What is visual search?
Loosely following William James, we can assert that everyone knows what visual search tasks are because every one does them all the time. Visual search tasks are those where one looks for something. Real world examples include search for tumors or other critical information in X-rays, search for the right piece of a jigsaw puzzle, or search for the correct key on the keyboard when you are still in the "hunt and peck" stage of typing.
In the lab, a visual search task might look something like Figure One. If you fixate on the * in Figure One, you will probably find an "X" immediately. It seems to "pop-out" of the display. However, if you are asked to find the letter "T", you may not see it until some sort of additional processing is performed. Assuming that you maintained fixation, the retinal image did not change. Your attention to the "T" changed your ability to identify it as a "T".
Figure 1. Fixating on the "*", find the X and T
Processing all items at once ("in parallel") provides enough information to allow us to differentiate an "X" from an "L". However, the need for some sort of covert deployment of attention in series from letter to letter in the search for the "T" indicates that we cannot fully process all of the visual stimuli in our field of view at one time (e.g. Tsotsos, 1990). Similar limitations appear in many places in cognitive processing.
It is important to distinguish covert deployment of attention from movements of the eyes. If you fixate on the * in Figure Two, you will find that, not only doesn't the "T" pop out, it cannot be identified until it is foveated. It is hidden from the viewer by the limitations of peripheral visual processing.
Figure 2. Find the T
You can identify the stimuli in Figure 1 while fixating the central "*". This is not to say that you did not move your eyes - only that you did not need to move your eyes. For most of the experiments discussed below, eye movements were uncontrolled. While interesting, eye movements are probably not the determining factor in visual searches of the sort discussed in this reivew - those with relatively large items spaced fairly widely to limit peripheral crowding effects (Levi, Klein & Aitsebaomo, 1985). For instance, when Klein & Farrell (1989) and Zelinsky (1993), using stimuli of this sort, had participants perform the search tasks with and without overt eye movements, they obtained the same pattern of RT data regardless of the presence or absence of eye movements. The eye movements were not random. They simply did not constrain the RTs even though eye movements and attentional deployments are intimately related (Hoffman & Subramaniam, 1995; Khurana & Kowler, 1987; Kowler, Anderson, Dosher & Blaser, 1995)
Excerpt from Wolfe, J. M. (1996). Visual search. In H. Pashler (Ed.), Attention. London, UK: University College London Press.