Training session

At the beginning of the first session, participants were trained to use a standardized numerical pain scale (NPS) to rate the magnitude/intensity of their heat pain experience (Staud, et al., 2006; Vierck, et al., 1997). The scale ranges from 0 to 100, in increments of 5, with verbal descriptors at intervals of 10: 0 – no sensation, 10 = warm, 20 = a barely painful sensation (i.e., pain threshold), 30 = very weak pain, 40 = weak pain, 50 = moderate pain, 60 = slightly strong pain, 70 = strong pain, 80 = very strong pain, 90 = nearly intolerable pain, and 100 = intolerable pain. Participants were shown the NPS scale continuously while they rated their sensations. Prior to the onset of the heat pain conditions, the anchors of the scale were read to the participants, and they were instructed to rate their general pain sensation numerically as the baseline in order to differentiate between somatic pain and heat pain. This scale has been preferred for discriminating levels of sensation and rating a tandem series of sensations (Staud, et al., 2007a; Vierck, et al., 1997).

After participants were instructed on the scale, a series of threshold and calibration tests were performed. All heat sensations were applied to the skin via an MR-compatible, Peltier thermode (Medoc®, Ramat Yishai, Israel). The Medoc device was programmed to control the temperature while the thermode was held by one of the experimenters and applied manually to the skin as needed for the specific test or stimulus. First, participants were asked to rate their pain sensations to three different temperatures (FM at 44 °C, 45 °C, 46 °C; NC at 45 °C, 46 °C, 47 °C), which were applied to the skin of the forearm via the thermode for 2 seconds. Ratings from this test served to confirm that participants could distinguish between the different temperatures and understood the rating scale. The thermode was then heated to 42 °C and contact with the thenar eminence was repeated eight times, every 3 seconds (i.e. a frequency of 0.33 Hz). For the series of contacts the experimenter was guided by audio cues (imperceptible to the participant) to indicate the duration (1.5 seconds) and timing onset of each contact. Participants were instructed to rate their pain from each contact, and also to rate the sensation on their hand at 15 seconds and 30 seconds after the last contact, to provide indications of after-sensations. This process was repeated with thermal stimuli at varying temperatures (NC at 46 °C, 50 °C, 44 °C, and 48 °C, FM at 44 °C, 48 °C, 42 °C, and 46 °C). This series of tests determined if the participant experienced temporal summation of C-fibre mediated second pain and guided our calibration of the appropriate temperature required to achieve a sensitivity-adjusted final NPS rating of 50 ± 10 units. Therefore, the optimal stimulus intensity was varied as a function of each subject’s TSSP sensitivity. A rating of 50 ± 10 NPS units was chosen because healthy participants were unlikely to experience prolonged peripheral or central hypersensitivity at this temperature after repeated trials (Staud, et al., 2007a).