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. The decreases in the mean delay value were observed at doses that did not decrease rate of responding or increase the latency to initiate a trial. Morphine did not affect the mean delay value despite marked effects on response latency. Scopolamine, like diazepam, decreased the mean delay value but only at doses that also decreased rate of responding and increased the latency to initiate a trial. Methylscopolamine only produced effects on the mean delay and response late at the highest dose tested (0.3 mg/kg), suggesting that the effects observed with scopolamine were centrally mediated. The only effect observed with physostigmine was a decrease in the mean delay value at the highest dose studied (0.1 mg/kg), a dose that also increased the latency to initiate a trial. When neostigmine was studied, the 0.1 mg/kg dose decreased the mean delay value and increased response latency, suggesting that the effects of physostigmine at this dose may be mediated peripherally. These results show that both diazepam and scopolamine disrupt matching accuracy. The results also suggest a greater specificity in the effect of diazepam compared to scopolamine because the separation between doses that disrupt matching accuracy and doses that suppress rate of responding is greater for diazepam than for scopolamine.
