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Jean-Francois Delvenne, Laura Kaddour, Julie Castronovo; An electrophysiological measure of visual short-term memory capacity within and across hemifields. Journal of Vision 2010;10(7):716. doi: 10.1167/10.7.716.
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Recent ERPs studies have identified a specific electrophysiological correlate of the contents of visual short-term memory (VSTM) (McCollough et al., 2007; Vogel & Machizawa, 2004). A sustained posterior negative wave was observed throughout the memory retention period, which was larger over the contralateral side of the brain (with respect to the position of the memory items in the visual field) relative to the ipsilateral side. Importantly, the amplitude of this contralateral delay activity (CDA) increased progressively with the number of items to be remembered, reaching an asymptotic limit at around 3-4 objects. This contralateral organization of visual memories raises the possibility that each hemisphere has its own capacity of storage: more items could be held in memory when they are split between the left and right hemifields as when they are all presented within a single hemifield. In the present study, we measured CDA amplitude in 15 participants while they remembered colored squares from either one hemifield or both hemifields. We found that the amplitude of the CDA was modulated by the total number of items held in memory, independently of their spatial distribution in the visual field. When individuals had to remember one side of the memory array, the CDA activity increased for arrays of one, two, and three items, but ceased to get larger for arrays of four items. However, when individuals had to memorize the items from both sides of the memory array, this contralateral activity reached its asymptotic limit for arrays of two items per side. These results suggest that despite being contralaterally organized, VSTM is limited by the number of objects from both hemifields. VSTM may consist of a pool of resources that can be allocated flexibly to one or both hemifields and allow a maximum of 3–4 objects to be maintained simultaneously.
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