Inhibition is picky: shape difference is a necessary condition for attentional inhibition of irrelevant objects.

The present study investigated the conditions for attentional amplification in the processing of relevant objects and for attentional inhibition in the processing of irrelevant objects. Participants reported the color of one of two superimposed objects that was specified by occlusion. Irrelevant color words were presented as part of the relevant object, as part of the irrelevant object, or in the background; the words were either congruent or incongruent with the color of the relevant object. The size of the congruency effects provided a measure of the (relative) strength of the processing of the objects and the background. Finally, the two objects had the same shape in one session and different shapes in another session. In both sessions, results showed larger Stroop effects from words belonging to the relevant object than from words in the background, indicating attentional amplification of the relevant object that was unaffected by object similarity. In contrast, smaller Stroop effects from words belonging to the irrelevant object than from words in the background occurred only when the two objects differed in shape. The latter result suggests that a shape difference is both a necessary and a sufficient condition for inhibiting the processing of an irrelevant visual object.