Dr Anna Grubert's Outputs (42)

The capacity limitations of multiple‐template visual search during task preparation and target selection (2024)
Journal Article
Grubert, A., Wang, Z., Williams, E., Jimenez, M., Remington, R., & Eimer, M. (online). The capacity limitations of multiple‐template visual search during task preparation and target selection. Psychophysiology, e14720. https://doi.org/10.1111/psyp.14720

Visual search is guided by mental representations of target‐defining features (attentional templates) that are activated in a preparatory fashion. It remains unknown how many templates can be maintained concurrently, and what kind of costs are associ... Read More about The capacity limitations of multiple‐template visual search during task preparation and target selection.

Attentional templates for target features versus locations (2024)
Journal Article
Jimenez, M., Wang, Z., & Grubert, A. (2024). Attentional templates for target features versus locations. Scientific Reports, 14(1), Article 22306. https://doi.org/10.1038/s41598-024-73656-6

Visual search is guided by visual working memory representations (i.e., attentional templates) that are activated prior to search and contain target-defining features (e.g., color). In the present study, we tested whether attentional templates can al... Read More about Attentional templates for target features versus locations.

Target switch costs in visual search arise during the preparatory activation of target templates (2024)
Journal Article
Grubert, A., Wang, Z., & Eimer, M. (online). Target switch costs in visual search arise during the preparatory activation of target templates. Psychophysiology, https://doi.org/10.1111/psyp.14658

Prior research on task switching has shown that the reconfiguration of stimulus–response mappings across trials is associated with behavioral switch costs. Here, we investigated the effects of switching representations of target‐defining features in... Read More about Target switch costs in visual search arise during the preparatory activation of target templates.

Do We Prepare for What We Predict? How Target Expectations Affect Preparatory Attentional Templates and Target Selection in Visual Search (2023)
Journal Article
Grubert, A., & Eimer, M. (2023). Do We Prepare for What We Predict? How Target Expectations Affect Preparatory Attentional Templates and Target Selection in Visual Search. The Journal of Cognitive Neuroscience, 35(12), 1919-1935. https://doi.org/10.1162/jocn_a_02054

Visual search is guided by representations of target-defining features (attentional templates) that are activated in a preparatory fashion. Here, we investigated whether these template activation processes are modulated by probabilistic expectations... Read More about Do We Prepare for What We Predict? How Target Expectations Affect Preparatory Attentional Templates and Target Selection in Visual Search.

Which processes dominate visual search: Bottom-up feature contrast, top-down tuning or trial history? (2023)
Journal Article
Becker, S. I., Grubert, A., Horstmann, G., & Ansorge, U. (2023). Which processes dominate visual search: Bottom-up feature contrast, top-down tuning or trial history?. Cognition, 236, Article 105420. https://doi.org/10.1016/j.cognition.2023.105420

Previous research has identified three mechanisms that guide visual attention: bottom-up feature contrasts, top-down tuning, and the trial history (e.g., priming effects). However, only few studies have simultaneously examined all three mechanisms. H... Read More about Which processes dominate visual search: Bottom-up feature contrast, top-down tuning or trial history?.

Capacity limitations in template-guided multiple color search (2021)
Journal Article
Kerzel, D., & Grubert, A. (2022). Capacity limitations in template-guided multiple color search. Psychonomic Bulletin and Review, 29(3), 901-909. https://doi.org/10.3758/s13423-021-02040-6

Visual selection of target objects relies on representations of their known features in visual working memory. These representations are referred to as attentional templates. We asked how the capacity of visual working memory relates to the maximal n... Read More about Capacity limitations in template-guided multiple color search.

Preparatory Template Activation during Search for Alternating Targets (2020)
Journal Article
Grubert, A., & Eimer, M. (2020). Preparatory Template Activation during Search for Alternating Targets. The Journal of Cognitive Neuroscience, 32(8), 1525-1535. https://doi.org/10.1162/jocn_a_01565

Visual search is guided by representations of target-defining features (attentional templates). We tracked the time course of template activation processes during the preparation for search in a task where the identity of color-defined search targets... Read More about Preparatory Template Activation during Search for Alternating Targets.

Dwelling on simple stimuli in visual search (2019)
Journal Article
Horstmann, G., Becker, S. I., & Grubert, A. (2020). Dwelling on simple stimuli in visual search. Attention, Perception, and Psychophysics, 82(2), 607-625. https://doi.org/10.3758/s13414-019-01872-8

Research and theories on visual search often focus on visual guidance to explain differences in search. Guidance is the tuning of attention to target features and facilitates search because distractors that do not show target features can be more eff... Read More about Dwelling on simple stimuli in visual search.

Suppression of salient stimuli inside the focus of attention (2018)
Journal Article
Kerzel, D., Barras, C., & Grubert, A. (2018). Suppression of salient stimuli inside the focus of attention. Biological Psychology, 139, 106-114. https://doi.org/10.1016/j.biopsycho.2018.10.010

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... Read More about Suppression of salient stimuli inside the focus of attention.

The time course of target template activation processes during preparation for visual search (2018)
Journal Article
Grubert, A., & Eimer, M. (2018). The time course of target template activation processes during preparation for visual search. Journal of Neuroscience, 38(44), 9527-9538. https://doi.org/10.1523/jneurosci.0409-18.2018

Search for target objects in visual scenes is guided by mental representations of target features (attentional templates). However, it is unknown when such templates are activated during each search episode and whether this can be controlled by tempo... Read More about The time course of target template activation processes during preparation for visual search.

Category-based attentional guidance can operate in parallel for multiple target objects (2018)
Journal Article
Jenkins, M., Grubert, A., & Eimer, M. (2018). Category-based attentional guidance can operate in parallel for multiple target objects. Biological Psychology, 135, 211-219. https://doi.org/10.1016/j.biopsycho.2018.04.006

The question whether the control of attention during visual search is always feature-based or can also be based on the category of objects remains unresolved. Here, we employed the N2pc component as an on-line marker for target selection processes to... Read More about Category-based attentional guidance can operate in parallel for multiple target objects.

The speed of voluntary and priority-driven shifts of visual attention (2018)
Journal Article
Jenkins, M., Grubert, A., & Eimer, M. (2018). The speed of voluntary and priority-driven shifts of visual attention. Journal of Experimental Psychology: Human Perception and Performance, 44(1), 27-37. https://doi.org/10.1037/xhp0000438

The question how fast spatial attention moves between different visual objects remains debated. We used electrophysiological measures to determine the speed of voluntary and visually guided shifts of attention. Participants shifted attention from a k... Read More about The speed of voluntary and priority-driven shifts of visual attention.

Target objects defined by a conjunction of colour and shape can be selected independently and in parallel (2017)
Journal Article
Jenkins, M., Grubert, A., & Eimer, M. (2017). Target objects defined by a conjunction of colour and shape can be selected independently and in parallel. Attention, Perception, and Psychophysics, 79(8), 2310-2326. https://doi.org/10.3758/s13414-017-1410-x

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 c... Read More about Target objects defined by a conjunction of colour and shape can be selected independently and in parallel.

Multivariate EEG analyses support high-resolution tracking of feature-based attentional selection (2017)
Journal Article
Fahrenfort, J. J., Grubert, A., Olivers, C. N., & Eimer, M. (2017). Multivariate EEG analyses support high-resolution tracking of feature-based attentional selection. Scientific Reports, 7, Article 1886. https://doi.org/10.1038/s41598-017-01911-0

The primary electrophysiological marker of feature-based selection is the N2pc, a lateralized posterior negativity emerging around 180–200 ms. As it relies on hemispheric differences, its ability to discriminate the locus of focal attention is severe... Read More about Multivariate EEG analyses support high-resolution tracking of feature-based attentional selection.

Intermodal attention shifts in multimodal working memory (2017)
Journal Article
Katus, T., Grubert, A., & Eimer, M. (2017). Intermodal attention shifts in multimodal working memory. The Journal of Cognitive Neuroscience, 29(4), 628-636. https://doi.org/10.1162/jocn_a_01072

Attention maintains task-relevant information in working memory (WM) in an active state. We investigated whether the attention-based maintenance of stimulus representations that were encoded through different modalities is flexibly controlled by top-... Read More about Intermodal attention shifts in multimodal working memory.

The control of single-color and multiple-color visual search by attentional templates in working memory and in long-term memory (2016)
Journal Article
Grubert, A., Carlisle, N., & Eimer, M. (2016). The control of single-color and multiple-color visual search by attentional templates in working memory and in long-term memory. The Journal of Cognitive Neuroscience, 28(12), 1947-1963. https://doi.org/10.1162/jocn_a_01020

The question whether target selection in visual search can be effectively controlled by simultaneous attentional templates for multiple features is still under dispute. We investigated whether multiple-color attentional guidance is possible when targ... Read More about The control of single-color and multiple-color visual search by attentional templates in working memory and in long-term memory.

Rapid attentional selection processes operate independently and in parallel for multiple targets (2016)
Journal Article
Grubert, A., & Eimer, M. (2016). Rapid attentional selection processes operate independently and in parallel for multiple targets. Biological Psychology, 121(Part A), 99-108. https://doi.org/10.1016/j.biopsycho.2016.10.012

The question whether multiple objects are selected serially or in parallel remains contentious. Previous studies employed the N2pc component as a marker of attentional selection to show that multiple selection processes can be activated concurrently.... Read More about Rapid attentional selection processes operate independently and in parallel for multiple targets.

Category-based guidance of spatial attention during visual search for feature conjunctions (2016)
Journal Article
Nako, R., Grubert, A., & Eimer, M. (2016). Category-based guidance of spatial attention during visual search for feature conjunctions. Journal of Experimental Psychology: Human Perception and Performance, 42(10), 1571-1686. https://doi.org/10.1037/xhp0000244

The question whether alphanumerical category is involved in the control of attentional target selection during visual search remains a contentious issue. We tested whether category-based attentional mechanisms would guide the allocation of attention... Read More about Category-based guidance of spatial attention during visual search for feature conjunctions.

Rapid parallel attentional selection can be controlled by shape and alphanumerical category (2016)
Journal Article
Jenkins, M., Grubert, A., & Eimer, M. (2016). Rapid parallel attentional selection can be controlled by shape and alphanumerical category. The Journal of Cognitive Neuroscience, 28(11), 1672-1687. https://doi.org/10.1162/jocn_a_00995

Previous research has shown that when two color-defined target objects appear in rapid succession at different locations, attention is deployed independently and in parallel to both targets. This study investigated whether this rapid simultaneous att... Read More about Rapid parallel attentional selection can be controlled by shape and alphanumerical category.

Rapid top-down control over template-guided attention shifts to multiple objects (2016)
Journal Article
Grubert, A., Fahrenfort, J., Olivers, C., & Eimer, M. (2017). Rapid top-down control over template-guided attention shifts to multiple objects. NeuroImage, 146, 843-858. https://doi.org/10.1016/j.neuroimage.2016.08.039

Previous research has shown that when observers search for targets defined by a particular colour, attention can be directed rapidly and independently to two target objects that appear in close temporal proximity. We investigated how such rapid atten... Read More about Rapid top-down control over template-guided attention shifts to multiple objects.

All set, indeed! N2pc components reveal simultaneous attentional control settings for multiple target colors (2016)
Journal Article
Grubert, A., & Eimer, M. (2016). All set, indeed! N2pc components reveal simultaneous attentional control settings for multiple target colors. Journal of Experimental Psychology: Human Perception and Performance, 42(8), 1215-1230. https://doi.org/10.1037/xhp0000221

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

Attentional guidance by relative features: Behavioral and electrophysiological evidence (2016)
Journal Article
Schönhammer, J., Grubert, A., Kerzel, D., & Becker, S. (2016). Attentional guidance by relative features: Behavioral and electrophysiological evidence. Psychophysiology, 53(7), 1074-1083. https://doi.org/10.1111/psyp.12645

Our ability to select task-relevant information from cluttered visual environments is widely believed to be due to our ability to tune attention to the particular elementary feature values of a sought-after target (e.g., red, orange, yellow). By cont... Read More about Attentional guidance by relative features: Behavioral and electrophysiological evidence.

The speed of serial attention shifts in visual search: Evidence from the N2pc component (2015)
Journal Article
Grubert, A., & Eimer, M. (2016). The speed of serial attention shifts in visual search: Evidence from the N2pc component. The Journal of Cognitive Neuroscience, 28(2), 319-332. https://doi.org/10.1162/jocn_a_00898

Finding target objects among distractors in visual search display is often assumed to be based on sequential movements of attention between different objects. However, the speed of such serial attention shifts is still under dispute. We employed a se... Read More about The speed of serial attention shifts in visual search: Evidence from the N2pc component.

Visual search is postponed during the period of the AB: An event-related potential study (2015)
Journal Article
Lagroix, H., Grubert, A., Spalek, T., Di Lollo, V., & Eimer, M. (2015). Visual search is postponed during the period of the AB: An event-related potential study. Psychophysiology, 52(8), 1031-1038. https://doi.org/10.1111/psyp.12435

In the phenomenon known as the attentional blink (AB), perception of the second of two rapidly sequential targets (T2) is impaired when presented shortly after the first (T1). Studies in which T2 consisted of a pop-out search array provided evidence... Read More about Visual search is postponed during the period of the AB: An event-related potential study.

Nasotemporal ERP differences: evidence for increased inhibition of temporal distractors (2015)
Journal Article
Huber-Huber, C., Grubert, A., Ansorge, U., & Eimer, M. (2015). Nasotemporal ERP differences: evidence for increased inhibition of temporal distractors. Journal of Neurophysiology, 113(7), 2210-2219. https://doi.org/10.1152/jn.00344.2014

Previous research has demonstrated behavioral advantages for stimuli in the temporal relative to the nasal visual hemifield. To investigate whether this nasotemporal asymmetry reflects a genuinely attentional bias, we recorded event-related potential... Read More about Nasotemporal ERP differences: evidence for increased inhibition of temporal distractors.

Rapid parallel attentional target selection in single-color and multiple-color visual search (2015)
Journal Article
Grubert, A., & Eimer, M. (2015). Rapid parallel attentional target selection in single-color and multiple-color visual search. Journal of Experimental Psychology: Human Perception and Performance, 41(1), 86-101. https://doi.org/10.1037/xhp0000019

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 mult... Read More about Rapid parallel attentional target selection in single-color and multiple-color visual search.

A dissociation between selective attention and conscious awareness in the representation of temporal order information (2015)
Journal Article
Eimer, M., & Grubert, A. (2015). A dissociation between selective attention and conscious awareness in the representation of temporal order information. Consciousness and Cognition, 35, 274-281. https://doi.org/10.1016/j.concog.2015.01.001

Previous electrophysiological studies have shown that attentional selection processes are highly sensitive to the temporal order of task-relevant visual events. When two successively presented colour-defined target stimuli are separated by a stimulus... Read More about A dissociation between selective attention and conscious awareness in the representation of temporal order information.

Does visual working memory represent the predicted locations of future target objects? An event-related brain potential study (2014)
Journal Article
Grubert, A., & Eimer, M. (2015). Does visual working memory represent the predicted locations of future target objects? An event-related brain potential study. Brain Research, 1626, 258-266. https://doi.org/10.1016/j.brainres.2014.10.011

During the maintenance of task-relevant objects in visual working memory, the contralateral delay activity (CDA) is elicited over the hemisphere opposite to the visual field where these objects are presented. The presence of this lateralised CDA comp... Read More about Does visual working memory represent the predicted locations of future target objects? An event-related brain potential study.

The gradual emergence of spatially selective target processing in visual search: From feature-specific to object-based attentional control (2014)
Journal Article
Eimer, M., & Grubert, A. (2014). The gradual emergence of spatially selective target processing in visual search: From feature-specific to object-based attentional control. Journal of Experimental Psychology: Human Perception and Performance, 40(5), 1819-1831. https://doi.org/10.1037/a0037387

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... Read More about The gradual emergence of spatially selective target processing in visual search: From feature-specific to object-based attentional control.

Distinct neural networks for target feature versus dimension changes in visual search, as revealed by EEG and fMRI (2014)
Journal Article
Becker, S., Grubert, A., & Dux, P. (2014). Distinct neural networks for target feature versus dimension changes in visual search, as revealed by EEG and fMRI. NeuroImage, 102(Part 2), 798-808. https://doi.org/10.1016/j.neuroimage.2014.08.058

In visual search, responses are slowed, from one trial to the next, both when the target dimension changes (e.g., from a color target to a size target) and when the target feature changes (e.g., from a red target to a green target) relative to being... Read More about Distinct neural networks for target feature versus dimension changes in visual search, as revealed by EEG and fMRI.

Electrophysiological evidence for a sensory recruitment model of somatosensory working memory (2014)
Journal Article
Katus, T., Grubert, A., & Eimer, M. (2015). Electrophysiological evidence for a sensory recruitment model of somatosensory working memory. Cerebral Cortex, 25(12), 4697-4703. https://doi.org/10.1093/cercor/bhu153

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-relate... Read More about Electrophysiological evidence for a sensory recruitment model of somatosensory working memory.

From features to dimensions: cognitive and motor development in pop-out search in children and young adults (2014)
Journal Article
Grubert, A., Indino, M., & Krummenacher, J. (2014). From features to dimensions: cognitive and motor development in pop-out search in children and young adults. Frontiers in Psychology, 5, Article 519. https://doi.org/10.3389/fpsyg.2014.00519

In an experiment involving a total of 124 participants, divided into eight age groups (6-, 8-, 10-, 12-, 14-, 16-, 18-, and 20-year-olds) the development of the processing components underlying visual search for pop-out targets was tracked. Participa... Read More about From features to dimensions: cognitive and motor development in pop-out search in children and young adults.

Salience-based integration of redundant signals in visual pop-out search: evidence from behavioral and electrophysiological measures (2014)
Journal Article
Krummenacher, J., Grubert, A., Töllner, T., & Müller, H. (2014). Salience-based integration of redundant signals in visual pop-out search: evidence from behavioral and electrophysiological measures. Journal of Vision, 14(3), Article 26. https://doi.org/10.1167/14.3.26

Behavioral and electrophysiological evidence is presented suggesting that, in visual search for feature singleton targets, multidimensional signals are integrated at a preselective stage of processing. Observers searched for a target that was consist... Read More about Salience-based integration of redundant signals in visual pop-out search: evidence from behavioral and electrophysiological measures.

Spatial attention can be allocated rapidly and in parallel to new visual objects (2014)
Journal Article
Eimer, M., & Grubert, A. (2014). Spatial attention can be allocated rapidly and in parallel to new visual objects. Current Biology, 24(2), 193-198. https://doi.org/10.1016/j.cub.2013.12.001

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 independ... Read More about Spatial attention can be allocated rapidly and in parallel to new visual objects.

Top-down task sets for combined features: behavioral and electrophysiological evidence for two stages in attentional object selection. (2013)
Journal Article
Kiss, M., Grubert, A., & Eimer, M. (2013). Top-down task sets for combined features: behavioral and electrophysiological evidence for two stages in attentional object selection. Attention, Perception, and Psychophysics, 75(2), 216-228. https://doi.org/10.3758/s13414-012-0391-z

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 se... Read More about Top-down task sets for combined features: behavioral and electrophysiological evidence for two stages in attentional object selection..

Happy with a difference, unhappy with an identity: observers' mood determines processing depth in visual search. (2013)
Journal Article
Grubert, A., Schmid, P., & Krummenacher, J. (2013). Happy with a difference, unhappy with an identity: observers' mood determines processing depth in visual search. Attention, Perception, and Psychophysics, 75(1), 41-52. https://doi.org/10.3758/s13414-012-0385-x

Visual search for feature targets was employed to investigate whether the mechanisms underlying visual selective attention are modulated by observers' mood. The effects of induced mood on overall mean reaction times and on changes and repetitions of... Read More about Happy with a difference, unhappy with an identity: observers' mood determines processing depth in visual search..

A unitary focus of spatial attention during attentional capture: Evidence from event-related brain potentials (2013)
Journal Article
Grubert, A., Righi, L., & Eimer, M. (2013). A unitary focus of spatial attention during attentional capture: Evidence from event-related brain potentials. Journal of Vision, 13(3), 1-11. https://doi.org/10.1167/13.3.9

We studied whether attentional capture in vision can be elicited simultaneously at multiple locations in the visual field or whether it is always restricted to a single location. Participants searched for color singleton targets in search arrays that... Read More about A unitary focus of spatial attention during attentional capture: Evidence from event-related brain potentials.

Qualitative differences in the guidance of attention during single-color and multiple-color visual search: behavioral and electrophysiological evidence. (2013)
Journal Article
Grubert, A., & Eimer, M. (2013). Qualitative differences in the guidance of attention during single-color and multiple-color visual search: behavioral and electrophysiological evidence. Journal of Experimental Psychology: Human Perception and Performance, 39(5), 1433-1442. https://doi.org/10.1037/a0031046

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-def... Read More about Qualitative differences in the guidance of attention during single-color and multiple-color visual search: behavioral and electrophysiological evidence..

Attentional capture by salient distractors during visual search is determined by temporal task demands. (2012)
Journal Article
Kiss, M., Grubert, A., Petersen, A., & Eimer, M. (2012). Attentional capture by salient distractors during visual search is determined by temporal task demands. The Journal of Cognitive Neuroscience, 24(3), 749-759. https://doi.org/10.1162/jocn_a_00127

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 s... Read More about Attentional capture by salient distractors during visual search is determined by temporal task demands..

Redundancy gains in pop-out visual search are determined by top-down task set: behavioral and electrophysiological evidence. (2011)
Journal Article
Grubert, A., Krummenacher, J., & Eimer, M. (2011). Redundancy gains in pop-out visual search are determined by top-down task set: behavioral and electrophysiological evidence. Journal of Vision, 11(14), 1-10. https://doi.org/10.1167/11.14.10

We combined behavioral and electrophysiological measures to find out whether redundancy gain effects in pop-out visual search are exclusively determined by bottom-up salience or are modulated by top-down task search goals. Search arrays contained fea... Read More about Redundancy gains in pop-out visual search are determined by top-down task set: behavioral and electrophysiological evidence..

Display probability modulates attentional capture by onset distractors. (2010)
Journal Article
Sayim, B., Grubert, A., Herzog, M., & Krummenacher, J. (2010). Display probability modulates attentional capture by onset distractors. Journal of Vision, 10(3), 1-8. https://doi.org/10.1167/10.3.10

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 t... Read More about Display probability modulates attentional capture by onset distractors..

Inter-trial and redundant-signals effects in visual search and discrimination tasks: separable pre-attentive and post-selective effects. (2010)
Journal Article
Krummenacher, J., Grubert, A., & Müller, H. (2010). Inter-trial and redundant-signals effects in visual search and discrimination tasks: separable pre-attentive and post-selective effects. Vision Research, 50(14), 1382-1395. https://doi.org/10.1016/j.visres.2010.04.006

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