Visual search
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Visual search is a type of perceptual task requiring attention and vigilance. Visual search involves an active scan of the visual environment for particular objects or features. Common examples include trying to locate a certain brand of cereal at the grocery store or a friend in a crowd (e.g. Where's Waldo), though the scientific study of visual search typically makes use of much more idealised or abstract search tasks.
The process of Visual Search is made more difficult by the occurrence of distracters. Distracters are non target stimuli that divert attention away from the target stimulus. In general, the more similar distracters are to the sought-after target, the more difficult the search task will become.
In addition to the effects of distracter/target similarity, the number of distracters often has an effect on the difficulty of the visual search task. The total number of targets and distractors in a given visual array is commonly referred to as the display size. The display size effect is the degree to which the display size hinders the visual search process. Depending on the type of visual search the magnitude of the display size effect can vary greatly, from effectively zero in searches for a feature singleton (below) to a large effect in conjunction search and other difficult search tasks.
Search tasks with a small display size effect may be referred to as "efficient" or "parallel" search tasks. Conversely, search tasks showing a large display size effect are termed "inefficient" or "serial" search.
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Types of search
Feature Search
Feature Search is the process of searching for targets defined by a single visual feature, such as color, size, orientation or shape.
Feature searches can often be easily conducted if the target stimulus has a unique visual feature (a "featural singleton") relative to the distracters. For instance, a red target is easily found if all the distracters are blue. In this case the object often appears to “pop out” of the display. This form of featural singleton search is almost invariably highly efficient; increasing the number of blue distracters does not substantially increase the time it takes to find a single red target.
Conjunction Search
Conjunction Search occurs when a target stimulus is defined not by any single visual feature, but by a combination of two or more features. An example is search for a red square among both blue squares and red circles: neither the single feature "red" nor the feature "square" is sufficient in isolation to uniquely specify the search target.
Conjunction searches are typically inefficient, with the time to complete the search task increasing linearly with the number of distractors. This behavior is as if the subject were forced to examine each item in the search array one at a time before deciding whether or not it was the search target, leading to the term "serial search".
Popular Search Theories
- Feature Integration Theory
- Similarity Theory
- Guided Search Theory
- Movement Filter Theory
References
- Wolfe, J M (1998) Visual Search Originally Published in Attention, H. Pashler (Ed.), London, UK: University College London Press, Fulltext
- J Theeuwes - Perception & Psychophysics, 1992 Fulltext
- Treisman A (1991) Visual perception of features and objects(Annual Report, 15 Sep. 1990- 14 Sep. 1991). 53 Behavioral Sciences(AH)
- Verghese, P (2001) Visual Search and Attention: a Signal Detection Theory approach. Neuron, 30 August 2001, vol. 31, no. 4, pp. 523-535(13).
