Sustained attention operates via dissociable neural mechanisms across different eccentric locations

In primates, foveal and peripheral vision have distinct neural architectures and functions. However, it has been debated if selective attention operates via the same or different neural mechanisms across eccentricities. We tested these alternative accounts by examining the effects of selective attention on the steady-state visually evoked potential (SSVEP) and the fronto-parietal signal measured via EEG from human subjects performing a sustained visuospatial attention task. With a negligible level of eye movements, both SSVEP and SND exhibited the heterogeneous patterns of attentional modulations across eccentricities. Specifically, the attentional modulations of these signals peaked at the parafoveal locations and such modulations wore off as visual stimuli appeared closer to the fovea or further away towards the periphery. However, with a relatively higher level of eye movements, the heterogeneous patterns of attentional modulations of these neural signals were less robust. These data demonstrate that the top-down influence of covert visuospatial attention on early sensory processing in human cortex depends on eccentricity and the level of saccadic responses. Taken together, the results suggest that sustained visuospatial attention operates differently across different eccentric locations, providing new understanding of how attention augments sensory representations regardless of where the attended stimulus appears.

Sustained attention operates via dissociable neural mechanisms across different eccentric locations

In primates, foveal and peripheral vision have distinct neural architectures and functions. However, it has been debated if selective attention operates via the same or different neural mechanisms across eccentricities. We tested these alternative accounts by examining the effects of selective attention on the steady-state visually evoked potential (SSVEP) and the fronto-parietal signal measured via EEG from human subjects performing a sustained visuospatial attention task. With a negligible level of eye movements, both SSVEP and SND exhibited the heterogeneous patterns of attentional modulations across eccentricities. Specifically, the attentional modulations of these signals peaked at the parafoveal locations and such modulations wore off as visual stimuli appeared closer to the fovea or further away towards the periphery. However, with a relatively higher level of eye movements, the heterogeneous patterns of attentional modulations of these neural signals were less robust. These data demonstrate that the top-down influence of covert visuospatial attention on early sensory processing in human cortex depends on eccentricity and the level of saccadic responses. Taken together, the results suggest that sustained visuospatial attention operates differently across different eccentric locations, providing new understanding of how attention augments sensory representations regardless of where the attended stimulus appears.

Dissociable Neural Mechanisms Underlie the Effects of Attention on Visual Appearance and Response Bias

A prominent theoretical framework spanning philosophy, psychology, and neuroscience holds that selective attention penetrates early stages of perceptual processing to alter the subjective visual experience of behaviorally relevant stimuli. For example, searching for a red apple at the grocery store might make the relevant color appear brighter and more saturated compared with seeing the exact same red apple while searching for a yellow banana. In contrast, recent proposals argue that data supporting attention-related changes in appearance reflect decision- and motor-level response biases without concurrent changes in perceptual experience. Here, we tested these accounts by evaluating attentional modulations of EEG responses recorded from male and female human subjects while they compared the perceived contrast of attended and unattended visual stimuli rendered at different levels of physical contrast. We found that attention enhanced the amplitude of the P1 component, an early evoked potential measured over visual cortex. A linking model based on signal detection theory suggests that response gain modulations of the P1 component track attention-induced changes in perceived contrast as measured with behavior. In contrast, attentional cues induced changes in the baseline amplitude of posterior alpha band oscillations (∼9-12 Hz), an effect that best accounts for cue-induced response biases, particularly when no stimuli are presented or when competing stimuli are similar and decisional uncertainty is high. The observation of dissociable neural markers that are linked to changes in subjective appearance and response bias supports a more unified theoretical account and demonstrates an approach to isolate subjective aspects of selective information processing.SIGNIFICANCE STATEMENT Does attention alter visual appearance, or does it simply induce response bias? In the present study, we examined these competing accounts using EEG and linking models based on signal detection theory. We found that response gain modulations of the visually evoked P1 component best accounted for attention-induced changes in visual appearance. In contrast, cue-induced baseline shifts in alpha band activity better explained response biases. Together, these results suggest that attention concurrently impacts visual appearance and response bias, and that these processes can be experimentally isolated.

Feedforward attentional selection in sensory cortex

Salient objects grab attention because they stand out from their surroundings. Whether this phenomenon is accomplished by bottom-up sensory processing or requires top-down guidance is debated. We tested these alternative hypotheses by measuring how early and in which cortical layer(s) neural spiking distinguished a target from a distractor. We measured synaptic and spiking activity across cortical columns in mid-level area V4 of male macaque monkeys performing visual search for a color singleton. A neural signature of attentional capture was observed in the earliest response in the input layer 4. The magnitude of this response predicted response time and accuracy. Errant behavior followed errant selection. Because this response preceded top-down influences and arose in the cortical layer not targeted by top-down connections, these findings demonstrate that feedforward activation of sensory cortex can underlie attentional priority.

Statistical learning speeds visual search: More efficient selection, or faster response?

Learning statistical regularities of target objects speeds visual search performance. However, we do not yet know whether this statistical learning effect is driven by biasing attentional selection at the early perceptual stage of processing, as theories of attention propose, or by improving the decision-making efficiency at a late response-related stage. Leveraging the high-temporal resolution of the event-related potential (ERP) technique, we had 16 human observers perform a visual search task where we inserted a fine-grained statistical regularity that the target shapes appeared in different colors with six unique probabilities. Observers unintentionally learned these regularities such that they were faster to report targets that appeared in more likely target colors. The observers' ERPs showed that this learning effect resulted in subjects making faster decisions about the target presence, and not by preferentially shifting attention to more rapidly select likely target colors, as is often assumed by the attentional selection account, supporting a post-selection account for statistical learning of the probabilistic regularities of target features. These results provide fundamental insights into the attentional control mechanisms of statistical learning. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Even affective changes induced by the global health crisis are insufficient to perturb the hyper-stability of visual long-term memory

Past studies of emotion and mood on memory have mostly focused on the learning of emotional material in the laboratory or on the consequences of a punctate catastrophic event. However, the influence of a long-lasting global condition on memory and learning has not been studied. The COVID-19 pandemic unfortunately offered a unique situation to observe the effects of prolonged, negative events on human memory for visual information. One thousand online subjects were asked to remember the details of real-world photographs of objects to enable fine-grained visual discriminations from novel within-category foils. Visual memory performance was invariant across time, regardless of the infection rate in the local or national population, or the subjects’ self-reported affective state using the Positive and Negative Affect Schedule (PANAS). Thus, visual memory provides the human brain with storage that is particularly resilient to changes in emotional state, even when those changes are experienced for months longer than any imaginable laboratory procedure.

Resolving the mesoscopic missing link: Biophysical modeling of EEG from cortical columns in primates

Event-related potentials (ERP) are among the most widely measured indices for studying human cognition. While their timing and magnitude provide valuable insights, their usefulness is limited by our understanding of their neural generators at the circuit level. Inverse source localization offers insights into such generators, but their solutions are not unique. To address this problem, scientists have assumed the source space generating such signals comprises a set of discrete equivalent current dipoles, representing the activity of small cortical regions. Based on this notion, theoretical studies have employed forward modeling of scalp potentials to understand how changes in circuit-level dynamics translate into macroscopic ERPs. However, experimental validation is lacking because it requires in vivo measurements of intracranial brain sources. Laminar local field potentials (LFP) offer a mechanism for estimating intracranial current sources. Yet, a theoretical link between LFPs and intracranial brain sources is missing. Here, we present a forward modeling approach for estimating mesoscopic intracranial brain sources from LFPs and predict their contribution to macroscopic ERPs. We evaluate the accuracy of this LFP-based representation of brain sources utilizing synthetic laminar neurophysiological measurements and then demonstrate the power of the approach in vivo to clarify the source of a representative cognitive ERP component. To that end, LFP was measured across the cortical layers of visual area V4 in macaque monkeys performing an attention demanding task. We show that area V4 generates dipoles through layer-specific transsynaptic currents that biophysically recapitulate the ERP component through the detailed forward modeling. The constraints imposed on EEG production by this method also revealed an important dissociation between computational and biophysical contributors. As such, this approach represents an important bridge between laminar microcircuitry, through the mesoscopic activity of cortical columns to the patterns of EEG we measure at the scalp.

Laminar microcircuitry of visual cortex producing attention-associated electric fields

Cognitive operations are widely studied by measuring electric fields through EEG and ECoG. However, despite their widespread use, the neural circuitry giving rise to these signals remains unknown because the functional architecture of cortical columns producing attention-associated electric fields has not been explored. Here, we detail the laminar cortical circuitry underlying an attention-associated electric field measured over posterior regions of the brain in humans and monkeys. First, we identified visual cortical area V4 as one plausible contributor to this attention-associated electric field through inverse modeling of cranial EEG in macaque monkeys performing a visual attention task. Next, we performed laminar neurophysiological recordings on the prelunate gyrus and identified the electric-field-producing dipoles as synaptic activity in distinct cortical layers of area V4. Specifically, activation in the extragranular layers of cortex resulted in the generation of the attention-associated dipole. Feature selectivity of a given cortical column determined the overall contribution to this electric field. Columns selective for the attended feature contributed more to the electric field than columns selective for a different feature. Last, the laminar profile of synaptic activity generated by V4 was sufficient to produce an attention-associated signal measurable outside of the column. These findings suggest that the top-down recipient cortical layers produce an attention-associated electric field that can be measured extracortically with the relative contribution of each column depending upon the underlying functional architecture.

Does motor noise contaminate estimates of the precision of visual working memory?

The continuous-report task, in which subjects report the color of visual working memory representation by clicking on a color wheel, has become the gold standard for measuring the precision and number of representations stored in visual working memory. This task requires fine motor control, typically with a mouse, but the precision of responses have been interpreted as being entirely due to the precision of the memory representations, without regard to the contribution of noise from the response effectors (i.e., motor control of the hand). Here we tested the seemingly likely possibility that motor noise contaminates our estimates of visual memory representations in the continuous-report task by simply asking subjects to complete the color wheel continuous-report task using either their dominant or non-dominant hand on different blocks of trials. We found that subjects took longer to complete the task with their non-dominant hand, but this did not affect the precision of their responses. Our findings suggest that this commonly used task to study visual memory may be relatively immune from contamination by motor noise at the output stage.

Induced Forgetting Is the Result of True Forgetting, Not Shifts in Decision-making Thresholds

Induced forgetting occurs when accessing an item in memory appears to harm memory representations of categorically related items. However, it is possible that the actual memory representations are unharmed. Instead, people may just change how they make decisions. Specifically, signal detection theory suggests this apparent forgetting may be due to participants shifting their decision criterion. Here, we used behavioral and electrophysiological measures to determine whether induced forgetting is truly due to changes in how items are represented or simply due to a shifting criterion. Participants' behavior and brain activity showed that induced forgetting was due to changes in the strength of the underlying representations, weighing against a criterion shift explanation of induced forgetting.

α-Band activity tracks a two-dimensional spotlight of attention during spatial working memory maintenance

Covert spatial attention is thought to facilitate the maintenance of locations in working memory, and EEG α-band activity (8-12 Hz) is proposed to track the focus of covert attention. Recent work has shown that multivariate patterns of α-band activity track the polar angle of remembered locations relative to fixation. However, a defining feature of covert spatial attention is that it facilitates processing in a specific region of the visual field, and prior work has not determined whether patterns of α-band activity track the two-dimensional (2-D) coordinates of remembered stimuli within a visual hemifield or are instead maximally sensitive to the polar angle of remembered locations around fixation. Here, we used a lateralized spatial estimation task, in which observers remembered the location of one or two target dots presented to one side of fixation, to test this question. By applying a linear discriminant classifier to the topography of α-band activity, we found that we were able to decode the location of remembered stimuli. Critically, model comparison revealed that the pattern of classifier choices observed across remembered positions was best explained by a model assuming that α-band activity tracks the 2-D coordinates of remembered locations rather than a model assuming that α-band activity tracks the polar angle of remembered locations relative to fixation. These results support the hypothesis that this α-band activity is involved in the spotlight of attention, and arises from mid- to lower-level visual areas involved in maintaining spatial locations in working memory.NEW & NOTEWORTHY A substantial body of work has shown that patterns of EEG α-band activity track the angular coordinates of attended and remembered stimuli around fixation, but whether these patterns track the two-dimensional coordinates of stimuli presented within a visual hemifield remains an open question. Here, we demonstrate that α-band activity tracks the two-dimensional coordinates of remembered stimuli within a hemifield, showing that α-band activity reflects a spotlight of attention focused on locations maintained in working memory.

Converging Evidence That Neural Plasticity Underlies Transcranial Direct-Current Stimulation

It is not definitely known how direct-current stimulation causes its long-lasting effects. Here, we tested the hypothesis that the long time course of transcranial direct-current stimulation (tDCS) is because of the electrical field increasing the plasticity of the brain tissue. If this is the case, then we should see tDCS effects when humans need to encode information into long-term memory, but not at other times. We tested this hypothesis by delivering tDCS to the ventral visual stream of human participants during different tasks (i.e., recognition memory vs. visual search) and at different times during a memory task. We found that tDCS improved memory encoding, and the neural correlates thereof, but not retrieval. We also found that tDCS did not change the efficiency of information processing during visual search for a certain target object, a task that does not require the formation of new connections in the brain but instead relies on attention and object recognition mechanisms. Thus, our findings support the hypothesis that direct-current stimulation modulates brain activity by changing the underlying plasticity of the tissue.

Stimulus-induced Alpha Suppression Tracks the Difficulty of Attentional Selection, Not Visual Working Memory Storage

Human alpha-band activity (8-12 Hz) has been proposed to index a variety of mechanisms during visual processing. Here, we distinguished between an account in which alpha suppression indexes selective attention versus an account in which it indexes subsequent working memory storage. We manipulated two aspects of the visual stimuli that perceptual attention is believed to mitigate before working memory storage: the potential interference from distractors and the size of the focus of attention. We found that the magnitude of alpha-band suppression tracked both of these aspects of the visual arrays. Thus, alpha-band activity after stimulus onset is clearly related to how the visual system deploys perceptual attention and appears to be distinct from mechanisms that store target representations in working memory.

What not to look for: Electrophysiological evidence that searchers prefer positive templates

To-be-attended information can be specified either with positive cues (I'll be wearing a blue shirt) or with negative cues (I won't be wearing a red shirt). Numerous experiments have found that positive cues help search more than negative cues. Given that negative cues produce smaller benefits compared to positive cues, it stands to reason that searchers may choose to use positive templates instead of negative templates if given the opportunity. Here, we evaluate this possibility with behavioral measures as well as by directly measuring the formation of positive and negative templates with event-related potentials. Analysis of the contralateral delay activity (CDA) elicited by cues revealed that positive and negative templates relied on working memory to the same extent, even when negative working memory templates could have been circumvented by relying on long-term memories of target colors. Whereas the CDA did not discriminate positive and negative templates, a CNV-like potential did, suggesting cognitive differences between positive and negative templates beyond visual working memory. However, when both positive and negative information were presented in each cue, participants preferred to make use of the positive cues, as indicated by a CDA contralateral to the positive color in negative cue blocks, and a lack of search benefits for positive- and negative-color cues relative to positive-color cues alone. Our results show that searchers elect to selectively encode only positive information into visual working memory when both positive and negative information are available.

Performance Monitoring during Visual Priming

Repetitive performance of single-feature (efficient or pop-out) visual search improves RTs and accuracy. This phenomenon, known as priming of pop-out, has been demonstrated in both humans and macaque monkeys. We investigated the relationship between performance monitoring and priming of pop-out. Neuronal activity in the supplementary eye field (SEF) contributes to performance monitoring and to the generation of performance monitoring signals in the EEG. To determine whether priming depends on performance monitoring, we investigated spiking activity in SEF as well as the concurrent EEG of two monkeys performing a priming of pop-out task. We found that SEF spiking did not modulate with priming. Surprisingly, concurrent EEG did covary with priming. Together, these results suggest that performance monitoring contributes to priming of pop-out. However, this performance monitoring seems not mediated by SEF. This dissociation suggests that EEG indices of performance monitoring arise from multiple, functionally distinct neural generators.

Do we remember templates better so that we can reject distractors better?

Feature Integration Theory proposed that attention shifted between target-like representations in our visual field. However, the nature of the representations that determined what was target like received less specification than the nature of the attention shifts. In recent years, visual search research has focused on the nature of the memory representations that we use to guide our shifts of attention. Sensitive measures of memory quality indicate that the template representations are remembered better than other, merely maintained, memories. Here we tested the hypothesis that we prepare for difficult search tasks by storing a higher fidelity target representation in working memory than we do when preparing for an easy search task. To test this hypothesis, we explicitly tested participants' memory of the target color they searched for (i.e., the attentional template) versus another memory that was not used to guide attention (i.e., an accessory representation) following blocks of searches with easy-to-find targets (i.e., distractors were homogeneously colored) to blocks of searches with hard-to-find targets (i.e., distractors were heterogeneously colored). Although homogeneous-distractor searches required minimal precision for distractor rejection, we found that templates were still remembered better than accessories, just like we found in a heterogeneous-distractor search. As a consequence, we suggest that stronger memories for templates likely reflects the need to decide whether new perceptual inputs match the template, and not an attempt to create a better template representation in anticipation of difficult searches.

The Contralateral Delay Activity Tracks the Sequential Loading of Objects into Visual Working Memory, Unlike Lateralized Alpha Oscillations

Visual working memory temporarily represents a continuous stream of task-relevant objects as we move through our environment performing tasks. Previous work has identified candidate neural mechanisms of visual working memory storage; however, we do not know which of these mechanisms enable the storage of objects as we sequentially encounter them in our environment. Here, we measured the contralateral delay activity (CDA) and lateralized alpha oscillations as human subjects were shown a series of objects that they needed to remember. The amplitude of CDA increased following the presentation of each to-be-remembered object, reaching asymptote at about three to four objects. In contrast, the concurrently measured lateralized alpha power remained constant with each additional object. Our results suggest that the CDA indexes the storage of objects in visual working memory, whereas lateralized alpha suppression indexes the focusing of attention on the to-be-remembered objects.

Electrophysiological and behavioral evidence for attentional up-regulation, but not down-regulation, when encoding pictures into long-term memory

Visual long-term memory allows us to store a virtually infinite amount of visual information (Brady, Konkle, Alvarez, & Oliva in Proceedings of the National Academy of Sciences of the United States of America, 105(38), 14325-14329, 2008; Standing in Quarterly Journal of Experimental Psychology, 25(2), 207-222, 1973). However, our ability to encode new visual information fluctuates from moment to moment. In Experiment 1, we tested the hypothesis that we have voluntary control over these periodic fluctuations in our ability to encode representations into visual long-term memory using a precueing paradigm combined with behavioral and electrophysiological indices of memory encoding. We found that visual memory encoding can be up-regulated, but it was much more difficult, if not impossible, to down-regulate encoding on a trial-by-trial basis. In Experiment 2, we tested the hypothesis that voluntary up-regulation of visual memory encoding for an item incurs a cost to memory encoding of other items by manipulating the cueing probability. Here, we found that, although the cueing benefit was constant for both low (20%) and high (50%) cueing probabilities, the benefit in the high cueing probability condition came with the overall impairment of memory encoding. Taken together, our findings demonstrate that top-down control of visual long-term memory encoding may be primarily to prioritize certain memories, but this prioritization has a cost and should not be overused to avoid its negative consequences.

Localization and Elimination of Attentional Dysfunction in Schizophrenia During Visual Search

Theories of the locus of visual selective attention dysfunction in schizophrenia propose that the deficits arise from either an inability to maintain working memory representations that guide attention, or difficulty focusing lower-level visual attention mechanisms. However, these theoretical accounts neglect the role of long-term memory representations in controlling attention. Here, we show that the control of visual attention is impaired in people with schizophrenia, and that this impairment is driven by an inability to shift top-down attentional control from working memory to long-term memory across practice. Next, we provide converging evidence for the source of attentional impairments in long-term memory by showing that noninvasive electrical stimulation of medial frontal cortex normalizes long-term memory related neural signatures and patients' behavior. Our findings suggest that long-term memory structures may be a source of impaired attentional selection in schizophrenia when visual attention is taxed during the processing of multi-object arrays.

Quantifying the Attentional Impact of Working Memory Matching Targets and Distractors

Various theoretical proposals have been put forward to explain how memory representations control attention during visual search. In this study, we use the first saccade on each trial as away to quantify the attentional impact of multiple types of representations held in working memory. Across two experiments, we found that a search target maintained in working memory was attended over 20 times more frequently than a non-memory-matching distractor. In addition, an item matching an additional object represented in working memory was attended 2 times more frequently than a non-memory matching distractor. These findings show that there is a measurable attentional impact of items maintained in working memory for a future task, however, such representations have a much weaker attentional impact than working memory representations of search targets.

Contralateral delay activity tracks the storage of visually presented letters and words

Electrophysiological studies have demonstrated that the maintenance of items in visual working memory (VWM) is indexed by the contralateral delay activity (CDA), which increases in amplitude as the number of objects to remember increases, plateauing at VWM capacity. Previous work has primarily utilized simple visual items, such as colored squares or picture stimuli. Despite the frequent use of verbal stimuli in seminal investigations of visual attention and memory, it is unknown whether temporary storage of letters and words also elicit a typical load-sensitive CDA. Given their close associations with language and phonological codes, it is possible that participants store these stimuli phonologically, and not visually. Participants completed a standard visual change-detection task while their ERPs were recorded. Experiment 1 compared the CDA elicited by colored squares compared to uppercase consonants, and Experiment 2 compared the CDA elicited by words compared to colored bars. Behavioral accuracy of change detection decreased with increasing set size for colored squares, letters, and words. We found that a capacity-limited CDA was present for colored squares, letters, and word arrays, suggesting that the visual codes for letters and words were maintained in VWM, despite the potential for transfer to verbal working memory. These results suggest that, despite their verbal associations, letters and words elicit the electrophysiological marker of VWM encoding and storage.

Prefrontal Control of Visual Distraction

Avoiding distraction by conspicuous but irrelevant stimuli is critical to accomplishing daily tasks. Regions of prefrontal cortex control attention by enhancing the representation of task-relevant information in sensory cortex, which can be measured in modulation of both single neurons and event-related electrical potentials (ERPs) on the cranial surface [1, 2]. When irrelevant information is particularly conspicuous, it can distract attention and interfere with the selection of behaviorally relevant information. Such distraction can be minimized via top-down control [3-5], but the cognitive and neural mechanisms giving rise to this control over distraction remain uncertain and debated [6-9]. Bridging neurophysiology to electrophysiology, we simultaneously recorded neurons in prefrontal cortex and ERPs over extrastriate visual cortex to track the processing of salient distractors during a visual search task. Critically, when the salient distractor was successfully ignored, but not otherwise, we observed robust suppression of salient distractor representations. Like target selection, the distractor suppression was observed in prefrontal cortex before it appeared over extrastriate cortical areas. Furthermore, all prefrontal neurons that showed suppression of the task-irrelevant distractor also contributed to selecting the target. This suggests a common prefrontal mechanism is responsible for both selecting task-relevant and suppressing task-irrelevant information in sensory cortex. Taken together, our results resolve a long-standing debate over the mechanisms that prevent distraction, and provide the first evidence directly linking suppressed neural firing in prefrontal cortex with surface ERP measures of distractor suppression.

Neural bases of automaticity

Automaticity allows us to perform tasks in a fast, efficient, and effortless manner after sufficient practice. Theories of automaticity propose that across practice processing transitions from being controlled by working memory to being controlled by long-term memory retrieval. Recent event-related potential (ERP) studies have sought to test this prediction, however, these experiments did not use the canonical paradigms used to study automaticity. Specifically, automaticity is typically studied using practice regimes with consistent mapping between targets and distractors and spaced practice with individual targets, features that these previous studies lacked. The aim of the present work was to examine whether the practice-induced shift from working memory to long-term memory inferred from subjects' ERPs is observed under the conditions in which automaticity is traditionally studied. We found that to be the case in 3 experiments, firmly supporting the predictions of theories. In addition, we found that the temporal distribution of practice (massed vs. spaced) modulates the shape of learning curves. The ERP data revealed that the switch to long-term memory is slower for spaced than massed practice, suggesting that memory systems are used in a strategic manner. This finding provides new constraints for theories of learning and automaticity. (PsycINFO Database Record

Personality correlates of individual differences in the recruitment of cognitive mechanisms when rewards are at stake

Individuals differ greatly in their sensitivity to rewards and punishments. In the extreme, these differences are implicated in a range of psychiatric disorders from addiction to depression. However, it is unclear how these differences influence the recruitment of attention, working memory, and long-term memory when responding to potential rewards. Here, we used a rewarded memory-guided visual search task and ERPs to examine the influence of individual differences in self-reported reward/punishment sensitivity, as measured by the Behavioral Inhibition System (BIS)/Behavioral Activation System (BAS) scales, on the recruitment of cognitive mechanisms in conditions of potential reward. Select subscales of the BAS, including the fun seeking and reward responsiveness scales, showed unique relationships with context updating to reward cues and working memory maintenance of potentially rewarded stimuli. In contrast, BIS scores showed unique relationships with deployment of attention at different points in the task. These results suggest that sensitivity to rewards (i.e., BAS) and to punishment (i.e., BIS) may play an important role in the recruitment of specific and distinct cognitive mechanisms in conditions of potential rewards.

Using transcranial direct-current stimulation (tDCS) to understand cognitive processing

Noninvasive brain stimulation methods are becoming increasingly common tools in the kit of the cognitive scientist. In particular, transcranial direct-current stimulation (tDCS) is showing great promise as a tool to causally manipulate the brain and understand how information is processed. The popularity of this method of brain stimulation is based on the fact that it is safe, inexpensive, its effects are long lasting, and you can increase the likelihood that neurons will fire near one electrode and decrease the likelihood that neurons will fire near another. However, this method of manipulating the brain to draw causal inferences is not without complication. Because tDCS methods continue to be refined and are not yet standardized, there are reports in the literature that show some striking inconsistencies. Primary among the complications of the technique is that the tDCS method uses two or more electrodes to pass current and all of these electrodes will have effects on the tissue underneath them. In this tutorial, we will share what we have learned about using tDCS to manipulate how the brain perceives, attends, remembers, and responds to information from our environment. Our goal is to provide a starting point for new users of tDCS and spur discussion of the standardization of methods to enhance replicability.

Dissociations Among Attention, Perception, and Awareness During Object-Substitution Masking

When a visual target object is surrounded by four dots that onset at the same time as the target but remain visible after the target terminates, the four dots dramatically impair target discrimination performance. This phenomenon is called object-substitution masking, reflecting the hypothesis that both the target and the four dots are identified, but the representation of the four dots replaces the representation of the target object before the target can be reported. The present study used the event-related potential technique to demonstrate that a target masked in this manner is identified by the visual system and triggers a shift of attention. However, by the time attention is shifted to the target, only the mask remains visible, leading to impaired behavioral detection performance. These findings support the object-substitution hypothesis and provide new evidence that perception, attention, and awareness can be dissociated.

Distinct neural mechanisms for spatially lateralized and spatially global visual working memory representations

Visual working memory (VWM) allows humans to actively maintain a limited amount of information. Whereas previous electrophysiological studies have found that lateralized event-related potentials (ERPs) track the maintenance of information in VWM, recent imaging experiments have shown that spatially global representations can be read out using the activity across the visual cortex. The goal of the present study was to determine whether both lateralized and spatially global electrophysiological signatures coexist. We first show that it is possible to simultaneously measure lateralized ERPs that track the number of items held in VWM from one visual hemfield and parietooccipital α (8-12 Hz) power over both hemispheres indexing spatially global VWM representations. Next, we replicated our findings and went on to show that this bilateral parietooccipital α power as well as the contralaterally biased ERP correlate of VWM carries a signal that can be used to decode the identity of the representations stored in VWM. Our findings not only unify observations across electrophysiology and imaging techniques but also suggest that ERPs and α-band oscillations index different neural mechanisms that map on to lateralized and spatially global representations, respectively.

Electrical Stimulation of Visual Cortex Can Immediately Improve Spatial Vision

We can improve human vision by correcting the optics of our lenses [1-3]. However, after the eye transduces the light, visual cortex has its own limitations that are challenging to correct [4]. Overcoming these limitations has typically involved innovative training regimes that improve vision across many days [5, 6]. In the present study, we wanted to determine whether it is possible to immediately improve the precision of spatial vision with noninvasive direct-current stimulation. Previous work suggested that visual processing could be modulated with such stimulation [7-9]. However, the short duration and variability of such effects made it seem unlikely that spatial vision could be improved for more than several minutes [7, 10]. Here we show that visual acuity in the parafoveal belt can be immediately improved by delivering noninvasive direct current to visual cortex. Twenty minutes of anodal stimulation improved subjects' vernier acuity by approximately 15% and increased the amplitude of the earliest visually evoked potentials in lockstep with the behavioral effects. When we reversed the orientation of the electric field, we impaired resolution and reduced the amplitude of visually evoked potentials. Next, we found that anodal stimulation improved acuity enough to be measurable with the relatively coarse Snellen test and that subjects with the poorest acuity benefited the most from stimulation. Finally, we found that stimulation-induced acuity improvements were accompanied by changes in contrast sensitivity at high spatial frequencies.

Fractionating Working Memory

In the present study, we required subjects to remember simple objects that were masked to interrupt consolidation and allow us to estimate the rate of information accrual in visual working memory. We compared a consolidation-baseline condition with a consolidation-during-maintenance condition in which subjects needed to remember a set of unmasked items and then were shown to-be-remembered masked items. We hypothesized that if the control processes of consolidation and maintenance are performed by common mechanisms, then consolidation should be less efficient when performed during maintenance than when performed alone. However, we found that an identical amount of information was encoded per unit time in the two conditions. These results indicate that working memory consolidation is not slowed by maintenance and suggest a two-step model of encoding in visual working memory.

Attention's Accelerator

How do people get attention to operate at peak efficiency in high-pressure situations? We tested the hypothesis that the general mechanism that allows this is the maintenance of multiple target representations in working and long-term memory. We recorded subjects' event-related potentials (ERPs) indexing the working memory and long-term memory representations used to control attention while performing visual search. We found that subjects used both types of memories to control attention when they performed the visual search task with a large reward at stake, or when they were cued to respond as fast as possible. However, under normal circumstances, one type of target memory was sufficient for slower task performance. The use of multiple types of memory representations appears to provide converging top-down control of attention, allowing people to step on the attentional accelerator in a variety of high-pressure situations.

Electrophysiological measurement of information flow during visual search

The temporal relationship between different stages of cognitive processing is long debated. This debate is ongoing, primarily because it is often difficult to measure the time course of multiple cognitive processes simultaneously. We employed a manipulation that allowed us to isolate ERP components related to perceptual processing, working memory, and response preparation, and then examined the temporal relationship between these components while observers performed a visual search task. We found that, when response speed and accuracy were equally stressed, our index of perceptual processing ended before both the transfer of information into working memory and response preparation began. However, when we stressed speed over accuracy, response preparation began before the completion of perceptual processing or transfer of information into working memory on trials with the fastest reaction times. These findings show that individuals can control the flow of information transmission between stages, either waiting for perceptual processing to be completed before preparing a response or configuring these stages to overlap in time.

Transient reduction of visual distraction following electrical stimulation of the prefrontal cortex

The ability to overcome distraction is critical to a number of goal-directed behaviors, but information that is not relevant to our goals often captures our attention and distracts us from the task at hand. Neuroimaging work has demonstrated that activity in specific regions of the lateral prefrontal cortex (PFC) is related to the suppression of distracting information, implicating PFC as a critical node in the goal-directed control network. In the current work we asked whether applying transcranial direct-current stimulation (tDCS) to PFC would influence the likelihood of attentional capture by salient, task-irrelevant visual information encountered during visual search. Our results showed that anodal stimulation, relative to sham or cathodal stimulation, led to a transient decrease in attentional capture lasting approximately 15 min after stimulation. This provides causal evidence that PFC is involved in goal-directed control over distraction, and provides a basis for using PFC stimulation as a causal tool to understand deficits in goal-directed control in both neurologically healthy and impaired populations.

Predicting and Improving Recognition Memory Using Multiple Electrophysiological Signals in Real Time

Although people are capable of storing a virtually infinite amount of information in memory, their ability to encode new information is far from perfect. The quality of encoding varies from moment to moment and renders some memories more accessible than others. Here, we were able to forecast the likelihood that a given item will be later recognized by monitoring two dissociable fluctuations of the electroencephalogram during encoding. Next, we identified individual items that were poorly encoded, using our electrophysiological measures in real time, and we successfully improved the efficacy of learning by having participants restudy these items. Thus, our memory forecasts using multiple electrophysiological signals demonstrate the feasibility and the effectiveness of using real-time monitoring of the moment-to-moment fluctuations of the quality of memory encoding to improve learning.

The surprising temporal specificity of direct-current stimulation

As studies increasingly use transcranial direct-current stimulation (tDCS) to manipulate brain activity, surprising results are emerging. Specifically, research combining tDCS with electrophysiology is showing that the long-lasting effects of tDCS can counter-intuitively influence specific neural mechanisms active for as little as 100 ms during the flow of human information processing.

Visualizing Trumps Vision in Training Attention

Mental imagery can have powerful training effects on behavior, but how this occurs is not well understood. Here we show that even a single instance of mental imagery can improve attentional selection of a target more effectively than actually practicing visual search. By recording subjects' brain activity, we found that these imagery-induced training effects were due to perceptual attention being more effectively focused on targets following imagined training. Next, we examined the downside of this potent training by changing the target after several trials of training attention with imagery and found that imagined search resulted in more potent interference than actual practice following these target changes. Finally, we found that proactive interference from task-irrelevant elements in the visual displays appears to underlie the superiority of imagined training relative to actual practice. Our findings demonstrate that visual attention mechanisms can be effectively trained to select target objects in the absence of visual input, and this results in more effective control of attention than practicing the task itself.

Forgetting induced by recognition of visual images

Retrieval-induced forgetting is a phenomenon in which groups of stimuli are initially learned, but then a subset of those stimuli are subsequently remembered via retrieval practice, causing the forgetting of the other initially learned items. This phenomenon has almost exclusively been studied using linguistic stimuli. The goal of the present study was to determine whether our memory for simultaneously learned visual stimuli was subject to a similar type of memory impairment. Participants were shown real-world objects, then they practiced recognizing a subset of these remembered objects, and finally their memory was tested for all learned objects. We found that practicing recognition of a subset of items resulted in forgetting of other objects in the group. However, impaired recognition did not spread to new objects belonging to the same category. Our findings have important implications for how our memories operate in real-world tasks, where remembering one object or aspect of a visual scene can cause us to forget other information encoded at the same time.

Visual working memory gives up attentional control early in learning: ruling out interhemispheric cancellation

Current research suggests that we can watch visual working memory surrender the control of attention early in the process of learning to search for a specific object. This inference is based on the observation that the contralateral delay activity (CDA) rapidly decreases in amplitude across trials when subjects search for the same target object. Here, we tested the alternative explanation that the role of visual working memory does not actually decline across learning, but instead lateralized representations accumulate in both hemispheres across trials and wash out the lateralized CDA. We show that the decline in CDA amplitude occurred even when the target objects were consistently lateralized to a single visual hemifield. Our findings demonstrate that reductions in the amplitude of the CDA during learning are not simply due to the dilution of the CDA from interhemispheric cancellation.

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.