The ability of humans to localise noxious stimuli

We have investigated the ability of humans to localise noxious stimuli on the dorsum of the hand. Pin-prick (non-penetrating needle prick), noxious heat (round 1 cm2 copper probe heated to 50 degrees C), mustard oil (100% applied topically in a small cotton ball, diameter 5 mm) and histamine (iontophoresis of 20 mC delivered to an area of 75 mm2) were applied to skin with intact innervation and during a differential nerve compression block of the superficial radial nerve when only C-fibres were conducting. The mean mislocalisation (+/- S.E.M.; n = 8) of all stimuli was 9.5 +/- 0.8 mm with normal nerve conduction and 8.9 +/- 1.2 during the differential nerve block. There was no significant difference between the noxious submodalities. By contrast, when nerve conduction was intact, purely tactile stimulation (7 mN von Frey hair) was significantly better localised having a mean error of 5.5 +/- 0.4 mm. We conclude that focal stimuli evoking itch or pain can be localised with high precision which is only marginally worse than for tactile stimuli. This suggests the existence of a somatotopical representation for noxious inputs in the brain similar to that found for tactile stimuli.