Effects of drugs on responding under concurrent fixed-interval schedules and concurrent fixed-ratio schedules.

Pigeons were trained to respond under concurrent fixed-interval fixed-interval (concurrent FI FI) schedules with Fl values (in seconds) of 30/300, 60/240, 60/60, 240/60 and 300/30. A second group was trained to respond under concurrent fixed-ratio fixed-ratio (concurrent FR FR) with FR values of 10/40, 10/20, 20/20, 20/10 and 40/10. Under the concurrent Fl Fl schedules, pigeons responded much less often on the key with the higher reinforcement density than would be predicted basing on a perfect matching of the ratio of responses made to the ratio of reinforcers delivered, and the pigeons also showed a bias towards responding on the left key. Pentobarbital, methamphetamine, morphine and phencyclidine had little effect on bias, but high doses of methamphetamine, morphine and phencyclidine decreased undermatching. Under the concurrent FR FR schedules, pigeons responded almost entirely on the key with the higher reinforcement density. Only pentobarbital consistently disrupted this pattern of responding.