Size matters in object-based attentional selection

To extract relevant information from a visual scene, selective attention can be deployed between objects or within a single object. It is well established that within-object attentional shifts are faster than between-object shifts. However, it is not known whether attentional shifts are influenced by the metric properties of objects. We manipulated object width in a visual target discrimination task similar to that of Egly et al. (1994). Subjects were presented with one of two types of displays: (1) two parallel rectangles or (2) two trapezoids, with both objects presented vertically or both horizontally. Rectangles were identical and of two possible widths (‘thin’ or ‘thick’). Trapezoids were identical but inverted in orientation, with the ‘thin’ base of one object adjacent to the ‘thick’ base of the other. One end of an object was cued and then participants performed a target discrimination task (target letters T or L). The cue was either valid (cue and target at same location) or invalid (cue and target at different locations within the same object or between objects). Faster RTs were observed for valid vs. invalid trials, and for within- vs. between-object locations. Interestingly, however, the object-based effect was modulated by object size. Moving attention within or between ‘thick’ objects or toward the ‘thick’ end of objects, resulted in significantly faster RTs than the corresponding shifts of attention involving ‘thin’ objects or ‘thin’ object parts. These results demonstrate that object width influences the distribution of spatial attention. We discuss our findings in the context of sensory enhancement and prioritization accounts of object-based attention. We also examine the relationship between attentional selection and movement execution, suggesting that deployment of object-based attention plays a role in determining how target size and distance constrain the time required to move from a starting position to a final target (Fitt's Law).