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Yaoda Xu, Marvin Chun; Grouping determines object-based selection in human inferior intra-parietal sulcus. Journal of Vision 2007;7(9):128. doi: 10.1167/7.9.128.
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Visual objects are important units in visual processing, and yet how the brain determines what constitutes as a discrete visual object remains largely unknown. Previously, in a visual short-term memory (VSTM) study, we showed that, regardless of object complexity, fMRI response in the inferior intra-parietal sulcus (IPS) increased as the number of objects at different spatial locations increased (up to about four objects) (Xu & Chun, 2006, Nature). This finding indicates that the inferior IPS may participate in visual object individuation and selection, and its activation may reflect the number of discrete visual objects represented in the brain. In the current VSTM study, we asked: If object locations are fixed, but the grouping between objects differs, would that change the number of discrete objects represented in the brain? Our display consisted of two filled black rectangles on either side of the central fixation, similar to Egly, Driver and Rafal (1994, JEP:G). Observers encoded in VSTM two gray shapes appeared on the black rectangles and decided whether a probe shape at test matched one of the remembered shapes. Although grouping by closure (rectangles) was irrelevant to the VSTM task and could be ignored, the inferior IPS activation was lower when the two shapes appeared on the two ends of the same rectangle than on the same end of both rectangles. With an increased VSTM load, this grouping by closure allowed more shape information to be retained in VSTM as reflected in behavioral VSTM performance and fMRI responses in the superior IPS, which correlate with VSTM capacity. Interestingly, grouping did not modulate responses in the lateral occipital complex, an area involved in shape processing. Thus, grouping changes the number of discrete objects represented, and the inferior IPS may play a key role in determining object-based visual processing in the brain.
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