Brain generators of laser-evoked potentials: from dipoles to functional significance

Alpha transcranial alternating current stimulation modulates pain anticipation and perception in a context-dependent manner

ABSTRACT

Pain perception is closely tied to the brain's anticipatory processes, particularly involving the suppression of sensorimotor α-oscillations, which reflect the system's readiness for incoming pain. Higher sensorimotor α-oscillation levels are correlated with lower pain sensitivity. Alpha transcranial alternating current stimulation (α-tACS) can enhance these oscillations, potentially reducing pain perception, with effects that may be sustained and influenced by the certainty of pain expectations. Hence, this study investigated the immediate and sustained effects of α-tACS on pain anticipation and perception, focusing on how these effects are shaped by the certainty of expectations. In a double-blind, sham-controlled design, 80 healthy participants underwent a 20-minute session of real or sham α-tACS over the right sensorimotor region. Behavioral and neural responses related to pain anticipation and perception were recorded before, immediately after, and 30 minutes poststimulation under both certain and uncertain conditions. Compared with sham stimulation, real α-tACS disrupted the habituation of laser-evoked potentials (N2-P2 complex), particularly under certain expectations, with effects persisting 30 minutes poststimulation. In anticipatory brain oscillations, real α-tACS enhanced somatosensory α1-oscillations and increased midfrontal θ-oscillations in conditions of certainty, with θ-oscillation modulation showing sustained effects. Mediation analysis revealed that α-tACS reduced pain reactivity by enhancing somatosensory α1-oscillations but increased pain reactivity through the enhancement of midfrontal θ-oscillations, with the latter effect being more pronounced. These findings suggest that while α-tACS may provide pain relief through somatosensory α-oscillation augmentation, its stronger and longer-lasting impact on midfrontal θ-oscillations could lead to hyperalgesia, particularly in the context of certain pain expectations.

Does intra-epidermal electrical stimulation activate mechano- and thermo-nociceptors? A discrimination approach

BACKGROUND

Objective laboratory tests are needed to diagnose lesions within the nociceptive system accurately. One approach is assessing pain-related evoked potentials (PREPs) in response to intra-epidermal electrical stimulation (IES). In this context, peripheral characterization of the specificity of nociceptor activation with IES is needed.

NEW METHOD

As IES directly depolarizes free nerve endings, it might allow a more comprehensive nociceptor activation than classical contact heat stimulation. Hence, this study aimed to investigate whether mechano-nociceptors are activated by IES. To test this hypothesis, a heat pain model was used to assess whether IES would show comparable pain hypersensitivity in the experimentally-induced area of secondary mechanical hyperalgesia (SMH), as known for pinprick but not for contact heat stimuli. Pain ratings and PREPs were recorded in response to 15 contact heat and pinprick stimuli as well as IES applied to the volar forearm before (PRE) and after (POST) a heat pain model inducing an area of SMH (EXP) or a control model (CTRL).

RESULTS AND COMPARISON WITH EXISTING METHODS

All 24 participants (25.5 ± 4.7 y, 10 f/14 m) presented with SMH in POST-EXP condition. Pain ratings were significantly increased in EXP versus CTRL for IES (p = 0.016) and pinprick (p = 0.006) but not for contact heat (p = 0.683). PREP NP-amplitude between EXP and CTRL was only increased in response to pinprick (p = 0.027), but not to IES (p = 0.547) and contact heat stimuli (p = 0.070).

CONCLUSIONS

Psychophysical assessments suggest mechano-nociceptor activation by IES, while PREPs do not support this assumption, indicating the predominant activation of thermo-nociceptors by IES.

EEG oscillations reveal neuroplastic changes in pain processing associated with long-term meditation

The experience of pain is a combined product of bottom-up and top-down influences mediated by attentional and emotional factors. Meditation states and traits are characterized by enhanced attention/emotion regulation and expanded self-awareness that can be expected to modify pain processing. The main objective of the present study was to explore the effects of long-term meditation on neural mechanisms of pain processing. EEG pain-related oscillations (PROs) were analysed in highly experienced practitioners and novices during a non-meditative resting state with respect to (a) local frequency-specific and temporal synchronizing characteristics to reflect mainly bottom-up mechanisms, (b) spatial synchronizing patterns to reflect the neural communication of noxious information, (c) pre-stimulus oscillations to reflect top-down mechanisms during pain expectancy, and (d) the P3b component of the pain-related potential to compare the emotional/cognitive reappraisal of pain events by expert and novice meditators. Main results demonstrated that in experienced (long-term) meditators as compared to non-experienced (short-term) meditators (1) the temporal and spatial synchronizations of multispectral (from theta-alpha to gamma) PROs were substantially suppressed at primary and secondary somatosensory regions contra-lateral to pain stimulation within 200 ms after noxious stimulus; (2) pre-stimulus alpha activity was significantly increased at the same regions, which predicted the suppressed synchronization of PROs in long-term meditators; (3) the decrease of the P3b component was non-significant. These novel observations provide evidence that even when subjected to pain outside of meditation, experienced meditators exhibit a pro-active top-down inhibition of somatosensory areas resulting in suppressed processing and communication of sensory information at early stages of painful input. The emotional/cognitive appraisal of pain is reduced but remains preserved revealing a capacity of experienced meditators to dissociate pro-active and reactive top-down processes during pain control.

Neurofeedback and attention modulate somatosensory alpha oscillations but not pain perception

Pain is closely linked to alpha oscillations (8 < 13 Hz) which are thought to represent a supra-modal, top-down mediated gating mechanism that shapes sensory processing. Consequently, alpha oscillations might also shape the cerebral processing of nociceptive input and eventually the perception of pain. To test this mechanistic hypothesis, we designed a sham-controlled and double-blind electroencephalography (EEG)-based neurofeedback study. In a short-term neurofeedback training protocol, healthy participants learned to up- and down-regulate somatosensory alpha oscillations using attention. Subsequently, we investigated how this manipulation impacts experimental pain applied during neurofeedback. Using Bayesian statistics and mediation analysis, we aimed to test whether alpha oscillations mediate attention effects on pain perception. The results showed that attention and neurofeedback successfully up- and down-regulated the asymmetry of somatosensory alpha oscillations. However, attention and neurofeedback did not modulate pain ratings or related brain responses. Accordingly, somatosensory alpha oscillations did not mediate attention effects on pain perception. Thus, our results challenge the hypothesis that somatosensory alpha oscillations shape pain perception. A causal relationship between alpha oscillations and pain perception might not exist or be more complex than hypothesized. Trial registration: Following Stage 1 acceptance, the study was registered at ClinicalTrials.gov NCT05570695.

Unraveling the physiological and psychosocial signatures of pain by machine learning

BACKGROUND

Pain is a complex subjective experience, strongly impacting health and quality of life. Despite many attempts to find effective solutions, present treatments are generic, often unsuccessful, and present significant side effects. Designing individualized therapies requires understanding of multidimensional pain experience, considering physical and emotional aspects. Current clinical pain assessments, relying on subjective one-dimensional numeric self-reports, fail to capture this complexity.

METHODS

To this aim, we exploited machine learning to disentangle physiological and psychosocial components shaping the pain experience. Clinical, psychosocial, and physiological data were collected from 118 chronic pain and healthy participants undergoing 40 pain trials (4,697 trials).

FINDINGS

To understand the objective response to nociception, we classified pain from the physiological signals (accuracy >0.87), extracting the most important biomarkers. Then, using multilevel mixed-effects models, we predicted the reported pain, quantifying the mismatch between subjective level and measured physiological response. From these models, we introduced two metrics: TIP (subjective index of pain) and Φ (physiological index). These represent possible added value in the clinical process, capturing psychosocial and physiological pain dimensions, respectively. Patients with high TIP are characterized by frequent sick leave from work and increased clinical depression and anxiety, factors associated with long-term disability and poor recovery, and are indicated for alternative treatments, such as psychological ones. By contrast, patients with high Φ show strong nociceptive pain components and could benefit more from pharmacotherapy.

CONCLUSIONS

TIP and Φ, explaining the multidimensionality of pain, might provide a new tool potentially leading to targeted treatments, thereby reducing the costs of inefficient generic therapies.

FUNDING

RESC-PainSense, SNSF-MOVE-IT197271.

Analgesic effects of high-frequency rTMS on pain anticipation and perception

Previous studies suggest that pain perception is greatly shaped by anticipation, with M1 and DLPFC involved in this process. We hypothesized that high-frequency rTMS targeting these regions could alter pain anticipation and thereby reduce pain perception. In a double-blind, sham-controlled study, healthy participants received 10 Hz rTMS to M1, DLPFC, or a sham treatment. Assessments were conducted before, immediately after, and 60 min after stimulation, including laser-evoked potentials, pain ratings, and anticipatory EEG. M1-rTMS immediately reduced laser-evoked P2 amplitude, increased sensorimotor high-frequency α-oscillation power, and accelerated peak alpha frequency in the midfrontal region during pain anticipation. In contrast, DLPFC-rTMS reduced the N2-P2 complex and pain ratings 60 min post-stimulation, an effect associated with prolonged microstate C duration during pain anticipation-a microstate linked to default mode network activity. Thus, M1-rTMS immediately modulates anticipatory α-oscillations and laser-evoked potentials, while DLPFC-rTMS induces delayed analgesic effects partially by modulating default mode network activity.

Understanding neuropathic pain: the role of neurophysiological tests in unveiling underlying mechanisms

Neuropathic pain, arising from lesions of the somatosensory nervous system, presents with diverse symptoms including ongoing pain, paroxysmal pain, and provoked pain, usually accompanied by sensory deficits. Understanding the pathophysiological mechanisms behind these symptoms is crucial for targeted treatment strategies. Neurophysiological techniques such as nerve conduction studies, reflexes, and evoked potentials help elucidate these mechanisms by assessing large myelinated non-nociceptive fibres and small nociceptive fibres. This argumentative review highlights the importance of tailored neurophysiological assessments for improving our understanding of the pathophysiological mechanisms behind neuropathic pain symptoms.

Clinical neurophysiology of neuropathic pain

Timely and accurate diagnosis of neuropathic pain is critical for optimizing therapeutic outcomes and minimizing treatment delays. According to current standards, the diagnosis of definite neuropathic pain requires objective confirmation of a lesion or disease affecting the somatosensory nervous system. This can be provided by specialized neurophysiological techniques as conventional methods like nerve conduction studies and somatosensory evoked potentials may not be sufficient as they do not assess pain pathways. These specialized techniques apply various stimuli, such as thermal, electrical, or mechanical, alongside assessments of spinal/cortical potential or electromyographic reflex recordings. The selection of techniques is guided by the patient's clinical history and examination. The most common neurophysiological tests used in clinical practice are pain-related evoked potentials (PREPs) providing an objective evaluation of nociceptive pathways. Four types of PREPs are employed: laser evoked potentials, contact-heat evoked potentials, intra-epidermal electrical stimulation evoked potentials, and pinprick evoked potentials, with the two former ones being the most robust and reliable ones. These techniques investigate small-diameter fibers, primarily Aδ-fibers, and spinothalamic tracts allowing the identification of peripheral or central nervous system lesions. Yet, they are limited in capturing neuronal mechanisms underlying neuropathic pain or in providing objective quantification of pain sensation. Two neurophysiological measures which investigate the pain system beyond its integrity are the nociceptive withdrawal reflex and the N13 component of somatosensory evoked potentials. Both of these methods are more commonly used in research than clinical practice, but they pose interesting approaches to quantify central sensitization, a key underlying mechanism of neuropathic pain. Future investigations in neuropathic pain are therefore warranted.

Could Relationship-Based Learnt Beliefs and Expectations Contribute to Physiological Vulnerability of Chronic Pain? Making a Case to Consider Attachment in Pain Research

Pain is an interpersonal and inherently social experience. Pain perception and administration of medical treatment all occur in a particular environmental and social context. Early environmental influences and early learning experiences and interactions condition the body's response to different threats (like pain), ultimately shaping the underlying neurophysiology. These early interactions and experiences also determine what situations are perceived as threatening, as well as our belief in our own ability to self-manage, and our belief in others to offer support, during perceived threats. These beliefs intrinsically drive the combination of behaviors that emerge in response to perceived threats, including pain. Such behaviors can be categorized into attachment styles. In this interdisciplinary review, we synthesize and summarize evidence from the neurobiological, psychobiological, psychosocial, and psychobehavioral fields, to describe how these beliefs are embedded in the brain's prediction models to generate a series of expectations/perceptions around the level of safety/threat in different contexts. As such, these beliefs may predict how one experiences and responds to pain, with potentially significant implications for the development and management of chronic pain. Little attention has been directed to the effect of adult attachment style on pain in research studies and in the clinical setting. Using interdisciplinary evidence, we argue why we think this interaction merits further consideration and research. PERSPECTIVE: This review explores the influence of attachment styles on pain perception, suggesting a link between social connections and chronic pain development. It aligns with recent calls to emphasize the social context in pain research and advocates for increased focus on adult attachment styles in research and clinical practice.

The effects of C-tactile stimulation on temporal summation of second pain: A study of the central and peripheral neural correlates

Affective touch is mediated by specialized receptors sensitive to gentle and slow touch called C-tactile afferents (CT). The activation of these receptors has shown promise in reducing subjective pain ratings, however, how this type of touch can affect central sensitization processes is poorly studied. This work aimed to investigate if affective touch is able to modulate pain sensitization and its electrophysiological correlates during Temporal Summation of Second Pain (TSSP), a phenomenon characterized by an increase in pain perception due to repeated noxious stimuli. Thirty-seven participants underwent a TSSP protocol involving three conditions: TSSP alone, TSSP during vibrotactile stimulation, and TSSP during CT stimulation (administered with a brush mounted in a robot arm). We measured subjective pain ratings, electroencephalographic (N2-P2 complex) and electrocardiographic activity during these conditions. Participants reported a significantly lower increase of pain during CT stimulation compared to vibrotactile stimulation, but not to TSSP alone. In addition, TSSP was reduced when administered in the ipsilateral arm compared to the other somatosensory stimulation. Subjective reports of attention towards painful stimuli, amplitude of the N2-P2 complex, and heart rate were also reduced during CT stimulation. Conclusion: Our results indicated that the activation of CT receptors may reduce sensitization compared to other types of somatosensory stimulation, which is possibly related to the reduction of the attention devoted to nociceptive stimulation. Our results suggest that activation of CT receptors may alleviate the occurrence of central pain sensitization.

Pain reflects the informational value of nociceptive inputs

ABSTRACT

Pain perception and its modulation are fundamental to human learning and adaptive behavior. This study investigated the hypothesis that pain perception is tied to pain's learning function. Thirty-one participants performed a threat conditioning task where certain cues were associated with a possibility of receiving a painful electric shock. The cues that signaled potential pain or safety were regularly changed, requiring participants to continually establish new associations. Using computational models, we quantified participants' pain expectations and prediction errors throughout the task and assessed their relationship with pain perception and electrophysiological responses. Our findings suggest that subjective pain perception increases with prediction error, that is, when pain was unexpected. Prediction errors were also related to physiological nociceptive responses, including the amplitude of nociceptive flexion reflex and electroencephalography markers of cortical nociceptive processing (N1-P2-evoked potential and gamma-band power). In addition, higher pain expectations were related to increased late event-related potential responses and alpha/beta decreases in amplitude during cue presentation. These results further strengthen the idea of a crucial link between pain and learning and suggest that understanding the influence of learning mechanisms in pain modulation could help us understand when and why pain perception is modulated in health and disease.

Novel neurophysiological evidence for preserved pain habituation across chronic pain conditions

OBJECTIVE

The present study aimed to investigate whether subjective and objective measures of pain habituation can be used as potential markers for central sensitization across various chronic pain patients.

METHODS

Two blocks of contact-heat stimuli were applied to a non-painful area in 93 chronic pain patients (low back pain, neuropathic pain, and complex regional pain syndrome) and 60 healthy controls (HC). Habituation of pain ratings, contact-heat evoked potentials (CHEP), and sympathetic skin responses (SSR) was measured.

RESULTS

There was no significant difference in any measure of pain habituation between patients and HC. Even patients with apparent clinical signs of central sensitization showed no reduced pain habituation. However, prolonged baseline CHEP and SSR latencies (stimulation block 1) were found in patients compared to HC (CHEP: Δ-latency = 23 ms, p = 0.012; SSR: Δ-latency = 100 ms, p = 0.022).

CONCLUSION

Given the performed multimodal neurophysiological testing protocol, we provide evidence indicating that pain habituation may be preserved in patients with chronic pain and thereby be of limited use as a sensitive marker for central sensitization. These results are discussed within the framework of the complex interactions between pro- and antinociceptive mechanism as well as methodological issues. The prolonged latencies of CHEP and SSR after stimulation in non-painful areas may indicate subclinical changes in the integrity of thermo-nociceptive afferents, or a shift towards antinociceptive activity. This shift could potentially affect the relay of ascending signals.

SIGNIFICANCE

Our findings challenge the prevailing views in the literature and may encourage further investigations into the peripheral and central components of pain habituation, using advanced multimodal neurophysiological techniques.

Understanding inter-individual variability of experimental pain habituation and conditioned pain modulation in healthy individuals

Although reduced experimental pain habituation is proposed as a proxy of diminished endogenous pain modulatory capacity in chronic pain, prior studies show contradictory findings. Even across healthy participants, pain habituation varies substantially, which may relate to another measure of endogenous pain modulation, i.e., conditioned pain modulation (CPM). Hence, this study investigated the relationship between pain habituation and CPM. Pain habituation was assessed in 45 healthy participants between two blocks of 15-20 contact-heat stimuli applied to the hand. Habituation of subjective pain ratings and objective neurophysiological readouts (contact-heat evoked potential (CHEP) and palmar sympathetic skin response (SSR)) was investigated. CPM was assessed by comparing heat pain thresholds before and after hand immersion in a noxious cold (9 °C) and lukewarm water bath (32 °C, to control for repeated measures effects). Pain habituation showed a large variability, with subjective but not objective pain habituation correlating with cold-induced CPM effects (r = 0.50; p = 0.025). This correlation was not observed for 'true' CPM effects (corrected for repeated measures effects) nor for CPM effects induced by a lukewarm water bath. These findings suggest that the observed variability in subjective pain habituation may be influenced by both descending endogenous pain modulation and peripheral adaptation processes associated with repeated measures. Objective pain habituation readouts, i.e., CHEPs and SSRs, capture different, complementary aspects of endogenous pain modulation.

Neural dynamics of pain modulation by emotional valence

Definitions of human pain acknowledge at least two dimensions of pain, affective and sensory, described as separable and thus potentially differentially modifiable. Using electroencephalography, we investigated perceptual and neural changes of emotional pain modulation in healthy individuals. Painful electrical stimuli were applied after presentation of priming emotional pictures (negative, neutral, positive) and followed by pain intensity and unpleasantness ratings. We found that perceptual and neural event-related potential responses to painful stimulation were significantly modulated by emotional valence. Specifically, pain unpleasantness but not pain intensity ratings were increased when pain was preceded by negative compared to neutral or positive pictures. Amplitudes of N2 were higher when pain was preceded by neutral compared to negative and positive pictures, and P2 amplitudes were higher for negative compared to neutral and positive pictures. In addition, a hierarchical regression analysis revealed that P2 alone and not N2, predicted pain perception. Finally, source analysis showed the anterior cingulate cortex and the thalamus as main spatial clusters accounting for the neural changes in pain processing. These findings provide evidence for a separation of the sensory and affective dimensions of pain and open new perspectives for mechanisms of pain modulation.

Neurophysiological investigations in a case of primary paroxysmal hemicrania-tic syndrome

BACKGROUND

The association between paroxysmal hemicrania (PH) and trigeminal neuralgia-the so-called PH-tic syndrome-has rarely been described. However, a correct diagnosis is crucial since both disorders require specific treatments. Little is known about pathophysiological mechanisms, and, to date, there are no electrophysiological studies in patients with PH-tic syndrome.

CASE

We describe the case of a 52-year-old man with a PH-tic syndrome successfully treated with an association of carbamazepine (1200 mg/day) and indomethacin (150 mg/die). Patient underwent trigeminal reflex testing, including blink and masseter inhibitory reflex, and laser-evoked potential (LEP) recording after supraorbital region stimulation in the affected and unaffected side. Both neurophysiological investigations resulted normal; LEPs failed to detect any latency asymmetry between both sides.

CONCLUSIONS

Neurophysiological findings demonstrate for the first time the integrity of somatosensory system in a primary PH-tic syndrome case. Central pathophysiological mechanisms and hypothalamic dysregulation may contribute to the development of this rare syndrome.

Noninvasive neuromodulation of subregions of the human insula differentially affect pain processing and heart-rate variability: a within-subjects pseudo-randomized trial

ABSTRACT

The insula is an intriguing target for pain modulation. Unfortunately, it lies deep to the cortex making spatially specific noninvasive access difficult. Here, we leverage the high spatial resolution and deep penetration depth of low-intensity focused ultrasound (LIFU) to nonsurgically modulate the anterior insula (AI) or posterior insula (PI) in humans for effect on subjective pain ratings, electroencephalographic (EEG) contact heat-evoked potentials, as well as autonomic measures including heart-rate variability (HRV). In a within-subjects, repeated-measures, pseudo-randomized trial design, 23 healthy volunteers received brief noxious heat pain stimuli to the dorsum of their right hand during continuous heart-rate, electrodermal, electrocardiography and EEG recording. Low-intensity focused ultrasound was delivered to the AI (anterior short gyrus), PI (posterior longus gyrus), or under an inert Sham condition. The primary outcome measure was pain rating. Low-intensity focused ultrasound to both AI and PI similarly reduced pain ratings but had differential effects on EEG activity. Low-intensity focused ultrasound to PI affected earlier EEG amplitudes, whereas LIFU to AI affected later EEG amplitudes. Only LIFU to the AI affected HRV as indexed by an increase in SD of N-N intervals and mean HRV low-frequency power. Taken together, LIFU is an effective noninvasive method to individually target subregions of the insula in humans for site-specific effects on brain biomarkers of pain processing and autonomic reactivity that translates to reduced perceived pain to a transient heat stimulus.

Habituation to pain: self-report, electroencephalography, and functional magnetic resonance imaging in healthy individuals. A scoping review and future recommendations

ABSTRACT

Habituation to pain is a fundamental learning process and important adaption. Yet, a comprehensive review of the current state of the field is lacking. Through a systematic search, 63 studies were included. Results address habituation to pain in healthy individuals based on self-report, electroencephalography, or functional magnetic resonance imaging. Our findings indicate a large variety in methods, experimental settings, and contexts, making habituation a ubiquitous phenomenon. Habituation to pain based on self-report studies shows a large influence of expectations, as well as the presence of individual differences. Furthermore, widespread neural effects, with sometimes opposing effects in self-report measures, are noted. Electroencephalography studies showed habituation of the N2-P2 amplitude, whereas functional magnetic resonance imaging studies showed decreasing activity during painful repeated stimulation in several identified brain areas (cingulate cortex and somatosensory cortices). Important considerations for the use of terminology, methodology, statistics, and individual differences are discussed. This review will aid our understanding of habituation to pain in healthy individuals and may lead the way to improving methods and designs for personalized treatment approaches in chronic pain patients.

Influence of skin type and laser wavelength on laser-evoked potentials

BACKGROUND

Infrared laser stimulation is a valuable tool in pain research, its primary application being the recording of laser-evoked brain potentials (LEPs). Different types of laser stimulators, varying in their skin penetrance, are likely to have a large influence on the LEPs, when stimulating different skin types. The aim of this study was to investigate how LEPs depend on laser type and skin location.

METHODS

Two different laser stimulators (CO2 and Nd:YAP) were used to compare LEPs in healthy subjects. Stimuli were delivered to the hand dorsum and palm to investigate the effects of skin type on the evoked responses. Stimulus-evoked brain responses were recorded using EEG and perceived intensity ratings were recorded. Computational modelling was used to investigate the observed differences.

RESULTS

LEPs evoked by stimulation of the hairy skin were similar between CO2 and Nd:YAP stimulation. In contrast, LEPs elicited from the palm were markedly different and barely present for CO2 stimulation. There was a significant interaction between laser type and skin type (RM-ANOVA, p < 0.05) likely due to smaller CO2 LEPs in the palm. CO2 stimuli to the palm also elicited significantly lower perceived intensities. The computational model showed that the observed differences were explainable by the laser absorption characteristics and skin thickness affecting the temperature profile at the dermo-epidermal junction (DEJ).

CONCLUSIONS

This study shows that LEP elicitation depends on the combination of laser penetrance and skin type. Low penetrance stimuli, from a CO2 laser, elicited significantly lower LEPs and perceived intensities in the palm.

SIGNIFICANCE

This study showed that the elicitation of laser-evoked potentials in healthy humans greatly depends on the combination of laser stimulator type and skin type. It was shown that high penetrance laser stimuli are capable of eliciting responses in both hairy and glabrous skin, whereas low penetrance stimuli barely elicited responses from the glabrous skin. Computational modelling was used to demonstrate that the results could be fully explained by the combination of laser type and skin thickness.

Pain-motor integration in chronic pain: A neurophysiological study

OBJECTIVE

Chronic pain may lead to functional changes in several brain regions, including the primary motor cortex (M1). Our neurophysiological study aimed to probe M1 plasticity, through a non-invasive transcranial magnetic stimulation protocol, in a cohort of patients with chronic pain.

METHODS

Twenty patients with chronic pain (age ± SD: 62.9 ± 9.9) and 20 age- and sex-matched healthy controls (age ± SD: 59.6 ± 15.8) were recruited. Standardized scales were used for the evaluation of pain severity. Neurophysiological measures included laser-evoked potentials (LEPs) and motor-evoked potentials (MEPs) collected at baseline and over 60 minutes following a standardized Laser-paired associative stimulation (Laser-PAS) protocol.

RESULTS

LEPs and MEPs were comparable in patients with chronic pain and controls. The pain threshold was lower in patients than in controls. Laser-PAS elicited decreased responses in patients with chronic pain. The response to Laser-PAS was similar in subgroups of patients with different chronic pain phenotypes.

CONCLUSIONS

M1 plasticity, as tested by Laser-PAS, is altered in patients with chronic pain, possibly reflecting abnormal pain-motor integration processes.

SIGNIFICANCE

Chronic pain is associated with a disorder of M1 plasticity raising from abnormal pain-motor integration.

I Feel! Therefore, I Am from Pain to Consciousness in DOC Patients

Pain assessment and management in patients with disorders of consciousness (DOC) is a challenging and important aspect of care, with implications for detecting consciousness and promoting recovery. This narrative review explores the role of pain in consciousness, the challenges of pain assessment, pharmacological treatment in DOC, and the implications of pain assessment when detecting changes in consciousness. The review discusses the Nociception Coma Scale and its revised version, which are behavioral scales used to assess pain in DOC patients, and the challenges and controversies surrounding the appropriate pharmacological treatment of pain in these patients. Moreover, we highlight recent evidence suggesting that an accurate pain assessment may predict changes in the level of consciousness in unresponsive wakefulness syndrome/vegetative state patients, underscoring the importance of ongoing pain management in these patients.

The Biology of Placebo and Nocebo Effects on Experimental and Chronic Pain: State of the Art

(1) Background: In recent years, placebo and nocebo effects have been extensively documented in different medical conditions, including pain. The scientific literature has provided strong evidence of how the psychosocial context accompanying the treatment administration can influence the therapeutic outcome positively (placebo effects) or negatively (nocebo effects). (2) Methods: This state-of-the-art paper aims to provide an updated overview of placebo and nocebo effects on pain. (3) Results: The most common study designs, the psychological mechanisms, and neurobiological/genetic determinants of these phenomena are discussed, focusing on the differences between positive and negative context effects on pain in experimental settings on healthy volunteers and in clinical settings on chronic pain patients. Finally, the last section describes the implications for clinical and research practice to maximize the medical and scientific routine and correctly interpret the results of research studies on placebo and nocebo effects. (4) Conclusions: While studies on healthy participants seem consistent and provide a clear picture of how the brain reacts to the context, there are no unique results of the occurrence and magnitude of placebo and nocebo effects in chronic pain patients, mainly due to the heterogeneity of pain. This opens up the need for future studies on the topic.

Non-invasive neuromodulation of sub-regions of the human insula differentially affect pain processing and heart-rate variability

The insula is a portion of the cerebral cortex folded deep within the lateral sulcus covered by the overlying opercula of the inferior frontal lobe and superior portion of the temporal lobe. The insula has been parsed into sub-regions based upon cytoarchitectonics and structural and functional connectivity with multiple lines of evidence supporting specific roles for each of these sub-regions in pain processing and interoception. In the past, causal interrogation of the insula was only possible in patients with surgically implanted electrodes. Here, we leverage the high spatial resolution combined with the deep penetration depth of low-intensity focused ultrasound (LIFU) to non-surgically modulate either the anterior insula (AI) or posterior insula (PI) in humans for effect on subjective pain ratings, electroencephalographic (EEG) contact head evoked potentials (CHEPs) and time-frequency power as well as autonomic measures including heart-rate variability (HRV) and electrodermal response (EDR). N = 23 healthy volunteers received brief noxious heat pain stimuli to the dorsum of their right hand during continuous heart-rate, EDR and EEG recording. LIFU was delivered to either the AI (anterior short gyrus), PI (posterior longus gyrus) or under an inert sham condition time-locked to the heat stimulus. Results demonstrate that single-element 500 kHz LIFU is capable of individually targeting specific gyri of the insula. LIFU to both AI and PI similarly reduced perceived pain ratings but had differential effects on EEG activity. LIFU to PI affected earlier EEG amplitudes around 300 milliseconds whereas LIFU to AI affected EEG amplitudes around 500 milliseconds. In addition, only LIFU to the AI affected HRV as indexed by an increase in standard deviation of N-N intervals (SDNN) and mean HRV low frequency power. There was no effect of LIFU to either AI or PI on EDR or blood pressure. Taken together, LIFU looks to be an effective method to individually target sub-regions of the insula in humans for site-specific effects on brain biomarkers of pain processing and autonomic reactivity that translates to reduced perceived pain to a transient heat stimulus. These data have implications for the treatment of chronic pain and several neuropsychological diseases like anxiety, depression and addiction that all demonstrate abnormal activity in the insula concomitant with dysregulated autonomic function.

Testosterone administration enhances the expectation and perception of painful and non-painful somatosensory stimuli

The influence of testosterone on pain perception remains inconsistent in the literature. This randomized, placebo-controlled, double-blind, crossover study investigated the effect of testosterone administration on perception and expectation of electrocutaneous stimulus. Thirty healthy male participants received a single dose of testosterone in one session and a placebo in the other session. For each session, they completed a pain-rating task in which a predictability cue was inserted before a painful or non-painful electocutaneous stimulus delivery, while neural activity was simultaneously recorded by a 64-channel electroencephalographic (EEG) system. Expected and perceived pain ratings, as well as event-related potentials (ERPs) to electocutaneous stimuli and prestimulus EEG oscillatory activities while expecting upcoming electocutaneous stimuli were comprehensively compared between testosterone and placebo sessions. Compared with the placebo session, participants in the testosterone session reported greater pain rating and exhibited greater amplitude of N1 component on ERPs when perceiving both painful and non-painful electrocutaneous stimuli. Mediation analysis revealed that testosterone enhanced the pain-intensity ratings via the N1 response to the electrocutaneous stimulus. Upon viewing the predictability cues after testosterone administration, expected pain intensity increased and spontaneous low-frequency α-oscillation power in the frontal region decreased. These results provide evidence that testosterone enhanced perception and expectation of somatosensory events, and that this was a general effect rather than pain-specific. A plausible explanation for these findings is that testosterone acts to increase vigilance and sustained attention levels, as evidenced by the decreased α-oscillation power. Thus, our findings support a causal role for testosterone in heightening the biological salience of incoming somatosensory information.

Habituation deficit of visual evoked potentials in migraine patients with hypermobile Ehlers-Danlos syndrome

Objectives

Migraine is one of the most frequent clinical manifestations of hypermobile Ehlers-Danlos syndrome (hEDS). The comorbidity between these two diseases has been only partially investigated. We aimed to observe whether neurophysiological alterations described in migraineurs in visual evoked potentials (VEPs) were present in hEDS patients with migraine.

Methods

We enrolled 22 hEDS patients with migraine (hEDS) and 22 non-hEDS patients with migraine (MIG), with and without aura (according to ICHD-3), as well as 22 healthy controls (HC). Repetitive pattern reversal (PR)-VEPs were recorded in basal conditions in all participants. During uninterrupted stimulation, 250 cortical responses were recorded (4,000 Hz sample rate) and divided into epochs of 300 ms after the stimulus. Cerebral responses were divided into five blocks. The habituation was calculated as the slope interpolating the amplitudes in each block, for both the N75-P100 and P100-N145 components of PR-VEP.

Results

We observed a significant habituation deficit of the P100-N145 component of PR-VEP in hEDS compared to HC (p = 0.002), unexpectedly more pronounced than in MIG. We observed only a slight habituation deficit of N75-P100 in hEDS, with a slope degree that was intermediate between MIG and HC.

Discussion

hEDS patients with migraine presented an interictal habituation deficit of both VEPs components like MIG. Pathophysiological aspects underlying the pathology could account for the peculiar pattern of habituation in hEDS patients with migraine characterized by a pronounced habituation deficit in the P100-N145 component and a less clear-cut habituation deficit in the N75-P100 component with respect to MIG.

ERP indicators of situational empathy pain

This study aimed to validate a recent conceptualization proposed by Coll and colleagues (2017a) that defines empathic response as a situational, cognitively complex process requiring emotion identification and affective sharing. Sixty right-handed women university students (18-29 years) voluntarily participated in the study. We measured ratings for empathy pain to assess the individual differences in empathy. At the same time, we collected peak amplitudes of the event-related potentials (ERPs) components to empathic stimulations of painful faces or hand stimuli and neutral images. Electrophysiological results proved that the P2, N170, N2, and P3 ERP components were associated with the modulation of empathic responses. Participants with low empathic responses (p < 0.05) disclosed a larger frontal central N2 for the painful hands than for painful faces (p < .05) and a reduced temporoparietal N170 for painful hands compared to neutral ones. Furthermore, our results highlighted higher frontal central P3a and P3b to painful stimuli than controls (p ≤ 0.01). We explained these findings assuming that in identifying the emotional value of a stimulus, the emotional content can modulate the reorientation of attention and the in-memory updating process associated with the empathic response. Results are in line with Coll and colleagues' conceptualization of the empathic response that includes two cognitive processes, the identification of emotions, and affective sharing, related to the recognition of the emotional state of the other in the self.

A novel thermoelectric device integrated with a psychophysical paradigm to study pain processing in human subjects

INTRODUCTION

Cerebral projections of nociceptive stimuli are of great interest as targets for neuromodulation in chronic pain. To study cerebral networks involved in processing noxious stimuli, researchers often rely on thermo-nociception to induce pain. However, various limitations exist in many pain-inducing techniques, such as not accounting for individual variations in pain and trial structure predictability.

METHODS

We propose an improved and reliable psychometric experimental method to evaluate human nociceptive processing to overcome some of these limitations. The developed testing paradigm leverages a custom-built, open-source, thermoelectric device (TED). The device construction and hardware are described. A maximum-likelihood adaptive algorithm is integrated into the TED software, facilitating individual psychometric functions representative of both hot and cold pain perception. In addition to testing only hot or cold thresholds, the TED may also be used to induce the thermal grill illusion (TGI), where the bars are set to alternating warm and cool temperatures.

RESULTS

Here, we validated the TED's capability to adjust between different temperatures and showed that the device quickly and automatically changes temperature without any experimenter input. We also validated the device and integrated psychometric pain task in 21 healthy human subjects. Hot and cold pain thresholds (HPT, CPT) were determined in human subjects with <1 °C of variation. Thresholds were anticorrelated, meaning a volunteer with a low CPT likely had a high HPT. We also showed how the TED can be used to induce the TGI.

CONCLUSION

The TED can induce thermo-nociception and provide probabilistic measures of hot and cold pain thresholds. Based on the findings presented, we discuss how the TED could be used to study thermo-nociceptive cerebral projections if paired with intracranial electrode monitoring.

Extensive sensorimotor training enhances nociceptive cortical responses in healthy individuals

BACKGROUND

Prolonged and repeated sensorimotor training is a crucial driver for promoting use-dependent plasticity, but also a main risk factor for developing musculoskeletal pain syndromes, yet the neural underpinnings that link repetitive movements to abnormal pain processing are unknown.

METHODS

Twenty healthy musicians, one of the best in vivo models to study use-dependent plasticity, and 20 healthy non-musicians were recruited. Perceptual thresholds, reaction times (RTs) and event-related potentials (ERPs) were recorded using nociceptive intra-epidermal and non-nociceptive transcutaneous electrical stimulation.

RESULTS

In response to comparable stimulus intensities, musicians compared to non-musicians showed larger non-nociceptive N140 (associated with higher activation of regions within the salience network), higher nociceptive N200 ERPs (associated with higher activation of regions within the sensorimotor network) and faster RTs to both stimuli. Non-musicians showed larger non-nociceptive P200 ERP. Notably, a similar P200 component prominently emerged during nociceptive stimulation in non-musicians. Across participants, larger N140 and N200 ERPs were associated with RTs, whereas the amount of daily practice in musicians explained non-nociceptive P200 and nociceptive P300 ERPs.

CONCLUSIONS

These novel findings indicate that the mechanisms by which extensive sensorimotor training promotes use-dependent plasticity in multisensory neural structures may also shape the neural signatures of nociceptive processing in healthy individuals.

SIGNIFICANCE

Repetitive sensorimotor training may increase the responsiveness of nociceptive evoked potentials. These novel data highlight the importance of repetitive sensorimotor practice as a contributing factor to the interindividual variability of nociceptive-related potentials.

Assessment of C Fibers Evoked Potentials in Healthy Subjects by Nd : YAP Laser

Introduction

Although laser stimuli activate both Ad- and C-fibres, the corresponding laser evoked potentials (LEPs) remain restricted to the Ad-fibers input, while the C-fibers related potential is hardly detectable.

Aims

To evaluate multichannel ultralate LEPs (U-LEPs) by using Nd : Yap laser pulses in healthy volunteers to stimulation of face and lower and upper limbs, in order to estimate the reliability of C-LEPs elicited from both trigeminal and somatic sites.

Methods

Twenty healthy volunteers participated in two stimulation sessions to record Aδ-LEPs and C-LEPs. We used a Nd : YAP Laser and 62 EEG recording electrodes. Stimuli parameters were set to activate either small myelinated (Aδ), eliciting purely warmth sensations, or unmyelinated (C) afferents, and eliciting pinprick sensations.

Results

At the trigeminal level, we obtained a negative-positive complex in a time interval compatible with the C fibers activation. In the somatic districts, the averaged responses consisted of an earlier negative-positive complex, followed by a later one. Single trials analysis of U-LEPs showed a maximal positive peak in a time interval in the range of C fibers. Topographical analysis of U-LEPs resembled that of LEPs. All subjects exhibited readable U-LEPs in at least 2 stimulated sites. Discussion. A purely warmth sensation seems to correspond to Aδ and C-fibers coactivation, at least in the somatic districts. While the related cortical waves seem hardly readable, their total absence could be a sign of systemic involvement of warm related C-fibers in specific clinical conditions.

Selective and replicable neuroimaging-based indicators of pain discriminability

Neural indicators of pain discriminability have far-reaching theoretical and clinical implications but have been largely overlooked previously. Here, to directly identify the neural basis of pain discriminability, we apply signal detection theory to three EEG (Datasets 1-3, total N = 366) and two fMRI (Datasets 4-5, total N = 399) datasets where participants receive transient stimuli of four sensory modalities (pain, touch, audition, and vision) and two intensities (high and low) and report perceptual ratings. Datasets 1 and 4 are used for exploration and others for validation. We find that most pain-evoked EEG and fMRI brain responses robustly encode pain discriminability, which is well replicated in validation datasets. The neural indicators are also pain selective since they cannot track tactile, auditory, or visual discriminability, even though perceptual ratings and sensory discriminability are well matched between modalities. Overall, we provide compelling evidence that pain-evoked brain responses can serve as replicable and selective neural indicators of pain discriminability.

The influence of social signals on the self-experience of pain: A neuroimaging review

Researchers in cognitive neuroscience have investigated extensively how psychological factors shape the processing and perception of pain using behavioral, physiological, and neuroimaging methods. However, social influences of pain, an essential part of biopsychosocial pain models, have received relatively little attention. This is particularly true for the neurobiological mechanisms underlying social modulations on pain. Therefore, this review discusses the findings of recent neuroimaging studies measuring the effects of social manipulations on pain perception (e.g., verbal and non-verbal social signals, social interaction style, conformity, social support, and sociocultural mediators). Finally, a schematic summary of the different social modulatory themes is presented.

Effect of Virtual Reality Hypnosis on Pain Threshold and Neurophysiological and Autonomic Biomarkers in Healthy Volunteers: Prospective Randomized Crossover Study

Background

Virtual reality hypnosis (VRH) is a promising tool to reduce pain. However, the benefits of VRH on pain perception and on the physiological expression of pain require further investigation.

Objective

In this study, we characterized the effects of VRH on the heat pain threshold among adult healthy volunteers while monitoring several physiological and autonomic functions.

Methods

Sixty healthy volunteers were prospectively included to receive nociceptive stimulations. The first set of thermal stimuli consisted of 20 stimulations at 60°C (duration 500 milliseconds) to trigger contact heat evoked potentials (CHEPs). The second set of thermal stimuli consisted of ramps (1°C/second) to determine the heat pain threshold of the participants. Electrocardiogram, skin conductance responses, respiration rate, as well as the analgesia nociception index were also recorded throughout the experiment.

Results

Data from 58 participants were analyzed. There was a small but significant increase in pain threshold in VRH (50.19°C, SD 1.98°C) compared to that in the control condition (mean 49.45°C, SD 1.87; P<.001, Wilcoxon matched-pairs signed-rank test; Cohen d=0.38). No significant effect of VRH on CHEPs and heart rate variability parameters was observed (all P>0.5; n=22 and n=52, respectively). During VRH, participants exhibited a clear reduction in their autonomic sympathetic tone, as shown by the lower number of nonspecific skin conductance peak responses (P<.001, two-way analysis of variance; n=39) and by an increase in the analgesia nociception index (P<.001, paired t-test; n=40).

Conclusions

The results obtained in this study support the idea that VRH administration is effective at increasing heat pain thresholds and impacts autonomic functions among healthy volunteers. As a nonpharmacological intervention, VRH has beneficial action on acute experimental heat pain. This beneficial action will need to be evaluated for the treatment of other types of pain, including chronic pain.

Early life inflammation is associated with spinal cord excitability and nociceptive sensitivity in human infants

Immune function and sensitivity to pain are closely related, but the association between early life inflammation and sensory nervous system development is poorly understood-especially in humans. Here, in term-born infants, we measure brain activity and reflex withdrawal activity (using EEG and EMG) and behavioural and physiological activity (using the PIPP-R score) to assess the impact of suspected early-onset neonatal infection on tactile- and noxious-evoked responses. We present evidence that neonatal inflammation (assessed by measuring C-reactive protein levels) is associated with increased spinal cord excitability and evoked brain activity following both tactile and noxious stimulation. There are early indications that this hyperalgesia could be maintained post-inflammation, supporting pre-clinical reports of early-life immune dysfunction influencing pain sensitivity in adults.

Just Breathe: Improving LEP Outcomes through Long Interval Breathing

Background: Laser-evoked potentials (LEPs) constitute an objective clinical diagnostic method used to investigate the functioning of the nociceptor system, including signaling in thin peripheral nerve fibers: Aδ and C fibers. There is preliminary evidence that phase locking LEPs with the breathing cycle can improve the parameters used to evaluate LEPs. Methods: We tested a simple breathing protocol as a low-cost improvement to LEP testing of the hands. Twenty healthy participants all underwent three variants of LEP protocols: following a video-guided twelve-second breathing instruction, watching a nature video, or using the classic LEP method of focusing on the hand being stimulated. Results: The breath protocol produced significantly shorter latencies as compared with the nature or classic protocol. It was also the least prone to artifacts and was deemed most acceptable by the subjects. There was no difference between the protocols regarding LEP amplitudes. Conclusions: Using a breathing video can be a simple, low-cost improvement for LEP testing in research and clinical diagnostics.

Central effects of galcanezumab in migraine: a pilot study on Steady State Visual Evoked Potentials and occipital hemodynamic response in migraine patients

BACKGROUND

The discovery of the prominent action of Calcitonin Gene Related Peptide -CGRP- on trigeminal afferents and meningeal vessels, opened a new era in migraine treatment. However, how the block of nociceptive afferents could act on central mechanisms of migraine is still not clear. In this pilot study we aimed to test the effect of 3 months Galcanezumab (CGA) therapy on occipital visual reactivity in migraine patients, using the Steady State Visual Evoked Potentials-SSVEPs and Functional Near Infrared Spectroscopy -fNIRS.

METHOD

Thirteen migraine patients underwent clinical and neurophysiological examination in basal condition (T0), 1 h after GCA injection (T1) and after 3 months of GCA treatment (T2). Ten healthy volunteers were also evaluated.

RESULTS

At T2, there was a reduction of headache frequency and disability. At T2, the EEG power significantly diminished as compared to T0 and T1 at occipital sites, and the topographical analysis confirmed a restoration of SSVEPs within normal values. The Oxyhemoglobin levels in occipital cortex, which were basically increased during visual stimulation in migraine patients, reverted to normal values at T2.

CONCLUSIONS

The present pilot study indicates that Galcanezumab could act on cortical targets located beyond the pain network, restoring the abnormal occipital reactivity. This effect could indicate the possible disease modifying properties of CGRP related monoclonal antibodies.

The influence of EEG oscillations, heart rate variability changes, and personality on self-pain and empathy for pain under placebo analgesia

We induced placebo analgesia (PA), a phenomenon explicitly attenuating the self-pain feeling, to assess whether this resulted in reduced empathy pain when witnessing a confederate undergoing such pain experience. We recorded EEG and electrocardiogram during a painful Control and PA treatment in healthy adults who rated their experienced pain and empathy for pain. We derived HRV changes and, using wavelet analysis of non-phase-locked event-related EEG oscillations, EEG spectral power differences for self-pain and other-pain conditions. First-hand PA reduced self-pain and self-unpleasantness, whereas we observed only a slight decrease in other unpleasantness. We derived linear combinations of HRV and EEG band power changes significantly associated with self-pain and empathy for pain changes using PCAs. Lower Behavioral Inhibition System scores predicted self-pain reduction through the mediating effect of a relative HR-slowing and a decreased midline ϑ-band (4-8 Hz) power factor moderated by lower Fight-Flight-Freeze System trait scores. In the other-pain condition, we detected a direct positive influence of Total Empathic Ability on the other-pain decline with a mediating role of the midline β2-band (22-30 Hz) power reduction. These findings suggest that PA modulation of first-hand versus other pain relies on functionally different physiological processes involving different personality traits.

Experience with opioids does not modify the brain network involved in expectations of placebo analgesia

Placebo analgesia (PA) is defined as a psychobiological phenomenon triggered by the information surrounding an analgesic drug instead of its inherent pharmacological properties. PA is hypothesized to be formed through either verbal suggestions or conditioning. The present study aims at disentangling the neural correlates of expectations effects with or without conditioning through prior experience using the model of PA.

We addressed this question by recruiting two groups of individuals holding comparable verbally‐induced expectations regarding morphine analgesia but either (i) with or (ii) without prior experience with opioids. We then contrasted the two groups' neurocognitive response to acute heat‐pain induction following the injection of sham morphine using electroencephalography (EEG). Topographic ERP analyses of the N2 and P2 pain evoked potential components allowed to test the hypothesis that PA involves distinct neural networks when induced by expectations with or without prior experience.

First, we confirmed that the two groups showed corresponding expectations of morphine analgesia (Hedges' g_s < .4 positive control criteria, g_s = .37 observed difference), and that our intervention induced a medium‐sized PA (Hedges' g_av ≥ .5 positive control, g_av = .6 observed PA). We then tested our hypothesis on the recruitment of different PA‐associated brain networks in individuals with versus without prior experience with opioids and found no evidence for a topographic N2 and P2 ERP components difference between the two groups. Our results thus suggest that in the presence of verbally‐induced expectations, modifications in the PA‐associated brain activity by conditioning are either absent or very small.

Neural processes responsible for the translation of sustained nociceptive inputs into subjective pain experience

Tracking and predicting the temporal structure of nociceptive inputs is crucial to promote survival, as proper and immediate reactions are necessary to avoid actual or potential bodily injury. Neural activities elicited by nociceptive stimuli with different temporal structures have been described, but the neural processes responsible for translating nociception into pain perception are not fully elucidated. To tap into this issue, we recorded electroencephalographic signals from 48 healthy participants receiving thermo-nociceptive stimuli with 3 different durations and 2 different intensities. We observed that pain perception and several brain responses are modulated by stimulus duration and intensity. Crucially, we identified 2 sustained brain responses that were related to the emergence of painful percepts: a low-frequency component (LFC, < 1 Hz) originated from the insula and anterior cingulate cortex, and an alpha-band event-related desynchronization (α-ERD, 8-13 Hz) generated from the sensorimotor cortex. These 2 sustained brain responses were highly coupled, with the α-oscillation amplitude that fluctuated with the LFC phase. Furthermore, the translation of stimulus duration into pain perception was serially mediated by α-ERD and LFC. The present study reveals how brain responses elicited by nociceptive stimulation reflect the complex processes occurring during the translation of nociceptive information into pain perception.

Effects of Exercise-induced Hypoalgesia and Its Neural Mechanisms

PURPOSE

Exercise-induced hypoalgesia is frequently documented in the literature. However, the underlying neural mechanism of this phenomenon remains unclear. Here, we explored the effects of different intensities of isometric exercise on pain perception with a randomized controlled design and investigated its neural mechanisms through tracing the dynamic changes of heat-evoked brain responses.

METHODS

Forty-eight participants were randomly assigned to one of the three groups with different exercise intensities (i.e., high, low, and control). Their subjective pain reports and brain responses elicited by heat stimuli before and after exercise were assessed.

RESULTS

We observed 1) the increased pressure pain thresholds and heat pain thresholds on the dorsal surface of the hand and the biceps brachii muscle of the exercised limb (closed to the contracting muscle), and the decreased pressure pain ratings at the indexed finger of the unexercised limb; 2) more reduction of pain sensitivity on both the biceps brachii muscle and the dorsal surface of the hand induced by the high-intensity isometric exercise than the low-intensity isometric exercise; and 3) both the high-intensity and the low-intensity isometric exercise induced the reduction of N2 amplitudes and N2-P2 peak-to-peak amplitudes, as well as the reduction of event-related potential magnitudes elicited by the heat stimuli on the exercised limb.

CONCLUSIONS

The hypoalgesic effects induced by the isometric exercise were not only localized to the moving part of the body but also can be extended to the distal part of the body. The exercise intensities play a vital role in modulating these effects. Exercise-induced hypoalgesia could be related to the modulation of nociceptive information transmission via a spinal gating mechanism and also rely on a top-down descending pain inhibitory mechanism.

Nociceptive Intra-epidermal Electric Stimulation Evokes Steady-State Responses in the Secondary Somatosensory Cortex

Recent studies have established the presence of nociceptive steady-state evoked potentials (SSEPs), generated in response to thermal or intra-epidermal electric stimuli. This study explores cortical sources and generation mechanisms of nociceptive SSEPs in response to intra-epidermal electric stimuli. Our method was to stimulate healthy volunteers (n = 22, all men) with 100 intra-epidermal pulse sequences. Each sequence had a duration of 8.5 s, and consisted of pulses with a pulse rate between 20 and 200 Hz, which was frequency modulated with a multisine waveform of 3, 7 and 13 Hz (n = 10, 1 excluded) or 3 and 7 Hz (n = 12, 1 excluded). As a result, evoked potentials in response to stimulation onset and contralateral SSEPs at 3 and 7 Hz were observed. The SSEPs at 3 and 7 Hz had an average time delay of 137 ms and 143 ms respectively. The evoked potential in response to stimulation onset had a contralateral minimum (N1) at 115 ms and a central maximum (P2) at 300 ms. Sources for the multisine SSEP at 3 and 7 Hz were found through beamforming near the primary and secondary somatosensory cortex. Sources for the N1 were found near the primary and secondary somatosensory cortex. Sources for the N2-P2 were found near the supplementary motor area. Harmonic and intermodulation frequencies in the SSEP power spectrum remained below a detectable level and no evidence for nonlinearity of nociceptive processing, i.e. processing of peripheral firing rate into cortical evoked potentials, was found.

Temporal–spectral signaling of sensory information and expectations in the cerebral processing of pain

Pain is not only shaped by sensory information but also by an individual’s expectations. Here, we investigated how commonly analyzed electroencephalography (EEG) responses to pain signal sensory information, expectations, and discrepancies thereof (prediction errors) in the processing of pain. Bayesian analysis confirmed that pain perception was shaped by objective sensory information and expectations. In contrast, EEG responses at different latencies (including the N1, N2, and P2 components) and frequencies (including alpha, beta, and gamma oscillations) were shaped by sensory information but not by expectations. Thus, EEG responses to pain are more involved in signaling sensory information than in signaling expectations or prediction errors. Expectation effects are obviously mediated by other brain mechanisms than the effects of sensory information on pain.

The perception of pain is shaped by somatosensory information about threat. However, pain is also influenced by an individual’s expectations. Such expectations can result in clinically relevant modulations and abnormalities of pain. In the brain, sensory information, expectations (predictions), and discrepancies thereof (prediction errors) are signaled by an extended network of brain areas which generate evoked potentials and oscillatory responses at different latencies and frequencies. However, a comprehensive picture of how evoked and oscillatory brain responses signal sensory information, predictions, and prediction errors in the processing of pain is lacking so far. Here, we therefore applied brief painful stimuli to 48 healthy human participants and independently modulated sensory information (stimulus intensity) and expectations of pain intensity while measuring brain activity using electroencephalography (EEG). Pain ratings confirmed that pain intensity was shaped by both sensory information and expectations. In contrast, Bayesian analyses revealed that stimulus-induced EEG responses at different latencies (the N1, N2, and P2 components) and frequencies (alpha, beta, and gamma oscillations) were shaped by sensory information but not by expectations. Expectations, however, shaped alpha and beta oscillations before the painful stimuli. These findings indicate that commonly analyzed EEG responses to painful stimuli are more involved in signaling sensory information than in signaling expectations or mismatches of sensory information and expectations. Moreover, they indicate that the effects of expectations on pain are served by brain mechanisms which differ from those conveying effects of sensory information on pain.

Cerebral processing of sharp mechanical pain measured with arterial spin labeling

INTRODUCTION

Arterial spin labeling (ASL) is a functional neuroimaging technique that has been frequently used to investigate acute pain states. A major advantage of ASL as opposed to blood-oxygen-level-dependent functional neuroimaging is its applicability for low-frequency designs. As such, ASL represents an interesting option for studies in which repeating an experimental event would reduce its ecological validity. Whereas most ASL pain studies so far have used thermal stimuli, to our knowledge, no ASL study so far has investigated pain responses to sharp mechanical pain.

METHODS

As a proof of concept, we investigated whether ASL has the sensitivity to detect brain activation within core areas of the nociceptive network in healthy controls following a single stimulation block based on 96 s of mechanical painful stimulation using a blunt blade.

RESULTS

We found significant increases in perfusion across many regions of the nociceptive network such as primary and secondary somatosensory cortices, premotor cortex, posterior insula, inferior parietal cortex, parietal operculum, temporal gyrus, temporo-occipital lobe, putamen, and the cerebellum. Contrary to our hypothesis, we did not find any significant increase within ACC, thalamus, or PFC. Moreover, we were able to detect a significant positive correlation between pain intensity ratings and pain-induced perfusion increase in the posterior insula.

CONCLUSION

We demonstrate that ASL is suited to investigate acute pain in a single event paradigm, although to detect activation within some regions of the nociceptive network, the sensitivity of our paradigm seemed to be limited. Regarding the posterior insula, our paradigm was sensitive enough to detect a correlation between pain intensity ratings and pain-induced perfusion increase. Previous experimental pain studies have proposed that intensity coding in this region may be restricted to thermal stimulation. Our result demonstrates that the posterior insula encodes intensity information for mechanical stimuli as well.

Intra-epidermal evoked potentials: A promising tool for spinal disorders?

OBJECTIVES

To test the robustness and signal-to-noise ratio of pain-related evoked potentials following intra-epidermal electrical stimulation (IES) compared to contact heat stimulation in healthy controls, and to explore the feasibility and potential added value of IES in the diagnosis of spinal disorders.

METHODS

Pain-related evoked potentials induced by IES (custom-made, non-invasive, concentric triple pin electrode with steel pins protruding 1 mm from the anode, triangularly separated by 7-10 mm respectively) and contact heat stimulation were compared in 30 healthy subjects. Stimuli were applied to four different body sites. Two IES intensities, i.e., high (individually adapted to contact heat painfulness) and low (1.5 times pain threshold), were used. Additionally, a 40-year-old patient with unilateral dissociated sensory loss due to a multi-segmental syringohydromyelia was assessed comparing IES and contact heat stimulation.

RESULTS

Both IES and contact heat stimulation led to robust pain-related evoked potentials recorded in all healthy subjects. Low intensity IES evoked potentials (14.1-38.0 µV) had similar amplitudes as contact heat evoked potentials (11.8-32.3 µV), while pain ratings on the numeric rating scale were lower for IES (0.8-2.5, compared to 1.5-3.9 for contact heat stimulation). High intensity IES led to evoked potentials with higher signal-to-noise ratio than low intensity IES and contact heat stimulation. The patient case showed impaired pain-related evoked potentials in segments with hypoalgesia for both IES modes. IES evoked potentials were preserved, with delayed latencies, while contact heat evoked potentials were abolished.

CONCLUSION

IES evoked robust pain-related cortical potentials, while being less painful in healthy controls. The improved signal-to-noise ratio supports the use of IES for objective segmental testing of nociceptive processing. This was highlighted in a spinal syndrome case, where IES as well as contact heat stimulation reliably detected impaired segmental nociception.

Different priming effects of empathy on neural processing associated with firsthand pain and nonpain perception

The shared-representation model of empathy is still debated. One of the major questions is whether empathy-eliciting stimuli depicting others' pain selectively activate the representations of self-pain. To address this issue, we assessed the priming effects of empathy-eliciting pictures on firsthand pain and nonpain perception, as well as its associated neural processing. In Experiment 1, when compared with nonpainful pictures depicting individuals' body parts with no injury, participants primed by painful pictures showing individuals' body parts with injury reported higher ratings for perceived intensity, unpleasantness, and salience of nociceptive and auditory stimuli, but they only exhibited increased N2 amplitude in response to nociceptive stimuli. In Experiment 2, the results from another group of participants replicated the observations of Experiment 1 and validated the findings in the non-nociceptive somatosensory modality. Importantly, participants' concern ratings for priming pictures predicted their unpleasantness ratings for subsequent nociceptive stimuli, while participants' attention ratings predicted their unpleasantness ratings for subsequent auditory and tactile stimuli. This finding implies that empathy for pain might influence pain and nonpain perception via different psychological mechanisms. In summary, our findings highlight the existence of pain-selective representations in empathy for pain and contribute to a better understanding of the shared-representation model of empathy.

Identifying Discomplete Spinal Lesions: New Evidence from Pain-Autonomic Interaction in Spinal Cord Injury

The clinical evaluation of spinal afferents is an important diagnostic and prognostic marker for neurological and functional recovery after spinal cord injury (SCI). Particularly important regarding neuropathic pain following SCI is the function of the spinothalamic tract (STT) conveying nociceptive and temperature information. Here, we investigated the added value of neurophysiological methods revealing discomplete STT lesions; that is, residual axonal sparing in clinically complete STT lesions. Specifically, clinical pinprick testing and thermal thresholds were compared with objective contact heat-evoked potentials (CHEPs) and a novel measure of pain-autonomic interaction employing heat-induced sympathetic skin responses (SSR). The test stimuli (i.e., contact heat, pinprick) were applied below the lesion level in 32 subjects with thoracic SCI while corresponding heat-evoked responses (i.e., CHEPs and SSR) were recorded above the lesion (i.e., scalp and hand, respectively). Readouts of STT function were related to neuropathic pain characteristics. In subjects with abolished pinprick sensation, measures of thermosensation (10%), CHEPs (33%), and SSR (48%) revealed residual STT function. Importantly, SSRs can be used as an objective readout and when abolished, no other proxy indicated residual STT function. No relationship was found between STT function readouts and spontaneous neuropathic pain intensity and extent. However, subjects with clinically preserved STT function presented more often with allodynia (54%) than subjects with discomplete (13%) or complete STT lesions (18%). In individuals with absent pinprick sensation, discomplete STT lesions can be revealed employing pain-autonomic measures. The improved sensitivity to discerning STT lesion completeness might support the investigation of its association with neuropathic pain following SCI.

Laser-evoked potentials recover gradually when using dorsal root ganglion stimulation, and this influences nociceptive pathways in neuropathic pain patients

OBJECTIVE

Dorsal root ganglion stimulation (DRGS) is able to relieve chronic neuropathic pain. There seems evidence that DRGS might achieve this by gradually influencing pain pathways. We used laser-evoked potentials (LEP) to verify our hypothesis that the recovery of the LEP may reflect DRGS-induced changes within the nociceptive system.

METHODS

Nine patients (mean age 56.8 years, range 36-77 years, two females) diagnosed with chronic neuropathic pain in the knee or groin were enrolled in the study. We measured each patient's LEP at the painful limb and contralateral control limb on the first, fourth, and seventh day after implantation of the DRGS system. We used the numeric rating scale (NRS) for the simultaneous pain assessment.

RESULTS

The LEP amplitude of the N2-P2 complex showed a significant increase on day 7 when compared to day 1 (Z = -2.666, p = 0.008) and to day 4 (Z = -2.547, p = 0.011), respectively. There was no significant difference in the N2-P2 complex amplitude between ON and OFF states during DRGS. The patients' NRS significantly decreased after 1 day (p = 0.007), 4 days (p = 0.007), and 7 days (p = 0.007) when compared to the baseline.

CONCLUSIONS

The results show that with DRGS, the LEP recovered gradually within 7 days in neuropathic pain patients. Therefore, reduction of the NRS in patients with chronic neuropathic pain might be due to DRGS-induced processes within the nociceptive system. These processes might indicate neuroplasticity mediated recovery of the LEP.

Effects of spatial attention and limb position on the cortical interaction of bilateral noxious inputs

Bilateral noxious inputs interact in the brain to provide a better representation of physical threat. In the present study, we investigated the effects of spatial attention and limb position on this interaction. Painful laser stimuli were applied randomly on the right hand or on both hands, while varying spatial attention (focal or overall) and limb position (hands near or far from each other). Pain perception and laser-evoked potentials (N1, N2, P2) were compared between conditions in 27 healthy volunteers. Compared with unilateral stimulation, bilateral stimulation increased pain (p = .004), the N2 (p = .0015) and P2 (p < .001) amplitude. The effects on pain and the P2 were greater when hands were in the near compared with the far position (p < .05). The effect on pain was also greater for overall compared with focal pain rating (p = .003). In addition, the N1 amplitude was greater for bilateral stimulation when hands were in the far compared with the near position (p = .01). These results show that increased brain responses and pain for bilateral compared with unilateral noxious stimulation are modulated differentially by spatial attention and limb position. This suggests that the integration of noxious inputs occurs through partially independent pain-related processes, that it is modulated by limb position, and that it is partially independent of pain perception. We propose that this is necessary to produce coordinated, flexible and adapted defensive responses.

Aerobic Exercise Attenuates Pain Sensitivity: An Event-Related Potential Study

In this study, electroencephalography (EEG) was utilized to explore the neurophysiological mechanisms of aerobic exercise-induced hypoalgesia (EIH) and provide a theoretical basis for the application of aerobic exercise in pain assessment and treatment. Forty-five healthy subjects were randomly divided into moderate-intensity aerobic exercise [70% heart rate reserve (HRR)], low-intensity aerobic exercise (50% HRR), or control groups (sitting). Aerobic exercise was performed with cycling. Pressure pain threshold (PPT), heat pain threshold (HPT), event-related potential (ERP) induced by contact heat stimulus and pain scoring were measured before and after the intervention. We found that moderate-intensity aerobic exercise can increase the PPT (rectus femoris: t = -2.71, p = 0.017; tibialis anterior muscle: t = -2.36, p = 0.033) and HPT (tibialis anterior muscle: t = -2.219, p = 0.044) of proximal intervention sites rather than distal sites, and decreased pain scorings of contact heat stimulus. After moderate-intensity aerobic exercise, alpha oscillation power reflecting the central descending inhibitory function was enhanced (t = -2.31, p < 0.05). Low-intensity aerobic exercise mainly reduced the pain unpleasantness rating (Block 1: t = 2.415, p = 0.030; Block 2: t = 3.287, p = 0.005; Block 4: t = 2.646, p = 0.019; Block 5: t = 2.567, p = 0.022). Aerobic exercise had an overall EIH effect. Its hypoalgesic effect was related to exercise intensity and affected by the site and type of pain stimulus. Moderate-intensity aerobic exercise effectively reduced the sensitivity to various painful stimuli, and low-intensity aerobic exercise selectively inhibited the negative emotional pain response. The hypoalgesic mechanism of aerobic exercise involves the enhancement of the central descending inhibitory function.