Research papers Spatial summation for pain perception: interaction of inhibitory and excitatory mechanisms Serge Marchand * , Pierre Arsenault De ´partement des Sciences de la Sante ´, Universite ´ du Que ´bec en Abitibi-Te ´miscamingue, 445, boulevard de l’Universite ´, Rouyn-Noranda, Quebe ´c, Canada J9X 5E4 Received 18 January 2001; received in revised form 29 June 2001; accepted 7 August 2001 Abstract To study the relation between size of the surface stimulated and perceived pain intensity (spatial summation effect), subjects sequentially immersed predetermined segments of the surface of their arm, between the fingertips and the shoulder, in circulating nociceptive hot water. Immersion sessions were of three types: (i) increasing session (immersion beginning at fingertips and increasing to shoulder); (ii) decreasing session (immersion beginning at shoulder and decreasing to fingertips); and (iii) whole arm 1 increasing session (preliminary immersion of the whole arm up to shoulder, followed by an increasing session from fingertips to shoulder). Results showed a positive spatial summation effect (pain perception positively correlated to the size of the surface stimulated) during both the decreasing session and the whole arm 1 increasing session. However, no spatial summation effect was found during the increasing session (fingertips to shoulder). In addition, pain perceived for a surface area was less intense during the decreasing session compared to the increasing session. One possible explanation for the lack of a spatial summation effect during the increasing session is that inhibitory mechanisms are gradually recruited at the same time as excitatory afferences, thus ‘canceling out’ any measurable spatial summation effect. The results obtained during the decreasing session and the whole arm 1 increasing session may be explained by inhibitory mechanisms being fully recruited at the beginning of the session with the immersion of the largest surface area (whole arm). The results are a shift of the pain perception curve and a positive relation between the surface stimulated and pain perception. q 2002 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved. Keywords: Spatial summation; Pain; Hot; Human; Diffuse noxious inhibitory control; Endogenous inhibitory mechanisms 1. Introduction Spatial summation can be defined as the effect of the size of the surface area stimulated on sensory threshold and perceived intensity. A positive spatial summation effect for non-painful thermal stimuli has been clearly demon- strated (Hardy and Oppel, 1937; Kenshalo et al., 1967; Stevens and Marks, 1971; Stevens et al., 1974). However, the results of psychophysical studies are contradictory regarding the existence of spatial summation for pain perception. Early studies systematically reported little or no spatial summation for painful cold and heat stimuli (Wolf and Hardy, 1941; Hardy, 1952; Green and Hardy, 1958; Hardy et al., 1959; Murgatroyd, 1964; Kenshalo et al., 1967), while later studies report spatial summation in response to different pain stimuli including thermally induced pain (Westcott et al., 1977; Machet-Pietropaoli and Chery-Croze, 1979; Kojo Pertovaara, 1987; Price et al., 1989; Douglass et al., 1992; Defrin and Urca, 1996). One possible explanation for these discrepant findings regarding spatial summation for pain perception is the recruitment of inhibitory mechanisms by nociceptive stimuli. Two factors are important for the recruitment of endogenous pain modulation mechanisms such as diffuse noxious inhibitory control (DNIC): the intensity of the conditioning stimulus and the surface area stimulated. The more intense the conditioning nociceptive stimuli, the more active the inhibitory response (Willer et al., 1984, 1989). Bouhassira et al. (1995) demonstrated the importance of the surface area stimulated in descending inhibition in rats. Nociceptive activity recorded in the spinothalamic tract increases with the size of the surface area stimulated up to a certain point after which the nociceptive activity gradually decreases despite the continued augmentation of the size of the surface area stimulated. In spinalized animals, however, a gradual decrease of nociceptive responses is not observed suggesting the involvement of endogenous pain modulation mechanisms such as DNIC. If by increasing the surface area stimulated, we gradually recruit inhibitory as well as excitatory mechanisms, a lack of correlation between the surface area stimulated and Pain 95 (2002) 201–206 0304-3959/02/$20.00 q 2002 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved. PII: S0304-3959(01)00399-2 www.elsevier.com/locate/pain * Corresponding author. E-mail address: serge.marchand@uqat.ca (S. Marchand).