Statistical Learning Induces Discrete Shifts in the Allocation of Working Memory Resources

Observers can voluntarily select which items are encoded into working memory, and the efficiency of this process strongly predicts memory capacity. Nevertheless, the present work suggests that voluntary intentions do not exclusively determine what is encoded into this online workspace. Observers indicated whether any items from a briefly stored sample display had changed. Unbeknown to observers, these changes were most likely to occur in a specific quadrant of the display (the dominant quadrant). Across 84 subjects and 5 groups of observers, change detection accuracy was significantly higher for items in the dominant quadrant, suggesting that memory encoding was biased towards the dominant quadrant. Only 9 of the 84 subjects were able to correctly specify the dominant quadrant when asked whether any location was more likely to contain the changed item, but more sensitive forced-choice procedures did reveal above-chance discrimination of the dominant quadrant. Nevertheless, because forced choice performance was unrelated to the size of the bias and no observer reported a biased encoding strategy, the bias was unlikely to depend on voluntary encoding strategies. The encoding bias was not due to a reduction in the response threshold for indicating changes in the dominant quadrant (Experiment 2). Finally, separate measures of the number and resolution of the representations in memory suggested that encoding was biased in a discrete slot-based fashion (Experiment 3). That is, although items in the dominant quadrant were more likely to be encoded into memory, mnemonic resolution for the favored items was not affected.