Conjunction and serial search

Search tasks of this type can be contrasted with a second type, conjunction search, [conjunction search visual search for a unique conjunction of two (or more) visual features such as colour and orientation (e.g. a red tilted line) from within an array of distractors, each of which manifests one of these features alone (e.g. red vertical lines and green tilted lines)] in which the target/distractor difference is not based on a single feature, but on conjunctions of features. For example, the target might be a vertical red line in an array of vertical blue lines and tilted red lines. In this scenario, search time for the target is not constant, but instead rises with the number of distractors. The observer apparently searches through the display serially, scanning each item (or small group of items) successively (serial search). [serial search a visual search task in which time to find the target increases with the number of items in the stimulus display, suggesting that the observer must be rocessing items serially, or sequentially] This kind of task might arise in real life when you have forgotten the location of your car in a large car park. You have to find a blue Ford amongst an array of cars of many makes and colours, where, for example, red Fords and blue Volkswagens are the distractors. The target does not pop out, but finding it requires effortful attentive scrutiny (Treisman & Gormican, 1988). When search times are compared for scenes in which a target is or is not present, the times rise with the number of visible items, but they rise twice as steeply when there is no target. This is probably because, when there is a target present (which can occur anywhere in the visual display), on average, the observer has to scan half the items in the display to find it. When there is no target, on the other hand, the observer has to scan all the items in the display in order to be sure that no target is present.