Clock with Roman numerals: Image Credit: @alexxsvch (CC0)
Rather than being confined to a perpetual present, humans can adopt flexible perspectives on time. We can place ourselves within past and future events (i.e., mental time travel), or step back and survey them as a whole from a distance (i.e., mental time watching). This flexibility may come from how we frame time using spatial frameworks, specifically allocentric (world-centered) and egocentric (self-centered) reference frames.
In spatial processing, allocentric and egocentric representations rely on different regions in the brain. The medial temporal lobe, including the hippocampus, supports allocentric representations, whereas the parietal cortex supports egocentric ones. These two representations can be transformed into one another. Egocentric inputs can be integrated into a unified allocentric map in the medial temporal lobe (bottom-up processing), while allocentric representations can be used to reconstruct diverse egocentric perspectives in the parietal cortex (top-down processing).
The same mechanisms also apply to temporal processing. The hippocampus may encode a stable, perspective-independent representation of event sequences (allocentric time), while the parietal cortex may represent these sequences in a perspective- and task-dependent manner (egocentric time).
To test this hypothesis, Xu et al. asked participants to view the same sequence of events from either an internal (mental time travel) or an external (mental time watching) perspective while undergoing fMRI scanning. Activity in the posterior parietal cortex increased with greater temporal distance in the external perspective but decreased in the internal perspective. In contrast, hippocampal activity increased with temporal distance in both perspectives. These findings suggest that the hippocampus stores event sequences in a stable, perspective-independent manner, whereas the posterior parietal cortex retrieves them flexibly, depending on perspective.
Overall, these results show how the human brain supports flexible thinking about time while maintaining stable memories. They align with conceptual metaphor theory from cognitive linguistics, which proposes that abstract domains such as time are structured through more concrete domains like space, thereby enabling flexible access. This study will be relevant across disciplines, including philosophy, psycholinguistics, evolutionary biology, and cognitive neuroscience, particularly those concerned with the interplay of space, time, and memory.
