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David E. Anderson, Edward Awh; Perceptual grouping enables multiple items to be stored within a single slot in visual working memory. Journal of Vision 2011;11(11):1246. doi: https://doi.org/10.1167/11.11.1246.
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Resolution in visual working memory (WM) declines monotonically with set size. Discrete resource models account for this relationship by positing a fixed set of slots that constrain the allocation of the resources that determine WM resolution. Critically, the discrete view predicts that precision will reach a stable asymptote once the item limit is exceeded because resources are allocated only to stored items. In line with this prediction, we recently observed that precision in an orientation WM task reached asymptote at a set size that was strongly predicted by behavioral and neural estimates of each observer's item limit. Here we used the same approach to examine whether perceptual grouping cues enable multiple items to be represented within a single slot. We measured WM performance in an orientation recall task using solid discs containing prominent gaps. On half of the trials, the gaps in pairs of items were collinear with respect to each other, such that an illusory rectangle was formed (grouped condition). Orientation was random during the other half (ungrouped condition). As in the previous work, WM precision reached a stable asymptote at a relatively low set size. Moreover, the set size at which this asymptote was achieved was approximately doubled in the grouped condition, suggesting that multi-item perceptual groups can be stored within a single discrete slot in visual WM. Across the grouped and ungrouped conditions, there was robust correlation between the set sizes at which asymptote occurred, suggesting that these asymptotes were the product of the same fixed item limit. Thus, the assignment of visual items to discrete slots in memory is done on the basis of scene-level representations that are sensitive to Gestalt grouping cues, rather than image-level representations in which each disc is individuated in visual WM.
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