The effects of morphine on chained and clocked fixed-interval schedule performance in pigeons.

The present experiment examined the role of stimulus functions and degree of stimulus control on the effects of drugs on behavior maintained by clocked fixed-interval (CFI) schedules. Three pigeons pecked keys under a multiple fixed-interval (FI) 1-min, CFI 1-min schedule of food presentation (the FI CFI condition). During the FI component, the key was lit amber. During the CFI component, the key colors changed in a regular manner across the interval. Three other pigeons pecked keys under a multiple schedule in which a three-link chained FI schedule alternated with a yoked-CFI schedule (the chain yoked-CFI condition). During the chain component, three successive FI 20-s links were associated with different key colors. During the yoked-CFI schedule, the clock stimuli were the same duration as, or 'yoked' to, the corresponding link in the chain component. Therefore, key colors changed at the same times as in the chain component, but independently of key pecking. All pigeons received a range of doses of morphine (1.0-17.0 mg/kg) and saline. Baseline rates of responding during the FI and chain components were generally higher than during the CFI and yoked-CFI components. Morphine decreased overall response rates during all components. During the FI component, morphine increased response rates at the beginning of the intervals. Morphine did not disrupt the patterns of responding maintained during the CFI, yoked-CFI, or chain components, despite the fact that the chained schedule required responding early in the intervals. This finding suggests that external stimulus control plays an important role in determining the drug effects on behavior maintained by CFI or chained schedules.