Gravity-dependent choice of frame of reference in the posterior parietal cortex.

Reference frame is an essential concept for spatial navigation. The principles underlying the choice of reference frames by neuronal ensembles, especially under natural conditions, which involve sloping terrains, remain unclear. The posterior parietal cortex (PPC) is a central region for navigation and movement control, and it contains several distinct types of reference frames. Here, we investigated the influence of gravity signals on egocentric tuning and allocentric head direction tuning in the mouse PPC by introducing a mismatch between visual input and gravitational signals by tilting the ground. We found that the allocentric head direction tuning and egocentric boundary bearing tuning were differentially controlled by altered gravity signals. Specifically, the allocentric head direction signals became stronger, whereas the egocentric boundary signal was weakened. However, across horizontal and tilted conditions, the head direction tuning was less consistent, as the head directional signal could be anchored to a visual or gravity frame of reference. As the mismatch between gravitational and visual inputs increased, the gravity-anchored head direction cells became dominant. Notably, individual head direction cells can switch between reference frames. Together, these results suggest that PPC neurons exhibit a highly flexible selection of reference frames based on task demands, which may allow efficient navigation strategies in natural environments.