Is it a matter of time or space to code serial order in working memory?

Many current verbal WM models agree on the existence of positional markers for binding items and their serial position. However, they diverge to define the nature of serial order coding: items could be represented according to a spatial or a temporal dimension. To confront these hypotheses, participants performed three tasks in fMRI: a spatial detection dot task, a temporal detection sound task and an order WM probe recognition task. In each task, we varied the position probed: start/left or end/right position. Multivoxel pattern analyses demonstrated same neural patterns at fronto-parietal level allowing to discriminate between both positions in the order WM and the temporal detection tasks (the start vs. the end of the WM list/temporal sequence). However, no prediction with the spatial task (left vs. right space) was observed suggesting a specific involvement of the temporal dimension use as an internal code to process serial order information in WM.