of visual search investigate the process of looking for and identifying the
presence or absence of a specific visual stimulus (a target) embedded among
other items (distracters). To date, most of the research questions that have
been asked in this area have been related to the very interesting finding that
visual searches for some features are much easier and faster than searches for
other features. Two natural
questions emerge from these findings: Which visual features are found quickly
and easily and which visual features are not?
And, of course, why are some visual searches easier than others? The
answers to both questions and an understanding of the mechanisms involved in
visual search will contribute to our understanding of the active nature of
visual information processing and to the perceptual organization of our visual
does this perceptual mechanism work? Although
low-level stimulus features such as color or size play a major role in
determining the efficiency (speed) of a visual search (Geisler &
Chou, 1995), other factors like familiarity also play a role (Lubow & Kaplan,
Cavanagh, & Green, 1994).
mentioned above, one fascinating aspect of our ability to perform a visual
search is that some physical characteristics of stimuli allow for easy and
efficient searches, whereas other stimuli result in difficult and time-consuming
searches. This property is often
referred to as salience, and objects that have high salience are perceived to
“pop-out” from their surroundings. It
appears that visual scenes can be processed in parallel, that is,
simultaneously, and pre-attentively for these items.
Low salience objects, on the other hand, require lengthy searches.
A search for low salience objects seems to take place in a serial format
where item-by-item processing is required. Sometimes simply changing which
stimulus is the target and which stimuli are the distracters changes the quality
of the search from one mode to another. For
example, Triesman & Souther (1985) found that searching for an “Q” among
“O”’s was an easy (parallel) task but that searching for a “O” among
“Q”’s was a more difficult (serial) task and we
demonstrate this with the demonstrations below.
Figure 14. This is an example of searching for a "Q" among a small array of "O's". Clicking on the button labeled "Start" will present the stimulus array for a brief 100msec. This is an example of a parallel search: the "Q" pops out and you have no difficulty seeing it despite the very short duration of the display. Now try the demonstration below:
Figure 15. Same as above, but now you are searching for a "Q" in an array of 36 other items. Again, click on "Start" to flash the array. Here, despite the fact that there is a 9-fold increase in the number of search items, you find the "Q" quickly. There is little if any influence of the number of items. O.k., now try the demonstrations below. In the first example you will be searching for a "O" among "Q's" in an array of 4 items.
Figure 16. Again, you have no difficulty identifying the presence of the target, an "O" in this case, when the array size is small. Now try the following:
Figure 17. Ouch! Were you lucky enough to see it? Not likely. In an array of items such as this the length of presentation must be considerably longer in order to allow people enough time to identify the presence or absence of a target "O". It does not pop-out.
of the additional visual features that pop-out are brightness (Gilchrist,
Humphreys, Riddock, & Neumann, 1997), color (D’Zmura,
1991), and motion (Nothdurft,
1993). Quite often, though, we are
searching for objects that can only be identified by the simultaneous presence
of two or more stimulus features. These
so-called conjunction searches have been studied for color and form (D’Zmura,
Lennie, & Tiana, 1997), motion and form (Muller & Found,
and orientation (Friedman-Hill & Wolfe,
1995), and for the conjunction of
two colors or two sizes (Wolfe,
conjunction searches are very difficult and require serial processing. A good
example is the children’s game “Where’s Waldo.” In this popular game, children search drawings of crowded
social events for one particular individual (Waldo) who is characteristically
dressed in a red and white-stripped sweater and cap, glasses, and dark hair.
Waldo must be distinguished from a crowd of “distractor” individuals
who possess some but not all of these features.
there has been extensive research on the topic of visual search over the last
decade, it is evident that there is still much to be learned about the basic
processes involved. The results of
dozens of visual search experiments (studying many different types of visual
features and feature conjunctions) has shown us that a sharp distinction
between serial and parallel processing may be too simplistic (Wolfe,
allocation of attention in visual search probably lies along a continuum, where
stimulus features and context determine search efficiency. This raises
interesting questions for future research.
For example, what situations result in optimal search efficiency? How
could the search context for a specific target be manipulated to maximize search
answers to these questions have a very practical value in applied settings.
That is, outside the laboratory in the real world.
What we have learned about visual search has quickly been applied to many
real world situations such as air traffic control (Vortac, Edwards, Fuller,
& Manning, 1993), driving (Lajunen,
Hakkarainen, & Summala, 1996; Summala,
Pasanen, Rasanen, & Sievanen, 1996), visual display design (Fisher
& Tanner, 1992) and how visual displays are monitored in the workplace (Liu,
1996). Today, questions regarding sensation and perception are
increasingly being applied to problems outside the laboratory.