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Yaoda Xu, Marvin M. Chun; Brain mechanisms supporting visual short-term memory for multi-feature objects. Journal of Vision 2006;6(6):1092. doi: 10.1167/6.6.1092.
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Recent brain imaging studies showed that responses in the superior intra-parietal sulcus (IPS) correlate with the number of object colors or shapes stored in visual short-term memory (VSTM). Because only single-feature objects were used in previous studies, it was not clear how multi-feature objects are stored. We measured whether superior IPS responses correlate with integrated object representations for which all features are stored or with object representations for which at least one feature is stored. To address this question, we conjoined a set of dissimilar color features with a set of similar shape features, and asked observers to retain such color and shape conjunctions. When both color and shape features are encoded together, behavioral measures yielded a VSTM capacity of about 3 colors and 1 shape. We examined responses in the superior IPS and two visual areas: the lateral occipital complex (LOC), which is involved in shape processing and whose activations also correlate with VSTM capacity for shapes; and area V4, which participates in color perception. Correlating with behavioral performance, V4 responses showed a capacity limit of 3 colors while LOC responses showed a capacity limit of 1 shape, indicating independent storage of color and shape features in these brain areas when both types of features are encoded together in VSTM. Superior IPS responses, interestingly, showed a capacity limit of 3, suggesting that this brain area represents the total number of remembered objects for which at least one feature is stored.
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