Lateral asymmetry in the hippocampal response to the remoteness of autobiographical memories

The time scale of hippocampal involvement in retrieving memories, particularly those more remote, is still a matter of debate. Some propose that the hippocampus is not involved in the retrieval of remote memories, whereas others assert that it is necessary for memory retrieval in perpetuity. Functional magnetic resonance imaging was used to examine the effect of remoteness on the neural basis of memory. We used a parametric event-related random-effects design in a large group of subjects to overcome some of the limitations of previous neuroimaging studies. We found that the hippocampi were significantly active during the retrieval of autobiographical mem-ories. Notably, the two hippocampi diverged in their responses to remoteness. The right hippocampus showed a temporal gradient, decreasing in activity the more remote the autobiographical memories. No such effect was apparent in the left hippocampus, suggesting its invariant involvement in remembering autobiographical events throughout the lifespan. The dorsal amygdalas showed a temporal gradient similar to the right hippocampus, but emotional valence and intensity were not directly associated with changes in activity. The current results indicate that consideration of lateral asymmetry may help to broaden the scope of theoretical interpretations concerning hippocampal involvement in remote memory. Key words: fMRI; hippocampus; temporal gradient; consolidation; episodic memory; amygdal