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Niklas Ihssen, David Linden, Kimron Shapiro; Increased vSTM for sequential displays - behavioural and neuronal dynamics. Journal of Vision 2009;9(8):570. doi: 10.1167/9.8.570.
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It has been argued that the number of items that can be maintained in visual short-term memory is limited to 3 or 4 objects (Luck and Vogel, 1997). This widely held view has been recently challenged by work of Fecteau and Shapiro (VSS 2008). They showed that splitting the to-be-remembered item display into two sequential arrays increases vSTM dramatically. Here, we report data that replicate the basic behavioural finding and aim to extend it to the neuronal domain. A change-detection paradigm was used in which participants had to remember briefly flashed displays of coloured squares and white shapes. Objects were presented at random spatial positions within a bilateral array of 18 placeholders, with coloured squares appearing in one visual hemifield and white shapes in the other hemifield. The two memory sets (squares and shapes) were shown either in two temporally separated displays (sequential condition) or in one single display (standard condition). A second display containing placeholders but no target items was introduced in the standard condition to control for perceptual load. As a main result, we found a strong increase in k (set-size dependent estimate of vSTM capacity, Cowan, 2001) for sequential versus standard displays. vSTM increased irrespective of the probed display's position in the maintenance phase (presented as 1st or 2nd display). To shed light on the neuronal mechanisms underlying these effects, we chose a two-fold approach: (1) Measuring hemodynamic responses of perceptual and memory-related brain areas in sequential versus standard trials with fMRI, (2) examining the Contralateral Delay Activity (CDA, Vogel & Machizawa, 2004), a lateralized ERP measure, which is sensitive to the number of items stored in vSTM.
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