Previous studies suggest that object-based effects are the product of attentional prioritization (Shomstein & Yantis, 2002, 2004; Drummond & Shomstein, 2008). What remains unclear, however, is whether prioritization enhances spatial locations (i.e., space-based representations) or surfaces that occupy them (i.e., object-based representations), and what effect target location certainty has on attentional allocation. In a traditional two-rectangle paradigm, the space- and object-based reference frames overlap, and therefore it is not known which representation is prioritized, or if both reference frames are prioritized to the same degree. To investigate the effects of prioritization on these representations, we used a dynamic display in which, following a spatial cue, the rectangles rotated 180?°. The target then appeared either in the cued spatial location, in the same- or different-object location, or in the cued object location. This manipulation successfully separated the spatial and object reference frames by creating non-matching spatial and object locations (Becker and Egeth, 2000). To examine the time course of attentional prioritization, space-based, object-based, and cued object effects were sampled at six SOAs, ranging from 200ms to 1500ms. Space-based effects exhibited a benefit at the shortest SOA, followed by an inhibition of return (IOR). The effect size was significantly reduced as compared to static displays, suggesting that spatial prioritization and object prioritization have an over-additive relationship. We found no object-based effects at any of the SOAs. Instead, only the cued object location received the benefit of prioritization (after 200ms), much the way certainty allows for prioritization of only the target location in static displays (in both cases, no object-based effects were present). The observed pattern of results indicates that attention tracks the cued object location through the rotation, eliminating the influence of whole objects and thereby eliminating object-based effects. Taken together these results place further constraints on the mechanisms of attentional allocation.
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