Interactions between spatial summation, 2-point discrimination and habituation of heat pain

Recently, spatial summation (SS) of two discrete noxious stimuli was found to occur at separation distances less than 10 cm in the forearm. Interestingly though, with larger separation distances there is 2-point discrimination (2PD) but not SS. However, previous studies have not examined the interactions between these spatial phenomena and temporal aspects of pain. Therefore, our aims were to (1) examine the inverse relationship between SS and 2PD in the leg, and (2) assess whether SS and 2PD of pain are affected by repetitive noxious stimulation. Twenty-four subjects received multiple series of 16 repeated noxious heat stimuli (22s inter-stimulus interval) at various intensities delivered with one or two thermal probes separated by either 0.4 cm or 15 cm. For each configuration, subjects rated the amount of perceived pain on a Visual Analogue Scale (VAS) (to evaluate SS) and stated the number of pain spots (to evaluate 2PD) following each stimulus. A significant SS of pain occurred only with the probe separation of 0.4 cm (in which perceived pain significantly increased compared with a single probe, p < 0.001) whereas 2PD occurred only at the probe separation of 15 cm (p < 0.001). When the stimulation temperature was fixed (but evoked different initial pain scores for different stimulation configurations), habituation occurred only with a single probe and two probes separated by 15 cm. However, when the initial pain score was fixed (but the stimulus temperature varied) habituation occurred with all stimulation configurations but significantly less for two probes separated by 0.4 cm. Sex was not a factor in SS and 2PD of pain, however there was greater habituation in females than males. In conclusion, SS of pain counteracts 2PD of pain and to a lesser extent, pain habituation.