Somatosensory & motor research
-
In the present study, we precisely and automatically measured the withdrawal latency to noxious radiant heat application in unrestrained male rats and in human subjects of both sexes, by means of the "plantar test" apparatus (Ugo Basile Biological Apparatus). The infrared light stimulus of this tool was applied underneath the hindpaws of rats and the middle fingers of human subjects. With one right and one left stimulation every 10 min, we observed a decrease in latency over a 40-min testing period in rats; the latency reached a mean value of 5.08 +/- 0.25 sec after 40 min with a 36-W stimulus, which corresponded to 46.5 degrees C. ⋯ Among humans, we noted gender differences, such as less sensitivity to the infrared noxious radiant heat for women, particularly during the menstrual period. A difference from rats was that there was no significant latency modification along the 40-min testing period for either women or men, with a mean latency of 5.61 +/- 0.18 sec (47.5 degrees C) for the women and 4.39 +/- 0.10 sec (45.5 degrees C) for the men. These data confirm the reliability of the plantar test in rats, and demonstrate the possible use of an infrared source in human subjects as a noxious heat stimulus; the withdrawal reaction to this stimulus is emphasized as a good index of nociception in humans.
-
The medial prefrontal cortex has been implicated in pain perception by recent anatomical, physiological, and functional imaging data demonstrating that frontal and anterior cingulate cortices receive inputs related to nociception; neurosurgical case reports suggest that lesions involving these areas may specifically reduce the affective or emotional component of chronic intractable pain. We examined this hypothesis more closely by assessing psychophysical ratings of (1) warmth, pain intensity, and unpleasantness evoked by phasic thermal stimuli, (2) tolerance to tonic cold stimuli, and (3) perceived intensity of visual stimuli, both before and after neurosurgical lesions of the fiber tracts connecting the frontal lobes to subcortical structures. A 22-year-old male, with no history of chronic pain, underwent psychophysical testing 3 days before, 5 days after, and 6 months after receiving bilateral lesions of the anterior internal capsule (aIC), performed as treatment for obsessive-compulsive disorder. ⋯ Magnetic resonance imaging 5 days following surgery revealed bilateral lesions and edema centered in the aIC, with some edema in the left frontal lobe. Those 6 months later showed substantially smaller lesions involving less than half of the aIC and no edema; pain ratings and cold-water tolerance measured at that time indicated a substantial return toward the patient's presurgical values. These data suggest that blocking subcortical input to the anterior cingulate and frontal cortices reduces both the perceived intensity and the unpleasantness of noxious stimuli; reduced cold tolerance times--in the face of decreased pain perception--may reflect a disinhibition of cortical control on spinal reflexes.(ABSTRACT TRUNCATED AT 400 WORDS)
-
The properties of a newly developed tonic heat pain model (THPM), which makes use of pulsating contact heat, were investigated in 18 young men. The most important feature of this model is that repetitive heat pulses with an intensity of 1 degree C above the individual pain threshold are employed. This approach was used to tailor the tonic pain stimulation to the individual pain sensitivity. ⋯ Moreover, there was absolutely no indication of a dichotomy between "pain-sensitive" and "pain-tolerant" individuals in the THPM, although such a dichotomy was evident in the CPT. This implies that the distinction between pain-sensitive and pain-tolerant individuals can be made only with the CPT, and that this distinction represents individual differences in peripheral vascular reactions to cold rather than in pain perception. In conclusion, the THPM appears to produce a stable and predictable temporal pattern of tonic pain with a predominant affective component, and to be suitable for application in the majority of individuals without causing undue discomfort.
-
Twenty-four healthy human subjects provided thresholds for their perception of pressure, sharpness, and pain. Mechanical forces were applied to the dorsal surface of the digits with flat-tipped probes of various sizes. Thresholds (expressed as force) increased with increasing probe size, as previously described. ⋯ In contrast, only 6% of subjects showed significant increases in sharpness or pressure thresholds over the same period. Thus, whereas most subjects exhibited stable pain thresholds, approximately one-fourth showed significant increases in pain threshold over time. We conclude that for evaluating regional dysesthesia or hemidysesthesia, a right-left difference in pain thresholds will provide a more sensitive and reliable measure than absolute pain threshold.
-
The gate control theory of pain (Melzack and Wall, 1965) suggests that tactile stimuli can decrease the perception of pain. We have found the reverse effect: Heat at levels that induce pain can substantially suppress tactile sensitivity, independently of shifts in attention or arousal. Ten human observers were stimulated by a tonic, pain-producing heat stimulus and vibrotactile stimuli (1, 10, and 100 Hz) coincidentally presented to the right thenar eminence. ⋯ The changes are not attributable to attentional or arousal shifts, since they were not associated with changes in auditory thresholds. Furthermore, the changes occurred just below the subjects' pain thresholds (where nociceptors are presumably activated). This over-twofold diminution of vibrotactile sensitivity suggests that heat stimuli capable of inducing pain can significantly diminish taction, perhaps through a "touch gate" in a manner similar to the gate control theory proposed for pain.