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Lingling Wang, Steven B. Most, James E. Hoffman; Contralateral delay activity is sensitive to the spatial distribution of items in working memory: An ERP study. Journal of Vision 2009;9(8):599. doi: 10.1167/9.8.599.
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© ARVO (1962-2015); The Authors (2016-present)
Recent studies have established a neurophysiological index sensitive to the number of items maintained in visual working memory (VWM). The amplitude of this ERP waveform - a widespread negative deflection appearing contralateral to the visual hemi-field containing VWM targets - approaches asymptote as the number of targets reaches VWM capacity and is known as contralateral delay activity (CDA; Vogel, McCollough, & Machizawa, 2005). In previous studies, VWM targets were always presented in different locations, a constraint imposed by their simultaneous appearance. Thus, it remains uncertain whether the CDA reflects the number of items encoded or, perhaps instead, the number of locations requiring attention. In the current study, participants maintained either 1 or 2 color squares in VWM; in the larger set size, targets were presented sequentially, so that they could appear either at the same or different locations. If the CDA is sensitive only to the number of objects, its amplitude should be the same regardless of whether the objects appeared in a single or multiple locations. In contrast, if the CDA is sensitive to the number of spatial locations, its amplitude should be the same for a single object and two objects presented in a single location, and in both of these conditions its amplitude should be smaller than when two objects are presented in different locations. The data were consistent with the latter prediction: CDA amplitude was highest for two squares appearing at different locations, while amplitudes in response to a single square and two squares appearing at the same location were equivalent. These results suggest that at least some components of the CDA are sensitive to the number of locations occupied by VWM targets rather than to the number of targets per se.
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