The amygdala but not the hippocampus is involved in pattern separation based on reward value

A total of 32 male Long-Evans rats were tested on a modified version of Flaherty, Turovsky, and Krauss's (1994) anticipatory contrast paradigm to assess pattern separation for reward value. Prior to testing, each rat received either a control, a hippocampal, or an amygdala lesion. In the home cage, each rat was allowed to drink a water solution containing 2% sucrose for 3 min followed by a water solution containing 32% sucrose for 3 min. Across 10 days of testing, the rats in each lesion group showed significantly increased anticipatory discriminability as a function of days. To assess the operation of a pattern separation mechanism, each rat was then tested using the same procedure except the 2% solution was followed by a 16% solution for 10 days and then by an 8% solution for 10 days. Control and hippocampal-lesioned rats continued to show high discriminability when the 2% solution was followed by a 16% solution; however, the amygdala-lesioned rats showed low anticipatory discriminability. On trials where the 2% sucrose solution was followed by an 8% sucrose solution, all groups showed low discriminability scores, suggesting that when two reward values are very similar even control animals are not able to separate the reward values in memory. However, the results of a preference task revealed that all groups can perceptually discriminate between a 2% and an 8% sucrose solution. The data suggest that the amygdala but not the hippocampus is involved in the separation of patterns based on reward value.