Swap errors in spatial working memory are guesses

In typical visual working memory tasks, participants report the color of a previously studied item at some probed location. Alternatively, in some recent studies, a color is probed and participants must report the item's location. There is a surprising difference between these tasks: in location reports participants almost never guess randomly as they do when reporting color, but often incorrectly report the locations of non-probed items. This finding has been taken as evidence for feature binding errors in memory, and evidence against discrete capacity models, which predict that pure guessing should occur. We test an alternative possibility: that non-target responses are guesses, but intelligent ones. In particular, when asked to report the location of an item for which participants have no memory, they may guess near locations where they know something was presented. Here we present false-probe trials in which a color is probed that was not actually studied, and find that the responses, which are necessarily guesses, are nonetheless centered around studied locations. Moreover, we find that the confidence ratings for non-target responses are low, and similar to confidence for uniformly distributed guesses. In a second experiment, we find that manipulating the retention interval, which is known to affect guess rates, changes the rate of these low-confidence non-target responses. These results suggest that the tendency to report locations of non-probed items reflects a good guessing strategy; not something fundamental about how features and objects are represented in working memory.