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Article Excerpt
INTRODUCTION
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). Consequently, these effects of distractor interference may be reduced to a considerable extent when targets appear within perceptually delineated regions of the display (e.g., by drawing a circle around the target), causing the region to appear as a distinct perceptual object on which to focus attention (e.g., Kramer & Jacobson, 1991).
The addition of perceptual boundaries to a display may also help searchers to maintain a better sense of where they have already looked, it has recently been suggested that observers fail to maintain a representation of the distractors that they have rejected in the course of search (Horowitz & Wolfe, 1998, 2001). Although other studies have refuted the notion of a fully "amnesic" visual search process (e.g., Peterson, Kramer, Wang, Irwin, & McCarley, 2001), it remains a reasonable assumption that searchers maintain a less-than-perfect memory for their search history. Perceptual boundaries may serve as landmarks according to which searchers may more easily assess the spatial relationships between the locations they have visited. Moreover, the mere presence of perceptual boundaries may encourage searchers to adopt a task-efficient scanpath--that is, one that appropriately reflects task constraints (e.g., visiting more important locations in the display before less important locations). The sensitivity of observers' scanpaths to the properties of the visual patterns they are assessing has been clearly demonstrated through the recording of eye movements (e.g., Noton & Stark, 1971).
With these points in mind, we reasoned that displays may be easier to search with the addition of perceptual cues that direct attention in accordance with task constraints. Such boundaries can help to define the regions that should be attended and ignored, allowing for the construction of a more efficient scanpath. It has previously been shown that partitioning search displays into quadrants provides little to no benefit to a non-task-directed visual search (Scinto et al., 1986). In fact, such boundaries may actually hinder performance by imposing a scanpath that counteracts the effects of bottom-up attentional guidance on the search process (Eriksen, 1955). However, if task requirements already constrain the path that search takes, perceptual boundaries that are consistent with this path could facilitate scanning along it.
In the current study, we sought to improve the efficiency with which observers searched for air track symbols within mock radar screens of the type presented in the Georgia Tech Aegis Simulation Platform (GT-ASP; Hodge et al., 1995), a task that simulates the duties of an anti-air warfare coordinator (AAWC) on a naval Aegis cruiser. A user operating the GT-ASP is required to consider several sources of information in order to identify unknown aircraft flying within the surveyed airspace displayed on a radarscope. A large part of this process involves simply scanning the radarscope for specific air tracks, the identities of which are indicated by the shapes of their symbols.
Global task constraints influence the pattern in which the user should scan the screen. AAWCs are instructed to identify unknown air tracks before they reach a 50-nautical-mile (NM; 92.6-km) range from the ownship, which is generally represented at the center of the radarscope. As a result, all other track characteristics being equal, closer tracks receive greater priority than do farther tracks. This distance-specific prioritization heuristic encourages users to search for targets in an inside-outside direction, first ensuring that targets are absent from regions close to the center before considering regions that lie farther away.
It is this inside-to-outside scanning process that we explored in the current set of experiments. In particular, we were interested in how this process might be facilitated by the addition of a range ring to the radarscope. A range ring is a centrally presented circle that delineates the region contained within a certain range from the ownship at the center of the scope. The most obvious benefit provided by the range ring is that it quickly indicates where range-specific boundaries lie, helping operators to determine how close an air track is to a given region. Many GT-ASP tasks do require range-specific decisions (e.g., "Has a track passed the 50-NM boundary?"), and range rings serve as crucial decision-making tools in these instances.
However, when range-specific decisions are not required, participants can generally follow the simple heuristic that "closer is more important." They need not know exactly where the 50-NM boundary lies in order to identify potentially dangerous air tracks appearing at a currently safe range; rather, they can rely on raw distance from the center and simply pursue tracks in an inside-to-outside pattern. Indeed, we have found that our participants only occasionally opt to...
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