Patrick Wu¹, Howard Egeth, Jonathan Flombaum; ¹Johns Hopkins University

We sought to understand the difference between conjunction and feature search using a limited-exposure localization task: A search display is shown for a variable amount of time, and after it disappears, the participant positions a cursor as close as possible to where they think the target was. Distance from the true location indexes what the participant was able to learn about the location of the target, and over many trials with different exposures, knowledge as a function of time can be traced. Participants were told that the target would always be a unique item, without specific foreknowledge of identifying features. In color-shape conjunction search, two experiments suggested that participants remain maximally uncertain about the location of a target during the first 300 ms of a trial. This is consistent with search being a process of first excluding stimuli while building a template for the target and finally estimating its position. In contrast, experiments with color pop-out showed immediate and rapidly increasing knowledge about the location of a target (following just 17 ms of exposure). This is consistent with a process that only needs to estimate the location of a signal. We also conducted experiments in which participants reported what the target was, following limited exposure and localization. The results suggested that participants often know what the target is—what it looks like—before knowing anything about where it is. Additional experiments demonstrate the utility of the limited-exposure localization approach for tracking the micro-genesis of search in attention capture conditions and in conditions that produce asymmetries.