Object-based attention: shifting or uncertainty?

The contribution of object-based attentional guidance to visual processing is widely accepted, and recent models of attentional selection now include both space- and object-based representations. Nevertheless, although the mechanism of space-based attentional selection has been well characterized, the mechanism underlying object-based attentional selection remains poorly understood. Recent attempts at identifying the mechanism giving rise to object-based attentional selection have put forth two conflicting alternatives. The first suggests that object-based effects are driven by attentional uncertainty (i.e., the attentional prioritization hypothesis), whereas the second suggests that objects guide attentional selection only when an attentional shift is necessitated (i.e., the attentional-shifting hypothesis). Here, we directly investigated which of the two suggested accounts drives object-based effects, by pitting attentional certainty against attentional shifting. In a series of four experiments, we manipulated the certainty of target location, shifting of attention, and the stimulus onset asynchrony. It was observed that object-based effects depended solely on certainty of the forthcoming target location. These results suggest that attentional prioritization, and not a mere shift of attention, gives rise to object-based guidance of attentional selection. In addition, these results lend further support to the attentional prioritization account of object-based attention and provide further constraints on the mechanisms of object-based selection.