An fMRI Analysis of the Human Hippocampus: Inference, Context, and Task Awareness

• Authors: Anthony J. Greene;William L. Gross;Catherine L. Elsinger;Stephen M. Rao
• Venue: Journal of Cognitive Neuroscience
• Year: 2006

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 resonance 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; several 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 dissociated 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, acquiring these “inner pairs” corresponded to greater hippocampal activation than the “outer pairs” (A B, D E) for all participants. However, a distinct pattern of hippocampal activity 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 because the premise pair acquisition necessary for inference is hippocampal-dependent; (4) We found B D related hippocampal activity at test that is anatomically consistent with preconsolidation recall effects shown in other studies.