The hippocampus is critical for encoding and retrieving semantic and episodic memories. Animal studies indicate that the hippocampus is also required for relational learning tasks. A prototypical relational learning task, and the one investigated in this experiment, using event-related functional magnetic res- onance imaging, is the transitive inference (TI) task. In the TI task, participants were to choose between A and B (A?B) and learned by trial and error to choose A (A > B). There were four such premise pairs during a training (A > B, B > C, C > D, D > E). These can be acquired distinctly or can be organized into a superordinate hierarchy (A > B > C > D > E), which would efficiently represent all the learned relations and allow inferences (e.g., B > D). At test there was no reinforcement: In addition to premise pairs, untrained pairings were introduced (e.g., A?E, B?D). Correctly inferring that B > D is taken as evidence for the formation of a superordinate hierarchy; sev- eral alternatives to the superordinate hierarchy hypothesis are considered. Awareness of the formation of this hierarchy was measured by a postscan questionnaire. Four main findings are reported: (1) Inferential performance and task awareness disso- ciated behaviorally and at the level of hemodynamic response; (2) As expected, performance on the inferred relation, B > D, corresponded to the ability to simultaneously acquire B > C and C > D premise pairs during training; (3) Interestingly, ac- quiring these ‘‘inner pairs’’ corresponded to greater hippo- campal activation than the ‘‘outer pairs’’ (A > B, D > E) for all participants. However, a distinct pattern of hippocampal ac- tivity for these inner pairs differentiated those able to perform the inferential discrimination, B > D, at test. Because these inner premise pairs require contextual discrimination (e.g., C is incorrect in the context of B but correct in the context of D), we argue that the TI task is hippocampal-dependent be- cause the premise pair acquisition necessary for inference is hippocampal-dependent; (4) We found B > D related hippo- campal activity at test that is anatomically consistent with pre- consolidation recall effects shown in other studies.
Greene AJ, Gross WL, Elsinger CL, Rao SM (2006). An FMRI analysis of the human hippocampus: inference, context, and task awareness. J Cogn Neurosci, 18(7), 1156-1173.