Figure 1. “Feel drug effect” ratings (left) and number of symbols drawn correctly on the digit-symbol substitution test (right) during and after the infusion of saline (open square), remifentanil (solid circle), and alfentanil (solid triangle). Targeted doses of remifentanil tested were 0.75 (LO), 1.5 (MED), and 3 (HI) ng/ml; targeted doses of alfentanil tested were 16 (LO), 32 (MED), and 64 (HI) ng/ml. Each infusion dose was tested for 40 min, and the total infusion period was 120 min, then a 3-h postinfusion period (recovery) ensued. Each symbol represents the mean response for 10 participants. Asterisks above a symbol indicate that the rating is significantly different from the corresponding saline infusion rating. Brackets above or below symbols indicate SEM.