Anna Grubert

Attentional Capture by Salient Distractors during Visual Search Is Determined by Temporal Task Demands

The question whether attentional capture by salient but task-irrelevant visual stimuli is triggered in a bottom–up fashion or depends on top–down task settings is still unresolved. Strong support for bottom–up capture was obtained in the additional singleton task, in which search arrays were visible until response onset. Equally strong evidence for top–down control of attentional capture was obtained in spatial cueing experiments in which display durations were very brief. To demonstrate the critical role of temporal task demands on salience-driven attentional capture, we measured ERP indicators of capture by task-irrelevant color singletons in search arrays that could also contain a shape target. In Experiment 1, all displays were visible until response onset. In Experiment 2, display duration was limited to 200 msec. With long display durations, color singleton distractors elicited an N2pc component that was followed by a late Pd component, suggesting that they triggered attentional capture, which was later replaced by location-specific inhibition. When search arrays were visible for only 200 msec, the distractor-elicited N2pc was eliminated and was replaced by a Pd component in the same time range, indicative of rapid suppression of capture. Results show that attentional capture by salient distractors can be inhibited for short-duration search displays, in which it would interfere with target processing. They demonstrate that salience-driven capture is not a purely bottom–up phenomenon but is subject to top–down control.

Spatial Attention Can Be Allocated Rapidly and in Parallel to New Visual Objects

In real-life visual environments, where multiple objects compete for processing, new objects that require immediate attention often appear when attention is already focused elsewhere. The question of whether spatial attention can be directed independently to different locations in the visual field remains controversial. Serial models assume a unitary attentional focus that is directed to one object at a time and moves rapidly between objects. According to parallel models, attention can be simultaneously allocated to several visual objects, but the distribution of attention cannot change rapidly when new objects arrive. Here we demonstrate the existence of a fast and flexible mechanism of attentional object selection, where focal attention is allocated in parallel and independently to different target objects. Using event-related brain potential (ERP) markers of visual attention, we show that when two targets appear in rapid succession at different locations, two separate foci of attention are established, each with its own independent time course. Attention can be maintained at its previous location while it is simultaneously allocated to a new target object. Our results challenge the view that the attentional focus is always unitary and that the spatial selection of multiple visual objects operates in a strictly serial fashion.

Top-down task sets for combined features: Behavioral and electrophysiological evidence for two stages in attentional object selection

We studied whether visual search for targets defined by a combination of features from different dimensions is guided by separately represented target features or by an integrated representation of the target objects. In Experiment 1, participants searched for target singleton bars that were defined by a specific combination of color (red or blue) and size (small or large). The target arrays were preceded by cue arrays that contained a spatially uninformative color/size singleton. Behavioral spatial-cueing effects indicative of attentional capture were triggered only by cues that matched both target-defining features, but not by partially target-matching cues, suggesting that attention was guided by integrated object representations. However, the presence of reliable N2pc components for partially matching cues demonstrated that these cues did capture attention, in line with independent feature-based guidance of attention. This dissociation between the electrophysiological and behavioral markers of attentional capture was confirmed in Experiment 2, in which targets were defined by a color/size disjunction. Our results suggest that the attentional selection of targets that are defined by a combination of features is a two-stage process: Attention is initially captured by all target-matching features, but is then rapidly withdrawn from nontarget objects that share some but not all features with the current target.

Electrophysiological Evidence for a Sensory Recruitment Model of Somatosensory Working Memory

Sensory recruitment models of working memory assume that information storage is mediated by the same cortical areas that are responsible for the perceptual processing of sensory signals. To test this assumption, we measured somatosensory event-related brain potentials (ERPs) during a tactile delayed match-to-sample task. Participants memorized a tactile sample set at one task-relevant hand to compare it with a subsequent test set on the same hand. During the retention period, a sustained negativity (tactile contralateral delay activity, tCDA) was elicited over primary somatosensory cortex contralateral to the relevant hand. The amplitude of this component increased with memory load and was sensitive to individual limitations in memory capacity, suggesting that the tCDA reflects the maintenance of tactile information in somatosensory working memory. The tCDA was preceded by a transient negativity (N2cc component) with a similar contralateral scalp distribution, which is likely to reflect selection of task-relevant tactile stimuli at the encoding stage. The temporal sequence of N2cc and tCDA components mirrors previous observations from ERP studies of working memory in vision. The finding that the sustained somatosensory delay period activity varies as a function of memory load supports a sensory recruitment model for spatial working memory in touch.

Inter-trial and redundant-signals effects in visual search and discrimination tasks: separable pre-attentive and post-selective effects.

Feature singleton search is faster when the target-defining dimension is repeated, rather than changed, across trials (Found & Müller, 1996). A similar dimension repetition benefit has been observed in a non-search (discrimination) task with a single stimulus (Mortier, Theeuwes, & Starreveld, 2005). Two experiments examined whether these effects in the two tasks originate from the same or different processing stages. Experiment 1 revealed differential feature-specific effects, and Experiment 2 differential processing of dimensionally redundant target signals between the two types of task. These dissociations support the existence of separable, pre-attentive and post-selective sources of inter-trial effects in the two tasks.

Qualitative Differences in the Guidance of Attention During Single-Color and Multiple-Color Visual Search: Behavioral and Electrophysiological Evidence

To find out whether attentional target selection can be effectively guided by top-down task sets for multiple colors, we measured behavioral and ERP markers of attentional target selection in an experiment where participants had to identify color-defined target digits that were accompanied by a single gray distractor object in the opposite visual field. In the One Color task, target color was constant. In the Two Color task, targets could have one of two equally likely colors. Color-guided target selection was less efficient during multiple-color relative to single-color search, and this was reflected by slower response times and delayed N2pc components. Nontarget-color items that were presented in half of all trials captured attention and gained access to working memory when participants searched for two colors, but were excluded from attentional processing in the One Color task. Results demonstrate qualitative differences in the guidance of attentional target selection between single-color and multiple-color visual search. They suggest that top-down attentional control can be applied much more effectively when it is based on a single feature-specific attentional template.

Display probability modulates attentional capture by onset distractors

Attention can be stimulus-driven and bottom-up or goal-driven and top-down. Bottom-up attention and, particularly, attentional capture are often thought to be strongly automatic, i.e., not modulable. For example, in visual search, it has been shown that salient distractors strongly attract attention even though observers were instructed to ignore them. However, it was also shown that the strength of distraction can be modulated by the display probabilities of the distractors. Hence, bottom-up attention seems not to be completely automatic. In these studies, the distractors were salient by color differences to the other items in the display. Such color distractors, however, do not necessarily trigger bottom-up attention. Here, we presented onset distractors, that is, distractors displayed after the onset of the other search items, which are thought to strongly elicit bottom-up attention and to capture eye movements. Varying the display probabilities of the onset distractors strongly modulated attentional capture. We suggest that modulation was due to statistical learning. This study adds further evidence that bottom-up processes are not completely automatic.

Rapid parallel attentional target selection in single-color and multiple-color visual search

Previous work has demonstrated that when targets are defined by a constant feature, attention can be directed rapidly and in parallel to sequentially presented target objects at different locations. We assessed how fast attention is allocated to multiple objects when this process cannot be controlled by a unique color-specific attentional template. N2pc components were measured as temporal markers of the attentional selection of 2 color-defined targets that were presented in rapid succession. Both targets either had the same color (one color task) or differed in color (two color task). Although there were small but systematic delays of target selection in the two color task relative to the one color task, attention was allocated extremely rapidly to both target objects in the two color task, which is inconsistent with the hypothesis that their selection was based on a slow switch between different color templates. Two follow-up experiments demonstrated that these delays did not reflect template switch costs, but were the result of competitive interactions between simultaneously active attentional templates. These results show that the control of focal attention during multiple-feature search operates much faster and more flexibly than is usually assumed.

The gradual emergence of spatially selective target processing in visual search: from feature-specific to object-based attentional control

To dissociate feature-based and object-based stages in the control of spatial attention during visual search, we employed the N2pc component as an electrophysiological marker of attentional object selection. Participants searched for a target object that was defined by a conjunction of color and shape. Some search displays contained the target or a nontarget object that matched either the target color or its shape among 3 nonmatching distractors. In other displays, the target and a partially target-matching nontarget object appeared together. N2pc results demonstrated that the initial stage of attentional object selection is controlled by local feature-specific signals. Attention is allocated in parallel and independently to objects with target-matching features during this early stage, irrespective of whether another target-matching object is simultaneously present elsewhere. From around 250 ms poststimulus, information is integrated across feature dimensions, and spatially selective attentional processing becomes object-based. These findings demonstrate that feature-based and object-based stages of attentional selectivity in visual search can be dissociated in real time.

All set, indeed! N2pc components reveal simultaneous attentional control settings for multiple target colors.

To study whether top-down attentional control processes can be set simultaneously for different visual features, we employed a spatial cueing procedure to measure behavioral and electrophysiological markers of task-set contingent attentional capture during search for targets defined by 1 or 2 possible colors (one-color and two-color tasks). Search arrays were preceded by spatially nonpredictive color singleton cues. Behavioral spatial cueing effects indicative of attentional capture were elicited only by target-matching but not by distractor-color cues. However, when search displays contained 1 target-color and 1 distractor-color object among gray nontargets, N2pc components were triggered not only by target-color but also by distractor-color cues both in the one-color and two-color task, demonstrating that task-set nonmatching items attracted attention. When search displays contained 6 items in 6 different colors, so that participants had to adopt a fully feature-specific task set, the N2pc to distractor-color cues was eliminated in both tasks, indicating that nonmatching items were now successfully excluded from attentional processing. These results demonstrate that when observers adopt a feature-specific search mode, attentional task sets can be configured flexibly for multiple features within the same dimension, resulting in the rapid allocation of attention to task-set matching objects only. (PsycINFO Database Record