The effects of morphine on fixed-interval patterning and temporal discrimination

Changes produced by drugs in response patterns under fixed-interval schedul es of reinforcement have been interpreted to result from changes in tempora l discrimination. To examine this possibility, this experiment determined t he effects of morphine on the response patterning of 4 pigeons during a fix ed-interval 1-min schedule of food delivery with interpolated temporal disc rimination trials. Twenty of the 50 total intervals were interrupted by cho ice trials. Pecks to one key color produced food if the interval was interr upted after a short time (after 2 or 4.64 s). Pecks to another key color pr oduced food if the interval was interrupted after a long time (after 24.99 or is s). Morphine (1.0 to 10.0 mg/kg) decreased the index of curvature (a measure of response patterning) during fixed intervals and accuracy during temporal discrimination trials. Accuracy was equally disrupted following sh ort and long sample durations. Although morphine disrupted temporal discrim ination in the context of a fixed-interval schedule, these effects are inco nsistent with interpretations of the disruption of response patterning as a selective overestimation of elapsed time. The effects of morphine may be r elated to the effects of more conventional external stimuli on response pat terning.