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Won Mok Shim, George A. Alvarez, Yuhong Jiang; Capacity limit of visual working memory in parietal cortex reflects capacity limit of spatial selection. Journal of Vision 2005;5(8):914. doi: 10.1167/5.8.914.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Recent neuroimaging studies have revealed that the activation seen in fMRI of the parietal cortex parallels behavioral performance in visual working memory (VWM) tasks: As the number of visual objects increases from 1 to 4, the posterior parietal cortex increases its activity monotonically but remains constant thereafter as does memory performance (Todd & Marois, 2004). However, the capacity limits of VWM can arise from two different sources, 1) holding attention on multiple locations in space and 2) maintaining identity information in memory. To determine which process causes the saturation of parietal activation, we conducted a VWM experiment in fMRI using two different modes of presentation: simultaneous (items presented simultaneously at different spatial locations) and sequential (items presented sequentially at the same location). Indexing spatial positions and storing identity information are both necessary in the simultaneous presentation, but indexing spatial locations is minimized in the sequential presentation, leaving only the memory component. Methods: 1, 2, 3, 4, or 6 colors were presented, either simultaneously or sequentially, for subjects to remember. We estimated capacity in each set size from behavioral data and compared them to the response functions of the parietal cortex. Results: While memory capacity was matched between simultaneous and sequential conditions, the parietal area showed differential activity. In particular, parietal activity as a function of set size paralleled behavioral performance in the simultaneous condition, but was insensitive to set size in the sequential condition. Conclusion: These results support the idea that VWM limitation reflects two dissociable components: maintaining spatial attention to the target locations and remembering their identities. These components have separable neural correlates. The parietal cortex contributes more to the maintenance of attention than to remembering each item's identity.
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