Attentional capture by globally defined objects

The abrupt appearance of a new perceptual object in the visual field typica lly captures visual attention. However, if attention is focused in advance on a different location, onsets can fail to capture attention (Yantis & Jon ides, 1990). In the present experiments, we investigated the extent to whic h the deployment of attention to the local level of a hierarchical scene ma y be affected by the abrupt appearance of a new object at the global level. Participants searched for a semi-disk target in an array of randomly orien ted segmented disks ("pacmen"). On half the trials, a subset of the segment ed disks induced a subjective square. On these critical trials, participant s were significantly slower to respond to the presence of a local target ev en though the local features of the display were qualitatively identical ac ross all conditions. This slowing was absent when outline pacmen were used (which do not induce subjective figures) and when the subjective square was perceptually old. When the participants' task was defined at the global le vel of the display, a new local element failed to capture attention, sugges ting an asymmetry in the ability of objects at different levels of a hierar chical scene to capture attention. In a control experiment, a new local ele ment captured attention, however, when the participants' task was defined a t the local level, indicating that the local item was in principle capable of capturing attention. It is argued that global objects capture attention because they convey important information about the environment that is not available at the local level.