Attentional Capture ABSTRACT: How likely are subjects to notice something salient and potentially relevant that they do not expect? Recently, several new paradigms exploring this question have found that, quite often, unexpected objects fail to capture attention. This, phenomenon known as ‘inattentional blindness’ has been brought forth by Simon (2000) who raised the intriguing possibility that salient stimuli, including the appearance of new objects, might not always capture attention in the real world. For example, a driver may fail to notice another car when trying to turn. With regards to this, in the context of driver attention, this (draft) proposal predicts that intattentional blindness may be the cause of the majority of automobile accidents, and that attentional capture may be improved by expanding the attentional set of the driver through training in virtual driving settings. This review first considers evidence for the effects of irrelevant features both on performance, by implicit attentional capture and on awareness, by explicit attentional capture.
Together studies of implicit attentional capture and recent studies of inattentional blindness can provide a fuller understanding of the varieties of attentional capture, and has important implications for real world driving situations. Two general definitions have been used in the study of attentional capture. Explicit attentional capture occurs when a salient and unattended stimulus draws attention, leading to awareness of its presence. Implicit attentional capture is revealed when a salient and irrelevant stimulus affects performance on another task, regardless of whether or not subjects are aware of the stimulus (Simon, 2000). Explicit attentional capture for example, occurs when someone across a room says our name or waves vigorously, and that stimulus signal sets itself apart from the background and we become aware of its source (Simon, 2000).
Typically, studies determine whether capture has occurred by asking subjects whether they noticed the critical stimulus. Several recent studies of explicit attentional capture have found that when observers are focused on some other object or event, they often fail to notice salient and distinctive objects, a phenomenon that is termed inattentional blindness (IB). Although the use of explicit reports was one of the first approaches used to study attentional capture , most studies have focused on implicit attentional capture. These studies make the critical stimulus irrelevant to the primary task and infer capture of attention based on different patterns of response times or eye movements. (Simon, 2000).
Four distinct paradigms have been used to explore implicit attentional capture by measuring the effects of an irrelevant stimulus on performance of a primary task visual search task. These have already been commonly discussed directly or indirectly before by researchers (eg. In PSY 375 lectures) so they will be briefly presented in a tabular form (See Appendix 1). The primary debate in the literature on implicit attentional capture focuses on which features might automatically draw attention regardless of the expectations and attentional set of the observer. Evidence from the Pre-cueing paradigm suggests that attentional capture does not occur in the absence of the appropriate attentional set.
Findings from each of the other paradigms suggest that stimulus-driven capture can occur, particularly by the abrupt onset of a new object. All of these studies explore the degree to which subjects can ignore something they know to be irrelevant. During performance of these tasks, observers often do not even notice the irrelevant feature despite its effects on their search performance (Yantis, 1993). In fact, even distinctive features that are presented below a subjective threshold for awareness can implicitly capture attention and affect performance (McCormick, 1997) . Evidence for implicit attentional capture is critical to understanding the mechanisms underlying visual search and for determining whether a perceptual event can automatically influence performance.
Furthermore, such implicit effects can have a dramatic influence on our execution of real-world tasks and goals. For example, much of our driving performance probably reflects implicit detection of salient events (such as cars turning or slowing down) leading to corresponding adjustments to our behavior. A good proportion of perception occurs without awareness, and we need to be able to adjust our behavior without necessarily becoming aware of the cause or even the need for adjustment (Simon, 2000). Do we automatically really become aware of salient events in our visual environment, particularly events that have behavioral consequences? Are we automatically become aware of a salient new object if it unexpectedly appeared in front of us? And, if so, would attentional capture allow us not only to modify our behavior implicitly in order to accomplish an existing goal but also to select a new behavioral goal? These are questions which need to be addressed in future research. Perhaps we may have had an auto-mobile accident and the other driver claimed he did not see us even though you were right in front of him. Although we might intuitively believe that unusual, unexpected and salient objects will capture attention, leading to awareness, they often do not.
In this example, driving performance might have been affected if our car implicitly captured attention, but that does not really tell us why he did not see us and it probably could not have prevented a collision. In most real-world settings, the critical question of interest is not whether an object will implicitly affect performance, but whether it will explicitly capture attention and reach awareness, thereby allowing us to modify our behavior and select new goals (Simon, 2000). Although much, if not most, of perception and performance occurs without awareness, we feel that when salient events occur, we should become aware of them so that we can intentionally change our behavior. The implicit attention capture paradigms explore how well observers can ignore something they expect but know to be irrelevant, whereas in explicit attentional capture, the critical question is how likely subjects are to notice something that is potentially relevant, but that they do not expect. Recent studies of explicit attentional capture reveal a surprising degree of blindness to salient or unusual events that we might expect to cap-ture attention. For example, observers often fail to notice surprisingly large, but unexpected changes to their visual world, such as a change to the identity of the central actor in a brief movie (Levin & Simons, 1997).
Most subjects intuitively believe that such changes should capture attention and be detected, both because of their magnitude and their potential behavioral relevance (Levin et al., 2000). More relevant for the focus on the driving situation, is the fact that people sometimes fail to notice an unexpected object or event altogether. This phenomenon is now commonly known as inattentional blindness (IB) (Mack & Rock, 1998) . Studies of inattentional blindness are among the few direct explorations of explicit attentional capture by complex visual events. Newby & Rock (1998) used a static inattentional blindness paradigm for studying explicit attentional capture. In their task, subjects decided which arm of a briefly presented cross was longer.
After several such trials, subjects viewed a critical trial during which another object unexpectedly appeared along with the cross. Afterwards, subjects were asked whether they had noticed anything that had not been present on the previous trials. When the cross appeared at fixation and the unexpected object appeared away from fixation, approximately 25% of subjects were inattentionally blind. This mean the unexpected object did not explicitly capture attention and they did not notice it. Interestingly though, when the cross appeared away from fixation and the unexpected object appeared at fixation, nearly 75% were inattentionally blind 28 . Even when the object was a different color or moved stroboscopically, observers were often inattentionally blind 28 .
These findings show that a salient new object does not always explicitly capture attention. Simon (2000) points out that even in the absence of explicit attentional capture, the object may still implicitly affect performance. Although most studies of explicit attentional capture focus on whether or not observers notice an unexpected object, even in the absence of awareness the object might still influence performance. An object might implicitly capture attention even when it fails to do so explicitly. Several studies using the static inattentional blindness paradigm have explored this question by examining whether observers show priming for the unexpected object that they did not notice.
For example, observers are more likely to complete word fragments with a word that had appeared in the display even if they had not reported seeing it (Mack & Rock, 1998). Furthermore, even unattended background information can influence performance. Moore and Egeth (1997) used a variant of the static IB paradigm in which the cross was replaced by two horizontal lines and subjects were asked to judge which was longer. Random dots appeared in the background of the display on each of the initial trials. On the critical trial, the dots were arranged to produce either the Ponzo or the Mller-Lyer illusion. Although subjects rarely noticed the pattern in the dots, their judgments of line length were clearly influenced by the illusions.
These effects suggest that attention was implicitly, but not explicitly, captured by the unexpected object. Although subjects could not report the configuration of the dots and in fact never noticed that they were grouped to form the illusion, their judgments were still influenced by the dot configuration. Further studies are needed to explore implicit attentional capture in the absence of explicit attentional capture, especially in the context of selective-looking paradigms. Although the findings of IB suggest that novel, distinctive objects do not necessarily explicitly capture attention, perhaps attentional capture failed in these experiments because the objects were static and presented too briefly (Simon, 2000). During the 1970s and 1980s, the ‘selective looking’ paradigm was developed as a visual analog of dichotic listening to explore the detection of sustained, dynamic, unexpected visual events (Becklen & Cervone, 1983) .
A dramatic example of inattentional blindness (IB) comes from a selective-looking study that used a display with two superimposed teams, each playing a ball game. When observers monitor one of the two overlapping teams and not the other (e.g. the three players wearing white shirts and not the three players wearing black shirts), they often failed to see a woman with an open umbrella appear from one side of the screen and walk across the display (Becklen & Cervone, 1983). The appearance of this new, salient object did not capture attention. Chabris & Simons (1999) set out to replicate and extend these studies and to revive the selective-looking paradigm as a tool for the study of attentional capture. As in the basketball-game studies, subjects counted the passes made by either the white team or the black team.
The two teams and the unexpected event were filmed separately and then superimposed into a single video display to replicate the original displays. After about 45 seconds of the display, while the subjects were performing the counting task, a woman carrying an open umbrella walked across the display and exited the other side five seconds later. As in the earlier study, many subjects did not notice the umbrella woman. Another set of conditions was used with a person wearing a gorilla suit. Again, Chabris & Simons (1999) found a great deal of IB. Although these studies suggest that salient new objects in complex displays do not explicitly capture attention, the degree of inattentional blindness could have been due to some oddity of the displays.
Partially transparent displays are not typical of our real-world visual experience, so they may have impaired subjects’ ability to detect the unexpected object. Chabris & Simon (1999) thus further tested subjects with a set of displays in which all of the players and the u …