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Posterior parietal cortex responds to set size in perceptual as well as working memory tasks
14th Annual Meeting of the Cognitive Neuroscience Society, D30
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It has recently been shown using fMRI that activity in the posterior parietal cortex (PPC) correlates with the limited number of objects that can be held in visual short term memory (VSTM) (Todd & Marois, 2004; Todd & Marois 2005; Xu & Chun, 2006). Here we replicate this finding (using set sizes of 1, 4 and 8 objects) and extend it to three other tasks that use a similar stimulus set but have no explicit memory requirement. The tasks were designed to manipulate attentional demands across space and time. In PPC we find a capacity-limited set-size effect in the VSTM task, but also in two of three other tasks which do not tax VSTM. In an anterior cingulateregion we find a similar effect but only in the memory task. In occipital regions, a capacity-limited effect is again seen in the VSTM task and one of the perceptual tasks, along with a linear effect of set-size in some of the perceptual tasks. These findings cannot easily be explained by behavioural performance measures or memory demands alone, and suggest a possible role of the PPC in segregating and maintaining distinct, task-relevant object representations, whether in short-term memory or the current perceptual scene. This is consistent with the idea that working memory is an emergent property of attention being flexibly allocated to internal representations (Postle, 2006).