T he world delivers more input than the visual system can handle. In response, the visual system has developed a variety of ways to limit the amount of information that it processes. For example, our foveated retina transmits the most detailed information from only a very small part of the visual eld. Because we cannot see the entire world at once, we must move our eyes in order to get that level of detail from a different portion of the visual eld. That retinal image still contains more information than the system can process. Visual selective attention is a set of mechanisms to restrict processing further to a subset of the input (e.g., a single location or object) at one time. As a consequence, if we want to know if a specic object is present, we will often need to search for it, even if it is easily visible. Thus, in Figure 5.1, the medium size, light gray diamond is perfectly visible but not immediately discovered until we search for and direct our attention to it.