Processing a visual display often requires a search for a target symbol embedded within a field of distractor symbols. There is still considerable disagreement as to why the difficulty of visual search increases as the similarity of targets and distractors increases (e.g., Duncan & Humphreys, 1989; Treisman, 1993; Wolfe, 1996). However, there is some consensus that only a limited amount of information can be fully analyzed at a given time in displays with relatively low signal-to-noise ratios. Finding a target symbol in such a display generally requires some amount of item-by-item or region-by-region processing, with observers repeatedly shifting the location of eye fixation and attentional focus to different locations in the display until the currently analyzed region contains the target and the perceptual representation of this signal surpasses some threshold level of activation.
Laboratory visual search paradigms generally entail the presentation of targets in random locations within experimental displays that may be searched in whatever manner the observer chooses. Of course, the perceptual organization of such displays may encourage a certain pattern in the sequence of ocular/attentional fixations, or scanpath (e.g., circular displays encourage circular sequences, blocks of text encourage left-to-right horizontal sequences). However, there is generally no principled reason for choosing a starting point in such tasks, and observers may often follow a roughly random scanpath for such searches (Scinto, Pillalamarri, & Karsh, 1986). In contrast, real-world visual search tasks often impose additional constraints on the scanning process. Locating a target symbol on a radar screen is one instance of a real-world search in which observers generally adopt a nonrandom scanning procedure; operators generally assess the composition of tracks in the display with specific information-seeking goals in mind (e.g., "How close is the target symbol to Position X?"). Finding a target in one region of the display may be more important than finding it in another region. It is this form of strategic "task-directed" search that we sought to understand better in the current set of experiments.
Following a prescribed scanpath shares many similarities with spatial precuing. Preexisting knowledge about the probable spatial locations in which target information will appear greatly aids visual processing. Numerous studies have demonstrated that participants' responses to probe stimuli are quicker and more accurate when the stimuli are presented at or near cued locations (e.g., Posner, Snyder, & Davidson, 1980). This "cue validity effect" (so called because enhancement occurs when cues validly predict target location) is generally attributed to the allocation of spatial attention to the cued area (Posner et al., 1980). Providing observers with a prespecified order in which to attend to different regions in a display should, therefore, have the same consequences as indicating those areas with spatial precues.
If following a prescribed scanpath ("Find target closest to Point X") encourages a sequence of ocular/attentional fixations that mimic precuing, then its effects may be enhanced by the addition of perceptual boundaries that delineate to-be-attended regions in the display. Although observers may be capable of confining their attention to an area of less than a visual degree under the right conditions (Nakayama & Mackeben, 1989), they typically experience considerable difficulty restricting attention to an unbounded region in a display. For example, observers generally find it challenging to respond to target stimuli flanked by distractors associated with different responses (Eriksen & Eriksen, 1974). This difficulty may arise partially because observers tend to focus their attention on entire perceptual objects (Duncan, 1984), and similar-looking target and distractor stimuli can appear to form a single perceptual group that encourages the allocation of such "object-based" attention (Baylis & Driver, 1992). …