Spatial discrimination thresholds for pain and touch in human hairy skin

The traditional concept that pain is poorly localized has been challenged by recent studies, where subjects were able to point to the stimulated spot on the skin with an accuracy of 10-20 mm. Pointing movements themselves, however, have errors of about 15 mm. To determine the limits of sensory performance of the nociceptive system independent of motor performance, point localization of heat pain (540 mJ punctate laser stimuli, 5 mm diameter), mechanical pain (256 mN punctate probe, 200 microm diameter), and touch (16 mN von Frey probe, 1.1 mm diameter) were tested in a two-alternative forced-choice paradigm in 12 healthy subjects. Stimuli were applied in randomized order to two parallel lines on the back of the hand (4-32 mm distance). The cumulative distribution functions for correct localization were of similar sigmoid shape for all test stimuli, indicating logarithmic normal distributions. The 75% correct localization threshold for painful heat was 8.6 mm (3.1 +/- 0.1 log2 units) and did not differ significantly from that of non-painful touch (9.0 mm, 3.2+/-0.2 log2 units). Localization of mechanically-induced pain (5.1 mm, 2.4 +/- 0.2 log2 units) was significantly more accurate than both heat pain and touch, possibly due to a synergism of two different sensory channels, the tactile channel and the nociceptive channel, which were activated simultaneously. For all three stimuli, discrimination was significantly better in radial-ulnar compared to proximal-distal direction, which might be related to oval receptive field shapes. Sequential spatial discrimination for touch was significantly better than simultaneous spatial discrimination tested with a grating orientation task (18.9 mm), but both were one order of magnitude worse than at the finger tip (1.3 mm, 0.4 +/- 0.1 log2 units). In conclusion, pain evoked by radiant heat pulses and touch evoked by von Frey probes were localized with similar precision on the back of the hand. These findings indicate that outside the tactile fovea at finger tips or lips the spatial discrimination capacities of the nociceptive and tactile systems are about equal.