The role of the hippocampus in instrumental conditioning

Considerable evidence suggests that, in instrumental conditioning, rats can encode both the specific action-outcome associations to which they are exp osed and the degree to which an action is causal in producing its associate d outcome. Three experiments assessed the involvement of the hippocampus in encoding these aspects of instrumental learning. In each study, rats with electrolytic lesions of the dorsal hippocampus and sham-lesioned controls w ere trained while hungry to press two levers, each of which delivered a uni que food outcome. Experiments 1A and 1B used an outcome devaluation procedu re to assess the effects of the lesion on encoding the action-outcome relat ionship. After training, one of the two outcomes was devalued using a speci fic satiety procedure, after which performance on the two levers was assess ed in a choice extinction test. The lesion had no detectable effect on eith er the acquisition of instrumental performance or on the rats' sensitivity to outcome devaluation; lesion and sham groups both reduced responding on t he lever associated with the devalued outcome compared with the other lever . In experiment 2, the sensitivity of hippocampal rats to the causal effica cy of their actions was assessed by selectively degrading the contingency b etween one of the actions and its associated outcome. Whereas sham rats sel ectively reduced performance on the lever for which the action-outcome cont ingency had been degraded, hippocampal rats did not. These results suggest that, in instrumental conditioning, lesions of the dorsal hippocampus selec tively impair the ability of rats to represent the causal relationship betw een an action and its consequences.