, results indicated a consistent inverse relationship between peritraumatic E2 levels and pain-intrusions during daily-life and Memory-Triggering-Task. Together, these data suggest that higher peritraumatic E2 levels could be associated with lower long-term visual trauma intrusions, as well as lower pain-intrusions, and thereby possibly constitute a protective biologic factor for PTSD and potentially also for chronic pain.