A comparison of interoceptive and exteroceptive discrimination in the pigeon

In pigeons performing a conditional discrimination under a second-order, color-tracking procedure, stimulus control of responding was established using a blinking versus a nonblinking light as exteroceptive stimuli (light-discrimination group). Another group performing under the same second-order schedule of reinforcement was trained to discriminate the interoceptive stimuli produced by an IM injection of 1.5 mg/kg phencyclidine (PCP) versus saline (drug-discrimination group). In the drug-discrimination group, administration of PCP or pentobarbital resulted in dose-dependent increases in PCP-appropriate responding, while, in general, d-amphetamine did not result in appreciable drug-appropriate responding. In the light-discrimination group, all three drugs over the same dose ranges resulted in decreased discriminative control over responding. In both groups, doses of PCP and pentobarbital which resulted in intermediate (30 to 70%) levels of stimulus-appropriate responding were associated with responding at a single key position rather than tracking a key color. In contrast, intermediate responding after d-amphetamine administration was not associated with position responding in either group. These results emphasize the similarity between discriminative control maintained by interoceptive drug stimuli and exteroceptive visual stimuli.