The representation of serial order in working memory: A matter of space or time?

The representation of serial order information is a fundamental aspect of verbal working memory (WM). However, the way our brain represents serial order information remains an open question. The spatial hypothesis considers that auditory-verbal serial order information is recoded using spatial codes, according to a left-to-right dimension for left-to-right readers. The temporal hypothesis considers the intervention of temporal codes for representing serial order information. This fMRI study contrasted the spatial and temporal hypotheses of WM serial order coding by administering an implicit spatial processing task involving the end-of-movement detection of a dot that stabilized on leftward, central or rightward endpoints, an implicit temporal processing task involving the detection of a target sound that occurred at early, middle and late timepoints of an auditory sequence, and a verbal WM task involving the encoding and retention of the serial order of 6-letter lists. We observed shared multivariate neural activity patterns for the encoding of WM serial position information and corresponding timepoints in the temporal processing task, and this in several frontal and parietal areas. Moreover, neural activity patterns in the left anterior IPS also distinguished the different spatial endpoints in the spatial processing task and predicted robustly corresponding serial position information in the WM task. These results provide support for implicit temporal and to a lesser extent, implicit spatial encoding of serial order information in verbal WM.