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Hrag Pailian, George A. Alvarez; Probing the Neurocognitive Architecture of Visual Working Memory by Enhancing Storage vs. Manipulation Abilities. Journal of Vision 2019;19(10):247. doi: https://doi.org/10.1167/19.10.247.
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In an ever-changing world, intelligent behavior is shaped by our abilities to store and manipulate representations in visual working memory (VWM). Previous work (Pailian & Alvarez, 2018; Pailian & Halberda, 2015) has demonstrated that memory storage relies on separate resources than memory updating, suggesting separate underlying neural mechanisms. Here, we probe the neural architecture of VWM, using trans-cranial direct current stimulation (tDCS) to upregulate neuronal activity in the right posterior parietal cortex (PPC) and determining its effects on memory storage vs. memory updating. Across two nearly identical sessions (counterbalanced order), we applied 20 minutes of either anodal or sham stimulation over the right PPC. During this period, participants completed a behavioral task, in which they were presented with four colored dots that were subsequently hidden by opaque occluders. On some trials, all occluders remained stationary, requiring participants to simply store color information. On other trials, pairs of occluders swapped positions, requiring participants to update the color-location bindings of the moving objects. Memory for a cued item was subsequently tested. Relative to sham stimulation, anodal-tDCS failed to improve manipulation-related performance (p=.20) across all individuals. However, significant improvements were observed for storage, such that accuracy increased by up to ~20%. This dissociation suggests that storage and manipulation rely on separate neural resources, and that factors determining manipulation ability are separate from storage constraints. Moreover, storage improvements were observed in low (p< .04) - but not high (p=.16)- memory individuals. The magnitude of this neuroenhancement scaled continuously with initial storage capacity (r=−.76, p=.001), and was driven by a reduction in misbinding errors. Neurostimulation techniques modulating different neural mechanisms may provide an important lens onto understanding how individuals vary in their storage and manipulation capacities, whether these behavioral limits reflect genuine cognitive maxima, and how storage and manipulation substrates/mechanisms interact to dynamically update visual information.
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