Latent memory traces for prospective items in visual working memory

Visual working memory is a capacity-limited cognitive system that allows for keeping task-relevant information available for goal-directed actions. When selecting a subset of items for encoding in working memory (e.g., pears, pasta, and yogurt from a shopping list), observers can be simultaneously exposed to other items (e.g., tomatoes and eggs, on the same list) that are not selected for imminent action (hereafter: ‘prospective items’). Here, we asked whether prior exposure to such prospective items facilitates subsequent visual working memory encoding of these items, when they are selected for imminent action later. We used a so-called ‘copy task’, in which participants reproduced an arrangement of colored polygons (the ‘model grid’), in an adjacent empty grid. During placement, prospective items (i.e., hitherto unplaced items) in the model grid either remained at a fixed position or were swapped. The latter condition hampered the buildup of memory traces for prospective items. In three experiments, using different approaches to manipulate the stability of prospective items, we consistently observed that - when prospective items remained stable - participants took less time inspecting the model when encoding these items in a later stage (compared to when they were swapped). This reduced inspection duration was not accompanied by a higher number of inspections or an increase in errors. We conclude that the memory system gradually builds up latent memory traces of items that are not selected for imminent action, thus increasing the efficiency of subsequent visual working memory encoding. The present work reveals one way in which the mnemonic system circumvents its capacity limitations to efficiently operate in a complex visual world.