Journal of Experimental Psychology: Human Perception and Performance 2014, Vol 40. No. 3, 968-982

© 2013 American Psychological Assoeiation 0096-]523/]4/$12.00 DOI: ]0.1037/a0035005

Voluntary Spatial Attention Induces Spatial Facilitation and Object-Centered Suppression Zhicheng Lin University of Minnesota and University of Washington Many daily activities require encoding spatial locations relative to a reference object (e.g., "leftness"), known as object-centered space. Integrating object-centered space and visual attention, this study reports a new form of attention called object-centered suppression, as revealed by a novel object-centered paradigm. Specifically, after cueing a location within an object (e.g., on the left), performance at 2 locations within another, uncued object was worse for the location that shared the same object-centered space as the cued location (e.g., on the left) than the location that did not (e.g., on the right). Because these 2 locations were equidistant to the cued location and because both appeared within the same object, the effect could not be explained by space-based or object-based accounts of attention. Alternative accounts based on attentional capture were also refuted. Instead, a novel object-centered Simon effect (stimulus-response interference) reveals automatic object-centered spatial coding, supporting an objectcentered account: when attention is disengaged from an invalidly cued location, a negative attention priority signal at the cued location is tagged and transferred across objects in an object-centered manner. Object-centered suppression therefore unveils a new functional footprint of voluntary spatial attention, integrating space-based and object-based selection through object-centered space. Keywords: object-based attention, object-centered Simon effect, object-centered space, object-centered suppression/inhibition, spatial attention

At any single moment, visual inputs to the eye far exceed neural processing capacity and only some are relevant to the current task. To sift the wheat from the chaff, spatial attention directs limited resources to prioritize the processing of certain information at specific locations (for summaries, see Pashler, 1998; Petersen & Posner, 2012). In daily life, spatial attention can be voluntarily deployed based on our expectation or knowledge of the incoming information. In the laboratory, this is induced by a precue, a cue that indicates the probable location of the forthcoming target with a high validity (e.g., 75% of the trials). Precues engage voluntary attention through two central processes: i) disengaging attention from the current focus (such as the fixation), and ii) orienting attention to a potentially informative location (such as the cued location). The general picture from the last few decades of research is that voluntary spatial attention leaves two functional footprints: spatial

facilitation (space-based attention) and object-based facilitation (object-based attention), as prominently revealed by the Posner cueing paradigm (Bashinski & Bacharach, 1980; Posner, 1980) and the two-rectangle paradigm (Fgly, Driver, & Rafal, 1994), respectively. In the Posner cueing paradigm, one of two locations is precued, and performance at the cued location is better than that at the uncued location, revealing spatial facilitation. Subsequent refinement of this paradigm, by incorporating object structures into a two-rectangle paradigm, reveals object-based facilitation that manifests as a same-object advantage. In this paradigm, subjects first view two rectangles with a precue at an end of a rectangle; after a delay, a target appears at one of three locations (Figure 1 left): on the majority of trials, the target appears at the cued location (valid); on the remaining, invalid trials, the target appears at an uncued location either within the cued object (same-object location) or within the uncued object (different-object location). Consistent with classic spatial facilitation, performance is better at the valid location than at the two invalid locations. In addition and more important, despite the two invalid locations being equidistant to the valid location, performance is better at the same-object location than at the different-object location, revealing a sameobject advantage.

This article was published Online First December 23, 2013. Zhicheng Lin, Department of Psychology, University of Minnesota, Twin Cities and Department of Psychology, University of Washington, Seattle. I thank the editor James R. Brockmole and four anonymous reviewers for comments. Part of this work was conducted as the author's Ph.D. dissertation research. I am indebted to Dr. S. He and Dr. S. Murray for laboratory accommodation and discussions. This work was supported by National Institutes of Health (NIH EY015261 and T32EB008389) and National Science Foundation (NSF BCS-0818588). Correspondence concerning this article should be addressed to Zhicheng Lin, Department of Psychology, University of Washington, Seattle, WA 98195. E-mail: [email protected]

The Current Study In the natural environment, however, space and objects are rarely in isolation. The current study therefore goes beyond space and objects by asking how spatial attention is deployed based on the intertwining nature of space and object representations: objectcentered space. In object-centered space, spatial locations are encoded in accordance with a reference object, such as the "front968

OBJECT-CENTERED SUPPRESSION

Object-based Attention valid same-object

different-object

969

Object-centered Attention valid

ID

IS

IS; invalid, same object-centered location ID; invalid, different object-centered iocation Figure 1. From object-based attention to object-centered attention. In the two-rectangle paradigm (left), performance at the same-object location as the cued location is better than performance at the different-object location—a same-object advantage. In the object-centered paradigm developed here, the cued object is shifted so that the two ends of the uncued object are equidistant to the cued location (right): one at the same object-centered location as the cue (IS) and the other at a different object-centered location (ID). A performance difference between the IS and ID locations would demonstrate object-centered attention (and cannot be attributed to space-based attention or object-based attention).

ness" of a driver within his car wherever he drives. Objectcentered spatial processing is at the root of visuospatial perception and cognition in daily life, ranging from writing, map reading, to communicating spatial positions. Moreover, recent evidence reveals that an item's object-centered space (e.g., left) is robustly tagged in its representation even when spatial information is taskirrelevant—its processing is modulated to a much larger extent by a distant item of the same object-centered space (e.g., on the left end of another object) than by the same equidistant stimulus of a different object-centered space (e.g., on the right end). Such automatic object-centered spatial processing is pervasive, intruding visual memory, priining, and backward masking (Lin & He, 2012; Lin & Murray, under review). Therefore the present study explores how attentional priority is tagged and transferred across objects in accordance with objectcentered space, by examining how precueing a spatial location within an object affects performance at locations within an uncued object. To do so, an object-centered paradigm is developed based on the two-rectangle paradigm; the cued object is shifted so that the two ends of the uncued object are now equidistant to the cued location. As a result, whereas one end of the uncued object shares the same object-centered space as the cued location, the other end has a different object-centered space. For example, in Figure 1 right, whereas one invalid location is on the left and thus of the iame object-centered space as the cued location (referred to as the IS location), the other invalid location is on the right and thus i/ifferent from the cued location (referred to as the ID location). Because the two invalid locations are equidistant to the cued location and because they appear within the same object, this crucial manipulation allows one to control for space-based attention and object-based attention—these two classic models would predict no difference between the IS and ID locations. Thus, any difference between them must refiect object-centered attention. If performance is better at the IS location than at the ID location, this would indicate object-centered facilitation, suggesting the tagging and transfer of a positive attentional bias from the cued location; if the reverse is true, it would be evidence for object-centered suppression from a negative attentional bias from the cued location.

To preview the results. Experiment 1 reveals that performance at the valid location was better than at the two invalid locations, refiecting a classic spatial facilitation effect; crucially, in invalid trials performance was worse at the IS location than at the ID location, providing evidence for a new object-centered suppression effect. Additional data from Experiments 2-4 rule out accounts based on attentional capture, indicate instead that the suppressive attentional signal originates from attentional disengagement from the invalidly cued location on invalid trials, and suggest that object-centered suppression is based on relatively abstract spatial representations. Taken together, object-centered suppression represents a new functional footprint of voluntary spatial attention, integrating space-based and object-based selection through objectcentered space.

Experiment 1: Object-Centered Suppression After Peripheral Cueing and Central Cueing Apparent motion has been routinely used as a natural, effective tool for establishing perceptual continuity, which is known to promote object-centered tagging and updating (Lin, 2013; Lin & He, 2012; Lin & Murray, 2013, under review; Ögmen, Otto, & Herzog, 2006). Thus, in Experiment 1, as Figure 2A illustrates, to evoke apparent motion of the contour jumping from the peripheral object to the central object, a red contour was presented surrounding the peripheral object first and then surrounding the central object later. The critical question was whether and how performance differed between the two locations within the same uncued object, equidistant to the cued location. To induce voluntary attention, a cue was presented that predicted the target location on 75% of the trials. In traditional studies of voluntary attention, cues can be presented in one of two ways; either as peripheral cueing or as central cueing. Whereas objectbased attention is more readily revealed in peripheral cueing than in central cueing (Goldsmith & Yeari, 2003; Macquistan, 1997), space-based attention is robust in both types of cueing (Posner, 1980). Therefore, to test whether any object-centered effect found

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V = valid location (75% of the trials) IS = invalid, same object-centered location (12.5%) ID = invalid, different object-centered location (12.5%)

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Figure 2. Spatial facilitation and object-centered suppression after peripheral cueing. (A) Procedure: Observers fixated on a central cross throughout the experiment, and indicated whether a number 2 or 5 was presented on each trial. (B) Design: The target could appear at the same location as the cue (V, on 75% of the trials), at the same object-centered location as the cue within the central uncued object (IS, 12.5%), or at a different object-centered location within the central uncued object (ID, 12.5%). Note that IS and ID were equidistant to the cue. (C) Results (« = 14): Two effects were observed: a spatial facilitation effect (responses were faster when the cue was valid than invalid) and an object-centered suppression effect (in the invalid conditions, responses were slower when the targets appeared at the same object-centered location as the cue than at a different object-centered location). Error bars: standard error of the mean (SEM). *, **, and ***: statistically significant difference at the level oíp < .05, .01, and .001, respectively. In this and subsequent figures, the white cues (dots in peripheral cueing: lines in central cueing) and white rectangle frames were in red in the actual experiments.

might depend on the cueing method used, both peripheral cueing and central cueing were used here, in separate sessions.

Method Subjects and apparatus. The number of subjects in this and subsequent experiments was predetermined to be 14, based on typical studies in object-based attention. Fourteen subjects pardcipated in Fxperiment 1(11 females, mean age 22.6) in return for money or course credit. One subject was replaced because of failure to follow the instruction (i.e., slower in the valid condidon than in the invalid condidon by 244.2 ms—had the subject used the cue as instructed, performance should have been better at the cued locadon than at the uncued locadons; see also Experiment 3). All had normal or corrected-to-normal vision and signed a consent form approved by the local institutional review board. The sdmuli were presented on a black-framed, gamma-corrected 22-in. CRT monitor (model = Hewlett-Packard pl230; refresh rate =

100 Hz; resoludon = 1024 X 768 pixels) using MATLAB (The MathWorks, Nadck, MA) and the Psychophysics Toolbox (Brainard, 1997; Pelli, 1997). Subjects sat approximately 57 cm from the monitor with their heads positioned in a chin rest in an almost dark room. Stimuli and procedure. To ensure proper fixation, as required in the main experiment, subjects took part in a fixadon training session first, in which they viewed a square patch of black and white noise that flickered in counterphase, with each pixel alternating between black and white across frames. Each eye movement during the viewing would lead to perception of a flash, and subjects were asked to minimize the perception of flashes, thereby training to maintain stable fixadon. After the fixation training, subjects participated in two cueing sessions, peripheral cueing and central cueing, in a within-subjects design, with the order counterbalanced between subjects. Each cueing session included 32 practice trials (in 1 block) and 384 experimental trials (in 6 blocks).

OBJECT-CENTERED SUPPRESSION

In peripheral cueing (Figure 2A), on each trial, after a 1200-ms presentation of a fixation cross (length = 0.62°; width = 0.08°; luminance = 40.1 cd/m~ or, when overlaid on the later gray rectangle, 80.2 cd/nr), two gray rectangles (size = 5.2° X 2.3°; luminance = 40.1 cd/nr) were presented simultaneously for 300 ms, one centered on the fixation, the other on one of four corners (shift from the fixation = ± 1.9° X ± 2.9°, outlined by a red rectangle with a width of 0.08°). During this 300-ms presentation, a red dot cue (size = 0.4° X 0.4°) was fiashed within the peripheral rectangle along the vertical meridian at the 200th ms for 40 ms. Immediately after the 300 ms presentation, the red outline disappeared from the peripheral rectangle and appeared instead around the central rectangle; 4 digitized numbers (6, 8, 9, and either 2 or 5; size = 0.75° X 1.0°; number center to rectangle center distance = 1.9°; luminance = 80.2 cd/m^) also appeared at the four ends of the two rectangles for 200 ms. Thus, the cue to target stimulus onset asynchrony (SOA) was 100 ms. The task was to indicate whether the target was 2 or 5 by pressing a left or right key using the index or middle finger as quickly and accurately as possible. Figure 2B illustrates the three cueing conditions. For 75% of the trials, the cue was valid (valid condition, "V"). For the remaining 25% of the trials, the cue was invalid, and the target

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appeared equally often at the left or right end of the central uncued rectangle (12.5% each); in other words, the target could appear at the same object-centered location as the cue (invalid same condition, "IS"), or at a different object-centered location as the cue (invalid different condition, "ID"). Note that the IS and ID locations were equidistant to the cue location. The target never appeared at the far end of the peripheral cued rectangle (which was always occupied by the number 8). The next trial began after subjects responded. Each incorrect response was followed by a minus sign on the fixation for 1000 ms. In central cueing (Figure 3A), no peripheral cue was presented; instead, after 500 ms of fixation presentation, the upper part or the lower part of the central fixation became red for 200 ms, serving as a central cue. Thus, the cue to target SOA was 1000 ms. Other aspects were the same as peripheral cueing. Data analysis. As is typical in studies of spatial attention, reaction times (RTs) were the main focus, with error rates also checked for potential speed-accuracy tradeoff. RT outliers were not excluded in the analysis. All data are available upon request. Repeated measures ANOVAs were conducted with the following two factors: cueing method (peripheral and central) and cue

Central Cues Target (2 or 5?)

V = valid location (75% of the trials) IS = invalid, same object-centered location (12.5%) ID = invalid, different object-centered location (12.5%)

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Figure 3. Spatial facilitation and object-centered suppression after central cueing. (A) Procedure: Observers fixated on a central cross throughout the experiment, and indicated whether a number 2 or 5 was presented on each trial. (B) Design: The target could appear at the cued location (V, on 75% of the trials), at the same object-centered location as the cue within the central uncued object (IS. 12.5%), or at a different object-centered location within the central uncued object (ID, 12.5%). (C) Results (n = 14): As in peripheral cueing, a spatial facilitation effect and an object-centered suppression effect were observed. Error bars: standard error of the mean (SEM). '. "*, and ***: statistically significant difference at the level of p < .05, .01, and .001, respectively.

972 validity (valid, IS, and ID). The normality and sphericity assumptions of ANOVAs were checked with the Shapiro-Wilk test and the Mauchly's test of sphericity, respectively. Two planned contrasts were carried out according to a priori predictions; i) performance would be better in the V condition than in the IS and ID conditions; ii) performance would differ between the IS and ID conditions. The second contrast was of central interest.

Results and Discussion There was a significant main effect of cue validity, F(2, 26) = 29.74, p < .001,1)1 = 0.70; error rates; F(2, 26) = 6.34, p = .006, Tip = 0.33. Planned contrasts revealed that i) RTs were faster at the valid location than at the IS and ID locations, F(l, 13) = 36.25, p < .001,1)1 = 0.74; error rates; F(l, 13) = 17.06, p = .001, rtj = 0.57, confirming a classic spatial cueing effect; ii) more important, RTs were slower at the IS location than at the ID location, F(l, 13) = 14.21,p = .002, •^l = 0.52; error rates: F(l, 13) = 0.03,p = .872, T|p < 0.01, indicating a new object-centered suppression effect. No significant effects were found for cueing method, F(l, 13) = 1.79,p = .203, Ti^ = 0.12; error rates; F(l, 13) = 1.25,p = .284, T|p = 0.09, or its interaction with cue validity, F(2, 26) = 1.17, p = .327, -n^ = 0.08; error rates; F(2, 26) = 1.25, p = .302, Tip = 0.09. Figures 2C and 3C depict separate results for peripheral cueing and central cueing, respectively (see also Table 1).' The spatial facilitation effect reflects a classic positive attention bias toward the cued location when cues are predictive of the target. The object-centered suppression effect, however, is a novel effect that emerges after voluntary spatial attention has been oriented to an incorrectly cued location (i.e., in invalid trials). The effect apparently reflects a negative attention bias when attention disengages from the invalid location, with the priority signal tagged and transferred to the uncued object in an object-centered manner. Given that the same pattem was observed for both peripheral cueing and central cueing, these results indicate that objectcentered suppression is not restricted to peripheral cueing or a short cue to target SOA, but is a general phenomenon.

Experiments 2A and 2B: The Roles of Apparent Motion and Contour Onset Two issues exist regarding the object-centered suppression effect. First, is contour apparent motion necessary for objectcentered suppression? Although perceptual correspondence across apparent motion is typically thought of as a necessary component for object-centered tagging across objects (Lin, 2013; Lin & He, 2012; Ögmen et al., 2006), two recent studies on iconic working memory and priming showed that object-centered visual representations, though stronger with apparent motion, are still operative without low-level image-based apparent motion (Lin & He, 2012; Lin & Murray, under review). These observations raise the possibility that apparent motion may not be necessary for objectcentered suppression as well. To test this possibility. Experiment 2A used the same central cueing procedure as in Experiment 1, but critically the contour's image-based apparent motion was removed by continuously presenting the red contour sun-ounding the peripheral object while the contour surrounding the central object was on the screen (Figure 4A). Second, might object-centered suppression be explained by attentional capture to the contour onset of the peripheral object? If

LIN attention was indeed captured to the peripheral object, it could potentially explain the advantage of the ID condition over the IS condition, as the ID location is closer to the peripheral object than the IS location is (Kravitz & Behrmann, 2008). To address this issue. Experiment 2B removed tiie contour onset altogether by simultaneously presenting the contours around both the central and peripheral objects (Figure 4C). As an added benefit, because this procedure also removed motion based on onset tracking, it afforded a direct test regarding the role of high-level motion interpretation in object-centered suppression—for example, in Experiment 2A (as well as Lin & He, 2012; Lin & Murray, under review), subjects might be able to infer motion by tracking visual onsets (Cavanagh, 1992; Lu & Sperling, 1995). Therefore, if object-centered suppression is contingent on lowlevel, image-based apparent motion, then one would expect to see no object-centered suppression in Experiment 2A (without apparent motion). Likewise, if object-centered suppression is attributable to attentional bias to the peripheral object, then one would expect no object-centered suppression in Experiment 2B (without contour onset).

Method Two new groups, 14 subjects each, participated in Experiment 2A (9 females, mean age 21.5) and Experiment 2B (9 females, mean age 19.4). One subject in Experiment 2B was replaced because of experimenter errors. Experiment 2A was the same as the central cueing in Experiment 1 except that the red contour on the peripheral object stayed on until the offset of the objects to remove contour apparent motion (Figure 4A). Experiment 2B was the same as Experiment 2A except that the peripheral contour onset was also removed (Figure 4C). In addition, the stimuli were presented on a black-framed 21-inch CRT monitor (model = Sony G520; refresh rate = 60 Hz; resolution = 1024 X 768 pixels) and subjects sat approximately 80 cm from the monitor.

Results and Discussion The results of Experiments 2A and 2B paralleled the findings in Experiment 1 (Figure 4B and Figure 4D; see also Table 1). One-way repeated measures ANOVAs on cue validity (valid, IS, and ID) revealed significant main effects of cue validity in Experiment 2A (without apparent motion; F(2, 26) = 7.58, p = .003, T]¡ = 0.37; error rates, F(2, 26) = 1.58, p = .225, -q^ = 0.11) and Experiment 2B (without contour onset; F(2, 26) = 6.34, p = .006, ' Our focus was on RTs. However, in the peripheral cueing condition, given that numerically the error rate in the ID condition was slightly higher than in the IS condition, a concern then was whether the RTs might be contaminated by speed-accuracy tradeoff. One way to address speedaccuracy tradeoff is to exclude those subjects with the reverse effect in error rates and only analyze the RT data from the remaining subjects, whose RTs effects would be free from speed-accuracy tradeoff (e.g., Lavie, Lin, Zokaei, & Thoma, 2009). Based on this logic, 7 subjects were excluded. The results from the remaining subjects paralleled the original pattern: RTs in the IS condition were significantly higher than in the ID condition (678.4 ms [error rate: 6.2%] vs. 639.1 ms [eiTor rate: 4.5%]: i(6) = 3.15, p = .020, d = 1.19).

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Table 1 Mean Reaction Times (SEM) and Error Rates (SEM) as a Function of Cue Validity (V = Valid; IS = Invalid, Same Object-Centered Location; ID = Invalid, Different Object-Centered Location) in Experiments 1 to 4 Cue validity Experiment Exp 1 (il = 14) Peripheral cues Central cues Exp 2A (n = 14) Exp 2B (n = 14) Exp 3 (n = 14) Exp 4(n = 14) Peripheral objects Different objects

Object-centered suppression

Measure

V

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ID

IS - ID

RT (ms) Error (%) RT (ms) Error (%) RT (ms) Error (%) RT (ms) Error (%) RT (ms) Error (%)

545.5 (23.9) 2.8 (0.6) 524.5(19.5) 3.0 (0.5) 539.3 (19.7) 4.5 (1.3) 555.8(21.6) 2.3 (0.5) 532.6 (22.6) 11.1(1.2)

642.4 (28.0) 5.4(1.2) 609.6 (32.2) 5.1 (1.2) 600.2 (25.5) 7.5 (2.0) 631.3(25.5) 3.7(1.0) 482.4(21.0) 8.8 (0.8)

615.2(28.5) 6.2(1.2) 567.8 (24.7) 4.5 (0.8) 581.6(25.1) 5.6 (L4) 590.4 (24.8) 4.3 (0.8) 476.2(21.4) 8.6(1.4)

27.2(10.7) -0.9 (0.9) 41.8(14.1) 0.6(1.4) 18.6(5.9) 1.8(1.5) 41.0(18.5) -0.6 (0.8) 6.2 (7.7) 0.2(1.2)

r(13) = f( 13) = r(13) = i(13) = f(13) = /(13) = f(13) = i(13) = i(13) = i(13) =

2.5, p = .025, d = 0.68 -1.0, p = .337, d = -0.27 3.0,p= .0Il,d = 0.79 0.4,/> = .668, i/ = 0.12 3.3,p= .006,^ = 0.89 1.2,p = .246, i/ = 0.33 2.2, p = .045, ¿ = 0.59 -0.8, p = .441, d= -0.21 O.8,p = .436,rf= 0.21 0.2,p = .854,rf= 0.05

RT (ms) Error (%) RT (ms) Error (%)

558.3(15.1) 1.8 (0.5) 536.4 (22.3) 2.2 (0.6)

785.3 (39.0) 6.5(1.3) 720.8 (24.9) 6.1 (1.6)

746.6 (27.0) 4.7 (0.9) 693.0 (23.6) 4.0(1.1)

38.7(17.6) 1.8(1.2) 27.9(15.8) 2.1(1.4)

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Tip = 0.33; error rates, F(2, 26) = 1.88, p = .173, tfj, = 0.13). For both conditions, planned contrasts revealed faster RTs at the valid location than at the IS and ID locations (Experiment 2A: F(l, 13) = 7.36,p = .018, TiJ = 0.36; error rates: F(l, 13) = 1.65,p = .222, Ti^ = 0.11; Experiment 2B: F(l, 13) = 7.23, p = .019, T)^ = 0.36; error rates: F(l, 13) = 2.34, p = .150, TI^ = 0.15), and slower RTs at the IS location than at the ID location (Experiment 2A: F(l, 13) = 11.00, p = .006, j]} = 0.46; error rates: F(l, 13) = 1.48,p = .246, Tip = 0.10; Experiment 2B: F(l, 13) = 4.90,p = .045, Ti^ = 0.27; error rates: F(l, 13) = 0.61, p = .447, TI^ = 0.05). Thus, evidently, object-centered suppression does not depend on image-based or tracking-based apparent motion, and could not be explained by attentional capture to the peripheral contour (object).

Experiment 3: Attentioual Disengagement Versus Center of Mass What underlies object-centered suppression then? Given that the effect is observed after an invalid cue—that is, when attention is disengaged from an invalidly cued location in the invalid trials— the effect is likely to result from an object-centered inhibitory tag generated by attentional disengagement. However, although Experiments 2A and 2B rule out accounts based on an attentional bias to the peripheral contour onset, conceivably one could still argue that, although attention is not pulled by the contour onset of the peripheral object, it could still be pulled toward its center of mass. If so, considering that the ID location is closer to the peripheral object than the IS location is, objectcentered suppression might be explained as attentional capture to center of mass (Kravitz & Behrmann, 2008). But this scenario seems unlikely: in both experiments attention was directly cued by top-down precues to the locations along the vertical meridian; in addition, the central object with its center of mass at the fixation was also presented at the same time as the peripheral object. Experiment 3 attempted to directly test which account could best explain the object-centered suppression effect. The same setup

Paired t test

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0.59 0.42 0.47 0.39

as Experiment 2A was used, but, critically, attentional cues were removed and the target was presented equally often at one of three locations (up/down, left, and right) on each trial. Given that no attentional cue was provided and that the target appeared more often within the central object (2/3 of the trials) than within the peripheral object (1/3), based on studies on probability-induced attention (Geng & Behrmann, 2002; Logan, 1998; Miller, 1988), one would expect subjects to prioritize attention toward the central object rather than toward the peripheral object. This central object bias was further encouraged because the left and right locations were closer to the fixation than the up and down locations were. Consequently, this modification reduced the occurrence of attentional disengagement from the peripheral object—attentional disengagement, by definition, would require an invalid cue to engage attention to the peripheral object in the first place (as in Experiments 1, 2A, and 2B). Therefore, given that the center-of-mass was preserved, the center-of-mass capture account would predict a strong objectcentered suppression effect as in Experiments 1, 2A, and 2B. On the contrary, because attentional disengagement from the peripheral object was now strongly reduced, the attentional disengagement account would predict little object-centered suppression.

Method A new group of 14 subjects (9 females, mean age 20.9) participated. The experiment was the same as Experiment 2A except that no cue was provided, and the target could appear equally often at the top/bottom location, the left location, and the right location (Figure 5A). In keeping with the terminology used in the previous experiments, the three locations were still named as before: "V" (33.3%, for the top or bottom location on the vertical meridian), "IS" (33.3%, for the left or right location within the central object that had the same object-centered space as the "V" location), and "ID" (33.3%, for the left or right location within the central object that had a different object-centered space as the "V" location).

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Cue validity

Figure 4. Object-centered suppression without apparent motion and contour onset. (A, B) Procedure and results (« = 14) for object-centered suppression without apparent motion: Observers fixated on a central cross throughout the experiment, and indicated whether a number 2 or 5 was presented on each trial. The target could appear at the cued location (V, on 75% of the trials), at the same object-centered location as the cue within the central uncued object (IS, 12.5%), or at a different object-centered location within the central uncued object (ID, 12.5%). As in Experiment 1, a spatial facilitation effect and an object-centered suppression effect were observed. (C, D) Procedure and results (n = 14) for object-centered suppression without contour onset: The procedure was the same as in (A) except for the removal of peripheral contour onset; the results paralleled those in (B). Error bars: standard error of the mean {SEM). *, '*, and *'*: statistically significant difference at the level of p < .05, .01, and .001, respectively.

Results and Discussion A one-way repeated measures ANOVA revealed a significant main effect of target location, F(2, 26) = 23.84, p < .001, -qj = 0.65; error rates; F(2, 26) = 3.01, /; = .067, -qj = 0.19. Planned contrasts revealed that i) RTs at the "IS" and "ID" locations (within the central object) were faster than at the "V" location (within the peripheral object), confirming that subjects prioritized attention toward the central object, F(l, 13) = 37.54, p < .001, Tl^ = 0.74; error rates; F(I, 13) = 6.7, p = .023, TI^ = 0.34; ii) critically, however, object-centered suppression was abolished, with no difference between the "IS" and "ID" locations, F(l, 13) = 0.65, p = .436, Tip = 0.05; error rates: F(l, 13) = 0.04, p = .854, -^l < 0.01. These results (Figure 5B and Table 1) thus do not support accounts based on attentional capture to the center-of-mass of the peripheral object, but they are consistent with the notion that object-centered suppression depends on attentional disengagement from an invalidly cued location.

Object-Centered Stimulus-Response Interference (Simon Effect): Reanalysis of Experiments 1, 2A, and 2B So far, the results reveal an object-centered suppression effect following voluntary attention; they support the account that the effect reflects an object-centered tagging and transfer of attentional priority signal across objects following attentional disengagement from an invalidly cued location, rather than an attentional bias to the peripheral object. Importantly, Experiments 1, 2A, and 2B were also designed to afford a direct test regarding object-centered representations against other alternative spatial representations. To provide a further test regarding this object-centered account, next I reanalyzed the data in these experiments. Specifically, I took advantage of stimulus-response interference—Simon effects (Simon & Small, 1969). In Simon effects, when the spatial location of the target on the computer screen matches its motor/response location on the keyboard (e.g., both on the "left"), they are congruent; when they mismatch (e.g., one on the "left" and the other on the "right"), they are incongruent. Congruency of the target location between the

OBJECT-CENTERED SUPPRESSION

B580 Without Attentional Disengagement

975 19 n = 14

16- error bars: SEM

550-

Timé' IS"

"ID"

Target location

"V"

"IS"

"ID"

Target location

Figure 5. No object-centered suppression without attentional disengagement. (A) Procedure: Observers fixated on a central cross throughout the experiment, and indicated whether a number 2 or 5 was presented on each trial. (B) Results (n = 14): No cue was provided, and the target could appear equally often at the top/bottom, left, or right location. In keeping with previous terminology, trials were still sorted into three conditions based on the target location—"V" (33.3% of the trials, for the top or bottom location on the vertical meridian), "IS" (33.3%, for the left or right location within the central object that had the same object-centered space as the "V" location), and "ID" (33.3%, for the left or right location within the central object that had a different object-centered space from the "V" location). Performance was worse when the target was at the "V" location than at the "IS" an "ID" locations, but no significant difference was found between the "IS" and "ID" locations. Error bars: standard error of the mean (SEM), ns: not statistically significant at the level of p < .05.

input map and the output map serves as an index of stimulusresponse interference (Figure 6A and 6B). Accordingly, three potential visuospatial schemes for target location coding were evaluated: (i) A traditional rednotopic spadal map, based on the central object. When the target appeared on the same side within the central object as the response did within the response map, the trial was congruent; otherwise, it was incongruent, regardless of the peripheral object (Figure 6B, left). Better performance in congruent trials than in incongruent trials would represent a classic Simon effect. (ii) An object-centered spadal map, centered within the peripheral object. When the target appeared within the peripheral object, its spatial locadon, though along the vertical meridian, could be referenced to the peripheral object, thereby producing an objectcentered congruency effect as determined by its relative position within the peripheral object (Figure 6B, middle). If so, this would constitute a novel form of Simon effect: an object-centered Simon effect. (iii) A spadal bias to the peripheral object. When the target appeared within the central object, attendon might be attracted to the peripheral object, biasing the response toward the peripheral object, so that a left peripheral object primed a left response whereas a right peripheral object primed a right response (Figure 6B, right). This could result from attendonal capture. Although there has been no direct evidence for the second effect (the object-centered Simon effect), theoredcally, the three spadal coding schemes are all viable and by no means mutually exclusive. The central goal here, then, was to examine which coding schemes were operadve in this new paradigm. As Figure 6C and Table 2 show, the data from both peripheral cueing (Fxperiment 1) and

central cueing (Experiments 1, 2A, and 2B) converged to provide evidence for a traditional, retinotopic Simon effect and a new, object-centered Simon effect, but, importantly, there was no evidence for a spadal bias to the peripheral object. Thus, by providing evidence for a novel object-centered Simon effect, these results confirm that an object-centered coding scheme rather than a bias to the peripheral object is in play in this object-centered paradigm.

Experiment 4: What Is the Object in Object-Centered Suppression? The results of Experiments 1 to 3 make a strong case for object-centered suppression, an inhibitory tag based on objectcentered spatial coding. Yet a quesdon lingers: What is the object in object-centered suppression that enables this effect? Two possible theoredcal proposals can be differentiated based on different emphases on features and space. On the one hand, object-centered suppression could depend strictly on episodic featural representations: the transfer of attentional priority signal from the cued object to the uncued object is determined by an image-based match between the two objects (such as featural match in color and shape; referred to as the episodic account). On the other hand, it could also depend on abstract spatial representations: featural match is irrelevant and only representations of relative spatial positions matter (i.e., whether the object-centered space is the same or not; referred to as the abstract account). Fxperiment 4 tested these proposals in two ways. The first test concerned a weak version of abstract object-centered suppression. Specifically, in Experiments 1, 2A, and 2B, object-centered suppression was observed for locations within an invalid, uncued object that was presented at the central fixadon. For testing the

LIN

976

A Motor/response spatial map

Context-attraction

Simon effect (stimuíus-response interference) 650 620 590

T

1

n.s. J '

congruent, + 1 SEM i:r::i incongruent, + 1 SEM 1

1

n.s. ***

560 530 500 10 7 4 1 Retinotopic

Object- Contextcentered attraction

Peripheral cues (n = 14)

Retinotopic

comparable to the tradition two-rectangle paradigm in objectbased attention (see Figure 1). The second test concerned a stronger version of abstract objectcentered suppression, examining whether attentional priority signal could still transfer from the cued object to the uncued object without featural match in color and shape. In this test, two distinct objects were used, one a red rectangle, the other a blue oval (referred to as the "two different objects" test; Figure 7C). Therefore, if object-centered suppression depends on episodic representations, then there would be no object-centered suppression in this test; if it depends on abstract spatial representations, an objectcentered suppression effect would be expected. The working hypothesis was that the same object-centered suppression effect should be observed for the "peripheral invalid objects" test; whether it manifested in the "two different objects" test was an open question.

Object- Contextcentered attraction

Central cues (n = 42)

Figure 6. Multiple stimulus-response interference effects (Simon effects). (A) Motor/response spatial map. (B) Conditions: Congruency of the target location between the perceptual map and the response map provided an index of stimulus-response interference: when the spatial location of the target matched its motor/response location, it was a congruent trial; when they mismatched, it was an incongment trial. Three spatial schemes were evaluated: i) retinotopic: when the target appeared within the central object, its spatial coding could be in reference to the central object (as to the retina); ii) object-centered: when the target appeared along the vertical meridian within the peripheral object, its spatial coding could be in reference to the peripheral object; and iii) context-attraction: when the target appeared within the central object, its spatial coding could assimilate the location of the peripheral object (as in priming). The panel of each coding scheme here contained all example trials superimposed rather than a single trial. (C) Results (n = 14 for peripheral cues; n = 42 for central cues): Eor both peripheral and central cues, retinotopic and object-centered Simon effects were observed, without evidence for context-attraction. Error bars: standard error of the mean (SEM). *, **, and ***: statistically significant difference at the level of p < .05, .01, and .001, respectively.

existence of object-centered suppression, such a design was optimal, for one needed to consider only two locations on the screen— left and right along the horizontal meridian—that were canonical "left" and "right" positions. Yet, this raised the question regarding whether the observed effect was specific to objects at the center or it was more general, manifesting within objects in the periphery as well. To test this position sensitivity in object-centered suppression—that is, beyond the canonical "left" and "right" positions at the center—in Experiment 4 the two objects were rearranged so that one was above the fixation and the other below it (referred to as the "peripheral invalid objects" test; Figure 7A). This modified configuration also rendered the object-centered paradigm more

Method A new group of 14 subjects (9 females, mean age 20.9) participated in both tests, first in the "peripheral invalid objects" test and then in the "two different objects" test. The stimuli were presented on a 19-inch CRT monitor (model = ViewSonic G90fB; refresh rate = 60 Hz; resolution = 1024 X 768 pixels) and subjects sat approximately 50 cm from the monitor. The first test (the weak version: "peripheral invalid objects") was the same as the central cueing in Experiment 1 except that the central fixation was now in between the two objects, equidistant to them (Figure 7A). Accordingly the invalid object was either above or below the fixation on each trial (as opposed to be centered at the central fixation). The second test (the stronger version: "two different objects") was the same as the first test except that two distinct objects were used during each trial (Figure 7C): a red rectangle (CIE xy coordinates for the red contour: x = 0.603, y = 0.328; luminance of the red contour = 10.3 cd/m^; luminance of the gray filled region = 11.5 cd/m^) as the cued object; a blue oval (CIE xy coordinates for the blue contour: x = Q. 152, y = 0.076; luminance of the blue contour = 6.1 cd/m^) as the uncued object. The luminance of the letters was 47.1 cdlrc?.

Results and Discussion Repeated measures ANOVA with two factors, test (weak and strong versions) and cue validity (valid, IS, and ID), revealed significant main effects that did not interact. The critical result was a significant main effect of cue validity, F(2, 26) = 33.74, p < .001,71^ = 0.72; error rates: E(2, 26) = 10.97,p < .001, i]¡ = 0.46, which did not depend on the test, F(2, 26) = 1.90, p = .170, TI^ = 0.13; error rates: F(2, 26) = 0.42, p = .664, i]j = 0.03. Planned contrasts revealed that i) RTs were faster at the valid location than at the IS and ID locations, F(l, 13) = 37.79, p < .001, ^i^ = 0.74; error rates: F(l, 13) = 37.08, p < .001, ti^ = 0.74; ii) more important, RTs were slower at the IS location than at the ID location, F(l, 13) = 6.30, p = .026, -qj = 0.33; error rates: F(l, 13) = 2.88,/? = .114,-n^ = 0.18. Figure 7B and 7D depict separate results for the two test (see also Table 1). The suppression effects of 38.7 ms for the "peripheral invalid objects" test and 27.9 ms for the "two different objects" test were similar to those in Experi-

OBJECT-CENTERED SUPPRESSION

977

Table 2 Mean Reaction Times (SEM) and Error Rates (SEM) as a Function of Congruency (C: Congruent; I: ¡ncongruent). Spatial Reference Frame (Retinotopic; Object-Centered; Context-Attraction), and Cueing (Peripheral Cues; Central Cues) based on the aggregate data from Experiments 1, 2A, and 2B Peripheral cues

Retinotopic Object-centered Context-attraction

Measure

C

I

RT (ms) Error (%) RT (ms) Error (%) RT (ms) Error (%)

614.7 (27.6) 3.7(1.1) 538.9 (24.4) 1.7(0.5) 632.7 (28.8) 6.7(1.2)

643.4 (28.8) 7.9(1.5) 552.3 (23.6) 4.0 (0.8) 624.9 (27.2) 4.9(1.3)

Central cues Paired t test

t(\3) = t(\3) = i(13) = «(13) = f(13)= i(13) =

ments 1 (central cueing), 2A, and 2B, which were 41.8 ms, 18.6 ms, and 41.0 ms, respectively. For the test factor, overall RTs were faster for the "two different objects" test than the "peripheral invalid objects" test (F(l, 13) = 15.11,p = .002, Ti^ = 0.54; error rates; F(l, 13) = 0.12,p = .731, Tip = 0.01), potentially reflecting a practice effect.

- 2 . 9 ,p = .013 - 3 . 5 , p = .004 - 2 . 5 , p = .027 - 3 . 6 ,p = .004 1.0,p = .318 1.6,p = .124

C

I

588.7(15.8) 3.1 (0.6) 531.8(11.8) 2.0 (0.4) 597.6(14.3) 5.7 (0.8)

605.5 (14.8) 7.1 (0.9) 548.2(12.2) 4.6 (0.7) 596.3(16.1) 4.5 (0.7)

f test F(l, 39) = F(l, 39) = 39) = ^ • ( 1 , 39) = F(l, 39) = F(\, 39) =

5.4, p = .025 23.6, p < . 0 0 1 24.0, p < .001 31.1,p