Research has shown that there are at least two kinds of visual selective attention: location based and object based. In the present study, we sought to determine the locus of spatially invariant object-based selection using a dual-task paradigm. In four experiments, observers performed an attention task (object feature report or visual search) with a concurrent memory task (object memory or spatial memory). Object memory was interfered with more by a concurrent object-based attention task than by a concurrent location-based attention task. However, this interference pattern was reversed for spatial memory, with greater interference by a location-based attention task than by an object-based attention task. These findings suggest that object-based attention and locationbased attention are functionally dissociable and that some forms of object-based selection operate within visual short-term memory.
Visual attention selects objects as well as locations; observers can identify two attributes of a single object more accurately than two attributes of two different objects. For example, when two superimposed objects were shown, observers were more accurate at reporting two target attributes from one object rather than from two different objects (Awh, Dhaliwal, Christensen, & Matsukura, 2001; Duncan, 1984; Vecera & Farah, 1994).
Despite numerous demonstrations of object-based effects of attention, the locus of object-based attention is less well established. Vecera and Farah (1994; Vecera, 1994; see also Awh et al., 2001) suggested that there were two forms of object-based selection. Objects could be selected from a relatively late selection stage that represents the shape of an object and combines that object's features. In such a representation, an object's shape (i.e., features and parts) is selected, not the object's location. Objects could also be selected from a relatively early grouped array representation, which allows locations to be selected by spatial attention. In this grouped array selection, object-based attention arises because locations within an object are perceptually grouped, whereas locations between objects are weakly grouped. Distinguishing these two forms of object-based selection has been difficult, and the literature has focused on a grouped array interpretation of object-based effects.
In the present experiments, we asked whether some object-based effects of attention can occur at a relatively late visual processing stage that operates after objects have disappeared and are no longer available to perceptual processes. One candidate for such a late object-based attention process is visual short-term memory (VSTM). VSTM holds a small number of items across an eye movement (Hollingworth, Richard, & Luck, 2008; Irwin, 1992; Irwin & Andrews, 1996) or across a temporal delay without an eye movement (Luck & Vogel, 1997; Vogel, Woodman, & Luck, 2001). VSTM can maintain approximately three to four objects, irrespective of the number of features that an object possesses (e.g., Awh, Barton, & Vogel, 2007; Gajewski & Brockmole, 2006; Luck & Vogel, 1997; Vogel et al., 2001; but see Alvarez & Cavanagh, 2004).
Previous research on the relationship between visual memory and attention has focused primarily on spatial memory and spatial attention. For example, spatial attention acts as a rehearsal mechanism for spatial memory. Observers are faster to perform perceptual discriminations on targets appearing at locations held in spatial memory than on targets appearing elsewhere (Awh, Jonides, & Reuter- Lorenz, 1998). Similarly, the rate of spatial visual search slows down when observers perform a concurrent spatial memory task (Oh & Kim, 2004; Woodman & Luck, 2004; but see Woodman, Vogel, & Luck, 2001).
Some research has investigated the relationship between object memory and object-based attention. Barnes, Nelson, and Reuter-Lorenz (2001) asked observers to maintain an object in visual memory while performing an object-based attention task that required them to decide which of two vertices of a hexagonal object was located higher than the other (a vertex judgment task). …