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.

Expectancy mediation of concurrent conditional discriminations.

Pigeons learned a pair of two-choice conditional discriminations concurrently. One was red/green identity matching. The other also employed red and green sample stimuli, but the choice alternatives were vertical vs. horizontal lines, with vertical being correct following red samples and horizontal being correct following green. Correct choices produced either food reward or a brief feedback tone. For the "congruent" group, a given sample reliably predicted a particular outcome (e.g., red-food, green-tone), regardless of whether the animal chose between colors or lines. For the "incongruent" group, the outcome that followed a particular sample was one thing on color-choice trials and the other thing on line-choice trials. For example, if a subject in the incongruent group received food for red matches and tone for green matches, then it received tone for correct vertical choices and food for correct horizontal choices. The congruent group reached significantly higher levels of correct performance on both problems than did the incongruent group. Moreover, these relative levels of performance were reversed when birds trained on the congruent procedure were switched to the incongruent procedure and vice versa, even though the stimulus-response requirements of both problems remained unchanged. The experiment provides further evidence that the outcome expectancies acquired in learning situations can be significant sources of stimulus control of instrumental behavior.

A simple technique for delivering liquids directly to the mouth of an unrestrained rat.

A simple surgical procedure is described that makes it possible to deliver liquids directly into the mouth of a freely mobile rat. Data presented show that an 8% sucrose solution delivered by this technique is an effective reinforcer in a variety of simple and discriminative schedules of reinforcement.