Spatial summation of heat pain: a reassessment

Heat pain threshold is commonly considered to be an 'absolute' value, which is not dependent on the area stimulated. In contrast, suprathreshold heat pain sensation has been shown to be highly dependent on the area stimulated, with considerable spatial summation demonstrated both within and between dermatomes. The present study sought to reevaluate two major issues: (a) Whether nociceptive thresholds are, indeed, independent of stimulation area. (b) Whether the spatial summation of suprathreshold heat pain sensation is independent of threshold changes. Using noxious heat we evaluated nociceptive thresholds and perceived pain intensity for contact areas of 0.25, 2.25, 6.25 and 15.36 cm2. Our results show that considerable spatial summation of heat pain thresholds is obtained and an apparent spatial summation of perceived intensity can also be observed. However, these apparent changes in perceived pain intensity can be accounted for completely by the changes in noxious heat thresholds. Furthermore, when using a stimulus configuration in which stimulation area was increased without changing nociceptive threshold, no spatial summation of perceived pain intensity was seen. Our results suggest that the spatial summation of perceived heat pain intensity can be attributed to reduced heat pain threshold. Furthermore, our findings stress the importance of determining pain thresholds in studies examining the psychophysics of suprathreshold noxious stimuli.