a0005 Attention : Change Blin dness and Inatt entional Blindnes s R A Rensink, University of British Columbia, Vancouver, BC, Canada ã 2009 Elsevier Inc. All rights reserved.

Glossary is false – there are severe limits to what we can consciously experience in everyday life. Much of g0005 Change blindness – The failure to visually the evidence for this claim has come from two experience changes that are easily seen once phenomena: change blindness (CB) and inatten- noticed. This failure therefore cannot be due tional blindness (IB). to physical factors such as poor visibility; CB refers to the failure of an observer to visu- perceptual factors must be responsible. ally experience changes that are easily seen once Focused attention is believed to be necessary noticed. This can happen even if the changes are to see change, with change blindness large, constantly repeat, and the observer has been resulting if such attention is not allocated to informed that they will occur. A related phenome- the object at the moment it changes. non is IB – the failure to visually experience an g0010 Diffuse attention – A type of attention that is object or event when attention is directed else- spread out over large areas of space. It is where. For example, observers may fail to notice believed to be space-based rather than an unexpected object that enters their visual field, object-based. even if this object is large, appears for several g0015 Focused attention – A type of attention seconds, and has important consequences for the restricted to small spatial extents. It is selection of action. believed to act on small areas of space or on Both phenomena involve a striking failure to relatively small objects. report an object or event that is easily seen once g0020 Implicit perception – Perception that takes noticed. As such, both are highly counterintuitive, place in the absence of conscious awareness not only in the subjective sense that observers have of the stimulus. It is generally believed to take difficulty believing they could fail so badly at place in the absence of any type of attention. seeing but also in the objective sense that these g0025 Inattentional blindness – The failure to findings challenge many existing ideas about how visually experience the appearance of an we see. But as counterintuitive as these phenom- object or event that is easily seen once ena are, progress has been made in understanding noticed. Attention (likely, diffuse attention) is them. Indeed, doing so has allowed us to better thought to be necessary for such an understand the limitations of human perception in experience. Inattentional blindness typically everyday life and to gain new insights into how our occurs when attention is diverted, such as visual systems create the picture of the world that when the observer engages in an we experience each moment our eyes are open. attentionally demanding task elsewhere, and does not expect the appearance of the object or event. Change Blindness Background The ability to see change is extremely useful in

s00 05 Introductio n coping with everyday life: we can monitor the movement of nearby automobiles (as drivers or p0005 As obser ver s, we generally have a strong impres- pedestrians), notice sudden changes in the posture sion of seeing everything in front of us at any or location of people in front of us, and notice that moment. But compelling as it is, this impression the sky is quickly darkening. Given the importance

1 2 Attention: Change Blindness and Inattentional Blindness

of perceiving change and the fact that most observers were found to be surprisingly poor at humans can survive reasonably well in the world, detecting changes made under such conditions. it follows that our ability to perceive change must The next wave of studies, begun in the 1970s, was be such that few events in the world escape our based on a more systematic examination of these notice. This agrees nicely with our impression that effects. This work uncovered a general limit to the we perceive at each moment most, if not all, ability to detect gap-contingent changes under a objects and events in front of us. wide variety of conditions; this eventually formed p0025 Failures to see change have long been noticed, the basis for the proposal of a limited-capacity visual but they were usually taken to be temporary aber- short-term memory (vSTM). Likewise, a general rations, with nothing useful to say about vision. lack of ability was found for detection of saccade- This attitude began to change in the early twenti- contingent changes, which was traced to a limited eth century, when film editors discovered an inter- transsaccadic memory. Both lines of research esting effect: when the audience moved their eyes were eventually linked by the proposal that trans- across the entire screen (e.g., when the hero exited saccadic memory and vSTM were in fact the same on the left side and the femme fatale entered on system. the right) almost any change made during this time A third wave began in the mid-1990s, extending (e.g., a blatant change of costume) would often go the methodology and results of earlier work in unnoticed. A similar blindness to change could be several ways. To begin with, stimuli were often induced by making it during a loud, sudden noise more complex and realistic: images of real-world (e.g., a gunshot), during which the audience would scenes or dynamic events were used in place of momentarily close their eyes. simple figures. Next, changes were often repeated p0030 The scientific study of this ef f ect began in the rather than occurring just once, allowing the use of mid-1950s, with work on position changes in dot time as well as accuracy to measure performance arrays and other simple stimuli. Here, a change in (Figure 1). Third, blindness was induced via sev- one of the items was typically made contingent on a eral new kinds of manipulation, such as making temporal gap that lasted several seconds. A separate changes during a film cut or during the appearance line of studies was also begun that investigated of sudden splats elsewhere in the image. Finally, all the perception of displacements made contingent these effects – as well as the earlier ones – were on an eye movement (or saccade). In all cases, accounted for by the proposal that attention is

One-shot paradigm Flicker paradigm(a)(b) f0005 Figure 1 Example of a technique to induce CB. Here the change is made during a brief gap between the original and the modified stimulus. (a) One-shot paradigm. A single alternation of the stimuli is used, and the observer must respond to it. Performance is measured by accuracy of detection (or identification or localization). (b) Flicker Au2 paradigm. Stimuli are continually cycled under the observer responds. Performance is measured by the time taken until the change is detected (or identified or localized). Both types of measurement paradigm can also be applied to other techniques, such as changes made during saccades or splats. Attention: Change Blindness and Inattentional Blindness 3

necessary to see change. This work therefore visual event. For example, when a person is seen showed that this CB is a general and robust phe- walking, their legs are seen at various positions nomenon that can last for several seconds, and can over time. This kind of perception suggests – be connected to known perceptual mechanisms. As although does not prove – that the underlying such, it supported the idea that change perception representation has a spatiotemporal continuity of is an important part of perception, and that study- some kind. In contrast, difference is based on an ing its failure can cast light on mechanisms central atemporal comparison of structures that may or to our experience of the world. may not exist simultaneously, for example, com- paring the height of two people. This kind of

s00 20 Conceptual Distinctions perception allows for the possible involvement of long-term memory elements that are only inter- p0045 A c lear under standing of CB – and its inver se, mittently engaged. change perception – has been slow to emerge. This distinction is important. Change detection This is partly due to the nature of change itself. and difference detection can be distinguished, at Although this concept appears simple, attempts to least conceptually; it is important to consider the formalize it over the years – from the earliest type of experiment used when discussing either one. Greek thinkers to modern philosophers – have Spotting the difference between two side-by-side generally encountered difficulties. However, a stimuli may not engage the same perceptual number of important distinctions have become mechanisms as detecting a change in successively clear, which have helped our understanding of CB. presented stimuli. Failure to make this distinction can lead to erroneous conclusions being drawn s00 25 Change versus motion about the mechanisms involved. p0050 The human perce ption of temporal variation is handled by two separate systems: a motion- detection system for variation in regards to a partic- Experimental Approaches and Results ular location and a change-detection system for variation in regards to structure. Temporal varia- All studies to date have been based on much the tions in the world activate both systems to some same design: an initial stimulus is first presented extent. For example, a moving automobile will acti- (e.g., a picture), an altered version is then shown vate motion detectors over the relevant part of the (e.g., some object in it is removed), and the ability visual field, although these will not be able to deter- of the observer to perceive the change is then mine whether there is an enduring spatiotemporal measured (see Figure 1). Various aspects of per- structure that remains constant. Conversely, a whir- ception have been explored in this manner based ling dust devil will be seen as a coherent moving on particular selections among a relatively small structure, even though there is nothing beyond set of design parameters. the flow of dusty air in space. The key challenge in perceiving motion and change is to separate out the Type of task contributions of these two systems, so that temporal CB is the failure to perceive a change. Since there variation is assigned to the correct substrate. are several different aspects to perception, there can be – at least in principle – several different s00 30 Change versus difference kinds (or levels) of blindness. These can be inves- p0055 Another impor tant distinction is that between tigated by giving the observer the appropriate kind change and difference. In both cases, reference is of task: to a particular structure – an object or event of some kind. However, while change refers to trans- . Detection. This is the most basic and the most formation of a single structure, difference refers to widely studied aspect, concerned with the sim- the comparison of two or more separate structures. ple noticing of a change. No properties of the p0060 More precisely, change is based on the proper- change itself are necessarily perceived: the ties of the same structure at separate points in observer is simply asked to report whether time, and can be perceived as a single dynamic it occurs. 4 Attention: Change Blindness and Inattentional Blindness

. Identification. This concerns reporting the change that is not consciously experienced. Sys- properties of the change, that is, seeing what tems used are almost always manual pointing or type it is (e.g., a change in color, a change in eye fixation. Both kinds of visuomotor response orientation). This can in principle be separated are faster than consciously mediated ones. They from detection: the observer could be asked to also appear to be more accurate, suggesting the guess the type of change even if the change existence of representations with higher-capac- itself was not detected. Identification of change ity memory. appears to be more difficult than detection, indicating that somewhat different mechanisms are involved. Attentional manipulation . Localization. This is concerned with reporting A central tenet in change perception is that atten- the location of the change. This can be tion is needed to consciously experience change. decoupled from detection and from identifica- In normal viewing, the local motion signals accom- tion, at least in principle. Relatively little work panying a change attract attention to its location, to date has been done on this aspect of percep- allowing the change to be seen immediately; this is tion. Some results suggest that a separate mem- why it helps to wave at a friend from across the ory for location may exist, although this has not room. If these local signals can be neutralized so yet been fully established. that the automatic drawing of attention cannot help, a time-consuming attentional scan of the

s00 45 Type of response display will be needed. The observer will conse- quently be blind to the change until attention is p0080 Another way to engage dif f erent mechanisms is to use different aspects of the response of an observer directed to the appropriate item. A variety of tech- to a change in the external world. These effectively niques have explored this: test different perceptual subsystems: . Gap-contingent techniques. The change is . Explicit percept. This is the approach used in made during a blank field or mask briefly dis- most perceptual experiments, involving the played between the original and altered stimu- conscious visual experience of the observer. lus, which swamps the local motion signal (see A high degree of blindness can usually be Figure 1). Observers are very poor at detecting induced. The proposal that attention is needed change if more than a few items are present; to see change is concerned with this type of results suggest only 3–4 items can be seen to experience. change at a time. . . Semiexplicit percept. Some observers can have Saccade-contingent techniques. The change is a ‘gut feeling’ for several seconds that change is made during a saccade of the eyes. Observers occurring, even though they do not yet see it are generally poor at detecting change if more (i.e., do not have a picture of it). The basis of than a few items are present; again, a limit of this is controversial, but it may involve nonat- 3–4 items is found. CB can also be induced for tentional or subattentional systems. position change with even one item present, . Implicit percept. Change that is not experi- provided no global frame of reference exists. . enced consciously may still be perceived Blink-contingent technique. The change is implicitly. If so, this must be measured by its made during an eyeblink. Again, observers are effect on other processes. For example, some generally poor at detecting such changes. Inter- studies indicate that an unseen change can estingly, blindness can be induced even if the influence forced-choice guessing about its pos- observer is fixating the changing item. . sible location. The existence of this form of Splat-contingent techniques. These make the perception is controversial, although evidence change at the same moment as the appearance for it appears to be increasing. of brief distractors (or splats) elsewhere in the . Visuomotor response. This involves the res- image. The blindness induced in this manner is ponse of a visually guided motor system to a relatively weak, but still exists, showing that it Attention: Change Blindness and Inattentional Blindness 5

can be induced even when the change is extent to which we can potentially miss important completely visible. changes in everyday life. Indeed, most people are . Gradual-change techniques. Here, the transi- unable to believe the extent to which they are tion between original and altered display is unable to see change – in essence, they suffer made slowly (e.g., over the course of several from ‘CB blindness.’ seconds). Observers generally have difficulty However, CB itself can also be harnessed as a detecting such changes, even though no disrup- powerful tool to investigate the mechanisms by tions are used. which we see. The exact conclusions obtained from such studies are still the subject of debate, The results from all these approaches are consis- but their general outlines are becoming clear. tent with the proposal that attention is needed to see change. The finding that at most 3–4 items can Visual attention and short-term memory be seen to change at a time is consistent with the All experiments on CB are consistent with the capacity of attention obtained via other techniques proposal that attention is needed to see change. such as attentional tracking. Experiments on carefully controlled stimuli sug- gest that 3–4 items can be seen to change at a time, s00 55 Perceptual set consistent with the limit on attention obtained via p0095 An impor tant par t of perce ption is the perce ptual other techniques, such as attentional tracking. set of the observer, which strongly affects the However, while a limit of some sort is involved, mechanisms engaged for a given task. The issue the nature of this limit is still somewhat unclear. of set is important for the question of which Contrary to subjective impression, change percep- mechanisms are involved in everyday vision, and tion is not an elementary process. Instead, it how these are related to performance as measured involves – at least in principle – a sequence of in the laboratory: several steps: enter the information into a memory . Intentional set. Observers are instructed to store, consolidate it into a form usable by expect a change of some kind. They are assumed subsequent processes, hold onto this for at least a to devote all their resources to detecting the few hundred milliseconds, compare it to the cur- change, which provides a way to determine per- rent stimulus at the appropriate location, clear the ceptual capacities. A further refinement is memory store, and then shift to the next item. controlling expectation for particular types of A limit on any of these steps would limit the entire change. Changes for presence and location are process, making it difficult to determine the rele- not affected by expectation of type, whereas vant step in a given situation. changes for color are. Most proposals for mechanisms have been . Incidental set. Observers are given some other couched in terms of either visual attention or task as their primary responsibility (e.g., count vSTM. Both are similar in the results they cite the number of sheep in an image); there is no and the mechanisms they propose. Part of this is mention of a possible change until after the task caused by the extensive overlap that appears to is over. The engagement of perceptual mechan- exist between the mechanisms associated with isms is believed to be more representative of attention and vSTM. Indeed, the difference may their use in everyday life. The degree of blind- be largely one of terminology, caused by the ness found under these conditions is higher vagueness in the definition of attention. As used than under intentional conditions, indicating in CB studies, attention is defined by the formation that relatively little is attended – or at least of representations coherent over space and time; remembered – in many real-life tasks. such representations are not that different from those posited as the basis of vSTM. In any event, interesting new issues are s00 60 Implications for Perceptual Mechanisms emerging. One is whether the limiting factor p0100 At one level, CB is an impor tant phenomenon in applies to the construction of the coherent repre- its own right: among other things, it illustrates the sentation, or to its maintenance once it is formed, 6 Attention: Change Blindness and Inattentional Blindness

or perhaps both. Another issue is the nature of the is unnecessary: a virtual representation could handle elements that are attended and held in vSTM. most if not all aspects of scene perception. Further- Much evidence suggests that these are proto- more, no results to date have clearly shown storage objects that already have a considerable local bind- of information beyond the relatively sparse informa- ing of features; if so, the function of attention and tion used for guidance and the contents of attention vSTM would be to create a representation with and vSTM. Some information about the prior state extended spatial and temporal coherence. Results of a changed item appears to be stored in a longer- also suggest that the 3–4 items are not indepen- term memory not used for the perception of change; dent, but, rather, may have a higher level of inter- the amount of this is information not known. More action such that their contents are pooled into a generally, many scenes might be stored in long-term single collection point (or nexus) that supports the memories of various kinds, but the information den- perception of an individual object. sity of each could still be quite low – perhaps a relatively limited amount of information from each s00 70 Scene perception of a dozen or so locations. p0130 Given that only a f ew items may have a coherent Whether the dynamic element is total or partial, representation at any time, our subjective impres- it nevertheless plays an important role in each indi- sion of seeing everything that happens in front of vidual’s perception of a given scene. CB has there- us cannot be correct. But care must be taken in the fore been useful as a tool to investigate individual particular inferences drawn. Since it deals only differences in perception: the faster a change is with dynamic quantities, CB cannot say anything detected, the more important it is deemed to be. about the static information that may or may not Studies have shown an effect of training – including be accumulated. On the other hand, it does show culture – on the encoding of objects and the impor- the existence of severe limits on the extent to tance attached to them, along with an influence of which changes are represented in those subsystems the particular task undertaken. There is also an accessible to conscious perception. emerging connection here with the design of inter- p0135 To account for the impression that we see all the active visual interfaces: these owe much of their changes that occur in front of us – not to mention success to the engagement of these dynamic mech- that we can actually react to many of these – it has anisms, and are therefore subject to many of the been proposed that scene perception is handled by same considerations regarding operator and task. a virtual representation. Here, coherent represen- tations of objects – needed for change perception – are created on a ‘just in time’ basis, that is, formed Inattentional Blindness whenever they are needed for a task and then dis- Background solved afterward. Coordination of this process can be achieved via a sparse schematic representation It has been known for thousands of years that of the scene – perhaps a dozen or so items, each people engaged in deep thought can fail to see with some properties – formed independently of something directly in front of their eyes. Such the coherent representations. Guidance could be blindness can be easily induced when an observer done on the basis of both high-level factors (e.g., intensely attends to some event, for example, wait- schemas that enable testing of expected objects) ing to see if an oncoming automobile will stop in and low-level factors (e.g., motion signals that time. Under such conditions, much of the visual draw attention to unexpected events). All results field can effectively disappear from consciousness, on CB are consistent with this proposal, and results even if it contains objects that are highly visible. from other areas (e.g., research on eye movements) It was long believed that such blindness might be appear consistent with this as well. due to the image of an unseen object falling onto p0140 The status of static scene information is still the periphery of the eye, which is relatively poor at unclear. In principle, it could be accumulated to perceiving form. But decades of research have create a dense description that would match our shown that location is unimportant – the key factor subjective impression. However, such accumulation is attention. Attention: Change Blindness and Inattentional Blindness 7

p0155 One of the ear liest lines of research that did manage to kindle interest. These differed from encountered this IB was the study of head-up dis- the earlier studies in several ways. First, they used plays for aircraft pilots, which superimpose opaque rather than superimposed (or transparent) instructions and information over a view of the stimuli, greatly reducing concerns about artificial- external world. Simulator studies in the early ity. Indeed, it was found that even large unex- 1960s discovered that when pilots carried out dif- pected events – such as the appearance of a ficult maneuvers requiring attention to outside human in a gorilla suit – were still not noticed by cues, they did not see the unexpected appearance most observers under these conditions. Second, in of instructions to alter course. Later studies addition to videos of events, techniques were also showed the converse effect: if pilots focused on developed based on brief-presented static images the displayed instructions, they often failed to see (Figure 2), which showed that IB could apply to highly visible aircraft unexpectedly rolled out onto both static and dynamic stimuli. Third, simple the runway in front of them. stimuli were often used in both the static and p0160 The fir st perce ptual studies to investigate this dynamic case, allowing more experimental control. effect took place in the 1970s. These used selective And finally, there was greater examination of the looking tasks, where observers were presented effects caused by the stimuli that were not seen with two superimposed visual events (e.g., a sports consciously. Together, these developments showed game and the face of someone talking) and asked that IB could indeed occur in everyday life, and to attend to just one of these. Results showed that that the mechanisms involved play a major role in observers could easily report what happened in the everyday perception. attended event, but did not do well in the other: they often missed large unexpected events, such as Conceptual Distinctions the appearance of a woman carrying an umbrella. This happened even when the unseen stimulus Work on the theoretical and experimental aspects was in the center of the visual field, showing that of IB have emphasized several conceptual distinc- the cause of this blindness effect was not optical, tions. Some apply not only to IB but are also but attentional. relevant to more general issues of awareness. p0165 Although this effect was surprising, the use of superimposed stimuli caused it to be considered Expression versus suppression somewhat artificial, and interest in it never really Given that attention is needed to see something (i.e., developed. But in the 1990s a new wave of studies consciously experience it), two general possibilities

(a) Non-critical trial (b) Critical trial f0010 Figure 2 Example of a technique to induce IB. A static test stimulus is presented while visual attention is focused on a primary task; here, the task is to report which of the two lines is longer. (a) Noncritical trial. The primary lines alone are presented for 200 ms, followed by a mask to prevent further processing. The observer must report which is the longer line. Two or three such trials are usually given to allow the observer to get into the appropriate mental state. (b) Critical trial. Along with the primary lines, an unexpected test stimulus is presented nearby. Blindness is measured by detection (or identification or localization) of the test stimulus. An additional round of trials is often presented, in which the test stimulus is now expected; this is believed to correspond to a condition of divided attention. 8 Attention: Change Blindness and Inattentional Blindness

exist as to how it acts. First, attention might cause Type of task selected stimuli to be expressed consciously, for As in the case of CB, several different aspects of example, they enter awareness by being activated perception can be distinguished, each of which can so as to exceed some threshold. In this view, atten- be tested by appropriate selection of task: tion does not just facilitate the conscious perception . Detection. This is the most basic aspect of per- of a stimulus – it also enables it. On the other hand, ception, concerned with the simple noticing of attention might act to selectively suppress some of something present; no properties of the stimu- the stimuli, so that they vanish from the conscious lus itself need be involved. For isolated static mind. (In both cases, the status of unattended items items that are small, that is, have an extent of is open: they might be weakly expressed in some less than about 1 of visual angle, observers way, or available implicitly.) It is worth noting often fail to see anything at all in conditions that the two possibilities are not exclusive: it could that induce IB. For larger items, however, be that both are used, with attention acting to detection remains generally good. For dynamic express – or at least emphasize – some items while events, informal observations suggest that the suppressing others. unattended event does not necessarily disap-

s00 95 Restricted versus unrestricted effects pear entirely – observers often notice ‘some- p0180 A related issue is the extent of any expression or thing else’ going on, but cannot say what it is. . suppression. It was once believed that everything Identification. This is concerned with deter- registered in the visual system was experienced mining various properties of the stimulus. For consciously. However, given that implicit percep- example, observers can be asked to report the tion of various kinds has been found, an important color or location of an item; in principle – issue is whether the selective effects underlying IB although not often in practice – this can be are restricted to conscious experience, or whether done whether or not it was detected. Observers they spill over to other aspects of perception. can usually report the color of a small item that p0185 For example, results show that attentional inhi- is detected; identification of shape appears to be bition of a given location will increase the amount more difficult. Detection without identification of blindness there. But does such inhibition atten- has been reported, but it has been suggested uate the input strongly enough to also affect that these are simply false positives. . implicit processes at that location? Virtually all Localization. Here, the task is to locate the test studies to date have assumed that selective effects stimulus that appears in the critical trial. In are restricted to conscious experience, but this principle, this can be tested independently of assumption has not been tested. detection and identification, although this is rarely done. Observers are able to report the

s01 00 Experimental Approaches and Results location of a small item that is detected, showing that there may be some interaction p0190 At the most general level, all studies of IB use the between these two aspects of perception. same basic approach. An observer is given a pri- mary task that engages their visual attention, an unexpected test object (or event) then appears in Type of response the display, and the response of the observer to that Performance can also be studied in terms of the appearance is then tested. Because expectation is kind of percept experienced. Operationally, this an important factor, several trials without a test can be done by testing different aspects of the stimulus are generally presented first. Results are observer’s response to the appearance of a stimu- taken from the trial in which the test stimulus first lus. These different aspects effectively involve dif- appears (the critical trial). After this, the object is ferent perceptual subsystems: no longer entirely unexpected, and the blindness levels in subsequent trials are usually much lower, . Explicit percept. This is the kind of percept consistent with the idea that attention is now used in most IB experiments. The observer is divided among the expected stimuli. asked to respond to their conscious experience, Attention: Change Blindness and Inattentional Blindness 9

that is, the picture they have of something. The electronically. The result is perceived as a set of usual definition of IB is in terms of the failure to transparent or ghostly images. Observers can eas- have such an experience; the proposal that ily attend to one of these, but can often miss events attention is needed to see is likewise concerned in the other. with this aspect. . Interspersed stimuli. Here, two different sets of . Implicit percept. Items that are not experienced stimuli are presented; all are opaque and appear consciously may still be perceived implicitly. on the same display (see Figure 2). Tests on Given that the observer is blind to the stimulus, static stimuli generally use this method. Obser- responses cannot be measured via direct subjec- vers are asked to carry out a primary task on tive experience; they must instead be based on one of the sets (e.g., make a length judgment on indirect effects. For example, relatively little a pair of lines); a test stimulus is presented near blindness is found for emotionally laden words these lines a few trials later. Dynamic events or pictures. This suggests that such words have have also been tested this way. A high degree of been perceived implicitly, with attention drawn to blindness can be found, even when the items in them on the basis of their meaning. Similarly, a the two sets of stimuli are intermingled. length illusion can occur in a set of lines perceived . Dichoptic presentation. Here, two independent consciously, even if the background lines that events are presented, each to a different eye; the induce the illusion are themselves not reported. observer is asked to pay attention to one of . Motor response to primary task. The response them. Events in the unattended eye often fail to the appearance of a test stimulus can be to be reported. In contrast with superimposed measured by its effect on the speed or accuracy stimuli, where observers often have an impres- of the primary task. This is essentially the sion of ‘something else’ going on, observers here approach developed to study attention capture, fail to experience anything of the unattended which measures the effect of a new item on set – it is simply not there. This may be related motor response times for the main task, regard- to the blindness experienced in binocular less of whether the new item is seen. (An inter- rivalry suppression. esting variation would be to determine if such effects are conditional on the experiential state Perceptual set of the test stimulus.) An important aspect of IB is the perceptual set of . Visuomotor response. Here, the appearance of an the observer. There are several ways this could item causes the eye to move toward it. This is influence performance: another approach developed in the area of atten- tion capture; again, this response is measured . Control of selectivity. This is the extent to which regardless of whether the item is seen. This selection – either expression or suppression – is might be useful for studying IB if measurement invoked in the primary task. For example, obser- were made conditional on whether the test stim- vers can be required to attend only to the white- ulus is consciously experienced. shirted players in a game, while ignoring the black-shirted players. In such tasks, blindness appears to be due at least in part to observers s01 15 Attentional manipulation suppressing the features of the ignored stimuli: p0205 An impor tant f actor in inducing IB is to present the greater the similarity of the test item to the the test stimulus while the observer has their visual ignored items, the greater the blindness. In non- attention engaged on an unrelated task. This has selective tasks, there is no need to screen out any been done in several ways: stimuli, at least up to the critical trial. Blindness . Superimposed stimuli. Two independent dyn- is still induced here, but it remains unknown amic events (e.g., a basketball game and two pairs whether this is due to a failure of expression or of hands playing a game) are presented simulta- an invocation of suppression. neously. In earlier studies, this was often done via . Control of capture. It has been suggested that half-silvered mirrors; more recent studies do this high-level control may be exerted over the kind 10 Attention: Change Blindness and Inattentional Blindness

of stimuli that can capture attention, and thus achieved via an attentional set which determines be seen. Attention is usually drawn by the the kinds of information that can capture attention, appearance of a new item or by the presence and perhaps also the kinds of information that can of a unique property. However, work on atten- enter conscious awareness. This development tion capture has shown that this can be over- potentially connects work on IB to work on atten- ridden by a high-level attentional set. Results tion capture. on IB are consistent with the proposal that an Another possible connection is with preatten- attentional set governs what is consciously seen, tive vision, usually studied by the rapid detection with distinctive stimuli experienced con- (or pop-out) of unique items in a display. Work sciously only if they fit into the observer’s here has pointed toward a considerable amount of expectations. processing achieved in the absence of attention. But this assumes that little or no attention is given to most items in a display at any moment. s01 25 Implications for Perceptual Mechanisms Results on IB suggest that this supposedly inatten- p0215 The finding that obser ver s can often f ail to see tional condition may be better viewed as a case of highly noticeable objects and events touches on diffuse attention, with the inattentional condition several issues concerning the way we cope with characterized as one where no conscious percep- our world. For example, it suggests that we might tion exists. not be aware of the extent to which we fail to see However, there are several findings that make various aspects of our immediate environment, these connections less than certain. For example, even if these are important and highly visible. an observer in an IB experiment can usually detect (This might be termed IB blindness.) This has the individual items in a group, although the obvious implications for tasks such as driving, grouped pattern itself cannot be identified. In where the ability to accurately perceive objects addition, the pop-out of a unique item in a group and events is literally a matter of life and death, occurs only if the group appears in the critical and where knowing about our limitations could trial; if this group is shown earlier in noncritical well affect how careful we are. trials (with the unique item the same as the p0220 Meanwhile, work on IB can also tell us a bout others), pop-out no longer occurs. So what is the the mechanisms involved in visual perception. In effect of the unique item here? If it is to draw particular, it can provide a unique perspective on attention to the group, why are the individual the mechanisms that underlie our conscious expe- elements already seen in the earlier, noncritical rience of the world. trials? And why should pop-out occur in one con- dition, but not the other?

s01 30 Visual attention p0225 All results to date are consistent with the proposal Scene perception that attention is needed to see an object or event. Relatively little work has studied the aspects of For example, the degree of blindness has been natural scenes perceived under conditions of inat- found to increase with distance from the center of tention. Studies using briefly presented scenes as the attended location, in accord with space-based test stimuli found that observers could usually models of attention. In addition, the greater the report the scene gist (i.e., its overall meaning, attentional load of the primary task, the greater such as being an office or a forest), along with the blindness to the test stimulus, which is consis- several objects of indeterminate description. tent with the proposal of a limited attentional Some confabulation is also found. This is broadly capacity. Indeed, the inability of an observer to similar to results on the ability of observers to follow more than one coherent event supports the rapidly perceive the gist of scenes from brief expo- proposal that only one complex object or event can sures of 100 ms or less, which is also likely done be attended at a time. with little or no focused attention. p0230 Results also provide tentative suppor t for the Individual differences in the coding of scenes and proposal that the high-level control of attention is events can be measured in terms of how blindness Attention: Change Blindness and Inattentional Blindness 11

varies with the primary task. For example, experts in (same first-order distributions), but with different basketball could better detect the appearance of an amounts of change (different second-order distri- unexpected object while they were attending to a butions) if the alternation rates are not the same. basketball game. This suggests that they had And just as CB can say little about first-order (static) encoded the scenes and events in a way that allowed information, IB can say little about second-order them to divert some of their attention to occasion- (changing) information. The two therefore refer to ally monitor other items, without seriously affecting largely complementary aspects of the visual world. performance on the primary task. This distinction has consequences for the per- p0255 The model of scene perce ption often used to ceptual mechanisms involved. For example, the account for results on IB is the perceptual cycle. kind of attention required for each aspect of per- Here, sustained attention is believed to activate the ception may be different – or at least, have different conscious percept of a stimulus; once this has been effects. The kind of attention involved in IB is space done, the stimulus has entered the cycle, and helps based: the degree of blindness increases with select the appropriate schema to determine what increasing distance from the center of attention. information to admit next. In contrast, stimuli It is also easily diverted – hence the common use that do not become part of the predictive cycle of a test stimulus that is completely unexpected. may never be seen at all. Such stimuli – especially In contrast, the kind of attention needed to per- low-level signals – can guide the process, but ceive change is object-based and is much less eas- do not enter awareness on their own. Similarly, ily diverted (or at least slower), since telling the information assembled at a preattentive level can observer that a change will occur – and even giv- also guide this process, although it too does not ing them practice at perceiving it – does not enter awareness automatically. This model has affect performance to any great extent. Loosely some similarities to the dynamic scene representa- speaking, IB might be identified with the absence tion often used to account for CB, as well as the of diffuse attention, and CB with the absence reentrant models used to account for conscious of focused attention. But a final determination of experience. this must await a better understanding of atten- tion itself.

s01 40 General Issues Blindness versus Amnesia

s01 45 Change Blindness versus Inattentional An important issue in regards to the status of these Blindness induced failures is whether they are failures of p0260 CB and IB both involve a f ailure to perceive things perception or of memory. It might be, for instance, that are easily seen once noticed, and both are that an observer did experience something under believed to be due to a lack of attention. It is conditions that induced CB or IB, but then forgot therefore likely they are related. But exactly how? it before they could make their report. If so, these p0265 Fir st of all, the dif f erence between CB and IB effects would not be forms of blindness, but forms does not depend on the kind of input. Both can be of amnesia. found using dynamic images, and both can be found In the case of CB, the resolution of this issue is using static images. And the particular contents of reasonably straightforward. Perception of change the input do not matter for either. Instead, the criti- is often measured by asking the observer to cal difference between the two is the status of the respond to the change as soon as they see it; all information under consideration. IB is entirely that is needed to trigger this is a minimal conscious concerned with first-order information – the simple experience. When observers are asked to respond presence of quantities. In contrast, CB involves to a single change, only a few hundred millise- second-order information – the transitions between conds exist between its presentation and the initi- these quantities. These can be separated: an alter- ation of the report (e.g., the pressing of a button). nating sequence of two images, say, could be experi- Thus, if the experience of change is forgotten in enced as a 50% presence of each image over time the absence of attention, it would be exceedingly 12 Attention: Change Blindness and Inattentional Blindness

brief and incapable of causing a response to be Visual Attention versus Visual Experience initiated. To all intents and purposes it would be CB and IB can be regarded as two forms of the as if it never existed, at least at the conscious level. perceptual failure created by the diversion of (Note that if taken seriously, the possibly of such a attentional resources. They can be distinguished fleeting perception would not be restricted to CB – at the functional level by the type of information it would apply to any failure of perception.) involved (second- or first-order information, p0285 The situation for IB is more complex. In one respectively). They also appear to be distinguished sense, this issue was resolved by the finding that by the type of attention involved (focused or dif- unattended – and thus unseen – items are indeed fuse) and the kinds of operations (e.g., comparison) perceived, in that they can indirectly affect aspects associated with these. This division may corre- of conscious experience. Entry into conscious spond to the two modes sometimes proposed for experience itself, however, is not addressed by conscious visual experience: an object mode asso- this. The observer is usually asked about a possible ciated with focused attention and a background item only after the primary task has been com- mode operating as default. Beyond this, however, pleted, which allows several seconds to possibly only partial and tentative conclusions can be forget it. And, the observer cannot be asked to drawn regarding the issue of how visual attention prepare to respond to the test stimulus, since this relates to conscious visual experience. sets up an expectation of the item, which severely In the case where attention of any kind is diminishes the attentional diversion, and thus the absent, there does not appear to be any conscious degree of blindness. experience of stimuli (second-order quantities for p0290 This issue has been grappled with in several focused attention; first-order quantities for dif- ways. One is to present a highly surprising or fuse). However, results still point to a considerable meaningful item, and hope that the observer will amount of processing being carried out. For exam- spontaneously report it. However, this has not ple, work on IB indicates that unattended – and generally been successful: even if a human walks therefore unseen – items can influence the percep- around in a gorilla suit or if an airplane is wheeled tion of attended items. Similarly, some models of out onto the runway where the pilot is about to CB posit low-level representations with a degree land, most observers still do not respond. It has of detail and feature binding (proto-objects) that been proposed that the observers do see the visual are formed in the absence of this kind of attention. elements, but do not assign meaning to them. In It is worth pointing out that observers in IB essence, this phenomenon becomes one of inatten- experiments often report that they can detect some- tional agnosia. (This may explain the reports thing about the nonselected stimuli, even though in some selective looking experiments, where they cannot always identify it. Importantly, this observers see ‘something else’ going on, but do kind of experience is found only in those experi- not know what it is.) ments involving superimposed or interspersed p0295 Another per spective derives from studies of the stimuli; for dichoptically presented stimuli, there is neural systems involved. Patients with lesions to a complete absence of perception of the nonselected particular parts of the cortex can suffer conditions event. This suggests that in the superimposed and such as neglect and extinction, in which attention interspersed conditions diffuse attention is given to cannot be easily allocated to objects. Such patients, nonselected events, with the main event given however, do not appear to experience forgetting – focused attention. If so, this would suggest that iden- they simply do not report perceiving such stimuli, tification and localization may require more focused even when asked with the object in full view. In attention (or related resource), and that both diffuse addition, functional imaging of the brains of nor- and focused attention may be allocated simulta- mal observers shows that words are not con- neously to different stimuli. sciously identified in the absence of attention, This proposal would be consistent with work on even if the observer is looking directly at them. CB. Focused attention is needed only for the per- These results make it highly likely that the failure ception of complex quantities such as change; is one of perception, and not memory. Attention: Change Blindness and Inattentional Blindness 13

background items not given focused attention Suggested Readings might still be seen, but only in regards to detection and perhaps a limited form of identification based Brockmole JR and Henderson JM (2005) Object appearance, disappearance, and attention prioritization in on relatively fragmented pieces of static items. real-world scenes. Psychonomic Bulletin & Review 12: 1061–1067. Irwin DE (1991) Information integration across saccadic eye movements. Cognitive Psychology 23: 420–456. s01 60 Conclusions Levin DT (2002) Change blindness blindness as visual metacognition. Journal of Consciousness Studies 9: p0320 Work on CB suggests that focused attention is 111–130. needed for the conscious experience of change: Luck SJ and Vogel EK (1997) The capacity of visual working memory for features and conjunctions. Nature 390: without it, observers will be blind to even large 279–280. changes, at least at the conscious level. Some abil- Mack A and Rock I (1998) Inattentional Blindness. ity to implicitly perceive change may exist; if so, Cambridge, MA: MIT Press. this does not appear to require focused attention. Moore CM and Egeth H (1997) Perception without attention: Evidence of grouping under conditions of inattention. p0325 Similar ly, work on IB suggests that diffuse atten- Journal of Experimental Psychology: Human Perception tion is needed to detect an unexpected object or and Performance 23: 339–352. event, that is, to see it as ‘something.’ Some results Most SB, Scholl BJ, Clifford E, and Simons DJ (2005) What you see is what you set: Sustained inattentional blindness indicate that further (attentional) processing may and the capture of awareness. Psychological Review 112: be needed to more completely identify or locate it. 217–242. There also appears to be some ability to pick up Neisser U and Becklen R (1975) Selective looking: Attending to visually significant events. Cognitive Psychology 7: and process information about static items in the 480–494. absence of any form of attention, even though such Rensink RA (2002) Change detection. Annual Review of items are not experienced consciously. Psychology 53: 245–277. Rensink RA, O’Regan JK, and Clark JJ (1997) To see or not p0330 Beyond this, our current understanding is poor. to see: The need for attention to perceive changes in More work is needed to expand our empirical scenes. Psychological Science 8: 368–373. knowledge of the basic phenomena. More work is Simons DJ (ed.) (2000) Change Blindness and Visual also needed on the basic conceptual issues involved, Memory. Hove, East Sussex, UK: Psychology Press. Simons DJ and Chabris CF (1999) Gorillas in our midst: in particular, on our understanding of terms such as Sustained inattentional blindness for dynamic events. attention and awareness. However, much exciting Perception 28: 1059–1074. progress is being made on these fronts. And some Simons DJ and Rensink RA (2005) Change blindness: Past, present, and future. Trends in Cognitive Sciences 9: of the most powerful sources of these new develop- 16–20. ments are the phenomena of CB and IB. Wilken P (ed.) (1999) Mack & Rock: Inattentional blindness. Psyche 5–7. http://psyche.cs.monash.edu.au/symposia/ See also: Attention, Awareness, and Neglect (00005); mack_rock/. Attention: Selective Attention and Consciousness Wolfe JM (1999) Inattentional amnesia. In: Coltheart V (ed.) (00007); Perception: Unconscious Influences on Fleeting Memories, pp. 71–94. Cambridge, MA: MIT Perceptual Interpretation (00060). Press.