2D mental whiteboards: Multidimensional spatial coding of serial order in verbal working memory

Previous work has shown that serial order in verbal working memory triggers the spontaneous formation of a mental line on which the sequence of items is coded. Recent studies show that the orientation (horizontal versus vertical) and coding direction (e.g., left-to right versus right-to-left) of this spatialization can flexibly be adapted to match the task context. Here we investigate whether people spatialize a sequence of verbal items using multiple spatial dimensions simultaneously if the task probes such strategy. In two experiments, participants had to encode a letter sequence and maintain it for later reproduction. In between, they performed a vowel-consonant classification task on items that were part of the memorized sequence to probe for spatialization. Classification occurred via horizontal or vertical saccade responses that varied unpredictably from trial to trial, with a fixed (Experiment 1) or variable (Experiment 2) stimulus-response mapping. Spatial biases were observed for both horizontally (early-left, late-right) and vertically (early-up, late-down) oriented classification trials (mixed within the same block), indicating that the item sequence was spatialized in a multidimensional format (i.e. exploiting both the vertical and horizontal axes). The horizontal and vertical biases were also correlated with each other. Overall, we show for the first time that task context can probe a strategy in which people spatially code a sequence of verbal items using multiple spatial dimensions simultaneously.