Neural Basis of Serial Scanning across Position Codes in Verbal Working Memory

Serial order processing is a core component of verbal working memory (WM), yet its neural mechanisms remain incompletely understood. Here, we examined how order information is instantiated in the brain during an order comparison task that temporally dissociated item/serial position retrieval from comparison processes. To this end, we acquired fMRI data while participants accessed a WM item and its serial position and subsequently performed order judgments. Univariate analyses revealed engagement of a frontoparietal WM network during WM item/serial position access. Decoding analyses within a set of functional and a priori ROIs showed that a subset of these regions, including left middle frontal gyrus and superior frontal gyrus, bilateral posterior intraparietal sulcus (IPS), left SMA, and left supramarginal gyrus, exhibited position-sensitive coding independent of item identity, consistent with recent proposals of distributed positional representations. Behavioral performance showed a reverse distance effect during order judgment, with slower responses for Distance 2 than Distance 1. Univariate fMRI analyses revealed a neural reverse distance effect in the left posterior IPS and right posterior hippocampus. Notably, the left posterior IPS showed both positional coding during WM access and distance-sensitive activity during order judgments, supporting positional coding models and suggesting that the brain serially scans positional representations in verbal WM.