Effects of Perceived Space on Spatial Attention

This study demonstrates that a perceptual illusion that alters the perceived length of two lines also affects spatial attention. We used a cuing method that was introduced to study spaceversus object-based attention. Two parallel lines of equal length were placed so that the distance between them was equal to the length of the lines. We then added a scene with depth cues to produce a strong illusion that one line was longer than the other. The results showed that spatial attention is distributed in space as it is perceived and altered by perceptual organization. These data have implications for assumptions concerning the spatial medium that guides attention and the role of depth and distance cues in spatial orienting, as well as for understanding attentional systems related to neuropsychological functions that respond to space and objects. Extensive use of a cuing paradigm designed by Posner (1980; Posner, Snyder, & Davidson, 1980) has convincingly shown that attention can be allocated to locations in space at the expense of other locations. Space in these studies is generally considered some type of spatial coordinate that defines locations for attentional allocation. Attention is directed at a cued location in this space to prepare for a target that is expected there and to ignore other locations where a target is not expected. When a cue appears in a location on one side of fixation, reaction time (RT) to detect a subsequent target is better when the target is in the cued location (valid) than when it is in an uncued location (invalid) the same distance from fixation (generally on the opposite side). Theoretically, the cue draws attention to its location, which speeds target detection at that location and slows target detection at other unattended locations. A large number of studies have used this method or a variant of it to address questions of spatial attention, including whether an attention spotlight moves through space, how spatial gradients are formed by attention, the basis of attentional pathology, areas of the brain involved in spatial orienting, the consequences of inhibition and facilitation of locations, the development of spatial attention, and more. More recent evidence has demonstrated that objects in the field can change the amount of performance decrement observed for targets in invalidly cued locations. For instance, Egly, Driver, and Rafal (1994) distinguished between spaceand object-based attentional systems by demonstrating that within-object decrements were less than betweenobject decrements. The difference between RT to detect targets at validly cued locations and RT to detect targets at invalidly cued locations (validity effect) varied depending on whether the target at an invalid location was in the same or a different object. Egly, Rafal, Driver, and Starrveveld (1994) went on to show a hemispheric difference in these effects in a split-brain patient. Objects presented directly to the patient’s left hemisphere showed the normal object-based effects, whereas objects presented directly to the patient’s right hemisphere showed none. Studies with neurological patients converged with these findings and implicated parietal-occipital lobes in this asymmetry (Egly, Driver, & Rafal, 1994). The investigators argued for two attentional systems, one that is associated with the left hemisphere and is sensitive to objects and one that is associated with the right hemisphere and is not sensitive to objects. In the present study, we demonstrate that under conditions in which depth cues change the perception of space itself, the magnitude of the validity effect between and within objects depends on the perceived spatial structure of the stimulus as a whole. We used a perceptual illusion to vary what we call perceived space (i.e., the explicit perception of distances and spatial relationships). Figure 1 shows the stimulus we used, which was adapted from an example published in Rock (1984). Although the dark line on the right side (inner corner) is perceived as longer than the one on the left (outer corner), in fact the two lines are of equal length. In addition, the distance between the two dark lines is equal to their length. For conventional purposes, we call these two dark lines objects even though they are simply parts of the walls. We adopt this convention because this type of stimulus arrangement is the same as that used by Egly, Driver, and Rafal (1994) to study object-based attention, except that we added depth cues to change the perceived lengths of the two lines.