Reevaluating the Sensory Account of Visual Working Memory Storage

Sensory recruitment in visual short-term memory: A systematic review and meta-analysis of sensory visual cortex interference using transcranial magnetic stimulation

Sensory visual areas are involved in encoding information in visual short-term memory (VSTM). Yet it remains unclear whether sensory visual cortex is a necessary component of the brain network for maintenance of information in VSTM. Here, we aimed to systematically review studies that have investigated the role of the sensory visual cortex in VSTM using transcranial magnetic stimulation (TMS) and to quantitatively explore these effects using meta-analyses. Fourteen studies were identified and reviewed. Eight studies provided sufficient data for meta-analysis. Two meta-analyses, one regarding the VSTM encoding phase (17 effect sizes) and one regarding the VSTM maintenance phase (15 effect sizes), two meta-regressions (32 effect sizes in each), and one exploratory meta-analysis were conducted. Our results indicate that the sensory visual cortex is similarly involved in both the encoding and maintenance VSTM phase. We suggest that some cases where evidence did not show significant TMS effects was due to low memory or perceptual task demands. Overall, these findings support the idea that sensory visual areas are part of the brain network responsible for successfully maintaining information in VSTM.

State-dependent effects of neural stimulation on brain function and cognition

Invasive and non-invasive brain stimulation methods are widely used in neuroscience to establish causal relationships between distinct brain regions and the sensory, cognitive and motor functions they subserve. When combined with concurrent brain imaging, such stimulation methods can reveal patterns of neuronal activity responsible for regulating simple and complex behaviours at the level of local circuits and across widespread networks. Understanding how fluctuations in physiological states and task demands might influence the effects of brain stimulation on neural activity and behaviour is at the heart of how we use these tools to understand cognition. Here we review the concept of such 'state-dependent' changes in brain activity in response to neural stimulation, and consider examples from research on altered states of consciousness (for example, sleep and anaesthesia) and from task-based manipulations of selective attention and working memory. We relate relevant findings from non-invasive methods used in humans to those obtained from direct electrical and optogenetic stimulation of neuronal ensembles in animal models. Given the widespread use of brain stimulation as a research tool in the laboratory and as a means of augmenting or restoring brain function, consideration of the influence of changing physiological and cognitive states is crucial for increasing the reliability of these interventions.

Attentional Prioritization of Complex, Naturalistic Stimuli Maintained in Working-Memory-A Dot-Probe Event-Related Potentials Study

Recent studies suggest that a stimulus actively maintained in working memory (WM) automatically captures visual attention when subsequently perceived. Such a WM-based guidance effect has been consistently observed for stimuli defined by simple features, such as color or orientation, but studies using more complex stimuli provided inconclusive results. Therefore, we investigated whether the WM-based guidance effect occurs also for naturalistic stimuli, whose identity is defined by multiple features and relations among them, specifically for faces and houses. The experiment consisted of multiple blocks in which participants (N = 28) either memorized or merely saw (WM or exposure condition, respectively) a template stimulus and then performed several dot-probe trials, with pairs of stimuli (template and control) presented laterally as distractors and followed by a target-asterisk. Analysis of reaction-times (RT) in the dot-probe task shows that memorized stimuli were prioritized by attention and points toward attention-hold, rather than capture, as a mechanism of attentional prioritization. Consistent with this interpretation, memorized items did not evoke a lateralized N2pc ERP component, thought to indicate attention shifts. However, in an exploratory ERP analysis we found evidence for a very early (100-200 ms post-stimulus) prioritization specific to the memorized faces, which is in line with the sensory recruitment theory of WM. In conclusion, our data provide evidence that complex stimuli are prioritized by attention when maintained in WM, and that the mechanism of such prioritization is based on a prolonged hold of spatial attention.

Delay activity during visual working memory: A meta-analysis of 30 fMRI experiments

Visual working memory refers to the temporary maintenance and manipulation of task-related visual information. Recent debate on the underlying neural substrates of visual working memory has focused on the delay period of relevant tasks. Persistent neural activity throughout the delay period has been recognized as a correlate of working memory, yet regions demonstrating sustained hemodynamic responses show inconsistency across individual studies. To develop a more precise understanding of delay-period activations during visual working memory, we conducted a coordinate-based meta-analysis on 30 fMRI experiments involving 515 healthy adults with a mean age of 25.65 years. The main analysis revealed a widespread frontoparietal network associated with delay-period activity, as well as activation in the right inferior temporal cortex. These findings were replicated using different meta-analytical algorithms and were shown to be robust against between-study heterogeneity and publication bias. Further meta-analyses on different subgroups of experiments with specific task demands and stimulus types revealed similar delay-period networks, with activations distributed across the frontal and parietal cortices. The roles of prefrontal regions, posterior parietal regions, and inferior temporal areas are reviewed and discussed in the context of content-specific storage. We conclude that cognitive operations that occur during the unfilled delay period in visual working memory tasks can be flexibly expressed across a frontoparietal-temporal network depending on experimental parameters.

An EEG-Based Investigation of the Effect of Perceived Observation on Visual Memory in Virtual Environments

The presence of external observers has been shown to affect performance on cognitive tasks, but the parameters of this impact for different types of tasks and the underlying neural dynamics are less understood. The current study examined the behavioral and brain activity effects of perceived observation on participants’ visual working memory (VWM) in a virtual reality (VR) classroom setting, using the task format as a moderating variable. Participants (n = 21) were equipped with a 57-channel EEG cap, and neural data were collected as they completed two VWM tasks under two observation conditions (observed and not observed) in a within-subjects experimental design. The “observation” condition was operationalized through the addition of a static human avatar in the VR classroom. The avatar’s presence was associated with a significant effect on extending the task response time, but no effect was found on task accuracy. This outcome may have been due to a ceiling effect, as the mean participant task scores were quite high. EEG data analysis supported the behavioral findings by showing consistent differences between the no-observation and observation conditions for one of the VWM tasks only. These neural differences were identified in the dorsolateral prefrontal cortex (dlPFC) and the occipital cortex (OC) regions, with higher theta-band activity occurring in the dlPFC during stimulus encoding and in the OC during response selection when the “observing” avatar was present. These findings provide evidence that perceived observation can inhibit performance during visual tasks by altering attentional focus, even in virtual contexts.

Representation of Color, Form, and their Conjunction across the Human Ventral Visual Pathway

Despite decades of research, our understanding of the relationship between color and form processing in the primate ventral visual pathway remains incomplete. Using fMRI multivoxel pattern analysis, we examined coding of color and form, using a simple form feature (orientation) and a mid-level form feature (curvature), in human ventral visual processing regions. We found that both color and form could be decoded from activity in early visual areas V1 to V4, as well as in the posterior color-selective region and shape-selective regions in ventral and lateral occipitotemporal cortex defined based on their univariate selectivity to color or shape, respectively (the central color region only showed color but not form decoding). Meanwhile, decoding biases towards one feature or the other existed in the color- and shape-selective regions, consistent with their univariate feature selectivity reported in past studies. Additional extensive analyses show that while all these regions contain independent (linearly additive) coding for both features, several early visual regions also encode the conjunction of color and the simple, but not the complex, form feature in a nonlinear, interactive manner. Taken together, the results show that color and form are encoded in a biased distributed and largely independent manner across ventral visual regions in the human brain.

Components of language processing and their long-term and working memory storage in the brain

There is a consensus that the temporal lobes are involved in representing various types of information critical for language processing, including phonological (i.e., speech sound), semantic (meaning), and orthographic (spelling) representations. An important question is whether the same regions that represent our long-term knowledge of phonology, semantics, and orthography are used to support the maintenance of these types of information in working memory (WM) (for instance, maintaining semantic information during sentence comprehension), or whether regions outside the temporal lobes provide the neural basis for WM maintenance in these domains. This review focuses on the issue of whether temporal lobe regions support WM for phonological information, with a brief discussion of related findings in the semantic and orthographic domains. Across all three domains, evidence from lesion-symptom mapping and functional neuroimaging indicates that parietal or frontal regions are critical for supporting WM, with different regions supporting WM in the three domains. The distinct regions in different domains argue against these regions as playing a general attentional role. The findings imply an interaction between the temporal lobe regions housing the long-term memory representations in these domains and the frontal and parietal regions needed to maintain these representations over time.

Multivariate FMRI Signatures of Learning in a Hebb Repetition Paradigm With Tone Sequences

Important information from the environment often arrives to the brain in temporally extended sequences. Language, music, actions, and complex events generally unfold over time. When such informational sequences exceed the limited capacity of working memory, the human brain relies on its ability to accumulate information in long-term memory over several encounters with a complex stimulus. A longstanding question in psychology and neuroscience is whether the neural structures associated with working memory storage—often viewed as capacity limited and temporary—have any builtin ability to store information across longer temporal delays. According to the classic Hebbian dual memory theory, temporally local “activity traces” underlie immediate perception and working memory, whereas “structural traces” undergird long-term learning. Here we examine whether brain structures known to be involved in working maintenance of auditory sequences, such as area Spt, also show evidence of memory persistence across trials. We used representational similarity analysis (RSA) and the Hebb repetition paradigm with supracapacity tonal sequences to test whether repeated sequences have distinguishable multivoxel activity patterns in the auditory-motor networks of the brain. We found that, indeed, area Spt and other nodes of the auditory dorsal stream show multivoxel patterns for tone sequences that become gradually more distinct with repetition during working memory for supracapacity tone-sequences. The findings suggest that the structures are important for working memory are not “blank slates,” wiped clean from moment to moment, but rather encode information in a way can lead to cross-trial persistence.

Decoding Color Visual Working Memory from EEG Signals Using Graph Convolutional Neural Networks

Color has an important role in object recognition and visual working memory (VWM). Decoding color VWM in the human brain is helpful to understand the mechanism of visual cognitive process and evaluate memory ability. Recently, several studies showed that color could be decoded from scalp electroencephalogram (EEG) signals during the encoding stage of VWM, which process visible information with strong neural coding. Whether color could be decoded from other VWM processing stages, especially the maintaining stage which processes invisible information, is still unknown. Here, we constructed an EEG color graph convolutional network model (ECo-GCN) to decode colors during different VWM stages. Based on graph convolutional networks, ECo-GCN considers the graph structure of EEG signals and may be more efficient in color decoding. We found that (1) decoding accuracies for colors during the encoding, early, and late maintaining stages were 81.58%, 79.36%, and 77.06%, respectively, exceeding those during the pre-stimuli stage (67.34%), and (2) the decoding accuracy during maintaining stage could predict participants' memory performance. The results suggest that EEG signals during the maintaining stage may be more sensitive than behavioral measurement to predict the VWM performance of human, and ECo-GCN provides an effective approach to explore human cognitive function.

Shielding working-memory representations from temporally predictable external interference

Protecting working-memory content from distracting external sensory inputs and intervening tasks is an ubiquitous demand in daily life. Here, we ask whether and how temporal expectations about external events can help mitigate effects of such interference during working-memory retention. We manipulated the temporal predictability of interfering items that occurred during the retention period of a visual working-memory task and report that temporal expectations reduce the detrimental influence of external interference on subsequent memory performance. Moreover, to determine if the protective effects of temporal expectations rely on distractor suppression or involve shielding of internal representations, we compared effects after irrelevant distractors that could be ignored vs. interrupters that required a response. Whereas distractor suppression may be sufficient to confer protection from predictable distractors, any benefits after interruption are likely to involve memory shielding. We found similar benefits of temporal expectations after both types of interference. We conclude that temporal expectations may play an important role in safeguarding behaviour based on working memory - acting through mechanisms that include the shielding of internal content from external interference.

Neural Substrates of Visual Perception and Working Memory: Two Sides of the Same Coin or Two Different Coins?

Visual perception occurs when a set of physical signals emanating from the environment enter the visual system and the brain interprets such signals as a percept. Visual working memory occurs when the brain produces and maintains a mental representation of a percept while the physical signals corresponding to that percept are not available. Early studies in humans and non-human primates demonstrated that lesions of the prefrontal cortex impair performance during visual working memory tasks but not during perceptual tasks. These studies attributed a fundamental role in working memory and a lesser role in visual perception to the prefrontal cortex. Indeed, single cell recording studies have found that neurons in the lateral prefrontal cortex of macaques encode working memory representations via persistent firing, validating the results of lesion studies. However, other studies have reported that neurons in some areas of the parietal and temporal lobe-classically associated with visual perception-similarly encode working memory representations via persistent firing. This prompted a line of enquiry about the role of the prefrontal and other associative cortices in working memory and perception. Here, we review evidence from single neuron studies in macaque monkeys examining working memory representations across different areas of the visual hierarchy and link them to studies examining the role of the same areas in visual perception. We conclude that neurons in early visual areas of both ventral (V1-V2-V4) and dorsal (V1-V3-MT) visual pathways of macaques mainly encode perceptual signals. On the other hand, areas downstream from V4 and MT contain subpopulations of neurons that encode both perceptual and/or working memory signals. Differences in cortical architecture (neuronal types, layer composition, and synaptic density and distribution) may be linked to the differential encoding of perceptual and working memory signals between early visual areas and higher association areas.

Intermittent theta burst stimulation over the parietal cortex has a significant neural effect on working memory

The crucial role of the parietal cortex in working memory (WM) storage has been identified by fMRI studies. However, it remains unknown whether repeated parietal intermittent theta‐burst stimulation (iTBS) can improve WM. In this within‐subject randomized controlled study, under the guidance of fMRI‐identified parietal activation in the left hemisphere, 22 healthy adults received real and sham iTBS sessions (five consecutive days, 600 pulses per day for each session) with an interval of 9 months between the two sessions. Electroencephalography signals of each subject before and after both iTBS sessions were collected during a change detection task. Changes in contralateral delay activity (CDA) and K‐score were then calculated to reflect neural and behavioral WM improvement. Repeated‐measures ANOVA suggested that real iTBS increased CDA more than the sham one (p = .011 for iTBS effect). Further analysis showed that this effect was more significant in the left hemisphere than in the right hemisphere (p = .029 for the hemisphere‐by‐iTBS interaction effect). Pearson correlation analyses showed significant correlations for two conditions between CDA changes in the left hemisphere and K score changes (ps <.05). In terms of the behavioral results, significant K score changes after real iTBS were observed for two conditions, but a repeated‐measures ANOVA showed a nonsignificant main effect of iTBS (p = .826). These results indicate that the current iTBS protocol is a promising way to improve WM capability based on the neural indicator (CDA) but further optimization is needed to produce a behavioral effect.

Transformative neural representations support long-term episodic memory

Memory is often conceived as a dynamic process that involves substantial transformations of mental representations. However, the neural mechanisms underlying these transformations and their role in memory formation and retrieval have only started to be elucidated. Combining intracranial EEG recordings with deep neural network models, we provide a detailed picture of the representational transformations from encoding to short-term memory maintenance and long-term memory retrieval that underlie successful episodic memory. We observed substantial representational transformations during encoding. Critically, more pronounced semantic representational formats predicted better subsequent long-term memory, and this effect was mediated by more consistent item-specific representations across encoding events. The representations were further transformed right after stimulus offset, and the representations during long-term memory retrieval were more similar to those during short-term maintenance than during encoding. Our results suggest that memory representations pass through multiple stages of transformations to achieve successful long-term memory formation and recall.

Visual working memory in aphantasia: Retained accuracy and capacity with a different strategy

Visual working memory paradigms involve retaining and manipulating visual information in mind over a period of seconds. Evidence suggests that visual imagery (sensory recruitment) is a strategy used by many to retain visual information during such tasks, leading some researchers to propose that visual imagery and visual working memory may be one and the same. If visual imagery is essential to visual working memory task performance there should be large ramifications for a special population of individuals who do not experience visual imagery, aphantasia. Here we assessed visual working memory task performance in this population using a number of different lab and clinical working memory tasks. We found no differences in capacity limits for visual, general number or spatial working memory for aphantasic individuals compared to controls. Further, aphantasic individuals showed no significant differences in performance on visual components of clinical working memory tests as compared to verbal components. However, there were significant differences in the reported strategies used by aphantasic individuals across all memory tasks. Additionally, aphantasic individual's visual memory accuracy did not demonstrate a significant oblique orientation effect, which is proposed to occur due to sensory recruitment, further supporting their non-visual imagery strategy reports. Taken together these data demonstrate that aphantasic individuals are not impaired on visual working memory tasks, suggesting visual imagery and working memory are not one and the same, with imagery (and sensory recruitment) being just one of the tools that can be used to solve visual working memory tasks.

Sensory Recruitment Revisited: Ipsilateral V1 Involved in Visual Working Memory

The "sensory recruitment hypothesis" posits an essential role of sensory cortices in working memory, beyond the well-accepted frontoparietal areas. Yet, this hypothesis has recently been challenged. In the present study, participants performed a delayed orientation recall task while high-spatial-resolution 3 T functional magnetic resonance imaging (fMRI) signals were measured in posterior cortices. A multivariate inverted encoding model approach was used to decode remembered orientations based on blood oxygen level-dependent fMRI signals from visual cortices during the delay period. We found that not only did activity in the contralateral primary visual cortex (V1) retain high-fidelity representations of the visual stimuli, but activity in the ipsilateral V1 also contained such orientation tuning. Moreover, although the encoded tuning was faded in the contralateral V1 during the late delay period, tuning information in the ipsilateral V1 remained sustained. Furthermore, the ipsilateral representation was presented in secondary visual cortex (V2) as well, but not in other higher-level visual areas. These results thus supported the sensory recruitment hypothesis and extended it to the ipsilateral sensory areas, which indicated the distributed involvement of visual areas in visual working memory.

Shared Representational Formats for Information Maintained in Working Memory and Information Retrieved from Long-Term Memory

Current theories propose that the short-term retention of information in working memory (WM) and the recall of information from long-term memory (LTM) are supported by overlapping neural mechanisms in occipital and parietal cortex. However, the extent of the shared representations between WM and LTM is unclear. We designed a spatial memory task that allowed us to directly compare the representations of remembered spatial information in WM and LTM with carefully matched behavioral response precision between tasks. Using multivariate pattern analyses on functional magnetic resonance imaging data, we show that visual memories were represented in a sensory-like code in both memory tasks across retinotopic regions in occipital and parietal cortex. Regions in lateral parietal cortex also encoded remembered locations in both tasks, but in a format that differed from sensory-evoked activity. These results suggest a striking correspondence in the format of representations maintained in WM and retrieved from LTM across occipital and parietal cortex. On the other hand, we also show that activity patterns in nearly all parietal regions, but not occipital regions, contained information that could discriminate between WM and LTM trials. Our data provide new evidence for theories of memory systems and the representation of mnemonic content.

Synchronization patterns reveal neuronal coding of working memory content

Neuronal oscillations are suggested to play an important role in auditory working memory (WM), but their contribution to content-specific representations has remained unclear. Here, we measure magnetoencephalography during a retro-cueing task with parametric ripple-sound stimuli, which are spectrotemporally similar to speech but resist non-auditory memory strategies. Using machine learning analyses, with rigorous between-subject cross-validation and non-parametric permutation testing, we show that memorized sound content is strongly represented in phase-synchronization patterns between subregions of auditory and frontoparietal cortices. These phase-synchronization patterns predict the memorized sound content steadily across the studied maintenance period. In addition to connectivity-based representations, there are indices of more local, “activity silent” representations in auditory cortices, where the decoding accuracy of WM content significantly increases after task-irrelevant “impulse stimuli.” Our results demonstrate that synchronization patterns across auditory sensory and association areas orchestrate neuronal coding of auditory WM content. This connectivity-based coding scheme could also extend beyond the auditory domain.

Mamashli et al. use machine learning analyses of human magnetoencephalography (MEG) recordings to study “working memory,” maintenance of information in mind over brief periods of time. Their results show that the human brain maintains working memory content in transient functional connectivity patterns across sensory and association areas.

Working memory representations in visual cortex mediate distraction effects

Although the contents of working memory can be decoded from visual cortex activity, these representations may play a limited role if they are not robust to distraction. We used model-based fMRI to estimate the impact of distracting visual tasks on working memory representations in several visual field maps in visual and frontoparietal association cortex. Here, we show distraction causes the fidelity of working memory representations to briefly dip when both the memorandum and distractor are jointly encoded by the population activities. Distraction induces small biases in memory errors which can be predicted by biases in neural decoding in early visual cortex, but not other regions. Although distraction briefly disrupts working memory representations, the widespread redundancy with which working memory information is encoded may protect against catastrophic loss. In early visual cortex, the neural representation of information in working memory and behavioral performance are intertwined, solidifying its importance in visual memory.

The relative roles of visual, parietal, and frontal cortex in working memory have been actively debated. Here, the authors show that distraction impacts visual working memory representations in primary visual areas, indicating that these regions play a key role in the maintenance of working memory.

Understanding occipital and parietal contributions to visual working memory: Commentary on Xu (2020)

In her commentary, Xu (2020) admonishes the reader that "To have a full understanding of the cognitive mechanisms underlying VWM [visual working memory], both behavioral and neural evidence needs to be taken into account. This is a must, and not a choice, for any study that attempts to capture the nature of VWM" (p. 11). Although we don't disagree with this statement, our overall assessment of this commentary is that it, itself, fails to satisfy several "musts" and, consequently, does not pose a serious challenge for the sensory recruitment framework for understanding visual working memory. These "musts" include accurately characterizing the framework being critiqued, not favoring verbal models and intuition at the expense of formal quantitative models, and providing even-handed interpretation of the work of others. We'll conclude with a summary of how the sensory recruitment framework can be incorporated into a broader working model of visual working memory.

Augmenting Frontal Dopamine Tone Enhances Maintenance over Gating Processes in Working Memory

The contents of working memory must be maintained in the face of distraction, but updated when appropriate. To manage these competing demands of stability and flexibility, maintained representations in working memory are complemented by distinct gating mechanisms that selectively transmit information into and out of memory stores. The operations of such dopamine-dependent gating systems in the midbrain and striatum and their complementary dopamine-dependent memory maintenance operations in the cortex may therefore be dissociable. If true, selective increases in cortical dopamine tone should preferentially enhance maintenance over gating mechanisms. To test this hypothesis, tolcapone, a catechol-O-methyltransferase inhibitor that preferentially increases cortical dopamine tone, was administered in a randomized, double-blind, placebo-controlled, within-subject fashion to 49 participants who completed a hierarchical working memory task that varied maintenance and gating demands. Tolcapone improved performance in a condition with higher maintenance requirements and reduced gating demands, reflected in a reduction in the slope of RTs across the distribution. Resting-state fMRI data demonstrated that the degree to which tolcapone improved performance in individual participants correlated with increased connectivity between a region important for stimulus response mappings (left dorsal premotor cortex) and cortical areas implicated in visual working memory, including the intraparietal sulcus and fusiform gyrus. Together, these results provide evidence that augmenting cortical dopamine tone preferentially improves working memory maintenance.

Alterations in neural oscillations related to working memory deficit in temporal lobe epilepsy

OBJECTIVE

Working memory (WM) is critical for higher level cognition, but the underlying neural mechanisms are not fully understood. Impaired WM affects routine daily activities and is observed in patients with temporal lobe epilepsy (TLE). This study investigated neural oscillations associated with different WM phases, to determine the specific neural activity linked with the phases of WM impairment.

METHODS

Patients with TLE (n = 52) and healthy volunteers (n = 35) completed a WM task, during which 34-channel electroencephalogram signals were recorded. Characteristic neural oscillation patterns during each WM phase were compared between the 2 groups.

RESULTS

Patients with TLE showed decreased theta power during the encoding phase of WM, which was associated with reduced accuracy in the WM task. Altered theta power in the frontal region of the brain during the encoding phase was associated with a longer reaction time.

CONCLUSIONS

Alterations in theta oscillation are related to WM impairment in patients with TLE and may serve as an early marker for evaluating WM deficits.

SIGNIFICANCE

This study provides an early marker for evaluating WM deficits in TLE.

Object-based selection in visual working memory

Attentional mechanisms in perception can operate over locations, features, or objects. However, people direct attention not only towards information in the external world, but also to information maintained in working memory. To what extent do perception and memory draw on similar selection properties? Here we examined whether principles of object-based attention can also hold true in visual working memory. Experiment 1 examined whether object structure guides selection independently of spatial distance. In a memory updating task, participants encoded two rectangular bars with colored ends before updating two colors during maintenance. Memory updates were faster for two equidistant colors on the same object than on different objects. Experiment 2 examined whether selection of a single object feature spreads to other features within the same object. Participants memorized two sequentially presented Gabors, and a retro-cue indicated which object and feature dimension (color or orientation) would be most relevant to the memory test. We found stronger effects of object selection than feature selection: accuracy was higher for the uncued feature in the same object than the cued feature in the other object. Together these findings demonstrate effects of object-based attention on visual working memory, at least when object-based representations are encouraged, and suggest shared attentional mechanisms across perception and memory.

Towards a better understanding of information storage in visual working memory

Chota and Van der Stigchel (this issue), Iamshchinina, Christophel, Gayet, and Rademaker (this issue), Lorenc and Sreenivasa (this issue), and Teng and Postle (this issue) each present a commentary regarding Xu (2020) where I conclude that sensory regions are nonessential for the storage of information in visual working memory (VWM). They argue instead that sensory regions are critical to VWM storage. Here I briefly reiterate some of the key evidence against this account, some of which has not been accounted by the four commentaries. I also provide a detailed reanalysis of why the main evidence supporting this account may be problematic. Collectively, existence evidence from human neuroimaging and TMS studies and that from monkey neurophysiology studies does not provide strong support for the sensory storage account of VWM. To form an accurate understanding of the distinctive role each brain region may play in perception and VWM as well as how they may interact to collectively support a VWM task, it is important that we properly survey and evaluate all the available evidence.

Investigating Object Files in Spatial Cueing

In spatial cueing, cues presented at target position (valid condition) can capture visual attention and facilitate responses to the target relative to cues presented away from target position (invalid condition). If cues and targets carry different features, the necessary updating of the object representation from the cue to the target display sometimes counteracts and even reverses facilitation in valid conditions, resulting in an inverted validity effect. Previous studies reached partly divergent conclusions regarding the conditions under which object-file updating occurs, and little is known about the exact nature of the processes involved. Object-file updating has so far been investigated by manipulating cue-target similarities in task-relevant target features, but other features that change between the cue and target displays might also contribute to object-file updating. This study examined the conditions under which object-file updating could counteract validity effects by systematically varying task-relevant (color), response-relevant (identity), and response-irrelevant (orientation) features between cue and target displays. The results illustrate that object-file updating is largely restricted to task-relevant features. In addition, the difficulty of the search task affects the degree to which object-file updating costs interact with spatial cueing.

Does the presence of more features in a bound representation in working memory require extra object-based attention?

Recent studies have examined the role of attention in retaining bound representations in working memory (WM) and found that object-based attention plays a pivotal role. However, no study has investigated whether maintaining bound representations with more features in WM requires extra object-based attention. We investigated this by examining whether a secondary task consuming object-based attention was more disruptive to the maintenance of bindings in WM when more features were stored per object. We instructed participants to memorize three bound representations in a WM task while manipulating the number of features (two vs. three features) contained in each representation. Moreover, we manipulated whether a secondary task consuming object-based attention was interpolated into the maintenance phase of WM. If extra object-based attention was required after the addition of an extra feature in the bound representation, the secondary task would result in a greater disruption of the three- rather than two-featured binding. In two experiments, we found that the added secondary task significantly impaired the binding performance, but the performance of the two- and three-featured bindings was disrupted to the same extent. These results suggest that the presence of more features in a bound representation in WM does not require extra object-based attention.

Decoding Concurrent Representations of Pitch and Location in Auditory Working Memory

Multivariate analyses of hemodynamic signals serve to identify the storage of specific stimulus contents in working memory (WM). Representations of visual stimuli have been demonstrated both in sensory regions and in higher cortical areas. While previous research has typically focused on the WM maintenance of a single content feature, it remains unclear whether two separate features of a single object can be decoded concurrently. Also, much less evidence exists for representations of auditory compared with visual stimulus features. To address these issues, human participants had to memorize both pitch and perceived location of one of two sample sounds. After a delay phase, they were asked to reproduce either pitch or location. At recall, both features showed comparable levels of discriminability. Region of interest (ROI)-based decoding of functional magnetic resonance imaging (fMRI) data during the delay phase revealed feature-selective activity for both pitch and location of a memorized sound in auditory cortex and superior parietal lobule. The latter region showed higher decoding accuracy for location than pitch. In addition, location could be decoded from angular and supramarginal gyrus and both superior and inferior frontal gyrus. The latter region also showed a trend for decoding of pitch. We found no region exclusively coding pitch memory information. In summary, the present study yielded evidence for concurrent representations of pitch and location of a single object both in sensory cortex and in hierarchically higher regions, pointing toward representation formats that enable feature integration within the same anatomic brain regions.SIGNIFICANCE STATEMENT Decoding of hemodynamic signals serves to identify brain regions involved in the storage of stimulus-specific information in working memory (WM). While to-be-remembered information typically consists of several features, most previous investigations have focused on the maintenance of one memorized feature belonging to one visual object. The present study assessed the concurrent storage of two features of the same object in auditory WM. We found that both pitch and location of memorized sounds were decodable both in early sensory areas, in higher-level superior parietal cortex and, to a lesser extent, in inferior frontal cortex. While auditory cortex is known to process different features in parallel, their concurrent representation in parietal regions may support the integration of object features in WM.

Corpus callosum morphology in major mental disorders: a magnetic resonance imaging study

Mental disorders diagnosis is based on specific clinical criteria. However, clinical studies found similarities and overlapping phenomenology across a variety of disorders, which suggests a common neurobiological substrate. Thus, there is a need to measure disease-related neuroanatomical similarities and differences across conditions. While structural alterations of the corpus callosum have been investigated in obsessive-compulsive disorder, schizophrenia, major depressive disorder and bipolar disorder, no study has addressed callosal aberrations in all diseases in a single study. Moreover, results from pairwise comparisons (patients vs. controls) show some inconsistencies, possibly related to the parcellation methods to divide the corpus callosum into subregions. The main aim of the present paper was to uncover highly localized callosal characteristics for each condition (i.e. obsessive-compulsive disorder, schizophrenia, major depressive disorder and bipolar disorder) as compared either to healthy control subjects or to each other. For this purpose, we did not rely on any sub-callosal parcellation method, but applied a well-validated approach measuring callosal thickness at 100 equidistant locations along the whole midline of the corpus callosum. One hundred and twenty patients (30 in each disorder) as well as 30 controls were recruited for the study. All groups were closely matched for age and gender, and the analyses were performed controlling for the impact of antipsychotic treatment and illness duration. There was a significant main effect of group along the whole callosal surface. Pairwise post hoc comparisons revealed that, compared to controls, patients with obsessive-compulsive disorder had the thinnest corpora callosa with significant effects almost on the entire callosal structure. Patients with schizophrenia also showed thinner corpora callosa than controls but effects were confined to the isthmus and the anterior part of the splenium. No significant differences were found in both major depressive disorder and bipolar disorder patients compared to controls. When comparing the disease groups to each other, the corpus callosum was thinner in obsessive-compulsive disorder patients than in any other group. The effect was evident across the entire corpus callosum, with the exception of the posterior body. Altogether, our study suggests that the corpus callosum is highly changed in obsessive-compulsive disorder, selectively changed in schizophrenia and not changed in bipolar disorder and major depressive disorder. These results shed light on callosal similarities and differences among mental disorders providing valuable insights regarding the involvement of the major brain commissural fibre tract in the pathophysiology of each specific mental illness.

The Microstructure of Attentional Control in the Dorsal Attention Network

The top-down control of attention involves command signals arising chiefly in the dorsal attention network (DAN) in frontal and parietal cortex and propagating to sensory cortex to enable the selective processing of incoming stimuli based on their behavioral relevance. Consistent with this view, the DAN is active during preparatory (anticipatory) attention for relevant events and objects, which, in vision, may be defined by different stimulus attributes including their spatial location, color, motion, or form. How this network is organized to support different forms of preparatory attention to different stimulus attributes remains unclear. We propose that, within the DAN, there exist functional microstructures (patterns of activity) specific for controlling attention based on the specific information to be attended. To test this, we contrasted preparatory attention to stimulus location (spatial attention) and to stimulus color (feature attention), and used multivoxel pattern analysis to characterize the corresponding patterns of activity within the DAN. We observed different multivoxel patterns of BOLD activation within the DAN for the control of spatial attention (attending left vs. right) and feature attention (attending red vs. green). These patterns of activity for spatial and feature attentional control showed limited overlap with each other within the DAN. Our findings thus support a model in which the DAN has different functional microstructures for distinctive forms of top-down control of visual attention.

The causal role of auditory cortex in auditory working memory

Working memory (WM), the ability to actively hold information in memory over a delay period of seconds, is a fundamental constituent of cognition. Delay-period activity in sensory cortices has been observed in WM tasks, but whether and when the activity plays a functional role for memory maintenance remains unclear. Here, we investigated the causal role of auditory cortex (AC) for memory maintenance in mice performing an auditory WM task. Electrophysiological recordings revealed that AC neurons were active not only during the presentation of the auditory stimulus but also early in the delay period. Furthermore, optogenetic suppression of neural activity in AC during the stimulus epoch and early delay period impaired WM performance, whereas suppression later in the delay period did not. Thus, AC is essential for information encoding and maintenance in auditory WM task, especially during the early delay period.

Perceptual Learning beyond Perception: Mnemonic Representation in Early Visual Cortex and Intraparietal Sulcus

The ability to discriminate between stimuli relies on a chain of neural operations associated with perception, memory and decision-making. Accumulating studies show learning-dependent plasticity in perception or decision-making, yet whether perceptual learning modifies mnemonic processing remains unclear. Here, we trained human participants of both sexes in an orientation discrimination task, while using functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) to separately examine training-induced changes in working memory (WM) representation. fMRI decoding revealed orientation-specific neural patterns during the delay period in primary visual cortex (V1) before, but not after, training, whereas neurodisruption of V1 during the delay period led to behavioral deficits in both phases. In contrast, both fMRI decoding and disruptive effect of TMS showed that intraparietal sulcus (IPS) represented WM content after, but not before, training. These results suggest that training does not affect the necessity of sensory area in representing WM information, consistent with the sensory recruitment hypothesis in WM, but likely alters the coding format of the stored stimulus in this region. On the other hand, training can render WM content to be maintained in higher-order parietal areas, complementing sensory area to support more robust maintenance of information.SIGNIFICANCE STATEMENT There has been accumulating progresses regarding experience-dependent plasticity in perception or decision-making, yet how perceptual experience moulds mnemonic processing of visual information remains less explored. Here, we provide novel findings that learning-dependent improvement of discriminability accompanies altered WM representation at different cortical levels. Critically, we suggest a role of training in modulating cortical locus of WM representation, providing a plausible explanation to reconcile the discrepant findings between human and animal studies regarding the recruitment of sensory or higher-order areas in WM.

Representation of visual numerosity information during working memory in humans: An fMRI decoding study

Both animal and human studies on numerosity have shown the importance of the parietal cortex for numerosity processing. However, most studies have focused on the perceptual processing of numerosity. Still, it is unclear how and where numerosity information is coded when this information is retained during a working memory delay phase. Such temporal storage could be realized by the same structures as perceptual processes, or be transformed to a more abstract representation, potentially involving prefrontal regions. FMRI decoding studies allow the identification of brain areas that exhibit multi‐voxel activation patterns specific to the content of working memory. Here, we used an assumption‐free searchlight‐decoding approach to test where numerosity‐specific codes can be found during a 12 s retention period. Participants (n = 24) performed a retro‐cue delayed match‐to‐sample task, in which numerosity information was presented as visual dot arrays. We found mnemonic numerosity‐specific activation in the right lateral portion of the intraparietal sulcus; an area well‐known for perceptual processing of numerosity. The applied retro‐cue design dissociated working memory delay activity from perceptual processes and showed that the intraparietal sulcus also maintained working memory representation independent of perception.

Ventromedial Prefrontal Cortex Drives the Prioritization of Self-Associated Stimuli in Working Memory

Humans show a pervasive bias for processing self- over other-related information, including in working memory (WM), where people prioritize the maintenance of self- (over other-) associated cues. To elucidate the neural mechanisms underlying this self-bias, we paired a self- versus other-associated spatial WM task with fMRI and transcranial direct current stimulation (tDCS) of human participants of both sexes. Maintaining self- (over other-) associated cues resulted in enhanced activity in classic WM regions (frontoparietal cortex), and in superior multivoxel pattern decoding of the cue locations from visual cortex. Moreover, ventromedial PFC (VMPFC) displayed enhanced functional connectivity with WM regions during maintenance of self-associated cues, which predicted individuals' behavioral self-prioritization effects. In a follow-up tDCS experiment, we targeted VMPFC with excitatory (anodal), inhibitory (cathodal), or sham tDCS. Cathodal tDCS eliminated the self-prioritization effect. These findings provide strong converging evidence for a causal role of VMPFC in driving self-prioritization effects in WM and provide a unique window into the interaction between social, self-referential processing and high-level cognitive control processes.SIGNIFICANCE STATEMENT People have a strong tendency to attend to self-related stimuli, such as their names. This self-bias extends to the automatic prioritization of arbitrarily self-associated stimuli held in working memory. Since working memory is central to high-level cognition, this bias could influence how we make decisions. It is therefore important to understand the underlying brain mechanisms. Here, we used neuroimaging and noninvasive neurostimulation techniques to show that the source of self-bias in working memory is the ventromedial PFC, which modulates activity in frontoparietal brain regions to produce prioritized representations of self-associated stimuli in sensory cortex. This work thus reveals a brain circuit underlying the socially motivated (self-referential) biasing of high-level cognitive processing.

Distraction in Visual Working Memory: Resistance is Not Futile

Over half a century of research focused on understanding how working memory is capacity constrained has overshadowed the fact that it is also remarkably resistant to interference. Protecting goal-relevant information from distraction is a cornerstone of cognitive function that involves a multifaceted collection of control processes and storage mechanisms. Here, we discuss recent advances in cognitive psychology and neuroscience that have produced new insights into the nature of visual working memory and its ability to resist distraction. We propose that distraction resistance should be an explicit component in any model of working memory and that understanding its behavioral and neural correlates is essential for building a comprehensive understanding of real-world memory function.

Real-world objects are not stored in holistic representations in visual working memory

When storing multiple objects in visual working memory, observers sometimes misattribute perceived features to incorrect locations or objects. These misattributions are called binding errors (or swaps) and have been previously demonstrated mostly in simple objects whose features are easy to encode independently and arbitrarily chosen, like colors and orientations. Here, we tested whether similar swaps can occur with real-world objects, where the connection between features is meaningful rather than arbitrary. In Experiments 1 and 2, observers were simultaneously shown four items from two object categories. Within a category, the two exemplars could be presented in either the same or different states (e.g., open/closed; full/empty). After a delay, both exemplars from one of the categories were probed, and participants had to recognize which exemplar went with which state. We found good memory for state information and exemplar information on their own, but a significant memory decrement for exemplar-state combinations, suggesting that binding was difficult for observers and swap errors occurred even for meaningful real-world objects. In Experiment 3, we used the same task, but in one-half of the trials, the locations of the exemplars were swapped at test. We found that there are more errors in general when the locations of exemplars were swapped. We concluded that the internal features of real-world objects are not perfectly bound in working memory, and location updates impair object and feature representations. Overall, we provide evidence that even real-world objects are not stored in an entirely unitized format in working memory.

Decoding Spatial Versus Non-spatial Processing in Auditory Working Memory

Objective

Research on visual working memory has shown that individual stimulus features are processed in both specialized sensory regions and higher cortical areas. Much less evidence exists for auditory working memory. Here, a main distinction has been proposed between the processing of spatial and non-spatial sound features. Our aim was to examine feature-specific activation patterns in auditory working memory.

Methods

We collected fMRI data while 28 healthy adults performed an auditory delayed match-to-sample task. Stimuli were abstract sounds characterized by both spatial and non-spatial information, i.e., interaural time delay and central frequency, respectively. In separate recording blocks, subjects had to memorize either the spatial or non-spatial feature, which had to be compared with a probe sound presented after a short delay. We performed both univariate and multivariate comparisons between spatial and non-spatial task blocks.

Results

Processing of spatial sound features elicited a higher activity in a small cluster in the superior parietal lobe than did sound pattern processing, whereas there was no significant activation difference for the opposite contrast. The multivariate analysis was applied using a whole-brain searchlight approach to identify feature-selective processing. The task-relevant auditory feature could be decoded from multiple brain regions including the auditory cortex, posterior temporal cortex, middle occipital gyrus, and extended parietal and frontal regions.

Conclusion

In summary, the lack of large univariate activation differences between spatial and non-spatial processing could be attributable to the identical stimulation in both tasks. In contrast, the whole-brain multivariate analysis identified feature-specific activation patterns in widespread cortical regions. This suggests that areas beyond the auditory dorsal and ventral streams contribute to working memory processing of auditory stimulus features.

Maintaining verbal short-term memory representations in non-perceptual parietal regions

Buffer accounts of verbal short-term memory (STM) assume dedicated buffers for maintaining different types of information (e.g., phonological, visual) whereas embedded processes accounts argue against the existence of buffers and claim that STM consists of the activated portion of long-term memory (LTM). We addressed this debate by determining whether STM recruits the same neural substrate as LTM, or whether additional regions are involved in short-term storage. Using fMRI with representational similarity analysis (RSA), we examined the representational correspondence of multi-voxel neural activation patterns with the theoretical predictions for the maintenance of both phonological and semantic codes in STM. We found that during the delay period of a phonological STM task, phonological representations could be decoded in the left supramarginal gyrus (SMG) but not the superior temporal gyrus (STG), a speech processing region, for word stimuli. Whereas the pattern in the SMG was specific to phonology, a different region in the left angular gyrus showed RSA decoding evidence for the retention of either phonological or semantic codes, depending on the task context. Taken together, the results provide clear support for a dedicated buffer account of phonological STM, although evidence for a semantic buffer is equivocal.

Propagation and update of auditory perceptual priors through alpha and theta rhythms

To maintain a continuous and coherent percept over time, the brain makes use of past sensory information to anticipate forthcoming stimuli. We recently showed that auditory experience of the immediate past is propagated through ear-specific reverberations, manifested as rhythmic fluctuations of decision bias at alpha frequencies. Here, we apply the same time-resolved behavioural method to investigate how perceptual performance changes over time under conditions of stimulus expectation and to examine the effect of unexpected events on behaviour. As in our previous study, participants were required to discriminate the ear-of-origin of a brief monaural pure tone embedded in uncorrelated dichotic white noise. We manipulated stimulus expectation by increasing the target probability in one ear to 80%. Consistent with our earlier findings, performance did not remain constant across trials, but varied rhythmically with delay from noise onset. Specifically, decision bias showed a similar oscillation at ~9 Hz, which depended on ear congruency between successive targets. This suggests rhythmic communication of auditory perceptual history occurs early and is not readily influenced by top-down expectations. In addition, we report a novel observation specific to infrequent, unexpected stimuli that gave rise to oscillations in accuracy at ~7.6 Hz one trial after the target occurred in the non-anticipated ear. This new behavioural oscillation may reflect a mechanism for updating the sensory representation once a prediction error has been detected.

Hi-def memories of lo-def scenes

The study of visual memory is typically concerned with an image's content: How well, and with what precision, we can recall which objects, people, or features we have seen in the past. But images also vary in their quality: The same object or scene may appear in an image that is sharp and highly resolved, or it may appear in an image that is blurry and faded. How do we remember those properties? Here six experiments demonstrate a new phenomenon of "vividness extension": a tendency to (mis)remember images as though they are "enhanced" versions of themselves - that is, sharper and higher quality than they actually appeared at the time of encoding. Subjects briefly saw images of scenes that varied in how blurry they were, and then adjusted a new image to be as blurry as the original. Unlike an old photograph that fades and blurs, subjects misremembered scenes as more vivid (i.e., less blurry) than those scenes had actually appeared moments earlier. Follow-up experiments extended this phenomenon to saturation and pixelation - with subjects recalling scenes as more colorful and resolved - and ruled out various forms of response bias. We suggest that memory misrepresents the quality of what we have seen, such that the world is remembered as more vivid than it is.

Rehearsal of tactile working memory: Premotor cortex recruits two dissociable neuronal content representations

Recent working memory (WM) research has focused on identifying brain regions that retain different types of mental content. Only few neuroimaging studies have explored the mechanism of attention-based refreshing, which is a type of rehearsal and is thought to implement the dynamic components of WM allowing for update of WM contents. Here, we took advantage of the distinct coding properties of the superior parietal lobe (SPL), which retains spatial layout information, and the right inferior frontal gyrus (IFG), which retains frequency information of vibrotactile stimuli during tactile WM. In an fMRI delayed match-to-sample task, participants had to internally rehearse sequences of spatial layouts or vibratory frequencies. Our results replicate the dissociation of SPL and IFG for the retention of layout and frequency information in terms of activation differences between conditions. Additionally, we found strong premotor cortex (PMC) activation during rehearsal of either stimulus type. To explore interactions between these regions we used dynamic causal modeling and found that activation within the network was best explained by a model that allows the PMC to drive activity in the SPL and IFG during rehearsal. This effect was content-specific, meaning that the PMC showed stronger influence on the SPL during pattern rehearsal and stronger influence on the IFG during frequency rehearsal. In line with previously established PMC contributions to sequence processing, our results suggest that it acts as a content-independent area that flexibly recruits content-specific regions to bring a WM item into the focus of attention during the rehearsal of tactile stimulus sequences.

The Role of Location-Context Binding in Nonspatial Visual Working Memory

Successful retrieval of an item from visual working memory (VWM) often requires an associated representation of the trial-unique context in which that item was presented. In experiment 1, fMRI of 16 male and female humans replicated a previous dissociation of the effects of manipulating memory load in comparison to the effects of manipulating context binding, by comparing VWM for one oriented line versus for three lines individuated by their location versus for three “heterogeneous” items drawn from different categories (orientation, color, and luminance): delay-period fMRI signal in frontal cortex and intraparietal sulcus (IPS) was sensitive to stimulus homogeneity rather than to memory load per se. Additionally, inspection of behavioral performance revealed a broad range of individual differences in the probability of responses to nontargets (also known as “swap errors”), and a post hoc comparison of high swap-error versus low swap-error groups generated several intriguing results: at recall, high swap-error subjects were seen to represent both the orientation and the location of the probed item less strongly, and with less differentiation from nonprobed items, and delay-period signal in IPS predicted behavioral and neural correlates of context binding at recall. In experiment 2, which was a preregistered replication, the 27 male and female humans were grouped into low and high swap-error groups by median split, and the results were broadly consistent with experiment 1. These results present a neural correlate of swap errors, and suggest that delay-period activity of the IPS may be more important for the operation of context binding than for representation per se of stimulus identity.

Cross-modal involvement of the primary somatosensory cortex in visual working memory: A repetitive TMS study

Recent literature suggests that the primary somatosensory cortex (S1), once thought to be a low-level area only modality-specific, is also involved in higher-level, cross-modal, cognitive functions. In particular, electrophysiological studies have highlighted that the cross-modal activation of this area may also extend to visual Working Memory (WM), being part of a mnemonic network specific for the temporary storage and manipulation of visual information concerning bodies and body-related actions. However, the causal recruitment of S1 in the WM network remains speculation. In the present study, by taking advantage of repetitive Transcranial Magnetic Stimulation (rTMS), we look for causal evidence that S1 is implicated in the retention of visual stimuli that are salient for this cortical area. To this purpose, in a first experiment, high-frequency (10 Hz) rTMS was delivered over S1 of the right hemisphere, and over two control sites, the right lateral occipital cortex (LOC) and the right dorsolateral prefrontal cortex (dlPFC), during the maintenance phase of a high-load delayed match-to-sample task in which body-related visual stimuli (non-symbolic hand gestures) have to be retained. In a second experiment, the specificity of S1 recruitment was deepened by using a version of the delayed match-to-sample task in which visual stimuli depict geometrical shapes (non-body related stimuli). Results show that rTMS perturbation of S1 activity leads to an enhancement of participants' performance that is selective for body-related visual stimuli; instead, the stimulation of the right LOC and dlPFC does not affect the temporary storage of body-related visual stimuli. These findings suggest that S1 may be recruited in visual WM when information to store (and recall) is salient for this area, corroborating models which suggest the existence of a dedicated mnemonic system for body-related information in which also somatosensory cortices play a key role, likely thanks to their cross-modal (visuo-tactile) properties.

Effects of visual short-term memory load and attentional demand on the contrast response function

Visual short-term memory (VSTM) load leads to impaired perception during maintenance. Here, we fitted the contrast response function to psychometric orientation discrimination data while also varying attention demand during maintenance to investigate: (1) whether VSTM load effects on perception are mediated by a modulation of the contrast threshold, consistent with contrast gain accounts, or by the function asymptote (1 lapse rate), consistent with response gain accounts; and (2) whether the VSTM load effects on the contrast response function depend on the availability of attentional resources. We manipulated VSTM load via the number of items in the memory set in a color and location VSTM task and assessed the contrast response function for an orientation discrimination task during maintenance. Attention demand was varied through spatial cuing of the orientation stimulus. Higher VSTM load increased the estimated contrast threshold of the contrast response function without affecting the estimated asymptote, but only when the discrimination task demanded attention. When attentional demand was reduced (in the cued conditions), the VSTM load effects on the contrast threshold were eliminated. The results suggest that VSTM load reduces perceptual sensitivity by increasing contrast thresholds, suggestive of a contrast gain modulation mechanism, as long as the perceptual discrimination task demands attention. These findings support recent claims that attentional resources are shared between perception and VSTM maintenance processes.

Revisit once more the sensory storage account of visual working memory

Recent work has highlighted the role of early visual areas in visual working memory (VWM) storage and put forward a sensory storage account of VWM. Using a distractor interference paradigm, however, we previolsy showed that the contribution of early visual areas to VWM storage may not be essential. Instead, higher cortical regions such as the posterior parietal cortex may play a more significant role in VWM storage. This is consistent with reviews of other available behavioral, neuroimaging and neurophysiology results. Recently, a number of studies brought forward new evidence regarding this debate. Here I review these new pieces of evidence in detail and show that there is still no strong and definitive evidence supporting an essential role of the early visual areas in VWM storage. Instead, converging evidence suggests that early visual areas may contribute to the decision stage of a VWM task by facilitating target and probe comparison. Aside from further clarifying this debate, it is also important to note that whether or not VWM storage uses a sensory code depends on how it is defined, and that behavioral interactions between VWM and perception tasks do not necessarily support the involvement of sensory regions in VWM storage.

Tracking stimulus representation across a 2-back visual working memory task

How does the neural representation of visual working memory content vary with behavioural priority? To address this, we recorded electroencephalography (EEG) while subjects performed a continuous-performance 2-back working memory task with oriented-grating stimuli. We tracked the transition of the neural representation of an item (n) from its initial encoding, to the status of 'unprioritized memory item' (UMI), and back to 'prioritized memory item', with multivariate inverted encoding modelling. Results showed that the representational format was remapped from its initially encoded format into a distinctive 'opposite' representational format when it became a UMI and then mapped back into its initial format when subsequently prioritized in anticipation of its comparison with item n + 2. Thus, contrary to the default assumption that the activity representing an item in working memory might simply get weaker when it is deprioritized, it may be that a process of priority-based remapping helps to protect remembered information when it is not in the focus of attention.

Neuroimaging and the localization of function in visual cognition

Several recent studies have interpreted multivariate evidence for stimulus-specific patterns of activity in parietal and/or frontal cortex as evidence for a representational function in those regions that is qualitatively similar to the representational functions of the visual system. Here we argue that although evidence that a brain system takes on a different configuration for each stimulus in a stimulus set is a necessary property for that system having a role in perceptual representation, such evidence is not specific for this function. Drawing on several recent examples from the recent literature, we offer alternative accounts for understanding stimulus-specificity in parietal and frontal cortex that are consistent with longstanding ideas that activity these regions is best understood as implementing control-related, rather than inherently representational, functions.

Beyond action observation: Neurobehavioral mechanisms of memory for visually perceived bodies and actions

Examining the processing of others' body-related information in the perceivers' brain (action observation) is a key topic in cognitive neuroscience. However, what happens beyond the perceptual stage, when the body is not within view and it is transformed into an associative form that can be stored, updated, and later recalled, remains poorly understood. Here we examine neurobehavioural evidence on the memory processing of visually perceived bodily stimuli (dynamic actions and images of bodies). The reviewed studies indicate that encoding and maintaining bodily stimuli in memory recruits the sensorimotor system. This process arises when bodily stimuli are either recalled through action recognition or reproduction. Interestingly, the memory capacity for these stimuli is rather limited: only 2 or 3 bodily stimuli can be simultaneously held in memory. Moreover, this process is disrupted by increasing concurrent bodily operations; i.e., moving one's body, seeing or memorising additional bodies. Overall, the evidence suggests that the neural circuitry allowing us to move and feel ourselves supports the encoding, retention, and memory recall of others' visually perceived bodies.