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1.  Reconciling conflicting electrophysiological findings on the guidance of attention by working memory 
Maintaining a representation in working memory has been proposed to be sufficient for the execution of top-down attentional control. Two recent electrophysiological studies that recorded event-related potentials (ERP) during similar paradigms have tested this proposal, but reported contradictory findings. The goal of the present study was to reconcile these previous reports. To this end, we used the stimuli from one study (Kumar, Soto, & Humphreys, 2009) combined with the task manipulations used in the other (Carlisle & Woodman, 2011b). We found that when an item matching a working memory representation was presented in a visual search array, we could use ERPs to quantify the size of the covert attention effect. When the working memory matches were consistently task irrelevant, we observed a weak attentional bias to these items. However, when this same item indicated the location of the search target, we found that the covert attention effect was approximately four times larger. This shows that simply maintaining a representation in working memory is not equivalent to having a top-down attentional set for that item. Our findings indicate that high-level goals mediate the relationship between the contents of working memory and perceptual attention.
PMCID: PMC3800228  PMID: 23918552
2.  Causal Control of Medial–Frontal Cortex Governs Electrophysiological and Behavioral Indices of Performance Monitoring and Learning 
The Journal of Neuroscience  2014;34(12):4214-4227.
Adaptive human behavior depends on the capacity to adjust cognitive processing after an error. Here we show that transcranial direct current stimulation of medial–frontal cortex provides causal control over the electrophysiological responses of the human brain to errors and feedback. Using one direction of current flow, we eliminated performance-monitoring activity, reduced behavioral adjustments after an error, and slowed learning. By reversing the current flow in the same subjects, we enhanced performance-monitoring activity, increased behavioral adjustments after an error, and sped learning. These beneficial effects fundamentally improved cognition for nearly 5 h after 20 min of noninvasive stimulation. The stimulation selectively influenced the potentials indexing error and feedback processing without changing potentials indexing mechanisms of perceptual or response processing. Our findings demonstrate that the functioning of mechanisms of cognitive control and learning can be up- or down-regulated using noninvasive stimulation of medial–frontal cortex in the human brain.
PMCID: PMC3960465  PMID: 24647942
executive control; learning; medial–frontal cortex; transcranial direct current stimulation
3.  The effect of visual search efficiency on response preparation: Neurophysiological evidence for discrete flow 
Psychological science  2008;19(2):128-136.
Many models assume that response time (RT) consists of successive processing stages, but they disagree about whether information is transmitted continuously or discretely between stages. We tested these alternative hypotheses by measuring when movement-related activity began in the frontal eye field (FEF) of macaque monkeys performing visual search. Previous work showed that RT was longer when visual neurons in FEF took longer to select the target, consistent with prolonged perceptual processing during less efficient search. We now report that the buildup of saccadic movement-related activity in FEF is delayed when visual search is more demanding. Variability in the delay of movement-related activity accounted for the difference in RT between search conditions and for the variability of RT within conditions. These findings provide neurophysiological support for the discrete transmission of information between perceptual and response stages or processing during visual search.
PMCID: PMC4154362  PMID: 18271860
4.  Visual-Spatial Attention Aids the Maintenance of Object Representations in Visual Working Memory 
Memory & cognition  2013;41(5):698-715.
Theories have proposed that the maintenance of object representations in visual working memory is aided by a spatial rehearsal mechanism. In this study, we used two different approaches to test the hypothesis that overt and covert visual-spatial attention mechanisms contribute to the maintenance of object representations in visual working memory. First, we tracked observers’ eye movements while remembering a variable number of objects during change-detection tasks. We observed that during the blank retention interval, participants spontaneously shifted gaze to the locations that the objects had occupied in the memory array. Next, we hypothesized that if attention mechanisms contribute to the maintenance of object representations, then drawing attention away from the object locations during the retention interval would impair object memory during these change-detection tasks. Supporting this prediction, we found that attending to the fixation point in anticipation of a brief probe stimulus during the retention interval reduced change-detection accuracy even on the trials in which no probe occurred. These findings support models of working memory in which visual-spatial selection mechanisms contribute to the maintenance of object representations.
PMCID: PMC3655125  PMID: 23371773
5.  Oscillatory coupling reveals the dynamic reorganization of large-scale neural networks as cognitive demands change 
Journal of cognitive neuroscience  2013;26(1):175-188.
Cognitive operations are thought to emerge from dynamic interactions between spatially distinct brain areas. Synchronization of oscillations has been proposed to regulate these interactions, but we do not know whether this large-scale synchronization can respond rapidly to changing cognitive demands. Here we show that as task demands change during a trial, multiple distinct networks are dynamically formed and reformed via oscillatory synchronization. Distinct frequency-coupled networks were rapidly formed to process reward value, maintain information in visual working memory, and deploy visual attention. Strong single-trial correlations showed that networks formed even before the presentation of imperative stimuli could predict the strength of subsequent networks, as well as the speed and accuracy of behavioral responses seconds later. These frequency-coupled networks better predicted single-trial behavior than either local oscillations or event-related potentials. Our findings demonstrate the rapid reorganization of networks formed by dynamic activity in response to changing task demands within a trial.
PMCID: PMC3990735  PMID: 23984947
cross-frequency coupling; synchrony; reward; visual working memory; visual attention
6.  When memory is not enough: Electrophysiological evidence for goal-dependent use of working memory representations in guiding visual attention 
Journal of cognitive neuroscience  2011;23(10):2650-2664.
Biased competition theory proposes that representations in working memory drive visual attention to select similar inputs. However, behavioral tests of this hypothesis have led to mixed results. These inconsistent findings could be due to the inability of behavioral measures to reliably detect the early, automatic effects on attentional deployment that the memory representations exert. Alternatively, executive mechanisms may govern how working memory representations influence attention based on higher-level goals. In the present study, we tested these hypotheses using the N2pc component of participants’ event-related potentials (ERPs) to directly measure the early deployments of covert attention. Participants searched for a target in an array that sometimes contained a memory-matching distractor. In Experiments 1–3, we manipulated the difficulty of the target discrimination and the proximity of distractors, but consistently observed that covert attention was deployed to the search targets and not the memory-matching distractors. In Experiment 4, we showed that when participants’ goal involved attending to memory-matching items that these items elicited a large and early N2pc. Our findings demonstrate that working memory representations alone are not sufficient to guide early deployments of visual attention to matching inputs and that goal-dependent executive control mediates the interactions between working memory representations and visual attention.
PMCID: PMC3981747  PMID: 21254796
Event related potentials; Attention: Visual; Memory: Working memory; Executive functions
7.  The Benefit of Forgetting 
Psychonomic bulletin & review  2013;20(2):348-355.
Recent research using change-detection tasks has shown that a directed-forgetting cue, indicating that a subset of the information stored in memory can be forgotten, significantly benefits the other information stored in visual working memory. How do these directed-forgetting cues aid the memory representations that are retained? We addressed this question in the present study by using a recall paradigm to measure the nature of the retained memory representations. Our results demonstrate that a directed-forgetting cue leads to higher fidelity representations of the remaining items and a lower probability of dropping these representations from memory. Next, we show that this is possible because the to-be-forgotten item is expelled from visual working memory following the cue allowing maintenance mechanisms to be focused on only the items that remain in visual working memory. Thus, the present findings show that cues to forget benefit the remaining information in visual working memory by fundamentally improving their quality relative to conditions in which just as many items are encoded but no cue is provided.
PMCID: PMC3593955  PMID: 23208769
8.  Viewing the dynamics and control of visual attention through the lens of electrophysiology 
Vision research  2013;80:7-18.
How we find what we are looking for in complex visual scenes is a seemingly simple ability that has taken half a century to unravel. The first study to use the term visual search showed that as the number of objects in a complex scene increases, observers’ reaction times increase proportionally (Green and Anderson, 1956). This observation suggests that our ability to process the objects in the scenes is limited in capacity. However, if it is known that the target will have a certain feature attribute, for example, that it will be red, then only an increase in the number of red items increases reaction time. This observation suggests that we can control which visual inputs receive the benefit of our limited capacity to recognize the objects, such as those defined by the color red, as the items we seek. The nature of the mechanisms that underlie these basic phenomena in the literature on visual search have been more difficult to definitively determine. In this paper, I discuss how electrophysiological methods have provided us with the necessary tools to understand the nature of the mechanisms that give rise to the effects observed in the first visual search paper. I begin by describing how recordings of event-related potentials from humans and nonhuman primates have shown us how attention is deployed to possible target items in complex visual scenes. Then, I will discuss how event-related potential experiments have allowed us to directly measure the memory representations that are used to guide these deployments of attention to items with target-defining features.
PMCID: PMC3594578  PMID: 23357579
visual attention; visual working memory; long-term memory; visual search; electrophysiology; event-related potentials
9.  Why is Information Displaced from Visual Working Memory during Visual Search? 
Visual cognition  2009;18(2):10.1080/13506280902734326.
Research has shown that performing visual search while maintaining representations in visual working memory displaces up to one object’s worth of information from memory. This memory displacement has previously been attributed to a nonspecific disruption of the memory representation by the mere presentation of the visual search array, and the goal of the present study was to determine whether it instead reflects the use of visual working memory in the actual search process. The first hypothesis tested was that working memory displacement occurs because observers preemptively discard about an object’s worth of information from visual working memory in anticipation of performing visual search. Second, we tested the hypothesis that on target-absent trials no information is displaced from visual working memory because no target is entered into memory when search is completed. Finally, we tested whether visual working memory displacement is due to the need to select a response to the search array. The findings rule out these alternative explanations. The present study supports the hypothesis that change-detection performance is impaired when a search array appears during the retention interval due to nonspecific disruption or masking.
PMCID: PMC3817820  PMID: 24204180
10.  Templates for rejection: Configuring attention to ignore task-irrelevant features 
Theories of attention are compatible with the idea that we can bias attention to avoid selecting objects that have known nontarget features. Although this may underlie several existing phenomena, the explicit guidance of attention away from known nontargets has yet to be demonstrated. Here we show that observers can use feature cues (i.e., color) to bias attention away from nontarget items during visual search. These negative cues were used to quickly instantiate a template for rejection that reliably facilitated search across the cue-to-search stimulus onset asynchronies (SOAs), although negative cues were not as potent as cues that guide attention toward target features. Furthermore, by varying the search set size we show a template for rejection is increasingly effective in facilitating search as scene complexity increases. Our findings demonstrate that knowing what not to look for can be used to configure attention to avoid certain features, complimenting what is known about setting attention to select certain target features.
PMCID: PMC3817824  PMID: 22468723
11.  Directed Forgetting and Directed Remembering in Visual Working Memory 
A defining characteristic of visual working memory is its limited capacity. This means that it is crucial to maintain only the most relevant information in visual working memory. However, empirical research is mixed as to whether it is possible to selectively maintain a subset of the information previously encoded into visual working memory. Here we examined the ability of subjects to use cues to either forget or remember a subset of the information already stored in visual working memory. In Experiment 1, participants were cued to either forget or remember one of two groups of colored squares during a change-detection task. We found that both types of cues aided performance in the visual working memory task, but that observers benefited more from a cue to remember than a cue to forget a subset of the objects. In Experiment 2, we show that the previous findings, which indicated that directed-forgetting cues are ineffective, were likely due to the presence of invalid cues that appear to cause observers to disregard such cues as unreliable. In Experiment 3, we recorded event-related potentials (ERPs) and show that an electrophysiological index of focused maintenance is elicited by cues that indicate which subset of information in visual working memory needs to be remembered, ruling out alternative explanations of the behavioral effects of retention-interval cues. The present findings demonstrate that observers can focus maintenance mechanisms on specific objects in visual working memory based on cues indicating future task relevance.
PMCID: PMC3817833  PMID: 22409182
12.  A Brief Introduction to the Use of Event-Related Potentials (ERPs) in Studies of Perception and Attention 
Attention, perception & psychophysics  2010;72(8):10.3758/APP.72.8.2031.
Due to the precise temporal resolution of electrophysiological recordings, the event-related potential (ERP) technique has proven particularly valuable for testing theories of perception and attention. Here, I provide a brief tutorial of the ERP technique for consumers of such research and those considering the use of human electrophysiology in their own work. My discussion begins with the basics regarding what brain activity ERPs measure and why they are well suited to reveal critical aspects of perceptual processing, attentional selection, and cognition that are unobservable with behavioral methods alone. I then review a number of important methodological issues and often forgotten facts that should be considered when evaluating or planning ERP experiments.
PMCID: PMC3816929  PMID: 21097848
13.  Direct Electrophysiological Measurement of Attentional Templates in Visual Working Memory 
Psychological science  2010;22(2):10.1177/0956797610395395.
PMCID: PMC3816932  PMID: 21193780
14.  Do the Contents of Visual Working Memory Automatically Influence Attentional Selection During Visual Search? 
In many theories of cognition, researchers propose that working memory and perception operate interactively. For example, in previous studies researchers have suggested that sensory inputs matching the contents of working memory will have an automatic advantage in the competition for processing resources. The authors tested this hypothesis by requiring observers to perform a visual search task while concurrently maintaining object representations in visual working memory. The hypothesis that working memory activation produces a simple but uncontrollable bias signal leads to the prediction that items matching the contents of working memory will automatically capture attention. However, no evidence for automatic attentional capture was obtained; instead, the participants avoided attending to these items. Thus, the contents of working memory can be used in a flexible manner for facilitation or inhibition of processing.
PMCID: PMC2048820  PMID: 17469973
attention; working memory; visual search; capture
15.  Homologous mechanisms of visuospatial working memory maintenance in macaque and human: Properties and sources 
The Journal of Neuroscience  2012;32(22):7711-7722.
Although areas of frontal cortex are thought to be critical for maintaining information in visuospatial working memory, the event-related potential (ERP) index of maintenance is found over posterior cortex in humans. In the present study, we reconcile these seemingly contradictory findings. Here we show that macaque monkeys and humans exhibit the same posterior ERP signature of working memory maintenance that predicts the precision of the memory-based behavioral responses. In addition, we show that the specific pattern of rhythmic oscillations in the alpha band, recently demonstrated to underlie the human visual working memory ERP component, is also present in monkeys. Next, we concurrently recorded intracranial local field potentials from two prefrontal and another frontal cortical area to determine their contribution to the surface potential indexing maintenance. The local fields in the two prefrontal areas, but not the cortex immediately posterior, exhibited amplitude modulations, timing, and relationships to behavior indicating that they contribute to the generation of the surface ERP component measured from the distal posterior electrodes. Rhythmic neural activity in the theta and gamma bands during maintenance provided converging support for the engagement of the same brain regions. These findings demonstrate that nonhuman primates have homologous electrophysiological signatures of visuospatial working memory to those of humans and that a distributed neural network, including frontal areas, underlies the posterior ERP index of visuospatial working memory maintenance.
PMCID: PMC3373257  PMID: 22649249
16.  Automatic and strategic effects in the guidance of attention by working memory representations 
Acta psychologica  2010;137(2):217-225.
Theories of visual attention suggest that working memory representations automatically guide attention toward memory-matching objects. Some empirical tests of this prediction have produced results consistent with working memory automatically guiding attention. However, others have shown that individuals can strategically control whether working memory representations guide visual attention. Previous studies have not independently measured automatic and strategic contributions to the interactions between working memory and attention. In this study, we used a classic manipulation of the probability of valid, neutral, and invalid cues to tease apart the nature of such interactions. This framework utilizes measures of reaction time (RT) to quantify the costs and benefits of attending to memory-matching items and infer the relative magnitudes of automatic and strategic effects. We found both costs and benefits even when the memory-matching item was no more likely to be the target than other items, indicating an automatic component of attentional guidance. However, the costs and benefits essentially doubled as the probability of a trial with a valid cue increased from 20% to 80%, demonstrating a potent strategic effect. We also show that the instructions given to participants led to a significant change in guidance distinct from the actual probability of events during the experiment. Together, these findings demonstrate that the influence of working memory representations on attention is driven by both automatic and strategic interactions.
PMCID: PMC2991492  PMID: 20643386
attention; working memory; cuing; automaticity; strategic control; PsychINFO classification 2346
17.  Event-related potentials elicited by errors during the stop-signal task. I: Macaque monkeys 
The error-related negativity (ERN) and positivity (Pe) are components of event-related potential (ERP) waveforms recorded from humans that are thought to reflect performance monitoring. Error-related signals have also been found in single-neuron responses and local-field potentials recorded in supplementary eye field and anterior cingulate cortex of macaque monkeys. However, the homology of these neural signals across species remains controversial. Here, we show that monkeys exhibit ERN and Pe components when they commit errors during a saccadic stop-signal task. The voltage distributions and current densities of these components were similar to those found in humans performing the same task. Subsequent analyses show that neither stimulus- nor response-related artifacts accounted for the error-ERPs. This demonstration of macaque homologues of the ERN and Pe forms a keystone in the bridge linking human and nonhuman primate studies on the neural basis of performance monitoring.
PMCID: PMC3241968  PMID: 22049407
18.  Flexibility in Visual Working Memory: Accurate Change Detection in the Face of Irrelevant Variations in Position 
Visual cognition  2012;20(1):1-28.
Many recent studies of visual working memory have used change-detection tasks in which subjects view sequential displays and are asked to report whether they are identical or if one object has changed. A key question is whether the memory system used to perform this task is sufficiently flexible to detect changes in object identity independent of spatial transformations, but previous research has yielded contradictory results. To address this issue, the present study compared standard change-detection tasks with tasks in which the objects varied in size or position between successive arrays. Performance was nearly identical across the standard and transformed tasks unless the task implicitly encouraged spatial encoding. These results resolve the discrepancies in prior studies and demonstrate that the visual working memory system can detect changes in object identity across spatial transformations.
PMCID: PMC3266348  PMID: 22287933
19.  The Cost of Accessing an Object's Feature Stored in Visual Working Memory 
Visual cognition  2011;19(1):1-12.
The effects of accessing or retrieving information held in working memory are poorly understood compared to what we know about the nature of information storage in this limited-capacity memory system. Previous studies of object-based attention have often relied upon memory-demanding tasks, and this work could indicate that accessing a piece of information in visual working memory may have deleterious effects upon the other representations being maintained. In the present study, we tested the hypothesis that accessing a feature of an object represented in visual working memory degrades the representations of the other stored objects’ features. Our findings support this hypothesis and point to important new questions about the nature of effects resulting from accessing information stored in visual working memory.
PMCID: PMC3017355  PMID: 21221413
visual working memory; object-based attention; working memory access
20.  Attentional Templates in Visual Working Memory 
Most theories of attention propose that we maintain attentional templates in visual working memory to control what information is selected. In the present study, we directly tested this proposal by measuring the contralateral-delay activity (CDA) of human event-related potentials (ERPs) during visual search tasks in which the target is cued on each trial. Here we show that the CDA can be used to measure the maintenance of attentional templates in visual working memory while processing complex visual scenes. In addition, this method allowed us to directly observe the shift from working memory to long-term memory representations controlling attention as learning occurred and experience accrued searching for the same target object. Our findings provide definitive support for several critical proposals made in theories of attention, learning, and automaticity.
PMCID: PMC3147306  PMID: 21697381
21.  A Cuing Study of the N2pc Component: An Index of Attentional Deployment to Objects Rather Than Spatial Locations 
Brain research  2009;1297:101-111.
Symbolic visual cues indicating the location of an upcoming target are believed to invoke endogenous shifts of attention to cued locations. In the present study, we investigated how visual attention is shifted during such cuing paradigms by recording event-related potentials (ERPs). We focused on a component known to index lateralized shifts of perceptual attention during visual search tasks, known as the N2pc component. The ERP data show that attention was shifted to a cued location in anticipation of a target shape when the location is marked by a placeholder-object (Experiments 1 and 2). However, when the possible locations were not marked by placeholder objects we found no evidence for an anticipatory shift of attention to the cued location (Experiment 3). These findings indicate that the perceptual-attention mechanism indexed by the N2pc is deployed to objects and not simply locations in space devoid of object structure.
PMCID: PMC2758329  PMID: 19682440
visual attention; attentional cuing; N2pc; event-related potentials; object-based attention
22.  Masked Targets Trigger Event-Related Potentials Indexing Shifts of Attention but Not Error Detection 
Psychophysiology  2010;47(3):410-414.
To carry out tasks with the highest possible efficiency we have developed executive mechanisms which monitor task performance and optimize cognitive processing. It has been hypothesized that these executive mechanisms operate even without conscious awareness to maximize their sensitivity to task-relevant outcomes. To test this hypothesis the present study examined the error-related negativity (ERN), an electrophysiological index of the performance monitoring neural circuitry, during masked visual search. The findings show that representations of target objects that are processed perceptually but not to the level of awareness fail to elicit an ERN despite the ability of these targets to elicit a shift of attention. These findings indicate that the performance monitoring mechanism indexed by the ERN requires target information to be processed to the level of awareness for a mismatch between stimulus and response to be detected.
PMCID: PMC2956465  PMID: 20070578
23.  Where do we store the memory representations that guide attention? 
Journal of Vision  2013;13(3):1.
During the last decade one of the most contentious and heavily studied topics in the attention literature has been the role that working memory representations play in controlling perceptual selection. The hypothesis has been advanced that to have attention select a certain perceptual input from the environment, we only need to represent that item in working memory. Here we summarize the work indicating that the relationship between what representations are maintained in working memory and what perceptual inputs are selected is not so simple. First, it appears that attentional selection is also determined by high-level task goals that mediate the relationship between working memory storage and attentional selection. Second, much of the recent work from our laboratory has focused on the role of long-term memory in controlling attentional selection. We review recent evidence supporting the proposal that working memory representations are critical during the initial configuration of attentional control settings, but that after those settings are established long-term memory representations play an important role in controlling which perceptual inputs are selected by mechanisms of attention.
PMCID: PMC3590103  PMID: 23444390
visual attention; visual working memory; long-term memory; event-related potentials
24.  Microcircuitry of Agranular Frontal Cortex: Testing the Generality of the Canonical Cortical Microcircuit 
The Journal of Neuroscience  2014;34(15):5355-5369.
We investigated whether a frontal area that lacks granular layer IV, supplementary eye field, exhibits features of laminar circuitry similar to those observed in primary sensory areas. We report, for the first time, visually evoked local field potentials (LFPs) and spiking activity recorded simultaneously across all layers of agranular frontal cortex using linear electrode arrays. We calculated current source density from the LFPs and compared the laminar organization of evolving sinks to those reported in sensory areas. Simultaneous, transient synaptic current sinks appeared first in layers III and V followed by more prolonged current sinks in layers I/II and VI. We also found no variation of single- or multi-unit visual response latency across layers, and putative pyramidal neurons and interneurons displayed similar response latencies. Many units exhibited pronounced discharge suppression that was strongest in superficial relative to deep layers. Maximum discharge suppression also occurred later in superficial than in deep layers. These results are discussed in the context of the canonical cortical microcircuit model originally formulated to describe early sensory cortex. The data indicate that agranular cortex resembles sensory areas in certain respects, but the cortical microcircuit is modified in nontrivial ways.
PMCID: PMC3983808  PMID: 24719113
agranular; current source density; laminar; microcircuitry; spike width; supplementary eye field
25.  Can we throw information out of visual working memory and does this leave informational residue in long-term memory? 
Can we entirely erase a temporary memory representation from mind? This question has been addressed in several recent studies that tested the specific hypothesis that a representation can be erased from visual working memory based on a cue that indicated that the representation was no longer necessary for the task. In addition to behavioral results that are consistent with the idea that we can throw information out of visual working memory, recent neurophysiological recordings support this proposal. However, given the infinite capacity of long-term memory, it is unclear whether throwing a representation out of visual working memory really removes its effects on memory entirely. In this paper, we advocate for an approach that examines our ability to erase memory representations from working memory, as well as possible traces that those erased representations leave in long-term memory.
PMCID: PMC3986564  PMID: 24782798
purging; process purity; directed-forgetting; visual working memory; long-term memory

Results 1-25 (31)