Dutriaux, L. | bioRxiv Science

bioRxiv2024

Neural Representation of Time across Complementary Reference Frames

Humans conceptualize time in terms of space, allowing flexible time construals from various perspectives. We can travel internally through a timeline to remember the past and imagine the future (i.e., mental time travel) or watch from an external standpoint to have a panoramic view of history (i.e., mental time watching). However, the neural mechanisms that support these flexible temporal construals remain unclear. To investigate this, we asked participants to learn a fictional religious ritual of 15 events. During fMRI scanning, they were guided to consider the event series from either an internal or external perspective in different tasks. Behavioral results confirmed the success of our manipulation, showing the expected symbolic distance effect in the internal-perspective task and the reverse effect in the external-perspective task. We found that the activation level in the posterior parietal cortex correlated positively with sequential distance in the external-perspective task but negatively in the internal-perspective task. In contrast, the activation level in the anterior hippocampus positively correlated with sequential distance regardless of the observers perspectives. These results suggest that the hippocampus stores the memory of the event sequences allocentrically in a perspective-agnostic manner. Conversely, the posterior parietal cortex retrieves event sequences egocentrically from the optimal perspective for the current task context. Such complementary allocentric and egocentric representations support both the stability of memory storage and the flexibility of time construals.

neuroscience↗

bioRxiv2023

Disentangling reference frames in the neural compass.

The neural system that encodes heading direction in humans is found consistently in the medial and superior parietal cortex and the entorhinal-retrosplenial circuit. However, it is still unclear whether heading direction in these different regions is represented within an allocentric or egocentric coordinate system. To investigate this problem, we first asked whether regions encoding (putatively) allocentric facing direction also encode (unambiguously) egocentric goal direction. Second, we assessed whether directional coding in these regions scaled with the preference for an allocentric perspective during everyday navigation. Before the experiment, participants learned different object maps in two geometrically similar rooms. In the MRI scanner, their task was to retrieve the egocentric position of a target object (e.g., Front, Left) relative to an imagined facing direction (e.g., North, West). Multivariate analyses showed, as predicted, that facing direction was encoded bilaterally in the superior parietal lobule (SPL), the retrosplenial complex (RSC), and the left entorhinal cortex (EC). Crucially, we found that the same voxels in the SPL and RSC also coded for egocentric goal direction. Moreover, when facing directions were expressed as egocentric bearings relative to a reference vector, activities for facing direction and egocentric direction were correlated, suggesting a common reference frame. Besides, only the left EC coded allocentric goal direction as a function of the subjects propensity to use allocentric strategies. Altogether, these results suggest that heading direction in the superior and medial parietal cortex is mediated by an egocentric code, whereas the entorhinal cortex encodes directions according to an allocentric reference frame.

neuroscience↗