Because of the known neurophysiological spatial receptive field organization of the visual system, attention is thought to be primarily space-based; that is, the information to which one attends is selected based on its location in the visual field. As a result, attention directed to these spatial locations allows an individual to more deeply and efficiently process selected information (Posner, 1980; Posner, Snyder, & Davidson, 1980). However, it has been demonstrated that the representational basis of attentional selection can also be object-based (Duncan, 1984; Egly, Driver, & Rafal, 1994). Object-based attention (OBA) generally leads to a preferential and simultaneous processing of visual information that is typically characterized by faster RTs and heightened accuracy to targets contained within the boundaries of an attended object compared to targets contained within the boundaries of an unattended object, otherwise known as a same-object advantage. As a result, information processing within the boundaries of an attended object occurs more rapidly compared with the information processing that occurs between objects.
In the first demonstration of object-based attention, observers were instructed to report pairs of features that exist on a single object or on two different objects. Observers were more accurate and faster at reporting pairs of features of the same object as opposed to pairs of features spanning different objects, demonstrating an attentional cost incurred from shifting attention away from the attended object (Duncan, 1984). A task was later developed in which both space-based and object-based attention could be measured simultaneously using a double-rectangle cueing paradigm (Egly, Driver, and Rafal, 1994). Shifts of attention within two parallel rectangles (oriented either vertically, to the left and right of the vertical meridian; or horizontally, above and below the horizontal meridian) are contrasted against shifts of attention between rectangles. In this task, a spatial cue (typically, a brightening of one end of one rectangle) appears briefly, after which a single target appears in one of three possible locations on the objects: (1) the cued location of the cued rectangle (“Valid location”), (2) a non-cued location of the cued rectangle (Invalid-same location), or (3) a non-cued location of the non-cued rectangle (Invalid-different location). Critically, the two invalid target locations are equidistant from the cue and, thus, allow for the measurement of object-based attention. Observers are faster to detect targets at the valid location on the cued rectangle compared with targets at the invalid-same location, a demonstration of space-based attentional selection. Critically, observers are also faster to detect targets at the invalid-same location than the invalid-different location, indicating that the cue draws observers’ attention to aspects of the cued object (not simply the cued location), producing an object-based attention effect that cannot be explained solely by space-based attention, since the invalid targets are equidistant from the cue.
Recent efforts have moved away from simply documenting instances of object-based attention to understanding the mechanisms that underlie object-based attentional selection. One postulated theory is the sensory enhancement account. Under this view, attention (in the form of a spatial gradient) is first centered on a cued location, which is followed by an automatic spreading of attentional resources within the boundaries of the cued object, ultimately improving the quality of an early sensory representation of the cued object as a whole (Chen & Cave, 2006, 2008; Richard, Lee, & Vecera, 2008). As a result, visual information within the boundaries of the cued object is enhanced relative to visual information within the boundaries of unattended objects due to biased competition (Desimone & Duncan, 1995). A second theory that was proposed to underlie object-based attentional selection is the object-specific attentional prioritization account (Shomstein & Yantis, 2002, 2004), in which target locations within an attended object are afforded higher priority than target locations in unattended objects. This is accomplished via an automatic spatial selection of a cued location and subsequent prioritization of attention from the cued location to areas in which the probability of the target appearing is higher (i.e., in the cued object) over locations in which the probability of the target appearing is lower (i.e., in the non-cued object). Under this view, object-based attention effects result from the unequal prioritization of attention to the invalid-same and invalid-different locations. As such, this explanation ultimately establishes the order in which an observer will search a visual display for the presence of a target, beginning first at the valid location, next at the invalid-same location, and finally at the invalid-different location (Greenberg et al., 2015). However, the attentional prioritization strategy is not as rudimentary as a visual search mechanism, because although both processes rely on the combined attentional priorities of items (Shomstein & Yantis, 2002; Wolfe, 1994), the former approach is further constrained by the perceptual objectness formed by the object’s boundaries (Shomstein, 2012; Greenberg et al., 2015).
A recent study (Pilz, Roggeveen, Creighton, Bennett, & Sekuler, 2012) demonstrated that the preferential processing of visual information as a result of object-based attention was modulated by the orientation of the two parallel rectangles. In this experiment, a large number of observers were presented with the double-rectangle cueing paradigm (Egly, Driver, & Rafal, 1994) and performed either a detection task or a discrimination task. Space-based attention effects were observed in horizontal and vertical rectangles, as evidenced by increased accuracy and faster RTs to the valid location compared with the invalid-same location. Object-based attention effects, however, were relatively small compared with the space-based effects and varied as a function of rectangle orientation. Overall RTs were also significantly slower for horizontally oriented rectangles compared with vertically oriented rectangles. Moreover, object-based attention effects were not observed (at the group level) for vertically oriented rectangles across three different experiments. This differs from many previous reports of object-based attention effects (for a review, see Chen, 2012) that do not show an effect of, or (generally) even explicitly test for, object orientation. The results of Pilz and colleagues (2012) showed that, for horizontally oriented rectangles, RTs to the invalid-same location were significantly faster than RTs to the invalid-different location, the same-object advantage that typically characterizes object-based attention effects. However, they also showed that, for vertically oriented rectangles, RTs to the invalid-same location were actually slower than RTs to the invalid-different location. This same-object cost in vertically oriented rectangles has occasionally been documented by others, as well (Davis & Holmes, 2005; Harrison & Feldman, 2009; Chen & Huang, 2015). To explain this effect of rectangle orientation on the same-object advantage, Pilz and colleagues (2012) postulated that attention may be more efficiently allocated parallel to the horizontal meridian than the vertical meridian, a phenomenon previously observed in visual search studies (Carrasco, Evert, Chang, & Katz, 1995; MacKeben, 1999). Recent work from our laboratory showed that the orientation effects observed in the Pilz et al. (2012) study disappear when controlling for shifts across the visual field meridians (Greenberg, et al., 2014). This suggests that effects of the meridians may be the cause of orientation differences reported in object-based attention studies using the double-rectangle cueing paradigm.
Anatomical and physiological evidence, however, may also provide an explanation for differences between the horizontal and vertical dimensions. It is well established that the left and right visual field representations are organized contralaterally. As a result, objects that appear in both visual hemifields, crossing the vertical meridian, are partially represented in corresponding retinotopic areas in the left and right cortical hemispheres; whereas objects appearing entirely within a single visual hemifield are represented fully in the corresponding contralateral hemisphere. Consequently, attention allocation along the horizontal meridian may be impaired due to an interhemispheric boundary imposed by the contralateral organization of visual space in the cortical hemispheres. Shifting attention horizontally across the vertical screen meridian may require hemispheric interactions and additional cortical processing that is not required when attention shifts occur entirely within a hemifield (Holtzman, Sidtis, Volpe, Wilson, & Gazzaniga, 1981; Reuter-Lorenz & Fendrich, 1992a).
However, lower and upper visual field representations of retinotopic areas within extrastriate cortex are also sequestered physiologically. Representations of the lower half of the visual field correspond to dorsal aspects of retinotopic visual cortex, whereas representations of the upper visual field correspond to ventral aspects of retinotopic visual cortex (Van Essen, 1985). Therefore, the horizontal meridian represents an intrahemispheric boundary (Sereno & Kosslyn, 1991), which may require additional cortical processing when shifting attention between upper and lower visual field representations. Our goal was to examine, behaviorally, whether either (or both) of these physiological segregations of the visual field can explain the observed effects of object orientation on OBA. To accomplish this, we measured shifts of attention from cued locations to invalid locations that either crossed or did not cross the vertical and horizontal screen meridians and how these shifts of attention varied within a single object and between two objects.