Real-world objects are not stored in holistic representations in visual working memory
During difficult foraging tasks, humans rarely switch between target categories, but switch frequently during easier foraging. Does this reflect fundamental limits on visual working memory (VWM) capacity or simply strategic choice due to effort? Our participants performed time-limited or unlimited foraging tasks where they tapped stimuli from 2 target categories while avoiding items from 2 distractor categories. These time limits should have no effect if capacity imposes limits on VWM representations but more flexible VWM could allow observers to use VWM according to task demands in each case. We found that with time limits, participants switched more frequently and switch-costs became much smaller than during unlimited foraging. Observers can therefore switch between complex (conjunction) target categories when needed. We propose that while maintaining many complex templates in working memory is effortful and observers avoid this, they can do so if this fits task demands, showing the flexibility of working memory representations used for visual exploration. This is in contrast with recent proposals, and we discuss the implications of these findings for theoretical accounts of working memory.
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.
Prevailing theories of visual working memory assume that each encoded item is stored or forgotten as a separate unit independent from other items. Here, we show that items are not independent, and that the recalled orientation of an individual item is strongly influenced by the summary statistical representation of all items (ensemble representation). We find that not only is memory for an individual orientation substantially biased towards the mean orientation, but the precision of memory for an individual item also closely tracks the precision with which people store the mean orientation (which is, in turn, correlated with the physical range of orientations). Thus, individual items are reported more precisely when items on a trial are more similar. Moreover, the narrower the range of orientations present on a trial, the more participants appear to rely on the mean orientation as representative of all individuals. This can be observed not only when the range is carefully controlled, but also shown even in randomly generated, unstructured displays, and after accounting for the possibility of location-based ‘swap’ errors. Our results suggest that the information about a set of items is represented hierarchically, and that ensemble information can be an important source of information to constrain uncertain information about individuals.
Eye-tracking is a non-invasive measure that has been repeatedly used for studying attention and related cognitive processes. While eye-tracking is not a direct measure of brain activity, it has been shown to reveal information about mental processes, that may not be easily accessible through other measures, such as problem solving strategies. Mental attentional capacity corresponds to the amount of information an individual can maintain and manipulate in mind (Pascual-Leone, J. ,1970); it is considered the central maturational component of working memory (Arsalidou, M., Pascual-Leone, J., & Johnson, J. ,2010). This construct has been found to be closely related to other aspects of cognitive competence and intelligence (Johnson et al., 2003). Research into relation between eye movements and mental attentional capacity across development at the moment is sparse and fragmented and no eye tracking studies have been conducted so far with parametric developmental measures, such as the colour matching tasks (Arsalidou, M., Pascual-Leone, J., & Johnson, J. ,2010), which would allow to dissociate changes in saccades and fixations related to working memory load (n = 6) from those related to interference control and trace the maturation of these two processes. The purpose of this study is to investigate the relation between eye-tracking indices (e.g., number of fixations) and mental attentional capacity. Data from adult participants showed significant differences between number of fixations per trial for different levels of mental attentional load. Additionally, analysis revealed significant negative correlation between number and duration of fixations and accuracy for both the balloons and the clowns versions of the task, with the correlation being stronger for the clowns version, which contains interference. Interestingly, for each difficulty level, children generate a similar number of fixations regardless of interfering features, whereas adults make fewer fixations when the task has less interfering features. This suggests that adults may have different strategies depending on the task. Increased number of fixations may indicate that children favor a visual-spatial strategy, whereas adults favor a verbal strategy.
Our interactions with the visual world are guided by attention and visual working memory. Things that we look for and those we ignore are stored as templates that reflect our goals and the tasks at hand. The nature of such templates has been widely debated. A recent proposal is that these templates can be thought of as probabilistic representations of task-relevant features. Crucially, such probabilistic templates should accurately reflect feature probabilities in the environment. Here we ask whether observers can quickly form a correct internal model of a complex (bimodal) distribution of distractor features. We assessed observers’ representations by measuring the slowing of visual search when target features unexpectedly match a distractor template. Distractor stimuli were heterogeneous, randomly drawn on each trial from a bimodal probability distribution. Using two targets on each trial, we tested whether observers encode the full distribution, only one peak of it, or the average of the two peaks. Search was slower when the two targets corresponded to the two modes of a previous distractor distribution than when one target was at one of the modes and another between them or outside the distribution range. Furthermore, targets on the modes were reported later than targets between the modes that, in turn, were reported later than targets outside this range. This shows that observers use a correct internal model, representing both distribution modes using templates based on the full probability distribution rather than just one peak or simple summary statistics. The findings further confirm that performance in odd-one out search with repeated distractors cannot be described by a simple decision rule. Our findings indicate that probabilistic visual working memory templates guiding attention, dynamically adapt to task requirements, accurately reflecting the probabilistic nature of the input.