Somatosensory & motor research
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Comparative Study
Pain additivity, diffuse noxious inhibitory controls, and attention: a functional measurement analysis.
This study utilized the methodology of Functional Measurement theory to investigate the additivity of painful and non-painful thermally induced experiences at one body site with those produced by brief noxious and innocuous electrical stimuli at another. Forty healthy young subjects were tested, using a Peltier thermode to induce tonic pain and an electrocutaneous stimulator for presenting phasic pain, under conditions of either full attention or visual/cognitive distraction (counting numerous light signals) in order to evaluate whether the summed effects are attributable to refocused attention. Six levels of intensity were combined in a factorial design for both tonic and phasic pain. ⋯ Distraction had a very small effect, suggesting that the "pain inhibits pain" phenomenon attributable to diffuse noxious inhibitory controls (DNIC) is not due to attentional processes. Our data also relate to issues regarding spatial summation across dermatomes and to adaptation level effects in pain, in which a strong painful experience serves as an anchor or comparison point by which others are judged. The psychophysical findings provide a perceptual foundation for clinical phenomena in which patients face with comorbid pain disorders.
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Two neuroimaging studies using fMRI were conducted in order to assess the cortical processes involved in the perception and suppression of pain. In the first study, 15 healthy subjects were stimulated with variable intensities of electrical pulses during a discrimination task. In the second study, the same subjects had to try to suppress the feeling of pain during tonic stimulation. ⋯ During this suppression task, activations changed from anterior to posterior insula; also there was a suppression of activity in the anterior cingulated cortex (ACC) and caudate nucleus. Subjects seem to be able to suppress to a certain degree the feeling of pain under constant (and previously painful) stimulation. The cortical correlate seems to be a shift of cerebral activation from anterior to posterior right insula and a suppression of activity in the ACC and caudate nucleus.
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The longstanding question of whether temperature is sensed via separate sensory systems for warmth and cold was investigated by measuring individual differences in perception of nonpainful heating and cooling. Sixty-two subjects gave separate ratings of the intensity of thermal sensations (warmth, cold) and nociceptive sensations (burning/stinging/pricking) produced by cooling (29 degrees C) or heating (37 degrees C) local regions of the forearm. Stimuli were delivered via a 4 x 4 array of 8 mm x 8 mm Peltier thermoelectric modules that enabled test temperatures to be presented sequentially to individual modules or simultaneously to the full array. ⋯ Intensity ratings for individual modules indicated that the number of responsive sites out of 16 was a poor predictor of temperature sensations but a significant predictor of nociceptive sensations. The very high correlation between ratings of thermal sensations conflicts with the classical view that warmth and cold are mediated by separate thermal modalities and implies that warm-sensitive and cold-sensitive spinothalamic pathways converge and undergo joint modulation in the central nervous system. Integration of thermal stimulation from the skin and body core within the thermoregulatory system is suggested as the possible source of this convergence.
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The spatial distribution of pressure sensitivity and muscle hardness was examined on normal muscle tissue and muscle tissue after induction of delayed onset muscle soreness (DOMS). The pressure sensitivity and muscle hardness were assessed at nine sites on the tibialis muscle from the proximal to distal tendon on two separate days. In total 37 healthy volunteers participated in three experiments. ⋯ In conclusion, within subjects the pressure sensitivity varies along the musculoskeletal unit. In DOMS, specific muscle belly sites were more sensitive to pressure stimulation. Muscle-tendon sites were harder compared to muscle belly sites.
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Comparative Study Clinical Trial
Sex differences in nociceptive withdrawal reflex and pain perception.
Experimentally induced pain often reveals sex differences, with higher pain sensitivity in females. The degree of differences has been shown to depend on the stimulation and assessment methods. Since sex differences in pain develop anywhere along the physiological and psychological components of the nociceptive system, we intended to compare the nociceptive flexion reflex (NFR) as a more physiological (spinal) aspect of pain procession to the verbal pain report of intensity and unpleasantness as the more psychological (cortical) aspect. ⋯ Females exhibited marked lower nociceptive flexion reflex thresholds than males, while the supra-threshold reflex response tailored to the individual reflex threshold did not show any significant differences. The verbal pain ratings, corrected for NFR threshold, were not found to differ significantly. The large sex differences in nociception that were present in NFR threshold but not in the pain ratings corroborate the hypothesis that spinal processes contribute substantially to sex differences in pain procession.