Neural population dynamics and temporal context cells in macaque medial parietal cortex support temporal order memory.

Episodic memory involves encoding and remembering the order of events experienced over time. Previous work examining the mechanisms of temporal order memories has focused primarily on the hippocampus and prefrontal cortices, with comparatively less attention paid to population-level memory signals in the medial posterior parietal cortex (mPPC). Combining in vivo multi-unit electrophysiology and a temporal order judgment task with naturalistic cinematic material in macaques, we show that population activity in mPPC exhibits temporally structured dynamics during both encoding and retrieval. During encoding, mPPC neuronal ensembles exhibit gradually evolving activity patterns consistent with temporal context representations embedded in the unfolding video episodes, whereas during retrieval these neurons engage in coordinated, synchronous activity preceding memory-guided decisions. Moreover, trial-by-trial similarity between population activity patterns during encoding and retrieval predicts temporal order judgment performance. A separate control experiment further ruled out eye saccades, fixation patterns, and scan paths as confounding factors contributing to the observed neural dynamics. Together, these findings suggest that mPPC contributes to temporal order memory through population-level representations that integrate temporally extended experience with retrieval-related decision processes, rather than through simple sensory-driven or motor-related responses.