Transfer from extero-to interoceptive reinforcement in the course of instrumental conditioning in rats.

The possibility of maintaining an ingestive instrumental reflex following the transfer from oral food reinforcement to different kinds of intragastric reinforcements (liquid food, water, hypertonic NaCl and morphine) was investigated. The experiment carried out on male hooded rats with previously implanted nasopharyngeal gastric tube showed that in all but the morphine reinforced group the performance gradually decreased showing a pattern of extinction similar to nonreinforced control group. Instrumental response was maintained in morphine reinforced rats although they were not drug addicted. The results obtained contradict the possibility of instrumental conditioning of ingestive reflexes reinforced interoceptively. They also question the drive-reduction hypothesis of instrumental conditioning, supplying further evidence in favor of the hedonistic view.