Visual working memory capacity does not modulate the feature-based information filtering in visual working memory

Background

The limited capacity of visual working memory (VWM) requires us to select the task relevant information and filter out the irrelevant information efficiently. Previous studies showed that the individual differences in VWM capacity dramatically influenced the way we filtered out the distracters displayed in distinct spatial-locations: low-capacity individuals were poorer at filtering them out than the high-capacity ones. However, when the target and distracting information pertain to the same object (i.e., multiple-featured object), whether the VWM capacity modulates the feature-based filtering remains unknown.

Methodology/Principal Findings

We explored this issue mainly based on one of our recent studies, in which we asked the participants to remember three colors of colored-shapes or colored-landolt-Cs while using two types of task irrelevant information. We found that the irrelevant high-discriminable information could not be filtered out during the extraction of VWM but the irrelevant fine-grained information could be. We added 8 extra participants to the original 16 participants and then split the overall 24 participants into low- and high-VWM capacity groups. We found that regardless of the VWM capacity, the irrelevant high-discriminable information was selected into VWM, whereas the irrelevant fine-grained information was filtered out. The latter finding was further corroborated in a second experiment in which the participants were required to remember one colored-landolt-C and a more strict control was exerted over the VWM capacity.

Conclusions/Significance

We conclude that VWM capacity did not modulate the feature-based filtering in VWM.     

Neural correlate of filtering of irrelevant information from visual working memory

In a dynamic environment stimulus task relevancy could be altered through time and it is not always possible to dissociate relevant and irrelevant objects from the very first moment they come to our sight. In such conditions, subjects need to retain maximum possible information in their WM until it is clear which items should be eliminated from WM to free attention and memory resources. Here, we examined the neural basis of irrelevant information filtering from WM by recording human ERP during a visual change detection task in which the stimulus irrelevancy was revealed in a later stage of the task forcing the subjects to keep all of the information in WM until test object set was presented. Assessing subjects' behaviour we found that subjects' RT was highly correlated with the number of irrelevant objects and not the relevant one, pointing to the notion that filtering, and not selection, process was used to handle the distracting effect of irrelevant objects. In addition we found that frontal N150 and parietal N200 peak latencies increased systematically as the amount of irrelevancy load increased. Interestingly, the peak latency of parietal N200, and not frontal N150, better correlated with subjects' RT. The difference between frontal N150 and parietal N200 peak latencies varied with the amount of irrelevancy load suggesting that functional connectivity between modules underlying fronto-parietal potentials vary concomitant with the irrelevancy load. These findings suggest the existence of two neural modules, responsible for irrelevant objects elimination, whose activity latency and functional connectivity depend on the number of irrelevant object.     

The relationship between visual working memory and attention: retention of precise colour information in the absence of effects on perceptual selection

We examined the conditions under which a feature value in visual working memory (VWM) recruits visual attention to matching stimuli. Previous work has suggested that VWM supports two qualitatively different states of representation: an active state that interacts with perceptual selection and a passive (or accessory) state that does not. An alternative hypothesis is that VWM supports a single form of representation, with the precision of feature memory controlling whether or not the representation interacts with perceptual selection. The results of three experiments supported the dual-state hypothesis. We established conditions under which participants retained a relatively precise representation of a parcticular colour. If the colour was immediately task relevant, it reliably recruited attention to matching stimuli. However, if the colour was not immediately task relevant, it failed to interact with perceptual selection. Feature maintenance in VWM is not necessarily equivalent with feature-based attentional selection.     

Mental imagery and visual working memory

Visual working memory provides an essential link between past and future events. Despite recent efforts, capacity limits, their genesis and the underlying neural structures of visual working memory remain unclear. Here we show that performance in visual working memory--but not iconic visual memory--can be predicted by the strength of mental imagery as assessed with binocular rivalry in a given individual. In addition, for individuals with strong imagery, modulating the background luminance diminished performance on visual working memory and imagery tasks, but not working memory for number strings. This suggests that luminance signals were disrupting sensory-based imagery mechanisms and not a general working memory system. Individuals with poor imagery still performed above chance in the visual working memory task, but their performance was not affected by the background luminance, suggesting a dichotomy in strategies for visual working memory: individuals with strong mental imagery rely on sensory-based imagery to support mnemonic performance, while those with poor imagery rely on different strategies. These findings could help reconcile current controversy regarding the mechanism and location of visual mnemonic storage.     

Age-related specificity of the processing of visual information in the system of working memory

In adults and seven- to eight-year-old children, event-related potentials (ERPs) were analyzed during quiet observation and detailed paired comparison of visual stimuli. In both age groups, we showed the differences in the initial stages (component N1) of sensory analysis in these situations. In adults, an increase in the negativity during the initial stages of analysis was observed in the caudal and central areas of the cortex during presentation of standard and test stimuli. In the frontal areas of the cortex, an increase in the negative potential was observed only in ERPs induced by the test stimulus. In children, an increase in the negativity at the initial phases of analysis of stimuli in the situation of working memory, as compared to quiet observation, was confined to the caudal areas of the cortex. Differential curves that characterize analysis of standard and test stimuli showed age-related differences in the initial and late phases of information processing under the conditions of working memory. In adults, the differential curves that characterize analysis of the standard stimulus were represented by negative phases, and the curves related to the test stimulus, by positive phases. In children, late phases of analysis of the standard and test stimuli had smaller differences as compared to adults: the late positive wave was predominant in the responses to both standard and test stimulus in the caudal areas of the cortex. In the frontal areas, there was no considerable increase in the amplitude of the late positive wave in response to the test stimulus. This fact, together with the absence of enhancement of initial negativity in the frontal areas, which reflects analysis of the test stimulus, indicates that the prefrontal cortex plays a smaller role in the comparison of the memory trace with the current information in seven- to eight-year-old children. The data obtained suggest that the central executive of working memory is not sufficiently mature in children aged seven to eight years.     

Brain mechanisms for extracting spatial information from smell

Forty years ago, von Békésy demonstrated that the spatial source of an odorant is determined by comparing input across nostrils, but it is unknown how this comparison is effected in the brain. To address this, we delivered odorants to the left or right of the nose, and contrasted olfactory left versus right localization with olfactory identification during brain imaging. We found nostril-specific responses in primary olfactory cortex that were predictive of the accuracy of left versus right localization, thus providing a neural substrate for the behavior described by von Békésy. Additionally, left versus right localization preferentially engaged a portion of the superior temporal gyrus previously implicated in visual and auditory localization, suggesting that localization information extracted from smell was then processed in a convergent brain system for spatial representation of multisensory inputs.     

Directing the mind's eye: prefrontal, inferior and medial temporal mechanisms for visual working memory

Human and nonhuman primates have a remarkable ability to recall, maintain and manipulate visual images in the absence of external sensory stimulation. Evidence from lesion, single-unit neurophysiological and neuroimaging studies shows that these visual working memory processes are consistently associated with sustained activity in object-selective inferior temporal neurons. Furthermore, results from these studies suggest that mnemonic activity in the inferior temporal cortex is, in turn, supported by top-down inputs from multimodal regions in prefrontal and medial temporal cortex, and under some circumstances, from the hippocampus.     

Priority-based transformations of stimulus representation in visual working memory

How does the brain prioritize among the contents of working memory (WM) to appropriately guide behavior? Previous work, employing inverted encoding modeling (IEM) of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) datasets, has shown that unprioritized memory items (UMI) are actively represented in the brain, but in a “flipped”, or opposite, format compared to prioritized memory items (PMI). To acquire independent evidence for such a priority-based representational transformation, and to explore underlying mechanisms, we trained recurrent neural networks (RNNs) with a long short-term memory (LSTM) architecture to perform a 2-back WM task. Visualization of LSTM hidden layer activity using Principal Component Analysis (PCA) confirmed that stimulus representations undergo a representational transformation–consistent with a flip—while transitioning from the functional status of UMI to PMI. Demixed (d)PCA of the same data identified two representational trajectories, one each within a UMI subspace and a PMI subspace, both undergoing a reversal of stimulus coding axes. dPCA of data from an EEG dataset also provided evidence for priority-based transformations of the representational code, albeit with some differences. This type of transformation could allow for retention of unprioritized information in WM while preventing it from interfering with concurrent behavior. The results from this initial exploration suggest that the algorithmic details of how this transformation is carried out by RNNs, versus by the human brain, may differ.     

High visual working memory capacity in trait social anxiety

Working memory capacity is one of the most important cognitive functions influencing individual traits, such as attentional control, fluid intelligence, and also psychopathological traits. Previous research suggests that anxiety is associated with impaired cognitive function, and studies have shown low verbal working memory capacity in individuals with high trait anxiety. However, the relationship between trait anxiety and visual working memory capacity is still unclear. Considering that people allocate visual attention more widely to detect danger under threat, visual working memory capacity might be higher in anxious people. In the present study, we show that visual working memory capacity increases as trait social anxiety increases by using a change detection task. When the demand to inhibit distractors increased, however, high visual working memory capacity diminished in individuals with social anxiety, and instead, impaired filtering of distractors was predicted by trait social anxiety. State anxiety was not correlated with visual working memory capacity. These results indicate that socially anxious people could potentially hold a large amount of information in working memory. However, because of an impaired cognitive function, they could not inhibit goal-irrelevant distractors and their performance decreased under highly demanding conditions.     

Refixation frequency and memory mechanisms in visual search

Visual search-looking for a target object in the presence of a number of distractor items-is an everyday activity for humans (for example, finding the car in a busy car park) and animals (for example, foraging for food). Our understanding of visual search has been enriched by an interdisciplinary effort using a wide range of research techniques including behavioural studies in humans [1], single-cell electrophysiology [2], transcranial magnetic stimulation [3], event-related potentials [4] and studies of patients with focal brain injury [5]. A central question is what kind of information controls the search process. Visual search is typically accompanied by a series of eye movements, and investigating the nature and location of fixations helps to identify the kind of information that might control the search process. It has already been demonstrated that objects are fixated if they are visually similar to the target [6]. Also, if an item has been fixated, it is less likely to be returned to on the subsequent saccade. This automatic process is referred to as inhibition of return (IOR [7,8]). Here, we investigated the role of memory for which items had been fixated previously. We found that, during search, subjects often refixated items that had been previously fixated. Although there were fewer return saccades than would be expected by chance, the number of refixations indicated limited functional memory, indeed the memory effects that were present may primarily be a result of IOR.     

Role of working memory in forming the cognitive visual set

It was found in healthy adult subjects (n = 90) that under conditions of complication of cognitive tasks (increasing number of relevant stimuli), the stability (rigidity) of the unconscious cognitive sets markedly rises. These changes in both verbal and nonverbal sets depend on the working memory loading. The revealed dependence of the motor reaction time to the probe stimulus on the sequence of the set stimuli in a context and on the set stage suggests that unconscious cognitive sets can regulate selective attention.     

Aging-related changes of EEG synchronization during a visual working memory task

Differences of EEG synchronization between normal old and young people during a working memory (WM) task were investigated. The synchronization likelihood (SL) is a novel method to assessed synchronization in multivariate time series for non-stationary systems. To evaluate this method to study the mechanisms of WM, we calculated the SL values in brain electrical activity for both resting state and task state. EEG signals were recorded from 14 young adults and 12 old adults during two different states, respectively. SL was used to measure EEG synchronization between 19 electrodes in delta, theta, alpha1, alpha2 and beta frequency bands. Bad task performance and significantly decreased EEG synchronization were found in old group compared to young group in alpha1, alpha2 and beta frequency bands during the WM task. Moreover, significantly decreased EEG synchronization in beta band in the elder was also detected during the resting state. The findings suggested that reduced EEG synchronization may be one of causes for WM capacity decline along with healthy aging.     

Differential roles for parietal and occipital cortices in visual working memory

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

Event-related potentials at different stages of the operation of visual working memory

Trace fixation and comparison with incoming information was studied using event-related potentials (ERPs) recorded from various cortical areas during passive viewing and matching of two consecutive pictures. Visual stimuli differing in the spatial location of elements (4 × 4 square patterns with random positions of 4 black and 12 white squares) and phonological stimuli (differently written letters) were used. Trace fixation was studied by comparing the ERPs generated in response to the first (reference) stimulus in the pair with those generated during passive viewing. Sensory analysis of the reference stimuli was observed in the time interval 128–196 ms. For the patterns presented, it was reflected by an increased amplitude of the N1 component in the caudal areas as compared with passive viewing. The phonological stimuli produced a higher amplitude of a positive wave in the frontotemporal area in the same time interval. Processing of subsequent information to be stored in memory was observed in the interval 232–452 ms. Processing of patterns was reflected by a decreased positivity, most pronounced in the left temporo-parieto-occipital area. Comparison of a trace with incoming information was studied by comparing the ERPs generated in response to the first (reference) and second (test) stimuli. The number of cortical areas involved in the sensory analysis of the test stimuli was larger than the number involved in the analysis of the reference stimuli. Comparison of the new information with the trace was reflected by an increased amplitude of the late positive wave (components P3, Pc, and Pc-Nc) in the frontocentral and caudal cortical areas. The topographic changes in the late positive components depended on the type of stimulus.