Visual marking: the influence of temporary changes on time-based visual selection

The preview benefit describes the finding that participants can prioritize the selection of new stimuli by the top-down inhibition of previously presented (previewed) items already in the field (Watson & Humphreys, 1997). Previous work has shown that if the old items undergo a permanent shape change when the new are added, then the old items recompete for selection with the new-the preview benefit is abolished. This is adaptive because it would be useful to stop ignoring items which change in potentially important ways. In contrast, temporary or repeating changes might be much less behaviorally relevant than permanent changes. Accordingly, we examined the effect of temporary changes (Experiment 1) and repeating changes (Experiment 2). Overall, the results showed that temporary changes could be partially but not fully ignored, and ignoring repeating changes became more difficult as the number of repetitions increased. The findings are discussed in terms of attentional prioritization mechanisms, maintenance of inhibitory representations, and the ecological properties of time-based selection.     

Visual marking and visual change

Five experiments investigated the types of changes that disrupt the preview effect--the benefit gained in difficult search tasks from presenting some distractors earlier in time. A shape change with or without an overall luminance change at the location of an old item was found to disrupt the preview effect, whereas an equivalent luminance change alone or an isoluminant color change was not disruptive. Results suggest that (a) relatively low-level visual changes may not be sufficient to abolish the benefit, (b) the benefit most likely occurs through inhibition applied to locations within a location master map, and (c) inhibition need not be applied to surface features of objects.     

Visual marking beside the mark: prioritizing selection by abrupt onsets

In a standard visual marking experiment, observers are presented with a display containing one set of elements (old elements) followed after a certain time interval by a second set of elements (new elements). The task of observers is to search for a target among the new elements. Typically, the time to find the target depends only on the number of new elements in the display and not on the number of old elements, showing that observers search only among the new elements. This effect of prioritizing new elements over old elements is explained in terms of top-down inhibition of old objects-that is, visual marking (Watson & Humphreys, 1997). The present study addressed whether this prioritizing is in fact mediated by top-down inhibition of old objects, as suggested by Watson and Humphreys (1997), or whether it is mediated by the abrupt onsets of the newly presented elements (Yantis & Jonides, 1984). In three experiments, the presentations of the old and new elements were or were not accompanied by a luminance change. The results showed that if new elements were equiluminant with the background, no visual marking occurred, suggesting that new elements must have a luminance onset in order to be prioritized over old elements. Implications for current theories on visual selection are discussed.     

Visual marking and the perception of salience in visual search

In the present study, the gap paradigm originally developed by Watson and Humphreys (1997) was used to investigate whether the process of visual marking can influence the perceptual salience of a target in visual search. Consistent with previous studies (Watson & Humphreys, 1997), the results showed that search was not affected by the presence of the preceding distractors when the target was relatively low in salience. This finding suggests that visual marking can increase the efficiency of visual search by decreasing the size of the search set. However, more important, the results also showed that search was affected by the presence of the preceding distractors when the target was relatively high in salience. This finding suggests that visual marking may be limited in its ability to increase the perceptual salience of the target. Together, the results of the present study suggest that the effectiveness of visual marking may vary as a function of search context.     

Visual marking beside the mark: Prioritizing selection by abrupt onsets

In a standard visual marking experiment, observers are presented with a display containing one set of elements (old elements) followed after a certain time interval by a second set of elements (new elements). The task of observers is to search for a target among the new elements. Typically, the time to find the target depends only on the number of new elements in the display and not on the number of old elements, showing that observers search only among the new elements. This effect of prioritizing new elements over old elements is explained in terms of top-down inhibition of old objects—that is, visual marking (Watson & Humphreys, 1997). The present study addressed whether this prioritizing is in fact mediated by top-down inhibition of old objects, as suggested by Watson and Humphreys (1997), or whether it is mediated by the abrupt onsets of the newly presented elements (Yantis & Jonides, 1984). In three experiments, the presentations of the old and new elements were or were not accompanied by a luminance change. The results showed that if new elements were equiluminant with the background, no visual marking occurred, suggesting that new elements must have a luminance onset in order to be prioritized over old elements. Implications for current theories on visual selection are discussed.     

Proneness to psychosis and selection of objects of visual attention: individual differences in visual marking

Deficits in early visual attention and perceptual organisation have frequently been shown to associate with both poor pre-morbid schizophrenia and those at a greater putative risk of psychosis. The nature of the deficit is unclear. The present study investigated the relationship between speed of visual marking and proneness to psychosis. 20 males and 20 females completed several tasks assessing speed of selection and de-selection of visual objects. As predicted, negative schizotypy was associated with poorer marking in males, but socially desirable responding potentially confounded this result. In addition, impulsive non-conformity swas associated with poorer visual marking, more prominently in females. These results are discussed in relation to possible mechanisms by which psychosis-proneness and impulsivity may restrict the top–down influences operating on early visual attention.     

Visual marking: a convergence of goal- and stimulus-driven processes during visual search

Watson and Humphreys (1997) proposed that visual marking is a goal-directed process that enhances visual search through the inhibition of old objects. In addition to the standard marking case with targets at new locations, included in Experiment 1 was a set of trials with targets always at old locations, as well as a set of trials with targets varying between new and old locations. The participants' performance when detecting the target at old locations was equivalent to their performance in the full-baseline condition when they knew the target would be at old locations, and was worse when the target appeared at old locations on 50% of the trials. Marking was observed when the target appeared at new locations. In Experiment 2, an offset paradigm was used to eliminate the influence of the salient abrupt-onset feature of the new objects. No significant benefits were found for targets at new locations in the absence of onsets at new locations. The results suggest that visual marking may be an attentional selection mechanism that significantly benefits visual search when (1) the observer has an appropriate search goal, (2) the goal necessitates inhibition of old objects, and (3) the new objects include a salient perceptual feature.     

Visual marking: evidence for inhibition using a probe-dot detection paradigm

Watson and Humphreys (1997, 1998) have recently demonstrated that new objects can be prioritized for visual attentional processing by the top-down attentional inhibition of old objects already in the field, a mechanism they called visual marking. The experiments reported here show that the detection of a dim probe dot is impaired when it falls at the location of an old object (Experiments 1 and 3) but that this occurs only in conditions in which it is advantageous for subjects to mark (inhibit) old objects (Experiment 2). These results further support previous work showing that visual marking is based on the inhibition of the locations of old objects and that visual marking can be flexibly applied (or withheld), depending on the goals of current behavior.     

Visual selection mediated by location: Selecting successive visual objects

In these experiments, each stimulus consists of a series of frames, each containing a target digit of one color and a distractor digit of another color. The task is to name the highest digit of the target color. Subjects make fewer errors when successive targets appear at the same location than when they appear at different locations, apparently because they select target objects by using a mechanism that is based on location. When successive targets appear at the same location, there is no need to “move” the selection mechanism to a new location, leaving more time to identify the stimuli. These experiments show that location-based selection is used even though selection by color would be more direct. They also demonstrate a method of measuring location-based selection that can be applied to a variety of visual tasks. Further experiments reveal that although location-based selection is used to identify a digit in the presence of a digit distractor, it is not used to identify a digit in the presence of a letter distractor, suggesting that this selection mechanism is not used in this situation to prevent interference among the basic features making up letters and digits, but to inhibit responses associated with the distractors.     

Parieto-occipital areas involved in efficient filtering in search: A time course analysis of visual marking using behavioural and functional imaging procedures

Search for a colour-form conjunction target can be facilitated by presenting one set of distractors prior to the second set of distractors and the target: the preview benefit (Watson & Humphreys, 1997). The early presentation of one set of distractors enables them to be efficiently filtered from search. We report two studies investigating the time course of the preview benefit. In Experiment 1 we use a standard reaction time analysis to show that the benefit has a relatively slow time course; old items need to precede the new set by 600 ms or more in order to be fully filtered from search. Furthermore, the reductions in reaction time across time in the preview condition varied nonlinearly with the display size, suggesting that old items were discounted from search in parallel. In Experiment 2 we examined the neural locus of this filtering effect over time, using positron emission tomography (PET). We show that regions of parieto-occipital cortex are selectively activated in a preview search condition relative to a detection baseline. These regions also increase in activation as the preview interval increases (and search then becomes easier), consistent with them modulating the parallel filtering of distractors from targets in spatial search. Interestingly, the same areas as those activated in preview search were also active in conjunction search relative to its own detection baseline. Thus these regions either modulate parallel filtering in conjunction search too, or they modulate different behavioural functions according to task constraints.     

Visual marking of old objects

Watson and Humphreys (1997) presented evidence that selection of new elements can be prioritized by on-line top-down attentional inhibition of old stimuli already in the visual field (visual marking). The experiments on which this evidence was based always presented old elements in green and new elements in blue. Because of this, instead of prioritizing new objects by inhibiting old objects, selection could have been based on color. The present experiment, which does not contain this confound, showed that visual marking is a strong and robust process that enables subjects to visually mark at least 15 old elements even when these elements have the same color as the new elements. The results indicate that preview of the elements is critical—not the fact that those elements contained a common feature.     

Visual marking and change detection

The preview benefit from prior exposure of response-irrelevant (distracter) objects in visual search has been accounted for in terms of top-down inhibition (i.e. visual marking), bottom-up abrupt onset capture, or asynchrony-dependent perceptual segregation. We assess the relative contribution of abrupt onset and visual marking in a paradigm combining visual search with a visual working memory task. We investigated time-based selection of multiple objects for storage in visual working memory, using a change detection paradigm (Luck and Vogel in Nature 390:279–281, 1997) with distracter preview. We varied preview exposure (short vs. long), in a series of three experiments. The contribution of asynchrony-related perceptual segregation was assessed across experiments by varying the complexity of the stimuli (colored squares, oriented bars and oriented T-shapes) and the type of change detection (color or orientation), resulting in different levels of perceptual segregation between visual elements. The results suggest that bottom-up abrupt onset, visual marking and perceptual segregation factors co-operate in time-based selection for storage in visual working memory.     

Visual marking inhibits singleton capture

This paper is concerned with how we prioritize the selection of new objects in visual scenes. We present four experiments investigating the effects of distractor previews on visual search through new objects. Participants viewed a set of to-be-ignored nontargets, with the task being to search for a target in a second set, added to the first after 1000ms. This second set could contain a salient feature singleton, defined in terms of its color, orientation, or both color and orientation. When the singleton was a distractor, search was slowed relative to when there was no singleton. Search was facilitated when the singleton was a target. Interestingly, both the interference and facilitation effects were modulated when the preview shared features with the singleton. Follow-up experiments showed that this reduction of singleton effects was not due to: (i) low-level sensory aspects of the displays, (ii) increased heterogeneity in the search set in the preview condition, or (iii) color-based grouping of old and new items. Instead, we suggest that there is an inhibitory carry-over from the first to the second set of items based on feature similarity. We suggest the suppression stems from a process termed visual marking, which suppresses irrelevant visual objects in anticipation of more relevant new objects (Watson & Humphreys, 1997). The findings argue against alternative explanations such as the automatic capture by abrupt new onsets account.     

Visual marking across eye blinks

Visual search for a conjunction target can be made efficient by presenting one initial set of distractors as a preview, prior to the onset of the other items in the search display Watson & Humphreys (Psychological Review 104:90–122, 1997). However, this “preview advantage” is lost if the initial items are offset for a brief period before onsetting again with the search display Kunar, Humphreys, & Smith (Psychological Science 14:181–185, 2003). Researchers have long disputed whether the preview advantage reflects a process of internally coding and suppressing the old items or of the onset of the new items capturing attention Donk & Theeuwes (Perception & Psychophysics 63:891–900, 2001). In this study, we assessed whether an internally driven blink (in which participants close their eyes) acts in the same manner as an external blink produced by offsetting and then onsetting the preview. In the novel blink conditions, participants searched feature, conjunction, and preview displays after being cued to blink their eyes. The search displays were presented during the eye blink, and so were immediately available once participants opened their eyes. Having participants make an eye blink generally slowed search but had no effect on the search slopes. In contrast, imposing an externally driven blink disrupted preview search. The data indicated that visual attention can compensate for internally driven blinks, and this does not lead to the loss of the representations of distractors across time. Moreover, efficient preview search occurred when the search items had no abrupt onsets, demonstrating that onsets of new search items are not critical for the preview benefit.     

Contingent visual marking by transients

Preview search is a phenomenon in which a set of new items can be searched with seemingly no interference from items already present in the display. The preview effect has been shown to occur only when the presentation of the new items is accompanied by a luminance change (Donk & Theeuwes, 2001). In a series of experiments, we extend the type of transients that can lead to a preview benefit to offsets and motion, and confirm Donk and Theeuwes's finding that equiluminant color changes do not lead to a preview effect. Like Donk and Theeuwes, we find that preview search does not occur when only the old items undergo a transient change, suggesting that the processes responsible for preview search are triggered when the new items undergo a change detectable by the magnocellular system. In addition, we find that irrelevant transients interfere with preview search only when they match the current attentional set (e.g., luminance change or motion). Results suggest that preview search is not the automatic capture of attention by transients, but rather is contingent on top-down control settings.     

Attentional set protects visual marking from visual transients

When some distractors (old items) appear before others (new items) in an inefficient visual search task, the old items are excluded from the search (visual marking). Previous studies have shown that changing the shape of old items eliminates this effect, suggesting that shape identity must be maintained for successful visual marking. However, the contribution of top-down target knowledge to the maintenance of visual marking under shape change conditions has not been systematically examined. The present study tested whether the vulnerability of visual marking to shape change is contingent on observers' attentional set, by manipulating compatibility of the set and the domains in which the change occurs. The results indicated that visual marking survived shape changes when the observer's attentional set was consistent with critical features between the old and new items. This protection was observed when the set was based on explicit instructions at the beginning of the experiment, and when the task set was implicitly carried over from the previous task. These results suggest that top-down processes play a role in maintaining memory templates by enhancing the grouping and suppression processes during visual search, despite disruptive bottom-up signals.     

Visual marking: selective attention to asynchronous temporal groups

In visual search, when a subset of distractors is previewed 1 s before the target and the remaining distractors, search speed is independent of the number of previewed items. This is visual marking. What allows old items to be marked? Four experiments show that marking is disrupted if the onset of the new items is accompanied by synchronous changes to the old items, but it is not disrupted by changes restricted to the background or by asynchronous changes to the old items. Further, behaviorally relevant old items can be prioritized over new items. Visual marking is based on temporal asynchrony between new and old items, which allows segregation of these items into 2 temporal groups. Attention is then selectively applied to 1 group.     

Measuring the time course of selection during visual search

In visual search tasks, observers can guide their attention towards items in the visual field that share features with the target item. In this series of studies, we examined the time course of guidance toward a subset of items that have the same color as the target item. Landolt Cs were placed on 16 colored disks. Fifteen distractor Cs had gaps facing up or down while one target C had a gap facing left or right. Observers searched for the target C and reported which side contained the gap as quickly as possible. In the absence of other information, observers must search at random through the Cs. However, during the trial, the disks changed colors. Twelve disks were now of one color and four disks were of another color. Observers knew that the target C would always be in the smaller color set. The experimental question was how quickly observers could guide their attention to the smaller color set. Results indicate that observers could not make instantaneous use of color information to guide the search, even when they knew which two colors would be appearing on every trial. In each study, it took participants 200–300 ms to fully utilize the color information once presented. Control studies replicated the finding with more saturated colors and with colored C stimuli (rather than Cs on colored disks). We conclude that segregation of a display by color for the purposes of guidance takes 200–300 ms to fully develop.     

Prioritization in visual search: visual marking is not dependent on a mnemonic search

Visual marking (VM) refers to our ability to completely exclude old items from search when new stimuli are presented in our visual field. We examined whether this ability reflects an attentional scan of the old items, possibly allowing observers to apply inhibition of return or maintain a memory representation of already seen locations. In four experiments, we compared performance in two search conditions. In the double-search (DS) condition, we required participants to pay attention to a first set of items by having them search for a target within the set. Subsequently, they had to search a second set while the old items remained in the field. In the VM condition, the participants expected the target only to be in the second (new) set. Selection of new items in the DS condition was relatively poor and was always worse than would be expected if only the new stimuli had been searched. In contrast, selection of the new items in the VM condition was good and was equal to what would be expected if there had been an exclusive search of the new stimuli. These results were not altered when differences in Set 1 difficulty, task switching, and response generation were controlled for. We conclude that the mechanism of VM is distinct from mnemonic and/or serial inhibition-of-return processes as involved in search, although we also discuss possible links to more global and flexible inhibition-of-return processes not necessarily related to search.     

Visual marking in moving displays: feature-based inhibition is not necessary

Visual marking is a mechanism by which new visual stimuli can gain a selection advantage by the top-down attentional inhibition of stimuli already in the field. Previous work (Olivers, Watson, & Humphreys, 1999) has shown that, for moving stimuli, there must be a unique feature difference between the old items and the new items for marking to occur. The present study shows that this constraint is not necessary if the local spatial relationships between the old moving items remain constant. It is proposed that, with a fixed configuration, the old moving items can be grouped to form a single object. An inhibitory template set up to represent the object then coordinates the application of inhibition to the individual stimuli. Implications for the theory and ecological flexibility of visual marking are discussed.