Contour grouping inside and outside of facial contexts

Acta Psychologica 114 (2003) 245–271 www.elsevier.com/locate/actpsy

Contour grouping inside and outside of facial contexts q James R. Pomerantz *, Alpna Agrawal 1, Stephen W. Jewell, Martha Jeong, Hana Khan 2, Sandra C. Lozano 3 Department of Psychology, Rice University, MS 25, P.O. Box 1892, Houston, TX 77251-1892, USA Received 4 November 2002; received in revised form 12 August 2003; accepted 25 August 2003

Abstract We examine how contours group in isolation compared with when they are embedded in face-like contexts. As previously shown, contours that seem to group by phenomenological observation also show powerful effects on task performance: with contours that group, selective attention to one while ignoring another is poor (as indexed by Garner Interference (GI), but not Stroop Interference), whereas divided attention across contours is good. With contours that do not group, however, the reverse happens. Here we test pairs of curved lines (parentheses) displayed either in isolation or within contexts including cartoon faces, where these curves may serve as mouths or eyebrows. The results with isolated contours replicate previous findings of poor selective attention, but within face-like contexts the same contours showed nearly perfect selective attention (i.e., zero GI). Thus, contour grouping was weaker inside than outside of faces, a finding that contrasts with the widely-held belief that faces are processed configurally, not by local features.
2003 Published by Elsevier B.V. PsycINFO classification: 2323; 2346; 2320; 2520 Keywords: Perceptual grouping; Selective attention; Gestalt; Emergent feature; Face perception; Object perception

q Portions of this research were presented at the 41st Annual Meeting of the Psychonomic Society, New Orleans, LA, USA, 17 November 2000. Pomerantz, J.R., Agrawal, A., and Lozano, S.C. ‘‘How can we know when selective attention fails?’’. * Corresponding author. Tel.: +1-713-348-3419. E-mail address: [email protected] (J.R. Pomerantz). 1 Now at the University of Texas School of Public Health. 2 Now at Baylor College of Medicine. 3 Now at Stanford University.

0001-6918/$ - see front matter
2003 Published by Elsevier B.V. doi:10.1016/j.actpsy.2003.08.004

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1. Introduction Among the oldest of Gestalt demonstrations of perceptual organization is the grouping of discrete line segments into unitary perceptual configurations. Fig. 1 shows one such demonstration, adapted from the work of Koffka (1963/1935, Fig. 45), revealing how alternating sets of bracket-like elements are organized by the perceiver into pairs, presumably based on such configural properties as symmetry, good continuation, and closure. This problem is both classic and important to this day because perception is so clearly driven by properties that span discrete elements and because, despite years of research, it is not yet clear how perception accomplishes the feat of grouping. Demonstrating and measuring grouping effects has proven challenging, but major advances have been made by a number of researchers, with Leeuwenberg (1971, 1978) among the very most influential. Others, some of whom are represented in this issue, include Hochberg (Hochberg, 1978, 2003; Hochberg & Brooks, 1960; Hochberg & MacAlister, 1953), Kubovy (e.g., Kubovy & Wagemans, 1995; Strother & Kubovy, 2003), Palmer (e.g., Palmer, 1999; Palmer, Brooks, & Nelson, 2003), Rock (1983) and Garner (1974). Here we use perceptual independence, as indexed by selective attention performance, to operationalize grouping (Pomerantz, Carson, & Feldman, 1994; Pomerantz & Garner, 1973; Pomerantz & Pristach, 1989). The logic is that if two elements form a perceptual configuration, then perceivers should not process those elements independently of one another. Conversely, if two elements do not group, their processing should be independent (or at least more independent). Using stimuli similar to those of Koffka that appear phenomenologically to form groups, our lab and others have shown by that grouped elements are not processed independently, in that perceivers cannot (or at least do not) attend to individual elements selectively. Rather, perceivers show large and reliable failures of selective attention with such stimuli. For example, when judging the direction of curvature of the left-hand member of a pair of parentheses such as ‘‘( )’’, perceivers are affected by the direction of curvature of the right-hand member. With stimuli that do not group, however, the same perceivers exhibit nearly perfect attentional selectivity (e.g., when the right-hand member is rotated 90). The present study extends this research by exploring several variations in such stimuli, including ones that lead into the realm of face perception. There are two reasons for this extension. First, faces are sometimes regarded as quintessential visual

Fig. 1. Demonstration adopted from Koffka (1963/1935), showing how alternating sets of bracket-like elements pair off into groups.

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configurations. Just a few suitably placed marks on paper can convey not only the perception of a face but also a clear sense of the identity, age, gender, and the emotional expression the face bears. In an effort to capitalize on our supposed ability to perceive configural relations in faces better than in other stimuli, cartoon faces have been used with some success for displaying multidimensional stimuli. Best known are the so-called Chernoff faces (Chernoff, 1973), which are available commercially for displaying multidimensional data sets deemed too complex for ordinary graphs. Second, there is evidence that faces may be Ôspecial,’ that is, may be processed differently from other visual stimuli. Specifically, faces sometimes show different effects of manipulations such as inversion or contrast reversal; there may be brain sites specific to processing faces; and certain patients show deficits that may be confined to the perception of faces (Bruce & Young, 1998; Farah, Wilson, Drain, & Tanaka, 1998; Kanwisher, McDermott, & Chun, 1997; Sergent, 1984; Yin, 1969). In addition, a consensus view exists that faces are perceived holistically or configurally, as opposed to being perceived via piecemeal parts or features, at least for faces presented upright and with normal contrast polarity. As Bartlett and his colleagues have summarized it in a recent review, ‘‘piecemeal features of upright faces are not encoded independently of each other’’ (Bartlett, Searcy, & Abdi, 2003). Although processing parts (such as the nose, the eyes, etc.) is possible, this stands as a secondary route for perceiving faces; holistic processing is believed to dominate. The present experiments aim to clarify how configurations of line segments are perceived, in or out of any context, while also shedding light how faces are perceived. We investigate the perception of contour pairs presented in isolation, and then compare those same pairs when they are placed in the context of other contours to form face-like and other configurations. The stimuli we employ are based on simple curved line segments that would rarely be perceived as parts of faces in most contexts, but in other contexts they take on the roles of mouths and brows, with the resulting configurations perceived almost universally as line-drawn, or cartoon, faces. 1.1. Grouping and selective attention Many experiments have shown that those visual elements that group perceptually, according to observers’ reports, are also not attended to selectively, one element at a time. Two prototypical sets of configurations that group are shown in Fig. 2. Panel A shows parenthesis pair sets, whereas Panel B shows sets of arrows and triangles. Panel A shows four pairs of parentheses. When observers are presented with just one pair, such as ‘‘((’’, and are asked to judge the direction of curvature of one member of the pair (e.g., the left hand parenthesis), they show sizeable effects of the direction of curvature of the other member of the pair (the right hand) even though that other member is irrelevant to their task. In particular, they show Garner Interference (GI), defined as an increase in reaction time (RT) and/or error rates to the relevant element caused by random trial-to-trial variation in the irrelevant element. For example, in making speeded responses to a series of parenthesis pairs when the

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Fig. 2. (A) Pairs of curved lines or simple parentheses that show failure (or absence) of selective attention, (B) configurations of line segments that form arrows and triangles and that similarly fail to show selective attention, (C) pairs of parentheses that lack EFs and show perfect selective attention.

direction of curvature on the right element is relevant, participants are faster and make fewer errors when the curvature of the left element remains constant over trials (Control condition) than when it varies unpredictably from trial to trial (Filtering condition). In short, GI is interference from irrelevant variation. Interestingly, perceivers rarely manifest another common form of selective attention failure, Stroop Interference (SI), with these stimuli. That is, in making speeded responses to the direction of curvature of one element, they typically are neither faster nor slower when the other, irrelevant element has the same direction of curvature (congruent trials) than when it has the opposite curvature (incongruent trials). Although such congruity or flanker effects (Eriksen & Eriksen, 1974) arise routinely with other stimuli such as side-by-side letters, they have not yet been observed with elements that group into distinctive configurations (Pomerantz et al., 1994). Thus, we may be able to diagnose the grouping of elements into unitary configurations using selective attention measures, but it is random variation on the irrelevant element or dimension (GI) rather than incongruence on the irrelevant element (SI; MacLeod, 1991, 1992; Stroop, 1935) that appears to be the key diagnostic for grouping (Pomerantz et al., 1994; Pomerantz, Pristach, & Carson, 1989; for more comparisons of SI with GI, see Melara & Mounts, 1993; and Pansky & Algom, 1999). The likely reasons for this outcome appear later in this paper. GI arises with other stimuli besides the pairs of curved lines in Fig. 2(A). It appears with arrangements of straight lines that form arrows and triangles, as illustrated in Fig. 2(B): In making speeded judgments of the orientation of the diagonal line in one of these stimuli, perceivers experience significant interference from random, trial-to-trial variation in the location of the irrelevant vertical line, and vice versa. This remains true even if we shorten or lengthen the line segments, which in turn alters seemingly important features of the configurations such as closure, terminators (end points), and intersections (Pomerantz & Pristach, 1989). Our research has shown that modifications in the parenthesis pairs that affect perceived grouping also produce corresponding changes in GI. For example, separating the curved elements horizontally reduces GI monotonically, as would be expected from grouping by proximity (Pomerantz & Schwaitzberg, 1975). Rotating one of the parentheses 90, which weakens or eliminates good continuation, symmetry,

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and closure between the elements, makes GI vanish entirely, as shown in Fig. 2(C) (Pomerantz & Garner, 1973). Fig. 3 shows a number of stimulus manipulations that we and others have performed on parentheses, many of which affect the success of attentional selectivity. We will revisit these in Section 5, but our point is that GI appears to be a sensitive and reliable indicator of grouping strength that is based on a plausible theoretical view of attention, as described next.

Fig. 3. A variety of parenthesis variations. The leftmost strip shows parenthesis pairs with varying vertical separation (offset), while the next panel to the right shows a corresponding horizontal offset. The remaining panels show the effects of factors including rotating one of the parentheses 90, of joining the pairs with lines, of adding lines that converts each parenthesis into a closed D, of color or lightness differences, of face-like contexts, of enclosure in common regions, and of absolute or relative size changes.

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1.2. Theory underlying grouping There are at least two ways to conceive of perceptual grouping. One view is that grouping acts like glue, binding elements together so that in the extreme case, they are available either as a whole or not at all. 4 Under this view, selective attention to an individual element in a group would be impossible, whereas divided attention across the group would be possible and, in fact, easy. The second view is that perceptual grouping involves neither glue nor any loss of independence of perceptual elements, but instead involves only the creation of extra emergent features (EFs) such as symmetry, closure, and good continuation. It is these EFs that make the whole different from the sum of its parts but they also leave the parts intact and accessible to perception. The EFs constitute new features that can be attended to along with (or independently of) the elements from which they emerge. 5 Based on earlier work (Pomerantz & Pristach, 1989), we believe the evidence favors the second view. Accordingly, our interpretation for GI with the stimuli in Fig. 2(A) and (B) rests on the presence of EFs arising from visual elements placed in close proximity to one another. We define EFs as salient features that arise from combinations of two or more elements but that do not reside within any single constituent element such as a single curve or line segment. Using a more stringent criterion, we could stipulate further that EFs must not only be salient but also be more salient than the features of the elements from which they emerge. Research on configural superiority effects (Pomerantz, 1981; Pomerantz, Sager, & Stoever, 1977) has demonstrated this increased saliency both with the parenthesis pairs and with the arrows and triangles of Fig. 2(A) and (B). Fig. 4 shows discriminations becoming much easier following the addition of a context, even though the elements comprising that context provide no task-relevant information in and of themselves. EFs are important in the selective attention tasks mentioned previously because in the Control conditions, perceivers can base their responses exclusively on these salient EFs whereas in the Filtering conditions they must attend to the less salient featural differences contained in the individual elements or contend with four whole stimuli rather than just two. To be concrete, when performing a Control task such

4 As an analogy, consider that some current computer graphics programs contain a command feature called ‘‘Group’’. This feature links multiple elements in such a way that an operation performed on any element, such as a change in size, position, color, or orientation, is extended to the entire group. Selective attention––operations targeted on one element only––are impossible within a group but are the norm between groups. Of course, less all-or-none forms of grouping are possible, such as one in which attention is simply spread more easily within a group or object than across groups (e.g., Duncan, 1984; Egly, Driver, & Rafal, 1994). 5 EFs are features that arise when separate stimulus components are combined in a single stimulus, that is, are not found in individual components. For some writers, the term ‘‘feature’’ implies a local, piecemeal component. An EF, however, is global or holistic in the sense that it spans two or more components. Complicating matters further, the term ‘‘configural’’ for some writers can mean something different from ‘‘holistic’’ in that an inter-part relationship or EF––such as proximity––can still be fairly local in that it may span only two components of a much larger stimulus.

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Fig. 4. Configural superiority effects (CSEs). Two CSEs are demonstrated using the Ôodd quadrant’ task of localizing the quadrant contain a stimulus different from the other three. For example, with the parentheses in the top row, the Target display contains an odd element in the southeast corner, whereas the Context display contains all identical elements and so provides no information by itself. When the Context is superimposed on the Target to produce the Composite display, the task becomes much easier with RTs sometimes reduced to 50% or less of their initial value. The bottom row shows a similar CSE with diagonal line segments. Several other such CSEs have now been found.

as discriminating ‘‘((’’ from ‘‘( )’’, perceivers may choose between two strategies: they may attend to just the differentiating right hand element, or they may attend to an emergent property (here symmetry, closure, good continuation, or parallelism) and perform the task using just that EF. When performing a Filtering task, however, such as discriminating ‘‘((’’ and ‘‘)(’’ from ‘‘( )’’ and ‘‘))’’, those EFs are of little or no value in deciding which response must be made. For example, equal numbers of vertically symmetric pairs are assigned to each response; the same is true of parallel pairs. As a result, in the Filtering task perceivers will be forced into a less efficient strategy. Either they must attend to a less salient individual parenthesis, or they must remember which two stimuli are assigned to one response and which two to the other (compared with one and one in the Control task). This processing difference is the genesis of GI. 1.3. Purpose of experiments The present experiments compared the grouping of curved line segments when they appear in face-like contexts with when they appear in non-facial configurations or in isolation. The key idea is shown in Fig. 5, where the parenthesis pairs of

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Fig. 5. Parenthesis pairs in and out of facial contexts. On the left, a set of four parenthesis pairs rotated to a horizontal orientation. On the right, the identical parenthesis pairs now embellished with three inscribed dots and a surrounding circle to create cartoon faces.

Fig. 2(A), now rotated to the horizontal, are accompanied by an otherwise equivalent set where each parenthesis pair (1) is surrounded by a circle, and (2) has three dots arranged in an equilateral triangle and placed between the curves. The latter stimuli are seen nearly universally as cartoon faces, with the parentheses assuming the roles of mouth and of eyebrows, and the three dots serving as eyes and nose. Variation in the direction of curvature of the parentheses produces salient and nameable changes in facial expression. Thus, parenthesis pairs in facial context may yield further EFs, beyond those that arise from the isolated pairs; specifically they may yield EFs associated with the expression of emotional states. 6 The specific purpose of these experiments was to assess the effect of facial context on perceived grouping, as indexed by GI and related performance measures. By testing parenthesis pairs both in isolation and in face-like contexts, we hope to detect any differences context may generate in the way the line segments are processed. Two outcomes, not mutually exclusive, were envisioned. First, given the additional EFs that might arise with face-like contexts (beyond the basic EFs such as symmetry, parallelism, and good continuation that arise from plain parenthesis pairs), we expected to see increases in GI with those stimuli. This follows from the notion introduced earlier that EFs may be responsible for GI. Assuming that these new facial EFs add to, rather than replace or mask, the pre-existing EFs, increased GI would reflect the additional salience of the discrimination among the various faces. Barring the possibility that the plain parenthesis pairs already group at maximum strength and thus that ceiling effects would prevent any increases in GI, our

6

The first use of stimuli like those in Fig. 5 was, to our knowledge, Pomerantz (1986, Fig. 1.7); a replica appears here as the leftmost of the two face-like stimuli in Fig. 3. In the interim, at least one other published article (Suzuki & Cavanagh, 1995) has used similar face-like stimuli in which parenthesis pairs serve as facial features. That study, which did not cite the 1986 work noted or the still earlier work on grouping and texture segmentation with parenthesis pairs, looked for popout effects (flat search functions) in visual search tasks and so did not look for GI or SI. Some of this study’s findings are relevant here, and even more relevant to our lab’s current work on faces and configural superiority effects.

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core prediction was that GI would increase. Just what any new EFs associated with the face-like stimuli might be is hard to say, but they could range from geometric properties that are common to any sort of configuration to properties linked with expressions and emotion that are probably idiosyncratic to faces. Second, given that faces may be special, we were prepared to find significant qualitative differences in the way the facial stimuli are processed compared with otherwise similar non-face stimuli. This could entail an overall strengthening of grouping as noted above. It could, however, involve specific changes in how curved lines are perceived and attended to. Following Leeuwenberg (1971, 1978), for example, line segments might be coded differently in different contexts, and the structural description of a line might differ in a face compared with a non-face. In the extreme case, if faces are processed in different neural areas, where different rules may apply to the processing of such configurations, then all bets are off as to how the face stimuli might behave. That said, however, recall that the consensus view in the literature on the perception of faces is that they are perceived holistically rather than by parts. Most of the research suggesting that faces are perceived holistically comes from studies using photographic images of faces, rather than the cartoons used here. Photographic images are richer in detail, with the attendant advantage that they are more realistic and disadvantage that it is difficult to know what properties of the photographs are being attended. Most of the research on face perception has used measures of holistic versus part-based processing that are substantially different than the selective attention measures used here. Our hope is that by using the simplest possible stimuli in which the dimensions of interest can be isolated or shown in other contexts, we might provide a different perspective on how faces are processed in human vision. It is safe to say that identifying the effective features used in visual perception has proven to be a great challenge, even for fairly simple geometric forms, so our hope here is that by using stimuli formed from simple, well-studied, and independently changeable contours that look face-like, we may learn more about both face processing and about the perception of grouped contours.

2. Method The 11 experiments reported here use a procedure employed previously (Pomerantz & Pristach, 1989) in which the only component to vary from experiment to experiment is the stimulus set employed. Different participants were used in each experiment to prevent biases or carryover effects that result from a within-subjects design where individual participants would see more than one stimulus set. In the present case, a within subjects design could bias participants into seeing stimuli as faces that they might otherwise not, and perhaps vice versa. Each experiment entailed four stimulus configurations that were tested in two main types of conditions: Control conditions (in which the irrelevant element remained constant) and Filtering conditions (in which the irrelevant element varied randomly from trial to trial). The arithmetic differences in RTs and error rates between Control and Filtering tasks define GI, i.e., the cost to performance from

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variation on the irrelevant element compared with that element remaining fixed over trials. In addition, these experiments measured SI. In both the Control and the Filtering tasks, exactly half of the stimuli contain an irrelevant element that is identical to (congruent with) the relevant, whereas the other half of the stimuli contain an irrelevant element different from (incongruent with) the relevant and so calling for the opposite response. The arithmetic differences in RTs and error rates between congruent and incongruent stimuli define SI (or Stroop-like interference, since our stimuli are not color words in different colored fonts). Thus, SI may be defined as the cost to performance from incongruence on the irrelevant element. As noted earlier, parenthesis pairs typically show near zero SI, but SI was measured here to determine if facial contexts would change this outcome. 2.1. Participants Each of the 11 experiments employed eight participants, drawn mostly from the undergraduate subject pool at Rice University, with a few additional participants paid for their services. They all had normal or corrected-to-normal vision, and they averaged 21–22 years in age. Each person participated in only one experiment. 2.2. Stimuli The stimulus sets for Experiments 1–11 are shown to scale in Fig. 6. Panel 1 shows the standard set of four parenthesis pairs, which has been tested many times but included here for replication and control purposes. Panel 2 shows these same pairs ro-

Fig. 6. The stimulus sets used in Experiments 1–11.

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tated 90; although this set has never before been tested to our knowledge, we had no reason to believe it would behave differently from the set in Panel 1. Panel 3 shows the same rotated parentheses surrounded by a circle of the same stroke width as the parentheses, along with a triangular arrangement of 3 dots intended to be suggestive of two eyes and a nose (although we did not mention any likeness to faces to participants in any of the experiments). Aside from the circle and dots, these stimuli were identical to those in Panel 2. Panel 4 shows the stimuli of Panel 3 rotated back 90 counterclockwise, so the parentheses duplicate the vertically oriented ones of Panel 1. Panel 5 shows the stimuli of Panel 3 rotated 180, creating upside down faces. Note that the only difference between the upright and the inverted faces is the inversion of the triangle of dots defining the eyes and nose. Panel 6 is a control for the faces of Panel 3 where the triplet of dots is removed, and Panel 7 is another control where the surrounding circle is removed instead. Panel 8 matches Panel 4 but without the three dots. Panel 9 and Panel 10 show respectively the 90 and 180 rotations of Panel 7, retaining the dots but omitting the circle. Lastly, Panel 11 shows a control for the faces in Panel 3 where the three dots have been rearranged into a horizontal linear arrangement intended to appear less like a face than the others. The actual stimulus appearing on any one trial was one of the four alternatives shown in each panel of Fig. 6. That is, it consisted of a single pair of parentheses, either alone or accompanied by the additional elements (one surrounding circle and/or the 3 dots). The stimuli were presented on the screen of a computer display monitor (Sony Trinitron 17 inch GS220) running at 640 · 480 resolution and controlled by an IBM compatible personal computer running E-Prime software under the Windows 98se operating system. The stimuli appeared on the screen until a response was made or until 2 s elapsed, whichever was briefer. The height of a single parenthesis on the screen was approximately 0.9 mm, and the distance between two identical parentheses was 1.7 mm. At a typical viewing distance of 50 cm, an individual parenthesis was thus about 1 tall, and a parenthesis pair would just fit inside a circle of diameter 2. The stimuli shown in Fig. 6 are all reproductions of the actual stimuli used, all drawn to the same scale as just described for the parentheses pairs. In four of the eight blocks of trials (see below), stimulus presentation always occurred centered on a central fixation point whereas in the other four blocks they occurred at random among 12 possible locations around an imaginary clock face that was
4.8 of visual angle in diameter. Given that the fovea is an instrument of selective attention, this manipulation was included to check the generality of our results between there and the periphery. 2.3. Tasks As noted above, there were two primary types of task, Filtering and Control, which differed only in whether the irrelevant parenthesis varied from trial to trial, for example between ‘‘(’’ and ‘‘)’’, or remained fixed at one level, for example ‘‘(’’, respectively. In both the Control and Filtering tasks, there was only one element that was relevant to the task, although we did not tell this to participants explicitly.

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Instead, we simply showed them which keyboard key they were to press for each stimulus; participants decided for themselves what information was relevant and what strategy to pursue. Our goal was to learn how perceivers process these configurations spontaneously, for example as wholes or as arrangements of discrete parts, without our biasing them through explicit instructions. In addition, we did not describe or name the stimuli for the participants, and we specifically avoided using the word ‘‘face’’; instead, the stimuli were simply displayed without labels or other characterizations. As elaborated upon later, for each stimulus set there were four versions of the Control task and two versions of the Filtering task. Each version contained 40 stimulus presentations (in addition to the practice presentations used to ensure accuracy), with each of the two or four possible stimuli presented an equal number of times in an unpredictable order. In addition to the four Control and two Filtering tasks, these experiments included two other types of conditions that will not be discussed here because they do not illuminate the issues in question. These are the two Correlated Elements conditions (in which the irrelevant element varies from trial to trial but in a manner perfectly correlated with the relevant element) and the one Condensation condition (in which the second element varies from trial-to-trial in a manner uncorrelated with the first element but where the correct response is tied to the conjunction of both elements; thus both are relevant). These conditions have been described previously (Pomerantz, 1983; Pomerantz & Pristach, 1989) but because they yield no new results and so shed no new light on the present research, they will not be discussed further here. In sum, there were a total of nine conditions (tasks) tested. 2.4. Design and procedure Participants were tested in individual cubicles, seated before a station housing a monitor and keyboard. Each participant completed nine blocks of nine conditions (tasks), with the first block regarded as practice and excluded from the analyses below. This first block was followed by eight complete blocks, each of which included all the experimental conditions, in an order counterbalanced over participants by a Latin square, to ensure the testing sequence affected all conditions identically. Each condition began with a screen indicating which stimuli would appear in that task and which of two buttons on the numeric keypad, the ‘‘1’’ or the ‘‘2’’ key, was the correct response for each stimulus. Instructions encouraged maximum speed of responding consistent with low error rates. Participants then passed through a series of eight practice trials (stimulus presentations) that included all possible stimuli in that condition, and they were required to achieve perfect accuracy during these trials before passing on to the task proper. One or more errors resulted in additional sets of eight practice trials until performance was error-free. During the task proper, each stimulus was presented and remained on the screen for either 2 s or until a response was made, whichever was briefer. A correct response led to a screen displaying the word ‘‘Correct’’ for 500 ms, followed at an ISI of 500 ms by a screen in which only a fixation cross was displayed at the center of the screen, followed in turn by the next

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stimulus. An error led to a screen stating that the wrong key had been pressed or that no response was detected, and this screen remained on for 1000 ms, to discourage participants from making excessive numbers of errors. As a reminder of the response assignments, small replicas of each stimulus for the current task remained on the screen well separated from the actual stimuli for the duration of each condition, with stimuli assigned to the ‘‘1’’ key shown at the lower left and stimuli assigned to the ‘‘2’’ key at the lower right. As noted earlier, there were four Control conditions, differentiated by which of the two parentheses was relevant (e.g., left versus right) and by the direction in which the irrelevant parenthesis pointed (e.g., leftward versus rightward). To illustrate, consider the stimuli in Fig. 6, Panel 1, in the Control condition where the left element was relevant and the right was held constant at ‘‘)’’. Here, participants would see a random alternation between the two stimuli on the righthand side of Panel 1, that is, ‘‘( )’’ and ‘‘))’’, with the two pairs appearing equally often, and with one assigned to the ‘‘1’’ key and the other to the ‘‘2’’ key. The key assignment was counterbalanced across conditions and participants. There were two Filtering conditions, differentiated by which of the two parentheses was relevant (e.g., left versus right element). To illustrate, consider the Filtering condition where the left element was relevant. Here, participants would see a random alternation among the four configurations, ‘‘( )’’, ‘‘((’’, ‘‘))’’, and ‘‘)(’’, with the first two assigned to the ‘‘1’’ key and the second two to the ‘‘2’’ key, and with the key assignment again counterbalanced across conditions and participants.

3. Results Analyses were performed across participants, conditions, and blocks on both the RTs for correct responses and the error rates for all trials, excluding those noted earlier that served as practice. The RTs were first treated to remove outliers, defined as any RT less than 200 ms or greater than 3 standard deviations above the grand mean RT, computed across all conditions, blocks, and participants. The key analyses consisted of repeated measures ANOVAs on treated RTs and error rates, performed separately for each experiment (i.e., for each stimulus set shown in Fig. 6). The three primary within-subjects factors in these ANOVAs consisted of (1) Control versus Filtering task, which assesses GI; (2) Congruent versus Incongruent irrelevant element, which assesses SI; and (3) Relevant element, which assesses any performance differences dependent on which parenthesis is relevant, that is, top or left parenthesis versus bottom or right. Although our primary interest was in the first two factors (i.e., Garner and Stroop), interactions with the third factor allowed for assessment of asymmetries of interference, such as whether Garner is as strong from the left element to the right as vice versa. The RTs and error rates from all 11 experiments are shown in Table 1. Each section shows eight RT means (and eight corresponding error rates beneath, in percentages) resulting from the 2 · 2 · 2 design of each experiment. GI (Filtering minus Controls) is calculated separately for each of the two relevant dimensions (e.g., left

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Table 1 RTs and error rates (below RTs, in percentages) for Experiments 1–11

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Table 1 (continued)

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Table 1 (continued)

For each Experiment, the 8 RTs and error rates following from the 2 · 2 · 2 design appear in boxes, with marginal means for GI and SI appearing outside the boxes. GI ¼ Garner Interference, SI ¼ Stroop Interference.

versus right) and separately for Congruous versus Incongruous stimuli; the same is true for SI (Incongruous minus Congruous), which is calculated separately for each of the two relevant dimensions and also separately for Control versus Filtering conditions.

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3.1. Experiment 1 The data from this experiment with standard parenthesis pairs are similar to those reported previously in the literature. These stimuli produced 97 ms of GI, [F ð17Þ ¼ 29:7, p < 0:001], accompanied by a parallel effect of 1.3 percentage points in error rates (ns). Recall that GI is a difference score, indicating that performance was worse in the Filtering than in the Control conditions. GI was approximately symmetric, with the left parenthesis receiving 93 ms interference from the right (mean of 79 and 108 in Table 1) versus 101 ms in the opposite direction (mean of 82 and 119), a non-significant difference. SI, which is often at or near zero for these stimuli, was measured at a higher-than-usual 29 ms [F ð1; 7Þ ¼ 13:3, p < 0:01] and 1.7 percentage points error rate (ns), and was nearly perfectly symmetric left to right. The ANOVA also indicated an interaction of Garner by Stroop on RTs [F ð1; 7Þ ¼ 8:61, p < 0:05] and on error rates [F ð1; 7Þ ¼ 11:6, p < 0:05], a common finding indicating that SI is larger in the Filtering than in the Control conditions (or equivalently, that GI is larger with Incongruous than with Congruous stimuli). 7 This experiment, like several of the others, yielded a main effect whereby responses were faster or more accurate with foveal than with peripheral presentation but no interactions with this factor even approaching significance arose, either here or in the other experiments following the Bonferroni adjustment for multiple post hoc comparisons. (For this reason, the foveal versus peripheral presentation factor will not be discussed further.) No other RT or error rate effects in this analysis were significant. 3.2. Experiment 2 The data from this experiment were similar to those from Experiment 1. GI remained strong at 86 ms (p < 0:01) and 1.6 percentage points in error rates (ns), and it was symmetric upper to lower. SI declined to a more typical yet still significant level of 6 ms [F ð1; 7Þ ¼ 6:44, p < 0:05] and 0.2 percentage points error rates (ns). The only other significant effect was a Garner · Stroop interaction, which was insignificant for RTs but was significant for error rates (p < 0:05). This interaction was small and ran in the opposite direction from the norm as noted above. We regard it as anomalous, particularly given that it did not appear in any of our other experiments or in any earlier pilot experiments. 3.3. Experiment 3 This experiment used stimuli that most perceivers see as face-like, and the data differed greatly from those of the first two experiments. GI dropped to just 1 ms and 0.1

7

To put this result in context, imagine a traditional color-word Stroop experiment in which ink color varies, say between red and green, but the word is always the same, say ‘‘green’’. Our lab has found that SI hovers near zero in this sort of situation where the irrelevant dimension does not vary.

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percentage points in error rate (both ns). This represents a more than 98% decline of the RT effect and more than 94% in the error rate effect of the first two experiments combined. SI remained low as well, at 7 ms and 0.5 percentage points error rate (both ns). In fact, there were no significant effects anywhere in this experiment. 3.4. Experiment 4 This experiment used the faces rotated 90. As with the upright faces, GI was miniscule at 2 ms and 0.1 percentage points error rate (both ns). SI was low as well but significant at 13 ms (p < 0:01), albeit not for error rates at 0.6 percentage points. No other effects proved significant. 3.5. Experiment 5 This experiment with inverted faces showed results similar to those of the preceding two experiments but with no significant effects at all. GI averaged 15 ms and 0.5 percentage points error rate (the latter approaching significance) while the corresponding SI effects averaged 5 ms and 0.2 percentage points. 3.6. Experiment 6 This experiment used faces with the triangle of dots removed. In informal reports, some participants reported these stimuli looking like faces whereas others compared them with baseballs, with the parentheses representing the curved stitching. The results, however, showed no significant effects. GI measured 17 ms and )0.2 percentage points error (meaning that fewer errors were made in the Filtering conditions than in the Controls), whereas SI measured 5 ms and 0.2 percentage points. 3.7. Experiment 7 The upright faces with the surrounding circle removed showed a nearly significant 13 ms of GI, along with a non-significant error effect going in the opposite direction. SI was at non-significant negative levels for both speed and accuracy. Reaching significance at 16 ms were faster responses when the upper curved segment was relevant compared with the lower, perhaps indicating that processing began at the top and worked downward; the corresponding error rate effect of 1 percentage point was insignificant. 3.8. Experiment 8 The parentheses surrounded by circles showed effects more similar to the plain parenthesis pairs of Experiment 1 than to the faces. Specifically, they showed significant GI at 72 ms (p < 0:05), although the corresponding error rate effect was insignificant. They also showed significant SI at 33 ms (p < 0:05), although again the error difference was not significant. Lastly, they showed a large Garner · Stroop

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interaction, indicating again that SI was larger in the Filtering (about 66 ms) than in the Control conditions (1 ms). 3.9. Experiment 9 The rotated faces without the surrounding circle showed only one significant effect, namely SI at 17 ms (insignificant with regard to errors). GI was insignificant at 8 ms and 0.2 percentage point error rate. 3.10. Experiment 10 The inverted faces minus the surrounding circle showed GI at 3 ms and 0 percentage points, with SI at 2 ms and 0.4 percentage point error, that is, no significant effects. 3.11. Experiment 11 With the triplet of dots moved from a triangle to a linear, horizontal arrangement, the results indicated an insignificant 4 ms of GI and a similarly insignificant 9 ms of SI, with insignificant error effects as well.

4. Discussion 4.1. Summary Let us summarize these results as succinctly as we can: 1. Simple parenthesis pairs show large amounts of symmetric GI when oriented vertically (as parentheses appear in text). When rotated 90 to the horizontal, GI drops a bit but still remains strong. The parentheses show much smaller levels of SI, although the smaller amounts shown are sometimes statistically significant (as has been reported previously; Pomerantz, 1991). 2. Converting these pairs into cartoon faces by adding a triangle of dots between the parentheses and a circle surrounding them abolishes all interference, Garner or Stroop, whether the faces are in normal, 90 rotated, or 180 rotated (inverted) orientation. 3. Placing just the triangle of dots between the parentheses (without the surrounding circle) is sufficient to eliminate GI, while SI remains non-significant or marginally significant. This holds true regardless of the orientation of the configuration. 4. Surrounding the parentheses with a circle (without the triangle of dots between them) produces an altogether different effect that depends on the orientation of the parentheses. When they are oriented vertically, GI is strong and symmetric, and smaller but significant amounts of SI appear. When they are oriented horizontally, however, GI falls to insignificant levels, and SI vanishes as well.

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5. Converting the triangle of dots into a linear arrangement while retaining the surrounding circle has no discernable effect, even though the resulting stimulus looks less face-like: GI and SI both remain absent. 4.2. Three possible explanations Taken together, there are three types of explanations that might account for these findings: (1) that faces are special and that, for some reason, GI does not arise when faces are processed; (2) that the mere addition of visual elements, whether they create a facial configuration or not, weakens or eliminates grouping, by distracting or diluting attention from elements that would otherwise configure or by interfering with the creation of EFs; and (3) more specifically, that the addition of elements between the contours that would ordinarily group weakens or destroys that grouping. The first explanation, that faces are special and are processed in a way that somehow avoids GI, may appear to be a non-starter at this point, because GI is similarly reduced by adding features that should not lead to face perception. Besides being ad hoc and lacking detail, this approach runs counter to claims that faces are processed configurally rather than as a conglomerate of separate elements. Recall the earlier summary of the literature by Bartlett and others that ‘‘piecemeal features of upright faces are not encoded independently of each other’’. Nonetheless, it is possible that by residing in a face, a curved line is coded differently or otherwise assumes an identity, say as a mouth or eyebrow, that allows it to be attended selectively better than it would in another context. In any case, this approach would help explain the absence of GI in Experiment 3. There, pairs of parentheses, which in isolation group strongly and within facial contexts appear to play a strong configural role in determining emotional expression, show perfect attentional selectivity at the individual parenthesis level––the epitome of independent encoding. Working against that interpretation, however, is the finding from Experiments 11 and 6, that stimuli which are non-facelike in some respect––for example, the eyes and nose are not properly arranged or are absent altogether––also exhibit no significant Garner. Similarly, the effects of rotation and inversion (e.g., Experiments 4 and 5) are not what one would expect from the literature on face perception. Countering that opposing view, however, are informal reports that many of our participants perceived the stimuli even in Experiment 11 (and to a lesser extent 6) as faces. Thus, it remains viable from the data here to maintain that face-like settings undermine the grouping of parenthesis pairs. A second approach holds that it is not a resemblance to faces that undermines grouping but rather the mere physical addition of new visual elements (curves and dots). If two elements, A and B, group, then the addition of a third element C could weaken the grouping between A and B in at least three sorts of ways besides creating a face. First, it could disrupt the perceptual interactions between A and B by weakening an EF. For example, if the pair A and B formed a symmetrical configuration, the addition of C could lead to asymmetry in the triplet. If A continued smoothly into B, the insertion of C could disrupt the detection of that good continuation. Second, C could influence the grouping between A and B by grouping itself with either A or B, which grouping could weaken or preclude entirely the grouping of A with B.

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These effects could occur irrespective of the placement of C. Pomerantz and Schwaitzberg (1975) showed that the grouping of the pair ‘‘((’’ is weakened by the addition of a third element, forming ‘‘(( )’’. The effects of C might be maximal, however, if C were place in between A and B, because there, C could not be excluded by means of, say, an attentional spotlight focused to include A and B. Third, the addition of C could weaken the grouping through some sort of masking of A and B, or through a diversion of attentional resources away from A and B (i.e., distraction or dilution of attention). In this case, the addition of a new element will weaken grouping whether that element destroys EFs or itself groups with the original elements. One explanation that clearly is not viable holds that the faces of Fig. 6, Panel 3 are processed as four unique, unanalyzed wholes, as though they were classified unto simple categories such as sad, happy, angry, and devilish. Such stimuli are called ‘‘nominal’’ (Garner, 1974), and they would not lead to the results found here. Specifically, nominal stimuli lead to very high levels of GI, because the Filtering tasks require mapping four stimuli onto two responses (compared with the Controls tasks, which have a simpler two to two mapping). Returning to the data, we see that a great reduction in GI occurred when the parentheses were joined by the intervening triplet of dots alone, but also that the greatest reduction––down to zero––occurred when the parentheses were also surrounded by a circle. This suggests additive effects where the more elements are added to the display, the worse the grouping (i.e., the better attention can be focused selectively). Merely surrounding the parentheses with a circle left Garner at much higher, and sometimes significant levels, supporting the notion that elements added between the parentheses are particularly disruptive of grouping. Placing contours within a surrounding circle might be expected to strengthen their grouping, via the principle of grouping by common region (Rock & Palmer, 1990; but see also Carson, 1992) while at the same time working via masking or distraction to reduce grouping. This could account for why the surrounding circle by itself was less effective than the triplet of dots at eliminating GI, especially when the parentheses were oriented vertically, which fosters stronger grouping than when they are oriented horizontally. In an effort to explore these notions, we replicated two of the experiments to assure ourselves that their results were reliable. Using a new set of participants, we repeated Experiments 2 and 6, to verify our findings with horizontally oriented pairs of parentheses both with (Experiment 6r) and without (Experiment 2r) the surrounding circle. The results for Experiment 2r replicated those from Experiment 2. GI did drop from 86 to 46 ms but remained marginally significant (p ¼ 0:05), while SI remained almost absent at 4 ms. In addition, a significant effect of relevant dimension was found, with 26 ms faster responses (p < 0:01) and 1.4 percentage points fewer errors (p < 0:05) when the upper parenthesis was relevant. This result strengthens the conclusion that grouping is stronger between vertically than horizontally oriented parenthesis pairs, a result that parallels the equivalent comparison with configural superiority effects (Pomerantz et al., 1977). Although there may be multiple reasons why grouping might be stronger with vertically oriented parentheses, our interpretation is that vertically oriented parentheses are difficult to process as individual

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elements because they are mirror reflections of one another about a vertical axis, and such stimuli are nearly always more difficult to discriminate. For that reason, EFs (such as symmetry and closure) become more valuable when they are available and will thus be attended to, with the resulting appearance of GI (see the Introduction for the underlying logic). Horizontally oriented parentheses are not so confusable because they are reflections about a horizontal axis; thus they benefit less from the availability of EFs and these pairs are more likely to be attended to as individual elements. The results from Experiment 6r replicated those from Experiment 6 in showing no significant GI or SI. There was a higher level of GI in Experiment 6r than in Experiment 6 (34 versus 17 ms), but it remained insignificant. The only significant results from Experiment 6r was an asymmetry in SI, which overall averaged an insignificant level of 9 ms: When the upper element was relevant, incongruity on the lower element produced 18 ms of SI, whereas in the reverse direction SI averaged 0 ms. In short, Experiment 6r confirms our earlier finding that horizontally oriented parenthesis pairs yield little or no GI or SI when circles surround them.

5. Conclusions In drawing conclusions from the present experiments, it may help to review several previous manipulations of the parenthesis pairs (some are shown in Fig. 3) and the effects they had on Garner (and in some cases Stroop) interference. Horizontal distance between parentheses: Separating normally oriented parenthesis pairs horizontally weakens GI monotonically, as might be expected if physical proximity were required for grouping to occur (Pomerantz & Schwaitzberg, 1975). Vertical distance between the parentheses: In research now underway, we find that offsetting the parentheses vertically yields non-monotonic effects, that is, Garner declines with increasing distance, but not in the same nearly linear fashion as with horizontal separation. This is probably because of factors relating to good continuation (also known as relatability; see Kellman & Shipley, 1991; Kubovy & Gepshtein, 2000) and rotational symmetry, because as one parenthesis is lowered vertically, the pair first loses relatability and axial symmetry, but then it gains rotational symmetry and regains reliability before losing both of these as it is lowered yet further. Rotation of one parenthesis 90: This eliminates GI entirely (Pomerantz & Garner, 1973). Rotation with face context: Putting the parentheses into faces. As reported by Pomerantz (1986), taking the stimuli in the preceding paragraph, which show no GI, and placing them into a face-like context (where one parenthesis serves as a mouth and the other as a nose––see Fig. 3) does not cause GI to arise. Thus, contours that do not group in isolation do not begin to group when placed in a face-like context.

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Rotation with intersection: Combining pairs where one parenthesis has been rotated and the two parentheses are made to intersect. The resulting stimuli resemble curved crosses. By virtue of the fact that the two parentheses touch one another physically, one might expect them now to group, because of grouping by uniform connectedness (Palmer & Rock, 1994). Consistent with that prediction, these parenthesis pairs do show moderate levels of GI. Degrouping by adding a third element: As noted above, adding a third parenthesis to a pair that groups can eliminate the GI between the original pair Pomerantz and Schwaitzberg (1975). Such ‘‘degrouping’’ is a classic Gestalt reorganization. Enclosure in boxes: Surrounding a pair of parentheses with a line-drawn box (cf. Rock & Palmer’s principle of common region) has mixed effects (Carson, 1992). Separating parentheses that would normally group into different regions weakens their grouping, albeit not to zero, whereas placing two parentheses that ordinarily would not group (e.g., where one has been rotated) does not make them group. Connecting the endpoints with vertical lines: Connecting the two endpoints of each parenthesis (converting each into a closed ‘‘D’’ form) leaves GI strong while SI continues to hover near zero (Pomerantz et al., 1994). Because the addition of the verticals converts the parenthesis pairs into two closed forms, one might expect these forms to be seen as separate objects, which would decrease GI and increase SI; but that does not happen. Connecting the endpoints with a diagonal: This manipulation, which makes the parenthesis pairs resemble a curvy letter N, reduces GI substantially but not completely. Thus, grouping persists even though a new element is added between the members of each pair and despite the fact that this element alters global properties such as symmetry and connectivity (Pomerantz, 1986). Connecting with a horizontal: This manipulation, which makes the parenthesis pairs resemble the letter H, is currently being tested. Instructions to attend to just one parenthesis: Instructing participants specifically to focus their attention on just one, relevant parenthesis weakens GI, suggesting that selective attention is in fact possible to an individual member of a parenthesis pair and thus that GI reflects not an involuntary glue but rather a voluntary deployment of attention to EFs. These instructions to focus also lead to modest levels of SI, normally absent with these stimuli (Pomerantz, 1991). Such SI–GI reciprocity again suggests that SI arises only when items break into separate elements that may be either congruent or incongruent. Face-like contexts: In the only other experiment examining facial contexts besides the ones reported here, Pomerantz (1986) found that placing parentheses that do not group (because one element has been rotated 90) into a face-like context (where one parenthesis serves as a mouth and the other as a nose) does not cause these elements to group (i.e., they continue to produce no GI). This brief review of previous findings constrains the range of possible explanations of the effects we observed in the present 11 experiments, as follows. First and most obviously, if attention were perfectly selective, neither GI nor SI would occur. Thus, both GI and SI could indicate a ‘‘failure’’ of selective attention

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or (stated more properly) the absence of selective attention, because perceivers may be dividing their attention deliberately, in which case it would be misleading to call this a failure. Second, GI and SI have different origins. GI may result either from the presence of EFs or from additional processing such as that required to analyze whole groups into component parts or to remember more complex stimulus-response mappings. Recall that in a Control task, perceivers may choose between attending to just the differentiating element or to an EF. In a Filtering task, such EFs are of little value and so perceivers must attend to less salient features or remember more complex stimulus-response assignments. This is the most likely cause of GI. SI, by contrast, may arise because of competition (say, in the form of inhibitory crosstalk) between stimulus elements (Pomerantz et al., 1989). When perceivers process parenthesis pairs, we find they typically have no greater difficulty with pairs where the two elements are incongruent––‘‘( )’’ and ‘‘)(’’– than with pairs where they are congruent––‘‘((’’ and ‘‘))’’. One reason for this is that if a pair such as ‘‘( )’’ is seen as a single object, there can be no SI because it takes two objects for congruence or incongruence to arise. Alternatively, and in keeping with Leeuwenberg’s approach, we can focus on the coding that may occur with these parenthesis pairs. Although we call the pair ‘‘( )’’ incongruent because the two parentheses curve in opposite directions, we might just as plausibly call them congruent because they are both convex curves with respect to the whole figure; in this sense, the pair ‘‘((’’ would be incongruent. Interestingly, in experiments with these pairs in which participants are instructed to focus their attention on just one of the two curves, we find that GI (which is normally large) declines substantially whereas SI (normally small or zero) climbs to significant levels (Pomerantz, 1991), probably because focusing attention on just one curve weakens grouping. As a result, the EFs are no longer available, and so GI declines, whereas the stimulus now is perceived as two objects that may be congruent or incongruent, and so SI emerges. Putting together the results from the basic 11 experiments reported here and the two replications, we conclude that placing two contours that ordinarily group when shown in isolation no longer group when placed in the context of other contours to create a face. We also conclude that there may be no effect of the facial context per se; that is, the effect of the face may amount to no more than the physical effects of adding new element to the display, irrespective of whether they yield a facial configuration. Specifically, the addition of as few as three dots between two parentheses suffices to eliminate GI, whereas the addition of a surrounding circle serves just to weaken GI, eliminating it only when it is weak to begin with, when the parentheses are oriented horizontally. If grouping is caused by operations that code geometric and other emergent relations between contours, it makes sense that adding material between two such contours could undermine that grouping. With regard to symmetry for example, placing a vertically asymmetric triangle of dots between two vertically symmetric parentheses could make it harder to detect that latter symmetry. If we view attention as a spotlight or search beam, it would be difficult to exclude from attention elements that are placed between other to-be-attended elements. A surrounding circle, however, could

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be excluded from attention, but only if the beam could be adjusted to just the proper diameter. A surrounding circle could undermine grouping for other reasons, however, ranging from its serving merely to distract or dilute attention to its generating a sensory masking effect. Thus, our conclusion is that face-like contexts undermine grouping, as indexed by GI, via more than one route. Our thinking going into these experiments was based on a different premise. Faces, as we noted earlier, are often regarded as the quintessential configuration. The nearly universal view is that faces are perceived holistically, at least as indexed by other techniques, which makes the present results unexpected indeed. Had our findings indicated a strengthening of grouping inside of facial context, we might have ignored masking and concluded that face contexts provide an even stronger level of grouping or configuration. The mass of evidence showing that faces are perceived as unitary wholes is thus all the more impressive. If holistic perception is so strong with faces, then why are the curves that define the mouths and eyebrows of the present stimuli processed as separable elements, with insignificant GI or SI between them? One possibility involves the artificiality of our stimuli. Most research on face perception has used more realistic stimuli, typically photographs of actual faces. Although more realistic than ours, they do lack the control over features that our cartoon-like faces have. For that reason, photographs are unlikely to allow one to compare grouping of the same elements inside and outside of facial contexts. Research underway in our lab is extending these experiments to the domain of configural superiority effects, as assessed by the odd quadrant method as shown in Fig. 4. Our results appear consistent with those reported here in showing that adding a non-informative context that places parentheses into face-like contexts undermines grouping. For example, although an upward curve is easier to locate among downward ones, if an identical second curve is added to each to create a face actually generates a configural inferiority effect wherein the smiling face is extremely hard to locate among frowning faces. We hope that by contributing these new findings on how cartoon faces are processed, we will learn more both about how faces in general are perceived and also about one of the great and lasting questions in all of vision research, how local elements group into larger configurations.

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