University of Alberta Dictionary of Cognitive Science: Pop Out

Visual search tasks are frequently used to study visual cognition. In such a task, a subject is usually presented a visual display consisting of a number of objects. In the odd man out version of this task, in one half of the trials one of the objects (the target) is different from all of the other objects (the distractors). In the other half of the trials, the only objects present are distractors. Subjects have to decide as quickly and accurately as possible whether a target is present in each display. The dependent measures in such tasks are search latency functions, which represent the time require to detect the presence or absence of a target as a function of the total number of display elements.

Pioneering work on visual search discovered the so-called pop out effect: the time required to detect the presence of a target that is characterized by one of a small number of unique features (e.g. color, orientation, contrast, motion) is largely independent of the number of distractor elements in a display, producing a search latency function that is essentially flat (Treisman & Gelade, 1980). This is because, regardless of the number of elements in the display, when the target is present it seems to pop out of the display, bringing itself immediately to attention. In contrast, the time to detect a target defined by a unique combination of features generally increases with the number of distractor items, producing search latency functions with positive slopes. The small number of properties that produce pop out in visual search are also the same properties for which visual modules have been discovered by neuroscientists (Livingstone & Hubel, 1988).

References:

Livingstone, M., & Hubel, D. (1988). Segregation of form, color, movement and depth: Anatomy, physiology, and perception. Science, 240, 740-750.
Treisman, A. M., & Gelade, G. (1980). A feature integration theory of attention. Cognitive psychology, 12, 97-136.

(Added September 2010)