II. Visual Search

When hunting concealed prey, most ground-feeding birds are engaging in visual search (see video clip of a pigeon searching for grain). It happens that search is  widely used in laboratory studies of visual perception. The laboratory search procedure requests the participant to find a target embedded in a multi-item display, which may comprise an array of alphanumeric symbols, forms, colors, or words. Performance is usually assessed under time pressure; either the display is flashed only briefly or the participant is instructed to respond as quickly as possible. An important theme of this chapter concerns the manner in which such laboratory data and resulting theories apply to natural foraging.

Some Background

The experimental study of search in humans has a rather different history than does comparable work in pigeons. Some current research shares common techniques, however, for which we can thank the computer, especially its ability to produce quickly an enormous variety of visual displays and to time events in milliseconds.

Early search studies with humans were motivated by applied considerations such as the design of radar and other instrumental displays. For example, Green and Anderson (1956) found that search for a particular numeral was speeded if the target had a unique color. Over the years, the search procedure has come to address some deeply theoretical issues concerning human visual information processing. In summarizing some of these, it is helpful to return to the distinction between perceptual and memory variables. Search efficiency is most obviously affected by display characteristics, such as the density of non-target items. However, search also depends on the number of potential targets. If, for example, the searcher is looking for any of several letters, he or she may have to refer back to memory where representations of the target set are stored. An important manipulation in search experiments is that of set size, where the set may refer to either to the number of items in the display or the number of targets to remembered. These manipulations, along with similarities among set members, have helped guide modern views about visual-information processing.

Perceptual Search

Human studies. An early and important observation in search studies concerned the relation between RT and display size. Under some conditions search time increases with more distractors (the red line in the figure to the right, which idealizes the distinction between processes affected by display size and those that are not), but in others RT seems to change very little with distractor set size (the blue line). These observations have given rise to a fundamental differentiation between information-processing styles. When the relation is increasing, the display items are said to be examined serially or one at time. When serial processing is literally the case, RT should have a linear relation to set size, indicating that overall search time is incremented by a fixed amount as each item is examined. When RT does not change with set size, processing is said to be parallel; that is, the searcher inspects all of the items at once; in such cases the search task is easy enough that focused attention is not required. In fact, truly invariant RTs are rare, but experimental manipulations distinguish clearly between the two types of function, and the descriptors are still in use.

Research has not only identified conditions associated with serial or parallel search but has also led to models detailing the ways in which humans process complex visual information. For example, manipulation of similarity among targets and distractors has proven useful. As would be expected, search is faster when the target is dissimilar to all distractors in the display, and in cases of sufficient dissimilarity search becomes almost parallel. A well-known analysis of this phenomenon suggests that parallel search occurs only when the target possesses a unique feature that distinguishes it from the other items (Treisman & Gelade, 1980). According to this analysis, there are a limited number of such features, which include specific colors and line orientations. To the observer a unique feature seems to pop out from the display whether or not it is the object of attention (see Figure 2). Treisman and Gelade suggested properties of the human neural system that determine which visual characteristics constitute features (also see Cook, 2001 for more on this in pigeons).

A number of models have attempted to relate such perceptual effects to stages of information processing. For example, Hoffman (1979) suggested that easy target-distractor discriminations are analyzed at an early level that deals with information in parallel. For difficult discriminations further levels of processing are required, and these are performed in serial. A modification of Hoffman’s model views the two processes as more closely blended in time (Wolfe, Cave, & Franzel, 1989).

Avian Studies. Search in birds as well as humans depends perceptual factors. In foraging situations similarity between prey and background is a natural condition, because such crypsis is beneficial to the prey. Studies that address foraging strategies are most likely to assess consummatory behavior, using observational techniques and prey appropriate to the species under investigation. For example, Dawkins (1971) observed chick feeding behavior when dyed grains were scattered on surfaces that were similar or different in color. Her findings showed that the birds initially consumed the concealed grains more slowly, although their speed increased with practice. Gendron (1986) reported a similar outcome in observations of quail selecting food pellets. Of course, these results are hardly surprising, and it should be noted that such comparisons were secondary to the main purpose of the cited research. The section on search image return to this and related work

Other research approaches substitute simple pecking for the full consummatory response. For example, experiments with jays required the birds to peck at photographic displays when the image included a moth; the moth, whenpresent, could appear against a similar or dissimilar natural background (Pietrewicz & Kamil, 1979). In our laboratory, pigeons peck at computerized displays made up of artificial symbols (See Figure 1) .

As in the human studies noted earlier, we have explored target concealment by varying the number of distractors and their similarity to the targets. Figure 4 illustrates some simpledisplay-size and similarity effects obtained in a pigeon study (Blough, 1992). The distractors were alphanumeric symbols. In a conspicuous or low-similarity condition the target was a filled heart. In intermediate and high-similarity conditions targets were a '/' and a 'B' respectively. As shown below, search for the filled form was faster overall, indicating that this target was the most distinctive. Further, the mode of search appeared to be parallel, contrasting with the seemingly serial style associated with the less distinctive targets.

Other perceptual factors include the way in which display items are grouped; for example, detection seems faster if distractors are alike (Blough & Blough, 1990). Natural crypsis is enhanced by the clutter characteristic within foraging patches.

It is interesting to ask whether there are certain natural "features" that command the pigeon's attention. Treisman and Gormican (1988) placed forms distinguished by a line or a gap in that category; that is, search appeared parallel for such forms, but was "serial" when the form was distinguished instead by the absence of a line or a gap. and this finding was replicated with humans in our search paradigm by Allan and Blough (1989). However, when pigeons performed a comparable search task, there was no evidence for such special properties. Of course, these findings do not rule out the possibility of special "pigeon features."

Memory Search

Human Studies. Memory assumes a special role in visual search when the observer’s task is to locate any of several targets. In such cases he or she will presumably refer to a list stored in memory, and the task can be labeled memory search. Of course, memory search occurs in everyday circumstances as well. A familiar example occurs when one tries to recall the name associated with an acquaintance’s face.

Again research has distinguished between parallel and serial processes, marked by set-size effects. In a search paradigm, Schneider & Shiffrin (1977) varied the strength of the memory demand. In advance of each trial participants viewed a list specifying potential target items; the size of this memory set was variable. Following a subsequent test display the viewer indicated whether or not one of those targets was present in the display. In a relatively easy "consistent-mapping" task, members of target set were the same throughout the session. In a more difficult "varied-mapping task," symbols that had been targets on one trial could appear as distractors on another. Search appeared to be parallel in the consistent condition; that is, the practiced observer's speed did not depend on the number of items in the list. In contrast, search time increased with the number of items in the variable condition.

One way of understanding the difference between consistent and varied memory sets invokes the distinction between working and reference memory. Working memory has a limited capacity and is transient; it holds new information that has just been supplied or old information recently refreshed. Reference memory is relatively permanent, representing a vast amount of previously acquired knowledge. Schneider and Shiffrin’s (1977) varied-set task taps working memory, because the searcher must address new information at the start of each trial. Information required for the consistent-mapping task could remain in reference memory, because the distinction between targets and distractors was constant throughout a session,. The differences in set-size functions are understandable if we assume that working memory is addressed by a serial process while reference memory is accessed by some form of parallel process (e.g. Logan & Stadler,1991.).

Closer scrutiny of RTs in a fixed-set task shows an overall reduction with practice (Shiffrin & Schneider, 1977). This change may be attributable to a gradual disengagement from working memory as the searcher learns the distinction between targets and distractors. If this interpretation, as well as those suggested above, are correct, the set-size effect should be present early-on and should diminish with experience as long as a consistent-mapping task is used. This changeover is difficult to capture in humans because of the speed with which it seems to occur.

Avian Studies. Because they consume varying grain types, foraging pigeons may also address a memory list of acceptable targets. It is also reasonable to distinguish between lists that reside in working and reference memories. Avian research that has operationalized this distinction has helped us understand set-size effects seen in the pigeon search task.

An early analysis (Honig, 1978) recalls the Shiffrin and Schneider (1977) distinction between fixed and variable memory sets. In particular, Honig distinguished between aspects of a task that varied between trials and those that remained fixed over a session. The matching-to-sample experiment provides a good illustration (see demo from Grant & Kelly, 2001). The sample changes from trial to trial, so its current value is variable and would be held in working memory. However, the relation between a given sample value and the associated response remains constant, and that information could remain in reference memory.

Several approaches have considered the capacity of pigeon working memory and the nature of information that is retained when the load is high. Chase (1983) showed, for example, that pigeons can correctly identify up to 3 items just seen; Shimp (1976) and Terrace (2001) have described the correspondence between what is remembered and the order in which information is presented. Such work helps to define the contents of working memory; the search procedure should allow us to address the manner in which they are addressed. In a "conditional" search procedure we attempted to specify the effects of set size on search RT (Blough & Blough, 1990). Here letter stimuli could be either targets or distractors; pre-trial cues informed the bird which letters would be targets in the forthcoming display. In some conditions the cue was mapped to a single letter; in others it was mapped to a set of several targets, only one of which appeared. The data suggest a serial mode; that is, RT increased with the number of targets cued, but the effects are small and need replication for a clear conclusion.

As in humans, pigeon long-term memory has a relatively large capacity. Tests of picture recognition, for example, indicate that birds can categorize large sets of photographic slides according to whether or not they have been seen previously (Vaughan & Greene, 1983).

It is relatively easy to conduct a fixed-set memory search task with pigeons, and we have explored this matter. In the display-search task illustrated earlier pigeons are rewarded for pecking at any of several target letters, which are drawn from a fixed set. Only one of those targets appears in any given display, and the number of letters in a set varies in different phases of a study. If pigeons as well as humans address reference memory in parallel, RT should not depend on set size. An early study (P. Blough, 1984) provided seemingly inconsistent results. Experiment 1 revealed an RT elevation associated with the larger of two set sizes. However, the effect was not replicated in Experiment 2, conducted with the same birds and some of the same target letters. We speculated that practice improved the efficiency with which the birds accessed a multiple-item list.

In subsequent work, Vreven and Blough (1998, Experiment 2) considered set-size and practice effects more fully. Birds learned to search for each of four targets, but in sessions that presented only a single letter or, in a mixed condition, any of four letters selected randomly. Test sessions then presented either single letters as well or, in a mixed condition where all four letters were presentedrandomly within a session. The data reveal a sizeable set-size effect during initial test sessions, but RTs for the two set sizes eventually converged (See Figure 5). This outcome is consistent with the notion that practice brings about changes in the way multiple memory items are processed and that a switch from working to reference memory may be involved.

It is logical to think that memory search would proceed in a different manner if some of the targets in a set were similar to each other. For example, a search task might employ targets that shared features with each other, but not with any of the distractors. In that case, the common feature, if identified, would constitute a single target, and the set would have a size of one. Our experiments were designed to avoid this situation, but it reasonably could occur in a foraging patch. Two search-image studies, to be described in the next section, illustrate cases in which birds seemed to generalize between targets.

Next section: Attention and the Search Image