ABSTRACT
In rodents representations of environmental positions follow a granularity gradient along the hippocampal and entorhinal anterior-posterior axis; with fine-grained representations most posteriorly. To investigate if such a gradient exists in humans, functional magnetic resonance imaging data were acquired during virtual environmental learning of the objects' positions and the association between the objects and room geometry. The Objects-room geometry binding led to increased activation throughout the hippocampus and in the posterior entorhinal cortex. Within subject comparisons related specifically to the level of spatial granularity of the object position encoding showed that activation in the posterior and intermediate hippocampus was highest for fine-grained and medium-grained representations, respectively. In addition, the level of fine granularity in the objects' positions encoded between subjects correlated with posterior hippocampal activation. For the anterior hippocampus increased activation was observed for coarse-grained representations as compared to failed encoding. Activation in anterior hippocampus correlated with the number of environments in which the objects positions were remembered when permitting a coarse representation of positions. In the entorhinal cortex, activation in the posterior part correlated with level of fine granularity for the objects' positions encoded between subjects, and activation in the posterior and intermediate entorhinal cortex increased for medium-grained representations. This demonstrates directly that positional granularity is represented in a graded manner along the anterior-posterior axis of the human hippocampus, and to some extent entorhinal cortex, with most fine-grained positional representations posteriorly.
