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Estimated remifentanil levels on a trial-by-trial basis. Mean (± SEM) whole-body remifentanil levels at choice for each trial under the (A) controlled and (C) uncontrolled reinforcer frequency schedules. Correlations between individual remifentanil-food exchange (a) and aggregated whole-body levels under the (B) controlled and (D) uncontrolled reinforcer frequency procedures for the different conditions. * indicates p < 0.05.
