Figure 8. Interactions of Spatially Remote Stimuli with Offset Analgesia.

The minimum VAS ratings during the time period following the change from 50°C (T2) to 49°C (T3) were averaged across all subjects (A, B, D, & E, means ± SEM) for both the three temperature stimulus trains and from time matched locations for constant 49°C stimuli. Percent analgesia was calculated by dividing the minimum VAS from the three temperature stimulus train by the minimum VAS from the constant 49°C for each series within subjects (C & F, means, ± SEM). A–C. Offset analgesia evoked simultaneously at two sites (b) does not summate beyond that evoked at one site (a) since the percent analgesia (c) was not different between conditions. D–F. Offset analgesia evoked by a proximal stimulus inhibits pain evoked by a distal noxious stimulus (d). However, a proximal noxious stimulus abolishes offset analgesia evoked at a distal location (e). Examination of the percent analgesia confirms these spatial interactions are asymmetric (f). Since these complex spatial interactions cannot be readily explained by a peripheral mechanism, offset analgesia must be, in part, mediated by central mechanisms. Abbreviations: 49°C - 49°C 30 second stimulus. 35°C-35°C 30 second stimulus. 3Temp - 49°C for 5 seconds, followed by 50°C for 5 seconds, followed by 49°C for 20 seconds. D and P represent Distal and proximal, respectively. N.S. symbolizes “not significant.”