The conditioned satiating effect of orosensory stimuli.

Prior to the introduction of sham feeding as a method for studying the controls of meal size, the dominant view was that gustatory stimulation activated the ingestion of palatable diets and postingestional stimulation inhibited it. Early sham feeding studies with rats challenged this view because they showed that, contrary to expectation, rats did not eat continuously the first time they were given a sham feeding test. They ate a larger meal than when tested under normal conditions but stopped eating and showed all the signs of satiety soon after. Only after two or more sham feeding tests did they eat continuously. Subsequent research, reviewed here, established that experience ingesting a diet under real feeding conditions leads to the development of a classically conditioned form of satiation based on an association between gustatory stimulation and some consequence of gastrointestinal stimulation by the ingested food. This conditioned orosensory satiating effect extinguishes when sham feeding occurs repeatedly without intervening real feeding tests. Thus gustatory stimulation both stimulates and inhibits meal size. An experimental implication of this finding is that intake during sham feeding must be shown to be maximal before sham feeding can be used to measure only the orosensory stimulation of the diet. Another implication is that the analysis of a change in meal size produced by some treatment should now include measurement of the potency of the conditioned orosensory satiating effect as well as the potencies of orosensory stimulation and postingestive negative feedback.