Neural signatures of serial order access in verbal working memory

Serial order working memory allows information to be maintained as structured sequences, yet the neural formats supporting access to serial position remain unclear. Behavioral evidence suggests that temporal order can be recoded into spatial representations, but serial order may also rely on representations that generalize beyond the spatial configuration of encoding. Here, participants memorized digit sequences presented either left-to-right or right-to-left, thereby reversing the mapping between ordinal position and external space while keeping retrieval cues centrally presented during fMRI. Behaviorally, spatial coding during retrieval was modulated by encoding direction. Multivoxel pattern analysis revealed direction-specific decoding in lateral parietal cortex: left inferior parietal cortex encoded serial position after left-to-right encoding, whereas right inferior parietal cortex did so after right-to-left encoding. In contrast, cross-direction decoding revealed direction-invariant effects in left supramarginal and precentral regions, suggesting serial-position information that generalized across spatial layouts. Finally, univariate parametric modulation revealed activity scaling with serial position in bilateral posterior parietal cortex, left anterior hippocampus, and action-control regions. Together, these findings indicate that serial order access in working memory relies on multiple neural signatures, including spatially context-dependent representations, direction-invariant ordinal codes, and graded process-related activity.