Somatotopic localization of thermal stimuli: I. A comparison of within- versus across-dermatomal separation of innocuous thermal stimuli.

Fourteen healthy subjects (4 males, 10 females) were asked to localize a thermal stimulus applied to the left distal forearm. Two temperature-controlled probes (1,2-cm2 area each) were strapped to the forearm, separated by 8 cm. The probes were positioned in one of three ways: (1) longitudinally within the C6 dermatome, (2) longitudinally within the C8 dermatomes, and (3) transversely with one probe within the C6 and one probe within the C8 dermatome. For any given stimulus configuration, cooling stimuli were localized significantly better than warming stimuli. For both warming and cooling, the transdermatomal configuration (transversely separated probes) provided significantly better localization than the intradermatomal configuration (longitudinally separated probes). Thus, the anisotropy that has been described for tactile spatial acuity is also present for thermal localization. These results suggest that cutaneous somatotopic information is integrated similarly for both and thermal stimuli, but differently within versus across dermatomes.