Object-based attention is "turned off" by top-down control

Object-based attention is manifested by better detection performance at a spatial location within an attended object, as compared to an unattended object (e.g., Egly, Driver, & Rafal, 1994). Shomstein and Yantis (2002) discussed two basic approaches that may account for this object-based effect. First, the early sensory enhancement account suggests that attention automatically spreads along the boundary of an object. Second, the attentional prioritization account proposes that higher attentional priority is given to locations that are within an attended object. Different mechanisms for object-based attention may predict different degrees of top-down control on object-based effects. For instance, attentional prioritization is thought to be largely configurable (e.g., Wolfe, 1994), whereas sensory enhancement effects appear to be relatively automatic. Previous studies have generally observed that object-based effects are reduced or even eliminated when focal attention is guided by other space-based, contextual information (Goldsmith & Yeari, 2003; Shomstein & Yantis, 2004; Yeari & Goldsmith, 2006). However, although object-based effects can be offset by deliberately engaging other spatial cues, this does not mean that the operation of object-based attention itself was moderated. Therefore, it remains an open question as to whether object-based attention can be modified by top-down control.
The present study is designed to address this issue. A direct way to answer this question is to create a situation in which object-based attention will impair a task. If an object-based effect is still observed in this situation, this would mean that object-based attention is obligatory; if the object-based effect is eliminated, this would mean object-based attention is modifiable by top-down processes.