Fig. 1. FI-FR chain procedure.

A) Schematic timeline of events during a single trial of FI5m-CRf(5m)-TO20m heterogeneous response chain. Arrows indicate theoretical lever-presses during the fixed-interval (FI) or continuous reinforcement (CRf) phase. B) Frequency of responding lawfully accelerates across the fixed-interval (left). Representative raster plot shows responses (black circles) for 8 rats during a single trial. The first response emitted after the interval elapsed (red circle) produced protraction of the drug-taking lever. Pattern of responding summarized as cumulative response record according to group means per 10s bin (solid trace, ±SEM shaded region). C) Cocaine-taking is typically front-loaded during the continuous reinforcement component. Event record depicts taking-lever responses (tick marks) across the 5min interval (top). Mean cocaine intake (mg/kg) is visualized in 15s bins across the access period (bottom; blue trace with filled AUC, ±SEM shading). D) Representative behavioral event record for a FI-FR test session, comprising 4 trials. Estimated brain [cocaine] is plotted on the right y-axis (dashed grey trace) to demonstrate spiking patterns of cocaine levels which approach zero prior to subsequent trial onset. Cumulative response output during each FI component is plotted on the left y-axis (red trace). Tick marks denote drug infusion events (blue trace). Unless otherwise noted, the first trial was used for all analyses.