Searching is a common human activity. We search for a book in the library by looking systematically along the shelves; we search for a friend in a crowd by looking for a familiar face; we search for a pin on the floor by crawling around and feeling for the pin. The common elements in search tasks are a known object that is the purpose of the search, some knowledge about how and where the object is hidden, and a deliberate concentration on the performance of search. In technical terms, search involves a target, prior-knowledge about the target-background relationship, and selective attention. Visual search implies that the visual perceptual system is being used to conduct the search.
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