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Visual Cognition Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/pvis20
Reversal of object-based benefits in visual attention a
Greg Davis & Amanda Holmes a
University of Cambridge, UK
b
Birkbeck College London, UK
b
Available online: 01 Oct 2010
To cite this article: Greg Davis & Amanda Holmes (2005): Reversal of object-based benefits in visual attention, Visual Cognition, 12:5, 817-846 To link to this article: http://dx.doi.org/10.1080/13506280444000247
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VISUAL COGNITION, 2005, 12 (5), 817±846
Reversal of object-based benefits in visual attention Greg Davis University of Cambridge, UK
Amanda Holmes Downloaded by [University of Cambridge] at 03:17 20 June 2012
Birkbeck College London, UK In divided-attention tasks, observers must make speeded (or near threshold accuracy) judgements concerning two target features in a display. Typically, when the two features belong to the same object they are more rapidly judged than when they belong to separate objects, a pattern of findings referred to here as a ``sameobject benefit''. However, we note here that many of these studies share common features, in particular the use of pre-exposed, outline, and/or overlapping objects, and their findings may not generalize to other types of display. Building substantially on previous work by Davis, Welch, Holmes, and Shepherd (2001), we show in four new studies that once these features are not present in a dividedattention task, no same-object benefits are reported. Rather we now find ``sameobject costs'', where features belonging to a single object are less rapidly judged than features belonging to separate objects.
In natural environments, our retinae may be stimulated by thousands of objectimages at any given moment, many of which will be irrelevant to us. Accordingly, human vision comprises mechanisms of visual attention that can prioritize the processing of particularly relevant information for the control of behaviour. Earlier models proposed that attention behaves as a zoom lens or spotlight, selectively enhancing perception within a circular region of visual space independently of the objects situated there. However, over the last two decades years, numerous behavioural and physiological studies have found that attention is ``object-based'', selecting whole visual ``object'' representations rather than circular regions of space (see Lavie & Driver, 1996, for a review). Some of the most compelling evidence that attention is object-based derives from the divided-attention paradigm, a behavioural measure in which observers must make speeded (or near threshold accuracy) judgements regarding two Please address all correspondence to Greg Davis, Department of Experimental Psychology, Downing Street, Cambridge CB2 3EB, UK. Email:
[email protected] We are grateful to Jacob Waldman-Khaira and two anonymous referees for feedback on the manuscript. This work was funded the Ministry of Defence (UK) and EPSRC (UK). # 2005 Psychology Press Ltd http://www.tandf.co.uk/journals/pp/13506285.html DOI:10.1080/13506280444000247
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target ``features'' (fragments of shape, colour, or texture) in each display. The crucial manipulation in such experiments concerns whether the two target features pertain to the same visual ``object'' or alternatively to two different objects. Typically, when the features belong to the same object, they are more rapidly or accurately judged than when they belong to the same objects, even when the spatial separation between the features is equated in the two conditions (such that spotlight models would predict equal performance). The performance advantages for features belonging to the same object are termed here ``sameobject benefits''. They constitute one of the core behavioural measures in cognitive science and have been found with a wide variety of tasks and stimulus conditions (e.g., Duncan, 1984; Lamy & Egeth, 2002; Lavie & Driver, 1996; Watson & Kramer, 1999). Figures 1A and 1B illustrate two possible displays from a divided-attention task in which observers might have to detect, for example, the presence of a squashed-ring feature (top-left of Figures 1A and 1B) and a notched ring feature (top right of both figures). Note that the two features are in the same spatial
Figure 1. (A and B) Typical two-target displays comprising two objects: In (A) both target features (the squashed ring element, top-left, and notched-ring element, top-right) appear on a single object; in (B) they appear on two separate objects. In many previous studies features belonging to the same object were more accurately judged than features from separated neighbouring objects, even when spatial factors are equated in the two cases. (C) Stimuli similar to those employed by Duncan (1984). (D) Stimuli after Lavie and Driver (1996). (E±G) Stimuli after Davis et al. (2001, Exp. 2), where target features were square or triangular notches removed from the objects: (E) A single-large-object stimulus, with vertically separated target features belonging to the same object; (F) two-small-object stimulus from Davis et al., with horizontally separated features; (G) two-small-object stimulus with vertically separated features belonging to separate objects.
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locations in the two displays, and could be encompassed with equal ease by an imagined attention spotlight. Thus, spotlight and zoom lens models would predict equal performance in the two cases. However, whereas in Figure 1A both target features appear on the same (upper) object, in Figure 1B the features appear on separate objects. In virtually all previous reports, these types of task have yielded performance benefits (shorter response timesÐRTsÐor lower error rates) when the features appear on the same object, compared to when they appear on two separate objects (same-object benefits). We begin our discussion with a brief review of some previous studies, all of which found same-object benefits, noting that aspects of stimuli and procedure common to those experiments that may have influenced the magnitude of sameobject benefits found. To anticipate our conclusions, we shall not dispute the validity of such findings. However, we shall present evidence that challenges their generality. Moreover, we shall then describe four experiments that demonstrate that once these common aspects of previous work are absent, sameobject benefits are often not found. Rather, we now find a reversal of this usual pattern of performance: A same-object cost.
COMMON ELEMENTS AMONG STUDIES OF OBJECT-BASED ATTENTION Use of outline stimuli The majority of previous divided-attention studies have employed simple linedrawing stimuli in which unfilled, outline objects are displayed. There is not space here for individual description of the many studies of object-based attention in the literature. However, to illustrate, consider some well-known examples. Duncan (1984) for example, employed stimuli comprising a tilted line overlapping an outline box (Figure 1C), similar to the stimuli of Lavie and Driver (1996) who examined attention thin, dashed overlapping lines (Figure 1D). More recent examples also employing outline stimuli include: Behrman, Zemel, and Mozer (1998), Kramer, Weber, and Watson (1997), and Lamy and Egeth (2002). Notably, this tendency to employ outline stimuli is also echoed in cueing studies of object-based attention, which show performance advantages for target features on the same object as a precue versus on neighbouring objects (Egly, Driver, & Rafal, 1994; Lamy & Egeth, 2002; Moore, Yantis, & Vaughn, 1998). We argue below that the use of outline stimuli may have played an important role in the finding of same object benefits in these studies.
Use of overlapping stimuli A second aspect common to some studies of object-based attention is the use of overlapping stimuli. In some cases, these are the same studies that employed outline objects (Duncan, 1984; Kramer et al., 1997; Lavie & Driver, 1996), while others have employed more naturalistic objects (e.g., Lamy, 2000; Law &
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Abrams, 2002). It has long been recognized, since the work of Edgar Rubin (1921), that when one region/object is perceived to be figural, adjacent regions/ objects tend to be perceived as ground. Thus, objects that share common borders may compete for attention in ways that other nonabutting/overlapping objects do not. We suggest here that such potential competition between overlapping objects may make it more difficult to attend two overlapping objects concurrently than two neighbouring objects, and may therefore have given rise to same-object benefits that may not occur when nonoverlapping objects are used.
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Extensive pre-exposure of objects One study with neither of the above characteristics was described by Atchley and Kramer (2001). They examined how rapidly observers could detect pairs of ``drips'' presented on 3-D objects that were filled (i.e., not outline shapes) and that did not overlap each other. However, Atchley and Kramer's experiments were characterized pre-exposure of the objects in a trial for 1500 ms before the target features were presented. As we discuss below, Davis, Welch, Holmes, and Shepherd (2001) have shown that, with their own stimuli, extensive preexposure of relevant objects gives rise to same-object benefits in stimuli that otherwise show very different patterns of effects. It therefore remains uncertain (at best) whether Atchley and Kramer's results would also hold without preexposure. This proposed influence of extensive pre-exposure is also a common feature of many cueing studies and may help to account for reliable same-object advantages found there.
Number of intervening luminance edges A second factor that may have influenced results in Atchley and Kramer's (2001) study concerns the smaller number of luminance edges lying directly between features when they belong to the same object versus to different objects. The extra luminance edges between features belonging to separate objects may have disrupted grouping of those features minimizing any potential evidence of a same-object cost. In our previous studies (Davis, 2001; Davis et al., 2001) and those presented here, we have kept the number of luminance edges lying directly between two target features constant across same-object and different-object conditions. In summary, many previous studies exhibit one or more of the characteristics listed above. In each case, we argue here, there is reason to suspect that these characteristics may have influenced the finding of same-object benefits. Other than the last of these (the greater number of luminance edges lying between features of different objects) these characteristics do not threaten the validity of same-object benefits in those studies. However, previous literature does leave unanswered the question as to whether studies that do not share these characteristics will also show same-object benefits. We show here that they do not.
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Rather, four studies employing nonoverlapping, nonoutline (filled) shapes in which there is no extensive pre-exposure of objects yield clear evidence of same-object costs: A reversal of the usual pattern. The current experiments stem from work described in two previous papers (Davis, 2001; Davis et al., 2001). In those reports, we claimed that under certain conditions same-object benefits can be reversed, such that features from the separate objects may be more easily attended together than features from the same object. These were the first claims in the literature that same-object costs in normal observers might be possible in the absence of confounds. One previous study, Cepeda and Kramer (1999) had also reported such a cost, but had concluded that it was due simply to a confound in their stimuli, whereby comparison of features within the same object required mental rotation, but comparison of features from separate objects did not. In contrast, Davis et al. (2001) concluded that same-object costs might be a real phenomenon requiring explanation in terms of within- and between-object binding processes. In their studies, Davis et al. (2001) employed stimuli comprising either one large object (Figure 1E) or two small objects (Figure 1F). Observers in those studies had to make speeded responses regarding whether two notches present in the object as soon as it was presented (triangular or rectangular) were of the same or different shape. In any given trial, both notches might be rectangular (requiring response ``same'': See Figure 1E), both triangular (requiring response ``same'': See Figure 1F) or one of each type (requiring response ``different'': See Figure 1G). In the two-object displays, the notches were either horizontally separated (e.g., Figure 1F) appearing on a single object or vertically separated appearing on two separate objects (e.g., Figure 1G). Similarly, in the one-object displays, the notches could be horizontally or vertically (Figure 1E) separated, but now always appeared on a single object. Note that the two crucial conditions to be compared both involved vertically separated features (the same versus different object factor in this comparison is not confounded with feature separation). In the one-object displays these belonged to the same large object, whereas in the two-object displays they belonged to separate objects. Davis et al. (2001) found, in two separate studies, that when vertically separated features belonged to the same large object (as in Figure 1E) they were less rapidly compared than when identical features (also vertically separated) belonged to separate objects (as in Figure 1G): A same-object cost. Horizontally separated features were not as important in these studies: They always belonged to the same object (in both one-object and two-object displays) and always yielded the same performance. Davis et al. (2001) have since extended these findings of same-object costs to displays comprising three large objects versus six small objects. These new displays comprised three smaller versions of the one-large-object and two-smallobject displays (see Figures 2A and 2B, respectively), and could be arranged in a triangle pattern or (Figures 2A and 2B) or an inverted triangle pattern (i.e., the
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Figure 2. Stimuli employed by Davis et al. (2001). (A) Typical three-object display in which the target features are vertically separated and appear on a single large object. In this case the two features are of different types (one square notch, one diagonal). (B) Six-object display where target features (one diagonal notch, one square notch) are vertically separated and appear on two separate small objects. (C) Three-object display (hi-pass filtered) from Davis (2001), in which the two target features (notches) are horizontally separated. (D) Six-object display from Davis (2001) in which target features are horizontally separated, appearing on a single small object.
displays were inverted relative to Figures 2A and 2B). The task was again to detect whether two notches (rectangular or triangular) were of the same or different shape. Davis et al. found the same results as before. Vertically separated features within a single large object of the three-object displays (Figure 2A) were less rapidly compared than vertically separated features belonging to separate, neighbouring objects within a pair of objects in the six-small-object displays (Figure 2B): A same-object cost.
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Intriguingly, however, while these particular conditions yielded robust sameobject costs other very similar conditions showed conventional same-object benefits. When the objects were pre-exposed on each trial for 2250 ms before the target notches were removed, a same-object benefit was found (also in an earlier study by Davis, Driver, Pavani, & Shepherd, 2000). Additionally, when Davis (2001) high-pass spatial-frequency filtered these stimuli such that they were much more similar to outline shapes used in other previous studies, he also found robust same-object benefits, even with no pre-exposure of objects. Together, these results suggest that only when objects are either pre-exposed, or outline (or possibly when they overlap) are same-object benefits always present. When nonoutline, nonoverlapping, nonpre-exposed objects are employed, sameobject costs are found. This conclusion runs counter to those of all previous divided-attention and cueing studies, and we must therefore consider whether some idiosyncratic features of these experiments are responsible for these new findings. In Experiments 1 and 2 of the current paper, we attempt to counter what must be the most serious concern regarding these experiments: Differential masking properties of single-large-object stimuli versus the two small-object stimuli. Note that these stimuli differed only in terms of a small ``linking segment'' in the centre of the single-large-object stimuli, which was absent in the two-object patterns. However, in principle this may have had a masking or cueing in the one-large object displays that was absent in the two-small-object stimuli, and could thus account for the same-object costs Davis et al. found for vertically separated features there (e.g., J. Duncan, personal communication). Such an effect could also have arisen in the three-large-object- versus six-small-objectdisplays of the Davis et al. (2001) studies. In our first study, we attempted to provide evidence against this potential account of the Davis et al. findings. We first noted that using high-pass spatial frequency filtered stimuli, Davis (2001) found no evidence of same-object costs (finding significant same-object benefits) even though all other aspects of the experiment were identical to Davis et al. (2001) which had found same object costs. This suggested that if the apparent same-object costs reported by Davis et al. (2001) were due to masking/cueing effects of linking segments in their displays, such masking/cueing cannot have been present to nearly the same degree in Davis (2001)'s filtered stimuli (where no apparent same object cost was found). Accordingly, if we could now demonstrate a same-object cost using stimuli with the same linking segments as the filtered stimuli of Davis (2001) this could not be readily attributed to masking by the linking segments. Figures 3A and 3B illustrate typical displays from the new study, which replicated the all aspect of the Davis et al. (2001) studies, other than changes to the stimuli. Note that the objects themselves are the same as the filtered patterns employed by Davis (2001). Now, however, most of the background behind the objects is white (rather than grey as in the Davis stimuli) except around the
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Figure 3. Stimuli employed in Experiment 1. (A) Typical three-object display in which the target features are vertically separated and appear on a single large object. In this case the two features are of the same type (both square). (B) Six-object display in which the two target features (notches) are vertically separated, appearing on two separate, neighbouring objects. In this case the two features are of different types (one square notch, one diagonal).
crucial linking segment regions in the three-large-object displays, and corresponding regions in the six-small-object displays, where it is grey as in the Davis stimuli (compare Figure 2C of the Davis stimuli to those of the new study in Figures 3A and 3B). Hence, the objects can generally be considered ``filled'' rather than ``outline'' objects and may thus be expected to yield same-object costs as described by Davis et al. (2001). These new stimuli therefore generate two clear predictions. If local masking/ cueing properties of the linking segments were responsible for the same-object costs that Davis et al. (2000) found, these effects should not now be present (or should be severely reduced) as they are identical to those used by Davis (2001) who found no such cost. However, if those same-object costs reflected whether target features belonged to the same or to different filled (nonoutline) objects, we should find same-object costs with these new stimuli.
EXPERIMENT 1: SAME-OBJECT COSTS AND LOCAL MASKING/CUEING PROPERTIES Figure 4A schematizes a typical trial in this new study, which replicated the conditions of Davis et al. (2001) but with modified stimuli. Following presentation of a fixation cross for 675 ms, three grey bars were displayed for 675 ms, before the objects (and target features) were presented (remaining until response). This gave the subjective impression that three large objects (or six small objects) had been presented in front of the three bars, helping to segment
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Figure 4. (A) Schematized trial in Experiment 1. Frame 1: Fixation cross (675 ms), frame 2: Sections of grey background display on which objects will appear (675 ms). Frame 3: Target features and objects presented (until response). (B) Average RTs (% error rates in parenthesesÐerror bars indicate 1 SD error) for conditions in Experiment 1 where both features appeared on a single large object in the three-large-object displays (filled squares) or on a pair of small objects in the six-smallobject displays (open circles). The left pair of data points indicates performance on trials where features were vertically separated (always belonging to the same object irrespective of display-type). The right pair of data points indicates performance on trials where features were vertically separated (belonging to same object in three-object-displays, but to separate objects in six-object displays).
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the objects from the grey background (where the crucial linking segments were located).
Method
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Observers. Eight observers from the Department Subject Panel were recruited. Three were female, five male, their ages ranging from 18 to 43 years, with a mean of 27.1 years. Each was paid seven pounds sterling. Displays. The stimuli were presented on a Sony 17-inch screen with a Power Macintosh G3 computer running ``Vscope'' experiment-generator software (Enns & Rensink, 1992). Figure 3A illustrates a typical target display in the three-large-objects condition, while Figures 3B and 4A illustrate typical six-small-object displays. These figures are drawn to scale: The actual stimuli measured 24 cm vertically and were viewed from a distance of approximately 50 cm. From this dimension, all other stimulus dimensions can be calculated given the scaled figure. A feedback symbol (+ or ±) followed each response, appearing centrally and subtending 0.8 degrees of visual angle. Note that in order to maintain the same procedure as that in the Davis et al. study, the displays were presented only in their illustrated orientations for this experiment (see Figure 3). They were not presented as other possible versions in which each large object or pair of small objects was rotated by 90 degrees. If such alternative orientations had been used in the original Davis et al. studies, this would have risked figure±ground reversal in the displays. For example, if each pair of small objects from those original studies (see Figure 2B) were to have been rotated 90 degrees, a white vase-like object might have been perceived as figure (due to symmetry around the vertical axis) and the dark objects (which we intended to be figural) would have been perceived as ground. In the current experiment, such figure±ground reversal was less likely, but we nonetheless employed only stimuli in their illustrated orientations to ensure that our current study was directly comparable to those of Davis et al. Procedure. Target features were ``notches'' cut from the objects and could be square or diagonal (see Figures 3A and 3B). These notches were equally likely to appear at any of the possible ``corner'' locations of any of the objects in a display, such that the position of one feature held no predictive value as to the likely position of the other. This meant that each cell in our analyses comprised 30 trials. Three-large-object displays comprised half the trials, and six-smallobject displays the other half, and the objects were equally likely to be presented in a triangular formation (e.g., Figures 3A and 3B), or an inverted triangular formation (e.g., Figure 4A). The order in which different trial types were presented was randomized. Each observer viewed 10 blocks of 60 trials, the first 4 blocks of which were excluded as practice. Note that this extensive practice
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was employed simply to make the current results comparable to our previous studies of same-object costs (Davis, 2001; Davis et al., 2001). All of the interpreted effects described here were also statistically significant in identical analyses when zero blocks of practice were excluded. Observers were instructed to focus their gaze at the centre of the screen, and to press the correct button, as quickly and accurately as possible, to indicate whether the two notches were the same or different in shape. Observers pressed one key on a computer keyboard when the two notches had the same shape (i.e., both diagonal, or both rectangular) and another key when the notches had different shapes (i.e., one diagonal, one rectangular), which was equally likely. Following each response, a +/7 feedback symbol was presented.
Results and discussion Most important for our current purposes, was performance on individual trial types where both target features had appeared on a single large object of the three-large-object displays or within a pair of small objects in the six-object displays. Horizontally separated features in these trials always belonged to the same object, in both six- and three-object displays. In contrast, vertically separated targets belonged to two small objects in the six-object displays (e.g., Figure 2B) but to one large object in the three-object displays (e.g., Figure 2A). Any same-object costs or benefits should be evident as a difference in performance between these last two conditions. As the features and objects had been presented simultaneously (i.e., no long pre-exposure of objects before feature presentation) and the objects were ``filled'' not outline, we expected to find a same-object cost, just as Davis et al. (2001) reported for the original stimuli. In our stimuli, such a cost should be evident as slower RTs for vertically separated features in the three-object displays (where the features appeared on a single large object) than for vertically separated features in the six-object displays (where the features appeared on separate small objects. The RTs for horizontally separated features should not differ in this same way between six- and three-object displays as they belonged to a single object in both cases. Figure 4B graphs inter-observer means of median RTs (with error rates in parentheses) for the one-large-object and two-small-object conditions (filled symbols versus open symbols respectively), separately for horizontally versus vertically separated targets. Inspection of Figure 4B suggests that RTs and errors are roughly equivalent overall in the one-large-object versus two-small-objects trials with any overall advantage holding in favour of the two-small-object displays and applying solely to vertically separated pairs of features. These data were analysed using a two-way within-observers ANOVA (display type: Features on a single large object in the three-object displays versus on a single small object or separate neighbouring objects in the six-object displays
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6 Horizontal/vertical target separation). This analysis yielded no main effect of one-large object versus two-small objects, F(1, 7) = 2.68, n.s., and no effect of horizontally versus vertically separated targets, F(1, 7) = 0.96, n.s. Crucially, however, these two factors interacted significantly, F(1, 7) = 6.64. p = .037. Planned comparisons revealed the source of this interaction. As we predicted, vertically separated targets in pairs of objects within the six-object displays (that pertained to separate objectsÐe.g., Figure 3B) were detected faster than vertically separated targets appearing on a single object in the three-object displays (see Figure 4A for an example of this latter condition), F(1, 7) = 9.45, p = .018. This effect did not arise for horizontally separated features in the two display types, F(1, 7) = 0.3, n.s. A similar analysis of % error rates found no significant terms that could threaten our interpretation of the RT data (all relevant Fs < 0.1). These new findings provide the first convincing evidence that previous reports of same-object costs in Davis et al. (2001) did not reflect local masking properties of ``linking segments'' in those displays. Rather, they seem to have reflected whether the features in a given display belonged to the same object versus to separate objects: a true same-object cost. However, given the potential importance of this conclusion we conducted one further study to ensure that cueing/masking properties of the stimuli employed by Davis et al. (2001) cannot account for their find of same-object costs.
EXPERIMENTS 2A AND 2B: PREVENTION OF SAME-OBJECT COSTS BY PERCEPTUAL COMPLETION Our second study examined the effects of perceptual shape completion upon same-object costs. In Experiment 2A, with a couple of exceptions, we replicated the conditions, task, and stimuli of Davis et al. (2001) Experiment 3, which employed displays of three pairs of small-objects (Figure 2B) versus three large objects (Figure 2A). In this new Experiment we added four small corner segments to each large object or pair of small objects in the displays employed by Davis et al. (e.g., as in Figures 5A and 5B). Additionally, a small amount of binocular disparity (when viewed via red±green spectacles) was added to the inside of the vertical bar of each of these corner segments and to the curved edges of the objects (indicated by lighter-grey colouring). While the extra physical elements added to the target display were very small, the disparity information they comprised caused observers to perceive a illusory white surfaces in the displays (indicated by cross-hatched regions for a single large object and pair of small objects in Figures 5C and 5D, respectively). These illusory surfaces appeared nearer to the observer than the objects in each display. Accordingly, both the single large objects (in three-object displays, e.g., Figure 5A) and the pairs of small objects (in the six-object displays, e.g., Figure 5B) now appeared subjectively to be single large square objects that were partially
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Figure 5. Typical target stimuli from Experiment 2A. (A) Three-large-object display in which the target features are horizontally separated from each other: Grey shading around the dark objects indicates where binocular disparity was manipulated. (B) Six-small-object display in which target features are vertically separated, appearing on a single (small) object. (C) Cartoon showing the regions of each large object in the three-object displays where illusory occluding surfaces were perceived. (D) Cartoon showing the region of each pair of small objects in the six-object displays where an illusory occluding surface was perceived.
occluded by the illusory surfaces. For the purposes of description we shall continue to refer to the conditions as ``two-small-objects'' versus ``one-largeobject'', although now of course, one large object was now perceived in both cases. If masking by the linking segments of the single-large-object patterns (absent in pairs of small objects) had been responsible for the same-object costs that Davis et al. (2001) found, we should find those same effects here, as the stimuli were physically very similar. However, if those findings had resulted from a true
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same-object cost (where two features from the same object are less well attended than two features from separate objects), we should not find the same results in this current study. Now, subjectively, both pairs of small objects and single large objects appeared to be single large, partially occluded objects, such that both conditions should suffer equally from any same-object cost.
Experiment 2A
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Method Observers. Nine new observers from the Department Subject Panel were recruited. Four were female, five male, their ages ranging from 21 to 31 years, with a mean of 26.5 years. Each was paid seven pounds sterling for participating. Displays, apparatus, procedure. All aspects of the method were identical to Experiment 1 with the exception of the stimuli. Figure 6A illustrates a typical trial sequence in a six-small-object trial, where the target features are vertically separated within a pair of small objects. Following presentation of a fixation cross (frame 1; 720 ms, not illustrated), three sets of stimuli were presented that generated perception of illusory occluding surfaces (frame 2; 160 ms). The tips of the black bars in the centre of each set comprised disparity information
Figure 6. Schematized trials in Experiment 2A. The ``placeholder'' stimuli in frame 2 elicited perception of illusory-figure occluders in both three- and six-object displays that continued during the target display, frame 3 (see text). (A) A six-small-object trial: Note that the illusory occluding surface caused these displays to be perceived as three large, partially occluded objects. (B) A threelarge-object trial.
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(indicated by light-grey colouring) so that the perceived illusory surface would appear at the same depth and location as in the following target displays. Finally, the objects were presented (see frame 3; present until response), being superimposed over the black bars in the previous frame. These objects appeared to be perceptually completed from single large, square objects partially occluded by the illusory surfaces. These figures are drawn to scale: The actual stimuli measured 24 cm vertically and were viewed from a distance of approximately 50 cm. From this dimension, all other stimulus dimensions can be calculated given the scaled figure. A feedback symbol (+ or 7) immediately followed each response, appearing centrally and subtending 0.8 degrees of visual angle. Figure 6B schematizes a corresponding trial in the three-large-object displays. Again three sets of stimuli were presented that elicited perception of illusory surfaces (frame 2). Now, the illusory surfaces were differently shaped (relative to the six-small-objects trials) so as to accommodate the shapes of single large objects presented in frame 3. When the three large objects were presented, they appeared to be single large square objects partially occluded by the illusory surfaces.
Experiment 2B To ensure that the predicted absence of a same-object advantage in Experiment 2A was not due to particular physical characteristics of the stimulus elements themselves, we replicated that experiment, with two minor changes to the corner elements added in frames 2 and 3. First, these elements were ``flipped'' vertically (see Figures 7A and 7B) and second, the disparity information was removed. Additionally, the bar elements that elicited perception of an illusory occluder in the placeholder display of Experiment 2A, were also altered, so as not to elicit any percept of an occluder. Now, although the target displays were physically very similar to those in Experiment 2A, no illusory, occluding surface was perceived, and neither the single-large objects (e.g., Figure 7A, frame 3) in the three-large-object displays or pairs of small objects (e.g., Figure 7B, frame 3) from the six-small-object displays underwent perceptual completion. Now that the six-object displays were perceived to comprise pairs of small objects, we expected to find same-object costs, as Davis et al. (2001) had done with very similar displays. Method Observers. Nine new observers from the Department Subject Panel were recruited. Five were female, four male, their ages ranging from 18 to 38 years, with a mean of 25.8 years. Each was paid seven pounds sterling for participating. Displays, apparatus, procedure. All aspects of the method were identical to Experiment 2A except for the stimuli employed. Figures 7A and 7B illustrate
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Figure 7. Schematized trials in Experiment 2B. Neither the ``placeholder'' stimuli in frame 2 nor the target stimuli in frame 3 elicited perception of illusory occluders. (A) A six-small-object trial. (B) A three-large-object trial.
typical six-small-objects trials and three-large-objects trials respectively. Note that relative to Experiment 2A, the corner elements in frames 2 and 3 have been ``flipped'' vertically relative to their original orientations in Experiment 2A, and the black bars in frame 2 have been similarly alter such that they do not yield perception of an illusory surface. Note that in all frames, no disparity information was manipulated, though these displays were still viewed via red± green spectacles to ensure equivalent colour adaptation in the two studies.
Results and discussion (Experiments 2A and 2B) In Experiment 2A, pairs of objects in the six-object appeared subjectively to be single large, partially occluded objects, due to perceptual completion behind an illusory occluder. Therefore, when comparing RTs for vertically separated features in the two types of display, we expected to find no disadvantage for features in the three-object displays (i.e., no same-object cost as in both conditions the features appeared to belong to one large object). In contrast for Experiment 2B, when no perceptual completion arose, we expected to find a two-target cost. That is, we should find slower RTs for vertically separated features in the three-object displays (appearing on one large object) than in the six-object displays (appearing on two separate objects). Figure 8A graphs, for Experiment 2A, interobserver means of median RTs (with error rates in parentheses) for the one-large-object and two-small-object conditions (filled symbols versus open symbols respectively), separately for horizontally (left data
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Figure 8. Average reaction times (% errors in parenthesesÐerror bars indicate 1 SD error) for conditions in (A) Experiment 2A (where illusory occluders cause perceptual completion of objects) and (B) Experiment 2B (no perceived occluder). Filled squares indicate performance for features appearing on a single large object in the three-large-object displays; open circles for features appearing on a pair of small objects in the six-small-object displays. In each graph, the left pair of data points indicates performance on trials where features were horizontally separated (always belonging to the same object irrespective of display type). The right pair of data points indicates performance on trials where features were vertically separated (belonging to same object in threeobject-displays, but to separate objects in six-object displays).
points) versus vertically separated (right data points) targets. As predicted, the data show a complete absence of any same-object cost for vertically separated features in the three-object displays relative to the six object displays. In fact, they display a trend in the opposite direction. Figure 8B graphs the results for Experiment 2B in the same format as Figure 2A. Note that, as predicted, there is now evidence of a same-object cost. RTs for vertically separated features are slower when they appear on a single object (in the three-object displays) than when they appear on two separate objects (in the six object displays). The RT data were analysed with a three-way, mixed ANOVA with the following factors- between-observers: Experiment (2A versus 2B); withinobserver: Display (six-object versus three-object), and feature separation (horizontal versus vertical). This found the following significant terms: A main effect of feature separation, F(1, 16) = 11.82, p = .01, an interaction between
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display and feature separation, F(1, 16) = 8.13, p = .02, and most crucially, a three-way interaction between experiment, feature-separation, and display, F(1, 16) = 7.75, p = .02. This three-way interaction demonstrates the manipulation of perceptual completion in Experiment 2A versus 2B had a significant effect on the pattern of RTs within six- and three-object displays in the two experiments. We then performed two partial ANOVAs to help interpret these results; there are two ways to divide up the factors in this experiment, one of which is described here, but we have ensured that the other way (analysing each experiment individually) also yields exactly the same conclusions. From the graphs it would appear that the important interaction derives largely from the following two results. First, that the experiment factor had little effect on the differences between six- and three-object displays as regards the horizontally separated features (left pair of data points in each graph: Note that RTs are slower in both experiments for the six-object displays, irrespective of experiment). This was confirmed in a two-way mixed ANOVA (factors: Experiment and display) on horizontally separated features conditions in the two experiments. This yielded a significant main effect of experiment, F(1, 16) = 4.81, p < .05, a near significant main effect of display, F(1, 16) = 4.24, p = .056, but importantly, no interaction between these factors F(1, 16) = 0.31, n.s. This lack of a significant interaction signalled that the manipulation of perceptual completion in Experiment 2A versus 2B had had no effect on the patterns of RTs in six- versus three-object displays for horizontally separated features. This was as expected, because the manipulation of perceptual completion did not affect the same-object or different-object status of horizontally separated features. In contrast, however, the graphs suggest that the Experiment factor did seem to affect the pattern of RTs for vertically separated features. In Experiment 2A, where perceptual completion arose such that all vertically separated features appeared to belong to the same object, irrespective of display, we find the same, general disadvantage for six-object displays as was present in both experiments for horizontally separated features. Remarkably, however, this general disadvantage (in all other conditions) for six-object displays, was reversed for vertically separated conditions in Experiment 2B. These last conditions were the only conditions where features appeared to fall on the same object in the threeobject displays, but on different objects in the six-object displays. Hence, if the slower RTs for three-object displays (features on same-object) versus for sixobject displays (features on separate objects) were to be statistically reliable, it would provide evidence of a robust same-object cost. To confirm that this was the case, we conducted an identical ANOVA on the vertically separated features conditions to that performed on the horizontally separated features. This yielded a near significant main effect of experiment, F(1, 16) = 3.39, p = .08, indicating that RTs were marginally significantly faster in Experiment 2A than Experiment 2B, and no main effect of display type, F(1, 16) = 2.04, p = .17. Most importantly for our purposes, however, we found a
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highly significant interaction between these two factors, F(1, 16) = 11.7, p = .0035. As predicted, the manipulation of perceptual completion had significantly affected RTs for vertically separated features. Planned comparisons revealed the source of this interaction. In Experiment 2B, where there was no perceptual completion and we expected to find a same-object cost, vertically separated features in three-object displays (belonging to the same object) were more slowly judged than features in six-object displays (belonging to separate objects), t(8) = 3.1, p < .02, a same-object cost. In Experiment 2A, however, where perceptual completion ensured that all features appeared to fall on the same object, we expected no evidence of a disadvantage for three-object displays compared to six-object displays. Indeed there was a numerical, though nonsignificant, t(8) = 1.6, n.s., advantage for three-object displays in these conditions. Similar analyses on error rates found no significant terms involving the experiment factor that could threaten our interpretation of the RT dataÐthe only F-ratio over 1 was F(1, 16) = 1.82, n.s.Ðwas an interaction between experiment and display type in a two-way ANOVA on vertically separated trials that held in the same direction as for the RTs. Experiments 2A and 2B were designed to be similar/identical in terms of masking by the linking segments in three-object displays. However, the patterns of results in the experiments were very different, suggesting a minimal role for such masking. In Experiment 2B, where features appeared on the same objects versus on separate objects, we found evidence of same-object costs: Vertically separated features were less rapidly compared in the three-object displays where they belonged to the same object, than in the six-object displays where they belonged to separate objects. In contrast for comparable conditions in Experiment 2A, where features always appeared to fall on a single object, but where masking properties of the linking segments in three-object displays were the same as in Experiment 2B, we found no evidence of a cost for vertically separated features in the three-object displays. Together, these new findings provide further evidence against masking explanations of the same-object costs reported by Davis et al. (2001). Masking by the linking segment alone seems unable to generate apparent same-object costs by impoverishing perception of three-object displays, as in Experiment 2A, where the linking segments were present, no such disadvantage arose. It is only when features belonging to the same- object are compared to features belonging to separate objects (as in Experiment 2B) that we find evidence for same-object costs. One further feature of these results deserves comment. The RTs for Experiment 2B are slower overall than those for Experiment 2A. This need not pose any threat to our interpretation of these studies, as we are primarily concerned here with the very different patterns of results rather than absolute RTs in the two experiments. However, this difference may still be of some potential interest. One possible explanation for this effect is apparent when viewing the displays. In Experiment 2B, the curved edges of the display seem, subjectively,
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to make the task of locating the target features more difficult. In contrast, for the displays of Experiment 2A, this does not appear to be the caseÐalthough the same curved edges are physically present they do not now appear belong to the display objects themselves, but rather to the occluding illusory figure(s) in the display. In these latter displays, each single, large object, or pair of two small objects in the displays appeared to be a partially occluded square comprising just four straight edges. Subjectively, the notches are easier to locate (in general) in these displays because they appear on very simple objects. It is perhaps, therefore, the perceptual simplicity of the objects on which the features appear which is responsible for this effect. In view of this potential effect of object complexity upon our tasks, we sought, in two further experiments, to test whether same-object costs could also be found in more conventional and simpler stimuli. In each case, we employed nonoutline, nonoverlapping stimuli, and included no lengthy pre-exposure of objects prior to the presentation of features, because as we suggested earlier, such characteristics my have biased previous studies towards showing sameobject benefits.
EXPERIMENT 3: SAME-OBJECT COSTS USING NEW STIMULI Most conventional divided-attention studies employ displays comprising two identical objects and do not compare performance on those displays to performance on single-large-object displays. In our new study, we employed stimuli such as these: See Figures 9A±D. The two objects were presented equally often in their vertical (e.g., Figure 9A) and horizontal (e.g., Figure 9B) orientations, and were displayed with two notches (rectangular or triangular) removed from two randomly specified corners. The observer's task, as with all the previous Davis et al. (2000, 2001) experiments, was to indicate as quickly and accurately as possible (by pressing the appropriate key on a computer keyboard) whether the two notches had the same shape (both rectangular/both triangular) or different (one of each type). The only factor of interest in this new study concerned whether the features belonged to the same object or to different objects: As the features and objects were presented simultaneously, we expected to find features belonging to the same object should be less rapidly compared than features from separate objects. If such a pattern were found (a same-object cost), it would also provide evidence that the two objects in the display had indeed been perceived as two separate objects rather than a single large ``X''.
Method Observers. Eight new observers from the Department Subject Panel were recruited. Six were female, two male, their ages ranging from 21 to 31 years, with a mean of 24 years. Each was paid seven pounds sterling.
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Figure 9. Stimuli employed in Experiment 3. (A and B) Displays in which target features appeared on the same object: In (A) they are same shape (both triangular); in (B) different shapes. (C and D) Displays in which target features appeared on separate objects. Note that in (A) and (C) features are horizontally separated from each other, whereas in (B) and (D) features are vertically separated from each other.
Displays. As for previous experiments, stimuli were presented on a Sony 17-inch screen with a Power Macintosh G3 computer running ``Vscope'' experiment-generator software (Enns & Rensink, 1992). Figure 9A illustrates a typical display in which both features appear on the same object, and Figure 9B a display in which features appear on separate objects. These figures are drawn
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to scale: The actual stimuli measured 23.5 cm vertically and were viewed from a distance of approximately 50 cm. From this dimension, all other stimulus dimensions can be calculated given the scaled figure. A feedback symbol (+ or 7) immediately followed each response, appearing centrally and subtending 0.8 degrees of visual angle. Procedure. Target features were either square or triangular notches (see Figures 9A±D) and were equally likely to appear at any of the four possible ``corner'' locations. Features were equally likely to be horizontally separated from each other (Figures 9A and 9C) or vertically separated (Figures 9B and 9D). The two objects in each display were equally likely to appear in the vertical orientation (e.g., Figures 9B and 9C) or in their horizontal orientation (e.g., Figure 9A and 9D) and were equally likely to belong to the same object (e.g., Figures 9A and 9B) or to separate objects (Figures 9C and 9D). In each trial, a fixation cross was presented for 675 ms before the target display, which was displayed until the observer responded. The order in which different trial types were presented was randomized. Each observer viewed 10 blocks of 60 trials, the first 4 blocks of which were excluded as practice. Observers were instructed to focus their gaze at the centre of the screen, and to press the correct button, as quickly and accurately as possible, to indicate whether the two notches were the same or different in shape. Observers pressed one key on a computer keyboard when the two notches had the same shape (i.e., both diagonal, or both rectangular) and another key when the notches had different shapes (i.e., one diagonal, one rectangular), which was equally likely. Following each response, a +/7 feedback symbol was presented.
Results and discussion The RT data were analysed in a three-way within-subject ANOVA, with factors of response (features same versus different), feature separation (vertical versus horizontal), and most importantly object (whether the features appeared on the same object versus on different objects). This analysis yielded significant main effects of response, F1(1, 7) = 5.73, p = .048 (743 versus 786 ms for same and different responses respectively) and feature separation, F1(1, 7) = 18.7, p < .01 (743 and 787 ms for horizontally versus vertically separated features). Additionally, and of most interest for us, was the main effect of object, F(1, 7) = 6.99, p = .033, in this last case, features on separate objects were more rapidly compared (754 ms, 5.3 % errors) than features appearing on the same object (776 ms, 5.4% errors). There were no significant interactions between these factors. However, by far the largest trend toward such an interaction, namely between object and feature separation, F(1, 7) = 3.06, p = .124, may be of interest. The corresponding interaction reaches significance in our final study (Experiment 4) and we discuss the potential implications in our description of
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those results. There were no significant effects or interactions in similar analyses of error rate (all Fs < 0.25). Experiment 3 yielded the clearest indication yet that robust same-object costs can be found in simple displays. However, a potential limitation remains. It may be that we only find same-object costs when the two target features in question belong to different ``parts'' of the same object (see Vecera, Behrmann, & McGoldrick, 2000, for a review). Note that such a finding would in no way invalidate our conclusion that features from the same object are sometimes harder to judge than features from separate objects. Rather, it would simply provide further evidence that features belonging to the same object ``part'' have special status (as claimed by Vecera et al., 2001). Either finding would be of interest. Therefore, in Experiment 4 we attempted to provide a first test of whether single ``part'' objects can also give rise to same-object costs.
EXPERIMENT 4: SAME-OBJECT COSTS FOR FEATURES APPEARING ON THE SAME ``PART'' In the previous three experiments (and in the studies reported by Davis et al., 2001) we have been careful to ensure that the same number of luminance edges lie between target features whether the features belong to the same object or to different objects. The advantage of this approach is that any advantages for attending features when they appear on the same object versus different objects cannot be attributed to the presence of a greater number of luminance edges in one condition or another. However, in order to achieve this, we have previously only compared features belonging to different ``parts'' within a single object versus to different objects. We have never compared features that both belong to an object that is constructed from just one ``part'' (e.g., a single bar) versus belong to different objects. Experiment 4 attempted this comparison, whilst ensuring that same-object and different-object conditions do not differ in terms of luminance boundaries lying between target features. In each trial of the new study, following presentation of a fixation cross for 720 ms, we then displayed a large cross in the centre of the screen for 720 ms (see Figure 10A). Four brown shapes were then presented that abutted the cross, such that they could be perceived as two vertical bars (Figure 10B) or two horizontal bars (Figure 10C). Target features, as in previous studies, were rectangular or triangular notches appearing at the outside corners (i.e., the corner of each brown segment furthest from the centre of the display, e.g., the top-left corner in the case of the top-left brown region). As in our previous studies, these notches were equidistant from each other irrespective of whether they belonged to the same bar versus to two separate bars and whether vertical or horizontal bars were presented. Features were equally likely to belong to the same object versus to different objects, irrespective of whether vertical or horizontal bars were presented.
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Figure 10. Stimuli from trials in Experiment 4. (A) Cross presented in frame 2. (B) Target display comprising two vertical rectangles, with horizontally separated target features (rectangular notches top-left and top-right) appearing on separate objects. (C) Display comprising two horizontal rectangles, with two horizontally separated target features (both diagonal notches, appearing on same object). (D) Display comprising two vertical rectangles, with vertically separated target features appearing on same object (this time notches are of different types: One triangular, one rectangular).
Note that due to the presence of the occluding cross, there were luminance edges lying directly in between pairs of target features both when they belonged to separate objects (e.g., Figure 10B) or to the same object (e.g., Figures 10C and 10D). Arguably, there were in fact fewer luminance edges lying between features when they belong to the same object than when they belonged to separate objects, and this should have acted against the possibility of finding a same-object cost. However, given our previous results we strongly predicted that features from the same object should still be more slowly compared than features from separate objects, as we were employing, nonoverlapping, nonoutline stimuli with no pre-exposure of objects. Note that our predictions were contrary to those that might be derived from any other previous studies other than Davis (2001) and Davis et al. (2001). In this new study, each object consisted of two physical segments, but was arguably one ``part'' perceptually, as they appeared to be simple vertical or horizontal rectangles rather than multisegment objects. Therefore, if we were to find a same-object cost in this new study, this could not be due to the fact that features appearing within an object fell in separate segments or parts of an object. As with Experiment 3, there was only one variable of interest: Whether the two target features belonged to the same object versus to different objects.
Method Observers. Ten new observers were recruited. Seven were female, two male, their ages ranging from 18 to 43 years, with a mean of 25.1 years. Each was paid seven pounds sterling. Displays. Stimuli were presented on an Apple Emac with a 17-inch screen, running ``Vscope'' experiment-generator software (Enns & Rensink, 1992).
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Figures 10A and 10C illustrate typical displays in which both features appear on the same object, and Figures 10B and 10C displays in which features appear on separate objects. These figures are drawn to scale: The actual stimuli measured 22 cm vertically and were viewed from a distance of approximately 50 cm. From this dimension, all other stimulus dimensions can be calculated given the scaled figure. A feedback symbol (+ or 7) immediately followed each response, appearing centrally and subtending 0.8 degrees of visual angle. Procedure. Target features were either square or triangular notches (see Figures 10A±D) and were equally likely to appear at any of the four possible ``corner'' locations. The two objects in each display were equally likely to appear in the vertical orientation (e.g., Figure 10A) or in their horizontal orientation (e.g., Figure 10B). In each trial, a fixation cross was presented for 675 ms before the target display, which was displayed until the observer responded. The order in which different trial types were presented was randomized. Each observer viewed 10 blocks of 60 trials, the first 4 blocks of which were excluded as practice. Observers were instructed to focus their gaze at the centre of the screen, and to press the correct button, as quickly and accurately as possible, to indicate whether the two notches were the same or different in shape. Observers pressed one key on a computer keyboard when the two notches had the same shape (i.e., both diagonal, or both rectangular) and another key when the notches had different shapes (i.e., one diagonal, one rectangular), which was equally likely. Following each response, a +/7 feedback symbol was presented.
Results and discussion As we predicted, median RTs were on average slower for trials where features appeared on the same object (592 ms, 5.0 % errors) than for trials where they appeared on two separate objects (577 ms, 5.4 % errors). This difference was confirmed in a three-way within-observers ANOVA with factors of response (same versus different features), feature separation (vertically versus horizontally separated), and object (whether the features fell on the same versus on different objects). This yielded a marginal main effect of response, F(1, 9) = 4.64, p = .06 (563 versus 583 ms for same and different responses respectively), no main effect of feature separation, F(1, 9) = 3.17, p = .11, and most importantly a main effect of object, F(1, 9) = 10.10, p = .01, a significant same-object cost. Our interpretation of this effect was complicated, however, by the one significant interaction involving the object and feature separation, F(1, 9) = 5.87, p = .039. Specifically, features belonging to the same object were more rapidly judged when they were horizontally separated than when they were vertically separated, F(1, 9) = 9.17, p = .014. In contrast, no such difference
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arose for features from separate objects, F(1, 9) = 0.16, n.s. We suggest here that this effect might reflect symmetry around the vertical axis, which Saiki (2000) has shown to play an important role in these types of task. When features from the same object are horizontally separated, observers may have been able to judge whether the two features were the same or different purely on the basis of the object's symmetry around the vertical axis. In contrast, when the features are vertically separated, observers could not employ vertical-axis symmetry (only horizontal-axis symmetry to which observers are less sensitive). Hence, symmetry might have given rise to an advantage for horizontally separated features. However, this effect is selective for features belonging to the same object. Features belonging to separate objects do not benefit from symmetry in this way, as symmetry appears to be a property only encoded efficiently within the features of single objects (see, e.g., Baylis & Driver, 1995). These effects were present in terms of numerical trends, but were not statistically significant, in Experiment 3. In Experiment 4, however, they significantly affected the size of the same-object cost. Whereas a highly significant same-object cost was present (using one-tailed, pairwise comparisons as the direction of the result was predicted) on trials where features were vertically separated, F(1, 9) = 20.77, p = .0007, no such effect was present for horizontally separated features, F1(1, 9) = 1.28, p = .15. The lack of effect for horizontally separated features presumably reflected the significant benefits of symmetry around the vertical axis which selectively improved performance for features from the same object (see our discussion above), hence diminishing the apparent size of the two object costs. This finding motivated identical pairwise comparisons of corresponding conditions in Experiment 3 (which had not been motivated previously, due to the absence of an interaction between object and feature separation terms in those analyses). In that study too, any trend was toward much reduced same-object costs for horizontally separated features. Whereas, pairwise comparisons now revealed an effect for both horizontally separated features, F(1, 9) = 3.62, p = .049, and vertically separated features, F(1, 9) = 19.15, p = .002, this effect appears to be more robust in the vertically separated case. Note that these effects in no way threaten our conclusion here regarding the existence of same-object costs. Rather, they might equally be considered to verify the object segmentation in our displays. Features from the same object have been shown in other studies, using different stimuli, to benefit more from vertical-axis symmetry than do features from separate objects (e.g., Baylis & Driver, 1995). We find the same effects in our experiments, suggesting that observers coded our displays in Experiments 3 and 4 in the way we had intended (i.e., as comprising two separate objects). One potential criticism of all object-based divided attention studies (including the current experiments) is that the two objects in a display may be coded as one large
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``object'' by observers. Such a criticism is always inadequate to account for same-object benefits or costsÐbut might alter our interpretation of them. It is therefore a significant benefit in Experiments 3 and 4 to find selective symmetry benefits, which add further support to our claim that two separate objects were perceived in each display. Analyses of error rate in Experiment 4 yielded two significant terms involving the object term, other Fs < 0.5, n.s. The first of these interactions was between response type (same versus different) and object (same versus different) factors, F(1, 9) = 4.5, p = .06. This reflected a tendency within trials where features appeared on separate objects for observers to make more errors when the features were the same versus when they were different, F(1, 9) = 9.92, p = .01, but an absence of such an effect for trials where features belonged to the same object, F(1, 9) = 0.02, n.s. However, these patterns of results did not threaten interpretation of the same-object cost present in the RTs as the object term in the analysis of error rate was not significant, F(1, 9) = 0.47, n.s. The second interaction, between object and feature separation, F(1, 9) = 7.94, p = .02, leant further support to our findings from the RT analyses. For vertically separated features, where vertical-axis symmetry could not play a role, we found same-object costs, F(1, 9) = 8.1, p = .04 (4.0% versus 6.0% errors for features on different objects versus same object respectively). In contrast, for horizontally separated trials (no significant effect was found), F(1, 9) = 3.6, p = .14. Indeed, the numerical trend for error rates in this second case was reversed relative to the vertically separated features. As for the RT analyses, we suggest this reflects the action of verticalaxis symmetry processing which selectively enhances processing for features within a single object (see Saiki, 2000, for a discussion of symmetry issues in relation to another study of object-based attention). The results from Experiments 3 and 4 yield further support for our contention that same-object costs can be found in a variety of stimuli. Moreover, in so far as a partially occluded rectangle (made up physically of two regions) can be considered to form a single object ``part'', these new findings also suggest that same-object costs do not only arise within multiple-part objects, but can also apply to features belonging to the same ``part''. Clearly, further research will be required to discover the boundary conditions of these effects. An additional aspect of results from Experiments 3 and 4 is that symmetry around the vertical axis exerts a powerful effect upon performance in divided-attention tasks such as these. It is plausible that symmetry around the vertical axis may be a further characteristic that such symmetry may be a further factor (other than the four we have listed in the introduction to this paper) that may help explain the absence of same-object costs in the literature (see Saiki, 2000). By including conditions in which symmetry can selectively enhance processing of features within the sameobject, previous studies may have inadvertently prevented same-object costs from becoming apparent.
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GENERAL DISCUSSION Together, these findings present a serious challenge to the generality of sameobject benefits, one of the major behavioural indicators in cognitive science. Such benefits have been found in numerous previous studies, but as we discussed in the introduction, those experiments shared particular properties that may be required for same-object benefits to arise. Many of those studies either employed outline stimuli, overlapping stimuli, pre-exposed objects, or a combination of these. When none of these factors are present, as in the paradigm of Davis et al. (2001), and the four experiments described here, no same-object benefits are found. On the contrary, in our studies, we have found same-object costs. Davis et al. (2001) were the first to claim that same-object costs can arise in divided attention tasks when no confounding factors are present. However, they provided only initial evidence for their claim. In the current studies we have added greatly to this evidence. In Experiments 1 and 2, we provided new support for the contention of Davis et al. that masking properties of ``linking segments'' in their single-large-object stimuli cannot account adequately for the same-object costs they found. In Experiments 3 and 4, we found further evidence for same-object costs in stimuli more similar to those employed in other previous studies. Additionally, Experiment 4 suggested that such same-object costs do not only arise for complex multisegment objects, but can also be found for features that appear on the same object ``part'' as each other. We shall not present a detailed argument here as to why the particular conditions employed by previous studies gave rise to same-object benefits. Such an argument has already been put forward by Davis (2001) and Davis et al. (2001), and bears some similarities to a view proposed by Humphreys (1998). Briefly, however, the argument runs as follows. Outline stimuli and extensive pre-exposure of objects before features are presented will tend to emphasize contributions to performance of sustained mechanisms in vision sensitive to high-spatial frequencies. Such mechanisms, found in what has been termed the P-pathway of vision (e.g., Livingstone & Hubel, 1988), are heavily implicated in the recognition of objects. Accordingly, the same-object benefits found in previous tasks may reflect strong within-object feature binding by mechanisms in that pathway. In contrast, Davis et al. argued that when filled-objects are presented simultaneously with target features, this emphasizes the contribution of mechanisms in a second (loosely segregated) channel, the magnocellular to dorsal-stream pathway, yielding same-object costs. Davis and colleagues suggested that the same-object costs they found reflected stronger binding of features from separate neighbouring objects than for features from the same object. This ``between-object'' feature binding might, in principle, support mechanisms governing the visual guidance of action by the dorsal stream. Irrespective of
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REVERSING SAME-OBJECT BENEFITS
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whether one accepts this possible account, however, our results do require explanation, and do not seem easily accounted for by conventional models of object-based selection such as that adopted by Duncan, Humphreys, and Ward (1997). We know of only two studies that have found conventional same-object benefits in the absence of any of the factors we suggest may give rise to sameobject benefits. The first, by Watson and Kramer (1999), employed stimuli where one of the target features was a diagonal section at the end of a vertical or horizontal bar: A same-object benefit was found even though the objects and features were presented simultaneously. However, the ease with which we found same-object costs using very similar objects in Experiment 3, suggests to us that the diagonal-section feature in the Watson and Kramer study (which is very different to the features normally used in divided attention studies) may have favoured same-object conditions. In order to encode the diagonal section as a ``bend'' in an object (as Watson & Kramer's observers were instructed to do), rather than simply a diagonal bar, encourages the observer to encode its relationship to the rest of the object, and thereby to attend other features belonging to the same object. Our explanation of the Watson and Kramer (1999) finding shares features in common with that suggested for the result of the second study, advanced by the authors themselves. Baylis and Driver (1995), found that when observers were asked to judge whether two edges were symmetrical, this was always more rapid when the edges belonged to one object than when they were from separate objects. Baylis and Driver suggested that the advantage for symmetry in the same-object conditions might have arisen because symmetry is a feature only encoded for different edges of the same object, not for edges belonging to separate objects. Thus, in the same-object case, observers were judging only one feature (an object's symmetry), whereas when the edges belonged to separate objects, they were judging two features (the shape of each edge). Any apparent same-object advantage may therefore have reflected the greater ease of judging one feature rather than two, as opposed to a general advantage for comparing features from the same object. In order for same-object costs to achieve the same status as conventional same-object benefits, they must first be widely replicated (another laboratory in the UK has already done so, employing very different stimuli and tasks; Boutsen and Humphreys, personal communication). If they do become so, and become widely accepted, they will inevitably add a further level of complexity when trying to understand object-based attention, providing a new challenge for cognitive neuroscience. However, while in some respects these findings may complicate matters, same-object costs could, in principle, also help to narrow down the likely candidate mechanisms underpinning conventional same-object benefits.
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