Comparative psychophysical characteristics of cutaneous CO2 laser and contact heat stimulation

Vigilance to Painful Laser Stimuli is Associated with Increased State Anxiety and Tense Arousal

Introduction

Pain is frequently accompanied by enhanced arousal and hypervigilance to painful sensations. Here, we describe our findings in an experimental vigilance task requiring healthy participants to indicate when randomly timed moderately painful stimuli occur in a long train of mildly painful stimuli.

Methods

During a continuous performance task with painful laser stimuli (CPTpain), 18 participants rated pain intensity, unpleasantness, and salience. We tested for a vigilance decrement over time using classical metrics including correct targets (hits), incorrectly identified non-targets (false alarms), hit reaction time, and false alarm reaction time. We measured state anxiety and tense arousal before and after the task.

Results

We found a vigilance decrement across four 12.5-minute blocks of painful laser stimuli in hits [F3,51=2.91; p=0.043; time block 1>block 4 (t=2.77; p=0.035)]. Both self-report state anxiety (tpaired,17=3.34; p=0.0039) and tense arousal (tpaired,17=3.20; p=0.0053) increased after the task. We found a vigilance decrement during our laser pain vigilance task consistent with vigilance decrements found in other stimulus modalities. Furthermore, state anxiety positively correlated with tense arousal.

Discussion

CPTpain acutely increased tense arousal and state anxiety, consistent with previous results implicating the reciprocal interaction of state anxiety and acute painful sensations and the role of pain in augmenting tense arousal. These results may indicate a psychological process which predisposes the hypervigilant to developing greater acute pain, resulting in positive feedback, greater pain and anxiety.

Temporal and spatial summation of laser heat stimuli in cultured nociceptive neurons of the rat

We studied the efficacy of a near-infrared laser (1475 nm) to activate rat dorsal root ganglion (DRG) neurons with short punctate radiant heat pulses (55 µm diameter) and investigated temporal and spatial summation properties for the transduction process for noxious heat at a subcellular level. Strength-duration curves (10–80 ms range) indicated a minimum power of 30.2mW for the induction of laser-induced calcium transients and a chronaxia of 13.9 ms. However, threshold energy increased with increasing stimulus duration suggesting substantial radial cooling of the laser spot. Increasing stimulus duration demonstrated suprathreshold intensity coding of calcium transients with less than linear gains (Stevens exponents 0.29/35mW, 0.38/60mW, 0.46/70mW). The competitive TRPV1 antagonist capsazepine blocked responses to short near-threshold stimuli and significantly reduced responses to longer duration suprathreshold heat. Heating 1/3 of the soma of a neuron was sufficient to induce calcium transients significantly above baseline (p < 0.05), but maximum amplitude was only achieved by centering the laser over the entire neuron. Heat-induced calcium increase was highest in heated cell parts but rapidly reached unstimulated areas reminiscent of spreading depolarization and opening of voltage-gated calcium channels. Full intracellular equilibrium took about 3 s, consistent with a diffusion process. In summary, we investigated transduction mechanisms for noxious laser heat pulses in native sensory neurons at milliseconds temporal and subcellular spatial resolution and characterized strength duration properties, intensity coding, and spatial summation within single neurons. Thermal excitation of parts of a nociceptor spread via both membrane depolarization and intracellular calcium diffusion.

Use of nociceptive threshold testing in cats in experimental and clinical settings: a qualitative review

OBJECTIVE

The objective of this study was to review the scientific articles on the use of nociceptive threshold testing (NTT) in cats and to summarize the clinical and experimental applications in this species.

DATABASES USED

Pertinent literature was searched with PubMed, Scopus, Web of Science, Universitätsbibliothek Basel (swissbib Basel Bern) and Google Scholar. The search was then refined manually based first on article titles and abstracts, and subsequently on full texts.

CONCLUSIONS

Of the four classical acute nociceptive models used for NTT, thermal and mechanical are most commonly used in cats. Thermal stimulation is applicable in experimental settings and has been used in pharmacodynamics studies assessing feline antinociception. Although mechanical stimulation is currently less used in cats, in the future it might play a role in the evaluation of clinical feline pain. However, the low response reliability after stimulus repetition within a narrow time interval represents a major limitation for the clinical use of mechanical thresholds in this species. Challenges remain when thermal thresholds are used to investigate analgesics that have the potential to affect skin temperature, such as opioids and α2-adrenergic agonists, and when a model of inflammatory pain is reproduced in experimental cats with the purpose of evaluating non-steroidal anti-inflammatory drugs as analgesics.

No temporal contrast enhancement of simple decreases in noxious heat

Offset analgesia (OA) studies have found that small decreases in the intensity of a tonic noxious heat stimulus yield a disproportionately large amount of pain relief. In the classic OA paradigm, the decrease in stimulus intensity is preceded by an increase of equal size from an initial noxious level. Although the majority of researchers believe this temporal sequence of two changes is important for eliciting OA, it has also been suggested that the temporal contrast mechanism underlying OA may enhance detection of simple, isolated decreases in noxious heat. To test whether decreases in noxious heat intensity, by themselves, are perceived better than increases of comparable sizes, we used an adaptive two-interval alternative forced choice task to find perceptual thresholds for increases and decreases in radiant and contact heat. Decreases in noxious heat were more difficult to perceive than increases of comparable sizes from the same initial temperature of 45°C. In contrast, decreases and increases were perceived equally well within a common range of noxious temperatures (i.e., when increases started from 45°C and decreases started from 47°C). In another task, participants rated the pain intensity of heat stimuli that randomly and unpredictably increased, decreased, or remained constant. Ratings of unpredictable stimulus decreases also showed no evidence of perceptual enhancement. Our results demonstrate that there is no temporal contrast enhancement of simple, isolated decreases in noxious heat intensity. Combined with previous OA findings, they suggest that long-lasting noxious stimuli that follow an increase-decrease pattern may be important for eliciting the OA effect.

NEW & NOTEWORTHY Previous research suggested that a small decrease in noxious heat intensity feels surprisingly large because of sensory enhancement of noxious stimulus offsets (a simplified form of “offset analgesia”). Using a two-alternative forced choice task where participants detected simple increases or decreases in noxious heat, we showed that decreases in noxious heat, by themselves, are no better perceived than increases of comparable sizes. This suggests that a decrease alone is not sufficient to elicit offset analgesia.

[Effects of morphine, butorphanol and levomethadone in different doses on thermal nociceptive thresholds in horses]

OBJECTIVE

Various opioids are available for use in equine medicine. Studies directly comparing their analgesic effects and side effects are rare. Therefore, the aim of this study was to compare the antinociceptive effect and the duration of analgesia of two different doses of morphine, butorphanol and levomethadone in horses.

MATERIAL AND METHODS

Eight adult, healthy horses were used for this randomized, placebo-controlled, blinded cross-over trail. Each horse received placebo (P = 0.9% saline) and morphine (M_0.1 = 0.1 mg/kg; M_0.2 = 0.2 mg/kg), butorphanol (B_0.1 = 0.1 mg/kg; B_0.2 = 0.2 mg/kg) and levomethadone (L_0.1 = 0.1 mg/kg; L_0.2 = 0.2 mg/kg) in a low and a high dose and with a wash-out period of 14 days. Thermal thresholds were determined by incremental contact heat applied to the skin at the withers. Single stimulations were performed 15 minutes prior and 10, 30, 60, 90, 120, 180, 240, 300, 360, 420, 540 and 1350 minutes after treatment. Threshold values, gastrointestinal auscultation score and horses' behavior were recorded. Data were analyzed with analysis of variance for repeated measurements (p < 0.05).

RESULTS

In group M_0.1, changes in thermal thresholds did not reach significance. Thermal threshold increased significantly in the groups M_0.2, B_0.1, B_0.2, L_0.1 and L_0.2 for 240, 90, 90, 60 and 300 minutes, respectively. Behavioural changes, increased locomotion and decreased bowel sounds as well as delayed time until defecation were noticed in all groups.

CONCLUSIONS

Levomethadone induced a dose-dependent increase and prolongation of analgesia, whereas with butorphanol there was no difference between dosages regarding duration and intensity of analgesia. Morphine provided detectable analgesia only in the high dose of 0.2 mg/kg.

CLINICAL RELEVANCE

Levomethadone and morphine in the low dose (0.1 mg/kg) produced only minor and short lived anti-nociception and further studies are necessary to give a profound dose recommendation for the use of these drugs in horses.

Guidelines and recommendations for assessment of somatosensory function in oro-facial pain conditions--a taskforce report

The goals of an international taskforce on somatosensory testing established by the Special Interest Group of Oro-facial Pain (SIG-OFP) under the International Association for the Study of Pain (IASP) were to (i) review the literature concerning assessment of somatosensory function in the oro-facial region in terms of techniques and test performance, (ii) provide guidelines for comprehensive and screening examination procedures, and (iii) give recommendations for future development of somatosensory testing specifically in the oro-facial region. Numerous qualitative and quantitative psychophysical techniques have been proposed and used in the description of oro-facial somatosensory function. The selection of technique includes time considerations because the most reliable and accurate methods require multiple repetitions of stimuli. Multiple-stimulus modalities (mechanical, thermal, electrical, chemical) have been applied to study oro-facial somatosensory function. A battery of different test stimuli is needed to obtain comprehensive information about the functional integrity of the various types of afferent nerve fibres. Based on the available literature, the German Neuropathic Pain Network test battery appears suitable for the study of somatosensory function within the oro-facial area as it is based on a wide variety of both qualitative and quantitative assessments of all cutaneous somatosensory modalities. Furthermore, these protocols have been thoroughly described and tested on multiple sites including the facial skin and intra-oral mucosa. Standardisation of both comprehensive and screening examination techniques is likely to improve the diagnostic accuracy and facilitate the understanding of neural mechanisms and somatosensory changes in different oro-facial pain conditions and may help to guide management.

Anatomofunctional organization of the insular cortex: a study using intracerebral electrical stimulation in epileptic patients

PURPOSE

Different lines of evidence suggest that the insular cortex has many important functional roles. Direct electrical stimulation (ES) of the human insular cortex during surgical procedures for epilepsy, functional imaging techniques, and lesion studies also occasionally induces clinical responses.

METHODS

In this study, we evaluated 25 patients with drug-refractory focal epilepsy by stereotactically implanting at least one electrode into the insular cortex using an oblique approach (transfrontal or transparietal). One hundred twenty-eight insular sites (each situated between two contiguous contacts within the same electrode) were examined within the gyral substructures. We located each stimulation site by fusing preimplantation three-dimensional (3D) magnetic resonance imaging (MRI) images with the postimplantation 3D computed tomography (CT) scans that revealed the electrode contacts.

RESULTS

Sixty-seven stimulations induced at least one clinical response. Stimulation from within the insular cortex evoked 83 responses, without evidence of afterdischarge in the insular or extrainsular regions. We classified the principal responses as sensory (paresthesias and localized warm sensations), motor, pain, auditory, oropharyngeal, speech disturbances (including speech arrest and reduced voice intensity) and neurovegetative phenomena, such as facial reddening, generalized sensations of warmth or cold, hypogastric sensations, anxiety attacks, respiratory accelerations, sensations of rotation, and nausea.

CONCLUSIONS

These findings may indicate a functional specificity for the insular gyri and show the need for exploring this structure during invasive presurgical evaluation of epileptic patients according to seizure manifestations.

The fine tuning of pain thresholds: a sophisticated double alarm system

Two distinctive features characterize the way in which sensations including pain, are evoked by heat: (1) a thermal stimulus is always progressive; (2) a painful stimulus activates two different types of nociceptors, connected to peripheral afferent fibers with medium and slow conduction velocities, namely Aδ- and C-fibers. In the light of a recent study in the rat, our objective was to develop an experimental paradigm in humans, based on the joint analysis of the stimulus and the response of the subject, to measure the thermal thresholds and latencies of pain elicited by Aδ- and C-fibers. For comparison, the same approach was applied to the sensation of warmth elicited by thermoreceptors. A CO₂ laser beam raised the temperature of the skin filmed by an infrared camera. The subject stopped the beam when he/she perceived pain. The thermal images were analyzed to provide four variables: true thresholds and latencies of pain triggered by heat via Aδ- and C-fibers. The psychophysical threshold of pain triggered by Aδ-fibers was always higher (2.5–3°C) than that triggered by C-fibers. The initial skin temperature did not influence these thresholds. The mean conduction velocities of the corresponding fibers were 13 and 0.8 m/s, respectively. The triggering of pain either by C- or by Aδ-fibers was piloted by several factors including the low/high rate of stimulation, the low/high base temperature of the skin, the short/long peripheral nerve path and some pharmacological manipulations (e.g. Capsaicin). Warming a large skin area increased the pain thresholds. Considering the warmth detection gave a different picture: the threshold was strongly influenced by the initial skin temperature and the subjects detected an average variation of 2.7°C, whatever the initial temperature. This is the first time that thresholds and latencies for pain elicited by both Aδ- and C-fibers from a given body region have been measured in the same experimental run. Such an approach illustrates the role of nociception as a “double level” and “double release” alarm system based on level detectors. By contrast, warmth detection was found to be based on difference detectors. It is hypothesized that pain results from a CNS build-up process resulting from population coding and strongly influenced by the background temperatures surrounding at large the stimulation site. We propose an alternative solution to the conventional methods that only measure a single “threshold of pain”, without knowing which of the two systems is involved.

Effect of lingual nerve block on burning mouth syndrome (stomatodynia): a randomized crossover trial

Burning mouth syndrome (stomatodynia) is associated with changes of a neuropathic nature the main location of which, peripheral or central, remains unknown. A randomised, double-blind crossover design was used to investigate the effects of lingual nerve block on spontaneous burning pain and a possible correlation with the effects of topical clonazepam, the patient's response to a psychological questionnaire, and the taste and heat thresholds. The spontaneous burning was measured with a visual analogue scale (VAS) just before and 15 min after injection. The decreases in VAS score after lidocaine or saline injection were not significantly different (2.7+/-3.9 and 2.0+/-2.6, respectively; n=20). However, two groups of patients could be identified: in a "peripheral group" (n=10) the VAS decrease due to lingual nerve injection was 4.3+/-3.1cm after lidocaine and 0.9+/-0.3 cm after saline (p=0.02). In a "central group" (n=7), there were an increase in pain intensity score (-0.8+/-2.6 cm) after lidocaine and a decrease (1.5+/-3.0 cm) after saline (p=0.15). An increase in the hospital anxiety and depression (HAD) score and a decreased taste sensitivity and heat pain threshold of painful oral area were seen in patients compared with age-and-sex-matched controls (p<0.05). Topical clonazepam treatment tended to be more effective (p=0.07) and HAD score lower (p<0.03) in the peripheral than in the central group. These results suggest that the neuropathic disorder associated with stomatodynia may be predominantly peripheral, central or mixed depending on the individual. Topical application of clonazepam and HAD may serve as indicators of which mechanism is dominating.

Middle short gyrus of the insula implicated in pain processing

Different lines of evidence have suggested an involvement of the insular cortex in pain processing. Direct electrical stimulation (ES) of the human insular cortex during surgical procedure sometimes induces painful sensations and painful stimuli induce activation of the insular cortex as shown by functional neuroimaging. Invasive evaluation of epileptic patients by deep brain stereotactically implanted electrodes provides an opportunity to analyze responses induced by ES of the insular cortex in awake and fully conscious patients. For this study, we included 25 patients suffering from drug refractory focal epilepsy with at least one electrode stereotactically implanted in the insular cortex using an oblique approach (transfrontal or transparietal). Out of the 83 responses induced by insular ES, eight (9.6%) were reported by five patients as painful sensations. Four were restricted to the cephalic region and four were felt on the ipsilateral or bilateral upper limbs, the shoulders and the trunk (pinprick sensations). The eight stimulation sites were anatomically localized via image fusion between pre-implantation 3D MRI and post-implantation 3D CT scans revealing the electrode contacts. All sites inducing painful sensations were restricted to the upper portion of the middle short gyrus of the insula. The findings of this study suggest that middle short gyrus is involved in the processing of pain-producing stimuli.

A novel modelling and experimental technique to predict and measure tissue temperature during CO2 laser stimuli for human pain studies

Laser nerve stimulation is now accepted as one of the preferred methods for applying painful stimuli to human skin during pain studies. One of the main concerns, however, is thermal damage to the skin. We present recent work based on using a CO2 laser with a remote infrared (IR) temperature sensor as a feedback system. A model for predicting the subcutaneous skin temperature derived from the signal from the IR detector allows us to accurately predict the laser parameters, thus maintaining an optimum pain stimulus whilst avoiding dangerous temperature levels, which could result in thermal damage. Another aim is to relate the modelling of the CO2 fibre laser interaction to the pain response and compare these results with practical measurements of the pain threshold for various stimulus parameters. The system will also allow us to maintain a constant skin temperature during the stimulus. Another aim of the experiments underway is to review the psychophysics for pain in human subjects, permitting an investigation of the relationship between temperature and perceived pain.

A reliable method for the preferential activation of C- or A-fibre heat nociceptors

There is strong evidence that A- and C-fibre nociceptors evoke significantly different sensory experiences, are differentially sensitive to pharmacological intervention, and play different roles in pain pathology. It is therefore of considerable interest to be able to selectively activate one fibre type or the other in studies of nociceptive processing. Here, we report significant modifications to a non-invasive technique, first described by Yeomans et al. [Pain 59 (1994) 85; Pain 68 (1996) 141; Pain 68 (1996) 133], which uses different rates of skin heating to preferentially activate A- or C-nociceptors. A copper disk (diameter: 4mm) was used to transfer heat evenly across the dorsal surface of the rat hindpaw. Initial experiments established the relationship between the temperature at the skin surface and the sub-epidermal temperature. Subsequently, the vanilloid capsaicin, which sensitises unmyelinated C-mechanoheat nociceptors, was shown to decrease the thresholds of reflex responses evoked by slow rates of heating. In contrast thresholds of responses to fast rates of skin heating were unchanged, indicating that nociceptors activated by this stimulus were capsaicin-insensitive A-fibre heat nociceptors.

The effects of A-fiber pressure block on perception and neurophysiological correlates of brief non-painful and painful CO2 laser stimuli in humans

This study examined the relative capacity of Adelta- and C-fibers to encode non-painful and painful brief CO(2) laser stimuli by comparing the effects of Adelta/C-fiber activation versus C-fiber activation alone. In nine normal subjects, brief CO(2) laser pulses of four different intensities (range 5.8-10.6mJ/mm(2)) were delivered at random on the first intermetacarpal zone of the dorsum of the hand. A-fiber pressure block of the superficial radial nerve was performed to fully isolate the activity of C-fibers. Quality and intensity (VAS) of percepts, reaction time (RT) and laser-evoked potentials (LEPs) were examined in baseline and A-fiber block conditions. During A-fiber block, absolute detection threshold increased dramatically from 4.8+/-1.8 to 10.9+/-4.8mJ/mm(2), proportion of detected stimuli decreased from 87% to 47% and proportion of pain reports from 39% to 10%. The quality of sensations became mainly 'light touch' and the 'pricking' sensation almost vanished. The stimulus-VAS curve shifted to the right and the slope was reduced. Signal Detection Theory analysis revealed that discrimination performance (P(A)) was significantly depressed and that response bias (B) evolved from a neutral towards a stoical attitude. Median RT increased from 492 to 1355ms. The late LEPs, attributed to the activation of Adelta-fibers, disappeared and ultra-late LEPs were recorded at Cz with a positivity peaking around 800ms. Collectively, these observations lead to the conclusion that Adelta-fibers are the main peripheral mediators for the perception of brief CO(2) laser stimuli and that they provide more sensory information than C-fibers.

A comparison of modality-specific somatosensory changes during menstruation in dysmenorrheic and nondysmenorrheic women

OBJECTIVE

The objective was to evaluate somatosensory thresholds to a multimodality stimulation regimen applied both within and outside areas of referred menstrual pain in dysmenorrheic women, over four phases of confirmed ovulatory cycles, and to compare them with thresholds in nondysmenorrheic women during menstruation.

DESIGN

Twenty dysmenorrheic women with menstrual pain scoring 5.45 +/- 0.39 cm (mean +/- standard error of mean) on a visual analog scale (10 cm) participated. Fifteen nondysmenorrheic women with a menstrual pain score of 0.4 +/- 0.2 cm participated as controls. Ovulation was confirmed by an enzyme-multiplied immunoassay technique. Menstrual pain was described with the McGill Pain Questionnaire. Areas within menstrual pain referral were two abdominal sites and the midline of the low back, and the arm and thigh were the control areas. The pressure pain threshold (PPT) and pinch pain threshold were determined by a hand-held electronic pressure algometer, the heat pain threshold (HPT) by a contact thermode, and the tactile threshold with von Frey hairs.

RESULTS

In dysmenorrheic women the McGill Pain Questionnaire showed a larger sensory and affective component of pain than the evaluative and miscellaneous groups. The HPT and PPT were lower in the menstrual phase than in the ovulatory, luteal, and premenstrual phases, both within and outside areas of referred menstrual pain (p <0.01), with a more pronounced decrease at the referral pain areas. The pinch pain threshold was lower in the menstrual phase than in the ovulatory phase (p <0.02), and the tactile threshold did not differ significantly across the menstrual phases or within any site. Dysmenorrheic women had a lower HPT at the control sites and a lower PPT at the abdomen, back, and control sites, than in those of nondysmenorrheic women in the menstrual phase.

CONCLUSIONS

The results show reduced somatosensory pain thresholds during menstruation to heat and pressure stimulation, both within and outside areas of referred menstrual pain in dysmenorrheic women. Dysmenorrheic women showed a lower HPT at the control sites and a lower PPT at all the sites than those for nondysmenorrheic women in the menstrual phase. The altered somatosensory thresholds may be dependent on a spinal mechanism of central hyperexcitability, induced by recurrent moderate to severe menstrual pain.

Reward circuitry activation by noxious thermal stimuli

Using functional magnetic resonance imaging (fMRI), we observed that noxious thermal stimuli (46 degrees C) produce significant signal change in putative reward circuitry as well as in classic pain circuitry. Increases in signal were observed in the sublenticular extended amygdala of the basal forebrain (SLEA) and the ventral tegmentum/periaqueductal gray (VT/PAG), while foci of increased signal and decreased signal were observed in the ventral striatum and nucleus accumbens (NAc). Early and late phases were observed for signals in most brain regions, with early activation in reward related regions such as the SLEA, VT/PAG, and ventral striatum. In contrast, structures associated with somatosensory perception, including SI somatosensory cortex, thalamus, and insula, showed delayed activation. These data support the notion that there may be a shared neural system for evaluation of aversive and rewarding stimuli.

The 980-nm diode laser as a new stimulant for laser evoked potentials studies

BACKGROUND AND OBJECTIVE

Lasers have been used as stimulators for creating pain response without stimulating mechanoreceptive fibers. Various laser systems are still under investigation on the quest for best laser system. Our objective was to test the feasibility of the 980-nm diode laser for LEP (laser evoked potentials) studies.

STUDY DESIGN/MATERIALS AND METHODS

Laser evoked potentials created by using the 980-nm diode laser were recorded by using standard electroencephalogram (EEG) techniques. The collimated laser beam was 3 mm in diameter. Stimulus duration was set to 200 msec. The power of laser stimulus exposed to the dorsum of the right hand of 10 healthy volunteer subjects (5 women and 5 men) was varied between 0 and 10 watts to determine the pain threshold. EEG signals during the exposure of 1.5 times the threshold value were recorded from scalp electrodes placed on areas Fz, Cz, Pz, C3, and C4 according to the international 10--20 system. The stimulus presented during the EEG recording was described as a bearable pain sensation like a pinprick perception by the subjects. After 0.1--30 Hz analog low-pass filtering, 100-msec prestimulus, and 900-msec poststimulus EEG epochs were recorded at 256 Hz sampling rate and evaluated statistically. Thirty stimuli were presented by randomly varying the interstimulus duration between 5 and 9 seconds.

RESULTS

Latency and amplitude values of LEPs were found in accordance with those reported in the literature.

CONCLUSION

The 980-nm diode laser used is a suitable stimulator for LEP studies.

[Acute pain measurement in animals. Part 1]

OBJECTIVE

To describe tests of nociception which appear in the "pre-clinical" literature.

DATA SOURCES

References obtained by computerized bibliographic research (Medline) and the authors' personal data.

DATA SYNTHESIS

Ethical problems arising from the study of the pain in awake animals, problems arising from the choice of stimulus and stimulus parameters and the quantification of responses are presented. Pain in animals can be estimated only by examining their reactions, but at the same time, the existence of a reaction does not necessarily mean that there is a concomitant sensation. A description of the signs of pain in mammals is proposed. A noxious stimulus can be defined by its physical nature, its site of application and what has previously happened to the tissues at this site. Electrical stimulation short-circuits the process of transduction at free nerve endings and is not specific; however it has the advantage that it can be applied suddenly and briefly and thus results in synchronised signals in the relevant primary afferent fibres which can be differentiated into A delta and C fibres. Heat selectively stimulates thermoreceptors and nociceptors, but the low calorific power of conventional stimulators restricts their usefulness. Radiant sources have the disadvantage of emitting waves in the visible and the adjacent infrared spectra, for which the skin is a poor absorber and good reflector. Thermodes have the disadvantage of activating mechanoreceptors and thermoreceptors simultaneously; furthermore, their capacity for transferring heat depends on the quality of contact with skin and thus on the pressure with which they are applied. These problems can be overcome by using CO2 lasers but even today, the cost of these is a major disadvantage. Chemical stimuli differ from those mentioned above by the progressive onset of their effectiveness, their duration of action and the fact that they are of an inescapable nature. Experimental models employing chemical stimuli are undoubtedly the most similar to acute clinical pain. A wide spectrum of reactions are observed in nociceptive tests, but in almost every case they involve motor responses. After defining the ideal characteristics of a nociceptive test, tests based on the use of short duration and longer duration stimuli are presented. In tests of phasic pain, reactions are evoked by thermal (tail-flick test, hot-plate test), mechanical or electrical (flinch-jump test, vocalisation test) stimuli. Tests of tonic pain employ injections of algogenic agents intradermally (formalin test) or intraperitoneally (writhing test) or even the dilation of hollow organs. All these tests will be critically appraised in a subsequent paper [1].

CONCLUSION

The tail-flick and hot-plate tests are the most used, but there is an increasing recourse to the formalin test and tests involving foot withdrawal after mechanical stimulation.

Determination of nerve conduction velocity of C-fibres in humans from thermal thresholds to contact heat (thermode) and from evoked brain potentials to radiant heat (CO2 laser)

This study was designed to estimate and compare nerve conduction velocity (NCV) of cutaneous heat-sensitive C-fibres obtained using two methods. The first is a method based on reaction times to different rates of temperature change produced by a large contact thermode (Thermotest). The second is a novel method based on ultra-late-evoked brain potentials to CO2 laser stimuli with tiny beam sections (< 0.25 mm2), allowing selective and direct activation of very slow conducting afferents. Both methods were applied on three sites of the right leg (foot, knee and thigh) of ten healthy subjects. When based on the reaction times to contact heat, NCV estimations were 0.4 +/- 0.22 m/s for the proximal segment (knee-thigh) and 0.6 +/- 0.23 m/s for the distal segment (foot-knee). When based on the difference in latency of the ultra-late positivity of laser-evoked brain potentials, NCV estimations were respectively 1.4 +/- 0.77 m/s and 1.2 +/- 0.55 m/s. For both methods, the difference in NCV between proximal and distal limb segments was not significant. Although both methods give NCV estimations within the range of C-fibres, the systematic difference between NCV obtained from each method may result from the activation of subpopulations of C-fibres with different NCV depending on the method of stimulation (low-threshold thermal receptors by the thermode and thermal nociceptors by the CO2 laser). Considering the difficulty of investigating peripheral fibres with slow conduction velocities (C-fibres) in humans, the methods used in the present study may be useful tools in both experimental and clinical situations.

Pain perception: is there a role for primary somatosensory cortex?

Anatomical, physiological, and lesion data implicate multiple cortical regions in the complex experience of pain. These regions include primary and secondary somatosensory cortices, anterior cingulate cortex, insular cortex, and regions of the frontal cortex. Nevertheless, the role of different cortical areas in pain processing is controversial, particularly that of primary somatosensory cortex (S1). Human brain-imaging studies do not consistently reveal pain-related activation of S1, and older studies of cortical lesions and cortical stimulation in humans did not uncover a clear role of S1 in the pain experience. Whereas studies from a number of laboratories show that S1 is activated during the presentation of noxious stimuli as well as in association with some pathological pain states, others do not report such activation. Several factors may contribute to the different results among studies. First, we have evidence demonstrating that S1 activation is highly modulated by cognitive factors that alter pain perception, including attention and previous experience. Second, the precise somatotopic organization of S1 may lead to small focal activations, which are degraded by sulcal anatomical variability when averaging data across subjects. Third, the probable mixed excitatory and inhibitory effects of nociceptive input to S1 could be disparately represented in different experimental paradigms. Finally, statistical considerations are important in interpreting negative findings in S1. We conclude that, when these factors are taken into account, the bulk of the evidence now strongly supports a prominent and highly modulated role for S1 cortex in the sensory aspects of pain, including localization and discrimination of pain intensity.