Contact Heat Stimuli Research Articles

Remembering the dynamic changes in pain intensity and unpleasantness: A psychophysical study

This study investigated the short-term memory of dynamic changes in acute pain using psychophysical methods. Pain intensity or unpleasantness induced by painful contact-heat stimuli of 8, 9, or 10s was rated continuously during the stimulus or after a 14-s delay using an electronic visual analog scale in 10 healthy volunteers. Because the continuous visual analog scale time courses contained large amounts of redundant information, a principal component analysis was applied to characterize the main features inherent to both the concurrent rating and retrospective evaluations. Three components explained about 90% of the total variance across all trials and subjects, with the first component reflecting the global perceptual profile, and the second and third components explaining finer perceptual aspects (eg, changes in slope at onset and offset and shifts in peak latency). We postulate that these 3 principal components may provide some information about the structure of the mental representations of what one perceives, stores, and remembers during the course of few seconds. Analysis performed on the components confirmed significant memory distortions and revealed that the discriminative information about pain dimensions in concurrent ratings was partly or completely lost in retrospective ratings. Importantly, our results highlight individual differences affecting these memory processes. These results provide further evidence of the important transformations underlying the processing of pain in explicit memory and raise fundamental questions about the conversion of dynamic nociceptive signals into a mental representation of pain in perception and memory.

The role of stimulation parameters on the conditioned pain modulation response

Background and purposeConditioned pain modulation (CPM) is a testing paradigm representing features of diffuse noxious inhibitory control. There is large diversity in the paradigms applied to induce CPM, and the consistency in CPM responses assessed by different paradigms is largely unknown. We aimed to characterize and explore the associations between the CPM responses assessed by different paradigms in the same cohort. MethodsThirty-three healthy middle-aged subjects underwent six CPM paradigms. The ‘test-stimuli’, consisted of thermal and mechanical modalities, using pain thresholds, suprathreshold pain and temporal summation types of measurements. The ‘conditioning-stimulus’ consisted of a contact heat stimulus applied to the thener of the left hand for 60s at an intensity of 46.5°C. ResultsLarge variability was observed among the responses to the different CPM paradigms. Surprisingly, no correlations were found between the various CPM responses. ConclusionsThe variability in the CPM responses may suggest that the capacity of pain modulation is a multifaceted trait, whose expression varies with the application of different CPM paradigms. ImplicationsConsidering that CPM responses may represent different processes when assessed by different paradigms, we encourage the use of more than one CPM paradigm.

Enhanced Pain Expectation in Migraine:EEG‐Based Evidence for Impaired Prefrontal Function

Dysexcitability characterizes the interictal migraineous brain. The main central expressions of this dysexcitability are decreased habituation and enhanced anticipation and attention to pain and other external sensory stimuli. This study evaluates the effects of anticipation on pain modulation and their neural correlates in migraine. In 39 migraineurs (20 migraine with aura [MWA] and 19 migraine without aura [MOA]) and 22 healthy controls, cortical responses to 2 successive trains of noxious contact-heat stimuli, presented in either predicted or unpredicted manner, were analyzed using standardized low-resolution electromagnetic tomography key. A lack of habituation to repeated predicted pain was associated with significantly increased pain-evoked potential amplitudes in MWAs (increase of 3.9 μV) and unchanged ones in MOAs (1.1 μV) but not in controls (decrease of 5 μV). Repeated unpredicted pain resulted in enhanced pain-evoked potential amplitudes in both MWA and MOA groups (increase of 5.5 μV and 4.4 μV, respectively) compared with controls (decrease of 0.2 μV). Source localization revealed reduced activations in the anterior-medial prefrontal cortices and subsequent increased somatosensory activity in migraineurs (P < .05). The prefrontal-somatosensory dysfunction positively correlated with lifetime headache duration (P < .05) and concern of upcoming migraine attacks (P < .05) in MWAs, and with frequency of migraine attacks in MOAs (P < .05). Our findings of impaired modulation of anticipated pain in migraine suggest a heightened state of anticipatory readiness combined with ineffective recruitment of prefrontal inhibitory pathways during experience of pain; the latter might account for the former, at least partially. In line, less efficient inhibitory capability is a plausible mechanistic explanation for patients' high concern about their upcoming migraine attacks.

The effect of nerve compression and capsaicin on contact heat-evoked potentials related to Aδ- and C-fibers

Brief noxious heat stimuli activate Aδ- and C-fibers and allow contact heat-evoked potentials (CHEPs) to be recorded from the scalp. Under normal conditions, only late responses related to Aδ-fibers can be recorded. This study aimed to demonstrate C-fiber responses to contact heat stimuli. A preferential A-fiber compression blockade of the superficial radial nerve was applied in 22 healthy subjects. Quality and intensity of heat-evoked pain and CHEPs were examined at baseline, during nerve compression, and during nerve compression with simultaneous application of topical capsaicin (5%). During the A-fiber block, three subjects had CHEPs with latencies below 400ms, eight subjects within 400–800ms and six subjects (29%) later than 800ms. Pain intensity to contact heat stimuli after compression was reduced and fewer subjects reported the heat stimuli as stinging. Following nerve compression and capsaicin application, ultralate CHEPs with latencies >800ms could be recorded in 13 subjects (62%), pain intensity to the contact heat stimuli was increased and the warm/hot-burning pain quality became more intense. The main results of our study are the demonstration of ultralate C-fiber-related CHEPs following A-fiber blockade in 29% of healthy subjects increasing to 62% when the blockade was combined with capsaicin. After blockade of Aδ-fibers we recorded responses with latencies in the range between the latencies of Aδ- and C-fibers suggesting release of Aδ-fibers with slower conduction velocity than normally recorded with CHEPs.

Contact heat and cold, mechanical, electrical and chemical stimuli to elicit small fiber-evoked potentials: Merits and limitations for basic science and clinical use

Laser-evoked potentials are the most extensively validated method to objectively assess nociceptive pathway function in humans. Here, we review merits and shortcomings of alternative techniques using different principles of stimulus generation to stimulate Aδ- or C-fibers. Fast ramp contact heat stimuli yield reproducible responses; however, stimulus location needs to be changed to reduce peripheral habituation, and the limited steepness of temperature ramps may result in response jitter and absence of averaged responses even in some healthy subjects. Inverse temperature ramps can serve to evoke cool evoked potentials to specifically test the cold pathway; the clinical impact of such findings is promising but uncertain to date, and availability of devices optimized for this purpose is currently limited. Mechanical stimuli excite low- or high-threshold mechanoreceptors depending on both the probe surface and the applied force. Electrical stimuli can be used to excite nerve fibers directly in the epidermis, the mucosa of the gut, or the tooth pulp. Principle limitation of the applicability of mechanical and electrical stimuli is the inevitable co-excitation of tactile (Aβ-) fibers. The nasal mucosa can be stimulated using pulsed-CO(2) air streams, which excite chemo-nociceptors; although these stimuli are specific to excite thin trigeminal afferents, their use is limited as it is restricted to a relatively small region. Current data do not allow a comparative analysis on their respective diagnostic values. Quantification of analgesic efficacy in healthy subjects has been established and may be useful in phase I and IIa clinical trials.

The effect of nerve compression and capsaicin on contact heat evoked potentials (CHEPs) related to Aδ and C fibers

Abstract Background/aims Brief noxious heat stimuli activate Aδ and C fibers, and contact heat evoked potentials (CHEPs) can be recorded from the scalp. Under standard conditions, late responses related to AS fibers can be recorded. This study examines C-fiber responses to contact heat stimuli. Methods A preferential A-fiber blockade by compression to the superficial radial nerve was applied in 22 healthy subjects. Quality and intensity of heat evoked pain (NRS, 0–10), and CHEPs were examined at baseline, during nerve compression, and during further nerve compression with topical capsaicin (5%). Results During the A-fiber blockade, 3 subjects had CHEPs with latencies below 400 ms, 8 subjects within 400–800 ms and 6 subjects later than 800 ms. Pain intensity to contact heat stimuli was reduced and fewer subjects reported the heat stimuli as stinging. Following acute capsaicin application, ultralate CHEPs with latencies &gt;800 ms could be recorded in 13 subjects, pain intensity to the contact heat stimuli was increased (p &lt;0.01) and more subjects reported the heat stimuli as being more warm/hot-burning. Conclusion The results indicate that following a compression to the superficial radial nerve, CHEPs compatible within complete A fibers or C fibers were recorded. Following sensitization with capsaicin, C-fiber responses were recorded in 62% of subjects.

Effects of perceived and exerted pain control on neural activity during pain relief in experimental heat hyperalgesia: A fMRI study

Perceived control over pain can attenuate pain perception by mechanisms of endogenous pain control and emotional reappraisal irrespective of whether this control is exerted or only perceived. Self-initiated termination of pain elicits different expectations of subsequent pain relief as compared to perceived pain control. It is unknown whether and how this perceived vs. exerted control on pain differs and affects subsequent pain relief. Using fMRI, we studied two factors of pain control on pain relief: the (i) sense of control (perceived control but no execution) and (ii) the execution of control (exerted control). To account for the impact of factual execution of pain control on pain relief we applied bearable short and hardly bearable long contact-heat stimuli which were applied either controllable or not. Using controllability as factor, there was dissociable neural activity during pain relief: following the perceived control condition neural activity was found in the orbitofrontal and mediofrontal cortex and, following the exerted control condition, in the anterolateral and dorsolateral prefrontal cortex and posterior parietal cortex. We conclude that (i) pain controllability has an impact on pain relief and (ii) the prefrontal cortex shows dissociable neural activity during pain relief following exerted vs. perceived pain control. This might reflect the higher grade of uncertainty during pain relief following perceived pain control mediated by the orbitofrontal and medial prefrontal cortex and processes of working memory and updating expectations during pain relief following exerted control mediated by the lateral prefrontal cortex.

Does Pain Necessarily Have an Affective Component? Negative Evidence from Blink Reflex Experiments

Experimental pain research has shown that the affective component of pain is influenced strongly by situational characteristics; affective pain processing appears to be particularly pronounced in situations that provoke a feeling of uncertainty and uncontrollability. To determine whether the affective component of pain can be completely abolished if a 'safe', particularly predictable stimulation paradigm is applied. Forty healthy volunteers recruited at the University of Bamberg (Bamberg, Germany) were assessed in two experiments. Tonic contact heat stimuli staged in three intensities (warmth, heat and pain) relative to the individual pain threshold was applied; these were predictable with regard to intensity and course, and the subjects had easy access to control. The startle reflex was assessed as an objective measure of affective response. In addition, the subjects provided unpleasantness ratings. To compare these results to a gold standard for affective response, affective pictures taken from the International Affective Picture System were presented during temperature stimulation in the second experiment. Both experiments showed no potentiation of the startle reflex under painful heat stimulation compared with the two nonpainful stimulus intensities (heat and warmth), although the painful stimulation was clearly rated as more unpleasant. Results suggest that it is possible to develop a 'safe' noxious stimulus, which is rated as clearly unpleasant, but lacks physiological indication of negative affect. This divergence might be explained by subjective ratings being influenced by the instructions. The possibility of reducing the pain affect by suggesting 'safety' may be of therapeutic interest.

Increased baseline temperature improves the acquisition of contact heat evoked potentials after spinal cord injury

ObjectiveTo investigate the effect of increasing the skin surface baseline temperature for contact heat evoked potentials (CHEPs). MethodsCHEPs were studied in healthy subjects and subjects with chronic cervical spinal cord injury (SCI) using a conventional 35°C (condition 1) or increased 42–45°C baseline temperature (condition 2). A third condition was used to standardize the contact heat stimulus duration from the different baseline temperatures. Changes in peak latency and N2P2 amplitude of the CHEPs and rating of perceived intensity were examined between conditions. ResultsIn healthy subjects, increasing the baseline temperature for contact heat stimulation significantly increased the rating of perceived intensity (conditions 2 and 3), as well as the amplitude of CHEPs (condition 2 only). Following SCI, an increased baseline temperature facilitated perception of contact heat stimulation and evoked potentials could be recorded from dermatomes that were insensitive to contact heat from a conventional baseline temperature. ConclusionsEnhancing the acquisition of CHEPs can be achieved by increasing the baseline temperature. This effect can be attributed, in part, to shortening the stimulation duration. SignificanceAfter SCI, increasing the baseline temperature for CHEPs in dermatomes with absent or diminished sensation improved the neurophysiological resolution of afferent sparing.

Pathophysiology of Neuropathic Pain in Type 2 Diabetes

OBJECTIVENeuropathic pain due to small-fiber sensory neuropathy in type 2 diabetes can be diagnosed by skin biopsy with quantification of intra-epidermal nerve fiber (IENF) density. There is, however, a lack of noninvasive physiological assessment. Contact heat–evoked potential (CHEP) is a newly developed approach to record cerebral responses of Aδ fiber–mediated thermonociceptive stimuli. We investigated the diagnostic role of CHEP.RESEARCH DESIGN AND METHODSFrom 2006 to 2009, there were 32 type 2 diabetic patients (20 males and 12 females, aged 51.63 ± 10.93 years) with skin denervation and neuropathic pain. CHEPs were recorded with heat stimulations at the distal leg, where skin biopsy was performed.RESULTSCHEP amplitude was reduced in patients compared with age- and sex-matched control subjects (14.8 ± 15.6 vs. 33.7 ± 10.1 μV, P < 0.001). Abnormal CHEP patterns (reduced amplitude or prolonged latency) were noted in 81.3% of these patients. The CHEP amplitude was the most significant parameter correlated with IENF density (P = 0.003) and pain perception to contact heat stimuli (P = 0.019) on multiple linear regression models. An excitability index was derived by calculating the ratio of the CHEP amplitude over the IENF density. This excitability index was higher in diabetic patients than in control subjects (P = 0.023), indicating enhanced brain activities in neuropathic pain. Among different neuropathic pain symptoms, the subgroup with evoked pain had higher CHEP amplitudes than the subgroup without evoked pain (P = 0.011).CONCLUSIONSCHEP offers a noninvasive approach to evaluate the degeneration of thermonociceptive nerves in diabetic neuropathy by providing physiological correlates of skin denervation and neuropathic pain.

Temporal summation of heat pain in humans: Evidence supporting thalamocortical modulation

Noxious cutaneous contact heat stimuli (48 °C) are perceived as increasingly painful when the stimulus duration is extended from 5 to 10 s, reflecting the temporal summation of central neuronal activity mediating heat pain. However, the sensation of increasing heat pain disappears, reaching a plateau as stimulus duration increases from 10 to 20 s. We used functional magnetic resonance imaging (fMRI) in 10 healthy subjects to determine if active central mechanisms could contribute to this psychophysical plateau. During heat pain durations ranging from 5 to 20 s, activation intensities in the bilateral orbitofrontal cortices and the activation volume in the left primary (S1) somatosensory cortex correlated only with perceived stimulus intensity and not with stimulus duration. Activation volumes increased with both stimulus duration and perceived intensity in the left lateral thalamus, posterior insula, inferior parietal cortex, and hippocampus. In contrast, during the psychophysical plateau, both the intensity and volume of thalamic and cortical activations in the right medial thalamus, right posterior insula, and left secondary (S2) somatosensory cortex continued to increase with stimulus duration but not with perceived stimulus intensity. Activation volumes in the left medial and right lateral thalamus, and the bilateral mid-anterior cingulate, left orbitofrontal, and right S2 cortices also increased only with stimulus duration. The increased activity of specific thalamic and cortical structures as stimulus duration, but not perceived intensity, increases is consistent with the recruitment of a thalamocortical mechanism that participates in the modulation of pain-related cortical responses and the temporal summation of heat pain.

Orbitofrontal disinhibition of pain in migraine with aura: An interictal EEG-mapping study

This study aimed to identify the cortical mechanisms underlying the processes of interictal dishabituation to experimental pain in subjects suffering from migraine with aura (MWA). In 21 subjects with MWA and 22 healthy controls, cortical responses to two successive trials of noxious contact-heat stimuli were analyzed using EEG-tomography software. When compared with controls, MWA patients showed significantly increased pain-evoked potential amplitudes accompanied by reduced activity in the orbitofrontal cortex (OFC) and increased activity in the pain matrix regions, including the primary somatosensory cortex (SI) (p < .05). Similarly to controls, MWA subjects displayed an inverse correlation between the OFC and SI activities, and positive interrelations between other pain-specific regions. The activity changes in the OFC negatively correlated with lifetime headache duration and longevity (p < .05). Reduced inhibitory functioning of the prefrontal cortex is a possible cause for disinhibition of the pain-related sensory cortices in migraine. The finding of OFC hypofunction over the disease course is in keeping with current concepts of migraine as a progressive brain disorder.

Optimizing the Measurement of Contact Heat Evoked Potentials

Our aim was to determine whether single trial averaging could improve quantification of contact heat evoked potentials measured from fixed position contact heat stimulation. Event-related brain potentials were measured in response to contact heat stimuli applied to the arm and the leg of 10 subjects via a circular thermode, using fixed and varied thermode positions at 41 degrees C and 51 degrees C. Contact heat evoked potentials were successfully recorded from varied position stimulation of the leg at 51 degrees C in 80% of subjects, but from only 60% of subjects using a fixed position. Contact heat evoked potentials were only identified in a small number of subjects when stimulating at 41 degrees C. The amplitude of the N2-P2 complex and pain intensity ratings were larger for the varied compared with the fixed thermode position and were also larger when stimulating the arm. Automated single trial analysis of the data resulted in larger amplitude of the N2-P2 complex than standard averaging. In conclusion, we have demonstrated that the reduced contact heat evoked potentials amplitude seen for fixed location stimulation can be improved with single-trial averaging.

Monosynaptically-evoked glutamatergic transmission though primary afferents is not enhanced by PAR-1 activation in the rat substantia gelatinosa

Brief noxious heat stimuli activate Aδ- and C-fibers and allow contact heat-evoked potentials (CHEPs) to be recorded from the scalp. Under normal conditions, only late responses related to Aδ-fibers can be recorded. This study aimed to demonstrate C-fiber responses to contact heat stimuli. A preferential A-fiber compression blockade of the superficial radial nerve was applied in 22 healthy subjects. Quality and intensity of heat-evoked pain and CHEPs were examined at baseline, during nerve compression, and during nerve compression with simultaneous application of topical capsaicin (5%). During the A-fiber block, three subjects had CHEPs with latencies below 400 ms, eight subjects within 400–800 ms and six subjects (29%) later than 800 ms. Pain intensity to contact heat stimuli after compression was reduced and fewer subjects reported the heat stimuli as stinging. Following nerve compression and capsaicin application, ultralate CHEPs with latencies >800 ms could be recorded in 13 subjects (62%), pain intensity to the contact heat stimuli was increased and the warm/hot-burning pain quality became more intense. The main results of our study are the demonstration of ultralate C-fiber-related CHEPs following A-fiber blockade in 29% of healthy subjects increasing to 62% when the blockade was combined with capsaicin. After blockade of Aδ-fibers we recorded responses with latencies in the range between the latencies of Aδ- and C-fibers suggesting release of Aδ-fibers with slower conduction velocity than normally recorded with CHEPs.

Patterns of contact heat evoked potentials (CHEP) in neuropathy with skin denervation: Correlation of CHEP amplitude with intraepidermal nerve fiber density

ObjectiveContact heat evoked potentials (CHEPs) provide an objective approach to investigate cerebral responses to thermal stimuli mediated by Aδ fibers. Skin denervation is often associated with reduced thermal sensibilities. We aimed to investigate the influences of skin denervation on CHEPs in neuropathic patients. MethodsCHEPs were recorded at the vertex area by applying contact heat stimuli of 51°C on the distal leg of neuropathic patients with sensory symptoms and pathological evidence of skin denervation in the distal leg. Patterns and parameters of CHEPs in the neuropathic group were compared with those in the control group of age- and gender-matched subjects. ResultsThere were 25 neuropathic patients with reduced intraepidermal fiber (IENF) density (1.46±1.70fibers/mm, range: 0–5.32). In the control group, well-defined averaged tracings of CHEPs with an initial negative peak (N-wave) followed by a positive peak (P-wave) were consistently recorded in all 25 subjects. The peripheral conduction velocities of CHEPs were 9.92±4.06m/s (range: 6.06–16.60), in the range of Aδ fibers. The group of neuropathic patients had markedly reduced N–P amplitudes (p<0.0001) and prolonged N-wave latencies (p=0.049) compared to the control group. IENF density was the only neuropathic parameter correlated with N–P amplitude on multiple linear regression analysis (p=0.010) compared to large-fiber parameters. ConclusionsIn neuropathic patients with pathological evidence of skin denervation, there were reduced amplitude and prolonged latencies in CHEPs mediated by Aδ fibers. The reduction of CHEP amplitude corresponded to the degree of skin denervation. SignificanceCHEP offers electrophysiological evidence of thermal responses and provides an objective, non-invasive approach to assess the physiological counterparts of skin denervation in neuropathic patients.

Objective Correlate of Subjective Pain Perception by Contact Heat-Evoked Potentials

The method of pain-evoked potentials has gained considerable acceptance over the last 3 decades regarding its objectivity, repeatability, and quantifiability. The present study explored whether the relationship between pain-evoked potentials and pain psychophysics obtained by contact heat stimuli is similar to those observed for the conventionally used laser stimulation. Evoked potentials (EPs) were recorded in response to contact heat stimuli at different body sites in 24 healthy volunteers. Stimuli at various temperatures were applied to the forearm (43 degrees C, 46 degrees C, 49 degrees C, and 52 degrees C) and leg (46 degrees C and 49 degrees C). The amplitudes of both components (N2 and P2) were strongly associated with the intensity of the applied stimuli and with subjective pain perception. Yet, regression analysis revealed pain perception and not stimulus intensity as the major contributing factor. A significant correlation was found between the forearm and the leg for both psychophysics and EPs amplitude. Contact heat can generate readily distinguishable evoked potentials on the scalp, consistent between upper and lower limbs. Although these potentials bear positive correlation with both stimulus intensity and pain magnitude, the latter is the main contributor to the evoked brain response.

Sex differences in endogenous pain modulation by distracting and painful conditioning stimulation

Sex differences in endogenous pain modulation were tested in healthy volunteers (32 men, 30 women). Painful contact heat stimuli were delivered to the right leg alone, and then in combination with various electrical conditioning stimuli delivered to the left forearm. Four conditioning protocols were applied to each subject in separate sessions: mild, non-painful (control); distracting; stressful-yet-non-painful; strongly painful. Thermal stimuli were rated on visual analog scales for pain intensity (INT) and unpleasantness (UNP). Distracting and painful conditioning stimuli significantly reduced heat pain INT and UNP ratings for both sexes, with significantly larger distraction effects on INT ratings for men than women ( p = 0.004). No sex differences in pain-evoked hypoalgesia were detected ( p > 0.05). The stress protocol did not consistently reduce heat pain ratings, possibly because the protocol was not sufficiently stressful to activate endogenous modulatory systems. Regression analysis revealed that the magnitude of pain-evoked hypoalgesia was predicted by the perceived distraction ( p = 0.003) and stress ( p = 0.04) produced by the painful conditioning stimulation, providing evidence that distraction and stress contribute to pain-evoked hypoalgesia. However, the contribution of stress to pain-evoked hypoalgesia differed by sex ( p = 0.02), with greater perceived stress associated with greater hypoalgesia in men and the opposite trend in women, suggesting sex differences in the mechanisms underlying pain-evoked hypoalgesia. This study provides indirect evidence that multiple neural mechanisms are involved in endogenous pain modulation and suggests that sex-specific aspects of these systems may contribute to greater pain sensitivity and higher prevalence of many chronic pain conditions among women.

Across- and within-session variability of ratings of painful contact heat stimuli

This study examined within- and across-session consistency of visual analog scale (VAS) pain intensity and unpleasantness ratings of contact heat stimuli in 64 subjects (32 male). Subjects participated in four sessions over 14 days, with three stimulus series per session. Two levels of painful heat (pain-lo: rated 40, and pain-hi: rated 70 on a 0–100 VAS) were delivered in randomized order during each series, with temperatures selected on an individual subject basis to equalize pain perception across subjects. Across-session ratings declined by the fourth session for both pain levels ( p = 0.01). Within-session ratings declined by the third series for both pain levels ( p < 0.001). While significant, changes in across- and within-session ratings were of small magnitude. Comparison of coefficients of variation (CVs) for across- and within-session ratings revealed that pain-lo ratings were more variable than pain-hi ratings ( p < 0.001). Across- and within-session CVs were highly correlated for each pain level (pain-lo p < 0.001; pain-hi p = 0.001), suggesting that variability of VAS ratings is a characteristic of individual subjects over both short and long time scales. Across- and within-session CVs were significantly negatively correlated with individual ratings of the stimuli, but were not correlated with demographic or psychosocial factors. Furthermore, sex did not impact consistency of ratings, demonstrating that neither sex is more variable in ratings than the other over time. Taken together, these findings suggest that VAS ratings of painful contact heat are relatively stable over time but the variability of these ratings is significantly impacted by the perceived intensity of the stimulus.

Common neural systems for contact heat and laser pain stimulation reveal higher‐level pain processing

Our current knowledge of pain-related neuronal responses is largely based on experimental pain studies using contact heat or nontactile laser painful stimulation. Both stimuli evoke pain, yet they differ considerably in their physical and perceptual properties. In sensory cortex, cerebral responses to either stimulus should therefore substantially differ. However, given that both stimuli evoke pain, we hypothesized that at a certain subset of cortical regions the different physical properties of the stimuli become less important and are therefore activated by both stimuli. In contrast, regions with clearly dissociable activity may belong to "lower-level" pain processing mechanisms depending on the physical properties of the administered stimuli. We used functional magnetic resonance (fMRI) to intraindividually compare pain-related activation patterns between laser and contact heat stimulation using four different intensities of laser and contact heat stimuli. Common and dissociable neural responses were identified by correlating perceived pain intensities with blood oxygenation level dependent (BOLD) signal changes. Only neuronal responses to stimuli that were perceived as painful were analyzed. Pain-related BOLD signal increases independent of stimulus modality were detected in the anterior insula, anterior cingulate cortex, medial secondary somatosensory cortex, and the prefrontal cortex. These similarities are likely to reflect higher-level pain processing, which is largely independent of the single physical parameters that determine the painful nature of the stimuli.

Trigeminal small-fibre function assessed with contact heat evoked potentials in humans

Contact heat stimuli have been reported to excite mechano-thermal nociceptors and to evoke brain potentials (CHEPs) from the limbs. We investigated whether contact heat evokes reproducible CHEPs from the trigeminal territory and may prove a reliable diagnostic tool in facial neuropathic pain. We applied contact heat stimuli to the perioral and supraorbital regions; CHEPs were recorded from the vertex in 20 controls and 2 patients with facial neuropathic pains, and reflex responses from the orbicularis oculi and masticatory muscles in 5 controls. We studied the correlation between CHEP data and perceptive ratings, site of stimulation, and age. Finally, we compared CHEPs with laser evoked potentials (LEPs). Contact heat stimuli at 51 °C evoked vertex potentials consisting of an NP complex similar to that elicited by laser pulses, though with a latency some 100-ms longer. Perioral stimulation yielded higher pain intensity ratings, shorter latency and larger amplitude CHEPs than supraorbital stimulation. CHEP data correlated significantly with age. Contact heat stimuli at 53 °C evoked a blink-like response in the relaxed orbicularis oculi muscle and a silent period in the contracted masseter muscle. In patients with facial neuropathic pain the CHEP abnormalities paralleled those seen with LEPs. We were unable to achieve reproducible signals related to C-receptor stimulation by contact heat stimuli at 41 °C in the ten subjects in whom they were tested. Contact heat stimulation, as well as laser stimulation, easily yields large-amplitude brain potentials and nociceptive reflexes, both related to the Aδ input. However CHEPs are not suitable for C-fibres potentials recording.