Excitotoxic lesions of the hippocampus disrupt runway but not consummatory contrast.

Rats shifted from a 12-pellet to a 1-pellet reward for running in a straight runway showed a decrease in start, run, and goal speed to levels below rats that received only the 1-pellet reward throughout training (a negative contrast effect). Contrast was greatest in the goal region of the runway. Rats with damage to the hippocampus produced by the excitotoxin ibotenic acid failed to show a negative contrast effect under these conditions. The same lesioned rats tested in a consummatory, contrast procedure following a shift from 32% to 4% sucrose showed a negative contrast effect equivalent to sham-lesioned rats. These data suggest that the hippocampus is necessary for behavioral outcomes based on encoding or comparison that affect approach behavior, but not for such outcomes that affect consummatory behavior.