The hippocampus and reward: effects of hippocampal lesions on progressive-ratio responding.

Two experiments investigated the effects of ibotenic-acid lesions of the hippocampus on food-rewarded performance under a progressive-ratio 10 schedule of reinforcement. The results of Experiment 1 indicated that rats with hippocampal lesions showed profound increases in breakpoint and enhancements in the efficiency of responding. In a second experiment the same rats were challenged with prefeeding, increases in the height of the response lever, and the substitution of sucrose- for grain-based reward pellets. Responding in both groups was similarly reduced by prefeeding and increases in the effortfulness of responding, but lesioned rats were significantly more responsive to the change to sucrose reward. Overall, the results indicated that hippocampal damage increased responding by enhancing the activational or hedonic properties of the delivered food pellets, while not affecting food-motivation or the motor capacity to respond.