The detectability, discriminability, and perceived magnitude of painful electrical shock

Master scaling of perceived intensity of touch, cold and warmth

A new approach is presented for scaling perceived intensity of touch, cold and warmth based on magnitude estimation. In this method named master scaling thenar is utilized as common reference area for scaling and calibrating perceived intensity. The master scaling is particularly well suited for clinical applications in which the stimulation in pain-affected body areas creates a complex perception (e.g., paradoxical heat for cold stimulation) and/or aberrant psychophysical functions for perceived intensity. The results from three different experiments showed that: (a) All patients and healthy subjects were able to scale adequately the perceived intensity of touch, cold, and warmth at unaffected body areas. (b) Thenar stimulations were shown to be adequate common references in the joint scaling of perceived intensity of other body areas in pain patients as well as healthy persons. (c) Individual thenar psychophysical functions can be used for screening patients and healthy persons with regard to their ability to scale perceived intensity of touch, cold and warmth. (d) Master scaled perceived intensity scales can be used for determining if various pain-unaffected body areas are normal or abnormal in patients and in healthy persons. (e) The interindividual variation in perceived intensity is considerably reduced after master scaling and approaches that of intraindividual variation as found in olfaction and hearing. Finally, empirically based thenar Master Functions of perceived intensity for touch, cold and warmth are proposed to be used in future sensory testing of patients, as well as of healthy persons.

Elevating pain thresholds in humans using depolarizing prepulses

Electrocutaneous stimulation is a potentially useful communication tool for applications in virtual reality, sensory substitution, and sensory augmentation. Many of these applications require the use of arrays of small electrodes. Stimulation through small electrodes is often painful, however, limiting the practicality of such arrays. The purpose of this study was to test a method for elevating the pain threshold to electrocutaneous stimulation through small (1-mm diameter) electrodes on the fingertip. We hypothesized that long, subthreshold, depolarizing prepulses (PP) would elevate the pain threshold so that a subsequent stimulus pulse (SP) would be less likely to be painful. We used psychophysical methods to measure the probability that an SP would be perceived as painful both by itself and when preceded by a PP that was 2, 4, 6, 8, or 10 dB lower in amplitude than the SP. We found that the PPs significantly increased the pain threshold, reducing the likelihood that the SP was painful (p < .0001). The dose effect of PP amplitude was also highly significant (p < .0001), with larger PPs elevating pain thresholds more. To our knowledge, this is the first report of PPs being used to elevate electrical stimulation thresholds in humans. PPs may be useful for selective inactivation of neural subpopulations in many human neuroprosthetic applications.

Saccades guided by somatosensory stimuli

The accuracy of somatosensory saccades defined by proprioceptive cues with and without an additional tactile stimulus was investigated over a wide range of stimulus amplitudes in 16 normal subjects. The present results confirm that somatosensory saccades are less accurate and more variable than visual saccades. Accuracy was minimal for saccades directed to hand distances of 40-50 deg and increased for larger stimulus amplitudes. The additional application of a tactile cue on the fingertip was not found to influence the accuracy of somatosensory saccades significantly.

Race and sex differences in cutaneous pain perception

OBJECTIVE

The purpose of this study was to determine race and sex differences in cutaneous pain perception.

METHODS

Pain perception was measured using a suprathreshold evaluation of pain intensity and pain unpleasantness to a series of thermal stimuli in 27 whites (14 men and 13 women) and 24 African Americans (12 men and 12 women). Blood pressure, depressive symptoms, anxiety state levels, and negative mood were assessed before pain testing to examine whether they might account for any sex or race differences in pain perception that emerged.

RESULTS

African Americans rated the stimuli as more unpleasant and showed a tendency to rate it as more intense than whites. Women showed a tendency to rate the stimuli as more unpleasant and more intense than men. In addition, systolic blood pressure was inversely related to pain intensity. After statistically adjusting for systolic blood pressure, sex differences in pain unpleasantness were reduced and sex differences in pain intensity were abolished; race differences were unaltered.

CONCLUSIONS

These differences in pain perception may be associated with different pain mechanisms: in the ease of sex, differences in opioid activity and baroreceptor-regulated pain systems; in the case of race, unmeasured psychological characteristics are suggested by the larger differences in ratings of pain unpleasantness than pain intensity.

Experimentally induced pain perception in men and women in the morning and evening

The literature regarding whether or not there are diurnal differences in pain perception in men and women is equivocal. The purpose of this study was to examine the influence of time of day on experimentally induced pain threshold in men and women. A secondary purpose was to measure selected psychological and physiological responses. Pressure (3000 gm force) was applied to the middle digit of the left forefinger for 2-min with the Forgione-Barber pain stimulator. Twenty-nine volunteers (women = 14; men = 15) completed two randomly assigned sessions between 6.00-8.00 in the AM and PM. Selected psychological variables (STAI,POMS) and physiological variables (BP, HR, TEMP) were assessed before application of the pressure stimulus. Data were analyzed with a 2x2 ANOVA. Results indicated that men had significantly higher (p<.05) systolic blood pressure and pain thresholds than women however, there was not a significant time of day effect for pain threshold. Significant time of day effects (p<.05) were found for systolic blood pressure and tympanic temperature. Heart rate, and tympanic temperature were found to be significantly higher (p<.05) in women in comparison to men. It is concluded that pain threshold did not differ in the AM and PM. Furthermore, men were found to have higher pain thresholds compared to the women.

Peripheral nerve excitation and plantar flexion force elicited by electrical stimulation in males and females

STUDY DESIGN

Two-group (gender) posttest only using a sample of convenience.

OBJECTIVES

Our study examined the effect of gender on selected stimulus properties, perceived pain thresholds, and maximally tolerated level of contraction of the plantar flexor muscle group.

BACKGROUND

There is a dearth of literature regarding gender as a factor that may influence the outcome following the application of neuromuscular electrical stimulation. Data from other disciplines suggest that males and females may not have similar tolerance to electrical stimulation.

METHODS AND MEASURES

Eleven women (mean age, 28.3 years +/- 5.6 years) and 9 men (mean age, 33.2 years +/- 6 years) participated in a single session of electrical stimulation. A pair of 4.5 x 4.5 cm, self-adhesive, synthetic, polymer electrodes was placed over the medial and lateral heads of the gastrocsoleus muscle group. Electrical stimulation was provided by a battery-powered pulsatile stimulator generating a symmetric biphasic waveform, 200-mu second phase duration, and pulse rate of 50 pulses per second. During testing, the subjects were seated on a specially constructed chair that incorporated a foot pedal attached to a piezoelectric force transducer to measure plantar flexion force. Stimulation amplitude was increased slowly until 4 thresholds (sensory, motor, pain, and maximal pain) were sequentially achieved. At each threshold, stimulus peak voltage, peak current, and phase charge used to elicit that threshold were recorded. Plantar flexion force was also recorded when stimulation was sufficient to achieve pain and maximal pain thresholds.

RESULTS

The peak voltage, peak current, and phase charge needed to elicit the 4 thresholds did not differ between women and men at any threshold examined (2-factor ANOVA; Newman-Keuls post hoc tests). However, significantly higher stimulus peak voltage, peak current, and phase charge were recorded for each of the 4 thresholds (sensory, motor, pain, and maximal pain tolerance) for both groups. The plantar flexion force elicited by electrical stimulation was significantly lower in female subjects than in male subjects at both pain threshold and maximally tolerated stimulation levels.

CONCLUSIONS

Gender can influence the magnitude of electrically induced plantar flexion contraction force. We hypothesize that females may require longer conditioning periods to achieve therapeutic levels of muscle contraction.

The prevalence and perception of pain amongst hospital in-patients

The aim of this paper is to describe the prevalence and perceptions of pain and pain management amongst hospital in-patients. A cross-sectional descriptive survey of 205 patients was conducted. Presence and severity of pain was assessed using verbal descriptor and visual analogue scales, and perceptions of pain were assessed using multi-item scales. Although the severity of pain reported was consistent across age groups and clinical areas, women in the study sample were significantly more likely to report high levels of pain than men. Differences in how men and women communicate their pain were observed, with women indicating that they were less willing to ask for help with their pain. Results suggest that pain continues to be an important problem for a large number of men and women in hospital, and that the experience of pain impacts negatively upon their well-being. Gender differences in the experience of and response to pain remain important considerations for clinical nurses who have major responsibilities for the management of pain in hospitalized patients.

The influence of film-induced mood on pain perception

It has been shown that a person's mood can influence pain tolerance. Films have been used as a means of inducing a desired mood. The effect on pain perception of film type and film length to induce mood was investigated. Previous research with brief humorous films had not indicated any unique advantage of humor over distraction approaches. Other recent research had indicated that after exposure to film stimulation there is a need to wait approximately 40 min before physiological changes can be obtained. Thus, the present study varied both film type and length and introduced a 30-min waiting period following the mood induction via film prior to exposure to cold-pressor pain. Two hundred subjects in nine different groups participated in the study. Three types of films were used: (1) humorous, (2) holocaust, (3) neutral. Three lengths of each type were also used: 15 min, 30 min, and 45 min. In addition, a tenth no-film group served as a control for the effects of a film. Each subject was given a baseline trial of cold-pressor pain, a trial immediately following the film and a trial 30 min later. Results indicated an advantage in increased pain tolerance for the humorous film and an increased pain tolerance for the longer film regardless of type only after the 30-min waiting period. Results were discussed from a pain theoretical perspective with emphasis placed on returning to psychological manipulations of the sensory aspects of pain and not just the cognitive/emotional/motivational dimensions.

Sex differences in the perception of noxious experimental stimuli: a meta-analysis

Fillingim and Maixner (Fillingim, R.B. and Maixner, W., Pain Forum, 4(4) (1995) 209-221) recently reviewed the body of literature examining possible sex differences in responses to experimentally induced noxious stimulation. Using a 'box score' methodology, they concluded the literature supports sex differences in response to noxious stimuli, with females displaying greater sensitivity. However, Berkley (Berkley, K.J., Pain Forum, 4(4) (1995) 225-227) suggested the failure of a number of studies to reach statistical significance suggests the effect may be small and of little practical significance. This study used meta-analytic methodology to provide quantitative evidence to address the question of the magnitude of these sex differences in response to experimentally induced pain. We found the effect size to range from large to moderate, depending on whether threshold or tolerance were measured and which method of stimulus administration was used. The values for pressure pain and electrical stimulation, for both threshold and tolerance measures, were the largest. For studies employing a threshold measure, the effect for thermal pain was smaller and more variable. The failures to reject the null hypothesis in a number of these studies appear to have been a function of lack of power from an insufficient number of subjects. Given the estimated effect size of 0.55 threshold or 0.57 for tolerance, 41 subjects per group are necessary to provide adequate power (0.70) to test for this difference. Of the 34 studies reviewed by Fillingim and Maixner, only seven were conducted with groups of this magnitude. The results of this study compels to caution authors to obtain adequate sample sizes and hope that this meta-analytic review can aid in the determination of sample size for future studies.

Sensory changes and pain after abdominal hysterectomy: a comparison of anesthetic supplementation with fentanyl versus magnesium or ketamine

Drugs interacting with opioid or N-methyl-D-aspartate (NMDA) receptors may have differing effects on post-surgical sensory changes, such as central inhibition or spinal excitation. We compared the effect of supplementing isoflurane/N2O/O2 anesthesia with an opioid agonist (fentanyl [n = 15]) or two drugs inhibiting the NMDA system differently (magnesium, ketamine [n = 15 in each group]) on sensory changes after abdominal hysterectomy. Electric sensation, pain detection, and pain tolerance thresholds were determined (preoperatively and 1, 4, 24 h, and 5 days postoperatively) in arm, thoracic, incision, and leg dermatomes together with pain scores and cumulative morphine consumption. Thresholds relative to the arm were derived to unmask segmental sensory changes hidden by generalized changes. Absolute thresholds were increased 1-24 h, returning to baseline on Day 5, without overall differences among drugs. Fentanyl thresholds were lower 1 h and higher 5 days postoperatively compared with magnesium and ketamine; thresholds were lower at 24 h for magnesium versus ketamine. Relative thresholds increased compared with baseline only with fentanyl (1-4 h); none decreased. Pain scores and morphine consumption were similar. Thus, all adjuvants suppressed spinal sensitization after surgery. Fentanyl showed the most, and magnesium the least, central sensory inhibition up to 5 days postoperatively, with different patterns of inhibition directly postsurgery versus later. Differences in sensory processing were not reflected in clinical measures.

IMPLICATIONS

We studied the effects on postsurgical sensory processing of general anesthesia supplemented by drugs affecting opioid or N-methyl-D-aspartate receptors using sensory thresholds. Generalized central sensory inhibition, differently affected by the drugs, predominated after surgery. All drugs suppressed spinal excitation. Clinical pain measures did not reflect sensory change.

Objective evidence of decreased pain perception in normotensives at risk for hypertension

Results from laboratory and naturalistic studies have demonstrated decreased subjective pain ratings in hypertensives and individuals at risk for hypertension. Based on previous evidence that the nociceptive withdrawal reflex may provide an objective index of pain threshold in humans, the present study examined the intensity of sural nerve stimulation required to elicit nociceptive withdrawal in offspring of hypertensives and normotensives. Participants included 60 men and 56 women who were normotensive, 18-23 years of age, and predominately Caucasian. To assess the nociceptive withdrawal reflex, ascending and descending intensities of electrical stimulation were applied over the sural nerve while electromyographic activity was recorded from the ipsilateral biceps femoris muscle. Analyses of the intensity of electrical stimulation required to reach the thresholds for nociceptive withdrawal and subjective pain revealed a pattern of hypoalgesia in individuals at risk for hypertension. First, significantly higher intensities were required to elicit nociceptive withdrawal in offspring of hypertensives versus normotensives. Second, offspring of hypertensives endured significantly more intense stimulation before reporting pain. Third, both parental history of hypertension and resting systolic blood pressure were significant independent predictors of stimulation intensity at nociceptive withdrawal reflex and subjective pain thresholds. These results confirm and extend previous observations of an association between risk for hypertension and hypoalgesia, and suggest that hypoalgesia should be examined as a potential predictor of progressive blood pressure increases in individuals at risk for hypertension.

The influence of resting blood pressure and gender on pain responses

Recent research suggests that resting blood pressure is inversely related to pain sensitivity, even among normotensives; however, most of these studies have included only male participants. To determine whether this hypoalgesic effect of blood pressure was also present in females, we investigated thermal and ischemic pain responses in a group of age-matched, normotensive females and males as a function of resting blood pressure. Thermal pain threshold and tolerance were determined, and a cross-modality thermal magnitude matching procedure was conducted, after which ischemic pain threshold and tolerance were determined using the submaximal effort tourniquet procedure. Systolic pressure, diastolic pressure, and heart rate were obtained using an automated blood pressure monitor with a pneumatic cuff positioned around the left ankle. Females provided higher normalized thermal magnitude estimates and a shorter time to ischemic pain tolerance, but no gender differences emerged on other pain measures. Systolic, diastolic, and mean arterial pressures were significantly correlated with thermal and ischemic pain responses among males but not females, with higher blood pressure being associated with lower pain sensitivity. After adjusting for resting blood pressure, the gender difference in normalized magnitude estimates was only marginally significant, and the gender difference in ischemic pain tolerance became nonsignificant. These findings are consistent with previous research indicating an inverse relationship between blood pressure and pain sensitivity. Additionally, the findings also suggest that blood pressure may partially moderate gender differences in pain sensitivity. Potential mechanisms and clinical implications of the current findings are discussed.

Gender differences in pain ratings and pupil reactions to painful pressure stimuli

In order to investigate gender differences in pain perception, the present study employed both a psychophysical and a psychophysiological measure. In experiment 1, 20 subjects rated the painfulness of 4 different levels of tonic pressure applied to their fingers using a verbally anchored categorization procedure. In general agreement with studies of pain threshold and tolerance, female subjects reported greater pain at high levels of stimulation, with no gender difference being evident at low pressure levels. In experiment 2, 16 different subjects were exposed to the same painful pressure stimuli while measuring their pupil reactions using infrared video pupillometry. The pupil dilations seen during the last 10 sec of the 20-sec pressure application turned out to be a highly significant indicator of pain intensity. When female and male subjects were compared on this measure, a similar divergent pattern as in the psychophysical data emerged, with female subjects showing greater pupil dilations at high pressure levels only. The fact that gender differences in pain perception can be demonstrated using an autonomic indicator of pain that is beyond voluntary control suggests that these differences reflect low-level sensory and/or affective components of pain rather than attitudinal or response-bias factors.

Sedation with intravenous infusions of propofol or thiopentone. Effects on pain perception

The aim of this study was to investigate pain perception during thiopentone or propofol infusions for sedation. Thirty ASA 1 or 2 patients received a two step infusion of either thiopentone (step 1: 1.25 mg.kg-1 followed by 2.5 mg.kg-1.h-1; step 2: 1.25 mg.kg-1 and 12.5 mg.kg-1.h-1; n = 15) or propofol (step 1: 0.5 mg.kg-1, 1 mg.kg-1.h-1; step 2: 0.5 mg.kg-1, 5 mg.kg-1.h-1; n = 15) for sedation. At control and 10 min after the start of each infusion dosage, reaction times and thermal pain detection thresholds were determined. We found no clinically or statistically significant depression of thermal pain detection thresholds during propofol or thiopentone infusions and these are, therefore, unlikely to be associated with clinically relevant hyperalgesia.

Baroreflex sensitivity during noxious stimulation in vasovagal reactors to blood donation

Baroreflex sensitivity was assessed at rest and during constrictive pain in healthy males with and without a history of vasovagal reactions to blood donation. Continuous recordings of cardiac inter-beat interval and finger arterial pressure were obtained at rest and during repeated presentations of constrictive thigh-cuff stimulation. Baroreflex sensitivity (in msec/mm Hg) was computed according to the spontaneous sequence analysis method. Results indicated that vasovagal reactors exhibited significantly lower descending baroreflex sensitivity across all periods, and significantly lower ascending baroreflex sensitivity at rest. Baseline differences in ascending baroreflex sensitivity were eliminated during thigh-cuff stimulation, as vasovagal reactors exhibited increases in ascending baroreflex sensitivity while non-reactors exhibited decreases. These findings suggest that susceptibility to vasovagal reactions may be associated with individual differences in baroreflex sensitivity.

Iontophoretically applied potassium ions as an experimental pain stimulus for investigating pain mechanisms

The present study investigated the psychophysical characteristics of potassium iontophoresis and its suitability as an experimental pain stimulus. Experiment 1 investigated the optimal duration of the pain stimulus for reliable reporting across repeated trials and whether the relationship between stimulus and subject response was linear, logarithmic, or a power function. In Experiment 2, the optimal interstimulus interval (ISI) was determined for reliable pain reporting, and stimulus history effects, both in terms of session effects and the effects of immediately preceding stimuli, were evaluated. In Experiment 3, potassium iontophoresis was compared with a sodium iontophoresis control. Linear functions described the stimulus-pain relationship best. No significant differences in the goodness-of-fit coefficients of determination, correlations, or coefficients of variation were found for the stimulus durations of 1, 2, and 4 sec. Significant stimulus history effects were found across a session, with adaptation and enhancement of responding for low- and moderate-intensity stimuli, respectively. The effects of the immediately preceding stimuli were suppression or enhancement of pain response, depending on the ISI, the preceding stimulus intensity, and the present stimulus intensity. Potassium iontophoresis was a significantly more effective pain stimulus than was sodium iontophoresis. It was concluded that potassium iontophoresis is a convenient and reliable experimental pain stimulus, which can be presented rapidly and repeatedly with minimal loss in consistency of subject pain report. Potassium iontophoresis provides a tool for investigating the neural modulation of pain in the relative absence of inflammation processes and tissue damage.

Psychophysics in the field: perception and memory for labor pain

Separate groups of women estimated the painfulness of labor contractions either immediately (perceptual judgments) or at time intervals varying from 8 to 48 h after their completion (memorial judgments). The pain judgments of individual uterine contractions were related functionally to the biometrically measured magnitude of these contractions. Perceived and remembered painful sensations were related to referent intensities by power functions governed, as a rule, by exponents greater than unity. Exponents for the memory functions were consistently greater than those for the perceptual functions. Psychophysics done in the field can help decide theoretical issues in addition to providing useful practical information.

Discriminability of electrocutaneous stimuli after topical anesthesia: detection-theory measurement of sensitivity to painful stimuli

In three experiments on the psychophysical measurement of pain, electrocutaneous currents were applied to the volar surface of the forearm. In the first experiment, a conventional category scaling method was compared with the rating method of signal detection. The results of both methods were analyzed in detection-theory terms to derive receiver operating characteristic curves and measures of the discriminability of adjacent currents. The rating method yielded larger discriminability values than the category scale did, and that method was therefore used in the subsequent experiments to examine the effect of a topical anesthetic on discriminability. When the stimuli were applied through surface electrodes, no effect of the topical anesthetic on discriminability was found, but when the stimuli were applied to a more localized area by intradermal needle electrodes, a dose-dependent effect of the anesthetic on discriminability occurred. For this experiment, the slope of the cumulative sensitivity function increased with increasing elapsed time since the removal of the anesthetic. This result is congruent with the theory that discriminability can serve as a measure of sensitivity to painful stimuli.

Electric shock, Part II: Nature and mechanisms of injury

Virtually every part of the body can be injured by electric current. The extent of injury to any given tissue will depend on many factors, including the nature of the tissue and the amount and duration of the electric current. In addition, cardiac and respiratory arrest can be induced by a number of mechanisms with little or no immediate tissue damage. Burns can be caused by the heating of tissue by electric current and by other mechanisms. Secondary trauma may result from falls, explosions, and other events initiated by electric forces. Nervous tissue has the least resistance to current flow and is thus more easily damaged. Part II in this 3-part series will discuss nervous tissue damage first, followed by discussions of damage to tissues of increasingly greater electric resistance. These are blood vessels, muscle, skin, and bone. Less common injuries are discussed last.

A noninvasive strategy for identifying and quantifying innocuous and nociceptive peripheral afferent activity evoked by nerve stimulation

This study evaluated the utility of the compound nerve action potential (CAP) and spinal nociceptive withdrawal (R3) reflex in identifying and quantifying peripheral afferent activity evoked by sural nerve stimulation in humans. The results of this work demonstrate that currents less than or equal to that which elicits a just-maximal CAP can be considered purely innocuous; provide further evidence that the R3 is an objective means of identifying noxious stimulus levels; and suggest that current provides the best noninvasive quantitative estimate of afferent activity throughout the innocuous and noxious range. This work also demonstrates that some of the individual variability in the psychophysical function can be attributed to peripheral factors that affect the amount of current reaching the nerve. It is important, therefore, that these peripheral factors be considered when studying individual differences in the psychophysical function generated by electrical stimulation.

Sex differences in responsiveness to painful and non-painful stimuli are dependent upon the stimulation method

Sex differences in thermo- and electrocutaneous responsiveness to painful and non-painful stimuli were investigated in 20 women and 20 men. Heat pain, warmth, and cold thresholds were assessed on the hand and foot with a Peltier thermode system. In addition, subjects used magnitude estimation to judge the sensation intensity evoked by temperatures ranging from 38 degrees C to 48 degrees C applied to the forearm. To measure detection, pain, and tolerance thresholds of electrocutaneous sensitivity, electrical pulses were administered to the hand. Magnitude estimates of sensation intensity were assessed for stimuli ranging from 0.5 mA to 4.0 mA. There were no sex differences in heat pain, warmth and cold thresholds. There were significant sex differences in electrical detection, pain and tolerance thresholds, with lower thresholds in women. Correspondingly, magnitude estimates were similar in women and men when using thermal stimuli while women judged stimuli from 2.5 mA on as more intense than men when using electrical stimuli. Despite these discrepancies, the measures for pain responsiveness from the two stimulation methods correlated significantly. In contrast, no significant correlations between the methods were found when considering the responsiveness to non-painful stimuli. The findings help to clarify controversies in the pain literature about sex differences. Results affirming and denying such differences could be obtained within a single sample, with stimulation method as the critical variable.

Individual differences in perceived pinch force and bite force

Six subjects made cross-modal matches of pinch force and bite force to an electrocutaneous stimulus. The electrocutaneous stimulus consisted of bursts of pulses, and the intensity of the stimulus was varied by changing the number of pulses per burst. All of the individual matching functions were fit well by power functions. The scaling constants and exponents of the power functions covaried systematically with the maximum pinch force for 5 of the 6 subjects. The relationship was consistent with the hypothesis that subjects perceive their physical maxima equally, in agreement with Borg's theory of relative perceived exertion. For both pinch force and bite force functions, the scale factors could be described by a single linear function of the exponents, suggesting that all of the matching functions converged at a single point with extreme values.

Psychophysical features of the transition from pure heat perception to heat pain perception

The psychophysical features of the transition from the pure heat to the heat pain range were studied in 25 healthy subjects (mean age 28.8 years). Thirty short heat stimuli from -1.6 degrees C to +1.6 degrees C relative to the pain threshold were applied to the thenar of the left hand with an apparatus containing a Peltier thermode (nine different temperatures at 0.4 degrees C intervals). The subjects rated the sensation intensity on a visual analogue scale. The resulting stimulus/sensation intensity relations could be explained equally well (same goodness of fit) by a model with a power function (PF) and by a model with two linear regression lines (TLR), one for stimulus intensities below and one for those above the pain threshold and intersecting at the pain threshold. The slopes of the TLR model were significantly larger above the pain threshold than below it. The PF model produced exponents between 1.8 and 1.9. We conclude that to describe the transition area, it is sufficient to use simple linear models for both the pure heat and the heat pain ranges.

Habituation to repeated painful and non-painful cutaneous stimuli: a quantitative psychophysical study

Repeated stimuli elicit progressively smaller responses and elevated sensory and/or pain thresholds (habituation). The present experiments were designed to determine the rate of habituation of perceptual responses to supraliminal painful and non-painful cutaneous stimuli. Changes in the perceived intensity of electrical stimuli applied to the digital nerves of the index finger were determined by a matching procedure in which subjects set the current applied to the index finger of one hand to match the perceived intensity of a stimulus train (5 pulses at 20 Hz) applied to the other index finger. Twenty-five volunteers took part in 7 experiments in which both non-painful (2.5 times the sensory threshold Ts) and painful (1.2 times the pain threshold Tp) stimulus trains were presented. Subjects were required to match the stimuli at 30 s intervals over a period of 7.5 min. The percentage change in matching current (Y) was fitted by the function Y = -20.7*[1-exp(-0.56*t)] for both painful and non-painful stimuli repeated at 2 Hz. Responses recovered completely within 2 min of cessation of the stimulation. The degree of habituation increased or decreased with the rate of stimulus presentation. These results did not depend on changes in afferent fibre recruitment or fatigue because the afferent volley on the median nerve remained constant throughout the period of stimulation. Thus perceptual responses to the perceived intensity of supraliminal painful and non-painful stimuli delivered to the index finger habituate to the same extent, and the extent of the habituation is a function of the frequency of presentation of the stimulus.

Sex differences in pain and thermal sensitivity: the role of body size

Sex differences in heat-pain and thermal sensitivity were investigated in 32 women (20 to 60 years of age) and 32 men (17 to 63 years of age) who had no somatosensory impairments. Pain thresholds were measured with stimuli of two different durations (phasic and tonic). Warmth and cold thresholds were assessed as indices of thermal sensitivity. Stimulation was applied to the hand and to the foot by an apparatus containing a Peltier thermode. There were no sex differences in heat-pain thresholds. Women had significantly lower warmth thresholds than men (more pronounced on the foot than on the hand), but similar cold thresholds. Measures of body size (weight, height) correlated much more strongly with thermal than with pain sensitivity, and helped to explain the sex difference in the warmth threshold. A reduction of sex differences to body-measure differences appears likely, but could not be demonstrated unequivocally.

Perceived locus and intensity of electrocutaneous stimulation

Two experiments investigated perceived locus and intensity for electrocutaneous stimulation. In Experiment 1, 21 subjects reported the perceived locus for various combinations of four electrode sites, two current directions, two pulse characteristics (single versus multiple), and two sensation levels (detection versus pain). In Experiment 2, 16 subjects reported the perceived locus and intensity for a wide range of current levels and two polarity conditions. The main results were 1) sensations were likely to be perceived under the cathode at detection levels, but under both electrodes at intense levels; 2) the "cathode" localization was gradually supplanted by "both" ("anode" and "cathode") localization with increasing current; 3) subjective intensity under the cathode was greater than that under the anode; 4) the effects of cathode position on perceived locus were found for only some pairs of electrodes. These results challenge the simple hypothesis that electrical stimulation of the skin through paired electrodes is perceived under the cathode.

Sex differences in the perception of noxious heat stimuli

This study compared pain perception in young male and female subjects, using experimental noxious heat stimuli. During 2 sessions, each of 40 subjects rated the magnitude of 120 heat stimuli, ranging from 45 degrees C to 50 degrees C. The study included a comparison of visual analogue and magnitude matching rating procedures, as well as a test of simulated analgesia, in which the range of stimuli presented during the 2 experimental sessions was shifted by 1 degree C. We found that females rated noxious heat stimuli as more intense than did males, independent of the gender of the experimenter or the type of rating scale. In addition, the data suggest that females discriminate among the painful heat intensities better than males. For example, female subjects showed significant between-session discrimination of noxious heat stimuli, while male subjects did not, and females produced steeper within-session stimulus-response functions than did males. These observed differences in nociceptive discrimination between males and females indicate that the sex-related variation in pain perception is probably related to sensory factors rather than differences in attitude or emotional response.

Magnitude estimates for electrical pulses: evidence for two neural mechanisms

The hypothesis that there are two neural mechanisms for electrocutaneous stimulation--one that is sensitive to low current and is adaptive to repeated stimulation and another that is responsive to high current and is less adaptive--was tested in a control and four main experiments. In the main experiments, magnitude estimates obtained for single electrical pulses (of 2-msec duration) were described by a simple power function for each combination of high- and low-current levels and 10 trial blocks. The results were: (1) The slope of the power function for low current was steeper than was that for high current; (2) for low current, the intercept of the power function decreased with increasing block, whereas for high current, it remained constant over blocks; (3) this decrease of the intercept for low current disappeared when judgmental blocks were separated by a rest period of 8 min; (4) the modulus did not affect the slope; (5) for a large modulus combined with low current, the intercept decreased rapidly over trial blocks, whereas for a small modulus combined with high current, the intercept increased over trial blocks. The first four findings support the two-mechanism hypothesis, but the last one may also be interpretable in terms of the regression to absolute scale values.