The neural mechanisms by which information is maintained in working memory are challenging to study, as such maintenance is not always correlated with detectable sustained neural activity. Perturbation of the memory network with a visual impulse has proven useful to probe memoranda in such activity-quiescent states. However, the impulse perturbation approach has so far been applied exclusively to spatially localized or spatially referenced stimuli, such as orientations or tone frequencies. It is unknown whether the neural maintenance mechanisms of non-spatial memoranda would be analogous. In the present study we therefore applied the impulse perturbation method to working memory for colours, which are intrinsically non-spatial stimuli, focusing on a set of pre-registered analyses. We analysed the EEG data of 30 participants who completed a delayed match-to-sample working memory task, in which one of the two items that were presented was retro-cued as task-relevant. We assessed the colour space underlying memory maintenance, and found that both cued and uncued colours were decodable from impulse-evoked activity, the latter in contrast to previous reports of working memory for orientation gratings. We furthermore examined colour decoding from ongoing oscillations in the alpha band, and found that cued items could be decoded therein, whereas uncued items could not, which might be mediated by attention. Overall, the outcomes suggest that subtle differences exist between the representation of colours and that of stimuli with spatial properties, but the present results also demonstrate that regardless of their specific neural state, both are accessible through visual impulse perturbation.
Working memory is known to be capacity limited and is therefore selective not only for what it encodes but also what it forgets. Explicit forgetting cues can be effectively used to free up capacity, but it is not clear how working memory adaptively forgets in the absence of explicit cues. An important implicit cue that may tune forgetting in working memory is the passage of time. When information becomes irrelevant more quickly, working memory should also forget information more quickly. In two delayed-estimation experiments, we systematically manipulated how probing probability changed over the course of a trial (i.e. the ‘probing’ hazard). In some blocks, probing hazard decreased over time, requiring participants to only briefly retain the memory item. In other blocks, the probing hazard increased or stayed flat. In line with our hypothesis, we found that participants adapted their forgetting speed to the probing dynamics of the working memory task. When the memory item quickly became irrelevant, forgetting speed was higher than in blocks where it increased or stayed flat. This effect seemed to be largely driven by participants completely erasing the item by the time it had become irrelevant. These findings suggest that implicit adaptations to the temporal structure of our environment tune forgetting speed in working memory, possibly contributing to the flexible allocation of limited working memory resources.
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