Set size effects on working memory precision are not due to an averaging of slots

Visual working memory is often characterized as a discrete system, where an item is either stored in memory or it is lost completely. As this theory predicts, increasing memory load primarily affects the probability that an item is in memory. However, the precision of items successfully stored in memory also decreases with memory load. The prominent explanation for this effect is the "slots-plus-averaging" model, which proposes that an item can be stored in replicate across multiple memory slots. Here, however, precision declined with set size even in iconic memory tasks that did not require working memory storage, ruling out such storage accounts. Moreover, whereas the slots-plus-averaging model predicts that precision effects should plateau at working memory capacity limits, precision continued to decline well beyond these limits in an iconic memory task, where the number of items available at test was far greater than working memory capacity. Precision also declined in tasks that did not require study items to be encoded simultaneously, ruling out perceptual limitations as the cause of set size effects on memory precision. Taken together, these results imply that set size effects on working memory precision do not stem from working memory storage processes, such as an averaging of slots, and are not due to perceptual limitations. This rejection of the prominent slots-plus-averaging model has implications for how contemporary models of discrete capacities theories can be improved, and how they might be rejected.