The impact of milk-derived unconditioned and conditioned negative feedbackon the microstructure of ingestive behavior

We describe at the microstructural level the impact of unconditioned and co nditioned negative feedback on the licking behavior of the rat. Six groups of rats were trained to ingest one of six different dilutions of sweetened milk (0.5:1, 1:1, 2:1, 4;1, 8:1, and 16:1, water to milk ratios) under real feeding conditions until intake was stable. Then they were given five sham -feeding tests with the same solution. We compared the size of the clusters (SC) and their number (NC) during the 15-min period when intake rate was d eclining in the real-feeding test with SC and NC during the corresponding 1 5-min period in the following sham-feeding test. Intake increased significa ntly over the five sham-feeding tests with the two highest concentrations, indicating the extinction of conditioned negative feedback. With these two solutions, we compared the microstructure of licking behavior in the first with that in the fifth sham-feeding test to determine if conditioned negati ve feedback affected SC or NC or both. The effect of both unconditioned and conditioned negative feedback on licking behavior was to decrease the numb er of clusters without affecting their size. We conclude that negative feed back derived from the accumulation of milk in the gastrointestinal tract de creases the probability of initiating a bout of licking during a pause. It has no effect on the ability to continue a bout of licking once it has begu n. (C) 2000 Elsevier Science Inc. All rights reserved.