The effects of salience on saccadic target selection

Two experiments were conducted to investigate the effects of saliency on saccadic target selection as a function of time. Participants were required to make a speeded saccade towards a target defined by a unique orientation presented concurrently with multiple nontargets and one distractor. Target and distractor were equally salient within the orientation dimension but varied in saliency in the colour dimension. Within the colour dimension, the target presented could be more, equally, or less salient than the distractor. The results showed that saliency played a large role early during processing while no effects of saliency were found in later processing. Results are discussed in terms of models on visual selection.     

Guidance of eye movements during conjunctive visual search: the distractor-ratio effect.

The distractor-ratio effect refers to the finding that search performance in a conjunctive visual search task depends on the relative frequency of two types or subsets of distractors when the total number of items in a display is fixed. Previously, Shen, Reingold, and Pomplun (2000) examined participants' patterns of eye movements in a distractor-ratio paradigm and demonstrated that on any given trial saccadic endpoints were biased towards the smaller subset of distractors and participants flexibly switched between different subsets across trials. The current study explored the boundary conditions of this tendency to flexibly search through a smaller subset of distractors by examining the influence of several manipulations known to modulate search efficiency, including stimulus discriminability (Experiment 1), within-dimension versus cross-dimension conjunction search and distractor heterogeneity (Experiment 2). The results indicated that the flexibility of visual guidance and saccadic bias exemplified by the distractor-ratio effect is a robust phenomenon that mediates search efficiency by adapting to changes in the relative informativeness of stimulus dimensions and features.     

Preattentive guidance of eye movements during triple conjunction search tasks: The effects of feature discriminability and saccadic amplitude

Eye movements were monitored during the performance of triple conjunction search tasks. Stimuli varied in color, shape, and orientation. Across trials, the target was either present or absent, and displays consisted of 6, 12, or 24 stimuli. Stimulus discriminability was manipulated for the shape dimension, with half of the participants seeing displays of Es and Fs (low-discriminability [LD] condition) and half seeing displays of Cs and Ts (high-discriminability [HD] condition). Participants in both conditions performed two search tasks. In the single-feature (SF) task, the target stimulus shared one feature with each of the distractors, whereas in the two-feature (TF) task, it shared two features with each distractor. An examination of saccadic endpoints revealed that participants were more likely to fixate on distractor stimuli sharing color (SF task) or color and shape (TF task) with the target. This was a robust finding, being observed across participants, saccades of different amplitudes and sequential position, and following short and long latencies to move. The extent to which participants made use of shape information increased with discriminability.     

Goal-driven modulation as a function of time in saccadic target selection

Four experiments were performed to investigate the contribution of goal-driven modulation in saccadic target selection as a function of time. Observers were required to make an eye movement to a prespecified target that was concurrently presented with multiple nontargets and possibly one distractor. Target and distractor were defined in different dimensions (orientation dimension and colour dimension in Experiment 1), or were both defined in the same dimension (i.e., both defined in the orientation dimension in Experiment 2, or both defined in the colour dimension in Experiments 3 and 4). The identities of target and distractor were switched over conditions. Speed–accuracy functions were computed to examine the full time course of selection in each condition. There were three major results. First, the ability to exert goal-driven control increased as a function of response latency. Second, this ability depended on the specific target–distractor combination, yet was not a function of whether target and distractor were defined within or across dimensions. Third, goal-driven control was available earlier when target and distractor were dissimilar than when they were similar. It was concluded that the influence of goal-driven control in visual selection is not all or none, but is of a continuous nature.     

Age-related increase in top-down activation of visual features

Previous research suggests that, during visual search and discrimination tasks, older adults place greater emphasis than younger adults on top-down attention. This experiment investigated the relative contribution of target activation and distractor inhibition to this age difference. Younger and older adults performed a singleton discrimination task in which either an E or an R target (colour singleton) was present among distractor letters. Relative to a baseline condition in which the colours of the targets and distractors remained constant, an age-related slowing of performance was evident when either the colour of the target or that of the distractors varied across trials. The age-related slowing was more pronounced in response to target colour variation, suggesting that older adults place relatively greater emphasis on the top-down activation of target features.     

Age-related increase in top-down activation of visual features

Previous research suggests that, during visual search and discrimination tasks, older adults place greater emphasis than younger adults on top-down attention. This experiment investigated the relative contribution of target activation and distractor inhibition to this age difference. Younger and older adults performed a singleton discrimination task in which either an E or an R target (colour singleton) was present among distractor letters. Relative to a baseline condition in which the colours of the targets and distractors remained constant, an age-related slowing of performance was evident when either the colour of the target or that of the distractors varied across trials. The age-related slowing was more pronounced in response to target colour variation, suggesting that older adults place relatively greater emphasis on the top-down activation of target features.     

Goal-driven modulation as a function of time in saccadic target selection

Four experiments were performed to investigate the contribution of goal-driven modulation in saccadic target selection as a function of time. Observers were required to make an eye movement to a prespecified target that was concurrently presented with multiple nontargets and possibly one distractor. Target and distractor were defined in different dimensions (orientation dimension and colour dimension in Experiment 1), or were both defined in the same dimension (i.e., both defined in the orientation dimension in Experiment 2, or both defined in the colour dimension in Experiments 3 and 4). The identities of target and distractor were switched over conditions. Speed-accuracy functions were computed to examine the full time course of selection in each condition. There were three major results. First, the ability to exert goal-driven control increased as a function of response latency. Second, this ability depended on the specific target-distractor combination, yet was not a function of whether target and distractor were defined within or across dimensions. Third, goal-driven control was available earlier when target and distractor were dissimilar than when they were similar. It was concluded that the influence of goal-driven control in visual selection is not all or none, but is of a continuous nature.     

数量化维度特征搜索的影响因素探索

有关数量化维度特征搜索的研究表明 ,目标搜索斜率 (反应时间对呈现项目数量函数的斜率 )是由目标与干扰子之间的差异决定的 ,然而以往研究中其差异的值取决于具体的实验条件。根据韦伯定律 ,笔者推测 ,目标搜索斜率应由目标与干扰子之间的相对差异 (C)决定 ,C为目标值 (T)与干扰子值 (D)的差值与该干扰子值的比值。当C值对应相等时 ,不同几何形状刺激的目标搜索斜率应无差异。为此笔者分别采用圆 (实验一 )和三角形 (实验二 )进行实验研究。实验结果支持了上述假设 ,并发现目标大于干扰子时的目标搜索斜率对C值的函数是单调递减函数     

Developmental Differences in the Influence of Distractors on Maintenance in Spatial Working Memory

This study examined whether attention to a location plays a role in the maintenance of locations in spatial working memory in young children as it does in adults. This study was the first to investigate whether distractors presented during the delay of a spatial working-memory task influenced young children’s memory responses. Across 2 experiments, 3- and 6-year-olds completed a spatial working-memory task featuring a static target location and distractor location. Results indicated a change from 3 years to 6 years of age in how distractors influenced memory. Six-year-olds’ memory responses were biased away from a distractor that was close to the target location and on the outside of the target location relative to the center of the monitor. Distractors that were far from the target or that were toward the center of the monitor relative to the target location had no effect. Three-year-olds’ responses were biased toward a distractor when the distractor was on the outside of the target location and farther from the target. Distractors that were near the target location or toward the center of the monitor had no effect. These biases provide evidence that young children’s maintenance of a location in memory is influenced by attention.     

Target objects defined by a conjunction of colour and shape can be selected independently and in parallel

It is generally assumed that during search for targets defined by a feature conjunction, attention is allocated sequentially to individual objects. We tested this hypothesis by tracking the time course of attentional processing biases with the N2pc component in tasks where observers searched for two targets defined by a colour/shape conjunction. In Experiment 1, two displays presented in rapid succession (100 ms or 10 ms SOA) each contained a target and a colour-matching or shape-matching distractor on opposite sides. Target objects in both displays elicited N2pc components of similar size that overlapped in time when the SOA was 10 ms, suggesting that attention was allocated in parallel to both targets. Analogous results were found in Experiment 2, where targets and partially matching distractors were both accompanied by an object without target-matching features. Colour-matching and shape-matching distractors also elicited N2pc components, and the target N2pc was initially identical to the sum of the two distractor N2pcs, suggesting that the initial phase of attentional object selection was guided independently by feature templates for target colour and shape. Beyond 230 ms after display onset, the target N2pc became superadditive, indicating that attentional selection processes now started to be sensitive to the presence of feature conjunctions. Results show that independent attentional selection processes can be activated in parallel by two target objects in situations where these objects are defined by a feature conjunction.     

A conjunctive feature similarity effect for visual search

Traditional models of visual search assume interitem similarity effects arise from within each feature dimension independently of other dimensions. In the present study, we examine whether distractor–distractor effects also depend on feature conjunctions (i.e., whether feature conjunctions form a separate “feature” dimension that influences interitem similarity). Spatial frequency and orientation feature dimensions were used to generate distractors. In the bound condition, the number of distractors sharing the same conjunction of features was higher than that in the unbound condition, but the sharing of features within frequency and orientation dimensions was the same across conditions. The results showed that the target was found more efficiently in the bound than in the unbound condition, indicating that distractor–distractor similarity is also influenced by conjunctive representations.     

The depth of distractor processing in search with clutter

Some search tasks involve looking for a category target in clutter. This is the task faced, for example, by a baggage screener looking for weapons in a suitcase. Such tasks presumably involve the segmentation and recognition of the target object, but it is unknown whether they also involve the segmentation and recognition of the distractor objects. To examine the depth of distractor processing in this task, we had observers search through cluttered displays composed of normal and chimerical distractors. The normal distractors were photographs of recognizable objects, while the chimerical distractors were created by interchanging parts between the normal objects. The obsever's task was to identify the display quadrant that contained an animal or a vehicle target. We varied the difficulty of the search task by varying target and distractor discriminability, target uncertainty, and target occlusion. Only when the target was partially occluded did we find an effect of distractor type. In this case, observers may have found the target through a process of mentally eliminating whole distractor objects. When the target was unoccluded, we found no evidence that observers selected and rejected whole distractors during search. This second result supports our previous claim that often the items for search in clutter are not whole objects.     

Not all visual memories are created equal

Two experiments examined how well the long-term visual memories of objects that are encountered multiple times during visual search are updated. Participants searched for a target two or four times (e.g., white cat) among distractors that shared the target's colour, category, or were unrelated while their eye movements were recorded. Following the search, a surprise visual memory test was given. With additional object presentations, only target memory reliably improved; distractor memory was unaffected by the number of object presentations. Regression analyses using the eye movement variables as predictors indicated that number of object presentations predicted target memory with no additional impact of other viewing measures. In contrast, distractor memory was best predicted by the viewing pattern on the distractor objects. Finally, Experiment 2 showed that target memory was influenced by number of target object presentations, not number of searches for the target. Each of these experiments demonstrates visual memory differences between target and distractor objects and may provide insight into representational differences in visual memory.     

Trial-by-trial adjustments of top-down set modulate oculomotor capture

The role of top-down control in visual search has been a subject of much debate. Recent research has focused on whether attentional and oculomotor capture by irrelevant salient distractors can be modulated through top-down control, and if so, whether top-down control can be rapidly initiated based on current task goals. In the present study, participants searched for a unique shape in an array containing otherwise homogeneous shapes. A cue prior to each trial indicated the probability that an irrelevant color singleton distractor would appear on that trial. Initial saccades were less likely to land on the target and participants took longer to initiate a saccade to the target when a color distractor was present than when it was absent; this cost was greatly reduced on trials in which the probability that a distractor would appear was high, as compared to when the probability was low. These results suggest that top-down control can modulate oculomotor capture in visual search, even in a singleton search task in which distractors are known to readily capture both attention and the eyes. Furthermore, the results show that top-down distractor suppression mechanisms can be initiated quickly in anticipation of irrelevant salient distractors and can be adjusted on a trial-by-trial basis.     

Action matters: The role of action plans and object affordances in selection for action

In a series of three experiments requiring selection of real objects for action, we investigated whether characteristics of the planned action and/or the “affordances” of target and distractor objects affected interference caused by distractors. In all ofthe experiments, the target object was selectedon the basis of colour and was presented alone or with a distractor object. We examined the effect of type of response (button press, grasping, or pointing), object affordances (compatibility with the acting hand, affordances for grasping or pointing), and target/distractor positions (left or right) on distractor interference (reaction time differences between trials with and without distractors). Different patterns of distractor interference were associated with different motor responses. In the button-press conditions of each experiment, distractor interference was largely determined by perceptual salience (e.g., proximity to initial visual fixation). In contrast, in tasks requiring action upon the objects in the array, distractors with handles caused greater interference than those without handles, irrespective of whether the intended action was pointing or grasping. Additionally, handled distractors were relatively more salient when their affordances for grasping were strong (handle direction compatible with the acting hand) than when affordances were weak. These data suggest that attentional highlighting of specific target and distractor features is a function of intended actions.     

Effects of unilateral distractors: A comparison of eye movement and key press responses

We compared the effects of a unilateral distractor on the latencies of voluntary eye movements and key press responses in order to elucidate some of the differences between these two types of response with respect to processing distractors. On each trial, participants indicated the colour of a central patch by making a left or a right response. The colour of the distractor either matched the target colour or matched the colour associated with the incorrect response. Similarly, the side of the distractor corresponded to the side of either the correct or the incorrect response. The results showed that side congruency exerted a main effect only on eye movement responses, but colour congruency affected eye movement and key press responses similarly. Moreover, when we made the target more distinct from the distractor, only distractors that appeared on the same side as the response elicited a colour congruency effect, for both response modalities. Wepropose that (1) the appearance of the distractor activates oculomotor cells, which facilitate a subsequent eye movement with the same direction vector, (2) the distinctiveness of the target determines whether colour and side congruency interact, and (3) spatial codes mediate the interaction between colour and side congruency that occurs when the target is sufficiently distinct from the distractor.     

Search for multiple targets of different colours: Misguided eye movements reveal a reduction of colour selectivity

Searching for two targets simultaneously is often less efficient than conducting two separate searches. Eye movements were tracked to understand this dual‐target cost. Findings are discussed in the context of security screening. In both single‐target and dual‐target search, displays contained one target at most. Stimuli were abstract shapes modelled after guns and other threat items. With these targets and distractors, colour information helped more in guiding search than shape information. When the two targets had different colours, distractors with colours different from either target were fixated more often in dual‐target search than in single‐target searches. Thus a dual‐target cost arose from a reduction in colour selectivity, reflecting limitations in the ability to represent two target features simultaneously and use them to guide search. Because of these limitations, performance in security searches may improve if each image is searched by two screeners, each specializing in a different category of threat item. Copyright © 2011 John Wiley & Sons, Ltd.     

Early top-down control over saccadic target selection: Evidence from a systematic salience difference manipulation

Previous research on the contribution of top-down control to saccadic target selection has suggested that eye movements, especially short-latency saccades, are primarily salience driven. The present study was designed to systematically examine top-down influences as a function of time and relative salience difference between target and distractor. Observers performed a saccadic selection task, requiring them to make an eye movement to an orientation-defined target, while ignoring a color-defined distractor. The salience of the distractor was varied (five levels), permitting the percentage of target and distractor fixations to be analyzed as a function of the salience difference between the target and distractor. This analysis revealed the same pattern of results for both the overall and the short-latency saccades: When the target and distractor were of comparable salience, the vast majority of saccades went directly to the target; even distractors somewhat more salient than the target led to significantly fewer distractor, as compared with target, fixations. To quantify the amount of top-down control applied, we estimated the point of equal selection probability for the target and distractor. Analyses of these estimates revealed that, to be selected with equal probability to the target, a distractor had to have a considerably greater bottom-up salience, as compared with the target. This difference suggests a strong contribution of top-down control to saccadic target selection—even for the earliest saccades.     

Top-down control of attention: it's gradual, practice-dependent, and hierarchically organized

When searching for a "pop-out" target, interference from a salient but irrelevant distractor can be reduced or even prevented under certain circumstances. Here, five experiments were conducted to further our understanding of three different aspects of top-down interference reduction: first, whether or not qualitatively different search modes can account for different reduction patterns; second, whether distractor practice plays a causal role in reduction; and third, how specific reduction is, that is, whether interference by intradimensional distractors can be reduced as effectively as interference by cross-dimensional distractors. The results provide evidence that interference reduction does not critically depend on the implementation of a feature search mode, but rather on practice with the distractor, that is, the acquisition of an effective suppression strategy. In addition, they suggest that interference reduction is based on hierarchically organized feature weighting ("dimension weighting"), rather than on completely independent feature weighting.     

Categorical effects in children's colour search: A cross‐linguistic comparison

In adults, visual search for a colour target is facilitated if the target and distractors fall in different colour categories (e.g. Daoutis, Pilling, & Davies, in press ). The present study explored category effects in children's colour search. The relationship between linguistic colour categories and perceptual categories was addressed by comparing native speakers of languages differing in the number of colour terms. Experiment 1 compared English and Kwanyama (Namibian) children aged 4 to 7 years on a visual search task, using target‐distractor pairs (blue‐green, blue‐purple, red‐pink) for which the Kwanyama did not have distinct names. The presence of a category advantage in the English, but not in the Kwanyama, suggested that linguistic boundaries may affect search performance. Experiment 2 examined visual search performance in the green‐yellow and the blue‐green region, in English and Himba (Namibian) 6‐year‐olds. The number of distractors was varied to assess search efficiency. Cross‐category search was more efficient than within‐category search in the English group, but this advantage was absent in the Himba. Increasing the number of distractors affected search speed in the English group, but not in the Himba. Overall, these findings suggest cross‐language differences in categorical effects on colour search, but also in the way the children performed the search. The nature of the category effect in search is discussed with respect to these findings.