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Niklas Ihssen, David Linden, Kimron Shapiro; A biased-competition account of VSTM capacity limits as revealed by fMRI. Journal of Vision 2010;10(7):721. doi: 10.1167/10.7.721.
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The capacity of visual short-term memory can be increased significantly when the to-be-remembered objects are presented sequentially across two displays, rather than simultaneously in one display (Ihssen, Linden, & Shapiro, VSS, 2009). Interestingly, a similar performance increase is observed when the (simultaneous) display is repeated. The present study sought to elucidate brain mechanisms underlying the sequential and repetition benefit. We used functional magnetic resonance imaging during a change detection task where participants had to maintain 8 different objects (colours and white shapes). Objects were presented either simultaneously (baseline condition), split into two temporally separated 4-object arrays (sequential condition), or presented twice (repetition condition). Importantly, conditions were matched for perceptual load and visual onset by filling the sequential 4-object arrays with placeholders and presenting a second ""empty"" placeholder array in the baseline condition. Whole-brain BOLD analyses revealed two main results: Relative to the baseline condition, both sequential and repeated conditions evoked stronger brain responses in extrastriate visual areas. Mirroring memory performance, BOLD amplification in these areas may relate to the higher number of object representations that are activated along the ventral pathway. In contrast to the occipital effects, two key regions of the frontoparietal attention/working memory network dissociated the sequential and repetition conditions. In the inferior parietal lobe and the frontal eye fields, sequential displays elicited reduced brain activation, relative to BOLD responses in the repetition and baseline conditions, which showed no difference. Implications of these findings are discussed within the framework of biased competition.
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