Attention Does Not Spread Automatically Along Objects: Evidence from the Pupillary Light Response

Objects influence attention allocation; when a location within an object is cued, participants react faster to targets appearing in a different location within this object than on a different object. Despite consistent demonstrations of this object-based effect, there is no agreement regarding its underlying mechanisms. To test the most common hypothesis that attention spreads automatically along the cued object, we utilized a continuous, response-free measurement of attentional allocation that relies on the modulation of the pupillary light response. In Experiments 1 and 2, attentional spreading was not encouraged because the target appeared often (60%) at the cued location and considerably less often at other locations (20% within the same object and 20% on another object). In Experiment 3, spreading was encouraged because the target appeared equally often in one of three possible locations within the cued object (cued-end, middle, uncued-end). In all experiments, we added gray-to-black and gray-to-white luminance gradients to the objects. By cueing the gray ends of the objects, we could track attention. If attention indeed spreads automatically along objects, then pupil size should be greater after the gray-to-dark object is cued because attention spreads towards darker areas of the object than when the gray-to-white object is cued, regardless of the target location probability. However, unequivocal evidence of attention spreading along the object was only found when spreading was encouraged. These findings do not support an automatic spreading of attention. Instead, they suggest that attentional spreading along the object is guided by cue-target contingencies.