Neural representation of temporal sequence and simultaneity in the retrieval of episodic memory

Abstract

Episodic memory retrieval requires one to recollect the simultaneously bounded information as well as the sequence of events in the order of occurrence. Previous studies reported the involvement of the hippocampal formation, dorsal striatum, and frontal lobe in processing temporal binding and sequence. But how the neural representations of simultaneity and sequence differ and how the length of temporal distance influences these representations have not yet been brought to light. In this study, we postulated that there would be a neural dissociation of episodic binding and sequence processing in the brain regions mentioned above. To test this hypothesis, we performed an event-related functional resonance imaging (fMRI) experiment on a task that combines components of episodic and associative memory paradigms, designed to compare the neurocognitive representations of temporal distances. Subjects were instructed to remember the order of objects presented on screen and were later asked to make temporal order judgment between two randomly paired objects. Univariate analysis suggested a functional dissociation of simultaneity and sequence in the anterior-posterior frontal lobe. Multivoxel pattern analysis (MVPA) classification revealed the caudate and anterior hippocampus to store dissociable representations of simultaneity and sequence. Regional differential pattern similarity analysis also revealed that the subiculum contains neural representations of sequential conditions clearly distinct from that of simultaneity and the entorhinal cortex to contain representations specific to short distances. Furthermore, representational similarity analysis (RSA) revealed that the caudate codes short temporal distances differently from other ranges, and the hippocampus codes consecutivity differently from other temporal conditions. These results suggest that the brain regions previously implicated in temporal memory, especially the hippocampal formation and the dorsal striatum, may specialize in their functions in processing simultaneously bounded and sequential information of episodic memory retrieval, and contain neural representations specific to temporal distances, which may form a basis of how we utilize temporal information for the complete retrieval of episodic memory.