The Effects of Morphine on Clocked Fixed-Interval Performance: Stimulus Function or Strength of Stimulus Control?

The present experiment was conducted to determine whether the modification of the effects of morphine by food deprivation would occur under conditions of strong stimulus control. Four pigeons pecked response keys under a multiple fixed-interval (FI) 5-min and clocked fixed-interval (CFI) 5-min schedule of food presentation. During the FI component, the key was lit green and the house light was on throughout. During the CFI component, the key light and house light settings changed in a regular manner as the interval elapsed. In all conditions, pigeons received a range of doses of morphine (1.0-10.0 mg/kg) and saline. In the first condition, pigeons were maintained at 80% of their free-feeding weights. When calculated over entire 5-min intervals, morphine produced dose-dependent decreases in the index of curvature (a measure of temporal patterning) during the FI component, but did not affect the index of curvature during the CFI component. A more detailed analysis revealed that the index of curvature during the last stimulus in the CFI sequence was reduced in a dose-dependent manner, similar to the effects obtained for the entire FI component. When the pigeons were maintained at 70% and 90% of their free-feeding weights, measures of the effects of morphine on temporal patterning did not differ as a function of body weight. The effects of morphine on rates of key-pecking, however, were related to body weight. For three of four pigeons, rates of key-pecking were not suppressed until higher doses, and increases in rates of key-pecking were more common at lower doses, when the pigeons were maintained at 70% of their free-feeding weights, than when they were maintained at 90% of their free-feeding weights. It is suggested that the effects of morphine on CFI performance can be understood in terms of the function of the stimuli that comprise the schedule, rather than the strength of the stimulus control engendered