Transitivity, flexibility, conjunctive representations, and the hippocampus. I. An empirical analysis

Abstract

After training on a set of four ordered, simultaneous, odor discrimination problems (A+B−, B+C−, C+D−, D+E), intact rats display transitivity: When tested on the novel combination BD, they choose B. Rats with damage to the hippocampus, however, do not show transitivity (Dusek and Eichenbaum, 1997. Proc Natl Acad Sci U S A 94:7109–7114). These results have been interpreted as support for the idea that the hippocampus is a relational memory storage system that enables the subject to make comparisons among representations of the individual problems and choose based on inferential logic. We provide evidence for a simpler explanation. Specifically, subjects make their choices based on the absolute excitatory value of the individual stimuli. This value determines the ability of that stimulus to attract a response. This conclusion emerged because after training on a five‐problem set (A+B−, B+C−, C+D−, D+E−, E+F−) rats preferred B when tested with BE, but not when tested with BD. The implication of these results for how to conceptualize the role of the hippocampus in transitive‐like phenomena is discussed. Hippocampus 2003;13:334–340.