Mapping visual attention with change blindness: new directions for a new method

Change blindness provides a new technique for mapping visual attention with unprecedented spatial and temporal resolution. Change blindness can occur when a brief full-field blank interferes with the detection of changes in a scene that occur during the blank. This interference can be overcome by attending to the location of a change. Because changes are detected at attended locations, but not at unattended locations, detection accuracy provides an indirect measure of the distribution of visual attention. The likelihood of detecting a new element in a scene provides a measure of the occurrence of attention at that element’s location. Potential new directions, advantages, and problems with this method are considered.     

Spatial maps of directed visual attention

Simple reaction times (RTs) to a visual target are facilitated when the target occurs at a location expected by an observer, and are slowed when the target occurs at the mirror-symmetric location contralateral to the expectancy (e.g., Posner, 1978; Posner, Snyder, & Davidson, 1980). The spatial extent of this attention effect was examined by inducing subjects to expect the target at one location and introducing occasional probe flashes at other locations throughout the visual field. The results indicated that RTs to these probes were equivalent to those obtained at the expected location so long as the probe was in the same hemifield as the subject's expectancy. Conversely, RTs to probes in the hemifield opposite the expectancy generated uniformly slower response times. These results were obtained when the expected location varied in eccentricity from 2 degrees to 16 degrees along the horizontal meridian. In addition, when the expected and unexpected locations were within the same hemifield, no expectancy effects were observed. Under these conditions, the frequently used metaphor that directed visual attention operates like a spatially restricted "beam" appears inaccurate. The implications of these findings for current views of directed attention are considered.     

Interlateral asymmetry in the time course of the effect of a peripheral prime stimulus

Evidence exists that both right and left hemisphere attentional mechanisms are mobilized when attention is directed to the right visual hemifield and only right hemisphere attentional mechanisms are mobilized when attention is directed to the left visual hemifield. This arrangement might lead to a rightward bias of automatic attention. The hypothesis was investigated by testing male volunteers, wherein a "location discrimination" reaction time task (Experiments 1 and 3) and a "location and shape discrimination" reaction time task (Experiments 2 and 4) were used. Unilateral (Experiments 1 and 2) and unilateral or bilateral (Experiments 3 and 4) peripheral visual prime stimuli were used to control attention. Reaction time to a small visual target stimulus in the same location or in the horizontally opposite location was evaluated. Stimulus onset asynchronies (SOAs) were 34, 50, 67, 83 and 100 ms. An important prime stimulus attentional effect was observed as early as 50 ms in the four experiments. In Experiments 2, 3 and 4, this effect was larger when the prime stimulus occurred in the right hemifield than when it occurred in the left hemifield for SOA 100 ms. In Experiment 4, when the prime stimulus occurred simultaneously in both hemifields, reaction time was faster for the right hemifield and for SOA 100 ms. These results indicate that automatic attention tends to favor the right side of space, particularly when identification of the target stimulus shape is required.     

Changes are not localized before they are explicitly detected

Change detection is in many ways analogous to visual search. Yet, unlike search, successful detection depends not on the salience of features within a scene, but on the difference between the original and modified scene. If, as in search, pre-attentive mechanisms guide attention to the change location, the change itself must produce a preattentively detectable signal. Despite recent evidence for implicit representation of change in the absence of conscious detection, few studies have yet explored whether attention is guided to a change location prior to explicit detection. In four “change blindness” experiments using several variants of the “flicker” task, we tested the hypothesis that implicit or preattentive mechanisms guide change localization prior to explicit detection. None of the experiments revealed improved localization of changes prior to explicit reports of detection, suggesting that implicit detection of change does not contribute to the eventual explicit localization of a change. Instead, change localization is essentially arbitrary, driven by the salience of features within scenes.     

Change detection in complex scenes: hemispheric contribution and the role of perceptual and semantic factors

The perceptual salience and semantic relevance of objects for the meaning of a scene were evaluated with multiple criteria and then manipulated in a change-detection experiment that used an original combination of one-shot and tachistoscopic divided-visual-field paradigms to study behavioural hemispheric asymmetry. Coloured drawings that depicted meaningful situations were presented centrally and very briefly (120 ms) and only the changes were lateralised by adding an object in the right or in the left visual hemifield. High salience and high relevance improved both response times (RTs) and accuracy, although the overall contribution of salience was greater than that of relevance. Moreover, only for low-salience changes did relevance affect speed. RTs were shorter when a change occurred in the left visual hemifield, suggesting a right-hemisphere advantage for detection of visual change. Also, men responded faster than women. The theoretical and methodological implications are discussed.     

Failures to see: attentive blank stares revealed by change blindness

Change blindness illustrates a remarkable limitation in visual processing by demonstrating that substantial changes in a visual scene can go undetected. Because these changes can ultimately be detected using top-down driven search processes, many theories assign a central role to spatial attention in overcoming change blindness. Surprisingly, it has been reported that change blindness can occur during blink-contingent changes even when observers fixate the changing location [O'Regan, J. K., Deubel, H., Clark, J. J., & Rensink, R. A. (2000). Picture changes during blinks: Looking without seeing and seeing without looking. Visual Cognition, 7, 191-212]. However, eye blinks produce a transient disruption of vision that is independent of any associated changes in the retinal image. We determined whether these 'attentive blank stares' could occur in the absence of blink-mediated visual suppression. Using a flicker change-blindness paradigm we confirm that despite direct attentive fixations, obvious scene changes often remain undetected. We conclude that change detection involves object or feature based attentional mechanisms, which can be 'misdirected' despite the allocation of spatial attention to the position of the change.     

Iconic memory for natural scenes: Evidence using a modified change-detection procedure

ABSTRACT

Change blindness for the contents of natural scenes suggests that only items that are attended while the scene is still visible are stored, leading some to characterize our visual experiences as sparse. Experiments on iconic memory for arrays of discrete symbols or objects, however, indicate observers have access to more visual information for at least several hundred milliseconds at offset of a display. In the experiment presented here, we demonstrate an iconic memory for complex natural or real-world scenes. Using a modified change detection task in which to-be changed objects are cued at offset of the scene, we show that more information from a natural scene is briefly stored than change blindness predicts and more than is contained in visual short-term memory. In our experiment, a cue appearing 0, 300, or 1000?msec after offset of the pre-change scene or at onset of the second scene presentation (a Post Cue) directed attention to the location of a possible change. Compared to a no-cue condition, subjects were significantly better at detecting changes and identifying what changed in the cue condition, with the cue having a diminishing effect as a function of time and no effect when its onset coincided with that of the second scene presentation. The results suggest that an iconic memory of a natural scene exists for at least 1000?msec after scene offset, from which subjects can access the identity of items in the pre-change scene. This implies that change blindness underestimates the amount of information available to the visual system from a brief glance of a natural scene.     

Orienting to Extinguished Signals in Hemispatial Neglect

This study tested for spatial orienting effects, without awareness to signals presented in the neglected hemifield of 2 hemispatial-neglect patients. The experiment adapted a spatial precuing paradigm for measuring the effects of visual attention. Contralesional orienting hastened subsequent target detection at the location of an extinguished precue. These findings validate a claim that orienting can occur independently of overt detection and indicate that location information is registered in the neglected field.     

The role of iconic memory in change-detection tasks

In three experiments, subjects attempted to detect the change of a single item in a visually presented array of items. Subjects' ability to detect a change was greatly reduced if a blank interstimulus interval (ISI) was inserted between the original array and an array in which one item had changed ('change blindness'). However, change detection improved when the location of the change was cued during the blank ISI. This suggests that people represent more information of a scene than change blindness might suggest. We test two possible hypotheses why, in the absence of a cue, this representation fails to produce good change detection. The first claims that the intervening events employed to create change blindness result in multiple neural transients which co-occur with the to-be-detected change. Poor detection rates occur because a serial search of all the transient locations is required to detect the change, during which time the representation of the original scene fades. The second claims that the occurrence of the second frame overwrites the representation of the first frame, unless that information is insulated against overwriting by attention. The results support the second hypothesis. We conclude that people may have a fairly rich visual representation of a scene while the scene is present, but fail to detect changes because they lack the ability to simultaneously represent two complete visual representations.     

From reorienting of attention to biased competition: Evidence from hemifield effects

When a distractor was presented simultaneously with or directly following a target, it produced more interference when it was presented in the same visual hemifield as the target than when it was presented in the opposite visual field. This result is interpreted in terms of biased competition; there is more competition between stimuli when they are presented in the same visual field, rather than in opposite visual fields. However, when the distractor was presented 125 msec or more prior to the target, this pattern was reversed. In those cases, there was more distractor interference when target and distractor were presented in opposite visual fields. This can be explained by assuming that attention was captured by the distractor, and that there was an additional cost of reorienting to a location in the opposite visual field.     

The spatial distribution of inhibition of return

Inhibition of return (IOR) refers to the finding that response times (RTs) are typically slower for targets at previously attended (cued) locations than for targets at novel (uncued) locations. Although previous research has indicated that IOR may spread beyond a cued location, the present study is the first to examine the spatial distribution of IOR with high spatial resolution over a large portion of the central visual field. This was done by using a typical IOR procedure (cue, delay, target) with 4 cue locations and 441 target locations (each separated by 1° of visual angle). The results indicate that IOR spreads beyond the cued location to affect the cued hemifield. However, the cues also produced a gradient of RTs throughout the visual field, with inhibition in the cued hemifield gradually giving way to facilitation in the hemifield opposite the cue.     

Evidence for preserved representations in change blindness

People often fail to detect large changes to scenes, provided that the changes occur during a visual disruption. This phenomenon, known as "change blindness," occurs both in the laboratory and in real-world situations in which changes occur unexpectedly. The pervasiveness of the inability to detect changes is consistent with the theoretical notion that we internally represent relatively little information from our visual world from one glance at a scene to the next. However, evidence for change blindness does not necessarily imply the absence of such a representation---people could also miss changes if they fail to compare an existing representation of the pre-change scene to the post-change scene. In three experiments, we show that people often do have a representation of some aspects of the pre-change scene even when they fail to report the change. And, in fact, they appear to "discover" this memory and can explicitly report details of a changed object in response to probing questions. The results of these real-world change detection studies are discussed in the context of broader claims about change blindness.     

How Are Different Properties of a Scene Encoded in Visual Memory?

Recent studies in scene perception suggest that much of what observers believe they see is not retained in visual memory. Depending on the roles they play in organizing the perception of a scene, different visual properties may require different amounts of attention to be incorporated into a mental representation of the scene. The goal of this study was to compare how three visual properties of scenes, colour, object position, and object presence, are encoded in visual memory. We used a variation on the change detection “flicker” task and measured the time to detect scene changes when: (1) a cue was provided regarding the type of change; and, (2) no cue was provided. We hypothesized that cueing would enhance the processing of visual properties that require more attention to be encoded into scene representations, whereas cueing would not have an effect for properties that are readily or automatically encoded in visual memory. In Experiment 1, we found that there was a cueing advantage for colour changes, but not for position or presence changes. In Experiment 2, we found the same cueing effect regardless of whether the colour change altered the configuration of the scene or not. These results are consistent with the idea that properties that typically help determine the configuration of the scene, for example, position and presence, are better encoded in scene representations than are surface properties such as colour.     

Saccade preparation inhibits reorienting to recently attended locations

We measured manual reaction time in normal human subjects to confirm that an eccentric visual signal has a biphasic effect on covert attention and eye movements. First, it summons attention and biases a saccade toward the signal; a subsequent inhibition of return then slows responses to signals at that location. A temporal hemifield dominance for inhibition of return was shown; this finding converges with observations in neurologic patients to suggest that it is mediated by midbrain pathways. Endogenous orienting of attention, from a central arrow cue, did not activate inhibition of return, whereas endogenous saccade preparation did so as effectively as an exogenous signal, even when no saccade was made. Inhibition of return is activated by midbrain oculomotor pathways and may function as a location "tagging" mechanism to optimize efficiency of visual search.     

Hemispheric visual atentional imbalance in patients with traumatic brain injury

We find a spatially asymmetric allocation of attention in patients with traumatic brain injury (TBI) despite the lack of obvious asymmetry in neurological indicators. Identification performance was measured for simple spatial patterns presented briefly to a locus 5 degrees into the left or right hemifield, after precuing attention to the same (ipsilateral) or opposite (cross-hemifield) side. Though the cue was non-predictive of target location overall, performance was significantly slower for cross than for ipsilateral trials in both patients and controls. We tested 21 TBI patients without overt focal brain damage and nine control subjects. Only patients demonstrated significantly worse performance for left side presentation in the ipsilateral condition. Furthermore, in the cross-hemifield condition, the left-right difference seen in TBI patients was significantly larger-reflecting a failure in producing a leftward attention shift. Again no significant difference was found in controls. These hemifield effects suggest an asymmetry in the ability of TBI patients in shifting attention to the left hemifield, whether from central fixation or from a cue in the contra-lateral hemifield. The results support basic hypotheses regarding visual attention: Attentional control may be asymmetric and attention may be a distributed, rather than localized cortical function.     

The capacity of visual short-term memory within and between hemifields

Visual short-term memory (VSTM) and attention are both thought to have a capacity limit of four items [e.g. Luck, S. J., & Vogel, E. K. (1997). The capacity of visual working memory for features and conjunctions. Nature, 309, 279-281; Pylyshyn, Z. W., & Storm, R. W. (1988). Tracking multiple independent targets: evidence for a parallel tracking mechanism. Spatial Vision, 3, 179-197.]. Using the multiple object visual tracking paradigm (MOT), it has recently been shown that twice as many items can be simultaneously attended when they are separated between two visual fields compared to when they are all presented within the same hemifield [Alvarez, G. A., & Cavanagh, P. (2004). Independent attention resources for the left and right visual hemifields (Abstract). Journal of Vision, 4(8), 29a.]. Does VSTM capacity also increase when the items to be remembered are distributed between the two visual fields? The current paper investigated this central issue in two different tasks, namely a color and spatial location change detection task, in which the items were displayed either in the two visual fields or in the same hemifield. The data revealed that only memory capacity for spatial locations and not colors increased when the items were separated between the two visual fields. These findings support the view of VSTM as a chain of capacity limited operations where the spatial selection of stimuli, which dominates in both spatial location VSTM and MOT, occupies the first place and shows independence between the two fields.     

Perceptual conditions necessary to induce change blindness

Change blindness is a failure to detect a change in an scene when the change occurs along with some visual disturbances. Disturbances are thought to play a delocalizing role that affects the saliency of the “target” transient signal coming from the change location, which would otherwise capture attention and render the change visible. For instance, it is hypothesized that the appearance of new objects in the “mudsplashes” paradigm generates transient signals that compete with the target object's transient signal for attracting attention. Thus, experiments using the mudsplashes paradigm do not rule out a possible role of object changes in capturing attention. Here, by reversing image contrast polarity, we develop a new paradigm to produce change blindness when a real global transient signal is the only visual event occurring, with no edges added or deleted except in the target object. The results show that transient signals, per se, are able to prevent change detection. However, abrupt transients are not necessary if object change occurs in the zero-contrast phase of a smoothly fading and reappearing image, leaving attention as the only common factor affecting all cases of change blindness.     

Modulation of the Directional Attention Deficit in Visual Neglect by Hemispatial Factors

Using a visuo-spatial cuing paradigm, Posner and collaborators (Posner, Cohen, & Rafal, 1982; Posner, Walker, Friedrich, & Rafal, 1984) reported that subjects with a parietal lobe lesion have difficulty in disengaging their visual attention from an invalidly precued location in the ipsilesional hemifield when the target they have to respond to is presented in the contralesional field. Later, these authors (Posner, Walker, Friedrich, & Rafal, 1987) proposed that this disengagement deficit is one involving spatial shifts of attention in a contralesional direction, irrespective of the visual hemifield in which the target is presented. This proposal of a directional disengagement deficit along the horizontal axis, present in either hemifield, contrasts with a report by Baynes, Holtzman, & Volpe (1986) showing, in right parietal subjects, a disengagement deficit for shifts of attention along the vertical axis that is only manifest in the contralesional hemifield. In the present report, we assessed the disengagement deficit of a neglect subject along the horizontal and vertical axes. Results show a disengagement deficit restricted to shifts of attention in the contralesional direction (horizontal axis), which is significant only in the contralesional visual hemifield. However, there is a clear trend for a directional disengagement deficit in the ipsilateral field. These observations indicate that the attention deficit present in neglect is directional and is modulated either by hemispatial factors or by the lateral target location in the visual field. On the basis of the present results, it is proposed that the deficit of parietal subjects may best be conceptualized as one of attentional capture for stimuli located on the contralesional side of the current focus of attention rather than one of disengagement.     

Covert visual orienting: Hemifield-activation can be mimicked by zoom lens and midlocation placement strategies

How is attention distributed over visual space when an observer expects a target to occur at one of several possible locations? Two experiments sought to understand the source of the conflict between studies leading to the notions of hemifield activation (Hughes and Zimba 1985) and attentional gradients (Downing and Pinker 1985; Shulman et al. 1985, 1986). Subjects were cued to attend one of the 4 corners of an imaginary square centered at fixation, allowing comparison of uncued locations in the cued and uncued hemifields. In one experiment marking of the 4 locations was varied to determine if providing a ‘target’ for attention is necessary to obtain within-hemifield gradients. RT was faster at the cued location than at the three uncued locations which had equivalent latencies, a pattern that was unaffected by marking of the potential target locations. This result, which is consistent with the notion of a gradient around the attended location is a strong disconfirmation of the hemifield activation hypothesis. The second experiment demonstrated that an unusual procedure for presenting the probe stimuli in Hughes and Zimba (1985) is at least partially responsible for their evidence for uniform hemifield activation. It is proposed that visual attention is directed to visuo-spatial channels with fixed structural properties, and that when attention to two locations is desired, the subject may attend a spatial channel located between them.     

Semantic guidance of attention within natural scenes

When viewing real-world scenes composed of a myriad of objects, detecting changes can be quite difficult, especially when transients are masked. In general, changes are noticed more quickly and accurately if they occur at the currently (or a recently) attended location. Here, we examine the effects of explicit and implicit semantic cues on the guidance of attention in a change detection task. Participants first attempted to read aloud a briefly presented prime word, then looked for a difference between two alternating versions of a real-world scene. Helpful primes named the object that changed, while misdirecting primes named another (unchanging) object in the picture. Robust effects were found for both explicit and implicit priming conditions, with helpful primes yielding faster change detection times than misdirecting or neutral primes. This demonstrates that observers are able to use higher order semantic information as a cue to guide attention within a natural scene, even when the semantic information is presented outside of explicit awareness.