Titrating matching-to-sample performance in pigeons: effects of diazepam, morphine, and cholinergic agents

Five adult, male White Carneau pigeons were trained to respond under a titrating matching-to-sample schedule of reinforcement. Under this titration schedule, each trial began with the presentation of a sample stimulus (red or green light) on the center key of a three-key pigeon chamber. Completion of 15 responses on the center key resulted in the termination of the stimulus presentation and the initiation of a delay period. The length of the delay changed as a function of the pigeon's performance. During the first five trials of each session, the delay was fixed at 3 s in length. On the sixth and all subsequent trials, the length of the delay was either increased, did not change, or decreased such that accuracy was maintained at approximately 80%. Following the delay, two of the three pigeon keys were transilluminated with different colored lights (red or green). A single response upon the key transilluminated with the same stimulus color as the sample stimulus resulted in the presentation of food. A response on the key transilluminated with the stimulus color that did not match the sample stimulus resulted in a time-out period. Using this procedure, the effects of two drugs of abuse, diazepam (0.03-3 mg/kg) and morphine (0.03-10 mg/kg), a muscarinic antagonist, scopolamine (0.003-0.3 mg/kg), the quaternary derivative of scopolamine, methylscopolamine (0.003-0.3 mg/kg), a cholinesterase inhibitor, physostigmine (0.003-0.1 mg/kg), and the quaternary derivative of physostigmine, neostigmine (0.003-0.1 mg/kg), were determined. Diazepam decreased matching accuracy such that a decrease in the mean delay value for the session was observed.(ABSTRACT TRUNCATED AT 250 WORDS)