Abstract
It is well established that participants respond faster to targets appearing on the same object as an attentional precue, compared with locations outside the cued object. Yet there is no agreement regarding the mechanisms underlying this object-based attentional effect. For instance, the ‘Attentional shifting’ account attributes this object-based effect to the higher cost of shifting attention between objects than within an object, while the ‘Attentional spreading’ account proposes that object-based effects arise due to the automatic spreading of attention along the object. We utilized a continuous measure of object-based attention, which relies on attentional modulations of the pupillary light response, to test these two accounts of object-based attention. To test the spreading account, we employed objects composed of white-to-gray and black-to-gray luminance gradients. If attention indeed spreads along the object, pupil size should be larger when the gray end of the black-to-gray object is cued than when the gray end of the white-to-gray object is cued. To test the shifting account, we displayed a single object that could be black or white. A precue appeared at one end of the object or in a location outside the object, and a target followed at the same location or in a different location – outside or inside the object, respectively. If disengaging attention from a location inside the object is indeed slower, then changes in pupil size should start later when the precue appears inside the object and the target appears outside the object than vice versa. We found evidence for the attentional shifting account; changes in the pupillary light response were observed later when the attentional shift involved disengaging from an object. In contrast, no evidence for involuntary spreading of attention was observed. Evidence for attentional spreading along the object emerged only when such spreading matched the observers’ goals.