Comparison of hyperalgesia induced by capsaicin injection and controlled heat injury: effect on temporal summation

The relationship between induction of central sensitization and facilitation of temporal summation to repetitive stimulation is still unclear. The aim of this study was to investigate temporal summation before and after the induction of secondary hyperalgesia by two different experimental methods: capsaicin injection and controlled heat injury. The effect of each injury model was assessed on a separate day with an interval of at least 5 days. Twelve healthy volunteers participated. Each experiment was performed using electrical, radiant heat, mechanical impact, and punctuate stimuli consecutively. The pain threshold (PT) to a single stimulus and the summation threshold to five repetitive stimuli for electrical (2?Hz) and radiant heat (0.83?Hz) were assessed within the secondary hyperalgesic area. The degree of temporal summation for stimulus intensities of 0.8, 1.0, and 1.2 times the baseline pain thresholds were evaluated by the increase in visual analogue scale (VAS) scores from the first to the fifth stimulus of the train. Further, the degrees of temporal summation were assessed for mechanical impact and punctuate stimuli within the primary and secondary hyperalgesic areas. The contra-lateral forearm served as control (no injury). The pain threshold and the summation threshold to electrical and heat stimuli decreased significantly within the secondary hyperalgesic area after the injury induced by both heat injury or capsaicin injection. However, there was no temporal summation for heat and electrical stimuli in either model. In contrast, for the mechanical impact and punctuate mechanical stimuli the degree of temporal summation was significantly facilitated in the secondary hyperalgesic areas compared with the baseline and the control arm in both models. In the primary hyperalgesic area, the degree of temporal summation was facilitated to mechanical impact and punctuate stimuli but only following the capsaicin injection. In conclusion, the temporal summation mechanism for mechanical stimuli was facilitated in the secondary hyperalgesic area.     

Comparison of hyperalgesia induced by capsaicin injection and controlled heat injury: effect on temporal summation

The relationship between induction of central sensitization and facilitation of temporal summation to repetitive stimulation is still unclear. The aim of this study was to investigate temporal summation before and after the induction of secondary hyperalgesia by two different experimental methods: capsaicin injection and controlled heat injury. The effect of each injury model was assessed on a separate day with an interval of at least 5 days. Twelve healthy volunteers participated. Each experiment was performed using electrical, radiant heat, mechanical impact, and punctuate stimuli consecutively. The pain threshold (PT) to a single stimulus and the summation threshold to five repetitive stimuli for electrical (2 Hz) and radiant heat (0.83 Hz) were assessed within the secondary hyperalgesic area. The degree of temporal summation for stimulus intensities of 0.8, 1.0, and 1.2 times the baseline pain thresholds were evaluated by the increase in visual analogue scale (VAS) scores from the first to the fifth stimulus of the train. Further, the degrees of temporal summation were assessed for mechanical impact and punctuate stimuli within the primary and secondary hyperalgesic areas. The contra-lateral forearm served as control (no injury). The pain threshold and the summation threshold to electrical and heat stimuli decreased significantly within the secondary hyperalgesic area after the injury induced by both heat injury or capsaicin injection. However, there was no temporal summation for heat and electrical stimuli in either model. In contrast, for the mechanical impact and punctuate mechanical stimuli the degree of temporal summation was significantly facilitated in the secondary hyperalgesic areas compared with the baseline and the control arm in both models. In the primary hyperalgesic area, the degree of temporal summation was facilitated to mechanical impact and punctuate stimuli but only following the capsaicin injection. In conclusion, the temporal summation mechanism for mechanical stimuli was facilitated in the secondary hyperalgesic area.     

Sex differences in nociceptive withdrawal reflex and pain perception

Experimentally induced pain often reveals sex differences, with higher pain sensitivity in females. The degree of differences has been shown to depend on the stimulation and assessment methods. Since sex differences in pain develop anywhere along the physiological and psychological components of the nociceptive system, we intended to compare the nociceptive flexion reflex (NFR) as a more physiological (spinal) aspect of pain procession to the verbal pain report of intensity and unpleasantness as the more psychological (cortical) aspect. Twenty female and twenty male healthy university students were investigated by use of nociceptive flexion reflex threshold (staircase method) after electrical stimulation of the N. suralis. Furthermore, we assessed supra-threshold reflex responses (latency, amplitude and area) by applying 10 stimuli 5 mA above reflex threshold. Following each stimulation, the subjects provided pain ratings of intensity and unpleasantness on a visual analogue scale. Females exhibited marked lower nociceptive flexion reflex thresholds than males, while the supra-threshold reflex response tailored to the individual reflex threshold did not show any significant differences. The verbal pain ratings, corrected for NFR threshold, were not found to differ significantly. The large sex differences in nociception that were present in NFR threshold but not in the pain ratings corroborate the hypothesis that spinal processes contribute substantially to sex differences in pain procession.     

Spatial summation of heat induced pain within and between dermatomes

The aim was to study spatial summation within and between ipsi- and contralateral dermatomes at different painful temperatures. For heat stimulation we used a computer controlled thermofoil based thermode. The thermode area could be varied in five discrete steps from 3.14 to 15.70 cm2. When we applied the stimuli within a dermatome, the mean heat pain threshold decreased significantly from 45.6 to 43.5 C as the area was increased from minimum (3.14 cm2) to maximum (15.70 cm2). When the areas were increased involving different dermatomes (both ipsi- or contralateral), we found similar decreases in pain threshold. Spatial summation was also found within and between dermatomes at supra-threshold temperatures (46, 48, 50 C).The study shows that spatial summation of pain is most likely a mechanism acting across segments and is existing from pain threshold to tolerance.     

Spatial summation of perceived pressure,sharpness and mechanically evoked cutaneous pain

Psychophysically, spatial summation can be demonstrated as a decrease in threshold accompanying an increased field of stimulation. The present study examined to what extent different mechanically evoked percepts (pressure, sharpness, and pain) show spatial summation. Various probes were used to apply prescribed forces to the dorsal surface of the digits of 19 healthy subjects. The threshold for three perceptual qualities showed differing degrees of spatial summation: sharpness showed no statistically significant spatial summation; pain demonstrated some significant summation (46% on average); pressure showed the greatest degree of spatial summation (76% on average). The lack of significant spatial summation for sharpness threshold is consistent with the theory that perceived sharpness can be evoked by near threshold activity of a single nociceptor. The modest amount of spatial summation for pain implies that distinctly suprathreshold activation of nociceptors is required for mechanically evoked pain perception, and such input summates centrally, but not completely. The greater spatial summation observed for pressure vs. pain thresholds implies a greater degree of central summation for slowly adapting mechanoreceptors vs. nociceptors.     

Subjects with Knee Osteoarthritis Exhibit Widespread Hyperalgesia to Pressure and Cold

Hyperalgesia to mechanical and thermal stimuli are characteristics of a range of disorders such as tennis elbow, whiplash and fibromyalgia. This study evaluated the presence of local and widespread mechanical and thermal hyperalgesia in individuals with knee osteoarthritis, compared to healthy control subjects. Twenty-three subjects with knee osteoarthritis and 23 healthy controls, matched for age, gender and body mass index, were recruited for the study. Volunteers with any additional chronic pain conditions were excluded. Pain thresholds to pressure, cold and heat were tested at the knee, ipsilateral heel and ipsilateral elbow, in randomized order, using standardised methodology. Significant between-groups differences for pressure pain and cold pain thresholds were found with osteoarthritic subjects demonstrating significantly increased sensitivity to both pressure (p = .018) and cold (p = .003) stimuli, compared with controls. A similar pattern of results extended to the pain-free ipsilateral ankle and elbow indicating widespread pressure and cold hyperalgesia. No significant differences were found between groups for heat pain threshold, although correlations showed that subjects with greater sensitivity to pressure pain were also likely to be more sensitive to both cold pain and heat pain. This study found widespread elevated pain thresholds in subjects with painful knee osteoarthritis, suggesting that altered nociceptive system processing may play a role in ongoing arthritic pain for some patients.     

Barrels IX-proceedings

The properties of a newly developed tonic heat pain model (THPM), which makes use of pulsating contact heat, were investigated in 18 young men. The most important feature of this model is that repetitive heat pulses with an intensity of 1°C above the individual pain threshold are employed. This approach was used to tailor the tonic pain stimulation to the individual pain sensitivity. In the first of two experiments, the effects of pulse frequencies ranging from 5 to 30 pulses per minute (ppm) on ratings of pain intensity and pain unpleasantness (visual analogue scales) were examined. At all frequencies, both ratings increased steadily over the 5-min test period. Frequencies of 15 ppm or more appeared to enhance pain intensity throughout the test period compared to the lower frequencies, but did not appear to alter pain unpleasantness. This suggests that only pain intensity is influenced by slow temporal summation and that a sort of frequency threshold exists for this kind of summation. In the second experiment, the THPM was compared to a well-established form of tonic pain stimulation, the compressor test (CPT); visual analogue scales were again used, and in addition the McGill Pain Questionnaire was employed. The CPT appeared to produce stronger tonic pain than the THPM. However, as is typical with tonic pain, both tonic pain models induced relatively higher values on the affective pain dimension than on the sensory pain dimension. The time course of pain was dynamic in the CPT, with an increase followed by a plateau phase, at least in those subjects who could tolerate the CPT for more than 60 sec. In contrast, as in the first experiment, the pain ratings in the THPM were characterized by a slow and steady increase over time. Moreover, there was absolutely no indication of a dichotomy between “pain-sensitive” and “pain-tolerant” individuals in the THPM, although such a dichotomy was evident in the CPT. This implies that the distinction between pain-sensitive and pain-tolerant individuals can be made only with the CPT, and that this distinction represents individual differences in peripheral vascular reactions to cold rather than in pain perception. In conclusion, the THPM appears to produce a stable and predictable temporal pattern of tonic pain with a predominant affective component, and to be suitable for application in the majority of individuals without causing undue discomfort.     

Augmented Pain Processing in Primary and Secondary Somatosensory Cortex in Fibromyalgia: A Magnetoencephalography Study Using Intra-Epidermal Electrical Stimulation

The aim of this study was to investigate augmented pain processing in the cortical somatosensory system in patients with fibromyalgia (FM). Cortical evoked responses were recorded in FM (n = 19) and healthy subjects (n = 21) using magnetoencephalography after noxious intra-epidermal electrical stimulation (IES) of the hand dorsum (pain rating 6 on a numeric rating scale, perceptually-equivalent). In addition, healthy subjects were stimulated using the amplitude corresponding to the average stimulus intensity rated 6 in patients with FM (intensity-equivalent). Quantitative sensory testing was performed on the hand dorsum or thenar muscle (neutral site) and over the trapezius muscle (tender point), using IES (thresholds, ratings, temporal summation of pain, stimulus-response curve) and mechanical stimuli (threshold, ratings). Increased amplitude of cortical responses was found in patients with FM as compared to healthy subjects. These included the contralateral primary (S1) and bilateral secondary somatosensory cortices (S2) in response to intensity-equivalent stimuli and the contralateral S1 and S2 in response to perceptually-equivalent stimuli. The amplitude of the contralateral S2 response in patients with FM was positively correlated with average pain intensity over the last week. Quantitative sensory testing results showed that patients with FM were more sensitive to painful IES as well as to mechanical stimulation, regardless of whether the stimulation site was the hand or the trapezius muscle. Interestingly, the slope of the stimulus-response relationship as well as temporal summation of pain in response to IES was not different between groups. Together, these results suggest that the observed pain augmentation in response to IES in patients with FM could be due to sensitization or disinhibition of the cortical somatosensory system. Since the S2 has been shown to play a role in higher-order functions, further studies are needed to clarify the role of augmented S2 response in clinical characteristics of FM.     

Responses to tonic heat pain in the ongoing EEG under conditions of controlled attention

To confirm the existence of an ongoing electroencephalogram (EEG) pattern that is truly suggestive of pain, tonic heat pain was induced by small heat pulses at 1?°C above the pain threshold and compared to slightly less intense tonic non-painful heat pulses at 1?°C below the pain threshold. Twenty healthy subjects rated the sensation intensity during thermal stimulation. Possible confounding effects of attention were thoroughly controlled for by testing in four conditions: (1) focus of attention directed ipsilateral or (2) contralateral to the side of the stimulation, (3) control without a side preference, and (4) no control of attention at all. EEG was recorded via eight leads according to the 10/20 convention. Absolute power was computed for the frequency bands delta (0.5–4?Hz), theta (4–8?Hz), alpha1 (8–11?Hz), alpha2 (11–14?Hz), beta1 (14–25?Hz), and beta2 (25–35?Hz). Ratings were clearly distinct between the heat and pain conditions and suggestive for heat and pain sensations. Manipulation of attention proved to be successful by producing effects on the ratings and on the EEG activity (with lower ratings and lower EEG activity (theta, beta1, 2) over central areas for side-focused attention). During pain stimulation, lower central alpha1 and alpha2 activity and higher right-parietal and right-occipital delta power were observed compared to heat stimulation. This EEG pattern was not influenced by the manipulation of attention. Since the two types of stimuli (pain, heat) were subjectively felt differently although stimulation intensities were nearby, we conclude that this EEG pattern is clearly suggestive of pain.     

Effect of pulsed electromagnetic field therapy on experimental pain: A double-blind,randomized study in healthy young adults

ABSTRACT

Previous studies suggested that pulsed electromagnetic field (PEMF) therapy can decrease pain. To date, however, it remains difficult to determine whether the analgesic effect observed in patients are attributable to a direct effect of PEMF on pain or to an indirect effect of PEMF on inflammation and healing. In the present study, we used an experimental pain paradigm to evaluate the direct effect of PEMF on pain intensity, pain unpleasantness, and temporal summation of pain. Twenty-four healthy subjects (mean age 22 ± 2 years; 9 males) participated in the experiment. Both real and sham PEMF were administered to every participant using a randomized, double-blind, cross-over design. For each visit, PEMF was applied for 10 minutes on the right forearm using a portable device. Experimental pain was evoked before (baseline) and after PEMF with a 9 cm² Pelletier-type thermode, applied on the right forearm (120 s stimulation; temperature individually adjusted to produce moderate baseline pain). Pain intensity and unpleasantness were evaluated using a 0–100 numerical pain rating scale. Temporal summation was evaluated by comparing pain intensity ratings obtained at the end of tonic nociceptive stimulation (120 s) with pain intensity ratings obtained after 60 s of stimulation. When compared to baseline, there was no change in pain intensity and unpleasantness following the application of real or sham PEMF. PEMF did not affect temporal summation. The present observations suggest that PEMF does not directly influence heat pain perception in healthy individuals.     

Sex differences in heat pain thresholds as a function of assessment method and rate of rise

Studies of sex differences in the responses to experimentally induced pain demonstrate greater pain sensitivity among females than males. However, studies investigating heat pain responses have produced inconsistent results. Differences in stimulus characteristics and assessment methods probably account for this variability. This study examined sex differences in the heat pain threshold as a function of two different assessment methods and varying rates of rise. Nineteen female and 18 male healthy volunteers underwent heat pain threshold assessment via the method of levels and the method of limits. In addition, both fast (4.0 C/s) and slow (0.5 C/s) rates of rise were used for the method of levels assessments. In order to examine the reliability of threshold values, each subject participated in two sessions, separated by approximately 8 days. Females evinced lower thresholds than males for the method of levels assessments with both slow and fast rates of rise ( ps < 0.05), while no sex differences emerged for the threshold assessed via the method of limits. Test-retest reliability coefficients were relatively high. However, thresholds generally increased significantly from session 1 to session 2. Between method correlations were generally low to moderate. These findings indicate that the method of levels may be more sensitive to sex differences than the more commonly used method of limits. Also, thresholds appear to increase from session 1 to session 2, and thresholds assessed via different methods are not strongly correlated. Potential implications of these results for experimental pain assessment are discussed.     

Modification of Electrical Pain Threshold by Voluntary Breathing-Controlled Electrical Stimulation (BreEStim) in Healthy Subjects

Background

Pain has a distinct sensory and affective (i.e., unpleasantness) component. BreEStim, during which electrical stimulation is delivered during voluntary breathing, has been shown to selectively reduce the affective component of post-amputation phantom pain. The objective was to examine whether BreEStim increases pain threshold such that subjects could have improved tolerance of sensation of painful stimuli.

Methods

Eleven pain-free healthy subjects (7 males, 4 females) participated in the study. All subjects received BreEStim (100 stimuli) and conventional electrical stimulation (EStim, 100 stimuli) to two acupuncture points (Neiguan and Weiguan) of the dominant hand in a random order. The two different treatments were provided at least three days apart. Painful, but tolerable electrical stimuli were delivered randomly during EStim, but were triggered by effortful inhalation during BreEStim. Measurements of tactile sensation threshold, electrical sensation and electrical pain thresholds, thermal (cold sensation, warm sensation, cold pain and heat pain) thresholds were recorded from the thenar eminence of both hands. These measurements were taken pre-intervention and 10−min post-intervention.

Results

There was no difference in the pre-intervention baseline measurement of all thresholds between BreEStim and EStim. The electrical pain threshold significantly increased after BreEStim (27.5±6.7% for the dominant hand and 28.5±10.8% for the non-dominant hand, respectively). The electrical pain threshold significantly decreased after EStim (9.1±2.8% for the dominant hand and 10.2±4.6% for the non–dominant hand, respectively) (F_{[1, 10]} = 30.992, p = .00024). There was no statistically significant change in other thresholds after BreEStim and EStim. The intensity of electrical stimuli was progressively increased, but no difference was found between BreEStim and EStim.

Conclusion

Voluntary breathing controlled electrical stimulation selectively increases electrical pain threshold, while conventional electrical stimulation selectively decreases electrical pain threshold. This may translate into improved pain control.     

Test–retest reliability of subjective supra-threshold scaling of multiple pressure-pain sensations among healthy individuals: a study using hydraulic pressure algometry

Abstract

Background: Supra-threshold scaling of multiple pressure-pain sensations involves delivery of varied stimulus intensities, either via stimulus-dependent or response-dependent manner, and recording of subjective pain ratings by participants. The focus of this study was to determine the intra- and inter-session reliability of pain intensity and pain unpleasantness ratings related to pressure-pain thresholds (PPTs) of just noticeable pain (JNP), weak pain (WP) and moderate pain (MP) among healthy individuals.

Methods: Fourteen healthy participants (eight women, six men) participated in three sessions of testing at varied intervals over the course of 72?h. In session one, a multiple random staircase method using hydraulic pressure algometry was used to measure PPT of JNP, WP and MP on thumbnail bed. In session 2, ratings of pain intensity and pain unpleasantness were recorded when stimuli at levels corresponding to PPT of JNP, WP and MP were repeatedly applied before and after 20?min of no intervention.

Results: Interclass correlation coefficient (ICC) values for pain ratings of JNP, WP and MP in intra-session reliability were 0.810, 0.826 and 0.881, respectively, whereas the values were 0.817, 0.792 and 0.910, respectively, for inter-session reliability. ICC values for pain unpleasantness were also highly consistent and repeatable. Temporal summation of pain intensity and pain unpleasantness were not related to the repeated application of pressure stimuli.

Conclusions: The findings indicate that the pain intensity and pain unpleasantness ratings for stimuli at levels equal to the thresholds of JNP, WP and MP have good intra- and inter-session reliability.

Significance: This study showed that both pain intensity and pain unpleasantness of JNP, WP and MP have good intra- and inter-session reliability and agreement. Furthermore, the temporal summation of pain or unpleasantness is not related to repeated application of pressure stimuli.

Abbreviations: JNP: Just noticeable pain; WP: Weak pain; MP: Moderate pain; PPTs: pressure-pain thresholds; HPA: Hydraulic pressure algometry; MRSM: multiple random staircase method     

Thermosensory intensity and affect throughout the perceptible range

The thermosensory system was evaluated psychophysically in 12 healthy volunteers, spanning the full range of tolerable temperatures. Subjects provided ratings of (1) perceived thermal intensity, (2) perceived pleasantness or unpleasantness, and (3) perceived pain intensity after placing either one hand or foot in a temperature controlled water bath. Of particular interest were the interrelationships among the three perceptual measures, and differences between heat and cold. The relationship between perceived intensity and (un)pleasantness was different for hot vs cold stimuli. Specifically, for a given perceived thermal intensity, cold stimuli were rated as less pleasant or more unpleasant than hot stimuli. Similarly, for a given pain intensity, cold stimuli were rated as more unpleasant than hot stimuli. As warm temperatures increased and as cold temperatures decreased, stimuli were perceived as being unpleasant before they were perceived as being painful. The difference in transition temperatures for unpleasantness vs pain for heat averaged 1.4 degrees C, while the same difference for cold averaged 5.6 degrees C. Thus, there was a fourfold difference in the range of unpleasant but non-painful cold vs hot temperatures. Pain intensity and unpleasantness ratings were significantly higher for heat stimuli applied to the foot vs hand. In contrast, there was no significant body site difference for pain intensity or unpleasantness ratings of cold stimuli. All of these results reveal important differences in the processing of cold vs hot stimuli. These differences could be exploited to differentiate processing relevant to discriminative vs affective components of somesthetic perception, in both the innocuous and noxious ranges.     

The summation properties of the somatosensory system demonstrated via a study of the tactile sensations evoked by an electrical current and by focused ultrasound

Sensational thresholds of the skin of fingers, palm and forearm have been determined in 5 healthy subjects ageing from 20 to 49 years. About 100 summation curves were obtained. Critical duration of the electrical stimuli remained constant in various areas. Critical duration of ultrasonic stimulation increased from fingers to forearm. The data obtained together with those found in literature suggest that critical duration of the stimulus is associated with the diameter of nonmyelin nervous fibers which are affected by the stimulus increasing with the increase in fiber diameter.     

Attenuation of vibrotactile spatial summation.

Masked and quiet thresholds at several frequencies of vibratory stimuli were measured as a function of contactor area. The test site was the left index finger; the masking site was the left little finger. The quiet threshold data were consistent with previous investigations: Low-frequency stimuli showed no spatial summation, whereas high-frequency stimuli did. In the presence of a masker, spatial summation was reduced or eliminated for high-frequency stimuli, i.e., the masked threshold was, under some conditions, independent of contactor area. Low-frequency stimuli continued to show no spatial summation in the presence of a masker. The attenuation of spatial summation appears to be a direct function of the intensity of the masking stimulus. Additional measurements with the left thenar eminence as the test site showed that spatial summation could be attenuated by a masker placed on a contralateral body site. The implications of the results for quantifying the effectiveness of a masking stimulus, for the duplex mechanoreceptor hypothesis, and for the nature of spatial summation on the skin are discussed.     

Effect of sex on perception of rectosigmoid stimuli in irritable bowel syndrome

In irritable bowel syndrome (IBS) patients, the relationship between sex and sensitivity to visceral stimuli is incompletely understood. Our aim was to evaluate the effect of sex on perceptual responses to visceral stimulation in IBS. Fifty-eight IBS patients (mean age 42+/-1 yr; 34 men, 24 women) and 26 healthy controls (mean age 38+/-3 yr; 9 men, 17 women) underwent barostat-assisted distensions of the rectum and sigmoid colon. Rectal discomfort thresholds were measured using a randomized, phasic distension paradigm before and after repeated noxious sigmoid stimulation (SIG, 60-mmHg pulses). Sex had a significant effect on rectal discomfort thresholds. Women with IBS were the most sensitive (lower thresholds [27+/-2.7 mmHg] and higher ratings), with significantly lower rectal discomfort thresholds compared with men with IBS (38+/-2.3 mmHg) and healthy women who were the least sensitive (41.9+/-3.2 mmHg; both P<0.01). There were no significant differences in rectal discomfort thresholds between healthy men (34+/-4.3 mmHg) and men with IBS. Across both IBS and control groups, women demonstrated a significant lowering of discomfort thresholds after noxious sigmoid stimulation (P<0.01), while men did not. Sex significantly influences perceptual sensitivity to rectosigmoid distension. Women show greater perceptual responses to this paradigm.     

Horizontal Body Position Reduces Cortical Pain-Related Processing: Evidence from Late ERPs

The present study investigated the influence of short-term horizontal body position on pain-related somatosensory processing, by measuring subjective and cortical responses to electrical pain stimulation. Twenty-eight healthy women were randomly assigned to either the experimental horizontal group (Bed Rest, BR) or to the sitting control group (Sitting Control, SC). After 90 minutes in either horizontal or sitting position, the individual pain thresholds were assessed and EEG/self-evaluations recorded during the administration of 180 stimuli delivered to the left forearm. Electrical pain stimuli, calibrated to subjects’ individual pain thresholds, consisted of two different intensity levels: no pain (40% below pain threshold) and pain (40% above pain threshold). Compared with control, BR condition significantly inhibited subjective sensitivity to painless stimuli, whereas electrophysiological results pointed to a reduced slow cortical wave (interval: 300-600 ms) at all stimulus intensities, and smaller amplitude in BR’s right vs. left prefrontal sites. sLORETA analysis revealed that cortical responses were associated with a decreased activation of superior frontal gyrus and anterior cingulate cortex (BA 6/24). Interestingly, BR group only showed significant negative correlations between self-evaluation of painful intensities and frontal cortical negativity, revealing increasingly differentiated responses in bed rest: indeed those BR participants who reported lower pain ratings, displayed reduced negativity within anterior regions. Taken together, results indicate that short-term horizontal position is able to inhibit a fronto-parietal pain network, particularly at the level of central prefrontal regions typically involved in cognitive, affective and motor aspects of pain processing.     

No Pain Relief with the Rubber Hand Illusion

The sense of body ownership can be easily disrupted during illusions and the most common illusion is the rubber hand illusion. An idea that is rapidly gaining popularity in clinical pain medicine is that body ownership illusions can be used to modify pathological pain sensations and induce analgesia. However, this idea has not been empirically evaluated. Two separate research laboratories undertook independent randomized repeated measures experiments, both designed to detect an effect of the rubber hand illusion on experimentally induced hand pain. In Experiment 1, 16 healthy volunteers rated the pain evoked by noxious heat stimuli (5 s duration; interstimulus interval 25 s) of set temperatures (47°, 48° and 49°C) during the rubber hand illusion or during a control condition. There was a main effect of stimulus temperature on pain ratings, but no main effect of condition (p = 0.32), nor a condition x temperature interaction (p = 0.31). In Experiment 2, 20 healthy volunteers underwent quantitative sensory testing to determine heat and cold pain thresholds during the rubber hand illusion or during a control condition. Secondary analyses involved heat and cold detection thresholds and paradoxical heat sensations. Again, there was no main effect of condition on heat pain threshold (p = 0.17), nor on cold pain threshold (p = 0.65), nor on any of the secondary measures (p<0.56 for all). We conclude that the rubber hand illusion does not induce analgesia.     

Spatial variation in sensitivity as a factor in measurements of spatial summation of warmth and cold

Perception of cutaneous heating and cooling depends strongly on stimulus size. Although this dependence has been attributed solely to spatial summation, topographical variations in temperature sensitivity may also play a role. These variations, which differentially affect perception of small stimuli, may have led to overestimation of spatial summation. This possibility was investigated by measuring detection thresholds and perceived intensity for heating and cooling on the volar surface of the forearm using a multiple-thermode stimulus array. By keeping the array in place throughout each testing session we were able to measure threshold sensitivity and suprathreshold responsiveness at eight individual sites and for combinations of these sites having total stimulus areas of 0.64-5.12 cm2. When spatial summation was calculated in the traditional way by averaging the data for all stimuli of each size, the results agreed closely with previous estimates of summation for warmth and cold. When calculations were based instead on the most sensitive test site for each stimulus size, estimates of summation were reduced by about two-thirds. This outcome indicates that the spatial heterogeneity of thermal sensitivity likely contributed to estimates of spatial summation reported in earlier psychophysical studies. A schematic model of cutaneous thermoreception is presented that shows how neural summation and the density of innervation may combine to produce the psychophysical effects of increasing stimulus size (spatial enhancement).