Spatial summation of pain increases logarithmically

Abstract

Pain intensity is difficult to predict. Mostly, because of modulatory processes underlying its formation. For example, when nociceptive stimulation occupies a larger body area, pain increases disproportionally. This modulation is called spatial summation of pain (SSp) and is responsible for coding pain intensity. To predict pain based on spatial variables, a profound understanding of the SSp effect is crucial. The aim of this study was i) to describe the SSp effect as a function of the size (or distance) of a stimulated area(s), ii) to investigate the effect of pain intensity on SSp and iii) to evaluate the influence of the SS type on the magnitude of SSp. Thirty-one healthy participants took part in a within-subject experiment. Participants were exposed to area- and distanced based SSp. In the former, electrocutaneous noxious stimuli were applied by up to 5 electrodes (5 areas) forming a line-like pattern at the ulnar side of the hand, while in the latter the same position and lengths of stimuli were used but only two electrodes were stimulated (5 separations). Each paradigm was repeated using pain of low, moderate and high intensity in a random and counterbalanced order. Each stimulus was assessed on a 0-100 scale. It was found that the pattern of increase in pain followed a logarithmic rather than a linear function. The dynamics of the pain increase were statistically different across pain intensities, with more summation occurring, if stimuli were calibrated to eliciting "high" pain. SSp was resistant to saturation in the area-based but not in the distance- based SSp, where 0.8cm separation between two electrodes produced a similar pain intensity as 1.6cm and 2.4cm. Results indicate that area-based SSp is more painful than distance-based SSp when low and moderate but not when high pain intensity is induced. Presented findings have important implications for all studies, in which the spatial dimension of pain is measured. When the area or separation between nociceptive stimulation increases, pain does not increase linearly. Furthermore, the pattern of the pain increase depends on i) intensity and ii) the number of sites of nociception. In conclusion, a logarithmic function should be considered when predicting the size of a nociceptive source. This pattern is indicative for inhibitory processes underlying SSp.