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

Visual short-term memory (VSTM) is a capacity limited resource, which is consistently estimated to hold about four visual items at a time. There is, however, debate in the literature about what constitutes an "item" and how resources are allocated within VSTM. Some research suggests information is stored in VSTM as discrete objects; however, there is also evidence suggesting that within-object features alter VSTM performance. The present study addresses the question of whether VSTM load effects reflect the number of discrete objects and/or the number of within-object features. An electrophysiological correlate of VSTM--the contralateral delay activity (CDA)--was measured while participants performed a lateralized change-detection task, in which to-be-remembered items varied in the number of features and locations. Each trial contained either a solitary simple feature (shape, color, or orientation) or one of two multifeature arrays: three features presented at three separate locations or three features bound at one location. While presenting multiple features--regardless of whether they are at discrete locations or bound within a single object--resulted in greater CDA amplitude relative to a solitary feature, there was a dissociation in the distribution of activity between the two multifeature conditions, such that the CDA at site P1/P2 was sensitive to the number of discrete objects, while activity at P7/P8 was most enhanced when multiple features were bound in one object. The findings demonstrate the inhomogeneity of the CDA and suggest this electrophysiological marker may reflect both discrete object individuation/separation and flexible feature-feature binding in VSTM.

Our previous work suggests that 2 colors can be consolidated into visual short-term memory (VSTM) in parallel without a loss of memory precision, whereas consolidation of 2 orientations is performed in a strictly serial manner. Those experiments compared VSTM performance for simultaneously and sequentially presented stimuli. However, there is still controversy about whether the bandwidth for consolidation is determined by the type of information. To further investigate this issue, here we measured electroencephalography while participants attempted to consolidate 1, 2 or 4 simultaneously presented colors (Experiment 1) or orientations (Experiment 2) under limited presentation times. We used the contralateral delay activity (CDA) as an electrophysiological marker of the number of items that were consolidated. For colored stimuli, the CDA amplitude increased between set-size 1 and 2 but did not further increase for set size 4. By contrast, for orientation, the CDA amplitude remained at the set size 1 amplitude as set size increased to 2 or 4 items. Furthermore, in a long exposure duration (300 ms) condition that did not limit the consolidation process, the CDA amplitude pattern indicated that VSTM capacity is limited to about 3 colored items and about 2 orientation items in our paradigm. Thus, the CDA effects observed in the short presentation time was not limited by VSTM storage, but rather by consolidation. These results are consistent with our previous behavioral research and suggest that the bandwidth of VSTM consolidation is determined by the stimulus feature. (PsycINFO Database Record

Working memory (WM) is limited in capacity, but it is controversial whether these capacity limitations are domain-general or are generated independently within separate modality-specific memory systems. These alternative accounts were tested in bimodal visual/tactile WM tasks. In Experiment 1, participants memorized the locations of simultaneously presented task-relevant visual and tactile stimuli. Visual and tactile WM load was manipulated independently (one, two, or three items per modality), and one modality was unpredictably tested after each trial. To track the activation of visual and tactile WM representations during the retention interval, the visual contralateral delay activity (CDA) and tactile CDA (tCDA) were measured over visual and somatosensory cortex, respectively. CDA and tCDA amplitudes were selectively affected by WM load in the corresponding (tactile or visual) modality. The CDA parametrically increased when visual load increased from one to two and to three items. The tCDA was enhanced when tactile load increased from one to two items and showed no further enhancement for three tactile items. Critically, these load effects were strictly modality-specific, as substantiated by Bayesian statistics. Increasing tactile load did not affect the visual CDA, and increasing visual load did not modulate the tCDA. Task performance at memory test was also unaffected by WM load in the other (untested) modality. This was confirmed in a second behavioral experiment where tactile and visual loads were either two or four items, unimodal baseline conditions were included, and participants performed a color change detection task in the visual modality. These results show that WM capacity is not limited by a domain-general mechanism that operates across sensory modalities. They suggest instead that WM storage is mediated by distributed modality-specific control mechanisms that are activated independently and in parallel during multisensory WM.

Contralateral delay activity does not reflect behavioral feature load in visual working memory

The serial and spatially extended nature of many real-world visual tasks suggests the need for control over the content of visual working memory (VWM). We examined the management of VWM in a task that required participants to prioritize individual objects for retention during scene viewing. There were 5 principal findings: (a) Strategic retention of task-relevant objects was effective and was dissociable from the current locus of visual attention; (b) strategic retention was implemented by protection from interference rather than by preferential encoding; (c) this prioritization was flexibly transferred to a new object as task demands changed; (d) no-longer-relevant items were efficiently eliminated from VWM; and (e) despite this level of control, attended and fixated objects were consolidated into VWM regardless of task relevance. These results are consistent with a model of VWM control in which each fixated object is automatically encoded into VWM, replacing a portion of the content in VWM. However, task-relevant objects can be selectively protected from replacement.

What goes through the gate? Exploring interference with visual feature binding

Introduction. Infants’ visual working memory (VWM) has a 3-item limit (Feigenson & Carey, 2003), but infants can "chunk" items using perceptually available properties, such as common features or spatial proximity, thereby overcoming that limit (Feigenson & Halberda, 2004; 2008). When such chunking cues are not available, adults can extract statistical regularities between visual arrays presented over time, and use these regularities to more efficiently group items in VWM (Brady, et al, 2009). Here we asked whether 13-month-old infants could also use statistical regularities between objects to increase VWM. Participants & Method. During 10 Familiarization trials, infants (N=21) saw 4 objects (red disk, blue cross, green pentagon, yellow squiggle) placed sequentially on a stage 2 at a time. In the No-Regularity condition, the objects were paired randomly on each Familiarization trial. In the Regularity condition, object identity was yoked such that, across Familiarization trials, the identity of one shape perfectly predicted the identity of the other. Next, infants saw 6 Test Trials in which all 4 objects were sequentially hidden behind an occluder, which was then removed to reveal either all 4 objects (Expected Outcome) or only 3 (Unexpected Outcome), on alternating trials. Results. As predicted by previous findings, infants in the No Regularity condition failed to look longer at the Unexpected Outcome when four objects were hidden and just three were revealed, F(1,10)=0.005, p=n.s.). This finding replicates earlier work demonstrating an upper limit on infants’ VWM. In contrast, infants in the Regularity condition increased their looking to the Unexpected Outcome, F(1,28)=6.37, p=0.024 (see Figure 1). This suggests that seeing statistical regularities between individual objects allowed infants to group or chunk the objects in VWM, and hence to remember more total information about the scene. Conclusions. Infants can chunk items in VWM based on the statistical regularities present as events unfold. Meeting abstract presented at VSS 2013

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

The limits of human visual short-term memory (VSTM) have been well documented, and recent neuroscientific studies suggest that VSTM performance is associated with activity in the posterior parietal cortex. Here we show that artificially elevating parietal activity via positively charged electric current through the skull can rapidly and effortlessly improve people's VSTM performance. This artificial improvement, however, comes with an interesting twist: it interacts with people's natural VSTM capability such that low performers who tend to remember less information benefitted from the stimulation, whereas high performers did not. This behavioral dichotomy is explained by event-related potentials around the parietal regions: low performers showed increased waveforms in N2pc and contralateral delay activity (CDA), which implies improvement in attention deployment and memory access in the current paradigm, respectively. Interestingly, these components are found during the presentation of the test array instead of the retention interval, from the parietal sites ipsilateral to the target location, thus suggesting that transcranial direct current stimulation (tDCS) was mainly improving one's ability to suppress no-change distractors located on the irrelevant side of the display during the comparison stage. The high performers, however, did not benefit from tDCS as they showed equally large waveforms in N2pc and CDA, or SPCN (sustained parietal contralateral negativity), before and after the stimulation such that electrical stimulation could not help any further, which also accurately accounts for our behavioral observations. Together, these results suggest that there is indeed a fixed upper limit in VSTM, but the low performers can benefit from neurostimulation to reach that maximum via enhanced comparison processes, and such behavioral improvement can be directly quantified and visualized by the magnitude of its associated electrophysiological waveforms.

How do verbal descriptions affect visual memory over the short and long term? Here we show for the first time that verbal labeling can boost visual memories, but the source of this benefit depends on whether representations are maintained over the short term in visual working memory or over the long term in visual long-term memory. Across three experiments, we contrasted color memory of randomly colored objects when participants labeled (a) the color, (b) the object, or (c) the color-object binding, to memory under an articulatory suppression condition inhibiting labeling. Memory was tested at two time points: after three objects (visual working memory) and at the end of the experiment (visual long-term memory). In Experiment 1, color labeling improved, whereas object labeling impaired, visual working memory in comparison to suppression. Visual long-term memory remained unchanged across conditions. Experiment 2 tested whether this was attributable to poor overall long-term learning by repeating the colored objects over three successive working memory trials. This increased performance over the short and long term, yet labeling did not change learning rate over repetitions or delayed memory performance, showing no long-term memory benefit. In Experiment 3, a labeling benefit was observed when the color-object binding was labeled both over the short and long term. Mixture modeling indicated that color-labeling benefits in visual working memory resulted from an increase of detailed visual memory, whereas long-term memory benefits accrued from categorical representations. Our findings point to dissociations on the role of language in visual working memory and visual long-term memory. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Previous research has demonstrated greater visual working memory (VWM) performance for real-world objects compared with simple features. Greater amplitudes of the contralateral delay activity (CDA)-a sustained event-related potential measured during the delay period of a VWM task-havealso been noted for meaningful stimuli, despite being thought of as a neural marker of a fixed working memory capacity. The current study aimed to elucidate the factors underlying improved memory performance for real-world objects by isolating the relative contributions of perceptual complexity (i.e., number of visual features) and conceptual meaning (i.e., availability of semantic, meaningful features). Participants (N = 22) performed a lateralized VWM task to test their memory of intact real-world objects, scrambled real-world objects and colours. The CDA was measured during both encoding and WM retention intervals (600-1000 ms and 1300-1700 ms poststimulus onset, respectively), and behavioural performance was estimated by using d' (memory strength in a two-alternative forced choice task). Behavioural results revealed significantly better performance within-subjects for real-world objects relative to scrambled objects and colours, with no difference between colours and scrambled objects. The amplitude of the CDA wasalso largest for intact real-world objects, with no difference in magnitude for scrambled objects and colours, during working memory maintenance. However, during memory encoding, both the colours and intact real-world objects had significantly greater amplitudes than scrambled objects and were comparable in magnitude. Overall, findings suggest that conceptual meaning (semantics) supports the memory benefit for real-world objects.

A rapidly growing body of work suggests that visual working memory (VWM) is fundamentally action oriented. Consistent with this, we recently showed that attention is more strongly biased by VWM representations of objects when we plan to act on those objects in the future. Using EEG and eye tracking, here, we investigated neurophysiological correlates of the interactions between VWM and action. Participants (n = 36) memorized a shape for a subsequent VWM test. At test, a probe was presented along with a secondary object. In the action condition, participants gripped the actual probe if it matched the memorized shape, whereas in the control condition, they gripped the secondary object. Crucially, during the VWM delay, participants engaged in a visual selection task, in which they located a target as fast as possible. The memorized shape could either encircle the target (congruent trials) or a distractor (incongruent trials). Replicating previous findings, we found that eye gaze was biased toward the VWM-matching shape and, importantly, more so when the shape was directly associated with an action plan. Moreover, the ERP results revealed that during the selection task, future action-relevant VWM-matching shapes elicited (1) a stronger Ppc (posterior positivity contralateral), signaling greater attentional saliency; (2) an earlier PD (distractor positivity) component, suggesting faster suppression; (3) a larger inverse (i.e., positive) sustained posterior contralateral negativity in incongruent trials, consistent with stronger suppression of action-associated distractors; and (4) an enhanced response-locked positivity over left motor regions, possibly indicating enhanced inhibition of the response associated with the memorized item during the interim task. Overall, these results suggest that action planning renders objects in VWM more attentionally salient, supporting the notion of selection-for-action in working memory.

During scene viewing, visual working memory (VWM) is used to retain information from recently attended and fixated objects. In the present study, I examined whether and how people can strategically control the content of VWM during scene viewing, prioritizing task-relevant objects for retention even as the eyes are directed to subsequent objects. Participants viewed a set of real-world objects presented serially within a 3-D rendered scene. One object in the sequence was cued by a tone as to-beremembered. At the end of the sequence, memory for the visual form of one object was tested. Participants exhibited tight control over the content of VWM, implementing prioritization after the encoding of an object into VWM, protecting that item from subsequent interference. Participants also successfully reallocated protection to subsequent objects, regardless of the duration of prioritization of the original item. Such strategic maintenance of objects in VWM is likely to play an important role in real-world visual behavior, especially when object information must be maintained across shifts of attention and the eyes to other objects (such as when comparing two spatially separated objects). Abstract Approved:________________________________________________________ Thesis Supervisor ________________________________________________________ Title and Department ________________________________________________________ DateApproved:________________________________________________________ Thesis Supervisor ________________________________________________________ Title and Department ________________________________________________________ Date STRATEGIC CONTROL OF VISUAL WORKING MEMORY DURING SCENE VIEWING by Ashleigh Monette Richard A thesis submitted in partial fulfillment of the requirements for the Doctor of Philosophy degree in Psychology in the Graduate College of The University of Iowa

In daily life, people frequently need to observe dynamic objects and temporarily maintain their representations in visual working memory (VWM). The present study explored the mechanism underlying the binding between perceptual features and locations of dynamic objects in VWM. In three experiments, we measured and compared the memory performance for feature-location binding of multiple dynamic and static objects. The results showed that the feature-location binding was impaired for the dynamic objects compared with the static objects. The impairment persisted when the global spatial configuration of the objects remained intact during the motion, as well as when the binding task was relatively easy, such as binding between single-feature objects and coarse locations. The results indicate that object features and locations are not maintained in VWM as well-integrated object files; rather, the formation of feature-location binding may require additional processes, which are disrupted by the constant change of locations in dynamic circumstances. We propose a consolidation process as possible underlying mechanism, and discuss factors that may influence the strength of feature-location binding in dynamic circumstances.

Time-resolved neuroimaging of visual short term memory consolidation by post-perceptual attention shifts