In all four studies, we applied thermal stimuli in randomized sequences of varying intensity (trials) to the left forearm of each participant during fMRI scanning. For imaging, we used a 1.5-T General Electric scanner in studies 1, 3, and 4 and a 3-T Phillips scanner in study 2.
Participants in study 1 underwent 12 trials at each of four intensities, which were calibrated for each person: innocuous warmth (defined with the use of self-report by the participant as level 1 on a 9-point visual-analogue scale [VAS], with a mean [±SD] temperature of 41.0±1.9°C) and three levels of painful heat (participant-defined levels 3, 5, and 7, with mean temperatures of 43.3±2.1°C, 45.4±1.71°C, and 47.1±0.98°C, respectively). Each trial consisted of a warning cue and anticipation period (8 seconds), stimulation (10 seconds), and a pain-recall and rating period (4 seconds), with periods of rest before and after recall.
Participants in study 2 underwent a total of 75 trials across six temperatures (44.3 to 49.3°C in 1°C increments). After each trial, participants judged whether the stimulus was painful. They subsequently judged nonpainful warmth on a 100-point VAS and pain intensity on a 100-point VAS. Ratings were coded from 0 to 99 for nonpain-ful events and from 100 to 200 for painful events.
Participants in study 3 underwent 32 trials, consisting of 8 trials with each of four stimulus types. We delivered noxious heat (46.6±1.7°C, denoted “painful”) and warmth that was near the pain threshold (39.9±2.8°C, denoted “warm”) at temperatures calibrated for each person. Each participant had recently experienced a romantic breakup and continued to feel intensely rejected. During scanning, participants viewed an image of their ex-partner (denoted as “rejecter” trials, which elicit social pain8 (link)) and an image of a close friend (denoted as “friend” trials).
Participants in study 4 received two intravenous infusions of remifentanil, a potent μ-opioid agonist, during fMRI scanning in two series of trials. In the open-infusion series, participants knew they received remifentanil, and in the hidden-infusion series, they were told that no drug was delivered, even though it had been administered. Remifentanil doses (mean dose, 0.043±0.01 μg per kilogram of body weight per minute) were individually calibrated before the session to elicit analgesia without sedation, and we estimated the brain concentration of the drug over time using a pharmacokinetic model.9 (link) We conducted 36 trials — 18 involving pain (mean temperature, 47.1±1.7°C) and 18 involving warmth (mean temperature, 41.2±2.6°C) — during each of the two infusion series. Drug infusion began partway through each series, after 6 trials, and ended after 24 trials. This design resulted in a continuously varying concentration of the drug over time during each infusion series.
Participants in study 1 underwent 12 trials at each of four intensities, which were calibrated for each person: innocuous warmth (defined with the use of self-report by the participant as level 1 on a 9-point visual-analogue scale [VAS], with a mean [±SD] temperature of 41.0±1.9°C) and three levels of painful heat (participant-defined levels 3, 5, and 7, with mean temperatures of 43.3±2.1°C, 45.4±1.71°C, and 47.1±0.98°C, respectively). Each trial consisted of a warning cue and anticipation period (8 seconds), stimulation (10 seconds), and a pain-recall and rating period (4 seconds), with periods of rest before and after recall.
Participants in study 2 underwent a total of 75 trials across six temperatures (44.3 to 49.3°C in 1°C increments). After each trial, participants judged whether the stimulus was painful. They subsequently judged nonpainful warmth on a 100-point VAS and pain intensity on a 100-point VAS. Ratings were coded from 0 to 99 for nonpain-ful events and from 100 to 200 for painful events.
Participants in study 3 underwent 32 trials, consisting of 8 trials with each of four stimulus types. We delivered noxious heat (46.6±1.7°C, denoted “painful”) and warmth that was near the pain threshold (39.9±2.8°C, denoted “warm”) at temperatures calibrated for each person. Each participant had recently experienced a romantic breakup and continued to feel intensely rejected. During scanning, participants viewed an image of their ex-partner (denoted as “rejecter” trials, which elicit social pain8 (link)) and an image of a close friend (denoted as “friend” trials).
Participants in study 4 received two intravenous infusions of remifentanil, a potent μ-opioid agonist, during fMRI scanning in two series of trials. In the open-infusion series, participants knew they received remifentanil, and in the hidden-infusion series, they were told that no drug was delivered, even though it had been administered. Remifentanil doses (mean dose, 0.043±0.01 μg per kilogram of body weight per minute) were individually calibrated before the session to elicit analgesia without sedation, and we estimated the brain concentration of the drug over time using a pharmacokinetic model.9 (link) We conducted 36 trials — 18 involving pain (mean temperature, 47.1±1.7°C) and 18 involving warmth (mean temperature, 41.2±2.6°C) — during each of the two infusion series. Drug infusion began partway through each series, after 6 trials, and ended after 24 trials. This design resulted in a continuously varying concentration of the drug over time during each infusion series.