Stimulus-driven shifts of attention to visual change

The visual world presents too much information to process at once. Instead, the visual system relies on several heuristics to prioritize some locations or objects over others. One of these mechanisms is a long-term bias to direct attention to sudden changes in the environment. We have all experienced this capture of our attention by sudden motion, or an unexpected new object. The first study in this thesis demonstrates that these shifts of attention are stimulus-driven: they occur even when sudden changes are irrelevant to the observer's current task and goals. The second and third studies in this thesis explore the types of visual changes that can capture our attention. According to the one prominent proposal, only one stimulus can robustly capture our attention---the abrupt appearance of a new perceptual object. In contrast, the second section shows that several kinds of sudden motion, as well as certain kinds of luminance changes, also strongly capture attention. Interestingly, while looming objects attract attention, receding objects do not appear to capture as strongly, suggesting that stimuli that signal potentially urgent events are weighted more heavily. The third section shows that new objects are not even sufficient to capture attention in the absence of a strong local luminance change. In contrast to the complex mechanisms required to detect new objects, the visual system appears to use simpler cues to direct attention to potentially important environmental changes.