Dissociations within human hippocampal subregions during encoding and retrieval of spatial information.

Although the hippocampus is critical for the formation and retrieval of spatial memories, it is unclear how subregions are differentially involved in these processes. Previous high-resolution functional magnetic resonance imaging (fMRI) studies have shown that CA2, CA3, and dentate gyrus (CA23DG) regions support the encoding of novel associations, whereas the subicular cortices support the retrieval of these learned associations. Whether these subregions are used in humans during encoding and retrieval of spatial information has yet to be explored. Using high-resolution fMRI (1.6 mm × 1.6-mm in-plane), we found that activity within the right CA23DG increased during encoding compared to retrieval. Conversely, right subicular activity increased during retrieval compared to encoding of spatial associations. These results are consistent with the previous studies illustrating dissociations within human hippocampal subregions and further suggest that these regions are similarly involved during the encoding and retrieval of spatial information.

Human hippocampal CA1 involvement during allocentric encoding of spatial information.

A central component of our ability to navigate an environment is the formation of a memory representation that is allocentric and thus independent of our starting point within that environment. Computational models and rodent electrophysiological recordings suggest a critical role for the CA1 subregion of the hippocampus in this type of coding; however, the hippocampal neural basis of spatial learning in humans remains unclear. We studied subjects learning virtual environments using high-resolution functional magnetic resonance imaging (1.6 mm x 1.6 mm in-plane) and computational unfolding to better visualize substructural changes in neural activity in the hippocampus. We show that the right posterior CA1 subregion is active and positively correlated with performance when subjects learn a spatial environment independent of starting point and direction. Altogether, our results demonstrate that the CA1 subregion is involved in our ability to learn a map-like representation of an environment.