The capacity and precision of visual working memory for object and ensembles
Previous research has documented the limited capacity of visual working memory (VWM) for color objects set at 3–5 items. Another line of research has shown that multiple objects can be stored in a compressed form of ensemble. However, existing data is more likely to testify that VWM can store no more than two such compressed units. But the nature of this discrepancy can be methodological: VWM for ensembles was never tested using methods that are applied in the research of VWM for objects. Here we have tested the capacity and precision of VWM for objects and ensembles using two standard methods — change detection and continuous report with a mixture model. We found that VWM for both types of units showed the similar capacity and precision when critical psychophysical parameters, such as foveal density and area are controlled. We also showed that this quantitative similarity between objects and ensembles is provided by a mechanism that represents each ensemble as a holistic VWM chunk as efficiently as it represents any single object.
It was previously shown that the features of individual items retrieved from visual working memory (VWM) are systematically biased towards the mean feature of a sample set (Brady & Alvarez, 2011), suggesting hierarchical encoding in VWM. In our work, we investigated how hierarchical representations are stored over time. Observers were shown four differently oriented triangles for 200 ms and, after 1-, 4-, or 7-second delay, they had to report either one individual orientation, or the average orientation of all triangles, rotating a probe circle. Before set presentations, observers were informed that they had to remember one particular orientation, all four individual orientations, or the average orientation. Using the mixture model (Zhang & Luck, 2008), we estimated a probability of a tested representation being in VWM and its precision, as well as a systematic bias that would indicate hierarchical encoding. We found a strong bias towards the mean in the “remember four” condition, which provides evidence for hierarchical encoding in VWM. Our main result was the absence of significant changes in retaining the elements of a hierarchical representation (the mean and individual features). This supports an idea that hierarchical representations are related to encoding, rather than storing in VWM. Both fidelity and the probability of an item being in memory decrease over time. It supports "Sudden Death" and "Gradual Decay" accounts for storing hierarchical representations.
The pattern of cortical functional connectivity in the source space was studied in a group of right handed adult participants (N = 44:17 women, 27 men, aged M = 29.61 ± 6.45 years). Participants retained the traces of realistic pictures of positive, neutral, and negative emotional valences in their working memory (WM) while performing the samedifferent task. Within the framework of this task, participants had to compare the initial picture against a target picture that followed after a specified delay. The coherence (COH) between the pairs of cortical sources chosen in advance according to fMRI data was estimated in the theta frequency range for the period preceding the initial stimulus, during the retention of the initial stimulus in WM, and during the rest interval between successive trials. Two distinct sets of functional links were found. The links of the first type that presumably reflected the involvement of sustained attention were between the dorsal anterior cingulate cortex, the prefrontal areas, and temporal areas of the right hemispheres. When compared to the rest period, the links of this type showed strengthening not only during the retention period but also during the period preceding the initial picture. The links of the second type presumably reflected a progressive neocortextohippocampus functional integration with increasing memory load and strength ened exclusively during the retention period. These links were between the parietal, temporal and prefrontal cortices in the lateral surface of both hemispheres with the additional inclusion of the posterior cingulate cortex and the medial parietal cortex in the left hemisphere. The impact of emotional valence on the strength and topography of the functional links of the second type was found. In the left hemisphere, the increase of strength of cortical interaction was more pronounced for the pictures of positive valence than for the pictures of either neutral or negative valences. When compared to the pictures of neutral valence, the retention of pictorial information of both positive and negative valence showed some extraneous integration of the cortical areas for the theta rhythm. This finding might be related to the additional load exerted by emotionally colored pictures onto the mechanisms of shorttime retention of visual information.
One of the important sources of failures in visual working memory (VWM) is that individual items can interfere with each other. Here, we tested how two causes of such interference—poor categorical distinctiveness and imperfect feature binding—interact. In three experiments, we showed low and high distinctive objects and tested VWM for objects alone, for locations alone and for object-location conjunctions. We found that low object distinctiveness impairs object recognition and increases the number of object-location binding errors. Also, we dissociated the probabilities that these binding errors are due to recognition impairment or a failure of correct binding. Results show that poor distinctiveness increases binding errors rate only due to lacking recognition but not to binding impairment. Together, our findings suggest that object distinction and object-location binding act upon different components of VWM and are separate sources of interference. This study was funded by RSCF #18-18-00334.
The question of whether visual working memory (VWM) stores individual features or bound objects as basic units is actively debated. Evidence exists for both feature-based and object-based storages, as well as hierarchically organized representations maintaining both types of information at different levels. One argument for feature-based storage is that features belonging to different dimensions (e.g., color and orientations) can be stored without interference suggesting independent capacities for every dimension. Here, we studied whether the lack of cross-dimensional interference reflects genuinely independent feature storages or mediated by common objects. In three experiments, participants remembered and recalled the colors and orientations of sets of objects. We independently manipulated set sizes within each feature dimension (making colors and orientations either identical or differing across objects). Critically, we assigned to-be-remembered colors and orientations either to same spatially integrated or to different spatially separated objects. We found that the precision and recall probability within each dimension was not affected by set size manipulations in a different dimension when the features belonged to integrated objects. However, manipulations with color set sizes did affect orientation memory when the features were separated. We conclude therefore that different feature dimensions can be encoded and stored independently but the advantage of the independent storages are mediated at the object-based level. This conclusion is consistent with the idea of hierarchically organized VWM.
The influence of emotional valence (positive, negative or neutral) of realistic images on the functioning of visual working memory (WM) was studied in adults (n= 40) and adolescents (n = 17). In adults, emotional coloring of stimuli increased the reaction time and decreased the accuracy of WM task performance. This effect was more pronounced for negative than for positive valence: the minimal reaction time was observed for the neutral stimuli, the maximal for the negative emotional stimuli, and significant differences in the reaction time were found between all three types of images. The accuracy was lower for negative stimuli than for either positive or neutral stimuli. Compared with adults, adolescents of age 14–16 showed lower indices of the performance accuracy and rate with neutral and positive stimuli in the WM task. In this group, no significant influence of the emotional valence of visual stimuli on the accuracy of WM task performance was found.