Resisting change: The influence of luminance changes on visual marking and the preview benefit

Visual search can benefit when one set of distractors is presented as a preview, prior to the appearance the second set of distractors plus the target (Watson & Humphreys, 1997). It has been shown that changing the shape of the old, previewed stimuli when the new items appear causes the old stimuli to recompete for selection with the new ones. In contrast, changing the luminance or color of the old stimuli has no detrimental effects. Here, we present five experiments that reassessed the effect of luminance changes in preview search. We show (1) that preview search is remarkably resistant to large changes in the absolute luminance of the old stimuli, even when those changes would ordinarily be sufficient to signal the appearance of a new object and draw attention (Experiments 1 and 2), and (2) that resistance to luminance changes can be bolstered by feature-based inhibitory processes (Experiments 3–5). These findings are discussed in terms of the possible ecological properties of time-based visual selection and possible mechanisms underlying the preview benefit.     

Fractionating the preview benefit in search: dual-task decomposition of visual marking by timing and modality

Providing participants with a preview of half the distractors in a visual search task facilitates performance. The present study examined the effects of secondary tasks on the preview benefit in search. Participants had to attend to a visual or an auditory stream of digits that began either (a) at the onset of the preview or (b) after the preview. Secondary tasks that onset with the preview disrupted the preview benefit irrespective of their modality. Only visual secondary tasks disrupted the benefit in the delayed condition. These selective interference effects suggest that the preview benefit can be fractionated into 2 components: an initialization component that involves modality-independent resources and a maintenance component that depends on visual resources. Results are discussed in relation to theoretical accounts of the preview benefit in search.     

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 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 working memory supports the inhibition of previously processed information: evidence from preview search

In four experiments we assessed whether visual working memory (VWM) maintains a record of previously processed visual information, allowing old information to be inhibited, and new information to be prioritized. Specifically, we evaluated whether VWM contributes to the inhibition (i.e., visual marking) of previewed distractors in a preview search. We evaluated this proposal by testing three predictions. First, Experiments 1 and 2 demonstrate that preview inhibition is more effective when the number of previewed distractors is below VWM capacity than above; an effect that can only be observed at small preview set sizes (Experiment 2A) and when observers are allowed to move their eyes freely (Experiment 2B). Second, Experiment 3 shows that, when quantified as the number of inhibited distractors, the magnitude of the preview effect is stable across different search difficulties. Third, Experiment 4 demonstrates that individual differences in preview inhibition are correlated with individual differences in VWM capacity. These findings provide converging evidence that VWM supports the inhibition of previewed distractors. More generally, these findings demonstrate how VWM contributes to the efficiency of human visual information processing--VWM prioritizes new information by inhibiting old information from being reselected for attention.     

Inhibition and anticipation in visual search: evidence from effects of color foreknowledge on preview search

We present four experiments in which we examined the effects of color mixing and prior target color knowledge on preview search (Watson & Humphreys, 1997). The task was to detect a target letter (an N or a Z) that appeared along with other new letters, when old distractors remained in the visual field. In some conditions, participants were told the target's color, in others, they were not. Foreknowledge of the target's color produced large improvements in search for both baseline and preview presentations (Experiment 1). For preview presentations, the magnitude of this effect was reduced if the target shared its color with a single colored set of previewed letters (Experiment 2). Removing this similarity across the displays greatly improved search efficiency (Experiment 3). In Experiment 4, we assessed and rejected the proposal that the effects reflected the probability that the target was carried by a particular color. We discuss the results in terms of separate effects of (1) inhibitory carryover from a preview color group and (2) an anticipatory set for a known target color.     

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: dissociating effects of new and old set size

Visual marking makes it possible to ignore old items during search. In a typical study, old items are previewed 1 s before adding an equal number of new items, one of which is the target. Previewing half of the items reduces the search slope relating response time (RT) to overall set size by half. However, this manipulation sometimes only reduces overall RT but not search slope (Experiment 1). By orthogonally varying the numbers of old and new items, Experiment 2 shows that old and new set sizes interactively affect visual marking. Given a constant new set size, the size of the old set has negligible effect on RT. However, increasing the new set size reduces the preview benefit in overall RT. Experiment 3 shows that this reduction may be restricted to paradigms that use temporal segregation cues. Studies should vary old and new set size orthogonally to avoid missing a visual marking effect where one may be present.     

Age-related effects in the marking of old objects in visual search

Two studies were performed to examine potential age-related differences in visual marking. Visual marking is a limited capacity process that enhances visual search through the inhibition of old objects. In Experiment 1, young and old adults performed three different search conditions: a full-element baseline, a half-element baseline, and a gap condition in which they searched through displays similar to the full-element baseline condition but with half of the letters presented before and the other half of the letters and the target presented after a 1000-ms gap. Both old and young adults displayed search slopes in the gap condition that were equivalent to slopes obtained in the half-element condition, suggesting that they were able to successfully inhibit the old letters. In Experiment 2, old and young adults also performed in three different visual search conditions but in this case with targets defined either by a form difference or by a conjunction of form and color. Both old and young adults showed a reduced slope in the gap as compared to the conjunction condition, suggesting inhibition of the old objects. The data are discussed in terms of age-related differences in the top-down control of attention in visual search.     

Visual change with moving displays: more evidence for color feature map inhibition during preview search

Preview search with moving stimuli was investigated. The stimuli moved in multiple directions, and preview items could change either their color or their shape before onset of the new (search) displays. In Experiments 1 and 2, the authors found that (a) a preview benefit occurred even when more than 5 moving items had to be ignored, and (b) color change, but not shape change, disrupted preview search in moving stimuli. In contrast, shape change, but not color change, disrupted preview search in static stimuli (Experiments 3 and 4). Results suggest that preview search with moving displays is influenced by inhibition of a color map, whereas preview search with static displays is influenced by inhibition of locations of old distractors.     

Revisiting preview search at isoluminance: new onsets are not necessary for the preview advantage

It has been argued that search performance under preview conditions relies on automatic capture by luminance onsets (Donk & Theeuwes, 2001). We present three experiments in which preview search was examined with both isoluminant and nonisoluminant items (e.g., as defined by luminance onsets). Experiment 1 provided evidence against the automatic capture of attention by onsets. Search benefited when onset previews were followed by new onset stimuli, as compared with a full-set baseline matched for the number of new onsets but in which half the distractors appeared simultaneously at isoluminance. Furthermore, both Experiments 1 and 2 established a preview advantage when isoluminant targets followed onset previews, when compared with appropriate full-set baselines. Experiment 3 replicated this result, while showing that the preview benefit was disrupted by dual-task interference. The data indicate that new onsets are not necessary to generate a preview advantage in search. We discuss the data in terms of search's benefiting from active inhibition of old onset-defined stimuli.     

Subset selective search on the basis of color and preview

In the preview paradigm observers are presented with one set of elements (the irrelevant set) followed by the addition of a second set among which the target is presented (the relevant set). Search efficiency in such a preview condition has been demonstrated to be higher than that in a full-baseline condition in which both sets are simultaneously presented, suggesting that a preview of the irrelevant set reduces its influence on the search process. However, numbers of irrelevant and relevant elements are typically not independently manipulated. Moreover, subset selective search also occurs when both sets are presented simultaneously but differ in color. The aim of the present study was to investigate how numbers of irrelevant and relevant elements contribute to preview search in the absence and presence of a color difference between subsets. In two experiments it was demonstrated that a preview reduced the influence of the number of irrelevant elements in the absence but not in the presence of a color difference between subsets. In the presence of a color difference, a preview lowered the effect of the number of relevant elements but only when the target was defined by a unique feature within the relevant set (Experiment 1); when the target was defined by a conjunction of features (Experiment 2), search efficiency as a function of the number of relevant elements was not modulated by a preview. Together the results are in line with the idea that subset selective search is based on different simultaneously operating mechanisms.     

The influence of irrelevant stimulus changes on stimulus and response repetition effects

In this study the influence of irrelevant stimulus changes from one trial to another in a serial reaction time task was investigated. Two experiments were performed in which subjects were required to respond to stimulus colour. Four colours were mapped on two response keys, so that colour and response repetition effects could be dissociated. In Experiment 1, the irrelevant stimulus dimension was location and in Experiment 2 it was shape. Both experiments were performed with a short and a long response-stimulus interval (RSI)-condition. In both experiments, the irrelevant dimension influenced the response repetition effect but not the colour repetition effect. In the reaction times, a response alternation effect was observed only when the irrelevant location of the stimulus changed in the long RSI-condition. The error rates showed a response alternation benefit for both irrelevant dimensions, in the short and the long RSI-condition. The benefit for response alternations is explained in terms of a response bias towards change that is triggered by a changing stimulus feature. We assume that the response bias is stronger for location than for colour and that accuracy is more sensitive to this bias than response latencies.     

Top-up search and the attentional blink: A two-stage account of the preview effect in search

An inefficient visual search task can be facilitated if half the distractor items are presented as a preview prior to the presentation of the remaining distractor items and target. This benefit in search is termed the preview effect. Recent research has found that a preview effect can still occur if the previewed items disappear before reappearing again just before the search items (the “top-up” procedure). In this paper we investigate the attentional demands of processing during the preview and the top-up periods. Experiment 1 found that if attention is withdrawn from the top-up stage then no preview effect occurs. Likewise if attention is withdrawn from the initial preview period then the preview effect is reduced (Experiment 2). The data suggest that the preview effect is dependent on attention being paid both to the initial display and also to the re-presentation of the old display before the search display appears. The data counter accounts of preview search in terms of automatic attention capture by new items or by inhibition of return. We discuss alternative accounts of the results, and in particular suggest an amalgamation of a temporal grouping and a visual marking account of preview search.     

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.     

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.     

Prioritizing new elements with a brief preview period: Evidence against visual marking

Watson and Humphreys (1997) have proposed that the ability to prioritize new elements over old elements involves a time-consuming process (of at least 400 msec) of active inhibition of the locations of the old elements, which they referred to asvisual marking. Recently, Donk and Theeuwes (2001) have suggested that prioritized selection of new over old elements is an instantaneous process related to the luminance change accompanying the appearance of the new elements. The aim of the present study was to test these two alternatives by investigating whether prioritized selection of new over old elements could be achieved with a very short preview of the old elements (50 msec). The results indicated that participants were able to prioritize selection of new over old elements when the new elements were presented with luminance onset whereas the old elements were not. New elements could not be prioritized if both the old and the new elements appeared with luminance onset. The results indicated that prioritization of new elements is based on an instantaneous process, rather than on a time-consuming process.     

Color grouping in space and time: evidence from negative color-based carryover effects in preview search

Five experiments addressed the role of color grouping in preview search (D. G. Watson & G. W. Humphreys, 1997). Experiment 1 used opposite color ratios of distractors in preview and second search displays, creating equal numbers of distractors in each color group in the final display. There was selective slowing for new targets carrying the majority color of the old items. This effect held when there was no bias in the preview and only the second search set had an uneven color ratio (Experiment 2). In Experiment 3, participants had foreknowledge of the target color, and effects were shown over and above those due to color biases. Experiment 4 demonstrated negative color carryover even when previews changed color. Experiment 5 showed reduced color carryover effects when previews were presented more briefly. Collectively, the results provide evidence for inhibitory carryover effects in preview search based on feature grouping.     

Modeling the effects of irrelevant speech on memory

The feature model (Nairne, 1990) is extended to account for the effects of irrelevant speech and concomitant interactions in immediate serial recall. In the feature model, both articulatory suppression and irrelevant speech are seen as adding noise to the memory representation, the difference being that articulatory suppression diverts more resources than does irrelevant speech. The addition of noise impairs recall because it reduces the probability of successful redintegration. When a competitor is incorrectly recalled, rather than the correct item, this competitor is recalled out of order, producing an increase in order errors. Six simulations are reported that show that the model accounts for (1) the impairment by both irrelevant speech and articulatory suppression, (2) the irrelevance of the phonological and semantic composition of the irrelevant speech, (3) greater disruption when the irrelevant speech tokens vary, (4) the abolition of the phonological similarity effect for visual, but not for auditory, items, (5) the abolition of the word length effect for both visual and auditory items, and (6) the abolition of the irrelevant speech effect under articulatory suppression for both visual and auditory items. The feature model is compared with the two other major views of irrelevant speech, the phonological store hypothesis and the changing state hypothesis.