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Viljami R. Salmela, Meri Lähde, Jussi Saarinen; Visual working memory for amplitude-modulated shapes. Journal of Vision 2012;12(6):2. doi: 10.1167/12.6.2.
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We investigated the trade-off between capacity and precision in visual working memory with two different tasks: delayed discrimination and recall. The stimuli were radial frequency patterns that require global pooling of local visual features. The thresholds in delayed amplitude discrimination were measured with a two-interval, forced-choice setup using the Quest procedure. In the recall experiment, the observers' task was to adjust the amplitude of a probe to match the amplitude of a cued item. For one item, the amplitude thresholds were low (0.01–0.05) and the adjustments precise (standard deviations, 0.03–0.05). As the number of items increased from one to six, there was a linear, 6-to-14-fold increase in the thresholds (0.14–0.29) and a 1.5-to-3-fold increase in the standard deviations (0.06–0.11). No sudden or complete breakdown in performance was observed for any subject. The results show a continuous trade-off between memory capacity and precision; six items can be discriminated with the same performance level (75% correct) as one item if the difference between the stimuli is set accordingly. Thus, the stimulus discriminability determines the capacity of visual working memory, and the trade-off between the capacity and precision is linear.
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