Verbal responding under a fixed-interval schedule: effects of D-amphetamine.

The purpose of this study was to characterize the effects of d-amphetamine (5-30 mg) on rate and distribution of verbal responding which was maintained under a fixed-interval (FI) schedule of reinforcement. Hired volunteer subjects simultaneously wrote and spoke narrative monologues; points were delivered under a multiple 5 min fixed-interval 1 min time out schedule for closure of a voice operated relay (VOR). Under placebo control conditions, most subjects paused for 2-4 minutes following the 1 min time out then spoke and wrote during later portions of the interval. d-Amphetamine increased both the number of words written and seconds of VOR closure in a dose-related manner. In subjects who showed typical FI response patterning, the drug generally decreased the length of early interval pausing and increased low control amounts of verbal responding which occurred early in the interval proportionately more than higher amounts of verbal responding seen during later portions of the interval. These drug effects on response patterning were generally similar to those seen in infrahuman species responding under fixed-interval schedules of reinforcement.