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Maha Adamo, Kristin Wilson, Morgan D. Barense, Susanne Ferber; Dissociating feature complexity from number of objects in VSTM storage using the contralateral delay activity. Journal of Vision 2010;10(7):719. doi: 10.1167/10.7.719.
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Many recent studies have examined the neural correlates of visual short-term memory (VSTM) maintenance using an ERP component known as the contralateral delay activity (CDA), whose amplitude corresponds to memory load within individuals and to memory capacity across individuals. The parietal distribution of the CDA makes it a particularly compelling locus of capacity-limited VSTM storage given that it overlaps with fMRI findings of feature- and location-based VSTM systems located in the superior and inferior intra-parietal sulcus. An under-explored question, however, is the extent to which the CDA indexes the feature complexity of items to be remembered or the number of objects/locations to be remembered, or both. We employed a lateralized change detection task in which the feature complexity and number of items to be remembered were independently manipulated. Items to be remembered were either simple features (shape, color, or orientation) or conjunctions of these features, and they were presented either at one location or at three locations. Behavioural results demonstrated that individuals performed comparably for simple features and conjunctions presented one at a time, while performance for simple features declined when three were presented at different locations relative to when they were conjoined in one object. We found that ERP amplitudes at the lateral, posterior sites that are typically measured in the CDA reflected the number of objects to be remembered, while more central, anterior sites indexed the complexity of the objects to be remembered. Thus, feature- and location-based systems in the parietal cortex can be dissociated even at the course spatial resolution of ERP.
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