Searching for Evidence of Transfer between Drug Facilitators

Four autoshaping experiments with pigeons examined the transfer of facilitation between drug stimuli. In Experiment 1 the presence and absence of methadone or phencyclidine signaled which of two keylight CSs were followed by grain (US). A different keylight CS was reinforced in the absence of the drugs. The drugs facilitated responding to the CS with which they were trained but did not facilitate responding to the CS reinforced in the presence of the other drug. Thus, transfer of facilitation across drug stimuli was not obtained. In Experiment 2 one of the drug facilitators was replaced by a tone. No evidence of transfer of facilitative function was obtained between the tone and the remaining drug facilitator. In Experiment 3 transfer was again not found using a design similar to that used in Experiment 1 except that the drugs did not simultaneously signal nonreinforcement of a keylight CS. In Experiment 4 the procedures of Experiment 1 were employed except that drug facilitators were replaced by distinct visual stimuli surrounding the response key. Clear evidence of transfer between the visual facilitators was obtained. The lack of transfer of modulatory control by drug stimuli in Experiments 1-3, coupled with its occurrence in Experiment 4 and previous studies, suggests that stimulus control by drugs may differ in important ways from stimulus control by conventional stimuli.