Salience-controlled visual search: Are the brightest and the least bright targets found by different processes?

It has been proposed that maximum and minimum targets (like the brightest or the least-bright item of a sample) are found by different search processes, the latter but not the former being sensitive to the withdrawal of focal attention. The present study could not establish such a systematic difference and suggests that variations in search performance are due to variations in target salience. Bright targets among dim distractors were generally more salient than dim targets among bright dis-tractors (Experiment 1). In both conditions, search performance was deteriorated when attention was withdrawn (Experiment 2). Performance became identical when maximum and minimum targets were matched in salience; this was also confirmed for dark items on bright background (Experiment 3). The data underline the importance of salience in visual search but do not support the assumption that maximum and minimum target searches are qualitatively different in their demands on focal attention. Salience and attention rather seem to resemble complementary functions in visual search; the less salient a target is the more attention is required to detect it.     

Salience drives overt selection of two equally relevant visual targets

In the present study, we investigated whether salience determines the sequence of selection when participants search for two equally relevant visual targets. To do this, attentional selection was tracked overtly as observers inspected two items of differing physical salience: one a highly salient color singleton, and the other a less salient shape singleton. Participants were instructed to make natural eye movements in order to determine whether two line segments contained within the two singletons were oriented in the same or in different directions. Because both singleton items were task-relevant, participants had no reason to inspect one item before the other. As expected, observers fixated both targets on the majority of trials. Critically, saccades to the color singleton preceded saccades to the less salient shape singleton on the majority of trials. This demonstrates that the order of attentional object selection is largely determined by stimulus salience when task relevance is equated.     

You’re looking for what? Comparing search for familiar,nameable objects to search for unfamiliar,novel objects

Searching for items in one’s environment often includes considerable reliance on semantic knowledge. The present study examines the importance of semantic information in visual and memory search, especially with respect to whether the items reside in long-term or working memory. In Experiment 1, participants engaged in hybrid visual memory search for items that were either highly familiar or novel. Importantly, the relatively large number of targets in this hybrid search task necessitated that targets be stored in some form of long-term memory. We found that search for familiar objects was more efficient than search for novel objects. In Experiment 2, we investigated search for familiar versus novel objects when the number of targets was low enough to be stored in working memory. We also manipulated how often participants in Experiment 2 were required to update their target (every trial vs. every block) in order to control for target templates that were stored in long-term memory as a result of repeated exposure over trials. We found no differences in search efficiency for familiar versus novel objects when templates were stored in working memory. Our results suggest that while semantic information may provide additional individuating features that are useful for object recognition in hybrid search, this information could be irrelevant or even distracting when searching for targets stored in working memory.     

The relationship between search efficiency and crowding

The role of target salience in crowding has remained controversial largely because salience usually escapes objective measurement. Here we address this problem using search efficiency as a measure of target salience. In separate experiments observers determined whether parafoveal arrays of vertical Gabor patterns contained targets having a unique colour, a unique direction of motion, and a unique temporal frequency. We analysed search efficiency in the conventional manner using reaction-time gradients (in seconds per item). We also considered accuracy gradients (in percent-correct per item). Crowding is typically quantified by comparing the acuity for a target within an array to the acuity for a target presented alone. We measured orientation acuity for determining whether a slightly tilted target was clockwise or anticlockwise of vertical. Targets with a unique colour or direction of motion were found to pop out, ie (with one exception) reaction-time and accuracy gradients were insignificantly different from zero. Acuity for these targets was significantly greater than acuity for targets whose neighbours had the same colour and direction of motion. Manipulation of temporal frequency produced a wide range of search efficiencies. For three of four observers we found a linear relationship between acuity and the accuracy gradient, shallow gradients being associated with high acuity. In general, we find that crowding is weakened when observers can find a parafoveally presented target quickly and accurately.     

Distractor familiarity leads to more efficient visual search for complex stimuli

Previous reports suggest that distractor familiarity plays an important role in determining visual search efficiency. However, the specific tasks used in those studies limit the extension of their findings to real-world situations and everyday images. In the present study, subjects engaged in a prolonged period of search experience as a control of their level of familiarity with a large set of target and distractor images. Reaction times and search slopes decreased dramatically over this period, especially for trials with a large target eccentricity and many distractors. Following extended practice, search among familiar distractors was more efficient than search among unfamiliar distractors. Furthermore, we found that familiar targets were located more efficiently than unfamiliar targets and that subjects were faster at locating targets that they had experienced in the majority of the search trials. These results show that prolonged visual experience facilitates processing of both target and distractor items during search.     

Comparing target detection errors in visual search and manually-assisted search

Subjects searched for low- or high-prevalence targets among static nonoverlapping items or items piled in heaps that could be moved using a computer mouse. We replicated the classical prevalence effect both in visual search and when unpacking items from heaps, with more target misses under low prevalence. Moreover, we replicated our previous finding that while unpacking, people often move the target item without noticing (the unpacking error) and determined that these errors also increase under low prevalence. On the basis of a comparison of item movements during the manually-assisted search and eye movements during static visual search, we suggest that low prevalence leads to broadly reduced diligence during search but that the locus of this reduced diligence depends on the nature of the task. In particular, while misses during visual search often arise from a failure to inspect all of the items, misses during manually-assisted search more often result from a failure to adequately inspect individual items. Indeed, during manually-assisted search, over 90 % of target misses occurred despite subjects having moved the target item during search.     

Spontaneous visual search during the first two years: Improvement with age but no evidence of efficient search

Efficient visual search, wherein reaction times to acquire targets are largely independent of array size, is commonly observed in adults. Evidence for efficient search in infants may imply that selective attention to visual features is similar across development. In the current cross-sectional eye-tracking study, we examined spontaneous visual search at 6, 12, 18, and 24 months. Infants were presented with Random arrays (one target among 7, 13, or 26 pseudorandomly distributed elements) and Circle arrays (one target among 4, 7, or 13 elements arranged in a circle). Contrary to predictions, we did not find evidence of efficient search among infants. With increasing array size, time-to-target increased, the proportion of targets fixated (analogous to accuracy) decreased, and the proportion of first looks to the target decreased for both types of array (ps < .001). For Random arrays, the proportion of first looks to the target was similar to chance for all ages and array sizes; for Circle arrays, it exceeded chance for some ages and array sizes. The proportion of targets fixated and first looks to target increased with age across display types (ps < .05). We also tested adults with the same stimuli under similar conditions; the adults showed evidence of efficient visual search. Possible explanations and implications are discussed.     

Salience-based progression of visual attention: Time course

In a visual search task, visual attention progresses from the most to the least salient item until a target is found. The time course of such salience-based progression remains unclear although the temporal deployment seems as crucial as the spatial deployment. The present study investigated how the general hierarchical pattern takes place. Healthy volunteers were presented with a primed visual search paradigm in which the primes consisted of three items differing in salience for either 33, 50 or 100 ms. Subsequently one of them became the target and participants had to make a judgment about it. In order to progress on the basis of salience, attention seemed first to be captured by the most salient item. Secondly, the first two most salient items seemed both encompassed within the attentional spotlight. At that time, the least salient item seemed inhibited, completing the stepwise progression of attention. In summary, our data suggest that the hierarchical pattern is built up over time through orienting and expansion of the spotlight, and inhibition of the least salient signals. Furthermore, complementary analyses revealed a proximity effect influencing the selection of the next location to visit. As predicted by different computational models, items located closely to the first visited location were more prone to attract attention than distant items but interestingly enough, this effect only applied to the least salient item suggesting a balance between salience and proximity criteria. Critically also, salience and proximity effects seem to have different time courses. Results are only partially explained by current visual attention models and require further investigation.     

Odd-item search by pigeons: Method,instrumentation, and uses

In odd-item visual search, subjects confront a display on which a number of stimulus items appear. All but one of these items are identical; the subject must respond to the one item (the target) that in some way differs from all the others (the distractors). The time required to find the target reflects the similarity between the target form and the distractor form. A matrix of search times for all possible pairs of a set of 20 or more items can be obtained in a single session. Such similarity matrices may reflect stimulus features, dimensions, and categories, among other things. A method is described through which pigeons learn odd-item search rapidly and perform with high accuracy despite the appearance of each form as a target on some trials and as a distractor on others. The paper also describes the essential apparatus and exemplifies displays and data.     

Hybrid search in the temporal domain: Evidence for rapid,serial logarithmic search through memory

In hybrid search, observers memorize a number of possible targets and then search for any of these in visual arrays of items. Wolfe (2012) has previously shown that the response times in hybrid search increase with the log of the memory set size. What enables this logarithmic search of memory? One possibility is a series of steps in which subsets of the memory set are compared to all items in the visual set simultaneously. In the present experiments, we presented single visual items sequentially in a rapid serial visual presentation (RSVP) display, eliminating the possibility of simultaneous testing of all items. We used a staircasing procedure to estimate the time necessary to effectively detect the target in the RSVP stream. Processing time increased in a log–linear fashion with the number of potential targets. This finding eliminates the class of models that require simultaneous comparison of some memory items to all (or many) items in the visual display. Experiment 3 showed that, similar to visual search, memory search efficiency in this paradigm is influenced by the similarity between the target set and the distractors. These results indicate that observers perform separate memory searches on each eligible item in the visual display. Moreover, it appears that memory search for one item can proceed while other items are being categorized as “eligible” or “not eligible.”     

Visual search in divided areas: Dividers initially interfere with and later facilitate visual search

A common search paradigm requires observers to search for a target among undivided spatial arrays of many items. Yet our visual environment is populated with items that are typically arranged within smaller (subdivided) spatial areas outlined by dividers (e.g., frames). It remains unclear how dividers impact visual search performance. In this study, we manipulated the presence and absence of frames and the number of frames subdividing search displays. Observers searched for a target O among Cs, a typically inefficient search task, and for a target C among Os, a typically efficient search. The results indicated that the presence of divider frames in a search display initially interferes with visual search tasks when targets are quickly detected (i.e., efficient search), leading to early interference; conversely, frames later facilitate visual search in tasks in which targets take longer to detect (i.e., inefficient search), leading to late facilitation. Such interference and facilitation appear only for conditions with a specific number of frames. Relative to previous studies of grouping (due to item proximity or similarity), these findings suggest that frame enclosures of multiple items may induce a grouping effect that influences search performance.     

Effects of stimulus frequency and reinforcement variables on reaction time

Pigeons pecked at one of two black forms, “+” or “O,” either of which could appear alone on a white computer monitor screen. In baseline series of sessions, each form appeared equally often, and two pecks at it produced food reinforcement on 10% of trials. Test series varied the relative probability or duration of reinforcement or frequency of appearance of the targets. Peck reaction times, measured from target onset to the first peck, were found to vary as a function of reinforcement probability but not as a function of relative target frequency or of reinforcement duration. Reaction times to the two targets remained approximately equal as long as the probability of reinforcement, per trial, was equal for the targets, even if the relative frequency of the targets differed by as much as 19 to 1. The results address issues raised in visual search experiments and indicate that attentional priming is unimportant when targets are easy to detect. The results also suggest that equalizing reinforcement probability per trial for all targets removes differential reinforcement as an important variable. That reaction time was sensitive to the probability but not the duration of reinforcement raises interesting questions about the processes reflected in reaction time compared with rate as a response measure.     

Target self-relevance speeds visual search responses but does not improve search efficiency

ABSTRACT

In a variety of contexts, arbitrarily associating one’s self with a stimulus improves performance relative to stimuli that are not self-associated, implying enhanced processing of self-associated stimuli (“self-relevance” effects). Self-relevance has been proposed to influence diverse aspects of cognition, including the perceptual prioritization of self-relevant stimuli (“self-prioritization” effects). We sought to elucidate the mechanisms of self-prioritization by using a visual search paradigm. In three experiments, subjects learned two stimulus-label combinations (SELF and OTHER), and then searched for one of those stimuli (cued by the label) on each trial, with a variable number of distractors present on each trial. We hypothesized that, if self-relevance enhances the perceptual salience of the stimuli pre-attentively, then the self-relevance of a target should result in improved search efficiency. In three experiments using conjunction-defined (Experiments 1–2) and feature-defined (Experiment 3) targets, we found that self-relevant targets were associated with overall faster responses than non-self-relevant targets (an intercept effect). However, the slopes of the search size by reaction time (RT) function were never significantly different between the self-relevant and non-self-relevant conditions, counter to the hypothesis that self-prioritization is pre-attentive. These results constitute novel evidence that self-relevance affects visual search performance, but they also cast doubt on the possibility that self-relevance enhances the perceptual salience of a target in a manner similar to physical manipulations. We propose that the self-relevance of a stimulus alters processing only after the self-relevant item has been attended.     

Effect of target separation on selective attention.

A visual search task, in which subjects searched circular stimulus displays for two instances of a prespecified target, was used to investigate the effects of target-separation on accuracy. When a comparison was made of the total number of targets correctly located at each separation, no significant differences were found, and this suggested that, within the range of separations (maximum of 2.53 degrees) examined, the selective processing of the relevant items was not influenced by the distance separating them. Also, assuming that the redundancy of target items increased the probability of a target being perceived, the differences between the number of first targets correctly located and the number correct in a single target condition, were in the predicted direction. However, they failed to reach significance.     

The spatial signal for saccadic eye movements emphasizes visual boundaries

By investigating the visual processing involved when saccades are made to newly appearing targets, we show that this processing is significantly nonlinear and that texture boundary information predominates. We used the global, or center-of-gravity, effect whereby a saccadic eye movement directed to a target consisting of a pair of elements has an amplitude intermediate between that of saccades directed to the individual elements. We measured the effect using target elements with different visual characteristics, including phase-reversal checkerboard targets that had the same space-average luminance as the background. The contribution to the center-of-gravity calculation was used to measure relative salience. We found that positive and negative contrast elements contribute almost equal weightings. Thus, salience, assessed in this way, is a highly nonlinear function of luminance. The salience of checkerboard targets was found to decrease as check size was decreased and increase as the overall size of the target was increased. Checkerboards with an empty center were as effective as were full checkerboards, showing the importance of boundaries in the salience signal.     

Visual search with meaningful and non-meaningful material

Two experiments are reported which attempt to assess the effects of variations in target word, context items and instructions on performance in a visual search task. In Experiment 1, subjects were required to search through context lists of three-letter nonsense syllables (of either high or low association value) for three-letter meaningful target words (of either high or low frequency). They were given either “positive” or “negative” instructions, i.e. were told either to pick out the meaningful word or to pick out the word which was not a nonsense syllable. The results showed that visual search times were significantly influenced by both frequency of target word and association value of context items. A significant interaction was observed between type of instructions and target word frequency. The design of Experiment 2 followed that of Experiment 1, with the exceptions that nonsense syllables now became target items, and meaningful words formed the contexts. Again, nonsense syllable association value and word frequency were found to be critical in determining visual search times.     

比较视觉搜索任务的认知老化研究

本研究通过增加组成图片的图形数量和改变目标图形的特征,考察老年人和大学生在比较视觉搜索任务中的眼动特点和图形特征加工的优先性。结果表明与青年人相比,老年人在比较视觉搜索过程中总注视时间和注视次数增加,平均眼跳幅度减少,对目标刺激的注视时间增加。老年人和青年人在目标刺激的注视过程中都表现出图形维度数量的优先性,即表现为颜色和形状都不同的目标刺激注视时间最短,颜色不同和形状不同的目标刺激在注视时间上差异不显著。说明老年人在比较视觉搜索过程中存在认知老化,在图形特征加工的优先性上结果和青年人一致,以上结果与年龄和研究任务有关。     

Understanding the contribution of target repetition and target expectation to the emergence of the prevalence effect in visual search

Behavior in visual search tasks is influenced by the proportion of trials on which a target is presented (the target prevalence). Previous research has shown that when target prevalence is low (2 % prevalence), participants tend to miss targets, as compared with higher prevalence levels (e.g., 50 % prevalence). There is an ongoing debate regarding the relative contributions of target repetition and the expectation that a target will occur in the emergence of prevalence effects. In order to disentangle these two factors, we went beyond previous studies by directly manipulating participants’ expectations regarding how likely a target was to appear on a given trial. This we achieved without using cues or feedback. Our results indicated that both target repetition and target expectation contribute to the emergence of the prevalence effect.     

Selection history-driven signal suppression

ABSTRACT

The control of attention is influenced by current goals, physical salience, and selection history. Under certain conditions, physically salient stimuli can be strategically suppressed below baseline levels, facilitating visual search for a target. It is unclear whether such signal suppression is a broad mechanism of selective information processing that extends to other sources of attentional priority evoked by task-irrelevant stimuli, or whether it is particular to physically salient perceptual signals. Using eye movements, in the present study we highlight a case where a former-target-colour distractor facilitates search for a target on a large percentage of trials. Our findings provide evidence that the principle of signal suppression extends to other sources of attentional priority beyond physical salience, and that selection history can be leveraged to strategically guide attention away from a stimulus.     

Adaptive but non-optimal visual search behavior with highlighted displays

Previous research [Fisher, D. L., & Tan, K. C. (1989). Visual displays: The highlighting paradox. Human Factors, 31(1), 17–30] suggested that making certain items visually salient, or highlighting, can speed performance in visual search tasks. But interface designers cannot always anticipate users’ intended targets, and highlighting non-target items can lead to performance decrements. An experiment presented suggests that people attend to highlighting less than what an algebraic visual search model of highlighted displays [Fisher, D. L., Coury, B. G., Tengs, T. O., & Duffy, S. A. (1989). Minimizing the time to search visual displays: The role of highlighting. Human Factors, 31(2), 167–182] predicts. Users adjust their visual search strategies by probability-matching to their visual environment. An ACT-R [Anderson, J. R., Bothell, D., Byrne, M. D., Douglass, S., Lebiere, C., & Quin, Y. (2004). An integrated theory of the mind. Psychological Review, 111, 1036–1060] model reproduced the major effects of the experiment and suggests that learning in this task occurs at very small cognitive and time scales.