What is the unit of visual attention? Object for selection, but Boolean map for access

In the past 20 years, numerous theories and findings have suggested that the unit of visual attention is the object. In this study, I first clarify 2 different meanings of unit of visual attention, namely the unit of access in the sense of measurement and the unit of selection in the sense of division. In accordance with this distinction, I argue that an object, as commonly described, is only the unit of selection. The unit of access is better characterized as a Boolean map (Huang & Pashler, 2007), that is, the linkage of a single feature value per dimension associated with a map (i.e., a set of locations). The experiments in this study demonstrated the following: (a) Grouping items into a single object (by connecting them) does not improve the perception of these items (Experiment 1); (b) same-object advantage exists only when the features to be perceived are different dimensions of a single Boolean map and not when they belong to different parts of an object (Experiments 2 and 3); (c) cuing the relevant feature does not help perception when the features to be perceived are different dimensions of a single Boolean map but does help significantly when these features belong to different parts of an object (Experiment 4); and (d) connection, as used in Experiments 1-4, is effective in affecting object structure (i.e., affecting the mechanism of selection) in both an enumeration and a tracking task (Experiments 5 and 6). The results of these experiments, together with data available in the literature, demonstrate that the unit of access is a Boolean map, not an object.