2-D Neural Geometry Underpins Hierarchical Organization of Sequence in Human Working Memory

Abstract

SummaryWorking memory (WM) is constructive in nature. Instead of passively retaining information, WM reorganizes complex sequences into hierarchically embedded chunks to overcome capacity limits and facilitate flexible behavior. To investigate the neural mechanisms underlying hierarchical reorganization in WM, we performed three electroencephalography (EEG) and magnetoencephalography (MEG) experiments, wherein humans retained in WM a temporal sequence of items, i.e., syllables, which are organized into chunks, i.e., multisyllabic words. We demonstrate that the 1-D sequence is represented by 2-D neural representational geometry in WM, with separate dimensions encoding item position within a chunk and chunk position in the sequence. Critically, this 2-D geometry correlates with WM behavior and is observed consistently in different experimental settings, even during tasks discouraging hierarchical reorganization in WM. Overall, these findings strongly support that complex sequences are reorganized into factorized multi-dimensional neural representational geometry in WM, which also speak to general structure-based organizational principles given WM’s involvement in many cognitive functions.