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-inter val (concurrent FI FI) schedules with FI 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 FI FI schedules, pi geons responded much less often on the key with the higher reinforcement de nsity 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, methamphet amine, morphine and phencyclidine had little effect on bias, but high doses of methamphetamine, morphine and phencyclidine decreased undermatching. Un der the concurrent FR FR schedules. pigeons responded almost entirely on th e key with the higher reinforcement density. Only pentobarbital consistentl y disrupted this pattern of responding. (C) 1999 Lippincott Williams & Wilk ins.