Search

In visual search tasks, targets are difficult to find when they are similar to the surrounding nontargets. In this scenario, it is optimal to tune attention to target features that maximize the difference between target and nontargets. We investigated whether the optimal tuning of attention is driven by biases arising from previously attended stimuli (i.e., trial history). Consistent with the effects of trial history, we found that optimal tuning was stronger when a single target–nontarget relation was repeated than when two target–nontarget relations alternated randomly. Detailed analysis of blocks with random alternation showed that optimal tuning was stronger when the target–nontarget relation probed in the current trial matched the relation in the previous trial. We evaluated several mechanisms that may underlie the effects of trial history, such as priming of attentional set, switch costs, and sensory adaptation. However, none of the accounts was able to fully account for the pattern of results. Public Significance Statement: Visual search is a common everyday activity. For instance, we often look for objects in a particular color. This task is difficult when the color of the target is similar to the surrounding colors. Observers perform surprisingly well in this situation, suggesting that they adjust attention optimally to maximize the difference between the color of the target and the surrounding colors. However, we show that this adjustment breaks down when observers attempt to find more than a single target. Therefore, to optimize performance, it may be best to search for only one object at a time. [ABSTRACT FROM AUTHOR]

Increasing the density or uniformity of nontarget stimuli appears to increase the saliency of singleton stimuli. Consequently, search times should be shorter. Surprisingly, however, effects of density or uniformity on search times were not always observed in detection tasks. We re-examined this finding with stimuli having two features, color and shape. Half of the participants indicated the presence or absence of a color singleton, and the other half indicated the presence or absence of a shape singleton. Density was changed by increasing the number of stimuli from 4 to 10. We found that the effects of density were either limited to target-absent trials or to target-present trials, which may explain previous failures to observe these effects. When color was the target feature, we found shorter RTs to dense than sparse displays on target-absent trials, but no difference on target-present trials. When shape was the target feature, it was the opposite. Concerning the uniformity of the nontargets, we found shorter RTs with uniform than mixed displays and this difference was larger on target-absent than target-present trials. These results are mostly consistent with the Guided Search Model. [ABSTRACT FROM AUTHOR]

In search arrays where the target is presented with similar nontarget stimuli, it is advantageous to shift the internal representation of the target features away from the nontarget features. According to optimal tuning theory (Navalpakkam & Itti, 2007), the shift of the attentional template increases the signal-to-noise ratio because the overlap of neural populations representing the target and nontarget features is reduced. While previous research has shown that the internal representation of the target is indeed shifted, there is little evidence in favor of a shift in attentional selectivity. To fill this gap, we used a cue–target paradigm where shorter reaction times (RTs) at cued than at uncued locations indicate attentional capture by the cue. Consistent with previous research, we found that attentional capture decreased with decreasing similarity between cue and target color. Importantly, target-similar cue colors closer to the nontarget colors captured attention less than target-similar cue colors further away from the nontarget colors, suggesting that attentional selectivity was biased away from the nontarget colors. The shift of attentional selectivity matched the shift of the memory representation of the target. Further, the bias in attentional capture was reduced when the nontarget colors were more distinct from the target. We discuss alternative accounts of the data, such as saliency-driven capture and the relational account of attentional capture (Becker, 2010), but conclude that optimal tuning theory provides the best explanation. Public Significance Statement: We often look for objects that are similar to the surrounding nontarget objects. For instance, the target may have a color that is similar to the other colors present in the display. The visual system operates efficiently in this situation, but the underlying mechanism is poorly understood. Previous research has suggested that observers may slightly modify the search goal in order to optimize performance. Instead of looking for the target, they look for a target that exaggerates the difference between target and nontargets. Understanding how human observers solve the task may be relevant for applied sciences, such as computer vision. [ABSTRACT FROM AUTHOR]

In a visual search task, sensory input is matched to a representation of the search target in visual working memory (VWM). This representation is referred to as attentional template. We investigated the conditions that allow for more than a single attentional template. The attentional template of color targets was measured by means of the contingent attentional capture paradigm. We found that attentional templates did not differ between search with 1 and 2 memorized target colors, suggesting that dual target search allowed for multiple attentional templates. In the same paradigm, we asked participants to memorize target and distractor color with equal precision. Both were presented before the search task. An attentional template was set up for the target, but not for the distractor color, suggesting that keeping a color in VWM does not automatically result in the creation of multiple attentional templates. Importantly, the precision of recall of the distractor color was worse than the precision of recall of the target color, regardless of instructions, suggesting that participants always allocated fewer VWM resources to the distractor color. Thus, 2 attentional templates may be set up, but only when the 2 colors receive an equal amount of resources in VWM (i.e., in dual target search). In contrast, when 1 item is deprioritized because of task demands, it receives fewer resources in VWM and multiple attentional templates cannot be established. Thus, unequal roles in the search task prevented the simultaneous operation of multiple attentional templates in VWM. [ABSTRACT FROM AUTHOR]

Voluntary attentional selection requires the match of sensory input to a stored representation of the target features. We compared the precision of attentional selection to the precision of the underlying memory representation of the target. To measure the precision of attentional selection, we used a cue-target paradigm where participants searched for a colored target. Typically, RTs are shorter at the cued compared to uncued locations when the cue has the same color as the target. In contrast, cueing effects are absent or even inverted when cue and target colors are dissimilar. By systematically varying the difference between cue and target color, we calculated a function relating cue color to cueing effects. The width of this function reflects the precision of attentional selection and was compared to the precision of judgments of the target color on a color wheel. The precision of the memory representation was far better than the precision of attentional selection. When the task was made more difficult by increasing the similarity between the target and the nontarget stimuli in the target display, the precision of attentional selection increased, but was still worse than the precision of memory. When the search task was made more difficult, we also observed that for dissimilar cue colors, RTs were slower at cued than at uncued locations (i.e., same location costs), suggesting that improvements in attentional selectivity were achieved by suppressing non-target colors. [ABSTRACT FROM AUTHOR]

According to the threat-capture hypothesis, irrelevant-but-salient stimuli interfere more with a visual search task when they are perceived as threatening. We investigated the neural basis for behavioural interference in conditions that promote attentional suppression of distracting stimuli (i.e., easy search with predictable targets). In Experiment 1, participants discriminated the shape of a neutral target (a flower), which competed for selection with a threat-related or neutral distractor (spider or leaf, respectively). In line with prior results, we observed larger behavioural interference from spider than leaf distractors. Electrophysiological recordings revealed a posterior positivity between 200 and 300 ms, the PD, suggesting that participants actively suppressed both leaf and spider distractors. Critically, the PD was delayed with spider compared to leaf distractors. Experiment 2 was a control experiment where we confirmed that the results depended on the execution of the peripheral search task. When participants performed a localization task on the fixation cross, the decisive results from Experiment 1 were not replicated despite equal peripheral stimulation. Our results indicate that the behavioural delay incurred by threatening stimuli is accompanied by a delay of suppressive mechanisms. Conversely, we found no evidence for initial capture followed by suppression that may be predicted by hypervigilance-avoidance theory. [ABSTRACT FROM AUTHOR]

Highlights • Observers judged whether two stimuli were same or different. • The same-different task induced a wide focus of attention. • The location of one of the two stimuli was salient. • The salient location inside the focus of attention was suppressed. • Suppression was evidenced by a P D or an attenuated N2pc. Abstract We investigated how attention is distributed when one of two attended stimuli stands out from the visual context. Participants judged whether the line orientations within two geometric shapes at two predictable locations were same or different, which induced a wide focus of attention around the two locations. One of the geometric shapes surrounding the lines could be a salient color or shape singleton but was irrelevant for the task. In Experiment 1, the salient and non-salient items were both placed on the horizontal midline. Electrophysiological recordings at posterior electrode locations PO7/8 revealed a positivity between 200 and 300 ms contralateral to the singleton, consistent with the occurrence of the P D. The P D is thought to reflect attentional suppression. In Experiment 2, one attended item was placed on the vertical meridian and the other one on a lateral position. Lateral line targets triggered robust N2pc components when there was no singleton present, reflecting attentional selection. However, this N2pc to lateralized line targets was abolished when a singleton was presented at the same lateral position, and conversely, was increased when a singleton was presented on the vertical position. This suggests that salient elements inside the focus of attention are suppressed and attention is enhanced at the other location. It can be concluded that salient elements inside the focus of attention do not capture attention, as bottom-up control of attention would propose, but that salient elements are suppressed, possibly to assure unbiased processing of equally relevant stimuli. [ABSTRACT FROM AUTHOR]

The orientation-bias hypothesis states that there is a bias to attend to the right visual hemifield (RVF) when there is spatial competition between stimuli in the left and right hemifield [Pollmann, S. (1996). A pop-out induced extinction-like phenomenon in neurologically intact subjects. Neuropsychologia, 34(5), 413- 425. doi:10.1016/0028-3932(95)00125-5]. In support of this hypothesis, stronger interference was reported for RVF distractors with contralateral targets. In contrast, previous studies using rapid serial visual presentation (RSVP) found stronger interference from distractors in the left visual hemifield (LVF). We used the additional singleton paradigm to test whether this discrepancy was due to the different distractor features that were employed (colour vs. orientation). Interference from the colour distractor with contralateral targets was larger in the RVF than in the LVF. However, the asymmetrical interference disappeared when observers had to search for an inconspicuous colour target instead of the inconspicuous shape target. We suggest that the LVF orienting-bias is limited to situations where search is driven by bottom-up saliency (singleton search) instead of top-down search goals (feature search). In contrast, analysis of the literature suggests the opposite for the LVF bias in RSVP tasks. Thus, the attentional asymmetry may depend on whether the task involves temporal or spatial competition, and whether search is based on bottom-up or top-down signals. [ABSTRACT FROM AUTHOR]

Search for a shape target is difficult when its shape is similar to the shape of the surrounding nontargets and easy when it is dissimilar. We asked whether interference from a salient but irrelevant color singleton depended on search difficulty as manipulated by target-nontarget similarity. We found that interference was strong in difficult searches, and the occurrence of an electrophysiological index of attentional selectivity (the N2pc component) confirmed that attention was captured by the distractor. In contrast, interference from the distractor was weak with easy searches, and the occurrence of the PD component confirmed that saliency signals from the distractor were suppressed. The results suggest that attentional suppression of salient but irrelevant distractors is only possible when search is efficient (i.e., with low target-nontarget similarity), otherwise, attentional capture occurs. Further, we analyzed the Ppc component, a positivity occurring between 100 and 200 ms after stimulus onset contralateral to a salient feature discontinuity. It has been suggested that the Ppc reflects bottom-up saliency signals, but we found the Ppc to occur only when the salient stimulus was unlikely to be selected, as in efficient searches. Thus, attentional requirements modulate the Ppc component. [ABSTRACT FROM AUTHOR]

We tested whether two known hemi-field asymmetries would affect visual search with face stimuli. Holistic processing of spatial configurations is better in the left hemi-field, reflecting a right hemisphere specialization, and object recognition is better in the upper visual field, reflecting stronger projections into the ventral stream. Faces tap into holistic processing and object recognition at the same time, which predicts better performance in the left and upper hemifield, respectively. In the first experiment, participants had to detect a face with a gaze direction different from the remaining faces. Participants were faster to respond when targets were presented in the left and upper hemi-field. The same pattern of results was observed when only the eye region was presented. In the second experiment, we turned the faces upside-down, which eliminated the typical spatial configuration of faces. The left hemi-field advantage disappeared, showing that it is related to holistic processing of faces, whereas the upper hemifield advantage related to object recognition persisted. Finally, we made the search task easier by asking observers to search for a face with open among closed eyes or vice versa. The easy search task eliminated the need for complex object recognition and, accordingly, the advantage of the upper visual field disappeared. Similarly, the left hemi-field advantage was attenuated. In sum, our findings show that both horizontal and vertical asymmetries affect the search for faces and can be selectively suppressed by changing characteristics of the stimuli. [ABSTRACT FROM AUTHOR]

Most theories of visual search maintain that attention is selectively tuned to the attributes of the search target (e.g., orange). Conversely, according to the relational account, attention is biased to the relative feature of the target (e.g., redder). However, previous studies that supported the relational account mainly measured mean response times. Hence, the results might not reflect early, perceptual mechanisms (e.g., signal enhancement) but later, decision-based mechanisms (channel selection). The current study tested the relational account against feature-specific theories in a spatial cueing task, in which the targets were backward-masked, and target identification accuracy was measured. The first experiment corroborated earlier results, demonstrating that relational effects are due to signal enhancement. In the second experiment, we chose highly discriminable colours along the blue-red continuum, and obtained results that were more consistent with broad feature-specific rather than relational tuning. The implications of these findings for current theories of attention are discussed. [ABSTRACT FROM AUTHOR]

In visual search for a shape target, interference from salient-but-irrelevant color singletons can be resisted in feature search mode, but not in singleton detection mode. In singleton detection mode, we observed a contralateral positivity (P D ) after 260–340 ms, suggesting that the salient distractor was suppressed. Because RTs in singleton detection mode increased when a distractor was present, we conclude that active suppression of distractors takes time. In feature search mode, no increase in RTs and no P D to the distractor was observed, showing that resistance to interference was not accomplished by suppression. Rather, the smaller N2pc to the target in feature search than in singleton detection mode suggests that enhancement of target features avoided interference. Thus, the strong top-down set in feature search mode eliminated the need to suppress the early attend-to-me signal (corresponding to the Ppc, from 160 to 210 ms) that was generated by salient stimuli independently of search mode. [ABSTRACT FROM AUTHOR]

We investigated the effects of task demands and individual differences on the allocation of attention. Using the same stimuli, participants indicated the orientation of a line contained in a shape singleton (identification task) or the presence of singletons (detection task). Shape singletons in the identification task elicited a contralateral negativity ( N2pc) whereas shape singletons in the detection task elicited a contralateral positivity ( Pd). We suggest that the reduction of attentional priority of a salient stimulus, reflected by the Pd, occurred more rapidly with the less demanding detection task. Further, fewer distractible participants showed a larger N2pc to lateral color distractors than highly distractible participants. We suggest that highly distractible participants developed compensatory mechanisms to suppress distracting stimuli. [ABSTRACT FROM AUTHOR]

Attentional capture by salient distractors has been confirmed by the occurrence of an N2pc to the salient distractor. To clarify some failures to replicate this finding, we varied target predictability to induce different search modes. In the unpredictable target condition, the target shape varied randomly from trial to trial, favoring singleton detection mode. In the predictable target condition, the target shape remained the same in a block of trials, favoring feature search mode. With unpredictable targets, we observed an N2pc toward the salient color distractor, confirming attentional capture in singleton search mode. With predictable targets, there was no N2pc to the salient distractor, but a distractor positivity ( Pd), suggesting distractor suppression. Also, differences emerged in the topographic segmentation of N2pc and Pd. Further, the amplitude of the N2pc toward the target was larger with predictable than with unpredictable targets. [ABSTRACT FROM AUTHOR]

Attentional templates are stored representations of target features that guide visual search. Target features may remain fixed or change on every trial, requiring sustained or transient templates, respectively. In separate blocks of trials, two sustained templates guide visual search as efficiently as two transient templates. In mixed blocks, however, the transient template interferes with the sustained template, impairing its efficiency in guiding visual search. Here, we hypothesized that the priority of the sustained template would increase when threatened by interference, eventually restoring efficient guidance of visual search. Participants memorized two possible target colors before the onset of the search display. At encoding, we assessed attentional selection of the two possible target colors with the N2pc. During subsequent maintenance, we measured the CDA as an index of resource allocation in working memory. In Experiment 1, the CDA was smaller with sustained than transient templates in separate blocks, but similar in mixed blocks. Thus, the sustained template received more working memory resources when maintained concurrently with an interfering transient template, suggesting that it was prioritized. In Experiment 2, the priority of the sustained template was further increased as it guided visual search in 80% of cases. The N2pc to possible target colors matching the sustained template was enhanced both at encoding and during visual search, thus eliminating interference from the transient template. Therefore, sustained templates are not necessarily less efficient than transient templates. Rather, prioritization through attentional selection at encoding and resource allocation during maintenance may restore efficient guidance of visual search. • Visual search is guided by attentional templates that are transient or sustained. • Transient templates interfere with sustained templates, impairing visual search. • We tested whether sustained templates are prioritized in the face of interference. • Neural measures of working memory (WM) were enhanced for sustained templates. • When threatened by interference, sustained templates receive more resources in WM. [ABSTRACT FROM AUTHOR]

When observers search for an object in the environment, they compare the incoming sensory information to the attentional template, a representation of the target in visual working memory (VWM). Previous studies have shown that visual search is more efficient when the attentional template is precise. We pursued the hypothesis that the attentional template in VWM is automatically complemented by features from long-term memory, possibly to increase its precision. At the beginning of the experiment, observers learned associations between shape and color. Then, we tested whether selecting one of these shapes was influenced by the previously associated color. To this end, we ran a saccadic selection task consisting of a memory and choice display. In the memory display, the target shape was presented at central fixation and participants were instructed to foveate this shape in the subsequent choice display. In the choice display, the target shape appeared together with a distractor shape at eccentric positions. Importantly, the target shape was colorless (gray) in the memory display so that only shape, but not color was loaded into VWM. However, saccades went more frequently to the target shape when it was shown in the learned color than when this color was shown in the distractor. Thus, the color of the target shape was reinstated from episodic memory to complement the attentional template in VWM. [ABSTRACT FROM AUTHOR]

Recent attentional capture studies with the spatial cueing paradigm often found that target-dissimilar precues resulted in longer RTs on valid than invalid cue trials. These same location costs were accompanied by a contralateral positivity over posterior electrodes from 200 to 300 ms, similar to a PD component. Same location costs and the PD have been linked to the inhibition of cues with a unique feature (singleton cues) that do not match the target feature. In some studies reporting same location costs, the cue was surrounded by other cues (i.e., the context cues) that matched the physical or relative feature of the target. We hypothesized that the context cues might have captured attention and might have elicited data patterns that mimicked the inhibitory effects. To disentangle inhibition of the singleton cue from capture by the context cues, we added gray cues to the cue array, which we considered neutral because gray matched neither the target nor the nontarget color. In four experiments, the results consistently showed that the context cues in the nonmatching cue condition captured attention, as reflected in shorter RTs compared to neutral cues and a substantial N2pc to lateralized context cues. By contrast, the evidence for inhibition of the singleton cue was rather weak. Therefore, same location costs and lateralized positivity in the event-related potential of participants in several recent studies probably reflected attentional capture by the context cues, not inhibition of the singleton cue. Public Significance Statement: How do we select relevant information from cluttered visual scenes? Many studies suggest that attentional control mechanisms facilitate processing of relevant and inhibit processing of irrelevant information. Several recent studies found evidence that supports the inhibition of irrelevant information. This study shows, however, that in a subset of these studies the findings are more likely associated with facilitation of context information than with inhibition. [ABSTRACT FROM AUTHOR]