Incomplete stimulus generalization from a mixture of d-amphetamine and morphine to different doses of the component drugs

Pigeons were trained to discriminate a mixture of 1.8 mg/kg morphine plus 1.0 mg/kg d-amphetamine from saline, and then tested for generalization to various doses of d-amphetamine and morphine, alone and in combination. The birds discriminated the drug mixture from saline with more than 90% responding on the drug key. The training doses of morphine (1.8 mg/kg) and d-amphetamine (1.0 mg/kg), as well as lower doses of these drugs, did not reliably generate responding on the drug key when given in combination with saline. Higher doses of morphine and d-amphetamine did generate responding on the drug key, but not as reliably as the training dose combination. Combination of the training dose of morphine with doses of d-amphetamine higher than those in the training dose combination resulted in somewhat less responding on the drug key than that seen with the training dose combination in one bird. These data and those from other experiments where animals were trained to discriminate drug mixtures can be characterized as the net effect of stimuli produced by the component drugs and interactions between the component drugs.