Methods. We employed a "flicker" technique, in which an original and a modified image (each of duration 240 ms) continually alternated, with a blank field (duration 80 ms) between each display. Images were all of real-world scenes. One of three kinds of change (appearance/disappearance, color, or translation) was made to an object or region in each scene. Changes were large and easily seen under normal conditions. Subjects viewed the flicker display, and pressed a key when they noticed the change.
Change blindness is the striking failure to see large changes that normally would be noticed easily. Over the past decade this phenomenon has greatly contributed to our understanding of attention, perception, and even consciousness. The surprising extent of change blindness explains its broad appeal, but its counterintuitive nature has also engendered confusions about the kinds of inferences that legitimately follow from it. Here we discuss the legitimate and the erroneous inferences that have been drawn, and offer a set of requirements (...) to help separate them. In doing so, we clarify the genuine contributions of change blindness research to our understanding of visual perception and awareness, and provide a glimpse of some ways in which change blindness might shape future research. (shrink)
Five aspects of visual change detection are reviewed. The first concerns the concept of _change_ itself, in particular the ways it differs from the related notions of _motion_ and _difference_. The second involves the various methodological approaches that have been developed to study change detection; it is shown that under a variety of conditions observers are often unable to see large changes directly in front of them. Next, it is argued that this "change blindness" indicates that focused attention is needed (...) to detect change, and that this can help map out the nature of visual attention. The fourth aspect concerns how these results affect our understanding of visual perceptionfor example, the proposal that a sparse, dynamic representation underlies much of our visual experience. Finally, a brief discussion is presented concerning the limits to our current understanding of change detection. (shrink)
One of the more powerful impressions created by vision is that of a coherent, richly-detailed world where everything is present simultaneously. Indeed, this impression is so compelling that we tend to ascribe these properties not only to the external world, but to our internal representations as well. But results from several recent experiments argue against this latter ascription. For example, changes in images of real-world scenes often go unnoticed when made during a saccade, flicker, blink, or movie cut. This "change (...) blindness" provides strong evidence against the idea that our brains contain a picture-like representation of the scene that is everywhere detailed and coherent. (shrink)
Large changes in a scene often become difficult to notice if made during an eye movement, image flicker, movie cut, or other such disturbance. It is argued here that this _change blindness_ can serve as a useful tool to explore various aspects of vision. This argument centers around the proposal that focused attention is needed for the explicit perception of change. Given this, the study of change perception can provide a useful way to determine the nature of visual attention, and (...) to cast new light on the way that it is?and is not?involved in visual perception. To illustrate the power of this approach, this paper surveys its use in exploring three different aspects of vision. The first concerns the general nature of _seeing_. To explain why change blindness can be easily induced in experiments but apparently not in everyday life, it is proposed that perception involves a _virtual representation_, where object representations do not accumulate, but are formed as needed. An architecture containing both attentional and nonattentional streams is proposed as a way to implement this scheme. The second aspect concerns the ability of observers to detect change even when they have no visual experience of it. This _sensing_ is found to take on at least two forms: detection without visual experience (but still with conscious awareness), and detection without any awareness at all. It is proposed that these are both due to the operation of a nonattentional visual stream. The final aspect considered is the nature of visual attention itself?the mechanisms involved when _scrutinizing_ items. Experiments using controlled stimuli show the existence of various limits on visual search for change. It is shown that these limits provide a powerful means to map out the attentional mechanisms involved. (shrink)
A set of visual search experiments tested the proposal that focused attention is needed to detect change. Displays were arrays of rectangles, with the target being the item that continually changed its orientation or contrast polarity. Five aspects of performance were examined: linearity of response, processing time, capacity, selectivity, and memory trace. Detection of change was found to be a self-terminating process requiring a time that increased linearly with the number of items in the display. Capacity for orientation was found (...) to be about 5 items, a value comparable to estimates of attentional capacity. Observers were able to filter out both static and dynamic variations in irrelevant properties. Analysis also indicated a memory for previously-attended locations. These results support the hypothesis that the process needed to detect change is much the same as the attentional process needed to detect complex static patterns. Interestingly, the features of orientation and polarity were found to be handled in somewhat different ways. Taken together, these results not only provide evidence that focused attention is needed to see change, but also show that change detection itself can provide new insights into the nature of attentional processing. (shrink)
Observers inspected normal, high quality color displays of everyday visual scenes while their eye movements were recorded. A large display change occurred each time an eye blink occurred. Display changes could either involve "Central Interest" or "Marginal Interest" locations, as determined from descriptions obtained from independent judges in a prior pilot experiment. Visual salience, as determined by luminance, color, and position of the Central and Marginal interest changes were equalized.
When brief blank fields are placed between alternating displays of an original and a modified scene, a striking failure of perception is induced: the changes become extremely difficult to notice, even when they are large, presented repeatedly, and the observer expects them to occur (Rensink, O'Regan, & Clark, 1997). To determine the mechanisms behind this induced "change blindness", four experiments examine its dependence on initial preview and on the nature of the interruptions used. Results support the proposal that representations at (...) the early stages of visual processing are highly volatile, and that focused attention is needed to stabilize them sufficiently to support the perception of change. (shrink)
The task of visual search is to determine as rapidly as possible whether a target item is present or absent in a display. Rapidly detected items are thought to contain features that correspond to primitive elements in the human visual system. In previous theories, it has been assumed that visual search is based on simple two-dimensional features in the image. However, visual search also has access to another level of representation, one that describes properties in the corresponding three-dimensional scene. Among (...) these properties are three dimensionality and the direction of lighting, but not viewing direction. These findings imply that the parallel processes of early vision are much more sophisticated than previously assumed. (shrink)
It has generally been assumed that rapid visual search is based on simple features and that spatial relations between features are irrelevant for this task. Seven experiments involving search for line drawings contradict this assumption; a major determinant of search is the presence of line junctions. Arrow- and Y-junctions were detected rapidly in isolation and when they were embedded in drawings of rectangular polyhedra. Search for T-junctions was considerably slower. Drawings containing T-junctions often gave rise to very slow search even (...) when distinguishing arrow- or Y-junctions were present. This sensitivity to line relations suggests that preattentive processes can extract 3-dimensional orientation from line drawings. A computational model is outlined for how this may be accomplished in early human vision. (shrink)
Previous theories of early vision have assumed that visual search is based on simple two-dimensional aspects of an image, such as the orientation of edges and lines. It is shown here that search can also be based on three-dimensional orientation of objects in the corresponding scene, provided that these objects are simple convex blocks. Direct comparison shows that image-based and scene-based orientation are similar in their ability to facilitate search. These findings support the hypothesis that scene-based properties are represented at (...) preattentive levels in early vision. (shrink)
A striking blindness to changes in real-world scenes can be induced using a variety of techniques (e.g., saccade-, blink-, or flicker-contingent change). The strength and robustness of this phenomenon points towards the involvement of mechanisms central to visual perception. It is proposed here that this induced change blindness can be explained by an..
Several recent investigations (Grimes, in press; McConkie and Currie, in preparation) report that large changes in images of natural scenes can remain unnoticed if these are made during saccades. We show here that similar massive effects can be obtained without synchronization to saccades. This is done via a "flicker" technique in which an original and an altered image (each of duration 240 ms) are repetitively alternated, with a blank field (duration 27 or 290 ms) between each display. One of four (...) kinds of change (color, left-right reflection, translation, or appearance/disappearance) were made in the foreground or background of each scene. Many of these changes were difficult to detect, even over long periods of observation (35 seconds). We believe that this is due to the spatially-distributed transient induced by the blank field, which swamps the localized flash that would otherwise draw attention to the changes; observers were therefore forced to rely on higher-level (probably non-iconic) representations of the scenes to detect the change. Our results indicate that the failure to notice scene changes during saccades is not due to saccade-specific mechanisms, but rather, involves more general mechanisms of visual attention. (shrink)
Ideomotor actions are behaviours that are unconsciously initiated and express a thought rather than a response to a sensory stimulus. The question examined here is whether ideomotor actions can also express nonconscious knowledge. We investigated this via the use of implicit long-term semantic memory, which is not available to conscious recall. We compared accuracy of answers to yes/no questions using both volitional report and ideomotor response . Results show that when participants believed they knew the answer, responses in the two (...) modalities were similar. But when they believed they were guessing, accuracy was at chance for volitional report , but significantly higher for Ouija response . These results indicate that implicit semantic memory can be expressed through ideomotor actions. They also suggest that this approach can provide an interesting new methodology for studying implicit processes in cognition. (shrink)
We show that cast shadows can have a significant influence on the speed of visual search. In particular, we find that search based on the shape of a region is affected when the region is darker than the background and corresponds to a shadow formed by lighting from above. Results support the proposal that an early-level system rapidly identifies regions as shadows and then discounts them, making their shapes more difficult to access. Several constraints used by this system are mapped (...) out, including constraints on the luminance and texture of the shadow region, and on the nature of the item casting the shadow. Among other things, this system is found to distinguish between line elements (items containing only edges) and surface elements (items containing visible surfaces), with only the latter deemed capable of casting a shadow. (shrink)
Pascal routines are described for performing and testing various timing and display operations on Macintosh computers. Millisecond timing of internal operations is described, as is a method to time inputs more accurately than tick timing. Techniques are also presented for placing arbitrary bit-image displays on the screen within one screen refresh. All routines are based on Toolbox procedures applicable to the entire range of Macintosh computers.
This paper discusses several key issues concerning consciousness and human vision. A brief overview is presented of recent developments in this area, including issues that have been resolved and issues that remain unsettled. Based on this, three Hilbert questions are proposed. These involve three related sets of issues: the kinds of visual experience that exist, the kinds of visual attention that exist, and the ways that these relate to each other.
A computational theory is developed that explains how line drawings of polyhedral objects can be interpreted rapidly and in parallel at early levels of human vision. The key idea is that a time-limited process can correctly recover much of the three-dimensional structure of these objects when split into concurrent streams, each concerned with a single aspect of scene structure.
Recent studies have shown that several scene-based properties can be determined rapidly and in parallel at preattentive levels, including surface convexity and concavity (Ramachandran, 1988), direction of illumination (Enns & Rensink, 1990), and three-dimensional orientation (Enns & Rensink, 1991). We show that in addition to these properties, preattentive vision is also sensitive to scene structure defined by shadows.
One of the more compelling beliefs about vision is that it is based on representations that are coherent and complete, with everything in the visual field described in great detail. However, changes made during a visual disturbance are found to be difficult to see, arguing against the idea that our brains contain a detailed, picture-like representation of the scene. Instead, it is argued here that a more dynamic, "just-in-time" representation is involved, one with deep similarities to the way that users (...) interact with external displays. It is further argued that these similarities can provide a basis for the design of intelligent display systems that can interact with humans in highly effective and novel ways. (shrink)
It has recently been demonstrated that early vision is capable of recovering several properties of the three-dimensional world. We describe a series of visual search experiments showing that such recovery includes a completion process that allows for the interpretation of objects that are partially occluded. Search for easily-detectable line segments is made much more difficult when they can be interpreted as the visible parts of a line that has been occluded by a three-dimensional object. We describe some of the conditions (...) under which this completion process takes place, such as its dependence on orientation, contrast, and spacing. We then show that fragments of three-dimensional objects can be completed in a similar way. These results extend what is known about rapid parallel scene interpretation -- in addition to assigning scene-based properties to image elements, early vision also constructs elements not present in the original image. (shrink)
Recent developments in vision science have resulted in several major changes in our understanding of human visual perception. For example, attention no longer appears necessary for "visual intelligence"--a large amount of sophisticated processing can be done without it. Scene perception no longer appears to involve static, general-purpose descriptions, but instead may involve dynamic representations whose content depends on the individual and the task. And vision itself no longer appears to be limited to the production of a conscious "picture"--it may also (...) guide processes outside the conscious awareness of the observer. (shrink)
In this paper we examine to what extent the lengths of the links in an animated articulated figure can be changed without the viewer being aware of the change. This is investigated in terms of a framework that emphasizes the role of attention in visual perception. We conducted a set of five experiments to establish bounds for the sen-sitivity to changes in length as a function of several parameters and the amount of attention available. We found that while length changes (...) of 3% can be perceived when the relevant links are given full attention, changes of over 20% can go unnoticed when attention is not focused in this way. These results provide general guidelines for algorithms that produce or process character motion data and also bring to light some of the potential gains that stand to be achieved with attention-based algorithms. (shrink)
Scene Perception is the visual perception of an environment as viewed by an observer at any given time. It includes not only the perception of individual objects, but also such things as their relative locations, and expectations about what other kinds of objects might be encountered. Given that scene perception is so effortless for most observers, it might be thought of as something easy to understand. However, the amount of effort required by a process often bears little relation to its (...) underlying complexity. A closer look shows that scene perception is a highly complex activity, and that any account of it must deal with several difficult issues: What exactly is a scene? What aspects of it do we represent? And what are the processes involved? Finding the answers to these questions has proven to be extraordinarily difficult. However, answers are being found, and a general understanding of scene perception is beginning to emerge. Interestingly, this emerging picture shows that much of our subjective experience as observers is highly misleading, at least in regards to the way that scene perception is carried out. In particular, the impression of a stable picture-like representation somewhere in our heads turns out to be largely an illusion. To see how this comes about, imagine a seashore where there is a sailboat, some rocks, some clouds, and perhaps a few other objects (see Figure 1). How do we perceive this scene? Intuitively, it seems that the set of objects in the environment would give rise to a corresponding set of representations in the observer. Thus, there would be detailed representations of the sailboat, clouds, etc., with each representation describing the identity, location, and 'meaning' of the item it refers to. In this view, the goal of scene perception is to form a literal re-presentation of the world, with all of its visible structure represented concurrently and in great detail everywhere. This representation then serves as the basis for all subsequent visual processing.. (shrink)
Focused attention is needed to perceive change (Rensink et al., 1997; Psychological Science, 8: 368-373) . But how much attentional processing is given to an item? And does this depend on the nature of the task?
This past decade has seen a great resurgence of interest in the perception of change. Change has, of course, long been recognized as a phenomenon worthy of study, and vision scientists have given their attention to it at various times in the past (for a review, see Rensink, 2002a). But things seem different this time around. This time, there is an emerging belief that instead of being just another visual ability, the perception of change may be something central to our (...) ‘visual life’, and that the mechanisms that underlie it may provide considerable insight into the operation of much of our visual system. This development may have been sparked by a number of factors: technology that allowed the easy creation of dynamic displays, a feeling in the air that it was time for something new, or it may have simply been a matter of chance. But once underway, this development was fueled by results, results that included both novel behavioral effects and new theoretical insights. Many of these centered around change blindness 1, the failure of observers to see large changes that are made contingent upon.. (shrink)
We present a rigorous way to evaluate the visual perception of correlation in scatterplots, based on classical psychophysical methods originally developed for simple properties such as brightness. Although scatterplots are graphically complex, the quantity they convey is relatively simple. As such, it may be possible to assess the perception of correlation in a similar way.
Purpose. Although observers easily extract the global meaning of natural scenes, it is often the case that they do not notice or remember all of their individual properties. It appears that some scene properties are more readily coded in mental representations than others. We tested the role of three different object properties - color, location, and presence/absence - in scene representations.
A new proof is presented of Tsotsos' result that the VISUAL MATCH problem is NP-complete when no (high-level) constraints are imposed on the search space. Like the proof given by Tsotsos, it is based on the polynomial reduction of the NP-complete problem KNAPSACK to VISUAL MATCH. Tsotsos' proof, however, involves limited-precision real numbers, which introduces an extra degree of complexity to his treatment. The reduction of KNAPSACK to VISUAL MATCH presented here makes no use of limited-precision numbers, leading to a (...) simpler and more direct proof of the result. (shrink)
Theories of human vision have generally assumed that the features underlying visual search and texture segmentation correspond to simple measurements made at the first stages of visual processing. In this paper, we describe a series of visual search experiments that refute this assumption. Using several variants of the Mueller-Lyer figure, we show that an illusion of length exists in preattentive vision -- search is easy when items contain line segments of equal length, but becomes difficult when these segments are adjusted (...) to have the same apparent length. This illusion cannot be reduced by selective inhibition of features, such as that used to facilitate the rapid detection of feature conjunctions. For example, subjects are unable to ignore the wings when making judgements of the test line, even when it is advantageous to do so. This rules out explanations based on interactions among the features themselves. We also show that spatial filtering cannot account for this illusion, since these effects are indifferent to the sign of contrast of the line segments and can occur for textured lines having the same first-order statistics. The illusion, however, can be explained by a model in which line length is determined via grouping operations acting at a level prior to the formation of preattentive features. (shrink)
Methods. Visual search experiments were carried out using simple black and white figures corresponding to shiny objects lit from various directions. These included, for example, depictions of cylinders with highlights positioned at various heights (see figure). Targets and distractors differed only in the arrangement of their constituent regions, allowing them to be distinguished by the position of the highlights on the corresponding objects.