Contents Of Working Memory Research Articles

Stimulus-Specific Visual Working Memory Representations in Human Cerebellar Lobule VIIb/VIIIa.

fMRI research has revealed that cerebellar lobule VIIb/VIIIa exhibits load-dependent activity that increases with the number of items held in visual working memory (VWM). However, it remains unclear whether these cerebellar responses reflect processes specific to VWM or more general visual attentional mechanisms. To investigate this question, we examined whether cerebellar activity during the delay period of a VWM task is selective for stimuli held in working memory. A sample of male and female human subjects performed a VWM continuous report task in which they were retroactively cued to remember the direction of motion of moving dot stimuli. Cerebellar lobule VIIb/VIIIa delay-period activation accurately decoded the direction of the remembered stimulus, as did frontal and parietal regions of the dorsal attention network. Arguing against a motor explanation, no other cerebellar area exhibited stimulus specificity, including the oculomotor vermis, a key area associated with eye movement control. Finer-scale analysis revealed that the medial portion of lobule VIIb and to a lesser degree the lateral most portion of lobules VIIb and VIIIa, which exhibit robust resting state connectivity with frontal and parietal regions of the dorsal attention network, encoded the identity of the remembered stimulus, while intermediate portions of lobule VIIb/VIIIa did not. These findings of stimulus-specific coding of VWM within lobule VIIb/VIIIa indicate for the first time that the distributed network responsible for the encoding and maintenance of mnemonic representations extends to the cerebellum.SIGNIFICANCE STATEMENT There is considerable debate concerning where in the brain the contents of visual working memory (VWM) are stored. To date, this literature has primarily focused on the role of regions located within cerebral cortex. There is growing evidence for cerebellar involvement in higher-order cognitive functions including working memory. While the cerebellum has been previously shown to be recruited by VWM paradigms, it is unclear whether any portion of cerebellum actively encodes and maintains mnemonic representations. The present study demonstrates that cerebellar lobule VIIb/VIIIa activity patterns are selective for remembered stimuli and that this selectivity persists in the absence of perceptual input. These findings provide novel evidence for the participation of cerebellar structures in the persistent storage of visual information.

Supplemental Material for Object-Based Attention Prioritizes Working Memory Contents at a Theta Rhythm

Supplemental Material for Object-Based Attention Prioritizes Working Memory Contents at a Theta Rhythm

Linguistic focus guides attention during the encoding and refreshing of Working Memory content

• Linguistic focus increases attention to the focused item during encoding in Working Memory. • The focused item receives more attention during retention if it remains relevant to the task. • When the task demands the sharing of attention, the above advantage disappears. • If prominence is marked non-conventionally, findings on attention allocation replicate. • Thus, focus marking is a grammatical device used to express prominence and guide attention. Focus is a linguistic device that marks a piece of information within an utterance as most relevant, as when emphasis is placed by the speaker on a word using phonological stress, special intonation, or prosodic prominence. The question addressed in the present study is whether the use of linguistic focus is best seen as a means of directing the listener’s attention. We investigated attention allocation on the part of the listener to linguistically focused elements in Working Memory in a series of eye-tracking experiments. We concentrated on two processes: the encoding of the focused element and its retention. Attentional load during encoding was measured by pupil dilation, and attention allocation during retention was estimated from fixations to locations of previously present visual stimuli on a blank screen. It was found that i) more attention was allocated during the processing of sentences with linguistic focus and ii) linguistically focused elements received more attention during memory retention. However, when the task demanded the sharing of attention, the advantage of the focused element during retention disappeared. Further experiments showed that when verbal stimuli whose prominence was not linguistically marked were presented, the patterns of attention allocation associated with linguistic focus during retention replicated. These results lend further support to the claim that linguistic focus is a grammaticalized means of expressing prominence, and as such, functions as an attention capturing device.

Failing to forget? Evidence for both impaired and preserved working memory control in older adults

ABSTRACT Voluntary forgetting is accomplished via top-down control over memory contents. Age-related declines in cognitive control may compromise voluntary forgetting. Using a working-memory variant of a directed forgetting task, we examined age differences in forgetting efficacy by analyzing direct measures of memory accuracy and two indirect measures of retention: proactive interference and semantic distortions. The directed forgetting effect in long-term memory was virtually absent in older adults. Further, compared to young adults, older adults recognized fewer to-be-remembered and more to-be-forgotten items in working memory. However, indirect measures of forgetting efficacy suggest some spared ability to control working memory contents in older adults: Both young and older adult participants exhibited reduced proactive interference for to-be-forgotten words (Experiment 1) and reduced semantic errors to to-be-forgotten list associates (Experiment 2) in working memory. Indirect memory measures of forgetting efficacy can provide a fuller understanding of spared and impaired control processes in older adults.

Flexible Working Memory Through Selective Gating and Attentional Tagging.

Working memory is essential: it serves to guide intelligent behavior of humans and nonhuman primates when task-relevant stimuli are no longer present to the senses. Moreover, complex tasks often require that multiple working memory representations can be flexibly and independently maintained, prioritized, and updated according to changing task demands. Thus far, neural network models of working memory have been unable to offer an integrative account of how such control mechanisms can be acquired in a biologically plausible manner. Here, we present WorkMATe, a neural network architecture that models cognitive control over working memory content and learns the appropriate control operations needed to solve complex working memory tasks. Key components of the model include a gated memory circuit that is controlled by internal actions, encoding sensory information through untrained connections, and a neural circuit that matches sensory inputs to memory content. The network is trained by means of a biologically plausible reinforcement learning rule that relies on attentional feedback and reward prediction errors to guide synaptic updates. We demonstrate that the model successfully acquires policies to solve classical working memory tasks, such as delayed recognition and delayed pro-saccade/anti-saccade tasks. In addition, the model solves much more complex tasks, including the hierarchical 12-AX task or the ABAB ordered recognition task, both of which demand an agent to independently store and updated multiple items separately in memory. Furthermore, the control strategies that the model acquires for these tasks subsequently generalize to new task contexts with novel stimuli, thus bringing symbolic production rule qualities to a neural network architecture. As such, WorkMATe provides a new solution for the neural implementation of flexible memory control.

How the content of visual working memory regulates the priority for access to visual awareness for memoranda with multiple features

How the content of visual working memory regulates the priority for access to visual awareness for memoranda with multiple features

Memory as Tinted Lens: Working Memory Contents Distort Perception

Memory as Tinted Lens: Working Memory Contents Distort Perception

Visual working memory content influences correspondence processes [Research data and description

Visual working memory content influences correspondence processes [Research data and description

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.

Reduced serial dependence suggests deficits in synaptic potentiation in anti-NMDAR encephalitis and schizophrenia

A mechanistic understanding of core cognitive processes, such as working memory, is crucial to addressing psychiatric symptoms in brain disorders. We propose a combined psychophysical and biophysical account of two symptomatologically related diseases, both linked to hypofunctional NMDARs: schizophrenia and autoimmune anti-NMDAR encephalitis. We first quantified shared working memory alterations in a delayed-response task. In both patient groups, we report a markedly reduced influence of previous stimuli on working memory contents, despite preserved memory precision. We then simulated this finding with NMDAR-dependent synaptic alterations in a microcircuit model of prefrontal cortex. Changes in cortical excitation destabilized within-trial memory maintenance and could not account for disrupted serial dependence in working memory. Rather, a quantitative fit between data and simulations supports alterations of an NMDAR-dependent memory mechanism operating on longer timescales, such as short-term potentiation.

The Case of the Cognitive (Opti)miser: Electrophysiological Correlates of Working Memory Maintenance Predict Demand Avoidance.

People are often considered cognitive misers. When given a free choice between two tasks, people tend to choose tasks requiring less cognitive effort. Such demand avoidance (DA) is associated with cognitive control, but it is still not clear to what extent individual differences in cognitive control can account for variations in DA. We sought to elucidate the relation between cognitive control and cognitive effort preferences by investigating the extent to which sustained neural activity in a task requiring cognitive control is correlated with DA. We hypothesized that neural measures of efficient filtering will predict individual variations in demand preferences. To test this hypothesis, we had participants perform a delayed-match-to-sample paradigm with their ERPs recorded, as well as a separate behavioral demand-selection task. We focused on the ERP correlates of cognitive filtering efficiency (CFE)-the ability to ignore task-irrelevant distractors during working memory maintenance-as it manifests in a modulation of the contralateral delay activity, an ERP correlate of cognitive control. As predicted, we found a significant positive correlation between CFE and DA. Individuals with high CFE tended to be significantly more demand avoidant than their low-CFE counterparts. Low-CFE individuals, in comparison, did not form distinct cognitive effort preferences. Overall, our results suggest that cognitive control over the contents of visual working memory contribute to individual differences in the expression of cognitive effort preferences. This further implies that these observed preferences are the product of sensitivity to cognitive task demands.

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.

A functional spiking-neuron model of activity-silent working memory in humans based on calcium-mediated short-term synaptic plasticity.

In this paper, we present a functional spiking-neuron model of human working memory (WM). This model combines neural firing for encoding of information with activity-silent maintenance. While it used to be widely assumed that information in WM is maintained through persistent recurrent activity, recent studies have shown that information can be maintained without persistent firing; instead, information can be stored in activity-silent states. A candidate mechanism underlying this type of storage is short-term synaptic plasticity (STSP), by which the strength of connections between neurons rapidly changes to encode new information. To demonstrate that STSP can lead to functional behavior, we integrated STSP by means of calcium-mediated synaptic facilitation in a large-scale spiking-neuron model and added a decision mechanism. The model was used to simulate a recent study that measured behavior and EEG activity of participants in three delayed-response tasks. In these tasks, one or two visual gratings had to be maintained in WM, and compared to subsequent probes. The original study demonstrated that WM contents and its priority status could be decoded from neural activity elicited by a task-irrelevant stimulus displayed during the activity-silent maintenance period. In support of our model, we show that it can perform these tasks, and that both its behavior as well as its neural representations are in agreement with the human data. We conclude that information in WM can be effectively maintained in activity-silent states by means of calcium-mediated STSP.

The varying nature of semantic effects in working memory

Several studies have demonstrated an influence of semantic knowledge on verbal working memory (WM) performance, such as shown by the observation of semantic relatedness (related vs. unrelated words) and word imageability (high vs. low imageability words) effects in working memory. The present study extends these observations by examining in four experiments the extent to which semantic knowledge can protect WM representations against interference. We assessed immediate serial recall performance for semantically related vs. unrelated word lists and for high vs. low imageability word lists, with memory lists being followed by an interfering task after encoding or not. Results show that semantic relatedness leads to a stronger protective effect against interference than word imageability at the item level. Furthermore, the semantic relatedness had a stronger impact on WM performance than word imageability; this was further supported by a meta-analysis of all relevant studies in the field. These results suggest that inter-item associative semantic knowledge can protect WM content against interference, but less so item-level semantic knowledge. This protective effect may result from between-item recurrent reactivation or from reduced cognitive load via the compression of memoranda into conceptual units, as further supported by a series of computational simulations.

Phase-dependent amplification of working memory content and performance

Successful working memory performance has been related to oscillatory mechanisms operating in low-frequency ranges. Yet, their mechanistic interaction with the distributed neural activity patterns representing the content of the memorized information remains unclear. Here, we record EEG during a working memory retention interval, while a task-irrelevant, high-intensity visual impulse stimulus is presented to boost the read-out of distributed neural activity related to the content held in working memory. Decoding of this activity with a linear classifier reveals significant modulations of classification accuracy by oscillatory phase in the theta/alpha ranges at the moment of impulse presentation. Additionally, behavioral accuracy is highest at the phases showing maximized decoding accuracy. At those phases, behavioral accuracy is higher in trials with the impulse compared to no-impulse trials. This constitutes the first evidence in humans that working memory information is maximized within limited phase ranges, and that phase-selective, sensory impulse stimulation can improve working memory.

Limitations of concurrently representing objects within view and in visual working memory

Representing visibly present stimuli is as limited in capacity as representing invisible stimuli in visual working memory (WM). In this study, we explored whether concurrently representing stimuli within view affects representing objects in visual WM, and if so, whether this effect is modulated by the storage states (active and silent state) of memory contents? In experiment 1, participants were asked to perform the change-detect task in a simultaneous-representing condition in which WM content and the continuously-visible stimuli in view were simultaneously represented, as well as a baseline condition in which only the representations of visual WM content were maintained. The results showed that the representations in visual WM would be impaired when the continuously-visible stimuli in view were concurrently represented, revealed by the reduced CDA amplitude and the lower behavior performance. In experiment 2, a dual-serial retro-cue paradigm was adopted to guide participants to maintain memory items in two different storage states, and the results revealed that simultaneously representing the continuously-visible stimuli and the WM content would only impair the WM representations in the active state. These evidences demonstrated that only the visual WM representations that were maintained in the active state would definitely share the limited resources with the representations of continuously-visible information, and further supported the dissociation between the active state and silent state of visual WM storage.

The spatial orienting of the focus of attention in working memory makes use of inhibition: Evidence by hemispheric asymmetries in posterior alpha oscillations

Retroactive cuing of information after encoding improves working memory performance. However, there is an ongoing debate on the contribution of target enhancement vs. distractor inhibition attentional sub-processes to this behavioral benefit. We investigated the electrophysiological correlates of retroactive attentional orienting by means of oscillatory EEG parameters. In order to disentangle excitatory and inhibitory attentional processes, the to-be-memorized information was presented in a way that posterior hemispheric asymmetries in oscillatory power could be unambiguously linked to lateral target vs. distractor processing. We found an increase of posterior alpha power (8–14 Hz) contralateral to the position of non-cued working memory content and a decrease of alpha power contralateral to cued positions. These effects were insensitive to the number of cued or non-cued items, supporting their relation to the spatial orienting of attention. Importantly, only the alpha power increase contralateral to non-cued positions differed reliably from the asymmetry in a neutral control condition, highlighting the importance of an inhibitory mechanism for the retroactive focusing of attention. Furthermore, the alpha power asymmetries relative to the positions of cued and non-cued items predicted the individual susceptibility to interference by irrelevant information during working memory retrieval. These findings indicate that the retroactive orienting of the focus of attention can bias the mental representations of non-spatial stimulus features stored in working memory and thereby promote a target-oriented retrieval process.

The pupillary light response reflects visual working memory content.

Recent studies have shown that the pupillary light response (PLR) is modulated by higher cognitive functions, presumably through activity in visual sensory brain areas. Here we use the PLR to test the involvement of sensory areas in visual working memory (VWM). In two experiments, participants memorized either bright or dark stimuli. We found that pupils were smaller when a prestimulus cue indicated that a bright stimulus should be memorized; this reflects a covert shift of attention during encoding of items into VWM. Crucially, we obtained the same result with a poststimulus cue, which shows that internal shifts of attention within VWM affect pupil size as well. Strikingly, the effect of VWM content on pupil size was most pronounced immediately after the poststimulus cue, and then dissipated. This suggests that a shift of attention within VWM momentarily activates an "active" memory representation, but that this representation quickly transforms into a "hidden" state that does not rely on sensory areas. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Modulation of the pupillary response by the content of visual working memory

Studies of selective attention during perception have revealed modulation of the pupillary response according to the brightness of task-relevant (attended) vs. -irrelevant (unattended) stimuli within a visual display. As a strong test of top-down modulation of the pupil response by selective attention, we asked whether changes in pupil diameter follow internal shifts of attention to memoranda of visual stimuli of different brightness maintained in working memory, in the absence of any visual stimulation. Across 3 studies, we reveal dilation of the pupil when participants orient attention to the memorandum of a dark grating relative to that of a bright grating. The effect occurs even when the attention-orienting cue is independent of stimulus brightness, and even when stimulus brightness is merely incidental and not required for the working-memory task of judging stimulus orientation. Furthermore, relative dilation and constriction of the pupil occurred dynamically and followed the changing temporal expectation that 1 or the other stimulus would be probed across the retention delay. The results provide surprising and consistent evidence that pupil responses are under top-down control by cognitive factors, even when there is no direct adaptive gain for such modulation, since no visual stimuli were presented or anticipated. The results also strengthen the view of sensory recruitment during working memory, suggesting even activation of sensory receptors. The thought-provoking corollary to our findings is that the pupils provide a reliable measure of what is in the focus of mind, thus giving a different meaning to old proverbs about the eyes being a window to the mind.

Electrophysiological evidence supports the role of sustained visuospatial attention in maintaining visual WM contents

Recent empirical and theoretical work suggests that there is a close relationship between visual working memory (WM) and visuospatial attention. Here, we investigated whether visuospatial attention was involved in maintaining object representations in visual WM. To this end, the alpha lateralization and contralateral delay activity (CDA) were analyzed as neural markers for visuospatial attention and visual WM storage, respectively. In the single-task condition, participants performed a grating change-detection task. To probe the role of visuospatial attention in maintaining WM contents, two color squares were presented above and below the fixation point during the retention interval, which remained visible until the detection display was present. In the dual-task condition, participants were required to maintain lateralized gratings while staring at the center-presented color squares, to detect possible subsequent color change. With this task, sustained visuospatial attention that guided to individual memory representations was disrupted. The behavioral data showed that, the insertion of secondary task significantly deteriorated WM performance. For electrophysiological data, we divided the retention interval into two stages, the early stage and late stage, bounded by the onset of the secondary task. We found that CDA amplitude was lower under the dual-task condition than the single-task condition during the late stage, but not the early stage, and the extent to which CDA reduced tracked the impaired memory performance at the individual level. Also, alpha lateralization only could be observed in the single-task condition of the late stage, and completely disappeared in the dual-task condition, indicating the disruption of visuospatial attention directed to memory representations. Individuals who experienced greater visuospatial attention disruption, as indicated by the alpha lateralization, had lower maintenance-associated neural activity (CDA), and suffered greater impairment of memory performance. These findings confirm that sustained visuospatial attention continues improving visual WM processing after the initial encoding phase, and most likely participates in this process by supporting the maintenance of representations in an active state.