Assessment of drug state dimensionality via drug-drug training and stimulus generalization testing

A procedure for determining whether different drugs share a common stimulus dimension is described. This procedure uses the presence of post-discrimination generalization gradient asymmetry as an indication that the training stimuli lay along a common stimulus dimension. Separate groups of hungry pigeons were trained to discriminate a 15 mg/kg dose of phenobarbital which was associated with frequent food reinforcement (S+) from each of 9 different drug conditions which were associated with infrequent reinforcement (S-). S- stimuli were selected to represent a drug from a completely different class (amphetamine), a drug with biphasic effects which may partially correspond with the effects of phenobarbital (delta 9-THC), and a drug from the same class as the S+ (pentobarbital). Following discrimination training subjects were tested for generalization to five dosage levels (5, 10, 15, 20, 25 mg/kg) of phenobarbital. Steep symmetrical generalization gradients around the S+ indicated that delta 9-THC and d-1-amphetamine were both quite discriminable from phenobarbital, and that they were perceived by subjects as representing stimulus dimensions different from phenobarbital, and that it was perceived as lying on a stimulus dimension common to phenobarbital. This procedure may allow better understanding of how different drug states are perceived by animals as similar or dissimilar.