The interactions between spatial summation and DNIC: effect of the distance between two painful stimuli and attentional factors on pain perception.

The ability of a painful stimulus to suppress pain in another, remote area (DNIC) has been intensely studied. However, the effect of the distance between the two painful stimuli and the attentional factors during the measurement of pain perception received minimal treatment. We evaluated the effect of these factors on DNIC and on the interaction between DNIC and spatial summation (SS) of pain. Subjects rated the intensity of a test stimulus (applied to one hand) alone and simultaneously with conditioning stimuli applied to four different locations; 5 and 30cm from the test stimulus on the same hand, the contralateral hand and contralateral leg. In each location, ratings were performed under three different instructions: summation, attention to test stimulus, attention to conditioning stimulus. The distance between the conditioning and test stimulus significantly affected pain perception (p<0.01) regardless of the instructions; SS occurred only at a distance of 5cm and DNIC occurred only in the remaining distances. DNIC's magnitude increased as the distance between the two stimuli increased (p<0.01). However, the instruction to summate attenuated DNIC and the DNIC instruction attenuated SS of pain. Attention to the conditioning stimulus induced a stronger DNIC than attention to the test stimulus (p<0.001). We conclude that (1) DNIC and SS of pain appear to be antagonistic processes. (2) DNIC is affected by the distance between two noxious stimuli and to a lesser extent, by attention. (3) The interaction between DNIC, SS and attention is complex and reflects the role of sensory-cognitive integration in pain perception.