Endogenous opioids have been implicated in mediating (placebo) analgesia, in reward processes, and in the regulation of socially relevant emotions. To explore their potential contributions to higher cognitive functions, we used a novel task with tachistoscopically presented (for 150 ms) pairs of meaningless figures. Healthy right-handed men judged the similarities and dissimilarities between the two figures on a visual analogue scale (VAS) in two separate runs. In a double-blind, between-subjects design, subjects were administered intravenously either 0.2-mg/kg naloxone or placebo 10 min prior to the task, and VAS judgments and response latencies were measured. We found a significant interaction between substance group and type of judgment: The magnitude of the similarity judgments was lower in the naloxone than in the placebo group, while dissimilarity judgments remained uninfluenced by the treatment. Reaction latencies and mood scores, assessed before and after substance administration, did not differ between the two groups, indicating that the findings did not rely on altered motor performance or motivation. We suggest that naloxone decreased the "similarity criterion" in comparative judgments, indicating its potentially modulatory effect on visual cognition. The task introduced here could be used for the implicit study and quantification of subtle affective-cognitive processes beyond the level of mere questionnaire data.
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