Abstract
When an attentional precue is presented at one location of an object followed by a target at a different location, reaction times are faster if the precue and target are on the same object than if they appear on different objects. Three different explanations were offered to account for this object-based advantage: attentional spreading, attentional prioritization, and attentional shifting. Here, we used attentional modulations of the pupillary light response to test these explanations. In Experiment 1, we presented two rectangles composed of a luminance gradient. The gradient ranged from white to gray in one rectangle and from black to gray in the other. The attentional precue always appeared at the gray end of one of the rectangles. If attention spreads from the cued location to the rest of the object (attentional spreading) the pupil should dilate when the precue appears on the gray-to-black rectangle and constrict when the precue appears on the gray-to-white rectangle. In Experiment 2, a single rectangle was presented on a gray background. The rectangle was either black or white. A precue appeared at one end of the rectangle or in a location outside the rectangle. The target could appear in the same location as the precue or in a different location – inside or outside the rectangle. If disengaging attention from a location inside the object is indeed slower than disengaging attention from a location outside the object (attentional shifting) then changes in pupil size should start later when the precue appears inside the rectangle while the target appears outside the rectangle. The pattern of results followed the attentional spreading account in Experiment 1, and the attentional shifting account in Experiment 2. This suggests that both mechanisms contribute to object-based effects.