Preference conditioning by concurrent diets with delayed proportional reinforcement.

Mildly food-deprived rats were presented at the same time either high- and low-carbohydrate diets or protein-containing and nonnutritive diets differing in flavor in parallel with nutrient composition. After a few days of these concurrent 10-minute presentations, the rats preferred the flavor of the richer or nutritive diet. Such preference conditioning also occurred when there was a 10-minute delay between the presentation of two flavors in nonnutritive diet and intubation of a mixture of nutrients in proportion to the flavor intakes. Thus, rats are able to perceive the difference between the nutritional aftereffects of two distinctive diets presented concurrently and to learn to respond differentially to associated flavors even when there was a delay as long as that used in earlier studies with pyloric closure. Therefore, the reinforcement that has been suggested to arise from gastric chemoreception can be attributed instead to delayed postgastric effects.