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



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. © 2003 Wiley-Liss, Inc.