Spatial Sensory Organization and Body Representation in Pain Perception

Interictal widespread pressure hyperalgesia and aura: associations with vestibular migraine in a cross-sectional study

Background

Patients with vestibular migraine (VM) exhibit higher levels of central sensitization and share similar disorder characteristics with migraine with vestibular symptoms (MwVS), except in terms of disability. These patients experience fluctuating mechanical pain thresholds and persistent vestibular symptoms even without a migraine attack.

Objective

This study aimed to investigate whether interictal allodynia or hyperalgesia can differentiate between VM, MwVS, and migraine only.

Methods

We conducted a cross-sectional study of patients with episodic migraine aged between 18 and 65 years, categorized into three groups. A questionnaire was used to collect and compare demographic and clinical variables. Interictal widespread pressure hyperalgesia (IWPH) was evaluated using the Manual Tender Point Survey. Patients with tender point counts ≥7 were classified as having IWPH.

Results

The study included 163 patients: 31 with VM, 54 with MwVS, and 78 with migraine without vestibular symptoms (migraine only). We found that aura (p = 0.042, odds ratio 3.50, 95% confidence interval 1.26-10.4), tender point count (p < 0.001, d = 0.889, median difference = 2), and IWPH (p = 0.002, odds ratio 5.3, 95% confidence interval 1.80-17.2) were significantly associated with VM compared to MwVS. Aura and IWPH were significantly associated with VM. However, there were no significant associations observed for interictal allodynia or hyperalgesia between the other two groups.

Conclusion

IWPH and aura are associated with VM, indicating their potential roles in its pathogenesis. These findings may contribute to the differential diagnosis and management of migraine, potentially leading to targeted treatment strategies.

Neuroanatomical correlates of peripersonal space: bridging the gap between perception, action, emotion and social cognition

Peripersonal space (PPS) is a construct referring to the portion of space immediately surrounding our bodies, where most of the interactions between the subject and the environment, including other individuals, take place. Decades of animal and human neuroscience research have revealed that the brain holds a separate representation of this region of space: this distinct spatial representation has evolved to ensure proper relevance to stimuli that are close to the body and prompt an appropriate behavioral response. The neural underpinnings of such construct have been thoroughly investigated by different generations of studies involving anatomical and electrophysiological investigations in animal models, and, recently, neuroimaging experiments in human subjects. Here, we provide a comprehensive anatomical overview of the anatomical circuitry underlying PPS representation in the human brain. Gathering evidence from multiple areas of research, we identified cortical and subcortical regions that are involved in specific aspects of PPS encoding.We show how these regions are part of segregated, yet integrated functional networks within the brain, which are in turn involved in higher-order integration of information. This wide-scale circuitry accounts for the relevance of PPS encoding in multiple brain functions, including not only motor planning and visuospatial attention but also emotional and social cognitive aspects. A complete characterization of these circuits may clarify the derangements of PPS representation observed in different neurological and neuropsychiatric diseases.

Body Image Measured via the Fremantle Awareness Questionnaire in Individuals With and Without Pain: A Systematic Review and Meta-Analysis

Research suggests that pain negatively affects body image, and body image may also influence reported pain levels. This review aims to summarize the literature on differences in body image distortion between individuals with pain compared to pain-free individuals. The review was conducted according to the Preferred Reporting Items for Systematic reviews and Meta-Analyses 2020 statement and an a priori preregistered protocol. The literature was searched using 5 electronic databases. Studies assessing body image with the Fremantle Awareness Questionnaire (FAQ) in individuals with and without pain were eligible for inclusion. Screening and selection of eligible studies were performed by independent reviewers. Methodological quality was assessed with the Joanna Briggs Institute critical appraisal tool. Meta-analyses, meta-correlations, and metamean analyses were performed using random-effect models. The primary outcome was the FAQ score; secondary outcomes were reported pain variables. Data from individuals with pain (n = 2277) and without pain (n = 615) were summarized. Significant body image distortions were found in individuals with pain compared to individuals without pain. Compared to pain-free individuals, the pain group rated significantly higher in the FAQ when experiencing back pain (standardized mean differences=1.33, 95% confidence interval=.88-1.77) or other body parts (standardized mean differences=1.25, 95% confidence interval=.51-1.99). The results of meta-correlation analyses confirmed the positive relationship between body image distortion and pain intensity (r = .31), pain at rest (r = .31), or pain during movement (r = .36), but not for pain duration. A difference in mean FAQ results was observed between individuals with pain in different areas (knee and back). PERSPECTIVE: This review confirms differences in body image distortion between pain and pain-free individuals. Pain intensity was correlated with altered body perception, but not pain duration. A moderate correlation was observed between body image distortion and reported pain variables. Body image was more impaired by knee pain than back pain. REGISTERED PROTOCOL AT PROSPERO: CRD42022309937; https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42022309937.

Combining various acupuncture therapies with multimodal analgesia to enhance postoperative pain management following total knee arthroplasty: a network meta-analysis of randomized controlled trials

Objective

This study aims to evaluate the efficacy and safety of various acupuncture treatments in conjunction with multimodal analgesia (MA) for managing postoperative pain and improving knee function in patients undergoing total knee arthroplasty (TKA), based on the findings from clinical research indicating the potential benefits of acupuncture-related therapies in this context.

Methods

We searched Web of Science, PubMed, SCI-hub, Embase, Cochrane Library, China Biology Medicine (CBM), China National Knowledge Infrastructure (CNKI), Wanfang Data, and Chinese Scientific Journal Database (VIP) to collect randomized controlled trials of acupuncture-related therapies for post-TKA pain. After independent screening and data extraction, the quality of the included literature was evaluated. The potential for bias in the studies incorporated in the analysis was assessed according to the guidelines outlined in the Cochrane Handbook 5.1. Network meta-analysis (NMA) was conducted using RevMan 5.4 and Stata 16.0 software, with primary outcome measures including visual analog scale (VAS), pain pressure threshold (PPT), hospital for special surgery knee score (HSS), and knee joint range of motion (ROM). Furthermore, the interventions were ranked based on the SUCRA value.

Results

We conducted an analysis of 41 qualifying studies encompassing 3,003 patients, examining the efficacy of four acupuncture therapies (acupuncture ACU, electroacupuncture EA, transcutaneous electrical acupoint stimulation TEAS, and auricular acupoint therapy AAT) in conjunction with multimodal analgesia (MA) and MA alone. The VAS results showed no significant difference in efficacy among the five interventions for VAS-3 score. However, TEAS+MA (SMD: 0.67; 95%CI: 0.01, 1.32) was more effective than MA alone for VAS-7 score. There was no significant difference in PPT score among the three interventions. ACU + MA (SMD: 6.45; 95%CI: 3.30, 9.60), EA + MA (SMD: 4.89; 95%CI: 1.46, 8.32), and TEAS+MA (SMD: 5.31; 95%CI: 0.85, 9.78) were found to be more effective than MA alone for HSS score. For ROM score, ACU + MA was more efficacious than EA + MA, TEAS+MA, and AAT + MA, MA. Regarding the incidence of postoperative adverse reactions, nausea and vomiting were more prevalent after using only MA. Additionally, the incidence of postoperative dizziness and drowsiness following ACU + MA (OR = 4.98; 95%CI: 1.01, 24.42) was observed to be higher compared to that after AAT + MA intervention. Similarly, the occurrence of dizziness and drowsiness after MA was found to be significantly higher compared to the following interventions: TEAS+MA (OR = 0.36; 95%CI: 0.18, 0.70) and AAT + MA (OR = 0.20; 95%CI: 0.08, 0.50). The SUCRA ranking indicated that ACU + MA, EA + MA, TEAS+MA, and AAT + MA displayed superior SUCRA scores for each outcome index, respectively.

Conclusion

For the clinical treatment of post-TKA pain, acupuncture-related therapies can be selected as a complementary and alternative therapy. EA + MA and TEAS+MA demonstrate superior efficacy in alleviating postoperative pain among TKA patients. ACU + MA is the optimal choice for promoting postoperative knee joint function recovery in TKA patients. AAT + MA is recommended for preventing postoperative adverse reactions.

Systematic review registration

https://www.crd.york.ac.uk/, identifier (CRD42023492859).

[Pain and its limits : A phenomenological consideration of chronic pain]

Chronic pain poses a significant global socio-medical challenge causing significant costs. It is only since the mid-20th century that pain syndromes have been considered diseases in their own right. According to the definition of the International Association for the Study of Pain, pain is a complex, context-dependent-and hence modifiable-phenomenon. The philosophical view on pain is no less multi-facetted and allows for a wide range of viewpoints. This analysis aims at a characterisation of pain including a-mainly phenomenological and enactivist-philosophical perspective. The discourse will be guided by the concept of the limit(s) of the lived body: what is the relationship between pain and the perception of the lived body's boundaries? Does a reciprocal influence exist? And may the perception be modified in order to reduce the patient's suffering? These musings will also clarify that neurosciences and philosophy are not competing sciences, but rather inform each other.

Tactile acuity improves during acute experimental pain of the limb

Introduction

Chronic pain is associated with poor tactile acuity, commonly measured with the 2-point discrimination (TPD) test. Although poor tactile acuity across chronic pain conditions is well established, less is known in acute pain.

Objective

Recent conflicting findings in experimentally induced neck and back pain led us to conduct a TPD investigation in experimentally induced limb pain. We hypothesised altered TPD during experimental upper limb pain, but we did not speculate on the direction of the change.

Methods

Thirty healthy subjects immersed their dominant hand in a circulating cold-water bath at 7°C (cold pressor test [CPT]). Two-point discrimination was measured at baseline (pre-CPT), during pain (during-CPT), and after withdrawal from the water (post-CPT) in 3 different sites: (1) the dominant forearm, (2) dominant arm and (3) contralateral forearm.

Results

Repeated-measures analysis of variance revealed a significant main effect of time (F(2,56) = 4.45, P = 0.02, η p 2 = 0.14) on TPD; in all 3 sites, TPD values decreased (ie, tactile acuity improved) during pain. Interestingly, the contralateral forearm followed a similar pattern to the dominant (ie, painful) forearm, and furthermore was the only site that exhibited any correlation with pain, albeit in an intriguing direction (r = 0.57, P = 0.001), ie, the greater the pain the worse the tactile acuity.

Conclusion

The improvements in tactile acuity during experimentally induced limb pain may reflect a protective response. The changes in the corresponding site in the contralateral limb may reflect a protective spinal cross talk. Such a response, together with the interesting relationship between tactile acuity and pain, warrant further inquiry.

Distinct neural signatures of multimodal resizing illusions

Illusory body resizing typically uses multisensory integration to change the perceived size of a body part. Previous studies associate these multisensory body illusions with frontal theta oscillations and parietal gamma oscillations for dis-integration and integration of multisensory signals, respectively. However, recent studies also support illusory changes of embodiment from unimodal visual stimuli. This preregistered study (N = 48) investigated differences between multisensory visuo-tactile and unimodal visual resizing illusions using EEG, to gain a more comprehensive understanding of the neural underpinnings of resizing illusions in a healthy population. We hypothesised (1) stronger illusion in multisensory compared to unimodal, and unimodal compared to incongruent (dis-integration) conditions, (2) greater parietal gamma during multisensory compared to unimodal, and (3) greater frontal theta during incongruent compared to baseline conditions. Subjective Illusory results partially support Hypothesis 1, showing a stronger illusion in multisensory compared to unimodal conditions, but finding no significant difference comparing unimodal to incongruent conditions. Results partially supported EEG hypotheses, finding increased parietal gamma activity comparing multisensory to unimodal visual conditions, happening at a later stage of the illusion when compared to previous rubber hand illusion EEG findings, whilst also finding increased parietal theta activity when comparing incongruent to non-illusion conditions. While results demonstrated that only 27% of participants experienced the stretching illusion with unimodal visual stimuli compared to 73% of participants experiencing the stretching illusion in the multisensory condition, further analysis suggested that those who experience visual-only illusions exhibit a different neural signature to those who do not, with activity focussed around frontal and parietal regions early on in the illusory manipulation, compared to activity focussed more over parietal regions and at a later point in the illusory manipulation for the full sample of participants. Our results replicate previous subjective experience findings and support the importance of multisensory integration for illusory changes in perceived body size, whilst adding to our understanding of the temporal onset of multisensory integration within resizing illusions, differing from that of rubber hand illusions.

Virtual embodiment in fibromyalgia

Chronic pain alters the experience of owning a body and leads to disturbances in bodily perception. We tested whether women with fibromyalgia (FM) are receptive to bodily illusions of owning a visible and progressively invisible body in immersive virtual reality (VR), and what modulates this experience. Twenty patients participated in two experimental sessions; each session included two conditions in a counterbalanced order. We found that patients with FM could indeed experience virtual embodiment. Sentiment analysis revealed significantly more positive reactions to the progressively invisible body, yet twice as many patients declared they preferred the illusion of a visible virtual body. A linear mixed model revealed that the strength of embodiment was positively associated with body perception disturbances and negatively associated with FM symptoms intensity. No effect of pain during the VR experience nor interoception awareness on embodiment was found. The results indicated that patients with FM are receptive to virtual bodily illusions and that the impact of the embodiment depends on affective reactions, the level of cognitive body distortions, and the intensity of symptoms. Importantly, there is a large variation among patients which should be considered in future VR-based interventions.

Thalamocortical Mechanisms Underlying Real and Imagined Acupuncture

Both acupuncture and imagery have shown potential for chronic pain management. However, the mechanisms underlying their analgesic effects remain unclear. This study aims to explore the thalamocortical mechanisms underlying acupuncture and video-guided acupuncture imagery treatment (VGAIT), a combination of acupuncture and guided imagery, using the resting-state functional connectivity (rsFC) of three thalamic subdivisions-the ventral posterolateral thalamus (VPL), mediodorsal thalamus (MD), and motor thalamus subregion (Mthal)-associated with somatosensory, limbic, and motor circuity. Twenty-seven healthy individuals participated in a within-subject randomized crossover design study. Results showed that compared to sham acupuncture, real acupuncture altered the rsFC between the thalamus and default mode network (DMN) (i.e., mPFC, PCC, and precuneus), as well as the prefrontal and somatosensory cortex (SI/SII). Compared to the VGAIT control, VGAIT demonstrated greater rsFC between the thalamus and key nodes within the interoceptive network (i.e., anterior insula, ACC, PFC, and SI/SII), as well as the motor and sensory cortices (i.e., M1, SMA, and temporal/occipital cortices). Furthermore, compared to real acupuncture, VGAIT demonstrated increased rsFC between the thalamus (VPL/MD/Mthal) and task-positive network (TPN). Further correlations between differences in rsFC and changes in the heat or pressure pain threshold were also observed. These findings suggest that both acupuncture- and VGAIT-induced analgesia are associated with thalamocortical networks. Elucidating the underlying mechanism of VGAIT and acupuncture may facilitate their development, particularly VGAIT, which may be used as a potential remote-delivered pain management approach.

The Biology of Chronic Pain and Its Implications for Pain Neuroscience Education: State of the Art

Pain is an individualized experience for the person suffering from chronic pain. Significant strides have been made in the last few decades in understanding various biological changes that coincide with chronic pain. This state-of-the-art overview looks at the current evidence related to the biology of chronic pain and the implications these findings have on the delivery of pain neuroscience education (PNE). The paper summarizes the various (epi)genetic, neural, endocrine, and immune factors discovered and explored in the scientific literature concerning chronic pain. Each of these biological factors has various implications for the content and delivery of PNE. We discuss the future directions these biological factors have for the clinical implementation of PNE by linking the importance of behavior change, optimizing the learning environment, and using an individualized multimodal treatment approach with PNE. In addition, future directions for research of PNE based on these biological factors are provided with importance placed on individualized patient-centered care and how PNE can be used with traditional modes of care and growing trends with other care methods. PNE was originally and continues to be rooted in understanding chronic pain biology and how that understanding can improve patient care and outcomes.

Recovery of pinch force sense after short-term fatigue

The aim of this study was to identify the exact origin of force sense and identify whether it arises centrally or peripherally. The present study was designed to analyze the effects of short-term fatigue on pinch force sense and the duration of these effects. During the fatigue protocol, twenty (10 men and 10 women; Mage = 22.0 years old) young Chinese participants were asked to squeeze maximally until the pinch grip force decreased to 50% of its maximal due to fatigue. Participants were instructed to produce the target force (10% of maximal voluntary isometric contraction) using the same hand before and after fatigue (immediately, 10, 30, 60, 180, 300 s). The results showed significantly higher absolute error immediately after fatigue (1.22 ± 1.06 N) than before fatigue (0.68 ± 0.34 N), and 60 s (0.76 ± 0.69 N), 180 s (0.67 ± 0.42 N), and 300 s (0.75 ± 0.37 N) after fatigue (all P < 0.05) but with no effect on the variable error (P > 0.05). It was also revealed that there was a significant overestimate of the constant error values before (0.32 ± 0.61 N) and immediately after fatigue (0.80 ± 1.38 N, all P < 0.05), while no significant overestimation or underestimation exceeded 300 s after fatigue (P > 0.05). Our study results revealed that short-term fatigue resulted in a significant decrease in force sense accuracy, but it did not affect force sense consistently; however, force sense accuracy recovered to a certain extent within 10 s and 30 s, whereas it recovered fully within 60 s, and force sense directivity improvement exceeded 300 s after fatigue. The present study shows that the sense of tension (peripherally) is also an important factor affecting force sense. Our study supports the view that the periphery is part of the origin of force sense.

Grip force makes wrist joint position sense worse

Background

The purpose of this study was to investigate how grip force affects wrist joint position sense.

Methods

Twenty-two healthy participants (11 men and 11 women) underwent an ipsilateral wrist joint reposition test at 2 distinct grip forces [0 and 15% of maximal voluntary isometric contraction (MVIC)] and 6 different wrist positions (pronation 24°, supination 24°, radial deviation 16°, ulnar deviation 16°, extension 32°, and flexion 32°).

Results

The findings demonstrated significantly elevated absolute error values at 15% MVIC (3.8 ± 0.3°) than at 0% MVIC grip force [3.1 ± 0.2°, t(20) = 2.303, P = 0.032].

Conclusion

These findings demonstrated that there was significantly worse proprioceptive accuracy at 15% MVIC than at 0% MVIC grip force. These results may contribute to a better comprehension of the mechanisms underlying wrist joint injuries, the development of preventative measures to lower the risk of injuries, and the best possible design of engineering or rehabilitation devices.

Dizziness and balance outcomes after two different postoperative rehabilitation approaches following neck surgery: analyses of a multicenter randomized controlled trial

BACKGROUND

Dizziness and balance problems are common symptoms in patients with cervical radiculopathy.

OBJECTIVE

To evaluate the effect of neck surgery postoperatively combined with either structured rehabilitation or standard approach in patients with cervical radiculopathy and dizziness and/or balance problems, and investigate factors influencing dizziness and balance at 6-month follow-up.

METHODS

Individuals (n = 149) with cervical radiculopathy and dizziness and/or balance problems were randomized preoperatively to structured postoperative rehabilitation or standard postoperative approach. Outcomes were intensity of dizziness and subjective balance, and clinical measures of balance.

RESULTS

Self-reported measures improved at three months (p ˂ 0.001 to p = .007) and the standing balance at six months (p = .008). No between-group differences. Baseline values, neck pain, and physical activity level explained 23-39% of the variance in 6-month outcomes for self-reported measures. Baseline values and physical activity level explained 71% of the variance in walking balance, and lower baseline scores were significantly associated with standing balance impairments (OR 0.876).

CONCLUSION

Patients improved significantly in dizziness and subjective balance intensity shortly after surgery, and in standing balance at 6 months, independent of postoperative rehabilitation. Neck pain, physical activity, and neck muscle function influenced dizziness and balance, although preoperative values and neck pain were of most importance for 6-month outcomes.

An illusion of disownership over one's own limb is associated with pain perception

Viewing one's body and even a fake/virtual body experienced as one's own has been suggested to modulate pain perception. However, what happens to pain perception when one's own body part is felt as not belonging to one? We designed a paradigm to induce an illusory feeling of disownership regarding one's limb, investigating whether the feeling affects pain threshold. Participants observed right-side images of their bodies from a third-person perspective via a head-mounted display in real-time. Following instructions, they moved their left hand while keeping their left elbow behind the upper body, so that the connection of their arm to the torso was not visible (test condition), or in front of it, so they could see the arm being part of them (control condition). Then, pain threshold was tested with a thermal stimulator. We found a significantly higher strength of disownership in the test condition than in the control condition. While there was no pain modulation within and between conditions, disownership ratings negatively correlated with pain-threshold changes, where the participants reporting explicit disownership showed lower pain-threshold changes than the others. The finding suggests that while multisensory disintegration had no modulatory effect, the individual sense of disownership was associated with pain perception.

Not my body, not my pain? Pain perception and placebo analgesia in individuals with body integrity dysphoria

Acute and chronic states of physical pain are inherently linked to our bodily perception. Bodily illusion paradigms have demonstrated that an experimentally induced sense of body disownership can modulate both acute and chronic pain. Insight into the relationship between enduring clinical alterations in body perception and pain is much more limited. The current study examined both pain perception and placebo analgesia in Body Integrity Dysphoria (BID), a clinical model of long-term alterations of bodily disownership: in its most commonly studied variant, people feel like a part of their body does not belong to them, leading to a desire for amputation of a physically healthy limb. Heat stimulations were applied before and after a placebo intervention (sham analgesic cream) to the desired and the undesired leg of 19 patients with BID with a unilateral leg amputation desire. Pain perception was assessed using pain thresholds, and ratings for pain intensity and pain unpleasantness. Results show that pain perception and placebo efficacy were lower for the undesired than for the desired leg, demonstrating a potential link between a clinical disorder of body ownership, pain perception, and placebo analgesia.

Limb Laterality Discrimination, Evoked Sensations and Somatosensory Behavior in Fibromyalgia Syndrome: A Cross-Sectional Study

The main objective of this study was to assess the status of body schema using limb laterality discrimination tasks and pain measurement variables of patients with FMS compared to healthy subjects. The secondary aim was to analyze the relationships between laterality discrimination with respect to somatosensory variables. Thirty female patients with FMS (with a mean age of 52.43 ± 11.82 years) and thirty healthy women (with a mean age of 47.93 ± 5.92 years) were recruited. The main outcome measures were laterality discrimination, referral of evoked sensations, pressure pain threshold and conditioned pain modulation. The main analysis showed that patients with FMS have a longer reaction time for laterality discrimination in hands (hands—20 images, t = 4.044, p < 0.0001, d = 1.04; hands—50 images t = 4.012, p < 0.0001, d = 1.31; feet—20 images t = 2.982, p < 0.01, d = 0.76; feet—50 images, t = 2.159, p < 0.05, d = 0.55). With regard the secondary analysis, patients with FM have higher mechanical hyperalgesia (t = −9.550; p < 0.0001, d = 2.51) and decreased response to conditioned pain modulation compared with healthy subjects (t = 15.519; p < 0.0001, d = 4.17). A positive correlation was found in patients with FMS between greater laterality discrimination ability and better function of conditioned pain modulation (hands r = 0.676, p < 0.0001; feet r = 0.485, p < 0.01). In conclusion, patients with FMS have a longer reaction time and lower accuracy for laterality discrimination, increased mechanical hyperalgesia and decreased conditioned pain modulation compared to healthy subjects. Finally, it seems that there is a positive correlation between greater laterality discrimination ability and better conditioned pain modulation function.

Hand size estimates of fibromyalgia patients are associated with clinical and experimental pain

Introduction

Simply inspecting one’s own body can reduce clinical pain and magnification of body parts can increase analgesia. Thus, body perceptions seem to play an important role for analgesia. Conversely, pain may also affect bodily perceptions. Therefore, we evaluated the effects of clinical and/or experimental pain on perceived hand size in fibromyalgia patients (FM) and healthy controls (HC).

Methods

To investigate the effects of chronic and/or acute pain on size perception we compared hand size estimates of 35 HC and 32 FM patients at baseline and during tonic mechanical pain stimuli applied to one ear lobe. Mechanical stimuli were adjusted for each individual pain sensitivity to achieve a rating of 4 ± 1 VAS (0–10) units. Photographs of each subject’s hands were digitally manipulated to produce a monotonic series of 5 images larger and 6 smaller than actual size which were then presented to the participants in ascending and descending order (total number of images: 12).

Results

FM and HC participants’ clinical pain ratings at baseline were 3.3 (3.1) and .3 (.8) VAS units, respectively. At baseline, FM participants selected significantly smaller hand images than HC as representative of their actual size (p < .02). During application of tonic experimental pain, the image size chosen to represent their actual hand size decreased significantly in FM participants and HC (p < .001) but this decrease was not different between groups (p > .05). Hand size estimates of FM participants correlated negatively with their clinical pain ratings (p < .04).

Conclusion

The decreased hand size perception of FM patients and HC was associated with their clinical and/or experimental pain, supporting the hypothesis that pain can result in visual body distortions.

The sensorimotor theory of pathological pain revisited

Harris (1999) proposed that pain can arise in the absence of tissue damage because changes in the cortical representation of the painful body part lead to incongruences between motor intention and sensory feedback. This idea, subsequently termed the sensorimotor theory of pain, has formed the basis for novel treatments for pathological pain. Here we review the evidence that people with pathological pain have changes to processes contributing to sensorimotor function: motor function, sensory feedback, cognitive representations of the body and its surrounding space, multisensory processing, and sensorimotor integration. Changes to sensorimotor processing are most evident in the form of motor deficits, sensory changes, and body representations distortions, and for Complex Regional Pain Syndrome (CRPS), fibromyalgia, and low back pain. Many sensorimotor changes are related to cortical processing, pain, and other clinical characteristics. However, there is very limited evidence that changes in sensorimotor processing actually lead to pain. We therefore propose that the theory is more appropriate for understanding why pain persists rather than how it arises.

Prolonged PGE2 treatment increased TTX-sensitive but not TTX-resistant sodium current in trigeminal ganglionic neurons

Prostaglandin E₂ (PGE₂) is an important inflammatory mediator for the initiation and maintenance of inflammatory and neuropathic pain. The acute effect of PGE₂ on sodium currents has been widely characterized in sensory neurons; however, the prolonged effect of PGE₂ remains to be determined. Here, we performed patch clamp recordings to evaluate the acute and prolonged effects of PGE₂ on sodium currents in trigeminal ganglionic (TG) neurons from male Sprague-Dawley rats. We found that 24-h treatment with PGE₂ (10 μM) increased the peak sodium current density by approximately 31% in a voltage-dependent manner and shifted the activation curve in a hyperpolarized direction but did not affect steady-state inactivation. Furthermore, treatment with PGE₂ for 24 h increased the current density of tetrodotoxin-sensitive (TTX-S) but not TTX-resistant (TTX-R) channels significantly. Interestingly, TTX-S current was increased mostly in medium-sized, but not in small-sized, neurons after 24 h of treatment with PGE₂. Moreover, the mRNA level of TTX-S Nav1.1 but not TTX-R Nav1.8 or Nav1.9 was significantly increased after 24 h of treatment with PGE_2. In contrast, 5-min treatment with PGE₂ (10 μM) increased the peak sodium current density by approximately 29% and increased TTX-R sodium currents, but not TTX-S currents, in both small- and medium-sized TG neurons. Our results presented a differential regulation of subtypes of sodium channels by acute and prolonged treatments of PGE₂, which may help to better understand the mechanism of PGE₂-mediated orofacial pain development.

Cross-cultural adaptation and validation of the simplified chinese version of the fremantle back awareness questionnaire in patients with low back Pain

PURPOSE

The Fremantle back awareness questionnaire (FreBAQ) was recently developed as simple and quick tool to assess back-specific body perception in Low back pain (LBP) patients. The aim of the present study was to translate and cross-culturally adapt the Fremantle back awareness questionnaire (FreBAQ) into a Simplified Chinese version (FreBAQ-C), and evaluate the reliability and validity of the FreBAQ-C in patients with non-specific Chronic Low back pain (CLBP).

METHODS

The FreBAQ was translated into Chinese according to established methods. Internal consistency was assessed according to Cronbach's alpha. Test-retest reliability was estimated by Intraclass correlation coefficient (ICC). Construct validity was evaluated by correlations between the FreBAQ-C and Visual analogue scale (VAS), Roland-Morris disability questionnaire (RDQ), Pain catastrophizing scale (PCS), Tampa scale for kinesiophobia (TSK) as well as Hospital anxiety and depression scale (HADS).

RESULTS

A total of 105 participants (38 males and 67 females) were included in this study with the mean age of 54.1 ± 15.6 years, mean duration of LBP of 6.8 ± 4.6 years. The FreBAQ-C total scores were well distributed, with no floor or ceiling effects. Internal consistency was excellent (Cronbach's alpha = 0.833). ICC of test-retest reliability was good (0.897, 95% confidence interval: 0.852-0.929). The limits of agreement (LOA) ranged from - 5.8 to 6.3. The Standard error of measurement (SEM) and Minimum detectable change (MDC) were 2.16 and 5.99. Construct validity was confirmed by significant correlation of The FreBAQ-C and VAS during motion (r = 0.274, p = 0.005) and rest (r = 0.243, p = 0.012), RDQ (r = 0.377, p < 0.001), PCS (r = 0.439, p < 0.001), and TSK(r = 0.311, p = 0.001).

CONCLUSIONS

The FreBAQ-C was demonstrated to have acceptable reliability and validity for patients with non-specific CLBP in Chinese mainland. It will allow evaluating body preception of the back in the Chinese population with CLBP.

Bodily Illusions and Motor Imagery in Fibromyalgia

Fibromyalgia (FM) is characterised by chronic, continuous, widespread pain, often associated with a sense of fatigue, non-restorative sleep and physical exhaustion. Due to the nature of this condition and the absence of other neurological issues potentially able to induce disorders in body representations per se, it represents a perfect model since it provides an opportunity to study the relationship between pain and the bodily self. Corporeal illusions were investigated in 60 participants with or without a diagnosis of FM by means of an ad hoc devised interview. In addition, motor imagery was investigated and illusions relating to body part movements and changes in body size, feelings of alienness, and sensations of body parts not belonging to one’s own body (disownership and somatoparaphrenic-like sensations) were found. Crucially, these symptoms do not correlate with any of the clinical measures of pain or functional deficits. The results showed that motor imagery was also impaired, and the severity of the deficits found correlated with the functional impairment of the participant. This indicates that disorders in body representations and motor imagery are part of the clinical expression of FM. However, while motor imagery seems to be linked to reduced autonomy and functional deficits, bodily illusions are independent and potentially represent a concurrent symptom.

Proprioceptive acuity is core for back awareness in chronic low back pain: Further analysis of the content validity of the Spanish version of the Fremantle Back Awareness Questionnaire

Treatments aimed at increasing self-perception may improve chronic low back pain (CLBP) symptomatology and present novel management approaches. Consequently, it is important to have valid, complete, and reliable tools for its assessment, and to understand which variables influence altered back awareness. We aimed to evaluate the face/content validity of the Spanish version of the Fremantle Back Awareness Questionnaire (FreBAQ-S) among people with and without CLBP, and to explore additional variables suggested to be involved in back awareness. A total of 264 individuals with CLBP and 128 healthy controls (HC) answered an online survey, including the FreBAQ-S, and questions regarding the completeness, comprehensibility, time-to-complete adequacy, and time spent completing it. If participants declared a lack of completeness, they had to report which aspects would be incorporated into the questionnaire to explore additional back-awareness-related variables. A statistically significant difference in completeness emerged between groups (p < 0.01). The questionnaire was comprehensible for more than 85% of participants, regardless of the group (p = 0.45). CLBP participants spent significantly more time in completing the questionnaire than controls (p < 0.01), but no differences were found between groups regarding the time-to-complete adequacy (p = 0.49). Regarding the back-awareness-related variables, 77 suggestions from CLBP group and seven from the HC were received. Most of them were related to proprioceptive acuity such as posture, weight, or movement patterns, among others. The FreBAQ-S demonstrated adequate face/content validity, completeness, comprehensibility, and adequate time of response. The feedback provided will help improve currently available assessment tools.

Two-point discrimination and judgment of laterality in individuals with chronic unilateral non-traumatic shoulder pain

BACKGROUND

Cortical representation of the painful body region may be disrupted in several chronic pain conditions. The two-point discrimination test (TPDT) and the Left/Right Judgement Task (LRJT) have been used to identify changes in the cortical body schema in several chronic pain conditions. However, it is unclear if these changes are present for all chronic pain mechanisms.

OBJECTIVES

To investigate the integrity of the body schema of the painful shoulder in patients with chronic unilateral nociceptive shoulder pain.

METHODS

The sample consisted of 52 individuals with chronic unilateral nociceptive shoulder pain. The TPDT was measured over the anterosuperior and lateral regions of both shoulders using a staircase method. Participants also performed judgment tests of shoulder and foot laterality. The comparison of the TPDT and LRJT was performed using the linear regression model with mixed effects.

RESULTS

There was no difference in TPDT in the anterosuperior and lateral regions when comparing the symptomatic and asymptomatic shoulders. There was no difference in the LRJT accuracy and response time between the symptomatic and asymptomatic shoulders. No differences were observed when comparing LRJT variables from symptomatic shoulder and foot.

CONCLUSION

Therefore, results do not provide clear evidence of altered body schema in chronic nociceptive unilateral shoulder pain. This suggest that alterations in body representations may depend on the primary pain mechanism.

Maladaptive reorganization following SCI: The role of body representation and multisensory integration

In this review we focus on maladaptive brain reorganization after spinal cord injury (SCI), including the development of neuropathic pain, and its relationship with impairments in body representation and multisensory integration. We will discuss the implications of altered sensorimotor interactions after SCI with and without neuropathic pain and possible deficits in multisensory integration and body representation. Within this framework we will examine published research findings focused on the use of bodily illusions to manipulate multisensory body representation to induce analgesic effects in heterogeneous chronic pain populations and in SCI-related neuropathic pain. We propose that the development and intensification of neuropathic pain after SCI is partly dependent on brain reorganization associated with dysfunctional multisensory integration processes and distorted body representation. We conclude this review by suggesting future research avenues that may lead to a better understanding of the complex mechanisms underlying the sense of the body after SCI, with a focus on cortical changes.

The iSTOPP study: Protocol for a proof-of-concept randomised clinical trial of sensory discrimination training in people with persistent neck pain

BACKGROUND

Neck pain can be associated with a reduction in tactile acuity that is thought to reflect disrupted sensory processing. Tactile acuity training may normalise sensory processing and improve symptoms. This proof-of-concept trial will assess the feasibility of a novel tactile acuity training method and whether this intervention improves tactile acuity in people with persistent neck pain.

METHODS

and analysis: In this two-arm randomised clinical proof-of-concept trial we will recruit participants with neck pain receiving usual care physiotherapy in a secondary outpatient healthcare setting. Thirty-six participants will be randomised 2:1 to receive four weeks of either tactile acuity training using the Imprint Tactile Acuity Device (iTAD) or a placebo intervention, in addition to usual care. The placebo intervention will consist of a de-activated TENS machine (iTENS) said to deliver a sub-threshold inhibitory therapy. Outcomes will be assessed at baseline, mid-treatment, and at 5-weeks and 2-months follow-up. The primary outcome tactile acuity will be evaluated using the two-point discrimination test and locognosia tests. Feasibility will be informed by recruitment and attrition rates, adherence, credibility of the interventions, treatment satisfaction and blinding. Pain intensity and anatomical spread will be analysed as secondary outcomes. The effect of iTAD training on tactile acuity will be assessed using a 2 (Group: iTAD vs. iTENS) x 4 (Time: baseline, mid-treatment, 5-week and 2-month outcome assessment) mixed ANOVA. Secondary outcomes including pain and pain spread, will be analysed with a focus on informing sample size calculations in future trials.

ETHICS AND DISSEMINATION

Risks associated with this study are minor. Usual care is not withheld, and participants consent to random allocation of either iTAD or iTENS. Potential benefits to participants include any benefit associated with the interventions and contributing to research that may assist people with chronic pain in the future. Trial results will be disseminated via academic journals and conference presentations. The study is approved by the Human Research Ethics Committee of Griffith University (2017/128).

Scanned optogenetic control of mammalian somatosensory input to map input-specific behavioral outputs

Somatosensory stimuli guide and shape behavior, from immediate protective reflexes to longer-term learning and higher-order processes related to pain and touch. However, somatosensory inputs are challenging to control in awake mammals due to the diversity and nature of contact stimuli. Application of cutaneous stimuli is currently limited to relatively imprecise methods as well as subjective behavioral measures. The strategy we present here overcomes these difficulties, achieving 'remote touch' with spatiotemporally precise and dynamic optogenetic stimulation by projecting light to a small defined area of skin. We mapped behavioral responses in freely behaving mice with specific nociceptor and low-threshold mechanoreceptor inputs. In nociceptors, sparse recruitment of single-action potentials shapes rapid protective pain-related behaviors, including coordinated head orientation and body repositioning that depend on the initial body pose. In contrast, activation of low-threshold mechanoreceptors elicited slow-onset behaviors and more subtle whole-body behaviors. The strategy can be used to define specific behavioral repertoires, examine the timing and nature of reflexes, and dissect sensory, motor, cognitive, and motivational processes guiding behavior.

Clinical evaluation of somatosensory integrity in people with chronic shoulder pain

PURPOSE

Indirect evaluation tests of somatosensory integrity, such as the left/right judgment task and two-point discrimination thresholds, are used to complement a clinical evaluation facilitating tailored rehabilitation programs for patients with chronic pain. This study aimed to compare performance on the left/right judgment task and two-point shoulder discrimination thresholds between people with chronic shoulder pain and asymptomatic controls. The study's secondary objective was to investigate whether there is a relationship between pain intensity and the painful area with TPDT and LRJT.

MATERIALS AND METHODS

It is a cross-sectional study with a control group. Volunteers participated in the left/right judgment task using the Recognise Shoulder^TM app, and two-point discrimination thresholds were performed using a Mitutoyo® digital caliper. The extent of the painful area, affectivity, and functionality were also determined to characterize the sample.

RESULTS

We assessed 52 symptomatic and 50 asymptomatic individuals. A between-group difference was observed in time response for the left/right judgment task (p = 0.025) and the two-point discrimination thresholds when tested over the shoulder's lateral region (p = 0.012). There was a significant positive weak correlation between pain intensity and TPDT test performance over the anterior shoulder (rs = 0.35; p = 0.01) and a positive weak correlation between pain area and LRJT response time (rs = 0.26; p = 0.04).

CONCLUSIONS

This study demonstrates a difference of small magnitude in the response time of the LRJT for the shoulder segment and TPDT in the lateral shoulder region between individuals with chronic shoulder pain and asymptomatic controls.

Is Right Angular Gyrus Involved in the Metric Component of the Mental Body Representation in Touch and Vision? A tDCS Study

Several studies have found in the sense of touch a good sensory modality by which to study body representation. Here, we address the “metric component of body representation”, a specific function developed to process the discrimination of tactile distances on the body. The literature suggests the involvement of the right angular gyrus (rAG) in processing the tactile metricity on the body. The question of this study is the following: is the rAG also responsible for the visual metric component of body representation? We used tDCS (anodal and sham) in 20 subjects who were administered an on-body distance discrimination task with both tactile and visual stimuli. They were also asked to perform the same task in a near-body condition. The results allow us to confirm the role of rAG in the estimation of tactile distances. Further, we also showed that rAG might be involved in the discrimination of distances on the body not only in tactile but also in visual modality. Finally, based on the significant effects of anodal stimulation even in a near-body visual discrimination task, we proposed a higher-order function of the AG in terms of a supramodal comparator of quantities.

A somatosensory cortex input to the caudal dorsolateral striatum controls comorbid anxiety in persistent pain

Chronic pain and anxiety symptoms are frequently encountered clinically, but the neural circuit mechanisms underlying the comorbid anxiety symptoms in pain (CASP) in context of chronic pain remain unclear. Using viral neuronal tracing in mice, we identified a previously unknown pathway whereby glutamatergic neurons from layer 5 of the hindlimb primary somatosensory cortex (S1) (Glu), a well-known brain region involved in pain processing, project to GABAergic neurons in the caudal dorsolateral striatum (GABA). In a persistent inflammatory pain model induced by complete Freund's adjuvant injection, enhanced excitation of the Glu→GABA pathway was found in mice exhibiting CASP. Reversing this pathway using chemogenetic or optogenetic approaches alleviated CASP. In addition, the optical activation of Glu terminals in the cDLS produced anxiety-like behaviors in naive mice. Overall, the current study demonstrates the putative importance of a novel Glu→GABA pathway in controlling at least some aspects of CASP.

Relationship of prosthesis ownership and phantom limb pain: results of a survey in 2383 limb amputees

ABSTRACT

Phantom limb pain (PLP) accounts for a significant reduction in quality of life and is difficult to treat. Prosthesis use has been shown to negatively covary with PLP. Recent research on body perception in amputees suggest that prosthesis ownership, defined as the extent to which a prosthesis is experienced as being part of the body rather than an artificial device foreign to the body, might interact with PLP. We used survey data from 2383 unilateral prosthesis-using upper-limb or lower-limb amputees and performed regression analyses to determine the relationship between prosthesis ownership and PLP. To test for specificity, we examined the role of prosthesis ownership also for residual limb pain (RLP) and nonpainful phantom limb sensations (npPLS). Prosthesis ownership was reduced in older participants and higher in lower-limb compared to upper-limb amputees. A longer residual limb and more frequent prosthesis use as well as a longer time since amputation also yielded higher values. Prostheses based on natural principles were associated with higher prosthesis ownership. Phantom limb pain and RLP were lower with higher prosthesis ownership, and RLP but not PLP was lower when prosthesis use was frequent. There were no significant associations for npPLS. The regression results differ in some aspects from those revealed by univariate analyses, emphasizing the importance of multivariate statistical approaches. Our findings provide insights into the interplay of body- and pain-related sensations after amputation, and could help to develop new treatment approaches for both PLP and RLP.

A Review on Mitigating Fear and Aggression in Dogs and Cats in a Veterinary Setting

Simple Summary

The majority of dogs and cats are fearful during veterinary visits, and some individuals may show aggression as a result. We review ways to avoid negative experiences and promote positive emotions in animals visiting the veterinarian. Whenever an animal is in the practice, the veterinary team should endeavour to make the visit as pleasant as possible, by using non-threatening body language and by creating positive associations. High-value food (unless an animal needs to be fasted) or toys should be used generously throughout the visit. In the interaction with the animals, low-stress handling methods, brief pauses and adjusting the procedure based on the animal’s body language help them to feel secure. Distractions can be used to minimise perceived pain such as from injections. If a known painful area needs to be treated, pain killers are advised. For animals that are very fearful, several medication options are available that can be given prior to the veterinary visit to help them with their fears. With reward-based training, animals can learn to accept veterinary procedures. A stress-free veterinary visit benefits all involved parties—the animals, their owners, as well as the veterinary team.

Abstract

A high proportion of dogs and cats are fearful during veterinary visits, which in some cases may escalate into aggression. Here, we discuss factors that contribute to negative emotions in a veterinary setting and how these can be addressed. We briefly summarise the available evidence for the interventions discussed. The set-up of the waiting area (e.g., spatial dividers; elevated places for cat carriers), tailoring the examination and the treatment to the individual, considerate handling (minimal restraint when possible, avoiding leaning over or cornering animals) and offering high-value food or toys throughout the visit can promote security and, ideally, positive associations. Desensitisation and counterconditioning are highly recommended, both to prevent and address existing negative emotions. Short-term pain from injections can be minimised by using tactile and cognitive distractions and topical analgesics, which are also indicated for painful procedures such as ear cleanings. Recommendations for handling fearful animals to minimise aggressive responses are discussed. However, anxiolytics or sedation should be used whenever there is a risk of traumatising an animal or for safety reasons. Stress-reducing measures can decrease fear and stress in patients and consequently their owners, thus strengthening the relationship with the clients as well as increasing the professional satisfaction of veterinary staff.

Hierarchical models of pain: Inference, information-seeking, and adaptive control

Computational models of pain consider how the brain processes nociceptive information and allow mapping neural circuits and networks to cognition and behaviour. To date, they have generally have assumed two largely independent processes: perceptual inference, typically modelled as an approximate Bayesian process, and action control, typically modelled as a reinforcement learning process. However, inference and control are intertwined in complex ways, challenging the clarity of this distinction. Here, we consider how they may comprise a parallel hierarchical architecture that combines inference, information-seeking, and adaptive value-based control. This sheds light on the complex neural architecture of the pain system, and takes us closer to understanding from where pain 'arises' in the brain.

Another's pain in my brain: No evidence that placebo analgesia affects the sensory-discriminative component in empathy for pain

The shared representations account of empathy suggests that sharing other people's emotions relies on neural processes similar to those engaged when directly experiencing such emotions. Recent research corroborated this by showing that placebo analgesia induced for first-hand pain resulted in reduced pain empathy and decreased activation in shared neural networks. However, those studies did not report any placebo-related variation of somatosensory engagement during pain empathy. The experimental paradigms used in these studies did not direct attention towards a specific body part in pain, which may explain the absence of effects for somatosensation. The main objective of this preregistered study was to implement a paradigm overcoming this limitation, and to investigate whether placebo analgesia may also modulate the sensory-discriminative component of empathy for pain. We induced a localized, first-hand placebo analgesia effect in the right hand of 45 participants by means of a placebo gel and conditioning techniques, and compared this to the left hand as a control condition. Participants underwent a pain task in the MRI scanner, receiving painful or non-painful electrical stimulation on their left or right hand, or witnessing another person receiving such stimulation. In contrast to a robust localized placebo analgesia effect for self-experienced pain, the empathy condition showed no differences between the two hands, neither for behavioral nor neural responses. We thus report no evidence for somatosensory sharing in empathy, while replicating previous studies showing overlapping brain activity in the affective-motivational component for first-hand and empathy for pain. Hence, in a more rigorous test aiming to overcome limitations of previous work, we again find no causal evidence for the engagement of somatosensory sharing in empathy. Our study refines the understanding of the neural underpinnings of empathy for pain, and the use of placebo analgesia in investigating such models.

Spontaneous sensations reveal distorted body perception in complex regional pain syndrome (CRPS)

Distortions of body representation have been reported in Complex Regional Pain Syndrome (CRPS). The perception of sensations arising without external triggers (spontaneous sensations or SPS) was assessed here as a means of investigating distortions of body representation and awareness in CRPS. To avoid confounds between CRPS symptoms and SPS, lower-limb CRPS patients were included, whereas SPS were tested on the hands. Patients and controls were required to focus on their hands and to report the spatial and qualitative characteristics of SPS arising there. We found an ipsilateral decrease in the perception of thermal, pain-related and surface/mechanical SPS, as well as in the number of SPS-sensitive areas. The latter finding was predicted by decreased body awareness as assessed through questionnaires. A bilateral decrease in the perception of paresis-like SPS was also observed. Finally, the ipsilateral spatial distribution of SPS frequency and intensity underwent a shift from the fingers towards the lower parts of the palm. CRPS is likely to distort patient's body perception and awareness of the entire half-body ipsilateral to the affected limb, and even of both sides. Such disturbances are not manifested solely as a decrease in sensitivity, but sometimes as shifts in the spatial distribution of sensitivity.

Explicit and Implicit Own's Body and Space Perception in Painful Musculoskeletal Disorders and Rheumatic Diseases: A Systematic Scoping Review

Background: Pain and body perception are essentially two subjective mutually influencing experiences. However, in the field of musculoskeletal disorders and rheumatic diseases we lack of a comprehensive knowledge about the relationship between body perception dysfunctions and pain or disability. We systematically mapped the literature published about the topics of: (a) somatoperception; (b) body ownership; and (c) perception of space, analysing the relationship with pain and disability. The results were organized around the two main topics of the assessment and treatment of perceptual dysfunctions.

Methods: This scoping review followed the six-stage methodology suggested by Arksey and O'Malley. Ten electronic databases and grey literature were systematically searched. The PRISMA Extension for Scoping Reviews was used for reporting results. Two reviewers with different background, independently performed study screening and selection, and one author performed data extraction, that was checked by a second reviewer.

Results: Thirty-seven studies fulfilled the eligibility criteria. The majority of studies (68%) concerned the assessment methodology, and the remaining 32% investigated the effects of therapeutic interventions. Research designs, methodologies adopted, and settings varied considerably across studies. Evidence of distorted body experience were found mainly for explicit somatoperception, especially in studies adopting self-administered questionnaire and subjective measures, highlighting in some cases the presence of sub-groups with different perceptual features. Almost half of the intervention studies (42%) provided therapeutic approaches combining more than one perceptual task, or sensory-motor tasks together with perceptual strategies, thus it was difficult to estimate the relative effectiveness of each single therapeutic component.

Conclusions: To our knowledge, this is the first attempt to systematically map and summarize this research area in the field of musculoskeletal disorders and rheumatic diseases. Although methodological limitations limit the validity of the evidence obtained, some strategies of assessment tested and therapeutic strategies proposed represent useful starting points for future research. This review highlights preliminary evidence, strengths, and limitations of the literature published about the research questions, identifying key points that remain opened to be addressed, and make suggestions for future research studies. Body representation, as well as pain perception and treatment, can be better understood if an enlarged perspective including body and space perception is considered.

Nuclear Factor I/A Controls A-fiber Nociceptor Development

Noxious mechanical information is transmitted through molecularly distinct nociceptors, with pinprick-evoked sharp sensitivity via A-fiber nociceptors marked by developmental expression of the neuropeptide Y receptor 2 (Npy2r) and von Frey filament-evoked punctate pressure information via unmyelinated C fiber nociceptors marked by MrgprD. However, the molecular programs controlling their development are only beginning to be understood. Here we demonstrate that Npy2r-expressing sensory neurons are in fact divided into two groups, based on transient or persistent Npy2r expression. Npy2r-transient neurons are myelinated, likely including A-fiber nociceptors, whereas Npy2r-persistent ones belong to unmyelinated pruriceptors that co-express Nppb. We then showed that the transcription factors NFIA and Runx1 are necessary for the development of Npy2r-transient A-fiber nociceptors and MrgprD⁺ C-fiber nociceptors, respectively. Behaviorally, mice with conditional knockout of Nfia, but not Runx1 showed a marked attenuation of pinprick-evoked nocifensive responses. Our studies therefore identify a transcription factor controlling the development of myelinated nociceptors.

Seeing or not Seeing Where Your Hands Are. The Influence of Visual Feedback About Hand Position on the Interaction Between Nociceptive and Visual Stimuli

Examining the mechanisms underlying crossmodal interaction between nociceptive and visual stimuli is crucial to understand how humans handle potential bodily threats in their environment. It has recently been shown that nociceptive stimuli can affect the perception of visual stimuli, provided that they occur close together in external space. The present study addresses the question whether these crossmodal interactions between nociceptive and visual stimuli are mediated by the visually perceived proximity between the visual stimuli and the limb on which nociceptive stimuli are applied, by manipulating the presence vs. absence of visual feedback about the position of the stimulated limb. Participants performed temporal order judgments on pairs of visual stimuli, shortly preceded by nociceptive stimuli, either applied on one hand or both hands simultaneously. The hands were placed near the visual stimuli and could either be seen directly, seen through a glass barrier, or hidden from sight with a wooden board. Unilateral nociceptive stimuli induced spatial biases to the advantage of visual stimuli presented near the stimulated hand, which were greater in the conditions in which the hands were seen than in the condition in which vision was prevented. Spatial biases were not modulated by the presence of the glass barrier, minimizing the possibility that the differential effect between the vision and no-vision conditions is solely due to the presence of the barrier between the hands and the visual stimuli. These findings highlight the importance of visual feedback for determining spatial mapping between nociceptive and visual stimuli for crossmodal interaction.

Testing the exteroceptive function of nociception: The role of visual experience in shaping the spatial representations of nociceptive inputs

Adequately localizing pain is crucial to protect the body against physical damage and react to the stimulus in external space having caused such damage. Accordingly, it is hypothesized that nociceptive inputs are remapped from a somatotopic reference frame, representing the skin surface, towards a spatiotopic frame, representing the body parts in external space. This ability is thought to be developed and shaped by early visual experience. To test this hypothesis, normally sighted and early blind participants performed temporal order judgment tasks during which they judged which of two nociceptive stimuli applied on each hand's dorsum was perceived as first delivered. Crucially, tasks were performed with the hands either in an uncrossed posture or crossed over body midline. While early blinds were not affected by the posture, performances of the normally sighted participants decreased in the crossed condition relative to the uncrossed condition. This indicates that nociceptive stimuli were automatically remapped into a spatiotopic representation that interfered with somatotopy in normally sighted individuals, whereas early blinds seemed to mostly rely on a somatotopic representation to localize nociceptive inputs. Accordingly, the plasticity of the nociceptive system would not purely depend on bodily experiences but also on crossmodal interactions between nociception and vision during early sensory experience.

Virtual reality and chronic low back pain

AIM

Chronic low back pain (CLBP) is a highly prevalent and significant cause of disability which is often resistant to pharmacological management. Virtual reality (VR) is an emerging technology with the potential to influence CLBP, and has been suggested as an alternative to opioids for pain management. VR is a goalfocused, computer-simulated reality allowing modification of the user's experience of their perceived world.

MATERIALS/METHODS

A narrative review of peer-reviewed literature using a systematic search strategy, and sole reviewer for data extraction.

CONCLUSIONS

VR has demonstrated effectiveness in reducing acute, experimental and chronic pain. This review describes the theoretical basis of the therapeutic effects of VR on CLBP via three distinct mechanisms: distraction, neuromodulation and graded exposure therapy. Furthermore, clinical application will be considered, including discussion of ethical issues associated with the technology.Implications for rehabilitationVirtual reality (VR) is suggested as an alternative for opioids in the management of acute and chronic pain.The therapeutic mechanisms of VR in chronic low back pain (CLBP) are equivocal but include distraction, neuromodulation of body perception and graded exposure therapy.VR may show greater efficacy in patients with CLBP with associated kinesiophobia.VR may show greater effect with increased immersion.

Visuo-tactile stimulation, but not type of movement, modulates pain during the vision of a moving virtual limb

Aim: Evidence has revealed a relationship between pain and the observation of limb movement, but it is unknown whether different types of movements have diverse modulating effects. In this immersive virtual reality study, we explored the effect of the vision of different virtual arm movements (arm vs wrist) on heat pain threshold of healthy participants. Patients & methods: 40 healthy participants underwent four conditions in virtual reality, while heat pain thresholds were measured. Visuo-tactile stimulation was used to attempt to modulate the feeling of virtual limb ownership while the participants kept their arms still. Results: Effects on pain threshold were present for type of stimulation but not type of movement. Conclusion: The type of observed movement does not appear to influence pain modulation, at least not during acute pain states.

Caloric vestibular stimulation has no effect on perceived body size

It has been suggested that the vestibular system not only plays a role for our sense of balance and postural control but also might modulate higher-order body representations, such as the perceived shape and size of our body. Recent findings using virtual reality (VR) to realistically manipulate the length of whole extremities of first person biometric avatars under vestibular stimulation did not support this assumption. It has been discussed that these negative findings were due to the availability of visual feedback on the subjects' virtual arms and legs. The present study tested this hypothesis by excluding the latter information. A newly recruited group of healthy subjects had to adjust the position of blocks in 3D space of a VR scenario such that they had the feeling that they could just touch them with their left/right hand/heel. Caloric vestibular stimulation did not alter perceived size of own extremities. Findings suggest that vestibular signals do not serve to scale the internal representation of (large parts of) our body's metric properties. This is in obvious contrast to the egocentric representation of our body midline which allows us to perceive and adjust the position of our body with respect to the surroundings. These two qualia appear to belong to different systems of body representation in humans.

Experimentally induced pain does not influence updating of peripersonal space and body representations following tool-use

Representations of the body and peripersonal space can be distorted for people with some chronic pain conditions. Experimental pain induction can give rise to similar, but transient distortions in healthy individuals. However, spatial and bodily representations are dynamic, and constantly update as we interact with objects in our environment. It is unclear whether induced pain disrupts the mechanisms involved in updating these representations. In the present study, we sought to investigate the effect of induced pain on the updating of peripersonal space and body representations during and following tool-use. We compared performance under three conditions (pain, active placebo, neutral) on a visuotactile crossmodal congruency task and a tactile distance judgement task to measure updating of peripersonal space and body representations, respectively. Consistent with previous findings, the difference in crossmodal interference from visual distractors in the same compared to opposite visual field to the tactile target was less when tools were crossed than uncrossed. This suggests an extension of peripersonal space to incorporate the tips of the tools. Also consistent with previous findings, estimates of the felt tactile distance judgements decreased after active tool-use. In contrast to our predictions, however, we found no evidence that pain interfered with performance on either task when compared to the control conditions. Our findings suggest that the updating of peripersonal space and body representations is not disrupted by induced pain. That is, experiencing acute pain does not give rise to distorted representations of the body and peripersonal space that can be present in people with chronic pain conditions.

Unimodal and crossmodal extinction of nociceptive stimuli in healthy volunteers

Nociception, the physiological mechanisms specifically processing information about noxious and potentially painful stimuli, has the double function to warn about potential body damages (interoception) and about the cause of such potential damages (exteroception). The exteroceptive function is thought to rely on multisensory integration between somatic and extra-somatic stimuli, provided that extra-somatic stimuli occur near the stimulated body area. To corroborate this hypothesis, we succeeded to show in healthy volunteers that the perception of nociceptive stimuli applied on one hand can be extinguished, as compared to single presentation, by the simultaneous application of nociceptive stimuli on the opposite hand, as well as by the presentation of visual stimuli near the opposite hand. On the contrary, visual stimuli presented near the same stimulated hand facilitated the perception of nociceptive stimuli. This nociceptive extinction phenomenon indicates that the perception of noxious events does not merely rely on the specific activation of the nociceptive system, but also depends on other sensory experiences about the body and the space around it.

Attenuation of Pain Perception Induced by the Rubber Hand Illusion

Adaptive behavior usually requires accurate representations of body positions and ownership, which rely on integration of multiple sources of sensory information. The rubber hand illusion (RHI) presents a compelling example demonstrating that the combination of visual and tactile signals strongly influences the subjective experience of body ownership. However, it still remains unclear how the perception of body ownership in turn alters other aspects of sensory processing, such as pain perception. In the present study, we examined whether the RHI could modulate the subjective experience of pain. We set three conditions corresponding to different levels of ownership of the rubber hand: the synchronous condition in which the rubber and the real hand were simultaneously stroked; the asynchronous condition in which the two hands were asynchronously stroked; the own-hand-only condition in which only the real hand was stroked. Results from the screening experiment indicated that subjects experienced the stronger RHI in the synchronous condition, compared with the strength of RHI in the other two conditions. In the main experiment, subjects were requested to report the intensity and unpleasantness of pain evoked by laser stimuli under the three stroking conditions. Results showed that pain ratings were significantly lower under the synchronous condition than those under the other two conditions, suggesting the RHI could induce a significant analgesic effect. Furthermore, the correlation analysis showed that the degree of the analgesic effect was positively correlated with the RHI strength across individuals. Taken together, these results suggest an analgesic effect of the RHI and support the potential usage of visual illusions in future translational research on pain.