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 ti tration schedule, each trial began with the presentation of a sample s timulus (red or green light) on the center key of a three-key pigeon c hamber. Completion of 15 responses on the center key resulted in the t ermination 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 w as 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 decrease d such that accuracy was maintained at approximately 80%. Following th e delay, two of the three pigeon keys were transilluminated with diffe rent colored lights (red or green). A single response upon the key tra nsilluminated with the same stimulus color as the sample stimulus resu lted in the presentation of food. A response on the key transilluminat ed with the stimulus color that did not match the sample stimulus resu lted in a time-out period. Using this procedure, the effects of two dr ugs 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 d erivative of scopolamine, methylscopolamine (0.003-0.3 mg/kg), a choli nesterase inhibitor, physostigmine (0.003-0.1 mg/kg), and the quaterna ry derivative of physostigmine, neostigmine (0.003-0.1 mg/kg), were de termined. Diazepam decreased matching accuracy such that a decrease in the mean delay value for the session was observed. The decreases in t he mean delay value were observed at doses that did not decrease rate of responding or increase the latency to initiate a trial. Morphine di d not affect the mean delay value despite marked effects on response l atency. Scopolamine, like diazepam, decreased the mean delay value but only at doses that also decreased rate of responding and increased th e latency to initiate a trial. Methylscopolamine only produced effects on the mean delay and response late at the highest dose tested (0.3 m g/kg), suggesting that the effects observed with scopolamine were cent rally mediated. The only effect observed with physostigmine was a decr ease 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 neost igmine was studied, the 0.1 mg/kg dose decreased the mean delay value and increased response latency, suggesting that the effects of physost igmine at this dose may be mediated peripherally. These results show t hat both diazepam and scopolamine disrupt matching accuracy. The resul ts also suggest a greater specificity in the effect of diazepam compar ed to scopolamine because the separation between doses that disrupt ma tching accuracy and doses that suppress rate of responding is greater for diazepam than for scopolamine.