Schedule-dependent effect of d-amphetamine on pausing by pigeons.

Two pigeons were exposed to a multiple schedule of grain reinforcement where key-pecks were reinforced according to a fixed ratio schedule in the presence of one stimulus and a fixed interval schedule in another stimulus. The fixed interval was adjusted to match average interreinforcer time for the fixed ratio schedule. d-Amphetamine decreased overall rate of responding in the fixed ratio schedule -- primarily by increasing pausing, especially just after a change from the fixed interval component. Rate of responding in the fixed interval schedule was increased for one bird and little affected for the other bird by d-Amphetamine. Again, changes in pausing primarily determined rate change. Since pausing was differently affected, a schedule-dependent rather than rate-dependent effect was indicated.