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Yaoda Xu, Marvin M. Chun; Representing objects in visual short-term memory: The roles of the human intra-parietal sulcus and the lateral occipital complex. Journal of Vision 2005;5(8):609. doi: 10.1167/5.8.609.
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An unresolved issue in visual short-term memory (VSTM) research concerns whether capacity is determined by a fixed number of slots or whether it is dependent on the complexity of the encoded objects. The intra-parietal sulcus (IPS) plays a central role in VSTM based on evidence that fMRI activations in IPS are modulated by the number of objects that observers can maintain in VSTM (Todd & Marois, 2004). In an event-related fMRI study, we varied shape encoding difficulty to manipulate VSTM capacity, and examined how IPS activity was modulated by such changes. In addition to IPS, we also examined activations in the lateral occipital complex (LOC), an area involved in object shape processing. We used a standard change detection paradigm in which observers were shown 1, 2, 3, 4, or 6 objects, and after a brief delay, detected a possible shape feature change to one of the objects. With the same set of objects, we examined VSTM for two types of shape features: In the easy condition, observers remembered whether each object in the display contained a hole or not; and in the hard condition, observers remembered the shape contour of each object. This task yielded a VSTM capacity of about 3 to 4 objects in the easy condition and about 2 objects in the hard condition. We found that activations in the LOC, but not IPS, followed the number of objects that observers could hold in VSTM: Activations in the LOC asymptoted at 3 objects for the easy condition and at 2 objects for the hard condition, whereas activations in the IPS asymptoted at 3 objects for both conditions. These results suggest that: (1) The IPS may index the number of objects present in a display in a slot-like representation with fixed capacity that determines the maximum number of objects an observer can maintain in VSTM; and (2) the LOC supports object representations in VSTM using a resource-like representation with variable capacity that is determined by the nature of the visual features encoded.
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