Spatial summation of heat pain in males and females

Sex differences in pain sensitivity have been found to vary between considerable and negligible. It has appeared that the pain stimulation method is critical in this context. It was assumed this might be due to the different degrees of spatial summation associated with the different pain stimulus modalities. Hence, sex differences were investigated in spatial summation of heat pain in 20 healthy women and 20 healthy men of similar age. Pain thresholds were assessed by a tracking procedure and responses to supra-threshold pain stimulation by numerical ratings. Heat stimuli were administered by a thermode with contact areas of 1, 3, 6 and 10 cm2. Pain thresholds were significantly higher with smaller areas stimulated than with larger ones. No significant effect of area was found for the ratings of the supra-threshold stimuli, the intensities of which were tailored to the individual pain threshold. Consequently, spatial summation of heat pain appeared to result mainly in a shift of the pain threshold on the ordinate and not a change of slope of the stimulus-response function in the pain range. In neither of the two pain parameters were there any sex differences. Therefore, the present study demonstrated that sex differences in spatial summation of heat pain are unlikely.     

Spatial summation of heat induced pain within and between dermatomes

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

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

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

Spatial summation in the tactile sensory system: Probability summation and neural integration

Psychophysical thresholds for the detection of a 300-Hz burst of vibration applied to the thenar eminence were measured for stimuli applied to the skin through 1.5?cm² and through 0.05?cm² contactors. Thresholds were approximately 13?dB lower when the area of the contactor was 1.5?cm² than when it was 0.05?cm². The difference between the thresholds measured with the large and small contactors was significantly reduced when only the lowest thresholds obtained in the testing sessions were considered. This result supports the hypothesis that one component of spatial summation in the P channel is probability summation. In addition, threshold measurements within a session were less variable when measured with the 1.5?cm² contactor. We conclude that spatial summation in the P channel is a joint function of two processes that occur as the areal extent of the stimulus increases: probability summation in which the probability of exceeding the psychophysical detection threshold increases as the number of receptors of varying sensitivities increases, and neural integration in which neural activity originating from separate receptors is combined within the central nervous system rendering the channel more sensitive to the stimulus.     

Spatial summation in the tactile sensory system: probability summation and neural integration

Psychophysical thresholds for the detection of a 300-Hz burst of vibration applied to the thenar eminence were measured for stimuli applied to the skin through 1.5 cm2 and through 0.05 cm2 contactors. Thresholds were approximately 13 dB lower when the area of the contactor was 1.5 cm2 than when it was 0.05 cm2. The difference between the thresholds measured with the large and small contactors was significantly reduced when only the lowest thresholds obtained in the testing sessions were considered. This result supports the hypothesis that one component of spatial summation in the P channel is probability summation. In addition, threshold measurements within a session were less variable when measured with the 1.5 cm2 contactor. We conclude that spatial summation in the P channel is a joint function of two processes that occur as the areal extent of the stimulus increases: probability summation in which the probability of exceeding the psychophysical detection threshold increases as the number of receptors of varying sensitivities increases, and neural integration in which neural activity originating from separate receptors is combined within the central nervous system rendering the channel more sensitive to the stimulus.     

Spatial pattern summation is phase-insensitive in the fovea but not in the periphery.

We measured the effect of phase coherence on the detectability of a chain of Gabor patches. The stimuli were elongated patterns consisting of 1-8 vertical Gabor patches, aligned vertically and either in phase or 180 deg out of phase from their immediate neighbors. In the fovea, the phase configuration had no effect on detection threshold, suggesting a full-wave rectification operation mediating foveal spatial summation. At 5 deg in the periphery, detection thresholds for the in-phase configuration were lower than those for the alternating-phase configuration, to an extent compatible with a half-wave rectification operation mediating peripheral spatial summation.     

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

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

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

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

Spatial summation in taste: NaCl thresholds and stimulated area on the anterior human tongue

Spatial summation has been demonstrated in several sensory modalities.In this study, spatial summation of sensitivity to NaCl wasinvestigated as a function of stimulated area. In a two-alternativeforced choice procedure, a filter paper stimulation method wasused to determine the sensitivity to, and the detection probabilitiesof, five low-intensity NaCl stimuli. Each of the stimuli waspresented 32 times in two conditions, a circular area (ø9 mm) and half that circle in four counterbalanced orientations.Only one side of the tongue was used. The results showed that(a) the summation found was partial; (b) the observed relationshipbetween threshold intensity and area obeyed Piper's Law, and(c) detection probabilities increased according to chance. Adhoc analysis of orientation yielded sensitivity differencesalong the vertical but not the horizontal axis.     

Lean mass mediates the relation between temporal summation of pain and sex in young healthy adults

Background

Previous studies have shown that women experience greater temporal summation (TS) of pain than men using a repetitive thermal stimulus. These studies, however, did not individualize the thermal stimulus to each subject’s thermal pain sensitivity. The aim of this study was to investigate sex differences in TS using an individualized protocol and potential mediators that have been shown to influence TS including physical activity and body composition.

Methods

Fifty young healthy men and women (21 men) participated in the study. Subjects completed TS testing on the right forearm using a repetitive thermal stimulus at a temperature that the subject reported 6/10 pain. Other testing included body composition (lunar iDXA), activity monitoring (Actigraph), and Pain Catastrophizing Scale (PCS).

Results

Women reported greater TS than men (p?=?0.019), and TS was correlated with right arm lean mass (r?=???0.36, p?=?0.01) and magnification subscale of PCS (r?=???0.32, p?=?0.03). Mediation analysis showed a complete mediation for the relation between sex and TS by right arm lean mass (indirect effect = 2.33, 95% BCa CI [0.42, 4.58]) after controlling for the temperature, the magnification subscale of PCS, and the average time spent in moderate to vigorous physical activity.

Conclusion

The results of this study suggest that lean mass is a contributing factor to the sex differences in TS. Future studies should investigate whether interventions that increase lean mass have a positive effect on TS.