Exploring Change Blindness: New Insights into the Nature and Role of Visual Attention, Study notes of Architecture

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Ronald A. Rensink

University of British Columbia

In L. Itti, G. Rees, and J.K. Tsotsos (eds). Neurobiology of Attention. (pp. 76-81). 2005.

San Diego, CA: Elsevier.

Abstract

Large changes that occur in clear view of an observer can become difficult to notice if made during an eye movement, blink, or other such disturbance. This change blindness is consistent with the proposal that focused visual attention is necessary to see change, with a change becoming difficult to notice whenever conditions prevent attention from being automatically drawn to it. It is shown here how the phenomenon of change blindness can provide new results on the nature of visual attention, including estimates of its capacity and the extent to which it can bind visual properties into coherent descriptions. It is also shown how the resultant characterization of attention can in turn provide new insights into the role that it plays in the perception of scenes and events.

Another important distinction is that between the perception of dynamic change (i.e., seeing a change as a dynamic visual event) and the inference of completed change (i.e., noticing that something has changed at some time in the past, without a phenomenological experience of anything dynamic). During the perception of dynamic change, the spatiotemporal continuity of the internal representation is maintained. In contrast, the perception of completed change does not require such continuity; in principle, it could be carried out simply by a comparison of the currently-visible structure with memory, requiring at most only an intermittent application of attention. Finally, it is also worth distinguishing between change and difference. Perception of difference is based on the lack of similarity in properties of two distinct structures. In contrast to change, difference involves no notion of temporal transformation; instead, similarity is defined via atemporal comparison. The question of whether attention is involved in the perception of difference (and perhaps of completed change) then reduces to the question of whether attention is needed for comparison.

B. Methodological Considerations

The design of any change-detection experiment must provide a way to decouple change, motion, and difference. To decouple change from motion, at least two strategies are possible. First, the change can be made gradually enough that the accompanying motion signal does not draw attention (e.g., Simons et al., 2000). Second, the change can be made contingent on an event (such as a brief flash, eye movement, or occlusion) that creates a global motion signal that can swamp the localized signal associated with the change (e.g., Rensink et al., 1997). Decoupling change from difference requires separating the effects of visual attention from the effects of long-term memory. One strategy is to have observers detect changes as soon as possible, thereby minimizing the contribution of memory. Another possibility is to have the observer respond differentially to the perception of dynamic change (which presumably relies on

attention) and the inference of completed change (which presumably relies on a longer-term visual memory).

Techniques have been developed that incorporate most of these considerations into their design. Two examples are shown in Figure 1. Figure 1a shows the one-shot paradigm, in which an image is briefly presented, followed by a brief blank or mask, and then followed by a second display, possibly containing a changed version of the first. Performance here is measured by the accuracy of change detection. Figure 1b shows the flicker paradigm, where the two displays continually alternate until the observer reports the presence or absence of the change. The measure here is the time taken to detect the change. Note that these variants correspond to the use of brief and extended displays in visual search experiments on static stimuli [see VISUAL SEARCH], with the target being a spatiotemporal pattern rather than a purely spatial one. (For a more extensive review, see Rensink, 2002.)

(a) one-shot paradigm (b) flicker paradigm

Figure 1. Examples of techniques used to induce change blindness. (a) One-shot paradigm. Here, the observer views a single alternation of displays, with a brief blank or mask between them. The task of the observer is to detect (or identify) the change; performance is measured via accuracy of response. (b) Flicker paradigm. Here, the observer views a continual cycling of displays, with a brief blank or mask after each display. The task of the observer is to detect (or identify) the change; performance is measured via response time. Both approaches can also be applied to other kinds of change, such as those made during eye movements or blinks.

reliably detect a non-changing target, only one item can be attended at a time. (Note that this explanation is similar to that used to explain search asymmetry, where detecting the presence of a basic property is far easier than detecting its absence. [see VISUAL SEARCH].)

• pools information from proto-objects
• maintains short-term memory
  • array of rapidly-generated, volatile structures that contain local estimates of scene structure

Nexus

• feedforward: collect information
• feedback: stabilize proto-objects

Links

Proto-objects

Figure 2. Pooling of attended information. Attended items are linked to a single nexus. (a) When searching for the presence of change, the nexus signal will either be 1 (target present) or 0 (target absent). A relatively strong signal therefore exists, even when information from several links is collected. (b) When searching for the absence of change, the nexus signal will either be n-1 (target present) or n (target absent). If all items are attended, n would be about 4, and the resulting signal would be quite weak; to obtain a strong signal, the nexus must collect information from only one link at a time.

These limits also cast light on the nature of the bottleneck involved. If 4 items can be held by attention, detecting the absence of change in any of them should be easy: Simply compare each with its counterpart in the image; even if only one comparison can be made at a time, all items could eventually be compared. The finding that this is not possible indicates that the bottleneck is not the number of comparisons that can be made, but rather, the number and nature of the representations constructed.

B. Independence of Attentional Complexes

Given that several items can be held by attention at any one time, how are the corresponding complexes^1 related to each other? It may be that each is independent of the others (Pylyshyn & Storm, 1988); on the other hand, a higher- level structure may somehow link them, imposing constraints upon their operation (Rensink, 2002). Results from change-detection studies support the latter view. For example, the constraint that only 1 non-change can be detected at a time would not exist if complexes were independent entities—each complex could simply be tested in turn, leading to a limit of at least 4 items. Additional evidence is the blindness found for switches of colors among tracked items (Saiki, 2003) and for switches of property assignments in static items containing multiple properties (Wheeler & Treisman, 2002), even when only 2 or 3 items were involved. If complexes were independent, and if properties were correctly bound to them, detection of such changes should be easy. The low level of performance actually found is compatible with some migration of properties among the attended items, a natural consequence of a pooled signal

C. Contents of Attentional Complexes

Another issue of interest is the content of an attentional complex, i.e., the number of basic properties it contains, and the amount of detail for each property. Change-blindness studies indicate that this content is usually sparse, with only a small number of properties represented. For example, observers can miss large changes in an object even when it is attended, suggesting that the corresponding complex may be far from a complete representation of that object (Levin & Simons, 1997). Moreover, it appears that complexes are held in coherent form only as long as they are attended, falling apart when attention is withdrawn (Wolfe, 1999). At least four simple properties—such as orientation, color, size, and curvature—can be simultaneously represented (Luck & Vogel, 1997), apparently via the concurrent coding of different kinds of properties (Wheeler & Treisman,

that are both highly detailed and coherent. To reconcile this with the coherent, detailed picture of the world that we experience, it has been proposed that scene perception is based on a sparse, dynamic “just-in-time” system that creates object representations when (and only when) they are needed. If this co-ordination were done correctly, this virtual representation would appear to higher-level processes as if "real", i.e., as if all objects simultaneously have detailed, coherent representations (Rensink, 2002).

A. Triadic Architecture

One possible implementation of a virtual representation is the triadic architecture (Rensink, 2002) shown in Figure 3. This is composed of three systems: (i) an early system that continually generates simple visual elements, (ii) an attentional system that enters a subset of these into a coherent representation of an object, and (iii) a nonattentional system that determines such things as the meaning (or gist ) of the scene, and the spatial arrangement (or layout ) of items in it.

Gist

Proto-objects

Coherence field

Setting (nonattentional) Object (attentional)

Layout

Early (nonattentional)

Figure 3. Triadic architecture. Thin lines indicate information flow; thick lines control. Here, visual perception is carried out by three largely independent systems: (i) an early system concerned with the formation of (unattended) elements rapidly and in parallel across the visual field, (ii) an object system concerned with the formation of coherent representations (complexes) via attention, and (iii) a nonattentional setting system that enables attentional guidance via high-level knowledge. These enable effective management of attention (and therefore conscious perception) via a combination of high- and low-level control.

Here, the constantly-regenerating elements in the early system provide a rapid estimate of scene gist and layout. Attention is controlled both by high- level considerations (knowledge) and by low-level considerations (salience of individual items) to create representations of the appropriate objects at the appropriate time. The objects so formed could then be used in turn as the basis of further attention guidance. As such, scene perception would involve a continually-circulating flow of information between low-level representations containing retinal input and higher-level representations containing knowledge about the scene [See ATTENTION AND SCENE UNDERSTANDING.]

B. Observer Intention

Given the dynamic nature of scene representation, perception for a given task must rely on attentional management —i.e., deploying attention as effectively as possible. An important factor here is the degree to which the observer expects a change, and believes that reporting it is relevant. The degree of change blindness found is much higher when the observer does not expect a change (being asked to report it afterwards), although some ability to detect change still remains (Levin & Simons, 1997). This supports the view that the only properties put into coherent form (or at least compared) are those needed for the task at hand. Another important factor is the type of change expected. Detection of orientation change is unaffected by irrelevant variations in contrast sign, again indicating that only those properties needed for the immediate task are encoded (Rensink, 2002). More generally, observers appear to be sensitive only to changes in those properties relevant to the task being carried out at the moment the change was made (see Rensink, 2002).

C. High-level Knowledge

Attentional management is heavily dependent on the high-level knowledge of the observer. One way this can influence perception is via the particular representations available. For example, detection of change was better for objects learned at a specific rather than a general level (Archambault et al., 1999). This

IV. CONCLUSIONS

Visual attention appears to be critical for the creation (and perhaps maintenance) of internal representations with a spatiotemporal coherence that in some sense matches that of the external object(s) they describe. Change blindness reflects the ability of visual attention to create (and perhaps maintain) such representational structures. Results to date on the nature and role of attention are consistent with—and in places extend—results obtained using other approaches. Looked at more broadly, the study of change blindness is the first stage of investigation into the more general issue of the perception of organized spatiotemporal patterns, such as movements and events. Based on the results obtained so far, it is likely that the perception of such patterns will critically depend upon visual attention.

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