Neural Subspaces Encode Sequential Working Memory, but Neural Sequences Do Not
The neural mechanisms of multiple-item working memory are not well understood. In the current study, we address two competing hypotheses about the neural basis of sequential working memory: neural subspaces versus neural sequences. Using broadband MEG data from human participants, we applied dimensionality reduction and multivariate decoding techniques to test whether sequential items are maintained during the retention period through the reactivation of individual items in sequence (neural sequences), or by organizing them into distinct low-dimensional subspaces (neural subspaces). Our results revealed behaviorally relevant, low-dimensional neural subspaces that organized memory representations during the retention period but not during stimulus encoding, supporting the neural subspaces hypothesis. In contrast, we found no evidence of sequential neural replay during the delay period, contrary to predictions from the neural sequences hypothesis. Together, our findings suggest that sequential working memory is maintained through structured geometric organization in low-dimensional representational space, rather than through the sequential reactivation of individual items.