Effects of d-amphetamine in grouped versus isolated humans.

This study was designed to determine whether the subjective, behavioral or physiological effects of a stimulant drug in humans depend on whether subjects are tested under isolated or social conditions. Forty-two subjects were randomly assigned to either the Social (SOC) or Isolated (ISO) condition. SOC subjects participated in 4 h laboratory sessions in groups of 3 or 4, whereas ISO subjects participated in the sessions alone. All subjects participated in three sessions, during which they received capsules containing d-amphetamine (10 or 20 mg) or placebo, in mixed order under double blind conditions. Subjective, physiological and behavioral measures were obtained at regular intervals, d-amphetamine produced dose-related, prototypic stimulant effects on many measures, including self-reported mood states, behavioral indices and physiological measures. Most of these effects were unaffected by the setting in which subjects were tested (SOC vs ISO). However, body temperature was overall higher in the SOC group, and there was a trend for d-amphetamine to produce greater hyperthermic effects in the SOC group. In addition, 10 mg d-amphetamine increased heart rate in the SOC group but not in the ISO group. The results suggest that, like in laboratory animals, some of the effects of stimulants in humans are greater under aggregated conditions. However, unlike in the animal studies, this observed enhancement of the drug's effects under aggregated conditions was limited to physiological measures and did not apply to other subjective or behavioral measures.