A neurophysiological investigation of anticipation to pain in Parkinson's disease.

Chronic pain is common in people with Parkinson's disease and is often considered to be caused by the motor impairments associated with the disease. Altered top-down processing of pain characterises several chronic pain conditions and occurs when the cortex modifies nociceptive processing in the brain and spinal cord. This contrasts with bottom-up modulation of pain whereby nociceptive processing is modified on its way up to the brain. Although several studies have demonstrated altered bottom-up pain processing in Parkinson's, the contribution of enhanced anticipation to pain and atypical top-down processing of pain has not been fully explored. During the anticipation to noxious stimuli, EEG source localisation reported an increased activation in the midcingulate cortex and supplementary motor area in the Parkinson's disease group compared to the healthy control group during mid [-1,500 -1,000]-and late anticipation [-500 0], indicating enhanced cortical activity before noxious stimulation. The Parkinson's disease group was also more sensitive to the laser and required a lower voltage level to induce pain. This study provides evidence supporting the hypothesis that enhanced top-down processing of pain may contribute to the development of chronic pain in Parkinson's. Additional research to establish whether the altered anticipatory response is unique to noxious stimuli is required as no control stimulus was used within the current study. With further research to confirm these findings, our results inform a scientific rationale for novel treatment strategies of pain in Parkinson's disease, including mindfulness, cognitive therapies and other approaches targeted at improving top-down processing of pain.

Neural representations of aversive value encoding in pain catastrophizers.

Chronic pain is exacerbated by maladaptive cognition such as pain catastrophizing (PC). Biomarkers of PC mechanisms may aid precision medicine for chronic pain. Here, we investigate EEG biomarkers using mass univariate and multivariate (machine learning) approaches. We test theoretical notions that PC results from a combination of augmented aversive-value encoding ("magnification") and persistent expectations of pain ("rumination"). Healthy individuals with high or low levels of PC underwent an experimental pain model involving nociceptive laser stimuli preceded by cues predicting forthcoming pain intensity. Analysis of EEG acquired during the cue and laser stimulation provided event-related potentials (ERPs) identifying spatially and temporally-extended neural representations associated with pain catastrophizing. Specifically, differential neural responses to cues predicting high vs. low intensity pain (i.e. aversive value encoding) were larger in the high PC group, largely originating from mid-cingulate and superior parietal cortex. Multivariate spatiotemporal EEG patterns evoked from cues with high aversive value selectively and significantly differentiated the high PC from low PC group (64.6% classification accuracy). Regression analyses revealed that neural patterns classifying groups could be partially predicted (R2 = 28%) from those neural patterns classifying the aversive value of cues. In contrast, behavioural and EEG analyses did not provide evidence that PC modifies more persistent effects of prior expectation on pain perception and nociceptive responses. These findings support the hypothesis of magnification of aversive value encoding but not persistent expression of expectation in pain catastrophizers. Multivariate patterns of aversive value encoding provide promising biomarkers of maladaptive cognitive responses to chronic pain that have future potential for psychological treatment development and clinical stratification.

Flexible 3D-Printed EEG Electrodes.

For electroencephalography (EEG) in haired regions of the head, finger-based electrodes have been proposed in order to part the hair and make a direct contact with the scalp. Previous work has demonstrated 3D-printed fingered electrodes to allow personalisation and different configurations of electrodes to be used for different people or for different parts of the head. This paper presents flexible 3D-printed EEG electrodes for the first time. A flexible 3D printing element is now used, with three different base mechanical structures giving differently-shaped electrodes. To obtain improved sensing performance, the silver coatings used previously have been replaced with a silver/silver-chloride coating. This results in reduced electrode contact impedance and reduced contact noise. Detailed electro-mechanical testing is presented to demonstrate the performance of the operation of the new electrodes, particularly with regards to changes in conductivity under compression, together with on-person tests to demonstrate the recording of EEG signals.

Reductions in co-contraction following neuromuscular re-education in people with knee osteoarthritis.

BACKGROUND: Both increased knee muscle co-contraction and alterations in central pain processing have been suggested to play a role in knee osteoarthritis pain. However, current interventions do not target either of these mechanisms. The Alexander Technique provides neuromuscular re-education and may also influence anticipation of pain. This study therefore sought to investigate the potential clinical effectiveness of the AT intervention in the management of knee osteoarthritis and also to identify a possible mechanism of action. METHODS: A cohort of 21 participants with confirmed knee osteoarthritis were given 20 lessons of instruction in the Alexander Technique. In addition to clinical outcomes EMG data, quantifying knee muscle co-contraction and EEG data, characterising brain activity during anticipation of pain, were collected. All data were compared between baseline and post-intervention time points with a further 15-month clinical follow up. In addition, biomechanical data were collected from a healthy control group and compared with the data from the osteoarthritis subjects. RESULTS: Following AT instruction the mean WOMAC pain score reduced by 56 % from 9.6 to 4.2 (P < 0.01) and this reduction was maintained at 15 month follow up. There was a clear decrease in medial co-contraction at the end of the intervention, towards the levels observed in the healthy control group, both during a pre-contact phase of gait (p < 0.05) and during early stance (p < 0.01). However, no changes in pain-anticipatory brain activity were observed. Interestingly, decreases in WOMAC pain were associated with reductions in medial co-contraction during the pre-contact phase of gait. CONCLUSIONS: This is the first study to investigate the potential effectiveness of an intervention aimed at increasing awareness of muscle behaviour in the clinical management of knee osteoarthritis. These data suggest a complex relationship between muscle contraction, joint loading and pain and support the idea that excessive muscle co-contraction may be a maladaptive response in this patient group. Furthermore, these data provide evidence that, if the activation of certain muscles can be reduced during gait, this may lead to positive long-term clinical outcomes. This finding challenges clinical management models of knee osteoarthritis which focus primarily on muscle strengthening. TRIAL REGISTRATION: ISRCTN74086288 , 4th January 2016, retrospectively registered.

When the brain expects pain: common neural responses to pain anticipation are related to clinical pain and distress in fibromyalgia and osteoarthritis.

Supraspinal processes in humans can have a top-down enhancing effect on nociceptive processing in the brain and spinal cord. Studies have begun to suggest that such influences occur in conditions such as fibromyalgia (FM), but it is not clear whether this is unique to FM pain or common to other forms of chronic pain, such as that associated with osteoarthritis (OA). We assessed top-down processes by measuring anticipation-evoked potentials and their estimated sources, just prior (< 500 ms) to laser heat pain stimulation, in 16 patients with FM, 16 patients with OA and 15 healthy participants, by using whole-brain statistical parametric mapping. Clinical pain and psychological coping factors (pain catastrophizing, anxiety, and depression) were well matched between the patient groups, such that these did not confound our comparisons between FM and OA patients. For the same level of heat pain, insula activity was significantly higher in FM patients than in the other two groups during anticipation, and correlated with the intensity and extent of reported clinical pain. However, the same anticipatory insula activity also correlated with OA pain, and with the number of tender points across the two patient groups, suggesting common central mechanisms of tenderness. Activation in the dorsolateral prefrontal cortex was reduced during anticipation in both patient groups, and was related to less effective psychological coping. Our findings suggest common neural correlates of pain and tenderness in FM and OA that are enhanced in FM but not unique to this condition.

Placebo analgesia: cognition or perception.

Placebo analgesia has become a well-studied phenomenon that encompasses psychology, physiology and pharmacology. In this chapter we explore the complex interactions between these disciplines in order to argue that the placebo response is more than a simple change in perception but is a cognitive style driven by prior expectations. The expectation of treatment effect is shaped by prior information and prior experience which our brain uses to predict future events. In the case of placebo analgesia the prediction of pain relief overrules the actual feeling of pain leading to a decrease in pain sensation. This altered sensation can be attributed to personality traits, altered error monitoring processes, changes in anticipatory responses to pain and activation of the endogenous opioid system. In conclusion we discuss how altered sensory processing by descending pain modulation may play a part in placebo analgesia and how the loss of the brains prefrontal regions can make it impossible to have a placebo response.

Are We Any Closer to Understanding How Chronic Pain Develops? A Systematic Search and Critical Narrative Review of Existing Chronic Pain Vulnerability Models.

Identifying biopsychosocial factors underlying chronic pain vulnerability is essential for the design of preventative efforts. Multiple chronic pain vulnerability models exist, however, there is a lack of comprehensive evaluation of these models in the literature, potentially due to the lack of guidelines that specify the criteria by which these types of work should be assessed. In this work, we created evaluation criteria (based on the general goals of conceptual models), and we then used them to critically review the chronic pain vulnerability models available in the current peer-reviewed literature (identified through a systematic search). Particularly, we evaluated the models on the basis of conceptual clarity/specificity of measures, depth of description of aetiological and mechanistic factors, use of a whole system approach, and quality of the evidence associated with the models. We found nine conceptual models that have been explored in detail (eg, fear avoidance model, diathesis-stress model). These models excel at clarity and are supported mostly by self-report evidence of a psychological nature (anxiety sensitivity, pain catastrophizing, etc.), but provide little explanation of mechanistic and aetiological factors. In the future, models could be improved by complementing them with proposals from other models and exploring potential causal factors and mechanisms maintaining the condition. This task could be carried out through prospective cohort studies, and computational approaches, amongst others.

A feasibility study of pre-sleep audio and visual alpha brain entrainment for people with chronic pain and sleep disturbance.

Introduction: Chronic pain and sleep disturbance are bi-directionally related. Cortical electrical activity in the alpha frequency band can be enhanced with sensory stimulation via the phenomenon of entrainment, and may reduce pain perception. A smartphone based programme which delivers 10 Hz stimulation through flickering light or binaural beats was developed for use at night, pre-sleep, with the aim of improving night time pain and sleep and thereby subsequent pain and related daytime symptoms. The aim of this study was to assess the feasibility and give an indication of effect of this programme for individuals with chronic pain and sleep disturbance. Materials and methods: In a non-controlled feasibility study participants used audio or visual alpha entrainment for 30 min pre-sleep each night for 4 weeks, following a 1 week baseline period. The study was pre-registered at ClinicalTrials.gov with the ID NCT04176861. Results: 28 participants (79% female, mean age 45 years) completed the study with high levels of data completeness (86%) and intervention adherence (92%). Daily sleep diaries showed an increase compared to baseline in total sleep time of 29 min (p = 0.0033), reduction in sleep onset latency of 13 min (p = 0.0043), and increase in sleep efficiency of 4.7% (p = 0.0009). Daily 0-10 numerical rating scale of average pain at night improved by 0.5 points compared to baseline (p = 0.027). Standardised questionnaires showed significant within-participant improvements in sleep quality (change in median Global PSQI from 16 to 12.5), pain interference (change in median BPI Pain Interference from 7.5 to 6.8), fatigue (change in median MFI total score from 82.5 to 77), and depression and anxiety (change in median HADS depression score from 12 to 10.5 and anxiety from 13.5 to 11). Discussion: Pre-sleep use of a smartphone programme for alpha entrainment by audio or visual stimulation was feasible for individuals with chronic pain and sleep disturbance. The effect on symptoms requires further exploration in controlled studies.

Role of functional brain imaging in understanding rheumatic pain.

Rheumatic pain and, in particular, rheumatoid arthritis, osteoarthritis and fibromyalgia, are common and debilitating chronic pain syndromes. Recently, human functional neuroimaging, for example EEG, fMRI, and PET has begun to reveal some of the crucial central nervous system mechanisms underlying these diseases. The purpose of this review is to summarise current knowledge on the brain mechanisms of rheumatic pain revealed by functional neuroimaging techniques. The evidence suggests that two mechanisms may be largely responsible for the clinical pain associated with these rheumatic diseases: abnormalities in the medial pain system and/or central nervous system sensitisation and inhibition. If we can understand how functioning of the central nociceptive system becomes altered, even in the absence of peripheral nociceptive input, by using functional neuroimaging techniques, in the future we may be able to develop improved, more effective treatments for patients with chronic rheumatic pain.

Altered Pain Processing Associated with Administration of Dopamine Agonist and Antagonist in Healthy Volunteers.

Striatal dopamine dysfunction is associated with the altered top-down modulation of pain processing. The dopamine D2-like receptor family is a potential substrate for such effects due to its primary expression in the striatum, but evidence for this is currently lacking. Here, we investigated the effect of pharmacologically manipulating striatal dopamine D2 receptor activity on the anticipation and perception of acute pain stimuli in humans. Participants received visual cues that induced either certain or uncertain anticipation of two pain intensity levels delivered via a CO2 laser. Rating of the pain intensity and unpleasantness was recorded. Brain activity was recorded with EEG and analysed via source localisation to investigate neural activity during the anticipation and receipt of pain. Participants completed the experiment under three conditions, control (Sodium Chloride), D2 receptor agonist (Cabergoline), and D2 receptor antagonist (Amisulpride), in a repeated-measures, triple-crossover, double-blind study. The antagonist reduced an individuals' ability to distinguish between low and high pain following uncertain anticipation. The EEG source localisation showed that the agonist and antagonist reduced neural activations in specific brain regions associated with the sensory integration of salient stimuli during the anticipation and receipt of pain. During anticipation, the agonist reduced activity in the right mid-temporal region and the right angular gyrus, whilst the antagonist reduced activity within the right postcentral, right mid-temporal, and right inferior parietal regions. In comparison to control, the antagonist reduced activity within the insula during the receipt of pain, a key structure involved in the integration of the sensory and affective aspects of pain. Pain sensitivity and unpleasantness were not changed by D2R modulation. Our results support the notion that D2 receptor neurotransmission has a role in the top-down modulation of pain.

Frontal alpha asymmetry: A potential biomarker of approach-withdrawal motivation towards pain.

Pain-related catastrophising is a maladaptive coping strategy known to have a strong influence on clinical pain outcomes and treatment efficacy. Notwithstanding, little is known about its neurophysiological correlates. There is evidence to suggest catastrophising is associated with resting-state EEG frontal alpha asymmetry (FAA) patterns reflective of greater relative right frontal activity, which is known to be linked to withdrawal motivation and avoidance of aversive stimuli. The present study aims to investigate whether such a relationship occurs in the situational context of experimental pain. A placebo intervention was also included to evaluate effects of a potential pain-relieving intervention on FAA. 35 participants, including both chronic pain patients and healthy subjects, completed the Pain Catastrophising Scale (PCS) questionnaire followed by EEG recordings during cold pressor test (CPT)-induced tonic pain with or without prior application of placebo cream. There was a negative correlation between FAA and PCS-subscale helplessness scores, but not rumination or magnification, during the pre-placebo CPT condition. Moreover, FAA scores were shown to increase significantly in response to pain, indicative of greater relative left frontal activity that relates to approach-oriented behaviours. Placebo treatment elicited a decrease in FAA in low helplessness scorers, but no significant effects in individuals scoring above the mean on PCS-helplessness. These findings suggest that, during painful events, FAA may reflect the motivational drive to obtain reward of pain relief, which may be diminished in individuals who are prone to feel helpless about their pain. This study provides valuable insights into biomarkers of pain-related catastrophising and prospects of identifying promising targets of brain-based therapies for chronic pain management.

Alpha entrainment drives pain relief using visual stimulation in a sample of chronic pain patients: a proof-of-concept controlled study.

One-third of the population in the UK and worldwide struggle with chronic pain. Entraining brain alpha activity through noninvasive visual stimulation has been shown to reduce experimental pain in healthy volunteers. Neural oscillations entrainment offers a potential noninvasive and nonpharmacological intervention for patients with chronic pain, which can be delivered in the home setting and has the potential to reduce use of medications. However, evidence supporting its use in patients with chronic pain is lacking. This study explores whether (a) alpha entrainment increase alpha power in patients and (b) whether this increase in alpha correlates with analgesia. In total, 28 patients with chronic pain sat in a comfortable position and underwent 4-min visual stimulation using customised goggles at 10 Hz (alpha) and 7 Hz (control) frequency blocks in a randomised cross-over design. 64-channel electroencephalography and 11-point numeric rating scale pain intensity and pain unpleasantness scores were recorded before and after stimulation. Electroencephalography analysis revealed frontal alpha power was significantly higher when stimulating at 10 Hz when compared to 7 Hz. There was a significant positive correlation between increased frontal alpha and reduction in pain intensity (r = 0.33; P < 0.05) and pain unpleasantness (r = 0.40; P < 0.05) in the 10 Hz block. This study provides the first proof of concept that changes in alpha power resulting from entrainment correlate with an analgesic response in patients with chronic pain. Further studies are warranted to investigate dose-response parameters and equivalence to analgesia provided by medications.

Is Transcranial Direct Current Stimulation (tDCS) Effective for the Treatment of Pain in Fibromyalgia? A Systematic Review and Meta-Analysis.

BACKGROUND: Fibromyalgia is a debilitating condition characterized by chronic widespread pain. It is believed to be caused by dysfunction of the central nervous system (CNS) but current treatments are largely ineffective. Transcranial direct current stimulation (tDCS), a neuromodulation technique that targets the CNS, may offer a new line of treatment. OBJECTIVE: To systematically review the most up-to-date literature and perform a meta-analysis of the effects of tDCS on pain intensity in fibromyalgia. METHODS: The following databases were searched from inception: Medline (Ovid), PsychInfo, CINAHL, Cochrane Library, and Web of Science. Studies were eligible if they were randomized controlled trials, quasi-randomized trials, and nonrandomized. Crossover and parallel-group design studies were included. Risk of bias was assessed for all included studies. Meta-analysis was conducted on studies investigating pain intensity after tDCS in participants with fibromyalgia and analyzed using standardized mean difference and 95% confidence intervals. RESULTS: Fourteen clinical studies were included. Ten were controlled trials and 4 were within-subjects crossover studies. Meta-analysis of data from 8 controlled trials provides tentative evidence of pain reduction when active tDCS is delivered compared to sham. However, substantial statistical heterogeneity and high risk of bias of primary studies prevent more conclusive recommendations being made. CONCLUSIONS: tDCS is a safe intervention with the potential to lower pain intensity in fibromyalgia. However, there is a need for more empirical research of the neural target sites and optimum stimulation parameters to achieve the greatest effects before conducting further clinical studies. PERSPECTIVE: This systematic review and meta-analysis synthesizes current evidence for the clinical effectiveness of tDCS in the treatment of fibromyalgia pain. There is only tentative evidence of pain reduction when active tDCS is compared to sham. High heterogeneity and risk of bias across studies suggest a need for further empirical research.

Effects of neurofeedback in the management of chronic pain: A systematic review and meta-analysis of clinical trials.

BACKGROUND AND OBJECTIVE: Neurofeedback (NFB) provides real-time feedback about neurophysiological signals to patients, thereby encouraging modulation of pain-associated brain activity. This review aims to evaluate the effectiveness and safety of NFB in alleviating pain and pain-associated symptoms in chronic pain patients. METHODS: MEDLINE, PUBMED, Web of Science and PsycINFO databases were searched using the strategy: ("Neurofeedback" OR "EEG Biofeedback" OR "fMRI Biofeedback") AND ("Pain" or "Chronic Pain"). Clinical trials reporting changes in pain following electroencephalogram (EEG) or functional magnetic resonance imaging (fMRI) NFB in chronic pain patients were included. Only Randomized-controlled trials (RCT), non-randomized controlled trials (NRCT) and case series were included. Effect size was pooled for all RCTs in a meta-analysis. RESULTS: Twenty-one studies were included. Reduction in pain following NFB was reported by one high-quality RCT, five of six low-quality RCT or NRCT and 13 of 14 case-series. Pain reduction reported by studies ranged from 6% to 82%, with 10 studies reporting a clinically significant reduction in pain of >30%. The overall effect size was medium (cohen's d -0.76, 95% confidence interval -1.31 to -0.20). Studies were highly heterogeneous (Q [df = 5] = 18.46, p = .002, I2  = 73%). Improvements in depression, anxiety, fatigue and sleep were also seen in some studies. Common side-effects included headache, nausea and drowsiness. These generally did not lead to withdrawal of therapy except in one study. CONCLUSIONS: Neurofeedback is a safe and effective therapy with promising but largely low-quality evidence supporting its use in chronic pain. Further high-quality trials comparing different protocols is warranted to determine the most efficacious way to deliver NFB. SIGNIFICANCE: Neurofeedback is a novel neuromodulatory approach which can be used to reduce the severity of pain and pain-associated symptoms such as sleep disturbances, mood disturbances, fatigue and anxiety in a number of chronic pain conditions. It has a potential to provide integrative non-pharmacological management for chronic pain patients with pain refractory to pharmacological agents with high side-effect profiles. Further high-quality double-blinded randomized sham-controlled trials are needed in order to fully explore the potential of this therapy.

Entraining Alpha Activity Using Visual Stimulation in Patients With Chronic Musculoskeletal Pain: A Feasibility Study.

Entraining alpha activity with rhythmic visual, auditory, and electrical stimulation can reduce experimentally induced pain. However, evidence for alpha entrainment and pain reduction in patients with chronic pain is limited. This feasibility study investigated whether visual alpha stimulation can increase alpha power in patients with chronic musculoskeletal pain and, secondarily, if chronic pain was reduced following stimulation. In a within-subject design, 20 patients underwent 4-min periods of stimulation at 10 Hz (alpha), 7 Hz (high-theta, control), and 1 Hz (control) in a pseudo-randomized order. Patients underwent stimulation both sitting and standing and verbally rated their pain before and after each stimulation block on a 0-10 numerical rating scale. Global alpha power was significantly higher during 10 Hz compared to 1 Hz stimulation when patients were standing (t = -6.08, p < 0.001). On a more regional level, a significant increase of alpha power was found for 10 Hz stimulation in the right-middle and left-posterior region when patients were sitting. With respect to our secondary aim, no significant reduction of pain intensity and unpleasantness was found. However, only the alpha stimulation resulted in a minimal clinically important difference in at least 50% of participants for pain intensity (50%) and unpleasantness ratings (65%) in the sitting condition. This study provides initial evidence for the potential of visual stimulation as a means to enhance alpha activity in patients with chronic musculoskeletal pain. The brief period of stimulation was insufficient to reduce chronic pain significantly. This study is the first to provide evidence that a brief period of visual stimulation at alpha frequency can significantly increase alpha power in patients with chronic musculoskeletal pain. A further larger study is warranted to investigate optimal dose and individual stimulation parameters to achieve pain relief in these patients.

Using EEG Alpha States to Understand Learning During Alpha Neurofeedback Training for Chronic Pain.

OBJECTIVE: Alpha-neurofeedback (α-NFB) is a novel therapy which trains individuals to volitionally increase their alpha power to improve pain. Learning during NFB is commonly measured using static parameters such as mean alpha power. Considering the biphasic nature of alpha rhythm (high and low alpha), dynamic parameters describing the time spent by individuals in high alpha state and the pattern of transitioning between states might be more useful. Here, we quantify the changes during α-NFB for chronic pain in terms of dynamic changes in alpha states. METHODS: Four chronic pain and four healthy participants received five NFB sessions designed to increase frontal alpha power. Changes in pain resilience were measured using visual analogue scale (VAS) during repeated cold-pressor tests (CPT). Changes in alpha state static and dynamic parameters such as fractional occupancy (time in high alpha state), dwell time (length of high alpha state) and transition probability (probability of moving from low to high alpha state) were analyzed using Friedman's Test and correlated with changes in pain scores using Pearson's correlation. RESULTS: There was no significant change in mean frontal alpha power during NFB. There was a trend of an increase in fractional occupancy, mean dwell duration and transition probability of high alpha state over the five sessions in chronic pain patients only. Significant correlations were observed between change in pain scores and fractional occupancy (r = -0.45, p = 0.03), mean dwell time (r = -0.48, p = 0.04) and transition probability from a low to high state (r = -0.47, p = 0.03) in chronic pain patients but not in healthy participants. CONCLUSION: There is a differential effect between patients and healthy participants in terms of correlation between change in pain scores and alpha state parameters. Parameters providing a more precise description of the alpha power dynamics than the mean may help understand the therapeutic effect of neurofeedback on chronic pain.

Long-term temperature-related morbidity after brain damage: survivor-reported experiences.

PRIMARY OBJECTIVE: To determine whether temperature-related symptoms exist, long-term, in survivors of brain damage. RESEARCH DESIGN: Scoping exercise. METHODS AND PROCEDURES: A 'call for information', posted in the quarterly News bulletins of two major UK brain injury support groups, about any current or past temperature-related symptoms or problems that had developed since onset of brain damage. MAIN OUTCOMES AND RESULTS: Narratives from 41 survivors revealed the nature of temperature-related morbidity, ongoing, on average, for 8 years since brain damage. Twenty-five survivors reported problems specifically related to feelings of extreme heat; in a further eight, heat-related problems occurred with bouts of 'cold'. Feelings of extreme cold only were less common (n = 8). In 20 survivors, temperature problems had adverse effects on health and personal and social relationships. CONCLUSIONS: To the authors' knowledge, this is the first account of temperature-related problems in the form of narratives from survivors of a variety of brain injuries. The origin of abnormal somatic sensibility to temperature is currently unclear. It is plausible that the symptoms could be a 'variant' of the 'pain-thermoregulatory distress axis' well described after human stroke. In the meantime, apparently abnormal sensory and physical symptoms experienced by the survivors lack both explanation and remedy.

Psychosocial factors partially mediate the relationship between mechanical hyperalgesia and self-reported pain.

BACKGROUND AND AIMS: Amplification of sensory signalling within the nervous system along with psychosocial factors contributes to the variation and severity of knee pain. Quantitative sensory testing (QST) is a non-invasive test battery that assesses sensory perception of thermal, pressure, mechanical and vibration stimuli used in the assessment of pain. Psychosocial factors also have an important role in explaining the occurrence of pain. The aim was to determine whether QST measures were associated with self-reported pain, and whether those associations were mediated by psychosocial factors. METHODS: Participants with knee pain identified from a population-based cohort completed a tender point count and a reduced QST battery of thermal, mechanical and pressure pain thresholds, temporal summation, mechanical pain sensitivity (MPS), dynamic mechanical allodynia (DMA) and vibration detection threshold performed following the protocol by the German Research Network on Neuropathic Pain. QST assessments were performed at the most painful knee and opposite forearm (if pain-free). Participants were asked to score for their global and knee pain intensities within the past month (range 0-10), and complete questionnaire items investigating anxiety, depression, illness perceptions, pain catastrophising, and physical functioning. QST measures (independent variable) significantly correlated (Spearman's rho) with self-reported pain intensity (dependent variable) were included in structural equation models with psychosocial factors (latent mediators). RESULTS: Seventy-two participants were recruited with 61 participants (36 women; median age 64 years) with complete data included in subsequent analyses. Tender point count was significantly correlated with global pain intensity. DMA at the knee and MPS at the most painful knee and opposite pain-free forearm were significantly correlated with both global pain and knee pain intensities. Psychosocial factors including pain catastrophising sub-scales (rumination and helplessness) and illness perceptions (consequences and concern) were significant partial mediators of the association with global pain intensity when loaded on to a latent mediator for: tender point count [75% total effect; 95% confidence interval (CI) 22%, 100%]; MPS at the knee (49%; 12%, 86%); and DMA at the knee (63%; 5%, 100%). Latent psychosocial factors were also significant partial mediators of the association between pain intensity at the tested knee with MPS at the knee (30%; 2%, 58%), but not for DMA at the knee. CONCLUSIONS: Measures of mechanical hyperalgesia at the most painful knee and pain-free opposite forearm were associated with increased knee and global pain indicative of altered central processing. Psychosocial factors were significant partial mediators, highlighting the importance of the central integration of emotional processing in pain perception. IMPLICATIONS: Associations between mechanical hyperalgesia at the forearm and knee, psychosocial factors and increased levels of clinical global and knee pain intensity provide evidence of altered central processing as a key mechanism in knee pain, with psychological factors playing a key role in the expression of clinical pain.

Placebo analgesia is not due to compliance or habituation: EEG and behavioural evidence.

This study was designed to resolve whether experimental placebo responses are due to either increased compliance or habituation. We stimulated both forearms and recorded laser-evoked potentials from 18 healthy volunteers treated on one arm with a sham analgesic cream and an inactive cream on the other (treatment group), and 13 volunteers with an inactive cream on both arms (controls). The treatment group showed a significant reduction in the pain ratings and laser-evoked potentials with both the sham and inactive creams. The control group showed no evidence of habituation to the laser stimulus. The results indicate that the reduction in pain during experimental placebo response is unlikely to be due to sensory habituation or compliance with the experimental instructions.

Parietal cortex involvement in the localization of tactile and noxious mechanical stimuli: a transcranial magnetic stimulation study.

The cortical system underlying perceptual ability to localize tactile and noxious cutaneous stimuli in humans is still incompletely understood. We used transcranial magnetic stimulation (TMS) to transiently interfere with the function of the parietal cortex, at different times after the beginning of noxious or non-noxious mechanical stimulation of the hairy skin overlying the dorsal surface of the first metacarpal of the contralateral hand. Peripheral stimuli consisted of rounded (1mm diameter) or sharp (0.2 mm) metal tips; skin contact lasted on average 242 ms (noxious) and 228 ms (non-noxious). Brief (80 ms, 25 Hz) TMS trains, given at 150 ms after the onset of cutaneous stimulation, significantly impaired subjects' ability in localizing non-nociceptive, tactile input, an effect which was not observed when TMS was applied at 300 ms after cutaneous stimulation. In contrast, brief TMS trains given at 300 ms after the onset of cutaneous stimulation significantly impaired subjects' ability in localizing nociceptive input, an effect which was not observed when TMS was applied at 150 ms after cutaneous stimulation. No impairment in stimulus detection was found in comparison with control sham TMS. The timing of parietal TMS interference with the ability to localize tactile and painful stimuli is compatible with known time differences in the arrival of non-noxious and noxious information in the postcentral gyrus. On these grounds, our findings support the existence of overlapping cortical populations in the contralateral parietal lobe, exerting a role in spatial discriminative aspects of touch and mechanically induced pain.