The Human Hippocampus Contributes to Egocentric Coding of Distance to a Local Landmark

Abstract

Spatial navigation can depend on path integration or environmental cues (e.g., landmarks), which are thought to be integrated in hippocampal and entorhinal circuits. This study investigates the anatomical basis of path integration and navigation based on a single local landmark using an individual differences approach, since people vary substantially in their ability to navigate with path integration cues and landmarks. In two experiments, we dissociated the use of path integration and a local landmark in the same navigation task, and investigated whether morphological variability in the hippocampus and entorhinal cortex could explain behavioral variability in young healthy humans. In Experiment 1, participants navigated in a fully immersive virtual reality environment, with body-based cues available for path integration. The participants first walked through a series of posts before attempting to walk back to the remembered location of the first post. We found that gray matter volume of the hippocampus positively predicted behavioral accuracy of retrieving the targets distance in relation to the local landmark. Hippocampus also positively predicted path integration performance in terms of walking-distance to the target location. Experiment 2 was conducted in a desktop virtual environment, with no body-based cues available. Optic flow served as path integration cues, and participants were tested on their memory of a learned target location along a linear track. Consistent with Experiment 1, the results showed that hippocampal volume positively predicted performance on the targets distance in relation to the local landmark. In contrast to Experiment 1, there was no correlation between hippocampal volume and path integration performance. Together, our two experiments provide novel and converging evidence that the hippocampus plays an important role in encoding egocentric distance to a single local landmark during navigation, and they suggest a stronger hippocampal involvement when path integration is based on body-based compared to optic flow cues.