Disentangling reference frames in the neural compass.

The neural system that encodes heading direction in humans can be found in themedial and superior parietal cortex and the entorhinal-retrosplenial circuit.However, it is still unclear whether heading direction in these differentregions is represented within an allocentric or egocentric coordinate system. Toinvestigate this problem, we first asked whether regions encoding (putatively)allocentric facing direction also encode (unambiguously) egocentric goaldirection. Second, we assessed whether directional coding in these regionsscaled with the preference for an allocentric perspective during everydaynavigation. Before the experiment, participants learned different object maps intwo geometrically similar rooms. In the MRI scanner, their task was to retrievethe egocentric position of a target object (e.g., Front, Left) relative to animagined facing direction (e.g., North, West). Multivariate analyses showed, aspredicted, that facing direction was encoded bilaterally in the superiorparietal lobule (SPL), the retrosplenial complex (RSC), and the left entorhinalcortex (EC), a result that could be interpreted both allocentrically andegocentrically. Crucially, we found that the same voxels in the SPL and RSC alsocoded for egocentric goal direction but not for allocentric goal direction.Moreover, when facing directions were expressed as egocentric bearings relativeto a reference vector, activities for facing direction and egocentric goaldirection were correlated, suggesting a common reference frame. Besides, onlythe left EC coded allocentric goal direction as a function of thesubject's propensity to use allocentric strategies. Altogether, theseresults suggest that heading direction in the superior and medial parietalcortex is mediated by an egocentric code, whereas the entorhinal cortex encodesdirections according to an allocentric reference frame.