| Visual working memory is a fundamental cognitive subsystem that plays a key role in visually guided behavior. A distinctive characteristic of this system is its limited storage capacity. However, it is not known whether this limit reflects the storage of a discrete number of fixed-resolution representations or reflects a general resource that can be divided among an unbounded number of objects but at the cost of reduced resolution for each individual object. To explore these possibilities, I developed a quantitative model to define and isolate three independent factors in visual working memory storage---resolution, accuracy, and capacity---that together influence measures of storage capacity. Results from several color recall experiments consistently support a model of visual working memory capacity positing a small number of fixed-resolution slot-like representations. Across multiple experiments, this model was able to quantitatively account for performance across a range of set sizes and cuing probabilities with a single mechanism. When the number of objects is less than the number of slots, the slots serve as a simple type of resource, and performance depends on how many slots can be assigned to a given object. When the number of objects exceeds the number of slots, only a subset of the objects are stored in memory, and no information is retained about the other objects. When one of the objects was cued and then probed for recall more frequently than the others, the encoding probability of the uncued color was substantially smaller compared to the neutral trials on which each object was equally likely to be tested. Moreover, the uncued objects were remembered as accurately as the neutral trials. Taken together, these results suggest that the capacity is the only limit in working memory storage and that it determines both the upper and lower bounds of the resolutions of memory representations. Additional experiments explored the nature of the process that transforms fragile perceptual representations into durable working memory representations. The probability that an item will be consolidated increases over time whereas the precisions of the encoded working memory representation do not gradually increase. |
