Painful stimuli evoke different stimulus-response functions in the amygdala, prefrontal, insula and somatosensory cortex: a single-trial fMRI study

Only recently have neuroimaging studies moved away from describing regions activated by noxious stimuli and started to disentangle subprocesses within the nociceptive system. One approach to characterizing the role of individual regions is to record brain responses evoked by different stimulus intensities. We used such a parametric single-trial functional MRI design in combination with a thulium:yttrium-aluminium-granate infrared laser and investigated pain, stimulus intensity and stimulus awareness (i.e. pain-unrelated) responses in nine healthy volunteers. Four stimulus intensities, ranging from warm to painful (300-600 mJ), were applied in a randomized order and rated by the subjects on a five-point scale (P0-4). Regions in the dorsolateral prefrontal cortex and the intraparietal sulcus differentiated between P0 (not perceived) and P1 but exhibited no further signal increase with P2, and were related to stimulus perception and subsequent cognitive processing. Signal changes in the primary somatosensory cortex discriminated between non-painful trials (P0 and P1), linking this region to basic sensory processing. Pain-related regions in the secondary somatosensory cortex and insular cortex showed a response that did not distinguish between innocuous trials (P0 and P1) but showed a positive linear relationship with signal changes for painful trials (P2-4). This was also true for the amygdala, with the exception that, in P0 trials in which the stimulus was not perceived (i.e. 'uncertain' trials), the evoked signal changes were as great as in P3 trials, indicating that the amygdala is involved in coding 'uncertainty', as has been suggested previously in relation to classical conditioning.

Neurophysiological evaluation of pain

Neurophysiological techniques for the evaluation of pain in humans have made important advances in the last decade. A number of features of neuroanatomy and physiology of nociception qualifies pain as a multidimensional phenomenon which is rather unique among the sensory systems and which poses a number of technical and procedural requirements for its appropriate diagnostic assessment. Various stimulation techniques to induce defined pain in humans and used in combination with the methodology of evoked electrical brain potentials and magnetic fields are presented. Most recent knowledge gathered from scalp topography and dipole source analysis of pain-relevant evoked potentials and fields is discussed. Particular emphasis is put upon laser-evoked potentials and their application for diagnosis, pathophysiological description and monitoring of patients with neurological disorders and abnormal pain states. Future perspectives in this growing field of research are discussed briefly.

Subcortical structures involved in pain processing: evidence from single-trial fMRI

Pain is processed in multiple cortical and subcortical brain areas. Subcortical structures are substantially involved in different processes that are closely linked to pain processing, e.g. motor preparation, autonomic responses, affective components and learning. However, it is unclear to which extent nociceptive information is relayed to and processed in subcortical structures. We used single-trial functional magnetic resonance imaging (fMRI) to identify subcortical regions displaying hemodynamic responses to painful stimulation. Thulium-YAG (yttrium-aluminum-granate) laser evoked pain stimuli, which have no concomitant tactile component, were applied to either hand of healthy volunteers in a randomized order. This procedure allowed identification of areas displaying differential fMRI responses to right- and left-sided stimuli. Hippocampal complex, amygdala, red nucleus, brainstem and cerebellum were activated in response to painful stimuli. Structures related to the affective processing of pain showed bilateral activation, whereas structures involved in the generation of withdrawal behavior, namely red nucleus, putamen and cerebellum displayed differential (i.e. asymmetric) responses according to the side of stimulation. This suggests that spatial information about the nociceptive stimulus is made available in these structures for the guidance of defensive and withdrawal behavior.

Brain electrical source analysis of laser evoked potentials in response to painful trigeminal nerve stimulation

Cerebral generators of long latency brain potentials in response to painful heat stimuli were identified from potential distributions in 31 EEG leads, using the brain electrical source analysis (BESA) programme in the multiple spatio-temporal dipole mode. Data were taken from a study with 10 young healthy male subjects who participated in 3 identical sessions, 1 week apart, with 4 blocks of 40 stimuli (randomized intensities above mean pain threshold). Brief infrared laser heat pulses were applied to the right temple; laser evoked brain potentials (LEPs) were averaged over 40 stimuli per block. BESA was applied to the grand mean maps averaged over the 10 subjects, 3 sessions and 4 stimulus blocks per session, as well as to the individual maps. In all cases 4 generators could consistently be identified by BESA, which were able to explain up to 98.8% of the total variance in scalp distributions at certain time intervals: dipole I with a maximum activity at 106.3 msec in the contralateral somatosensory trigeminal cortex, 19.0 mm beneath the surface; dipole II with a maximum activity at 112.1 msec at the corresponding ipsilateral area at a depth of 13.6 mm; dipole III with a maximum activity at 130.4 msec in the frontal cortex; dipole IV with 2 relative maximum activities at 150.6 and 220.5 msec, localized centrally under the vertex at a depth of 33.1 mm, which described both the late vertex negativity and the consecutive positivity. BESA applied to the individual LEP maps of each individual and session yielded again 4 major generators with sites, strengths and orientations comparable to those of the grand mean evaluations. The standard deviation (S.D.) of site coordinates within subjects was less than 3 mm for dipoles I, II and IV (5 mm for dipole III). The between-subject standard deviation was considerably larger (15 mm), which was attributed to individual differences in head geometry, size and anatomy. Dipoles I and II are assumed to be generators in secondary somatosensory areas of the trigeminal nerve system with bilateral representation, though significantly stronger in the contralateral site. Dipole III in the frontal cortex may be related to attention and arousal processes, as well as to motor cortical initiation for eye movements and muscle effects. The central dipole IV describing all late activity between 150 and 220 msec is probably a representative of perceptual activation and cognitive information processing; it was located in deep midline brain structure, e.g., the cingular gyrus.

Late somatosensory evoked cerebral potentials in response to cutaneous heat stimuli

Late components of cerebral potentials evoked by brief heat pulses applied to various skin sites were used to monitor the afferent pathways of pain and temperature sensitivity. Radiation at 10.6 micron wave length generated by a CO2 laser stimulator predominantly activates superficial cutaneous A delta and C nociceptors and elicits late and ultralate cerebral potentials. This paper deals with the investigation of the component structure and topography of the A delta fibre mediated late potentials, which were compared with the corresponding late potentials in response to standard electrical nerve stimuli. In the upper limb both stimulus types evoked a large positive potential (nerve: 260 msec, skin: 390 msec latency), preceded by a negativity (nerve: 140 msec, skin: 250 msec). Whereas these components were always maximal at the vertex, an earlier negativity appeared over the somatosensory projection area (nerve: 70 msec, skin: 170 msec). After stimulation of the lower limb all latencies were delayed by 20-30 msec. As a rule, the heat-evoked potentials appeared about 100 msec later than the corresponding potentials after electrical nerve stimulation. Similarities in interpeak latencies and scalp topography indicated similar cerebral processing.

Responses of human cutaneous afferents to CO2 laser stimuli causing pain

Microelectroneurographic studies in man allow the comparison of stimulus induced activity in the single peripheral nerve unit with the subject's ratings of sensation. Relationships between stimulus intensity, single unit discharges, and pain ratings were investigated using a CO2 laser stimulator which delivers radiant heat pulses of 50 ms duration. Recordings were performed percutaneously from the radial nerve at the wrist. Receptor types were identified by their response to different stimulus modalities and by their reaction delay to electrical test stimuli within the receptive field. Receptive fields of identified units were stimulated with randomised series of different radiant heat intensities between half and double the individual pain threshold (5 to 20 W; stimulation area 64 mm2). The largest receptor class observed to be activated by CO2 laser stimuli were polymodal C-nociceptors. None of them was spontaneously active. High discharge rates up to 75/s were not necessarily associated with pain but, if pain was felt, the impulse trains usually lasted for more than 60 ms. Inter-spike intervals were distributed over a wide range between 8 and 145 ms with a peak at about 25 ms. This peak was only slightly shifted by increasing the stimulus intensity. Higher correlations were found between the number of spikes and stimulus intensity. Measures of Signal Detection Theory indicated that the single unit discharges discriminated stimulus intensities better than the subjects' ratings. These findings underline the importance of temporal summation in the processing of C-fibre input with a considerable loss of information in the nociceptive system.

Human cerebral potentials evoked by CO2 laser stimuli causing pain

Brief radiant heat pulses, generated by a CO2 laser, were used to activate slowly conducting afferents in the hairy skin in man. In order to isolate C-fibre responses a preferential A-fibre block was applied by pressure to the radial nerve at the wrist. Stimulus estimation and evoked cerebral potentials (EP), as well as reaction times, motor and sudomotor activity were recorded in response to each stimulus. With intact nerve, the single supra-threshold stimulus induced a double pain sensation: A first sharp and stinging component (mean reaction time 480 ms) was followed by a second burning component lasting for seconds (mean reaction time 1350 ms). Under A-fibre block only one sensation remained with characteristics and latencies of second pain. The heat pulse evoked potential consisted of a late vertex negativity at 240 ms (N240) followed by a prominent late positive peak at 370 ms (P370). Later activity was not reliably present. Under A-fibre block this late EP was replaced by an ultralate EP beyond 1000 ms, which in the conventional average looked like a slow halfwave of 800 ms duration. This potential was distinct from eye movements, skin potentials or muscle artefacts. With cross-correlation methods waveforms similar to the N240/P370 were detected in the latency range from 900 to 1500 ms during A-fibre block, indicating a much greater latency jitter of the ultralate EP. Latency corrected averaging with a modified Woody filter yielded a grand mean ultralate EP (N1050/P1250), the shape of which was surprisingly similar to the late EP (N240/P370). The similarity of these components indicates that both EPs may be secondary responses to afferent input into neural centers, onto which myelinated and unmyelinated fibres converge. Such convergence may also explain through the known mechanisms of short term habituation and selective attention, why ultralate EPs are not reliably present without peripheral nerve block.

Middle and long latency somatosensory evoked potentials after painful laser stimulation in patients with fibromyalgia syndrome

Ten female patients with fibromyalgia syndrome (FS) were investigated with laser evoked potentials (LEPs) after hand stimulations and compared with 10 female pain-free and age-matched control patients. FS patients exhibited significantly lower heat pain thresholds than controls (P < 0.05) and had higher amplitudes of LEP components N170 (P < 0.01) and P390 (P < 0.05) in response to intensities of 20 W (beam diameter 5 mm, duration 20 msec, wavelength 10.6 microns). N170 additionally appeared with a broader distribution over bilateral central, vertex and fronto-central leads which contrasted to the control group and studies in healthy subjects where N170 was much more restricted to central and midtemporal positions contralateral to the stimulated hand. Auditory stimuli interspersed between laser impulses that served to announce subjects to rate the perceived pain elicited auditory evoked potentials that were not different between groups indicating no differences of general vigilance level to account for observed LEP effects. P390 amplitude enhancement might indicate greater attention and cognitive processing of nociceptive stimuli in FS subjects. Effects upon N170 rather point to exogenous factors like peripheral and spinal sensitization or reduced cortical or subcortical inhibition of nociception.

Single trial fMRI reveals significant contralateral bias in responses to laser pain within thalamus and somatosensory cortices

Pain is processed in multiple brain areas, indicating the complexity of pain perception. The ability to locate pain plays a pivotal role in immediate defense and withdrawal behavior. However, how the brain localizes nociceptive information without additional information from somatotopically organized mechano-receptive pathways is not well understood. We used single-trial functional magnetic resonance imaging (fMRI) to assess hemodynamic responses to right and left painful stimulation. Thulium-YAG-(yttrium-aluminium-granate)-laser-evoked pain stimuli, without concomitant tactile component, were applied to either hand in a randomized order. A contralateral bias of the BOLD response was investigated to determine areas involved in the coding of the side of stimulation, which we observed in primary (SI) and secondary (SII) somatosensory cortex, insula, and the thalamus. This suggests that these structures provide spatial information of selective nociceptive stimuli. More importantly, this contralateral bias of activation allowed functionally segregated activations within the SII complex, the insula, and the thalamus. Only distinct subregions of the SII complex, the posterior insula and the lateral thalamus, but not the remaining SII complex, the anterior insula and the medial thalamus, showed a contralaterally biased representation of painful stimuli. This result supports the hypothesis that sensory-discriminative attributes of painful stimuli, such as those related to body side, are topospecifically represented within the forebrain projections of the nociceptive system and highlights the concept of functional segregation and specialization within these structures.

Evoked cerebral potential correlates of C-fibre activity in man

CO2 laser emitted radiant heat pulses of 20 ms duration were used to activate predominantly slowly conducting nociceptive cutaneous afferents in man. Stimuli of two-fold individual pain threshold caused stinging and burning pain and elicited cerebral potentials with latencies consistent with A delta-fibre activity. After preferential block of the myelinated nerve fibres by pressure only the burning pain remained with significantly increased reaction time (about 1433 ms). The A delta-fibre-induced evoked potential components disappeared, and a marked ultralate positive component became visible with mean peak latency of 1260 ms, consistent with C-fibre activity.

Cerebral potentials evoked by painful, laser stimuli in patients with syringomyelia

Brief cutaneous heat stimuli generated by a CO2 laser were used to elicit late somatosensory evoked cerebral potentials (SEPc) in 10 patients with syringomyelia. For comparison, early and late cerebral potentials in response to electrical nerve stimuli (SEPn) were recorded in the same session. In 8 patients with localized impairment of pain and temperature sensitivity we found complete absence of SEPc after stimulation of the affected area; in another patient with similar sensory deficits, the SEPc was grossly attenuated and delayed. In 1 patient with intact pain sensitivity but absent temperature sensitivity, a well defined SEPc could be recorded. Both early cortical SEPn and late SEPn in response to conventional nerve stimuli were normal in all patients and thus did not differentiate control and affected areas. These data indicate that alteration of SEPc correlates with altered pain sensitivity in patients with a circumscribed spinal lesion. SEPc may thus be used as a neurophysiological test in the assessment of hypalgesic dermatomes.

Effects of menthol and cold on histamine-induced itch and skin reactions in man

The effects of cooling and topical application of menthol on histamine-induced itch, wheal and flare reactions of the left lower arm were investigated in a threefold cross-over design with 15 healthy male volunteers. Lowering skin temperature by cooling from 32.8 +/- 0.3 degrees C to 29.7 +/- 0.5 degrees C reduced itch intensity from 260 +/- 47 units to 55 +/- 12 units (visual analogue scale) and flare diameters from 39.0 +/- 2.0 mm to 30.2 +/- 1.8 mm; wheal reactions were not affected. A similar reduction in itch was found under menthol (42 +/- 14 units) although skin temperature was not decreased. These findings suggest a central inhibitory effect of cold sensitive A-delta fibre activation on itch.

Principal component analysis of pain-related cerebral potentials to mechanical and electrical stimulation in man

Single trial event-related cerebral potentials (ERPs) in response to skin stimuli of various intensities and qualities in man were investigated in respect to their nociceptive information content. Electrical constant current stimuli (20 msec, 2 - 8 mA) and mechanical force controlled stimuli (20 msec, 0.8 - 3.2 N) were applied to the tip of the left middle finger. Four intensities of each stimulus quality were given, each intensity appearing 40 times in standardized randomized order. EEG segments (between 5 sec before and 500 msec after stimulus onset) were subjected to computer analysis. ERP wave form was shown to depend upon the amount of alpha waves in the prestimulus EEG. For analysis, only subjects with low power in the alpha band were selected. Principal component analysis was applied to all single trial ERPs measured using the variance-covariance matrix of association. Six principal components (PCs) were extracted accounting for about 90% of total variance. Five of the extracted PCs had well located loading maxima: PC1 (50 - 80 msec), PC4 (140 - 160 msec), PC3 (200 - 250 msec), PC4 (280 - 360 msec), PC5 (400 - 500 msec); PC6 appeared polyphasic. Analysis of variance of the mean PC scores revealed that one PC (PC1) discriminated between quality, and 4 PCs (PC1 - PC4) between quantity of stimulation. Eliminating effects of stimulus intensity resulted in two PCs (PC2, PC4) which distinguished exclusively between non-pain and pain. PCA applied to disjunctive subsets of ERPs, corresponding to the different experimental conditions, yielded practically identical sets of PCs, such that no specific ERP component emerged when pain was reported.

Pain related cerebral potentials: late and ultralate components

Brief CO2 laser radiant heat pulses activate both A delta- and C-fibres. In the evoked potential (EP) late and ultralate components can be seen as correlates of first and second pain. Usually the ultralate EP appears to be suppressed. It could be uncovered by a preferential A-fibre block, and in two neurological patients with tabes dorsalis and with a polyneuropathy involving myelinated fibre loss. Due to a strong latency jittering the shape of the ultralate component is distorted in the conventional average. Latency corrected averaging, adaptive filters or parametric spectral estimators are needed to analyze these EP components. As a result the filtered ultralate waveforms look very similar to the late EP components. Clinical application of CO2 laser EPs promises to nonivasively assess A delta- and C-fibre function.

The pain inhibiting pain effect: an electrophysiological study in humans

This study examines the counterirritation phenomenon of experimental pain in human subjects. Phasic pain induced by intracutaneous electrical stimuli was simultaneously applied with tonic pain induced by ischemic muscle work. Pain ratings, spontaneous EEG and evoked potentials were measured. We found a significant reduction of phasic pain ratings during and 10 min after tonic pain. The late somatosensory evoked potentials as neurophysiological correlates of phasic pain sensation were attenuated until 20 min after tonic pain offset. The extent of phasic pain relief due to concomitant tonic pain was small but significant, comparable to the effect of a regular systemic dose of a narco-analgesic drug in this experimental pain model. On the other hand, no modulations in the late components of the auditory evoked potential and the power spectrum of the spontaneous EEG were observed. These variables reflect the attention and vigilance of the subject and are well-known to be affected by opioids. The only exception was an increase of beta power, which might reflect hyperarousal during tonic pain. These results support the suggestion, that the analgesic effect of heterotopic noxious stimulation in humans is based on the activation of a specific inhibitory pain control system. Systemic release of endogenous opioids is unlikely to be involved, because the typical effects of opioids on the EEG were not observed.

New aspects of itch pathophysiology: component analysis of atopic itch using the 'Eppendorf Itch Questionnaire'

Itch represents a leading symptom in dermatological practice with many psychophysiological aspects. Instruments for qualitative registration of these central nervous factors and evaluation of therapeutic measures are still missing. We analyzed in detail the subjective itch sensation in 108 patients with acute atopic eczema with a new questionnaire developed in analogy to the McGill pain questionnaire. The descriptors with the highest load in atopic itch and the most frequent reaction patterns in atopic eczema patients were identified. Itch intensity (mean VAS 62%) and eczema severity (SCORAD mean 41 points) showed a different frequency distribution pattern with a correlation of r = 0.33 (p < 0.05). Principal component analysis of the itch questionnaire data was performed and compared with the standardized SCORAD severity index for the patients with atopic eczema. Three main factors of atopic itch explained 58% of the total variance: (1) 'suffering' (correlation with SCORAD, r = 0.6); (2) 'phasic intensity' (correlation with SCORAD, r = 0.4), and (3) 'ecstatic' component (associated with certain active reaction patterns). In conclusion, the complete description of itch has to consider different factors, which may be described on a more general level by three main components. Two of these are correlated with objective criteria of disease activity.

Pain-evoked blink reflex

The electrically evoked blink reflex (BR) consists of an ipsilateral R1 component (R1) at 11 ms and two bilateral components R2 at 33 ms and R3 at 83 ms. It is still unclear whether the R2 is mediated by activation of tactile or nociceptive afferents. For testing the nociceptive hypothesis, nociceptors of the supraorbital nerve were selectively activated by infrared laser stimuli in 10 subjects. Only painful laser stimuli evoked a bilateral early polyphasic BR response (LR2) at 71 ms. Stimulation of infraorbital and mental nerve dermatomes was equally effective. A late bilateral reflex response at 130 ms was occasionally observed. Regarding the nociceptor activation time of about 40 ms, onset latencies were within the range of the electrically evoked R2 and R3, respectively. The good accordance of R2 and LR2 may be due to activation of identical nociceptive fibers or to convergence of electrically evoked tactile and laser-elicited nociceptive input onto common multireceptive neurons.

Isoflurane induces dose-dependent changes of thalamic somatosensory information transfer

In spite of several reports about suppressive effects of volatile anesthetics on somatosensation, their neuronal mechanisms are largely unknown. The present study investigates somatosensory impulse transmission at the thalamic level in rats under varied concentrations of isoflurane by recordings of neuronal responses to mechanical stimulation of the body surface. Single-unit recordings of thalamo-cortical relay neurons (TCNs, third order neurons; n=28) and presumed trigemino-thalamic fibers (TTFs, second order neurons; n=7) were performed in the ventral posteromedial nucleus. Functional response characteristics were quantified following defined tactile stimulation (trapezoidal or vibratory deflection of sinus hairs or fur) applied to the neuronal receptive fields. End-tidal isoflurane concentration was increased in steps of 0.2% between 0.6% (baseline) and 2.0%. The response activity in all TCNs studied was suppressed in a dose-dependent manner (2.0% isoflurane decreased responses to 3. 5+/-1.1% of baseline; mean+/-S.E.M.); the response activity in TTFs was much less affected (decrease to 55.0+/-8.2%). Suppression of ongoing activity, however, was similar for both, TCNs and TTFs. Furthermore, in TCNs, the response characteristics changed with increasing isoflurane between 1.0% and 1.8%: tonic and sustained responses were converted to phasic on-responses. In contrast, the tonic and sustained response characteristics of TTFs were preserved even at higher isoflurane concentrations. The results indicate that isoflurane attenuates the output of somatosensory signals in the specific nucleus of the rat's thalamus, while its input is only marginally affected. The observed changes of thalamic neuronal response characteristics, at least in part, may cause the loss in sensory discrimination observed during general anesthesia.

Imipramine reduces experimental pain

In a homogeneous sample of 20 healthy male students, the analgesic effects of the tricyclic antidepressant imipramine (100 mg) were compared to those of the narcotic meperidine (150 mg) and a further tricyclic compound with assumed analgesic properties (fluradoline, 450 mg). Drugs were orally administered, using a placebo controlled, double-blind repeated measures Latin Square design. Phasic pain was induced by intracutaneous electrical shocks with random intensities and interstimulus intervals. Each stimulus block consisted of 80 stimuli and lasted for 20 min. Pain estimates, somatosensory evoked cerebral potentials (SSEPs) and power spectral density of the electroencephalogram (EEG) were measured under each drug condition. Under placebo, pain ratings and SSEP amplitudes were constant within the entire session lasting for approximately 4 h. Meperidine analgesia was evident within 30 min of drug application, reaching a maximum after about 90 min. Imipramine produced a comparable degree of pain reduction, however, with a delay of 2 h. Under both drugs, the decrease in pain ratings was accompanied by decreased amplitudes of the late components of the SSEP, as well as by a reduction in alpha activity and an enhancement of slow EEG waves. Effects of fluradoline on experimental pain could not be affirmed. These findings are discussed in terms of pain relief and decrease in vigilance.

Event-related potential correlates of interference between cognitive performance and tonic experimental pain

In this study, we examined cognitive function during experimental pain induced by an ischemic upper-arm tourniquet. During pain and control conditions, individuals performed a memory search task and an oddball task. Reaction time, errors, and event-related potentials in response to task stimuli were evaluated. Pain reduced accuracy and changed the response-type dependency of errors and the reaction time within the memory search task: false rejections but not false acceptances increased, and rejections were faster than acceptances during pain, whereas the opposite occurred during control conditions. The memory probes elicited an N275 that increased and a P300 that decreased in amplitude during pain. Pain also reduced amplitudes of P200 and P300 from the oddball task. N275 enhancement was greater in nonaffected than affected individuals, suggesting its association with focused attention that inhibited disruption by pain. P300 attenuation is interpreted as an indication that cognitive involvement in pain diminishes the attention resources allocated to a concurrent cognitive task.

Analgesic efficacy of low-dose ketamine. Somatosensory-evoked responses in relation to subjective pain ratings

BACKGROUND

Low-dose ketamine has been shown to exert analgesic effects. Whether ketamine-induced pain relief may be quantitated by somatosensory evoked cerebral potentials has not been established.

METHODS

Thirty healthy volunteers were assigned randomly to one of three groups. Subjects of group 1 (n = 10, control) were given saline as placebo. In groups 2 (n = 10) and 3 (n = 10), intravenous ketamine (0.25 mg. kg-1 and 0.50 mg. kg-1, respectively) was administered. The following variables were recorded at baseline and for 50 min after drug administration: electroencephalographic (EEG) data, somatosensory-evoked late cortical responses (SEP) elicited by intracutaneous stimulation of the fingertip (2-3 fold pain threshold), heart rate, mean arterial blood pressure, and end-tidal PETCO2 via a tight-fitting mask. Electroencephalographic spectral power in selected frequency bands and frequency percentiles were calculated from the spontaneous EEG segment preceding each somatosensory stimulus. Somatosensory-evoked late cortical response parameters were calculated from the respective poststimulus EEG segments. After recording of each EEG response, subjects were asked to rate the individual pain sensation.

RESULTS

In group 1, all variables did not change over time. Ketamine administration resulted in dose-dependent decreases in alpha-activity and increases in theta power (group 2: 190%, group 3: 440%). Electroencephalographic changes were not related to changes in pain perception. For the first 30 min after ketamine injection, a dose-dependent decrease of the long-latency N150-P250 somatosensory-evoked late cortical response component was observed (group 2: 15-20%; group 3: 25-30%). Subjective pain ratings were also different between groups, with a higher degree of pain relief in group 3 for the first 30 min. At the end of the observation period, pain relief and the N150-P250 amplitude were comparable in both ketamine groups.

CONCLUSIONS

These data indicate that pain relief induced by low-dose ketamine is dose-dependent for the first 30 min after bolus injection. Changes in pain perception may be quantitated by somatosensory-evoked cortical responses. Also, EEG changes are not specific for changes in nociception, but the increase in theta power may reflect the hypnotic effect of low-dose ketamine.

Modulation of pain-related somatosensory evoked potentials by general anesthesia

The aim of the present study was to assess if late somatosensory evoked cerebral potentials (SEPs) in response to painful electrical stimuli are a sensitive indicator for analgesic treatment during general anesthesia. For this purpose, a pain model developed for the quantification of drug-induced analgesia in awake volunteers was used in 10 patients scheduled for elective abdominal hysterectomy. Before induction of anesthesia, stimuli were adjusted to two and three times the pain threshold for each individual. Late auditory evoked potentials (AEPs, 30 dB hearing level) and spontaneous electroencephalogram were also evaluated. After control recordings, anesthetic treatments were varied in the following sequences: (a) 0.8% (end-tidal) halothane with 70% nitrous oxide (HN); (b) 0.8% halothane in oxygen (H1); (c) same anesthetic condition as in H1, but the SEP and AEP stimulus intensities were increased to 15 times pain threshold and to 70 dB hearing level, respectively (H2); and (d) fentanyl (0.25 mg) was given with 0.8% halothane in oxygen with no further change in stimulus intensities (HF). In treatments HN and H1, blood pressure and heart rate increases to pain stimuli were abolished, and SEPs and AEPs were both suppressed. Increasing the somatosensory stimulus intensity (treatment H2) stimulated heart rate and arterial pressure responses and again elicited the SEPs. However, AEP components remained suppressed with increased auditory stimulus intensity. Addition of fentanyl (HF) suppressed SEP amplitudes and stimulus-induced hemodynamic responses. Our results suggest that late SEPs in response to painful stimuli change with different analgesic levels.

Response plasticity of pain evoked reactions in man

Pain estimation (E), evoked cerebral potential (EP), electrooculogram (EOG), electromyogram of withdrawal reflex activity (EMG) and skin conductance reaction (SCR) were measured in response to electrical skin stimuli in 11 male volunteers (age 21-31 years). Constant current stimuli (20 msec; 2, 3, 6, 10 mA) were applied to a finger tip (averaged pain threshold at 5.1 mA). Interstimulus intervals (20-40 sec) and stimulus intensities were delivered in quasi-randomized order, each intensity appearing 40 times per session. Four sessions were repeated with intervals of two days between sessions. With randomized stimulus intensities, power spectral density functions of the prestimulus EEG indicated a stable non-specific arousal level between and within sessions. Under these conditions no significant response plasticity was found for E and EP, not in the prepain or the pain range, and also not between or within sessions. In contrast, the amplitudes of EOG, EMG and SCR decreased drastically with time, especially between the first and second session, and between the first 10 and second 10 stimuli of equal intensity in each session. EP and E remained highly correlated in repeated sessions. A EOG-EP contamination could be ruled out because of their different time course to repeated stimuli.

CO2 laser radiant heat pulses activate C nociceptors in man

Cerebral potentials evoked by noxious CO2 laser stimuli in man have been referred to nociceptive A delta units. This paper shows 1) that ultra short (5 - 50 ms) high power (less than or equal to 50 W) CO2 laser skin stimuli are able to activate nociceptive C units in man, and 2) that these C nociceptors have to be assumed to terminate in the very superficial skin layer (less than or equal to 300 microns depth).