The effects of delay duration on visual working memory for orientation

Dissecting the components of error in analogue report tasks

Over the last two decades, the analogue report task has become a standard method for measuring the fidelity of visual representations across research domains including perception, attention, and memory. Despite its widespread use, there has been no methodical investigation of the different task parameters that might contribute to response variability. To address this gap, we conducted two experiments manipulating components of a typical analogue report test of memory for colour hue. We found that human response errors were independently affected by changes in storage and maintenance requirements of the task, demonstrated by a strong effect of set size even in the absence of a memory delay. In contrast, response variability remained unaffected by physical size of the colour wheel, implying negligible contribution of motor noise to task performance, or by its chroma radius, highlighting non-uniformity of the standard colour space. Comparing analogue report to a matched forced-choice task, we found variation in adjustment criterion made a limited contribution to analogue report variability, becoming meaningful only with low representational noise. Our findings validate the analogue report task as a robust measure of representational fidelity for most purposes, while also quantifying non-representational sources of noise that would limit its reliability in specialized settings.

Aging decreases the precision of visual working memory

OBJECTIVES

As individuals age, cognitive abilities such as working memory (WM), decline. In the current study, we investigated the effect of age on WM, and elucidated sources of errors.

METHOD

A total of 102 healthy individuals, aged 18 to 71, participated in this research. We designed and administered a face-based visual WM task, collecting responses via a graded scale in a delayed match-to-sample reproduction task.

RESULTS

The error of participants increased significantly as they aged. Our analysis revealed a significant age-related rise in the standard deviation of error distribution. However, there was no significant change in uniform probability with age.

CONCLUSION

We found that WM performance declines through the lifespan. Investigating the sources of error, we found that the precision of WM decreased monotonously with age. The results also indicated that the probability of guessing the response as a measure of random response is not affected by age.

Linking Cognitive Integrity to Working Memory Dynamics in the Aging Human Brain

Aging is accompanied by a decline of working memory, an important cognitive capacity that involves stimulus-selective neural activity that persists after stimulus presentation. Here, we unraveled working memory dynamics in older human adults (male and female) including those diagnosed with mild cognitive impairment (MCI) using a combination of behavioral modeling, neuropsychological assessment, and MEG recordings of brain activity. Younger adults (male and female) were studied with behavioral modeling only. Participants performed a visuospatial delayed match-to-sample task under systematic manipulation of the delay and distance between sample and test stimuli. Their behavior (match/nonmatch decisions) was fit with a computational model permitting the dissociation of noise in the internal operations underlying the working memory performance from a strategic decision threshold. Task accuracy decreased with delay duration and sample/test proximity. When sample/test distances were small, older adults committed more false alarms than younger adults. The computational model explained the participants' behavior well. The model parameters reflecting internal noise (not decision threshold) correlated with the precision of stimulus-selective cortical activity measured with MEG during the delay interval. The model uncovered an increase specifically in working memory noise in older compared with younger participants. Furthermore, in the MCI group, but not in the older healthy controls, internal noise correlated with the participants' clinically assessed cognitive integrity. Our results are consistent with the idea that the stability of working memory contents deteriorates in aging, in a manner that is specifically linked to the overall cognitive integrity of individuals diagnosed with MCI.

Representation and computation in visual working memory

The ability to sustain internal representations of the sensory environment beyond immediate perception is a fundamental requirement of cognitive processing. In recent years, debates regarding the capacity and fidelity of the working memory (WM) system have advanced our understanding of the nature of these representations. In particular, there is growing recognition that WM representations are not merely imperfect copies of a perceived object or event. New experimental tools have revealed that observers possess richer information about the uncertainty in their memories and take advantage of environmental regularities to use limited memory resources optimally. Meanwhile, computational models of visuospatial WM formulated at different levels of implementation have converged on common principles relating capacity to variability and uncertainty. Here we review recent research on human WM from a computational perspective, including the neural mechanisms that support it.

A dynamic neural resource model bridges sensory and working memory

Probing memory of a complex visual image within a few hundred milliseconds after its disappearance reveals significantly greater fidelity of recall than if the probe is delayed by as little as a second. Classically interpreted, the former taps into a detailed but rapidly decaying visual sensory or 'iconic' memory (IM), while the latter relies on capacity-limited but comparatively stable visual working memory (VWM). While iconic decay and VWM capacity have been extensively studied independently, currently no single framework quantitatively accounts for the dynamics of memory fidelity over these time scales. Here, we extend a stationary neural population model of VWM with a temporal dimension, incorporating rapid sensory-driven accumulation of activity encoding each visual feature in memory, and a slower accumulation of internal error that causes memorized features to randomly drift over time. Instead of facilitating read-out from an independent sensory store, an early cue benefits recall by lifting the effective limit on VWM signal strength imposed when multiple items compete for representation, allowing memory for the cued item to be supplemented with information from the decaying sensory trace. Empirical measurements of human recall dynamics validate these predictions while excluding alternative model architectures. A key conclusion is that differences in capacity classically thought to distinguish IM and VWM are in fact contingent upon a single resource-limited WM store.

Nabilone Impairs Spatial and Verbal Working Memory in Healthy Volunteers

Background: Memory impairments and psychosis-like experiences can be adverse effects of cannabis use. However, reports on the cognitive impact of cannabis use are not consistent. There are also limited studies on the psychotomimetic effects of cannabinoid compounds to reveal the association between cannabis and psychosis. Therefore, we investigated the effect of acute cannabinoid intoxication on verbal working memory (VWM) and spatial working memory (SWM) following oral doses of the synthetic cannabinoid agonist, nabilone (1-2 mg, oral). We further investigated the effect of nabilone on psychosis-like experiences (schizotypy scores) and associations of schizotypy with VWM and SWM. Methods: Healthy participants (n=28) completed spatial and digit span tasks across different delay conditions (0, 6, 12, and 18 sec) after receiving nabilone (1-2 mg, PO) or placebo in a randomized, double-blind, counterbalanced, crossover manner. A subset of participants completed a short battery of schizotypy measures (n=25). Results: Nabilone impaired VWM (p=0.03, weak effect size η2=0.02) and SWM (p=0.00016, η2=0.08). Nabilone did not significantly change overall schizotypy scores. Schizotypy scores were negatively correlated with working memory (WM) averaged across all delays and both modalities, under placebo (ρ=-0.41, p=0.04). In addition, there were significant negative correlations between occasions of cannabis use and overall WM averaged scores across drug treatments (ρ=-0.49, p=0.007) and under placebo (ρ=-0.45, p=0.004). The results showed that the drug effect in the less frequent cannabis users was more pronounced on the SWM (p<0.01) and VWM (p<0.01), whereas there appeared to be little drug effect in the frequent cannabis users. Conclusion: Low doses of synthetic cannabinoid impaired SWM and VWM, indicating that exogenous activation of the cannabinoid system influences cognitive performance. Further, the results replicated previous findings that schizotypy is correlated with deficits in WM. Clinical Trial Registry Name: Nabilone and caffeine effects on the perceptions of visually, auditory, tactile and multimodal illusions in healthy volunteers. Clinical Trial Registration Number: CT-2018-CTN-02561 (Therapeutic Goods Administration Clinical Trial Registry) and ACTRN12618001292268 (The Australian New Zealand Clinical Trials Registry).

Corvids optimize working memory by categorizing continuous stimuli

Working memory (WM) is a crucial element of the higher cognition of primates and corvid songbirds. Despite its importance, WM has a severely limited capacity and is vulnerable to noise. In primates, attractor dynamics mitigate the effect of noise by discretizing continuous information. Yet, it remains unclear whether similar dynamics are seen in avian brains. Here, we show jackdaws (Corvus monedula) have similar behavioral biases as humans; memories are less precise and more biased as memory demands increase. Model-based analysis reveal discrete attractors are evenly spread across the stimulus space. Altogether, our comparative approach suggests attractor dynamics in primates and corvids mitigate the effect of noise by systematically drifting towards specific attractors. By demonstrating this effect in an evolutionary distant species, our results strengthen attractor dynamics as general, adaptive biological principle to efficiently use WM.

Reference induces biases in late visual processing

How we perceive a visual stimulus can be influenced by its surrounding context. For example, the presence of a reference skews the perception of a similar feature in a stimulus, a phenomenon called reference repulsion. Ongoing research so far remains inconclusive regarding the stage of visual information processing where such repulsion occurs. We examined the influence of a reference on late visual processing. We measured the repulsion effect caused by an orientation reference presented after an orientation ensemble stimulus. The participants' reported orientations were significantly biased away from the post-stimulus reference, displaying typical characteristics of reference repulsion. Moreover, explicit discrimination choices between the reference and the stimulus influenced the magnitudes of repulsion effects, which can be explained by an encoding-decoding model that differentiates the re-weighting of sensory representations in implicit and explicit processes. These results support the notion that reference repulsion may arise at a late decision-related stage of visual processing, where different sensory decoding strategies are employed depending on the specific task.

Attractor dynamics with activity-dependent plasticity capture human working memory across time scales

Most cognitive functions require the brain to maintain immediately preceding stimuli in working memory. Here, using a human working memory task with multiple delays, we test the hypothesis that working memories are stored in a discrete set of stable neuronal activity configurations called attractors. We show that while discrete attractor dynamics can approximate working memory on a single time scale, they fail to generalize across multiple timescales. This failure occurs because at longer delay intervals the responses contain more information about the stimuli than can be stored in a discrete attractor model. We present a modeling approach that combines discrete attractor dynamics with activity-dependent plasticity. This model successfully generalizes across all timescales and correctly predicts intertrial interactions. Thus, our findings suggest that discrete attractor dynamics are insufficient to model working memory and that activity-dependent plasticity improves durability of information storage in attractor systems.

Assessing the interaction between working memory and perception through time

Content maintained in visual working memory changes concurrent visual processing, suggesting that visual working memory may recruit an overlapping neural representation with visual perception. However, it remains unclear whether visual working memory representations persist as a sensory code through time, or are recoded later into an abstract code. Here, we directly contrasted a temporal decay + visual code account and a temporal decay + abstract code account within the temporal dynamics of the interaction between working memory and perception. By manipulating the ISI (inter-stimulus interval) between working memory encoding and a perceptual discrimination task, we found that task-relevant and therefore actively maintained perceptual information parametrically altered participants' ability to discriminate perceptual stimuli even 4 s after encoding, whereas task-irrelevant information caused only an acutely transient effect. While continuously present, the size of this shift in discrimination thresholds gradually decreased over time. Concomitantly, the size of the bias in working memory reports increased over time. The opposing directions of threshold and bias effects are consistent with the local maintenance of information in perceptual areas, explained by a temporal decay + visual code account. As the maintained representation decays over time, its ability to alter incoming perceptual signals decreases (reduced threshold effects) while its likelihood of being impacted by those same signals increases (increased bias effects). Altogether, these results suggest that the readout of working memory relies on a sensory representation at a cost of increased interference by ongoing perception.

The effects of caffeine and d-amphetamine on spatial span task in healthy participants

Studies that examined the effect of amphetamine or caffeine on spatial working memory (SWM) and verbal working memory (VWM) have used various tasks. However, there are no studies that have used spatial span tasks (SSTs) to assess the SWM effect of amphetamine and caffeine, although some studies have used digit span tasks (DST) to assess VWM. Previous reports also showed that increasing dopamine increases psychosis-like experiences (PLE, or schizotypy) scores which are in turn negatively associated with WM performance in people with high schizotypy and people with schizophrenia. Therefore, the present study aimed to examine the influence of d-amphetamine (0.45 mg/kg, PO), a dopamine releasing stimulant, on SST, DST, and on PLE in healthy volunteers. In a separate study, we examined the effect of caffeine, a nonspecific adenosine receptor antagonist with stimulant properties, on similar tasks.

METHODS

Healthy participants (N = 40) took part in two randomized, double-blind, counter-balanced placebo-controlled cross-over pilot studies: The first group (N = 20) with d-amphetamine (0.45 mg/kg, PO) and the second group (N = 20) with caffeine (200 mg, PO). Spatial span and digit span were examined under four delay conditions (0, 2, 4, 8 s). PLE were assessed using several scales measuring various aspects of psychosis and schizotypy.

RESULTS

We failed to find an effect of d-amphetamine or caffeine on SWM or VWM, relative to placebo. However, d-amphetamine increased a composite score of psychosis-like experiences (p = 0.0005), specifically: Scores on Brief Psychiatric Rating Scale, Perceptual Aberrations Scale, and Magical Ideation Scale were increased following d-amphetamine. The degree of change in PLE following d-amphetamine negatively and significantly correlated with changes in SWM, mainly at the longest delay condition of 8 s (r = -0.58, p = 0.006).

CONCLUSION

The present results showed that moderate-high dose of d-amphetamine and moderate dose of caffeine do not directly affect performances on DST or SST. However, the results indicate that d-amphetamine indirectly influences SWM, through its effect on psychosis-like experiences.

TRIAL REGISTRATION

CLINICAL TRIAL REGISTRATION NUMBER

CT-2018-CTN-02561 (Therapeutic Goods Administration Clinical Trial Registry) and ACTRN12618001292268 (The Australian New Zealand Clinical Trials Registry) for caffeine study, and ACTRN12608000610336 for d-amphetamine study.

There Is no Theory-Free Measure of "Swaps" in Visual Working Memory Experiments

Visual working memory is highly limited, and its capacity is tied to many indices of cognitive function. For this reason, there is much interest in understanding its architecture and the sources of its limited capacity. As part of this research effort, researchers often attempt to decompose visual working memory errors into different kinds of errors, with different origins. One of the most common kinds of memory error is referred to as a "swap," where people report a value that closely resembles an item that was not probed (e.g., an incorrect, non-target item). This is typically assumed to reflect confusions, like location binding errors, which result in the wrong item being reported. Capturing swap rates reliably and validly is of great importance because it permits researchers to accurately decompose different sources of memory errors and elucidate the processes that give rise to them. Here, we ask whether different visual working memory models yield robust and consistent estimates of swap rates. This is a major gap in the literature because in both empirical and modeling work, researchers measure swaps without motivating their choice of swap model. Therefore, we use extensive parameter recovery simulations with three mainstream swap models to demonstrate how the choice of measurement model can result in very large differences in estimated swap rates. We find that these choices can have major implications for how swap rates are estimated to change across conditions. In particular, each of the three models we consider can lead to differential quantitative and qualitative interpretations of the data. Our work serves as a cautionary note to researchers as well as a guide for model-based measurement of visual working memory processes.

An adaptive perspective on visual working memory distortions

When holding multiple items in visual working memory, representations of individual items are often attracted to, or repelled from, each other. While this is empirically well-established, existing frameworks do not account for both types of distortions, which appear to be in opposition. Here, we demonstrate that both types of memory distortion may confer functional benefits under different circumstances. When there are many items to remember and subjects are near their capacity to accurately remember each item individually, memories for each item become more similar (attraction). However, when remembering smaller sets of highly similar but discernible items, memory for each item becomes more distinct (repulsion), possibly to support better discrimination. Importantly, this repulsion grows stronger with longer delays, suggesting that it dynamically evolves in memory and is not just a differentiation process that occurs during encoding. Furthermore, both attraction and repulsion occur even in tasks designed to mitigate response bias concerns, suggesting they are genuine changes in memory representations. Together, these results are in line with the theory that attraction biases act to stabilize memory signals by capitalizing on information about an entire group of items, whereas repulsion biases reflect a tradeoff between maintaining accurate but distinct representations. Both biases suggest that human memory systems may sacrifice veridical representations in favor of representations that better support specific behavioral goals. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Trial-by-trial mouse trajectory predicts variance in precision across working memory representations: A critical reanalysis of Hao et al. (2021)

Multiple representations in visual working memory (VWM) can vary in mnemonic precision. This inhomogeneity of VWM precision has received some support from recent studies with the whole-report procedure, in which all memory items are recalled in free or forced orders. Recently, Hao et al. (2021, Cognition, 214, 104739) added a novel item-selection stage before each memory recall and found smaller between-trial variance in mouse trajectory during the selection stage in free-recall condition as compared with forced recall, which was taken as evidence for less between-item interference and the resulting precision benefit under free recall. Here, we reanalyzed the original dataset with a different analytic approach and attempted independent hypothesis testing focusing on within-trial trajectory deviations. We found that the direction of trial-by-trial trajectory bias for the first to-be-recalled item was predictive of the relative mnemonic precision of the remaining items. Critically, this relationship was only present for forced recall but not for free recall. Hierarchical Bayesian modeling of recall errors further identified that this relationship was selectively driven by VWM precision. Together, our reanalysis provides evidence for the source of between-item interference and its direct association with variable precision of VWM representations, and further highlights the novel methodological benefits of probing memory decisional processes using mouse trajectory data.

Subjective confidence reflects representation of Bayesian probability in cortex

What gives rise to the human sense of confidence? Here, we tested the Bayesian hypothesis that confidence is based on a probability distribution represented in neural population activity. We implemented several computational models of confidence, and tested their predictions using psychophysics and fMRI. Using a generative model-based fMRI decoding approach, we extracted probability distributions from neural population activity in human visual cortex. We found that subjective confidence tracks the shape of the decoded distribution. That is, when sensory evidence was more precise, as indicated by the decoded distribution, observers reported higher levels of confidence. We furthermore found that neural activity in the insula, anterior cingulate, and prefrontal cortex was linked to both the shape of the decoded distribution and reported confidence, in ways consistent with the Bayesian model. Altogether, our findings support recent statistical theories of confidence and suggest that probabilistic information guides the computation of one’s sense of confidence.

Temporal dynamics of implicit memory underlying serial dependence

Serial dependence is the effect in which the immediately preceding trial influences participants' responses to the current stimulus. But for how long does this bias last in the absence of interference from other stimuli? Here, we had 20 healthy young adult participants (12 women) perform a coincident timing task using different inter-trial intervals to characterize the serial dependence effect as the time between trials increases. Our results show that serial dependence abruptly decreases from 0.1 s to 1 s inter-trial interval, but it remains pronounced after that for up to 8 s. In addition, participants' response variability slightly decreases over longer intervals. We discuss these results in light of recent models suggesting that serial dependence might rely on a short-term memory trace kept through changes in synaptic weights, which might explain its long duration and apparent stability over time.

Sequence structure organizes items in varied latent states of working memory neural network

In memory experiences, events do not exist independently but are linked with each other via structure-based organization. Structure context largely influences memory behavior, but how it is implemented in the brain remains unknown. Here, we combined magnetoencephalogram (MEG) recordings, computational modeling, and impulse-response approaches to probe the latent states when subjects held a list of items in working memory (WM). We demonstrate that sequence context reorganizes WM items into distinct latent states, that is, being reactivated at different latencies during WM retention, and the reactivation profiles further correlate with recency behavior. In contrast, memorizing the same list of items without sequence task requirements weakens the recency effect and elicits comparable neural reactivations. Computational modeling further reveals a dominant function of sequence context, instead of passive memory decaying, in characterizing recency effect. Taken together, sequence structure context shapes the way WM items are stored in the human brain and essentially influences memory behavior.

Errors in visuospatial working memory across space and time

Visuospatial working memory (VSWM) involves cortical regions along the dorsal visual pathway, which are topographically organized with respect to the visual space. However, it remains unclear how such functional organization may constrain VSWM behavior across space and time. Here, we systematically mapped VSWM performance across the 2-dimensional (2D) space in various retention intervals in human subjects using the memory-guided and visually guided saccade tasks in two experiments. Relative to visually guided saccades, memory-guided saccades showed significant increases in unsystematic errors, or response variability, with increasing target eccentricity (3°-13° of visual angle). Unsystematic errors also increased with increasing delay (1.5-3 s, Experiment 1; 0.5-5 s, Experiment 2), while there was little or no interaction between delay and eccentricity. Continuous bump attractor modeling suggested neurophysiological and functional organization factors in the increasing unsystematic errors in VSWM across space and time. These findings indicate that: (1) VSWM representation may be limited by the functional topology of the visual pathway for the 2D space; (2) Unsystematic errors may reflect accumulated noise from memory maintenance while systematic errors may originate from non-mnemonic processes such as noisy sensorimotor transformation; (3) There may be independent mechanisms supporting the spatial and temporal processing of VSWM.

Free-recall benefit, inhomogeneity and between-item interference in working memory

We investigated visual working memory (VWM) with a whole-report task, where participants were asked to sequentially recall all the items in an order either chosen by themselves (free recall) or randomly chosen by the computer (forced recall). Comparisons between free and forced recalls helped us understand important but largely neglected aspects of VWM, such as inhomogeneity (different levels of precision) and between-item interference. One unique part of our task was the introduction of a separate item-selection stage before each recall, during which participants located the next item to recall. Their mouse trajectory was recorded and served as a dynamic measure of between-item interference over time. We found a free-recall benefit: the overall precision of all items is higher in free recall than in forced recall. Meanwhile, during item-selection, free recall is associated with faster localization of the target and less interference from the other items in memory. We also found evidence for inhomogeneity and discuss the connection of inhomogeneity and between-item interference to the free-recall benefit.

Gradual decay and sudden death of short-term memory for pitch

The ability to discriminate frequency differences between pure tones declines as the duration of the interstimulus interval (ISI) increases. The conventional explanation for this finding is that pitch representations gradually decay from auditory short-term memory. Gradual decay means that internal noise increases with increasing ISI duration. Another possibility is that pitch representations experience "sudden death," disappearing without a trace from memory. Sudden death means that listeners guess (respond at random) more often when the ISIs are longer. Since internal noise and guessing probabilities influence the shape of psychometric functions in different ways, they can be estimated simultaneously. Eleven amateur musicians performed a two-interval, two-alternative forced-choice frequency-discrimination task. The frequencies of the first tones were roved, and frequency differences and ISI durations were manipulated across trials. Data were analyzed using Bayesian models that simultaneously estimated internal noise and guessing probabilities. On average across listeners, internal noise increased monotonically as a function of increasing ISI duration, suggesting that gradual decay occurred. The guessing rate decreased with an increasing ISI duration between 0.5 and 2 s but then increased with further increases in ISI duration, suggesting that sudden death occurred but perhaps only at longer ISIs. Results are problematic for decay-only models of discrimination and contrast with those from a study on visual short-term memory, which found that over similar durations, visual representations experienced little gradual decay yet substantial sudden death.

Delay-period activity in frontal, parietal, and occipital cortex tracks noise and biases in visual working memory

Working memory is imprecise, and these imprecisions can be explained by the combined influences of random diffusive error and systematic drift toward a set of stable states ("attractors"). However, the neural correlates of diffusion and drift remain unknown. Here, we investigated how delay-period activity in frontal and parietal cortex, which is known to correlate with the decline in behavioral memory precision observed with increasing memory load, might relate to diffusion and drift. We analyzed data from an existing experiment in which subjects performed delayed recall for line orientation, at different loads, during functional magnetic resonance imaging (fMRI) scanning. To quantify the influence of drift and diffusion, we modeled subjects' behavior using a discrete attractor model and calculated within-subject correlation between frontal and parietal delay-period activity and whole-trial estimates of drift and diffusion. We found that although increases in frontal and parietal activity were associated with increases in both diffusion and drift, diffusion explained the most variance in frontal and parietal delay-period activity. In comparison, a subsequent whole-brain regression analysis showed that drift, rather than diffusion, explained the most variance in delay-period activity in lateral occipital cortex. These results are consistent with a model of the differential recruitment of general frontoparietal mechanisms in response to diffusive noise and of stimulus-specific biases in occipital cortex.

Crows control working memory before and after stimulus encoding

The capacity of working memory is limited and this limit is comparable in crows and primates. To maximize this resource, humans use attention to select only relevant information for maintenance. Interestingly, attention-cues are effective not only before but also after the presentation of to-be-remembered stimuli, highlighting control mechanisms beyond sensory selection. Here we explore if crows are also capable of these forms of control over working memory. Two crows (Corvus corone) were trained to memorize two, four or six visual stimuli. Comparable to our previous results, the crows showed a decrease in performance with increasing working memory load. Using attention cues, we indicated the critical stimulus on a given trial. These cues were either presented before (pre-cue) or after sample-presentation (retro-cue). On other trials no cue was given as to which stimulus was critical. We found that both pre- and retro-cues enhance the performance of the birds. These results show that crows, like humans, can utilize attention to select relevant stimuli for maintenance in working memory. Importantly, crows can also utilize cues to make the most of their working memory capacity even after the stimuli are already held in working memory. This strongly implies that crows can engage in efficient control over working memory.

Error-correcting dynamics in visual working memory

Working memory is critical to cognition, decoupling behavior from the immediate world. Yet, it is imperfect; internal noise introduces errors into memory representations. Such errors have been shown to accumulate over time and increase with the number of items simultaneously held in working memory. Here, we show that discrete attractor dynamics mitigate the impact of noise on working memory. These dynamics pull memories towards a few stable representations in mnemonic space, inducing a bias in memory representations but reducing the effect of random diffusion. Model-based and model-free analyses of human and monkey behavior show that discrete attractor dynamics account for the distribution, bias, and precision of working memory reports. Furthermore, attractor dynamics are adaptive. They increase in strength as noise increases with memory load and experiments in humans show these dynamics adapt to the statistics of the environment, such that memories drift towards contextually-predicted values. Together, our results suggest attractor dynamics mitigate errors in working memory by counteracting noise and integrating contextual information into memories.

Neural representations in working memory are susceptible to internal noise, which scales with memory load. Here, the authors show that attractor dynamics mitigate the influence of internal noise by pulling memories towards a few stable representations.

Separating memoranda in depth increases visual working memory performance

Visual working memory is the mechanism supporting the continued maintenance of information after sensory inputs are removed. Although the capacity of visual working memory is limited, memoranda that are spaced farther apart on a 2-D display are easier to remember, potentially because neural representations are more distinct within retinotopically organized areas of visual cortex during memory encoding, maintenance, or retrieval. The impact on memory of spatial separability in depth is less clear, even though depth information is essential to guiding interactions with objects in the environment. On one account, separating memoranda in depth may facilitate performance if interference between items is reduced. However, depth information must be inferred indirectly from the 2-D retinal image, and less is known about how visual cortex represents depth. Thus, an alternative possibility is that separation in depth does not attenuate between-items interference; it may even impair performance, as attention must be distributed across a larger volume of 3-D space. We tested these alternatives using a stereo display while participants remembered the colors of stimuli presented either near or far in the 2-D plane or in depth. Increasing separation in-plane and in depth both enhanced performance. Furthermore, participants who were better able to utilize stereo depth cues showed larger benefits when memoranda were separated in depth, particularly for large memory arrays. The observation that spatial separation in the inferred 3-D structure of the environment improves memory performance, as is the case in 2-D environments, suggests that separating memoranda in depth might reduce neural competition by utilizing cortically separable resources.

Drift in Neural Population Activity Causes Working Memory to Deteriorate Over Time

Short-term memories are thought to be maintained in the form of sustained spiking activity in neural populations. Decreases in recall precision observed with increasing number of memorized items can be accounted for by a limit on total spiking activity, resulting in fewer spikes contributing to the representation of each individual item. Longer retention intervals likewise reduce recall precision, but it is unknown what changes in population activity produce this effect. One possibility is that spiking activity becomes attenuated over time, such that the same mechanism accounts for both effects of set size and retention duration. Alternatively, reduced performance may be caused by drift in the encoded value over time, without a decrease in overall spiking activity. Human participants of either sex performed a variable-delay cued recall task with a saccadic response, providing a precise measure of recall latency. Based on a spike integration model of decision making, if the effects of set size and retention duration are both caused by decreased spiking activity, we would predict a fixed relationship between recall precision and response latency across conditions. In contrast, the drift hypothesis predicts no systematic changes in latency with increasing delays. Our results show both an increase in latency with set size, and a decrease in response precision with longer delays within each set size, but no systematic increase in latency for increasing delay durations. These results were quantitatively reproduced by a model based on a limited neural resource in which working memories drift rather than decay with time.

SIGNIFICANCE STATEMENT Rapid deterioration over seconds is a defining feature of short-term memory, but what mechanism drives this degradation of internal representations? Here, we extend a successful population coding model of working memory by introducing possible mechanisms of delay effects. We show that a decay in neural signal over time predicts that the time required for memory retrieval will increase with delay, whereas a random drift in the stored value predicts no effect of delay on retrieval time. Testing these predictions in a multi-item memory task with an eye movement response, we identified drift as a key mechanism of memory decline. These results provide evidence for a dynamic spiking basis for working memory, in contrast to recent proposals of activity-silent storage.