Fig. 2.

A) Baseline responding across Conditioning and Test weeks for Experiment 2. Operant responding declined in both Paired and Unpaired groups as a function of CS interval (Pre-CS, CS 1–10, CS 11–20). Rats in the Unpaired group showed a modest increase in responding across conditioning weeks, and this increase was not seen in the Paired group (*p < 0.05, treatment × conditioning week interaction). B) Responding on Conditioning Days across weeks. There was no significant change in responding in the Pre-CS interval across Conditioning Weeks. A transient decrease in responding was noted in the CS 1–10 interval when the CS was first introduced to the Unpaired group ($p < 0.05 vs. Pre-CS interval in the corresponding Conditioning Week. This effect was masked in the Paired groups that also received naloxone-precipitated withdrawal (1.0 mg/kg) immediately prior to CS exposure. The decline in responding in the Unpaired group during Conditioning Weeks 1–2 was not present in the CS 11–20 interval; the Paired group showed essentially 100% suppression of responding in the CS 11–20 interval in all Conditioning Weeks.