Contralateral delay activity tracks object identity information in visual short term memory

Search flavor labels in beverages: An electrophysiological investigation of color-flavor congruency and association strength in visual search

Individuals are apt to link various characteristics of an object or event through different sensory experiences. We conducted two electrophysiological experiments to investigate the effects of color-flavor congruency and association strength on visual search efficiency and the in-depth cognitive mechanisms underlying multisensory processes. Participants were prompted with a flavor label and asked to identify the primed flavor from four beverage bottle images. Experiment 1 focused on color-flavor congruency and noted faster searches for congruent targets than incongruent ones. EEG data exhibited smaller N2, larger P3 and LPC, and increased parietal-occipital midline (POM) alpha power for incongruent targets than congruent ones. Experiment 2 manipulated color-flavor association strength within each flavor. Behavioral findings showed that searches for targets with weak association strength took longer than those with strong association strength. Moreover, time-frequency analysis displayed that the former evoked greater frontal midline (FM) theta power and higher alpha power than the latter. Altogether, our research indicated that (1) color expectations based on prior experience can automatically guide people's attentional selection, (2) the color-flavor congruency and association strength impact the visual search efficiency via distinct pathways, and (3) theta and alpha activities make a pivotal role in unraveling multisensory information processing. These findings shed some light on the intricate cognitive processes involved in crossmodal visual search and the underlying neurocognitive dynamics.

ERPs and alpha oscillations track the encoding and maintenance of object-based representations in visual working memory

When memorizing an integrated object such as a Kanizsa figure, the completion of parts into a coherent whole is attained by grouping processes which render a whole-object representation in visual working memory (VWM). The present study measured event-related potentials (ERPs) and oscillatory amplitudes to track these processes of encoding and representing multiple features of an object in VWM. To this end, a change detection task was performed, which required observers to memorize both the orientations and colors of six "pacman" items while inducing configurations of the pacmen that systematically varied in terms of their grouping strength. The results revealed an effect of object configuration in VWM despite physically constant visual input: change detection for both orientation and color features was more accurate with increased grouping strength. At the electrophysiological level, the lateralized ERPs and alpha activity mirrored this behavioral pattern. Perception of the orientation features gave rise to the encoding of a grouped object as reflected by the amplitudes of the Ppc. The grouped object structure, in turn, modulated attention to both orientation and color features as indicated by the enhanced N1pc and N2pc. Finally, during item retention, the representation of individual objects and the concurrent allocation of attention to these memorized objects were modulated by grouping, as reflected by variations in the CDA amplitude and a concurrent lateralized alpha suppression, respectively. These results indicate that memorizing multiple features of grouped, to-be-integrated objects involves multiple, sequential stages of processing, providing support for a hierarchical model of object representations in VWM.

Threat of shock increases distractor susceptibility during the short-term maintenance of visual information

Elevated arousal in anxiety is thought to affect attention control. To test this, we designed a visual short-term memory (VSTM) task to examine distractor suppression during periods of threat and no-threat. We hypothesized that threat would impair performance when subjects had to filter out large numbers of distractors. The VSTM task required subjects to attend to one array of squares while ignoring a separate array. The number of target and distractor squares varied systematically, with high (four squares) and low (two squares) target and distractor conditions. This study comprised two separate experiments. Experiment 1 used startle responses and white noise as to directly measure threat-induced anxiety. Experiment 2 used BOLD to measure brain responses. For Experiment 1, subjects showed significantly larger startle responses during threat compared to safe period, supporting the validity of the threat manipulation. For Experiment 2, we found that accuracy was affected by threat, such that the distractor load negatively impacted accuracy only in the threat condition. We also found threat-related differences in parietal cortex activity. Overall, these findings suggest that threat affects distractor susceptibility, impairing filtering of distracting information. This effect is possibly mediated by hyperarousal of parietal cortex during threat.

Threat of shock increases distractor susceptibility during the short-term maintenance of visual information

BACKGROUND

Work on anxiety related attention control deficits suggests that elevated arousal impacts the ability to filter out distractors. To test this, we designed a task to look at distractor suppression during periods of threat. We administered trials of a visual short-term memory (VSTM) task, during periods of unpredictable threat, and hypothesized that threat would impair performance during trials where subjects were required to filter out large numbers of distractors.

METHOD

Experiment 1 involved fifteen healthy participants who completed one study visit. They performed four runs of a VSTM task comprising 32 trials each. Participants were presented with an arrow indicating left or right, followed by an array of squares. They were instructed to remember the target side and disregard the distractors on the off-target side. A subsequent target square was shown, and participants indicated whether it matched one of the previously presented target squares. The trial conditions included 50% matches and 50% mismatches, with an equal distribution of left and right targets. The number of target and distractor squares varied systematically, with high (4 squares) and low (2 squares) target and distractor conditions. Trials alternated between periods of safety and threat, with startle responses recorded using electromyography (EMG) following white noise presentations. Experiment 2 involved twenty-seven healthy participants who completed the same VSTM task inside an MRI scanner during a single study visit. The procedure mirrored that of Experiment 1, except for the absence of white noise presentations.

RESULTS

For Experiment 1, subjects showed significantly larger startle responses during threat compared to safe period, supporting the validity of the threat manipulation. However, results suggested that the white noise probes interfered with performance. For Experiment 2, we found that both accuracy was affected by threat, such that distractor load negatively impacted accuracy only in the threat condition.

CONCLUSION

Overall, these findings suggest that threat affects distractor susceptibility during the short-term maintenance of visual information. The presence of threat makes it more difficult to filter out distracting information. We believe that this is related to hyperarousal of parietal cortex, which has been observed during unpredictable threat.

The impact of retro-cue validity on working memory representation: Evidence from electroencephalograms

Visual working memory (VWM) performance can be improved by retrospectively cueing an item. The validity of retro-cues has an impact on the mechanisms underlying the retro-cue effect, but how non-cued representations are handled under different retro-cue validity conditions is not yet clear. Here, we used electroencephalograms to investigate whether retro-cue validity can affect the fate of non-cued representations in VWM. The participants were required to perform a change-detection task using a retro-cue with 80% or 20% validity. Contralateral delay activity and the lateralized alpha power were used to assess memory storage and selective attention, respectively. The retro-cue could redirect selective attention to the cued item under both validity conditions; however, the participants maintained the non-cued representations under the low-validity condition but dropped them from VWM under the high-validity condition. These results suggest that the maintenance of non-cued representations in VWM is affected by the expectation of cue validity and may be partially strategically driven. DATA AVAILABILITY: The datasets generated/analyzed during this study and experimental script have been added to https://osf.io/qtwc9/.

Spatial Organization in Self-Initiated Visual Working Memory

Ample research in visual working memory (VWM) has demonstrated that the memorized items are maintained in integrated spatial configurations, even when the spatial context is task irrelevant. These insights were obtained in studies in which participants were provided with the information they memorized. However, the encoding of provided information is only one aspect of memory. In everyday life, individuals often construct their own memory representations, an aspect of memory we have previously termed self-initiated (SI) working memory. In this study, we employed a SI VWM task in which participants selected the visual targets they memorized. The spatial locations of the targets were task irrelevant. Nevertheless, we were interested to see whether participants would construct spatially structured memory representations, which would suggest that they intended to maintain the visual targets as integrated spatial configurations. The results of two experiments demonstrated that participants constructed spatially structured configurations relative to random displays. Specifically, participants selected visual targets in close spatial proximity and constructed spatial sequences with short distances and fewer path crossings. When asked to construct configurations for a hypothetical competitor in a memory contest, participants disrupted the spatial structure by selecting visual targets that were further apart and by increasing the distances between them, which suggests that these characteristics were under their control. At the end of each experiment, participants provided verbal descriptions of the strategies they used to construct the memory displays. While the spatial structure of the SI memory representations was robust, it was absent from the participants' explicit descriptions, which focused on non-spatial strategies. Participants reported selecting items based, most frequently, on semantic categories and visual features. Taken together, these results demonstrated that participants had access to the metacognitive knowledge on the spatial structure of VWM representations, knowledge they manipulated to construct memory representations that enhanced or disrupted memory performance. While having a profound impact on behavior, this metacognitive knowledge on spatial structure remained implicit, as it was absent from the participants' verbal reports. Viewed from a larger perspective, this study explores how individuals interact with the world by actively structuring their surroundings to maximize cognitive performance.

Contralateral delay activity during temporal order memory

In everyday life, we constantly need to remember the temporal sequence of visual events over short periods of time, for example, when making sense of others' actions or watching a movie. While there is increasing knowledge available on neural mechanisms underlying visual working memory (VWM) regarding the identity and spatial location of objects, less is known about how the brain encodes and retains information on temporal sequences. Here, we investigate whether the contralateral-delay activity (CDA), a well-studied electroencephalographic (EEG) component associated with VWM of object identity, also reflects the encoding and retention of temporal order. In two independent experiments, we presented participants with a sequence of four or six images, followed by a 1 s retention period. Participants judged temporal order by indicating whether a subsequently presented probe image was originally displayed during the first or the second half of the sequence. As a main novel result, we report the emergence of a contralateral negativity already following the presentation of the first item of the sequence, which increases over the course of a trial with every presented item, up to a limit of four items. We further observed no differences in the CDA during the temporal-order task compared to one obtained during a task concerning the spatial location of the presented items. Since the characteristics of the CDA appear to be highly similar between different encoded feature dimensions and increases as additional items are being encoded, we suggest this component might be a general characteristic of various types of VWM.

Set similarity modulates object tracking in dynamic environments

Based on the observation that sports teams rely on colored jerseys to define group membership, we examined how grouping by similarity affected observers' abilities to track a "ball" target passed between 20 colored circle "players" divided into two color "teams" of 10 players each, or five color teams of four players each. Observers were more accurate and exerted less effort (indexed by pupil diameter) when their task was to count the number of times any player gained possession of the ball versus when they had to count only the possessions by a given color team, especially when counting the possessions of one team when players were grouped into fewer teams of more individual members each. Overall, results confirm previous reports of costs for segregating a larger set into smaller subsets and suggest that grouping by similarity facilitates processing at the set level.

Evidence for the beneficial effect of perceptual grouping on visual working memory: an empirical study on illusory contour and a meta-analytic study

The capacity of visual working memory (VWM) is found to be extremely limited. Past research shows that VWM can be facilitated by Gestalt principles of grouping, however, it remains controversial whether factors like the type of Gestalt principles, the characteristics of stimuli and the nature of experimental design could affect the beneficial effect of grouping. In particular, studies have shown that perceptual grouping could improve memory performance for a feature that is relevant for grouping, but it is unclear whether the same improvement exists for a feature that is irrelevant for grouping. In this article, an empirical study and a meta-analytic study were conducted to investigate the effect of perceptual grouping on VWM. In the empirical study, we examined the grouping effect by employing a Kanizsa illusion in which memory items were grouped by illusory contour. We found that the memory performance was improved for the grouped items even though the tested feature was grouping irrelevant, and the improvement was not significantly different from the effect of grouping by physical connectedness or by solid occlusion. In the meta-analytic study, we systematically and quantitatively examined the effect of perceptual grouping on VWM by pulling the results from all eligible studies, and found that the beneficial grouping effect was robust but the magnitude of the effect can be affected by several moderators. Factors like the types of grouping methods, the duration and the layout of the memory display, and the characteristics of the tested feature moderated the grouping effect, whereas whether employing a cue or a verbal suppression task did not. Our study suggests that the underlying mechanism of the grouping benefit may be distinct with regard to grouping relevancy of the to-be-stored feature. The grouping effect on VWM may be independent of attention for a grouping relevant feature, but may rely on attentional prioritization for a grouping irrelevant feature.

The impact of visual working memory capacity on the filtering efficiency of emotional face distractors

Emotional faces can serve as distractors for visual working memory (VWM) tasks. An event-related potential called contralateral delay activity (CDA) can measure the filtering efficiency of face distractors. Previous studies have investigated the influence of VWM capacity on filtering efficiency of simple neutral distractors but not of face distractors. We measured the CDA indicative of emotional face filtering during a VWM task related to facial identity. VWM capacity was measured in a separate colour change detection task, and participants were divided to high- and low-capacity groups. The high-capacity group was able to filter out distractors similarly irrespective of its facial emotion. In contrast, the low-capacity group failed in filtering the neutral and angry face distractors, while the filtering was efficient for the happy face distractors. The results indicate that potentially threatening faces are particularly difficult to filter if VWM capacity is limited.

Gestalt grouping cues can improve filtering performance in visual working memory

As part of filtering irrelevant information from entering visual working memory (VWM) and selecting only the relevant information for further processing the system should first tag the pieces of information as relevant or irrelevant. We manipulated difficulty of tagging items as relevant or irrelevant by applying perceptual grouping cues to investigate if it can improve filtering performance in VWM. Participants performed a change-detection task with three targets, six targets, or three targets and three distractors (filtering condition) in the memory display, and were asked to remember the colors (Experiments 1-2) or the orientations (Experiments 3-5) of the targets and ignore the distractors. In the filtering conditions, either the targets (Experiments 1, 3, and 4) or the distractors (Experiments 2 and 5) formed an illusory object (a Kanizsa triangle), appeared in a triangle-like configuration (grouping by proximity), or appeared at random positions (non-grouping). Grouping the targets improved filtering performance relative to non-grouping. Moreover, the illusory object cue further improved filtering performance beyond a proximity cue, but only when the cue was compatible with the task. When the distractors were grouped, the proximity cue improved filtering performance, and the illusory object cue, despite being a potent grouping cue, failed to improve filtering performance when it was compatible with the task. We suggest that the grouping cues advanced tagging of the grouped items. Yet, when the grouping cue strongly enhanced processing of the distractors, the tagging failed, such that the preliminary process of estimating incoming items led to full processing of the grouped items.

A two-phase model of resource allocation in visual working memory

Two broad theories of visual working memory (VWM) storage have emerged from current research, a discrete slot-based theory and a continuous resource theory. However, neither the discrete slot-based theory or continuous resource theory clearly stipulates how the mental commodity for VWM (discrete slot or continuous resource) is allocated. Allocation may be based on the number of items via stimulus-driven factors, or it may be based on task demands via voluntary control. Previous studies have obtained conflicting results regarding the automaticity versus controllability of such allocation. In the current study, we propose a two-phase allocation model, in which the mental commodity could be allocated only by stimulus-driven factors in the early consolidation phase. However, when there is sufficient time to complete the early phase, allocation can enter the late consolidation phase, where it can be flexibly and voluntarily controlled according to task demands. In an orientation recall task, we instructed participants to store either fewer items at high-precision or more items at low-precision. In 3 experiments, we systematically manipulated memory set size and exposure duration. We did not find an effect of task demands when the set size was high and exposure duration was short. However, when we either decreased the set size or increased the exposure duration, we found a trade-off between the number and precision of VWM representations. These results can be explained by a two-phase model, which can also account for previous conflicting findings in the literature. (PsycINFO Database Record

Neural activity reveals perceptual grouping in working memory

There is extensive evidence that the contralateral delay activity (CDA), a scalp recorded event-related brain potential, provides a reliable index of the number of objects held in visual working memory. Here we present evidence that the CDA not only indexes visual object working memory, but also the number of locations held in spatial working memory. In addition, we demonstrate that the CDA can be predictably modulated by the type of encoding strategy employed. When individual locations were held in working memory, the pattern of CDA modulation mimicked previous findings for visual object working memory. Specifically, CDA amplitude increased monotonically until working memory capacity was reached. However, when participants were instructed to group individual locations to form a constellation, the CDA was prolonged and reached an asymptote at two locations. This result provides neural evidence for the formation of a unitary representation of multiple spatial locations.

Holding Biological Motion in Working Memory: An fMRI Study

Holding biological motion (BM), the movements of animate entities, in working memory (WM) is important to our daily life activities. However, the neural substrates underlying the WM processing of BM remain largely unknown. Employing the functional magnetic resonance imaging (fMRI) technique, the current study directly investigated this issue. We used point-light BM animations as the tested stimuli, and explored the neural substrates involved in encoding and retaining BM information in WM. Participants were required to remember two or four BM stimuli in a change-detection task. We first defined a set of potential brain regions devoted to the BM processing in WM in one experiment. We then conducted the second fMRI experiment, and performed time-course analysis over the pre-defined regions, which allowed us to differentiate the encoding and maintenance phases of WM. The results showed that a set of brain regions were involved in encoding BM into WM, including the middle frontal gyrus, inferior frontal gyrus, superior parietal lobule, inferior parietal lobule, superior temporal sulcus, fusiform gyrus, and middle occipital gyrus. However, only the middle frontal gyrus, inferior frontal gyrus, superior parietal lobule, and inferior parietal lobule were involved in retaining BM into WM. These results suggest that an overlapped network exists between the WM encoding and maintenance for BM; however, retaining BM in WM predominately relies on the mirror neuron system.

Contralateral delay activity tracks the influence of Gestalt grouping principles on active visual working memory representations

Recent studies have demonstrated that factors influencing perception, such as Gestalt grouping cues, can influence the storage of information in visual working memory (VWM). In some cases, stationary cues, such as stimulus similarity, lead to superior VWM performance. However, the neural correlates underlying these benefits to VWM performance remain unclear. One neural index, the contralateral delay activity (CDA), is an event-related potential that shows increased amplitude according to the number of items held in VWM and asymptotes at an individual's VWM capacity limit. Here, we applied the CDA to determine whether previously reported behavioral benefits supplied by similarity, proximity, and uniform connectedness were reflected as a neural savings such that the CDA amplitude was reduced when these cues were present. We implemented VWM change-detection tasks with arrays including similarity and proximity (Experiment 1); uniform connectedness (Experiments 2a and 2b); and similarity/proximity and uniform connectedness (Experiment 3). The results indicated that when there was a behavioral benefit to VWM, this was echoed by a reduction in CDA amplitude, which suggests more efficient processing. However, not all perceptual grouping cues provided a VWM benefit in the same measure (e.g., accuracy) or of the same magnitude. We also found unexpected interactions between cues. We observed a mixed bag of effects, suggesting that these powerful perceptual grouping benefits are not as predictable in VWM. The current findings indicate that when grouping cues produce behavioral benefits, there is a parallel reduction in the neural resources required to maintain grouped items within VWM.

The contralateral delay activity as a neural measure of visual working memory

The contralateral delay activity (CDA) is a negative slow wave sensitive to the number of objects maintained in visual working memory (VWM). In recent years, a growing number of labs started to use the CDA in order to investigate VWM, leading to many fascinating discoveries. Here, we discuss the recent developments and contribution of the CDA in various research fields. Importantly, we report two meta-analyses that unequivocally validate the relationship between the set-size increase in the CDA amplitude and the individual VWM capacity, and between the CDA and filtering efficiency. We further discuss how the CDA was used to study the role of VWM in visual search, multiple object tracking, grouping, binding, and whether VWM capacity allocation is determined by the items' resolution or instead by the number of objects regardless of their complexity. In addition, we report how the CDA has been used to characterize specific VWM deficits in special populations.

The effects of direction similarity in visual working memory: Behavioural and event-related potential studies

Object similarity can improve visual working memory (VWM) performance in the change-detection task, but impair the recognition performance when it occurs at retrieval of VWM in the recognition task. The effect of direction similarity is an issue that has not been well resolved. Furthermore, electrophysiological evidence in support of the mechanisms that underlie the effects of similarity is still scarce. In the current study, we conducted three behavioural experiments to examine the effects of direction similarity on memory performance with regard to both the encoding and retrieval phases of VWM and one event-related potential (ERP) experiment to explore the neural signatures of direction similarity in VWM. Our behavioural studies indicated that direction similarity improved performance when it occurred at the encoding phase but impaired performance when it occurred at the retrieval phase. Moreover, the ERP experiment showed that the amplitude of the contralateral delay activity (CDA) increased with the increasing set size for similar but not dissimilar directions. In addition, the CDA amplitude for similar directions was lower than that for dissimilar directions at set size 2. Taken together, these findings suggest that direction similarity at encoding has a positive effect on VWM performance and at retrieval has a negative effect. Given that VWM capacity depends on information load and the number of objects, the positive effect of similarity may be attributed to reduced information load of memory objects.

Organization principles in visual working memory: Evidence from sequential stimulus display

Although the mechanisms of visual working memory (VWM) have been studied extensively in recent years, the active property of VWM has received less attention. In the current study, we examined how VWM integrates sequentially presented stimuli by focusing on the role of Gestalt principles, which are important organizing principles in perceptual integration. We manipulated the level of Gestalt cues among three or four sequentially presented objects that were memorized. The Gestalt principle could not emerge unless all the objects appeared together. We distinguished two hypotheses: a perception-alike hypothesis and an encoding-specificity hypothesis. The former predicts that the Gestalt cue will play a role in information integration within VWM; the latter predicts that the Gestalt cue will not operate within VWM. In four experiments, we demonstrated that collinearity (Experiment 1) and closure (Experiment 2) cues significantly improved VWM performance, and this facilitation was not affected by the testing manner (Experiment 3) or by adding extra colors to the memorized objects (Experiment 4). Finally, we re-established the Gestalt cue benefit with similarity cues (Experiment 5). These findings together suggest that VWM realizes and uses potential Gestalt principles within the stored representations, supporting a perception-alike hypothesis.

Retrospective cues based on object features improve visual working memory performance in older adults

Research with younger adults has shown that retrospective cues can be used to orient top-down attention toward relevant items in working memory. We examined whether older adults could take advantage of these cues to improve memory performance. Younger and older adults were presented with visual arrays of five colored shapes; during maintenance, participants were presented either with an informative cue based on an object feature (here, object shape or color) that would be probed, or with an uninformative, neutral cue. Although older adults were less accurate overall, both age groups benefited from the presentation of an informative, feature-based cue relative to a neutral cue. Surprisingly, we also observed differences in the effectiveness of shape versus color cues and their effects upon post-cue memory load. These results suggest that older adults can use top-down attention to remove irrelevant items from visual working memory, provided that task-relevant features function as cues.

Precision requirements do not affect the allocation of visual working memory capacity

There has been a debate about whether allocation of visual working memory (VWM) capacity was flexible. One of the key points about this issue is whether complexity has an effect on the capacity, and one of the critical features of complex objects is higher requirements on the encoding precision than simple objects. Thus we investigated the influence of precision requirements on the allocation of VWM capacity resources, by comparing VWM capacity under different levels of sample-test similarity in a change-detection task. If the VWM capacity is limited by a fixed number of items, then the capacity should not be affected by precision requirements; however, if the capacity is allocated flexibly, then precision requirements should influence the capacity. Cowan's K and amplitude of contralateral delay activity (CDA) were used as behavioral and neurophysiological measures of VWM capacity, respectively. Cowan's K for high-precision discrimination was calculated on the basis of the accuracy of a small number of large-change trials inserted into high-precision blocks. This approach avoided the confounder of different test-phase difficulties between the low- and high-precision conditions and controlled for errors during the test phase. The results showed no effect of precision requirements on VWM capacity. However, analysis of the late positive component (LPC) amplitude indicated that higher precision requirements indeed caused more top-down control over VWM retention. These results support the hypothesis that VWM is limited by a fixed number of items.

Visual working memory capacity for color is independent of representation resolution

Background

The relationship between visual working memory (VWM) capacity and resolution of representation have been extensively investigated. Several recent ERP studies using orientation (or arrow) stimuli suggest that there is an inverse relationship between VWM capacity and representation resolution. However, different results have been obtained in studies using color stimuli. This could be due to important differences in the experimental paradigms used in previous studies.

Methodology/Principal Findings

We examined whether the same relationship between capacity and resolution holds for color information. Participants performed a color change detection task while their electroencephalography was recorded. We manipulated representation resolution by asking participants to detect either a salient change (low-resolution) or a subtle change (high-resolution) in color. We used an ERP component known as contralateral delay activity (CDA) to index the amount of information maintained in VWM. The result demonstrated the same pattern for both low- and high-resolution conditions, with no difference between conditions.

Conclusions/Significance

This result suggests that VWM always represents a fixed number of approximately 3–4 colors regardless of the resolution of representation.

More target features in visual working memory leads to poorer search guidance: evidence from contralateral delay activity

The visual-search literature has assumed that the top-down target representation used to guide search resides in visual working memory (VWM). We directly tested this assumption using contralateral delay activity (CDA) to estimate the VWM load imposed by the target representation. In Experiment 1, observers previewed four photorealistic objects and were cued to remember the two objects appearing to the left or right of central fixation; Experiment 2 was identical except that observers previewed two photorealistic objects and were cued to remember one. CDA was measured during a delay following preview offset but before onset of a four-object search array. One of the targets was always present, and observers were asked to make an eye movement to it and press a button. We found lower magnitude CDA on trials when the initial search saccade was directed to the target (strong guidance) compared to when it was not (weak guidance). This difference also tended to be larger shortly before search-display onset and was largely unaffected by VWM item-capacity limits or number of previews. Moreover, the difference between mean strong- and weak-guidance CDA was proportional to the increase in search time between mean strong-and weak-guidance trials (as measured by time-to-target and reaction-time difference scores). Contrary to most search models, our data suggest that trials resulting in the maintenance of more target features results in poor search guidance to a target. We interpret these counterintuitive findings as evidence for strong search guidance using a small set of highly discriminative target features that remain after pruning from a larger set of features, with the load imposed on VWM varying with this feature-consolidation process.

Building blocks of visual working memory: objects or Boolean maps?

The nature of the building blocks of information in visual working memory (VWM) is a fundamental issue that has not been well resolved. Most researchers take objects as the building blocks, although this perspective has received criticism. The objects could be physically separated ones (strict object hypothesis) or hierarchical objects created from separated individuals (broad object hypothesis). Meanwhile, a newly proposed Boolean map theory for visual attention suggests that Boolean maps may be the building blocks of VWM (Boolean map hypothesis); this perspective could explain many critical findings of VWM. However, no previous study has examined these hypotheses. We explored this issue by focusing on a critical point on which they make distinct predictions. We asked participants to remember two distinct objects (2-object), three distinct objects (3-object), or three objects with repeated information (mixed-3-object, e.g., one red bar and two green bars, green bars could be represented as one hierarchical object) and adopted contralateral delay activity (CDA) to tap into the maintenance phase of VWM. The mixed-3-object condition could generate two Boolean maps, three objects, or three objects most of the time (hierarchical objects are created in certain trials, retaining two objects). Simple orientations (Experiment 1) and colors (Experiments 2 and 3) were used as stimuli. Although the CDA of the mixed-3-object condition was slightly lower than that of the 3-object condition, no significant difference was revealed between them. Both conditions displayed significantly higher CDAs than the 2-object condition. These findings support the broad object hypothesis. We further suggest that Boolean maps might be the unit for retrieval/comparison in VWM.

Differential roles for parietal and occipital cortices in visual working memory

Visual working memory (VWM) is known as a highly capacity-limited cognitive system that can hold 3–4 items. Recent studies have demonstrated that activity in the intraparietal sulcus (IPS) and occipital cortices correlates with the number of representations held in VWM. However, differences among those regions are poorly understood, particularly when task-irrelevant items are to be ignored. The present fMRI-based study investigated whether memory load-sensitive regions such as the IPS and occipital cortices respond differently to task-relevant information. Using a change detection task in which participants are required to remember pre-specified targets, here we show that while the IPS exhibited comparable responses to both targets and distractors, the dorsal occipital cortex manifested significantly weaker responses to an array containing distractors than to an array containing only targets, despite that the number of objects presented was the same for the two arrays. These results suggest that parietal and occipital cortices engage differently in distractor processing and that the dorsal occipital, rather than parietal, activity appears to reflect output of stimulus filtering and selection based on behavioral relevance.

Toward the neural mechanisms of reduced working memory capacity in schizophrenia

People with schizophrenia (PSZ) demonstrate reliable reductions in working memory (WM) capacity (i.e., the number of objects that can be held in memory). The present study asked whether WM impairments in PSZ can be explained by the same neural mechanisms that underlie individual differences in WM capacity among healthy individuals. Specifically, we examined event-related potentials in PSZ and healthy matched controls during a change detection task that required the storage of multiple objects in WM. The amplitude of contralateral delay activity (CDA), which correlates with WM capacity in healthy individuals, was larger in controls than in PSZ for memory loads of 3 and 5 objects, but larger in PSZ than in controls for a memory load of 1. This same pattern was found in the subgroups of PSZ and controls with an equivalent WM capacity. Moreover, the increase in CDA amplitude was correlated with individual differences in capacity in controls, but not in PSZ. These results demonstrate that WM impairment in PSZ is not associated with the same patterns of neural activity that characterize low WM capacity in healthy individuals. We propose that WM impairment in PSZ instead reflects a specific impairment in the ability to distribute attention broadly.