Journal of Experimental Psychology: Copyright 2000 by the American Psychological Association, Inc. Human Perception and Performance 0096-1523/00/$5.00 DOI: 10.1037//0096-1523.26.1.424 2000, Vol. 26, No. 1,424-433

Occlusion, Symmetry, and Object-Based Attention: Comment on Behrmann, Zemel, and Mozer (1998)

Jun Saiki Nagoya University

The validity of M. Behrmarm, R. Zemel, and M. Mozer's (1998) finding that object-based attention can be directed toward occluded objects is examined in 3 experiments. In M. Behrmann et al.'s (1998) original study, participants made speeded judgments of whether the numbers of bumps attached to 2 arms of an X shape were the same or different. The 2 sets of bumps belonged either to a single object, 2 different objects, or 2 separated parts of an occluded object. Unfortunately, this objecthood manipulation was confounded by the symmetry of the stimuli. Experiment 1 replicated M. Behrmann et al.'s main results using identical stimuli. Experiments 2a and 2b dissociated objecthood from symmetry. The results suggest that the effects of object-based attention found by M. Behrmann et al. are largely due to symmetry. The stimuli used in M. Behrmann et al. are not appropriate for examining the relation between object-based attention and occlusion.

Recently, researchers have found that visual attention can be directed toward objects (Baylis & Driver, 1993; Driver & Baylis, 1989; Duncan, 1984; Kramer & Jacobson, 1991) in addition to selected locations in the visual field (e.g., Eriksen & St. James, 1986; LaBerge & Brown, 1989; Posner, 1980). For example, Duncan (1984) showed that participants report two properties of a visual display more accurately when they belong to the same object than when they belong to different objects. Unfortunately, much of this research has been limited by the lack of a clear definition for the term object. Defining the term object toward which object-based atten- tion can be directed is very important because it reveals various constraints and boundary conditions on attention and shape perception mechanisms as well as relations between these two processes. What kinds of objects can serve as the focus of attention?

A recent study by Behrmann, Zemel, and Mozer (1998) was an attempt to specify objects used by object-based attentional mechanism. The main question addressed in their study was whether visual attention can be directed to an object whose parts are separated by occlusion in the image. They conducted a series of experiments and concluded that visual attention can be directed to an amodally completed object. However, their experimental stimuli have some confounding factors. This article first reviews Behrrnann et al.'s (1998) experiments and then reports experiments that examine whether their findings are due to effects of amodal

This work was partially supported by Grants 09710049 and 10559010 from Grants-in-Aid for Scientific Research for Japanese Ministry of Education, Science, and Culture and a grant from Nissan Science Foundation. I thank Jesse Choplin, John Hummel, Philip Kellman, Brian Stankiewicz, and Jacqueline Zimmerman for their helpful comments.

Correspondence concerning this article should be addressed to Jun Saiki, Graduate School of Human Irfformatics, Nagoya Univer- sity, Furo-cho, Chikusa-ku, Nagoya, Japan 464-8601. Electronic mail may be sent to saiki@cog.human.nagoya-u.ac.jp.

completion or to a confounding factor, that is, an effect of symmetry.

Object-Based Attention to an Occluded Object in Behrmann et al. (1998)

Behrmann et al. (1998) conducted a series of experiments asking participants to make comparative judgments of two features in a display. Displays were line drawings of x-like shapes with four arms, two of which had either two or three bumps at the end. Figure 1 shows examples of displays used in Behrmann et al. The participants' task was to judge as accurately and quickly as possible whether the number of bumps at the two arms was the same or different. Their displays can be interpreted as two parts overlapping each other. In their first experiment, all displays were made up of two rectangles crossing each other (called X displays, shown in the left two columns of Figure 1), and depending on the locations of two sets of bumps, there were three different types of displays: In the single-object condition (top row of Figure 1), two sets of bumps belong to a single rectangle; in the two-object condition (middle row of Figure 1), they belong to two different rectangles; and in the occluded condition (bottom row of Figure 1), bumps are located to two ends of one rectangle, but the rectangle was occluded by the other rectangle. The occluded condition was critical in that bumps belong to spatially separated parts (short arms), but amodal completion provides an interpretation that these two parts belong to a single object. Behrmann et al. found that participants' judgments were faster in both the single-object and occluded conditions than in the two- object condition. In particular, the performance in the occluded condition was significantly better than that in the two-object condition and was close to that in the single- object condition, which led Behrmann et al. to conclude that object-based attention can be directed to an amodally completed object.

However, as Behrrnann et al. (1998) themselves admitted,

424

OBSERVATIONS 4 2 5

a. Single-object

A. X-shape Displays B. V-shape Displays Same Different Same Different

b. Two-object

c. Occlusion

Figure 1. Examples of the stimuli used in Behrmann, Zemel, and Mozer (1998). Panel A illustrates X displays, and Panel B illustrates V displays. Rows correspond to three stimulus types: single-object, two-object, and occluded. V displays were used in Experiment 1.

the first experiment with X displays had some obvious problems. Among them, the collinearity of bumps con- founds their manipulation: Bumps are located collinearly in the single-object and occluded condition, whereas they are not located collinearly in the two-object condition. To get around of this problem, Behrmann et al. modified the displays and conducted another experiment. In the new stimulus set, called Vdisplay, displays were made up of two overlapping V shapes, one rotated 180 °, with their apices overlapping, which as a whole formed an X shape, and again three types were defined in terms of location of bumps. Examples of single-object, two-object, and occluded condi- tions are shown in the right two columns of Figure 1. One important modification in this stimulus set was that bumps were always located two arms next to each other so that collinearity was no longer confounded with display type. Behrmann et al. found essentially the same pattern of results with the V displays.

Although the results with the Vdisplays appear to provide stronger evidence for object-based attention toward an occluded object, a closer examination of the V displays shows that they still have another confounding factor as well. In the V displays, symmetry of a whole display is confounded with display type. I As the top and bottom rows of the right two columns of Figure 1 show, same displays of the single-object and occluded conditions are symmetric in terms of an axis dividing two sets of bumps (e.g., the vertical axis of these objects), whereas different displays are not symmetric because of the different shapes of two bump sites. In other words, in the single-object and occluded conditions, symmetry and same or different are perfectly correlated. Thus, participants may use a strategy such that if a display as a whole is symmetric, then they respond "same" and otherwise respond "different." On the other hand, in the

two-object condition, the display as a whole is always asymmetric regardless of the number of bumps. The symme- try strategy does not work with the two-object condition, which forces participants to compare the numbers of bumps. Availability of an additional cue (symmetry) in the single- object and occluded conditions may be the reason Behrmann et al. obtained object-based attention effects.

Is It an Effect o f Occlusion or Symmetry?

It is known that the visual system is quite sensitive to symmetry of objects and shapes (Baylis & Driver, 1995a, 1995b; Corbalis & Roldan, 1974; Mach, 1885/1959; Rock, 1983). Since the classic demonstration by Mach (1885/ 1959), it has been shown that symmetry is detected more easily than repetition (Baylis & Driver, 1995a, 1995b; Corbalis & Roldan, 1974). Furthermore, symmetry is a determinant of figure and ground organization such that symmetric regions are likely to be interpreted as a figure and that edges are assigned to them (Rock, 1983; see Baylis & Driver, 1995a, for a systematic empirical study on this topic). Thus, an alternative explanation of Behrmann et al.'s (1998) object-based attention effect, which is based on symmetry of displays, needs to be considered seriously

1 One should note that though Behrmann et al. (1998) did n o t

mention it, the objectness manipulation of the X displays was also confounded with symmetry. Both single and occluded displays were symmetric with a diagonal axis, but two-object displays are

asymmetric. Thus, confound of objectness with symmetry is the property of both X and V displays. Behrmann et al.'s V displays eliminated the confound of collinearity but not the confound of symmetry.

426 OBSERVATIONS

before one can really conclude that object-based attention can be directed toward occluded objects. In fact, an account based on symmetry would seem to have some advantages in explaining Behrrnarm et al. 's (1998) data. First, the symme- try account is consistent with the result that there is little difference in response time (RT) between the single-object and the occluded conditions because in terms of symmetry there is no difference between these two conditions. On the other hand, to explain the lack of difference between the single-object and occluded conditions, the amodal comple- tion account needs to assume either that amodal completion occurs instantaneously or that the time for amodal comple- tion somehow does not reflect the response time in Behr- mann et al. 's task. Considering a recent finding by Sekuler and Palmer (1992) showing that amodal completion takes as long as 200 ms, the validity of these assumptions is not warranted. Furthermore, the amodal completion account has some difficulty in explaining why the pattern of results was identical between the two experiments in Behrrnann et al., even though phenomenological impression of amodal completion was quite different between X displays and V displays. As can be seen clearly in Figure 1, the occluded V displays have little phenomenological impression of occlu- sion, whereas the occluded X displays have a compelling impression of occlusion. If amodal completion is respon- sible for the results o f Behrmann et al.'s experiments, why did such a strong difference in impressions have no effects on participants' performance? Again, the symmetry account fits better with the data in this respect. I f the symmetry (and additionally collinearity in the case o f Experiment 1 of Behrmann et al.) is a main factor of the participants' performance, the same pattern of results between the two experiments is not a surprise at all.

On the basis of the arguments above, this study conducted an empirical test of whether Behrmann et al. 's (1998) effect of amodal completion was really an effect of object-based attention or not. I carded out this test by investigating whether the effect o f amodal completion occurs even when displays are asymmetric. I modified the V displays to be asymmetric by making one ann with bumps longer and another arm with bumps shorter, as shown in Figure 2. These asymmetric V displays as a whole are always asymmetric; thus, participants cannot use symmetry as a cue to make same/different judgments. Note that the asymmetric V displays preserve the three display types (single-object, two-object, and occluded) intact. The amodal completion account predicts that the participants' performance should be essentially identical to those in previous experiments: Both single-object and occlusion conditions will show signifi- cantly better performance than the two-object condition. By contrast, the symmetry account predicts that the effect of amodai completion will disappear with the asymmetric V displays. Because the amodal completion and symmetry accounts are not mutually exclusive, it is possible that both factors are responsible for the effect of amodal completion in Behrmann et al. In that case, the effect of amodal completion will be reduced but will still be significant with the asymmetric V displays.

a. Single-object

b. Two-object

c. Occlusion

Asymmetric Displays

Same Different

Figure 2. Examples of the asymmetric displays used in Experi- ment 2. Rows correspond to three stimulus types: single-object, two-object, and occluded. Symmetric displays used in Experiment 2 were identical to the V displays used in Experiment 1.

Expe r imen t 1: Repl ica t ion

The purpose of Experiment 1 is to replicate the basic findings of Behrmann et al. (1998) by using the same stimulus set.

Method

Participants. Sixteen undergraduate students at the University of California, Los Angeles, participated in the experiment for credit in introductory psychology courses. All had normal or corrected-to- normal vision.

Materials. The set of stimuli was identical to that in Behrmann et al.'s (1998) Experiments 2 and 3. The stimuli consisted of displays made up of two overlapping V shapes, one rotated 180 °, with their apices overlapping, which as a whole formed an X shape. Two of the four ends of X had a set of features or bumps. The number of bumps at each end was either two or three. Depending on the locations of bumps, there were three different conditions in terms of stimulus types. In the single-object condition, bumps appeared at each end of a single nonoccluded V (see Figure 1). In the two-object condition, bumps appeared at one end of the two different Vs (Figure 1). In the occluded condition, bumps appeared at each end of a single occluded V (Figure 1). As shown in Figure 1, bumps always appeared two ends next to each other, but the positions of bumps were counterbalanced such that they appeared equally often at the top, bottom, left, or right. Also the orientations of Vs were counterbalanced such that the nonoccluded V faced top, bottom, right, and left equally often.

The displays were presented as black and white line drawings on a white background. The display subtended a visual angle of 4.8 ° with a viewing distance of approximately 50 cm. Each ann of X was 3.1 cm in length and 2.5 cm in width.

The participants' task was to judge whether the number of bumps on the two ends of the display was the same or different. There were equal numbers of same and different trials. On the same trials, there

OBSERVATIONS 4 2 7

were either two (2-2 trials) or three bumps (3-3 trials) at the two ends, and the number of 2-2 trials and 3-3 trials was equal in each of the three conditions. On different trials, there were two bumps on one end and three bumps on the other end, and the locations of these bumps were counterbalanced. In all, there were 48 different displays used in this experiment, composed of 3 stimulus types, 4 combinations of number of bumps, and 4 different orientations of Vs.

Procedure. Participants ran individually on a Macintosh l]Fx computer with a color monitor. Each trial began with a fixation cross at the center of the screen. After 500 ms, a figure was presented and remained on the screen until a participant made a response. The exact location of the figure on the screen varied across trials. The center of each figure was randomly selected from a 1.9- × 1.9-cm square area of the center of the screen. The participant's task was to judge, as quickly and accurately as possible, whether the number of bumps was the same or different on the two ends. RTs were measured from the onset of the figure presentation. Responses were followed by correct feedback. The intertrial interval was 1,000 ms following a correct response and 3,000 ms following an incorrect response.

There were 12 blocks of 32 trials, leaving each of the 48 possible displays to be presented twice every three blocks. Thus, both judgment (same or different) and stimulus type (single-object, occluded, or two-object) are mixed within each block, and the order of displays was randomized. The first three blocks were practice blocks and were not included in the analyses. Thus, 288 trials were used in the data analysis. At the end of each block, the participant received summary feedback telling his or her percentage correct and mean RT for the block. The participant initiated the next block with a key press.

Results and Discussion

Error trials were excluded from the RT analyses. The median RT for each condition was used in the analysis. Figure 3 shows a mean median RT for each condition both for same and different trials. Although the different trials

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SINGLE

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Figure 3. Mean response times (RTs) for "same" and "different" responses as a function of stimulus type in Experiment 1. Error bars represent standard errors for each condition, and mean error rates ate denoted in parentheses above each bar.

showed a different pattern from those in Behrmann et al . 's (1998) experiments, the pattern of same trials was very similar to those in Behrmann et al. A 2 (judgment: same vs. differen0 × 3 (stimulus type: single-object, occluded, two-objec0 within-subjects analysis o f variance (ANOVA) revealed a significant interaction o f judgment and stimulus type, F(2, 30) = 13.22, MSE = 679.03, p < .001. Main effects of judgment, F(1, 15) = 1.64, MSE = 1885.35, p > .2, and of stimulus type, F(2, 30) = 0.19, MSE = 354.44, p > .5, were not statistically significant. Because there was a significant interaction, same trials and different trials were analyzed separately. For same trials, a one-way ANOVA showed a significant main effect of stimulus type, F(2, 30) = 9.13, MSE = 511.63,p < .05. A T u k e y post hoc test with an alpha level of .05 showed that the two-object condition had a significantly longer RT than the single and occluded condi- tions. For different trials, a one-way ANOVA showed a significant main effect of stimulus type, F(2, 30) = 8.38, MSE = 521.83, p < .005. However, a Tukey post hoe test showed that both occlusion and single conditions had significantly longer RTs than the two-object condition.

Error rates are shown in parentheses in Figure 3. Error rates were generally higher in this experiment than in Behrmann et al. 's (1998) study. A 2 X 3 within-subjects ANOVA revealed a marginally significant main effect o f judgment, F(1, 15) = 3.44, MSE = 0.0006, p = .084, suggesting that error rates tended to be higher in the same judgment, and a marginally significant interaction of judg- ment and stimulus type, F(2, 30) = 3.26, MSE = 0.0003, p = .052. A main effect of stimulus type was not significant, F < 1. Separate analyses of the same and different trials showed that the effect of stimulus type was significant for the same trials, F(2, 30) = 3.74, MSE = 0.0003, p < .05, but not significant for the different trials, F(2, 30) = 1.02. The significant effect of stimulus type in the same trials showed that the effect found with RT was not due to speed-accuracy trade-off, because the two-object condition showed the highest error rate.

The most important result of Behrmann et al. (1998), a significantly slower RT for the two-object condition than the other two conditions in the same trials, was replicated in this experiment. Indeed, this pattern of results with the same trials appears to be quite robust. By contrast, the pattern of results with the different trials was rather different from those in Behrmann et al. 's experiments. The RT for the two-object condition was significantly faster than the other two conditions. I should point out here, however, that the pattern of results with different trials was not stable even in Behrmann et al . 's original study. The result with the different trials in their Experiment 3 did not show any advantage of the single and occluded conditions over the two-object condition, which is more similar to the results of this study than to their own Experiment 2. Thus, the discrepancy in the results with different trials between this study and Behrmann et al. does not seem to be critical to the main arguments. Although more than single factors are probably involved, there are a number of possible explanations for the pattern of results with different trials in this experiment and Behrmann et al. 's Experiment 3. Two of them are worth mentioning

428 OBSERVATIONS

here. First, the pattern of RT results for trials with differing numbers o f bumps may reflect a Stroop-like interference as Behrmann et al. suggested to explain their strange RT pattern for trials with differing numbers of bumps in Experiment 3. Because participants are responding "differ- ent" when the set of bumps are on the "same" object in the occluded and single conditions, RTs in these conditions may become longer than the two-object condition, in which there is no such interference. In fact, this interference interpreta- tion fits better with my results than Behrmann et al.'s results. A second explanation is that objecthood is confounded with symmetry in Experiment 1 in this study and Experiment 3 in Behrmann et al. A strategy of responding "same" when a stimulus is symmetric and "different" when a stimulus is asymmetric would predict the observed pattern of results. The symmetry of the stimuli was determined by two factors: number of bumps and stimulus type. The single and occluded stimuli with the same number of bumps are symmetric, and the two-object stimuli with different number of bumps are asymmetric in terms of both factors. These two types of stimuli fit the symmetry strategy well, which produce short RTs. By contrast, the number of bumps and stimulus type provide conflicting cues regarding the symme- try for the other types of stimuli. Such cue conflicts may slow down RTs for single and occluding stimuli with different number of bumps and for two-object stimuli with the same number of bumps. As shown earlier, because of the confound between symmetry and objectness in this experi- ment and Behrmann et al.'s, one cannot tell which is a more plausible explanation of the results. Thus, Experiment 2 examined which factor, objectness or symmetry, is respon- sible for the object-based attention effect found in this experiment and previous experiments. I dissociated the

objectness and symmetry by introducing asymmetric V stimuli.

Exper imen t 2a: A Test o f S y m m e t r y Effect

Method

Participants. Sixteen undergraduate students at the University of California, Los Angeles, participated in the experiment for credit in introductory psychology courses. All had normal or corrected-to- normal vision.

Materials. Stimuli were composed of two sets of displays: symmetric displays and asymmetric displays. The symmetric displays were identical to those used in Experiment 1. The asymmetric displays were created from the corresponding symmet- ric displays in the following way. Out of the two arms with bumps, the arm on left side was stretched by 1.0 cm, and the arm on the right side was shrunk by 1.0 cm. As a result, as shown in Figure 2, the overall display became asymmetric. In all, there were 96 different displays used in this experiment, 48 symmetric displays and 48 asymmetric displays, and each set was composed of 3 stimulus types, 4 combinations of number of bumps, and 4 different orientations of Vs.

Procedure. Procedure was identical to that of Experiment 1 except for the following changes. There were 18 blocks of 32 trials. Every 3 blocks, the 96 possible displays were presented once, and the order of displays was randomized. Thus, judgment (same or dif- ferent), symmetry (symmetric or asymmetric), and stimulus type (single-object, occluded, or two-object) were mixed within each block. The first 3 blocks were practice blocks and were not in- cluded in the analyses. Thus, 480 trials were subjected to data analysis.

Results and Discussion

Figure 4 shows a mean of median RTs for each condition both for same trials and different trials. Overall, the results

A. Symmetry Condition

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600

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JUDGMENT JUDGMENT

Figure 4. Mean response times (RTs) for "same" and "different" responses as a function of stimulus type in Experiment 2a. Panel A denotes the symmetry condition, and Panel B denotes the asymmetry condition. Error bars represent standard errors for each condition, and mean error rates are denoted in parentheses above each bar.

OBSERVATIONS 429

showed a quite different pattern from those of Experiment 1, suggesting that the object-based attention effect found in Experiment 1 was largely due to a confounding variable, symmetry. In particular, in the asymmetry conditions, there was no effect of stimulus type for both same and different trials. A 2 (judgment: same vs. different) × 2 (symmetry: symmetry vs. asymmetry) × 3 (stimulus type: single-object, occluded, two-object) within-subjects ANOVA revealed a significant main effect of judgment, F(1, 15) = 6.92, MSE = 3072.07, p < .025; a significant main effect of symmetry, F(1, 15) = 5.89, MSE = 657.52, p < .05; a significant interaction of response and symmetry, F(1, 15) = 69.56, M S E = 532.96, p < .0001; and a marginally significant three-way interaction, F(2, 30) = 3.03, MSE = 362.88, p = .063. In general, same trials showed faster RTs than the different trials, and symmetric conditions showed faster RTs than the asymmetric conditions. A strongly significant interaction of judgment and symmetry suggests that the symmetric conditions showed faster RTs than the asymmet- ric conditions for the same trials, whereas the pattern was opposite for the different trials, such that the asymmetric conditions showed faster RTs. Because there were signifi- cant interactions, symmetric and asymmetric conditions were analyzed separately. For the symmetric condition, a 2 (judgment) × 3 (stimulus type) ANOVA regcealed a signifi- cant main effect of judgment, F(1, 15) = 38.22, MSE = 1497.31, p < .0001; a significant main effect of stimulus type, F(2, 30) = 4.05, MSE = 504.98, p < .05; and a significant interaction of judgment and stimulus type, F(2, 30) = 5.34, M S E = 416180, p < .025. Further analysis suggests that an effect of stimulus type was significant with symmetric different trials, F(2, 30) = 6.88, M S E = 492.41, p < .005, but it was not significant with symmetric same trials, F(2, 30) = 2.06, MSE = 429.37, p > .1. For the asymmetric condition, a 2 × 3 ANOVA showed that neither the main effects nor the interaction was significant, F < 1.

Error rates are shown in parentheses in Figure 4. A 2 (judgment: same vs. different) × 2 (symmetry: symmetry vs. asymmetry) × 3 (stimulus type: single-object, occluded, two-object) within-subjects ANOVA revealed a significant main effect of judgment, F(1, 15) = 4.84, MSE = 0,0006, p < .5, suggesting that error rates tended to be higher in the same judgment, a significant main effect of symmetry, F(1, 15) = 5.21, MSE = .001, p < .05, suggesting that symmetric condition showed fewer errors, and a significant interaction of judgment and symmetry, F(1, 15) = 8.22, MSE = 0.0003, p = .025. More important, the pattern of error rates does not allow me to interpret the lack of effects of stimulus type with RT data as a speed-accuracy trade-off. Indeed, for the asymmetric same trials, the two-object condition showed a lower mean error rate than the other conditions.

The most important result of this experiment was a complete lack of object-based attention effect in the asym- metric conditions. Although the null result cannot guarantee the lack of object-based attention effect, a comparison of the results of Experiments 1 and 2 clearly suggests that the object-based attention effect found in Experiment 1 and Behrmann et al. (1998) is critically dependent on the

symmetry of the displays. The pattern of results in the symmetric condition in this experiment was similar to that in Experiment 1, but the object-based attention effect was much smaller, This probably reflects the fact that symmetry is much less salient in Experiment 2 than in Experiment 1 because participants could use symmetry strategy in only half the trials in Experiment 2.

Experiment 2b: The Classification Task Used by Behrmann et al. (1998)

Although the results of Experiment 2a clearly suggest that symmetry of the displays, rather than amodal completion, was crucial for the object-based attention effect found in Experiment 1 and Behrmann et al. (1998), it is unclear whether there is any functional equivalence between the asymmetric occluded and single displays, as Behrmann et al. observed. Following Behrmann et al., the functional equiva- lence between the single and occluded displays was evalu- ated using a speeded classification task. If single and occluded displays are functionally equivalent (namely, the occluded displays are psychologically more similar to the single displays than to the two-object displays), then classifi- cation RTs are expected to be faster when the single and occluded displays belong to the same category than when the two-object and occluded displays belong to the same category. Indeed, in their Experiments lc and 2b, Behrmann et al. found that the classification time of the occluded displays were much faster when they were grouped with the single displays than when they were grouped with the two-object displays and that classification time was not significantly different between the single and occluded displays. These results suggest that the occluded and single displays are functionally equivalent.

However, because the symmetric displays used by Behr- mann et al. (1998) had a confound between symmetry and amodal completion, the functional equivalence found in their study does not necessarily reflect amodal completion. Although symmetry and response mapping are not corre- lated in the classification task, it is still the case that occluded displays are more similar to the single displays than to the two-object displays on the basis of symmetry relations (because half of the occluded and single displays share a property of symmetry). It is well known that classification performance is a function of psychological similarities among exemplars (Medin & Schaffer, 1978; Nosofsky, 1986); thus, one should examine what properties of displays determine the classification performance.

To examine whether amodal completion determined the functional equivalence of single and occluded displays in the classification task in Behrmann et al. (1998), I again used the asymmetric displays in Experiment 2b. If Behrmann et al.'s results in their Experiments lc and 2b reflect amodal completion, then the results of this experiment should be similar to them. This will imply that the results of Experi- ment 2a of this study are mainly due to a task-specific strategy of using symmetry relations. By contrast, if Behr- mann et al.'s results reflect mainly symmetry even in the classification task, then the results of this experiment should

430 OBSERVATIONS

be different from theirs because the symmetry is no longer available as a cue for classifications in this experiment. This latter result will imply that their displays themselves do not allow the effects of amodal completion to be examined.

Method

Participants. Twenty graduate students at the Nagoya Univer- sity participated in the experiment as volunteers. All had normal or corrected-to-normal vision. They were consecutively assigned to either Group 1 or Group 2, and their order was counterbalanced.

Materials. Stimuli were the asymmetric displays of three stimulus types, single-object, occluded, and two-object, used in Experiment 2a. In all, there were 48 different displays used in this experiment, and each stimulus type was composed of four combina- tions of number of bumps and four different orientations of Vs.

Design and procedure. Participants were instructed that they were to perform a categorization task and were shown a sample set of displays in which the category assignment was demonstrated. Participants in Group 1 were instructed to classify single and occluded objects into one category and the two-object display into a second category, whereas participants in Group 2 were instructed to classify single objects into one category and occluded and two-object displays into a second category. The categories were assigned to labels A and B, and the labels for the categories were counterbalanced in both groups. The categorization decision was made by pressing keys on the two-key box. Key assignment was also counterbalanced across participants. Participants completed three blocks of 96 trials followed by 24 practice trials.

Results and Discussion

Figure 5 shows a mean of median RTs for each condition. For the sake of comparison, Figure 5 also shows the available data from Experiment 2b of Behrmann et al. (1998). Overall, the results are inconsistent with those in Behrmann et al. Following Behrmann et al.'s analyses, I

compared RTs for single and occluded conditions in both groups. Participants in Group 1 (occluded assigned with single) categorized the single displays significantly faster than the occluded display (521 ms vs. 541 ms, respectively), and those in Group 2 (occluded assigned with two-object) also showed the same tendency (489 ms and 501 ms, respectively), though the difference in RT was smaller. This pattern of results was confirmed by a repeated-measures ANOVA with 4 within-subject variables: category type (occluded with single or with two-object), stimulus type (single-object, occluded, or two-object), number of bumps (same or different), and blocks (1, 2, or 3), which revealed significant main effects of stimulus type, F(2, 38) = 16.77, MSE = 3833.82, p < .0001, and category type, F(1, 19) = 19.43, MSE = 15244.95, p < .0001, but no significant interaction of these two factors, F(2, 38) = 1.03, p > . 1. By contrast, Behrmann et al.'s Experiment 2b, which used the symmetric V displays, revealed that participants in Group 1 showed quicker RTs in the occluded condition than in the single condition, whereas those in Group 2 showed the opposite pattern, exemplified by a significant interaction of category type and stimulus type. Thus, the classification of the occluded displays depends on whether they are assigned with the single- or two-object displays when the displays are symmetric, whereas there is no such dependency when the displays are asymmetric. As far as the asymmetric displays used in this experiment are concerned, there was no evidence for the functional equivalence between the single and occluded displays. This implies that the lack of object- based attention effect in Experiment 2a of this study was not due to the task-specific strategy of using symmetry relations in comparative judgments. Rather, the object-based attention effect reported by Behrmann et al. is likely to reflect the effect of symmetry, not amodal completion.

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450 Occluded with Occluded with Single Two

Category Type

Figure 5. Mean response times (RTs) for speeded classification responses as a function of category type in Experiment 2b. Panel A denotes the data of this study, and Panel B is a reproduction of the corresponding data from Experiment 2b of Behrmann et al. (1998) for comparison. Error bars represent standard errors for each condition. The standard errors and data for the two-object condition were not available in Behrmann et al. (1998).

OBSERVATIONS 431

Table 1 Mean Rating Scores of Subjective Impression of Occlusion for the Occluded Displays as a Function of Symmetry and Display Type (X Display and V Display)

Display type

Symmetry X display V display of display M SD M SD

Symmetry 5.76 0.95 2.68 0.99 Asymmetry 5.53 0.88 1.89 1.03

There are two alternative explanations of the discrepancy between classification data in Behrmann et al. (1998) and this study. One is that what the classification task reflected was the functional equivalence mediated by symmetry relations, not by amodai completion. The other is that Behrmann et al.'s findings really reflected amodal comple- tion, not simply symmetry, because symmetry itself is a crucial cue for amodal completion. To examine the validity of the second claim, I directly asked an independent group of participants to rate the subjective impression of occlusion with the occluded displays used in this study (symmetric and asymmetric, X and V displays). If the symmetry relation leads to amodal completion in the symmetry V display, then there should be a large difference in subjective impression of occlusion between symmetric and asymmetric V displays, and the symmetric V displays should not differ substantially in subjective impression of occlusion than the X displays, which should have strong impression of occlusion. By contrast, if the amodal completion is not related to the effect found in Behrmann et al., then there should be a large difference in the impression of occlusion between X display and Vdisplay, regardless of symmetry.

Twenty undergraduate students at the Nagoya Municipal University rated the subjective impression of occlusion on a 7-point scale ranging from 1 (no impression) to 7 (strong impression) with the occluded displays. There were 16 displays used for each participant, composed of four condi- tions defined by symmetry (symmetric and asymmetric) and display type (X display and V display). Table 1 shows mean ratings for the conditions. Clearly, the symmetric V display did not show strong impression of occlusion, and the mean rating score was much closer to that for the asymmetric V displays than those for the X displays. Although there was a significant difference in rating between symmetric and asymmetric V displays, it was much smaller than the difference between symmetric X and V displays. Thus, it is difficult to explain the pattern of results in the speeded judgment data (Experiments 1 and 2a of this study, and Experiment la of Behrmann et al., 1998) from the pattern of subjective impression ratings. A simpler and more coherent explanation of the available data from this study and Behrmann et al. (1998) seems that as far as V displays are concerned, the pattern of results with the speeded feature- comparison task and classification task reflected the symme- try relations of the displays, and amodal completion has few effects on participants' performance, if any.2

General Discussion

The results of the experiments in this study do not support Behrmarm et al.'s (1998) claim that object-based attention can be directed to an occluded object. The finding of Experiment 2 clearly suggests that the significantly faster RT for the occluded condition than for the two-object condition in Behrmann et al.'s Experiments 2 and 3 and Experiment 1 in this study is largely due to participants' use of symmetry as a cue for same or different judgments. Furthermore, the results of Experiment 2 suggest that even simple object- based attention effect defined as an advantage of the single-object condition over the two-object condition was largely due to symmetry of stimuli, because this effect also disappeared in the asymmetric condition.

Behrmann et al. (1998) argued that symmetry is unlikely to be driving the effect, and they based this argument on the results of their Experiment 2b and a speculation that when the central region of the display was removed so that bumps were separated, the single-object advantage would disappear (see Figure 13 of Behrmann et al., 1998). I showed that the functional equivalence between single-object and occluded displays disappeared with the asymmetric displays, suggest- ing that the results of Experiment 2b in Behrmann et al. were also mainly due to the effect of symmetry. Regarding their speculation on the separated displays, the predicted disappear- ance of the single-object advantage can also be interpreted by symmetry. By removing the central region of the single- and two-object displays of Experiment 1 of Behrmann et al. as shown in their Figures 13b and 13c, not only is objectness removed, but also the main axis of symmetry becomes ambiguous. Namely, the display in Figure 13b is symmetric with a diagonal axis, and the display in Figure 13c is symmetric with the vertical axis. Furthermore, Behrmann et al.'s predictions about the effect of perceptual learning on the performance with the separated displays can be inter- preted by symmetry in the same way. They predicted that after participants experienced the task with X displays, RTs for Figure 13b would become faster than those for Figure 13c, whereas after they experienced the task with V displays,

2 One may notice another general difference between this study and Behrmann et al. (1998). RTs in experiments in this study were faster than those in corresponding experiments in Behrmann et al. There are several possible factors contributing this difference. First of all, except for Experiment 2b of this study, there were more practice trials (96 ~als) in this study than in Behrmann et al. (24 trials), which certainly leads to faster mean RTs. Second, response devices (two-key box vs. keyboard) and response methods (using two thumbs vs. using two index fingers) were different. Third, in Experiment 2b of this study, participants were graduate students who had some experiences with RT experiments, whereas Debt- mann et al.'s participants appeared to be naive to psychology experiments. Finally, the error rates in this study were a little higher than those in Behrmann et al. (1998), suggesting that the partici- pants in this study sacrificed accuracy to obtain faster RTs, although the error rates in this study were by no means anomalous. In summary, the difference in overall mean RTs between the two studies does not seem to be critical to the main arguments of this study.

432 OBSERVATIONS

the pattern would be reversed. These predicted changes can be explained by weighting diagonal or vertical axes of symmetry depending on the preceding task: Experiences with X displays may lead participants to put more weight on diagonal axes, and experiences with V displays may lead them to put more weight on the vertical axis.

Of course, the predicted disappearance of single-object advantage with separated symmetric displays may mean that symmetry is not a sufficient condition for the single-object advantage. However, even so, the fact that symmetry is not a sufficient condition does not imply that the single-object advantage of Behrmann et al. (1998) is not mediated by symmetry of the displays. It is likely that symmetry detec- tion is easier within a single object than between multiple objects (Baylis & Driver, 1995a; Driver & Baylis, 1995); thus, symmetry may pronounce a substantial effect only with a single contiguous display. Indeed, the results of this study, in particular no difference in RT among asymmetric single, occluded, and two-object displays, suggest that participants treat the whole display as an object.

However, one should note that this study does not necessarily falsify the role of amodal completion in object- based attcntional mechanism. Findings of this study re- vealed that stimulus sets used in Behrmann et al. (1998) are not appropriate for a test of the role of amodal completion. First of all, as Behrmann et al. admitted, the occluded version of their V stimuli does not provide a good phenom- enal impression of occlusion to participants, which suggests that participants may not interpret their (and our) occluded stimuli as occluded. Besides phenomenal impressions, there are some properties of the occluded V stimuli that may have weakened participants' interpretation of occlusion. In the occluded V stimuli, two line segments of occluding part are collinear with the line segments of occluded part, which is likely to weaken the interpretation of occlusion (Kellman & Shipley, 1991). Moreover, the use of line drawings also weakened the interpretation of occlusion, compared with the use of filled patterns.

On the contrary to Behrmann et al.'s (1998) claim, an effect of object-based attention toward an occluded object, if any, is not robust. Further study is necessary to investigate whether object-based attention can be directed toward an occluded object, and if so, under what conditions such an effect occurs. As for the former question, recently Moore, Yantis, and Vaughn (1998) presented some evidence for object-based attention toward an occluded object. Moore et al. (1998) used a paradigm developed by Egly, Driver, and Rafal (1994), which is similar to Posner's spatial cueing paradigm. In Egly et al.'s (1994) paradigm, displays were two rectangles, and one of the corners was cued by brightening before the target was given. The most important finding was that target detection time was shorter when the cue and target belonged to the same rectangle than when they belong to different rectangles. Moore et al. found this object-based attention effect even when a large rectangle occluded the center of the two rectangles. At this point, it is not clear why a modified spatial cueing paradigm in Moore et al. revealed an object-based attention effect, whereas a feature-comparison paradigm in this study failed in obtain-

ing the effect. These two paradigms may reflect different attentional mechanisms: Spatial cueing paradigm concerns mainly movement or spreading of attention, whereas the feature-comparison paradigm concerns mainly attentional selection. Another possibility is that the use of binocular depth cue used by Moore et al. facilitated the amodal completion, suggesting that the lack of object-based atten- tion effect in this study is mainly due to the lack of binocular cues that facilitate amodal completion. The presence or absence of binocular depth cue may be important because a recent study by Atchley, Kramer, and Theeuwes (1997) obtained an object-based benefit of attention switching in depth when binocular depth cues were provided, whereas there was no such benefit without binocular depth cue.

Beyond the issue of object-based attention and occlusion, the results of Experiment 2 raise questions to the issue of object-based attention in general. Experiment 2 suggests that as far as Behrmann et al.'s (1998) stimuli are concerned, even the basic object-based attention effect may be an artifact of stimulus symmetry. On the contrary to Behrmann et al.'s claim that object-based attention effect is a robust one, it seems that there are certain limitations as for when object-based attention effects are obtained. So far, object- based attention research has focused on demonstrating object-based attention effects (Baylis & Driver, 1993; Dun- can, 1984; Egly et al., 1994) and is not interested in specifying boundary conditions for the object-based atten- tion effects. Specifying when an object-based attention effect occurs and when it does not is very important because it can inform us about what are objects for object-based attentional system. Despite my (and Behrmann et al.'s) intuition that V shapes (or in the asymmetric displays, tilted L shapes) are objects, the object-based attentional system may treat a whole display as an object.

In summary, the main question that Behrmann et al. (1998) raised still remains unanswered. This study suggests that to answer this question, careful controls for various confounding variables are necessary, and one should make sure that the stimulus set leads participants to perform amodal completion.

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Received June 9, 1998 Revision received January 4, 1999

Accepted February 11, 1999 •