Fig. 1.

Dose-response functions for the number of saline (solid triangles), cocaine (solid circles), and remifentanil (open circles) injections in each of three monkeys and the group means. Each data point is the average of three stable sessions from the initial drug-lever pairing and the average of three stable sessions from the reversed drug-lever pairing. Error bars are one standard error of the mean (+SEM). The x-axis represents each concurrent-access condition beginning in the leftmost panel when different doses of cocaine (C) and saline (S) were concurrently available. The second panel shows conditions when saline and various doses of remifentanil (R) were concurrently available, and subsequent panels show conditions when cocaine (0.5, 0.1, and 0.2 mg/kg/inj, respectively) was concurrently available with various doses of remifentanil. Also note the different y-axes for different subjects.