Cross-trial priming in visual search for singleton conjunction targets: role of repeated target and distractor features

Kristjánsson, Wang, and Nakayama (2002) demonstrated that visual search for conjunctively defined targets can be substantially expedited ("primed") when target and distractor features are repeated on consecutive trials. Two experiments were conducted to examine whether the search response time (RT) facilitation on target-present trials results from repetition of target-defining features, distractor features, or both. The experiments used a multiple conjunctive search paradigm (adapted from Kristjánsson et al., 2002), in which the target and distractor features were varied (i.e., repeated) independently of each other across successive trials. The RT facilitation was numerically largest when both target and distractor features were repeated, but not significantly larger than that when only distractor features were repeated. This indicates that cross-trial priming effects in conjunctive visual search result mainly from the repetition of distractor, rather than target, features.     

Cross-dimensional interference and cross-trial inhibition

In two experiments, we examined whether one source of cross-dimensional interference in visual search involves cross-trial position priming. In Experiment 1, we demonstrated cross-dimensional interference in search for an orientation-defined target: Search for a left-tilted target among right-tilted non-targets was disrupted by the presence of a singleton color distractor. In all conditions, search was facilitated when a target was presented at the same position as a target in the previous trial (positive position priming). In addition, there were negative effects of position priming on orientation targets that fell on the same side as singleton distractors on the previous trial. In Experiment 2, to examine the impact of negative position priming on cross-dimensional interference, trials with and without singleton distractors were presented in a single trial block. The chance of a singleton distractor's being present on a preceding trial was then equated across displays when the distractor was and when it was not subsequently present. Cross-dimensional interference was eliminated under this mixed presentation condition, suggesting that the cost of cross-dimensional interference was not determined by the stimulus-driven factors in the current trial, at least when a limited number of target and distractor locations was used. We conclude that top-down selection is possible during visual search, and this leads to inhibition of the location of salient distractors. The cost of this is slowed detection of targets at inhibited locations on subsequent trials.     

Parallel coding of conjunctions in visual search

Two experiments investigated whether the conjunctive nature of nontarget items influenced search for a conjunction target. Each experiment consisted of two conditions. In both conditions, the target item was a red bar tilted to the right, among white tilted bars and vertical red bars. As well as color and orientation, display items also differed in terms of size. Size was irrelevant to search in that the size of the target varied randomly from trial to trial. In one condition, the size of items correlated with the other attributes of display items (e.g., all red items were big and all white items were small). In the other condition, the size of items varied randomly (i.e., some red items were small and some were big, and some white items were big and some were small). Search was more efficient in the size-correlated condition, consistent with the parallel coding of conjunctions in visual search     

Target uncertainty does not lead to more distraction by singletons: intertrial priming does

In two experiments, we examined why a singleton distractor has a stronger interfering effect in visual search when the target identity is uncertain. When participants searched for a shape, a color singleton distractor had a larger slowing effect in a mixed block, in which the target shape could change from trial to trial, than in a pure block, in which the target shape remained the same. Importantly, this increased singleton distractor effect could be traced back entirely to intertrial priming, since the increased costs occurred only on trials in which the target and the singleton distractor swapped identity (Experiment 1, allowing for priming between targets and singleton distractors) or on trials in which the target alone changed identity while the singleton distractor remained constant (Experiment 2, allowing for priming between targets only). This suggests that target uncertainty itself does not lead to strategic changes in the attentional selection of singletons. Instead, selection is affected by relatively automatic priming mechanisms that may be enhanced by competition for attention.     

Sequential priming of 3-D perceptual organization

In four experiments, the effects of sequential priming on the perceptual organization of complex three-dimensional (3-D) displays were examined. Observers were asked to view stereoscopic arrays and to search an embedded subset of items for an odd-colored target while 3-D orientation of the stimuli was varied randomly between trials. Search times decreased reliably when 3-D stimulus orientation was unchanged on consecutive trials, indicating substantial sequential priming by 3-D stimulus layout. The priming was nonsensory and was independent of priming by additional stimulus characteristics. Finally, priming by 3-D layout was unaffected by observers' foreknowledge of display orientation. Results indicate that perceptual organization of 3-D stimuli is guided by a short-term trace of 3-D spatial relationships between stimuli.     

Selective attention and search images in pigeons

In a visual search task, pigeons detected targets when pretrial visual cues or blocked trial sequences signaled the target's identity. Sequential priming was robust over a wide range of intertrial intervals, but visual priming was unstable when the delay between cue offset and display onset was varied. Larger target set sizes enhanced sequential, but not visual, priming. Sequential priming did not depend on display size over the range of relatively large displays used. However, ambiguously cued targets in small displays were detected more quickly than primed targets in large displays. These findings suggest that naturalistic selection biases, or "search images," may be attributable to sequential priming and that the common attentional mechanism has moderately selective properties.     

Priming of luminance-defined motion direction in visual search

Features that we have recently attended to strongly influence how we allocate visual attention across a subsequently viewed visual scene. Here, we investigate the characteristics of any such repetition effects during visual search for Gabor patch targets drifting in the odd direction relative to a set of distractors. The results indicate that repetition of motion direction has a strong effect upon subsequent allocation of attention. This was the case for judgments of a target’s presence or absence, of a target’s location, and of the color of a target drifting in the odd direction. Furthermore, distractor repetition on its own can facilitate search performance on subsequent trials, indicating that the benefits of repetition of motion direction are not confined to repetition of target features. We also show that motion direction need not be the target-defining dimension throughout a trial block for motion priming to occur, but that priming can build up with only one presentation of a given target direction, even within blocks of trials where the target may, unpredictably, be defined by a different feature (color, in this case), showing that dimensional-weighting accounts cannot, on their own, account for motion direction priming patterns. Finally, we show by randomizing the set size between trials that priming of motion direction can decrease search rates in visual search.     

Attentional priming and visual search in pigeons

Advance information about a target's identity improved visual search efficiency in pigeons. Experiments 1 and 2 compared information supplied by visual cues with information supplied by trial sequences. Reaction times (RTs) were lower when visual cues signaled a single target rather than two. RTs were (Experiment 1) or accuracy improved (Experiment 2) when a sequence of trials presented a single target rather than a mixture of 2. Experiments 3, 4, and 5 considered the selectivity of visual priming by introducing probe trials that reversed the usual cue-target relationship. RT was higher following such miscues than following the usual 1- or 2- target cuing relationships (Experiment 3); the miscuing effect persisted over variations in the target's concealment (Experiments 4 and 5), but did not occur when the target was presented alone (Experiment 4). The findings indicate that priming modifies an attentional mechanism and suggest that this effect accounts for search images.     

Flexible target representations underlie repetition priming in visual search

Repeating target features across consecutive visual search trials can influence response time. Three experiments are reported that show that this repetition priming is dependent on the exact stimulus configuration and feature type. In Experiment 1 the target was defined relative to distractors by its orientation and target colour was task irrelevant. The roles played by these features were reversed in Experiment 2. Repeating target colour across trials speeded responses in both Experiments 1 and 2. Repeating orientation significantly slowed responses in Experiment 1 but had no significant effect in Experiment 2. Existing feature-based and episodic memory-based explanations of priming cannot account for these data, but instead more flexible explanations are required. Experiment 3 showed that significant positive priming results from repeating distractor-defining orientation, indicating that priming of pop-out effects previously reported in the literature may result from combined target and distractor effects.     

The unification of mind: Integration of hemispheric semantic processing

Seventy-six participants performed a visual half-field lexical decision task at two different stimulus onset asynchronies (50 or 750 ms). Word targets were primed either by a highly associated word (e.g., CLEAN-DIRTY), a weakly associated word (e.g., CLEAN-TIDY), or an unrelated word (e.g., CLEAN-FAMILY) projected to either the same or opposite visual field (VF) as the target. In the short SOA, RVF-left hemisphere primes resulted in high associate priming regardless of target location (ipsilateral or contralateral to the prime) whereas LVF-right hemisphere primes produced both high and low associate priming across both target location conditions. In the long SOA condition, contralateral priming patterns converged, demonstrating only high associate priming in both VF locations. The results of this study demonstrate the critical role of interhemispheric transfer in semantic processing and indicate a need to elaborate current models of semantic processing.     

The boundary conditions of priming of visual search: From passive viewing through task-relevant working memory load

Priming of visual search has a dominating effect upon attentional shifts and is thought to play a decisive role in visual stability. Despite this importance, the nature of the memory underlying priming remains controversial. To understand more fully the necessary conditions for priming, we contrasted passive versus active viewing of visual search arrays. There was no priming from passive viewing of search arrays, while it was strong for active search of the same displays. Displays requiring no search resulted in no priming, again showing that search is needed for priming to occur. Finally, we introduced working memory load during visual search in an effort to disrupt priming. The memorized items had either the same colors as or different colors from the visual search items. Retaining items in working memory inhibited priming of the working memory task-relevant colors, while little interference was observed for unrelated colors. The picture that emerges of priming is that it requires active attentional processing of the search items in addition to the operation of visual working memory, where the task relevance of the working memory load plays a key role.     

The principle of good continuation in space and time can guide visual search in absence of priming or contextual cueing

Previous research has shown that repetition of the same target features and/or spatial location over time can improve detection, discrimination, and identification. It has also been shown that repetition of the same distractor features or spatial layout can similarly improve search performance. Thus, it appears that target and distractor features and positions are stored in memory and used to guide visual processes such as object recognition and search. Here we introduce a new paradigm for manipulating the sequential structure of target position across trials independently of target features, position priming, and contextual configuration. Results show that facilitation or inhibition in visual search occurs when the target appears at an implicitly expected or unexpected location, respectively, according to the Gestalt's principle of good continuation of the target's successive positions across trials. These findings provide evidence that the simple rule of good continuation can act as a spatiotemporal cue and guide where to attend while searching for the target.     

Exploring conflict- and target-related movement of visual attention

Intermixing trials of a visual search task with trials of a modified flanker task, the authors investigated whether the presentation of conflicting distractors at only one side (left or right) of a target stimulus triggers shifts of visual attention towards the contralateral side. Search time patterns provided evidence for lateral attention shifts only when participants performed the flanker task under an instruction assumed to widen the focus of attention, demonstrating that instruction-based control settings of an otherwise identical task can impact performance in an unrelated task. Contrasting conditions with response-related and response-unrelated distractors showed that shifting attention does not depend on response conflict and may be explained as stimulus-conflict-related withdrawal or target-related deployment of attention.     

The mechanism of priming: episodic retrieval or priming of pop-out?

Previous studies indicate that priming affects attentional processes, facilitating processes of target detection and selection on repetition trials. However, the results are so far compatible with two different attentional views that propose entirely different mechanisms to account for priming. The priming of pop-out hypothesis explains priming by feature weighting processes that lead to more frequent selections of nontarget items on switch trials. According to the episodic retrieval account, switch trials conversely lead to temporal delays in retrieving priority rules that specify the target. The results from two eye tracking experiments clearly favour the priming of pop-out hypothesis: Switching the target and nontarget features leads to more frequent selection of nontargets, without affecting the time-course of saccades to a great extent. The results from two more control experiments demonstrate that the same results can be obtained in a visual search task that allows only covert attention shifts. This indicates that eye movements can reliably indicate covert attention shifts in visual search.     

Top-down imagery overrides the influence of selection history effects

The present study investigated whether color imagery could override the representations of the prevalent selection history effect termed Priming of Pop-out (PoP), which is constituted by faster responding when the target color is repeated rather than switched across trials of color singleton search. Participants imagined a color in the interval between trials of a color singleton search task that could be the same as or different to the previous target color, and they were to rate the vividness of these representations following each imagery event. It was revealed that when highly vivid imagery was reported, the PoP effect was attenuated relative to less vivid forms of it (and absent in two out of three experiments), and that color imagery eliminated the build-up of priming following consecutive target color repeats. Overall, the present findings suggest the representations of the selection history system can be overridden by top-down imagery.     

Task-irrelevant direct gaze facilitates visual search for deviant facial expression

The detection of emotional expression is important particularly when the expression is directed towards the viewer. Therefore, we conjectured that the efficiency in visual search for deviant emotional expression is modulated by gaze direction, which is one of the primary clues for encoding the focus of social attention. To examine this hypothesis, two visual search tasks were conducted. In Emotional Face Search, the participants were required to detect an emotional expression amongst distractor faces with neutral expression; in Neutral Face Search they were required to detect a neutral target among emotional distractors. The results revealed that target detection was accelerated when the target face had direct gaze compared to averted gaze for fearful, angry, and neutral targets, but no effect of distractor gaze direction was observed. An additional experiment including multiple display sizes has shown a shallower search slope in search for a target face with direct gaze than that with averted gaze, indicating that the advantage of a target face with direct gaze is attributable to efficient orientation of attention towards target faces. These results indicate that direct gaze facilitates detection of target face in visual scenery even when gaze discrimination is not the primary task at hand.     

You detect while I search: examining visual search efficiency in a joint search task

Numerous factors impact attentional allocation, with behaviour being strongly influenced by the interaction between individual intent and our visual environment. Traditionally, visual search efficiency has been studied under solo search conditions. Here, we propose a novel joint search paradigm where one individual controls the visual input available to another individual via a gaze contingent window (e.g., Participant 1 controls the window with their eye movements and Participant 2 – in an adjoining room – sees only stimuli that Participant 1 is fixating and responds to the target accordingly). Pairs of participants completed three blocks of a detection task that required them to: (1) search and detect the target individually, (2) search the display while their partner performed the detection task, or (3) detect while their partner searched. Search was most accurate when the person detecting was doing so for the second time while the person controlling the visual input was doing so for the first time, even when compared to participants with advanced solo or joint task experience (Experiments 2 and 3). Through surrendering control of one’s search strategy, we posit that there is a benefit of a reduced working memory load for the detector resulting in more accurate search. This paradigm creates a counterintuitive speed/accuracy trade-off which combines the heightened ability that comes from task experience (discrimination task) with the slower performance times associated with a novel task (the initial search) to create a potentially more efficient method of visual search.     

Learning efficient visual search for stimuli containing diagnostic spatial configurations and color-shape conjunctions

Visual search is often slow and difficult for complex stimuli such as feature conjunctions. Search efficiency, however, can improve with training. Search for stimuli that can be identified by the spatial configuration of two elements (e.g., the relative position of two colored shapes) improves dramatically within a few hundred trials of practice. Several recent imaging studies have identified neural correlates of this learning, but it remains unclear what stimulus properties participants learn to use to search efficiently. Influential models, such as reverse hierarchy theory, propose two major possibilities: learning to use information contained in low-level image statistics (e.g., single features at particular retinotopic locations) or in high-level characteristics (e.g., feature conjunctions) of the task-relevant stimuli. In a series of experiments, we tested these two hypotheses, which make different predictions about the effect of various stimulus manipulations after training. We find relatively small effects of manipulating low-level properties of the stimuli (e.g., changing their retinotopic location) and some conjunctive properties (e.g., color-position), whereas the effects of manipulating other conjunctive properties (e.g., color-shape) are larger. Overall, the findings suggest conjunction learning involving such stimuli might be an emergent phenomenon that reflects multiple different learning processes, each of which capitalizes on different types of information contained in the stimuli. We also show that both targets and distractors are learned, and that reversing learned target and distractor identities impairs performance. This suggests that participants do not merely learn to discriminate target and distractor stimuli, they also learn stimulus identity mappings that contribute to performance improvements.     

Cross-trial priming of element positions in visual pop-out search is dependent on stimulus arrangement

Two experiments examined cross-trial positional priming (V. Maljkovic & K. Nakayama, 1994, 1996, 2000) in visual pop-out search. Experiment 1 used regularly arranged target and distractor displays, as in previous studies. Reaction times were expedited when the target appeared at a previous target location (facilitation relative to neutral baseline) and slowed when the target appeared at a previous distractor location (inhibition). In contrast to facilitation, inhibition emerged only after extended practice. Experiment 2 revealed reduced facilitatory and no inhibitory priming when the elements' spatial arrangement was made irregular, indicating that positional--in particular, inhibitory--priming critically depends on the configuration of the display elements across sequences of trials. These results are discussed with respect to the role of the context for cross-trial priming in visual pop-out search.     

Memory benefits during visual search depend on difficulty

In three experiments we explored whether memory for previous locations of search items influences search efficiency more as the difficulty of exhaustive search increases. Difficulty was manipulated by varying item eccentricity and item similarity (discriminability). Participants searched through items placed at three levels of eccentricity. The search displays were either identical on every trial (repeated condition) or the items were randomly reorganised from trial to trial (random condition), and search items were either relatively easy or difficult to discriminate from each other. Search was both faster and more efficient (i.e., search slopes were shallower) in the repeated condition than in the random condition. More importantly, this advantage for repeated displays was greater (1) for items that were more difficult to discriminate and (2) for eccentric targets when items were easily discriminable. Thus, increasing target eccentricity and reducing item discriminability both increase the influence of memory during search.