Spatial summation of thermal sensations depends on skin type and skin sensitivity

The objective of the present study was to examine the extent to which spatial summation (SS) of thermal senses is affected by skin type and skin thermal sensitivity. A total of 19 healthy subjects underwent measurements of warm- and cold-sensation threshold (WST and CST) with a large (9 cm²) and small (2.25 cm²) stimulation area, within the glabrous (palm) and hairy skin (dorsal surface) of the hand. SS of WST was also measured in warm-sensitive and warm-insensitive hairy skin sites. WST and CST significantly increased as stimulation area decreased (at a similar amount), in both hairy and glabrous skin. SS of CST in the glabrous skin was larger than that of hairy skin. A significant SS of WS existed in both warm-sensitive and warm-insensitive sites but the amount of SS was larger in warm-insensitive sites. Sex did not affect any of the factors tested. The similar amount of SS for WST and CST suggest that despite possible differences in receptor density, these two sub-systems share common features. Based on the stimulation areas used herein and on receptive-field (RF) sizes, SS of WST and CST appears to occur within RF of a single neuron. The larger magnitude of SS in the glabrous than hairy skin might suggest a larger integration of sensory information from the former, possibly due to a greater functional role of the palm.