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NeuroImage 40 (2008) 1849–1856

Representing connected and disconnected shapes in human inferiorintraparietal sulcus

Yaoda Xu⁎

Department of Psychology, Yale University, Box 208205, New Haven, CT 06520-8205, USA

Received 2 October 2007; revised 28 January 2008; accepted 9 February 2008Available online 10 March 2008

Although human lesion data have indicated the importance of theparietal cortex in object-based representations, our understanding ofparietal object grouping and selection mechanisms in normal observersremains largely incomplete. This study manipulated the groupingbetween shapes and found that fMRI response from the inferiorintraparietal sulcus (IPS) was higher for the disconnected (ungrouped)than for the connected (grouped) shapes in a task in which observerssimply watched the displays and performed a simple image motionjitter detection task. These results replicated similar findings from aprevious study employing a different paradigm and showed that theinferior IPS plays an important role in tracking the grouping betweenvisual elements during visual perception. Assuming that a lowerresponse corresponds to a greater ease of representation, these resultsmay explain why after parietal brain lesions grouped visual elementsare easier to perceive than ungrouped ones.© 2008 Elsevier Inc. All rights reserved.

Keywords:Visual grouping; Object representation; Parietal cortex; Perception;Vision

A typical visual scene that we encounter in everyday life isusually filled with a huge amount of visual information. Toefficiently extract useful information from such visual scenes andto use it to guide behavior and thought, visual input needs to beorganized into discrete units that can be selectively attended to andprocessed. An important such selection unit in visual processing isthe visual object (see a review by Scholl, 2001). Grouping betweenvisual elements by various Gestalt principles, such as connectednessand closure (Wertheimer, 1950; see also Palmer, 1999), is believed toform the basis of object-based selection and shape conscious visualperception (e.g., Egly et al., 1994; Scholl et al., 2001; Waston andKramer, 1999; Xu, 2002, 2006). Human brain lesion studies haveprovided important insights regarding the neural mechanismsunderlying object-base representations. For example, after bilateral

⁎ Fax: +1 203 432 9621.E-mail address: yaoda.xu@yale.edu.Available online on ScienceDirect (www.sciencedirect.com).

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occipital–parietal lesions that result in Balint's syndrome (Balint,1909), observers could still perceive a single complex object, buttheir ability to perceive the presence of multiple visual objects wasseverely impaired (Balint, 1909; Coslett and Saffran, 1991;Friedman-Hill et al., 1995). Likewise, after unilateral parietallesions, observers' ability to perceive the presence of two objects,one on each side of the space, was greatly improved by connectingthe two objects with a bar, forming one big object with two partsinstead of two separated objects (Mattingley et al., 1997; see alsoGilchrist et al., 1996;Ward et al., 1994). Such lesion data point to theimportance of the parietal cortex in object-based representations, butour understanding of these parietal object grouping and selectionmechanisms in normal observers remains largely incomplete.

Parietal brain responses have been associated with the number ofvisual objects actively represented in the mind, including those fromthe inferior intraparietal sulcus (IPS), which participates in attention-related processing (e.g., Wojciulik and Kanwisher, 1999; Kourtziand Kanwisher, 2000; Culham et al., 2001), and the superior IPS,whose response correlates with the number of objects successfullystored in visual short-termmemory (VSTM; Todd andMarois, 2004,20051; Xu and Chun, 2006; see also Vogel and Machizawa, 2004).For example, when observers retained variable numbers of objectshapes in VSTM, inferior IPS fMRI activation increased linearlywith display set size and plateaued at about a set size of fourregardless of object complexity. Activations from the superior IPSalso increased linearly with display set size but plateaued at themaximal number of objects held in VSTM as determined by objectcomplexity (Xu and Chun, 2006). Thus, a fixed number of objectsare first represented and selected by the inferior IPS via their spatiallocations, and depending on their complexity, a subset of theselected objects are then retained in VSTM with great detail by thesuperior IPS. Activities in these parietal mechanisms thus reflect the

1 Although the IPS region reported by Todd and Marois (2004, 2005)encompassed both the inferior and the superior IPS, the mean Talairachcoordinates reported for this brain region were located in the superior IPS.Moreover, when Xu and Chun (2006) manipulated object complexity, onlythe superior IPS response correlated with VSTM capacity.

1850 Y. Xu / NeuroImage 40 (2008) 1849–1856

number of discrete visual objects represented in the mind at differentstages of visual processing.

Using these parietal responses as neural markers for objecthood,in a recent study, Xu and Chun (2007) manipulated the groupingbetween shapes and examined parietal responses for grouped andungrouped shapes in a VSTM task. They found that grouped shapeselicited lower fMRI responses than ungrouped shapes in the inferiorIPS, even when grouping was task irrelevant. This relative ease ofrepresenting grouped shapes allowed more shape information to bepassed onto later stages of visual processing and resulted in betterbehavioral VSTM performance and higher responses in the superiorIPS (which correspond to VSTM capacity) for the grouped than forthe ungrouped shapes.

In Xu and Chun (2007), observers were required to retain visualinformation in VSTM. In everyday visual perception, however,observers are not always required to do so. In fact, behavioral studiesreported that grouping influenced visual performance even whenobservers were unaware of the presence of such groupings (e.g.,Moore and Egeth, 1997; Driver et al., 2001; Chan and Chua, 2003).Does the inferior IPS grouping response depend on the VSTM taskemployed by Xu and Chun (2007), or does it reflect the encoding of

Fig. 1. Example stimuli used in the experiment: (A) connected shapes, (B) disdisconnected-shape images were constructed by rearranging the three shapes indispersion. Half of the phase scrambled images were generated from the connecte

visual grouping in general? Moreover, because only grouping byclosure was examined in Xu and Chun (2007), can the inferior IPSgrouping response be shown with a different Gestalt grouping cue?To replicate and extend the findings of Xu and Chun (2007), in thisstudy, grouping by connectedness was examined in a task in whichobservers simply watched different types of displays. Specifically,observers viewed either a block of sequentially presented connectedshapes or a block of sequentially presented disconnected shapesand detected an occasional jiggling movement of the entire display(Fig. 1). When shapes were connected, they were grouped togetherand could be considered as parts of a single object; and when theywere disconnected, they were ungrouped and could be considered asdifferent objects. If inferior IPS tracks the number of discrete objectspresent in a visual display, then its response should be higher for thedisconnected than for the connected shapes, similar to that found inXu and Chun (2007).

In addition to the inferior IPS, response from the lateral occipitalcomplex (LOC) was also examined. LOC participates in visual ob-ject shape processing and conscious object perception (e.g., Grill-Spector et al., 1998, 2000; Kourtzi and Kanwisher, 2000, 2001;Malach et al., 1995). Although LOC's response amplitude also

connected shapes, (C) white noise, and (D) phase scrambled shapes. Thethe corresponding connected-shape images, equating for overall spatial

d shapes, and the other half were generated from the disconnected shapes.

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increased as the number of shapes increased in the display in aVSTM task and plateaued at the maximal number of objects held inVSTM (Xu and Chun, 2006), Xu and Chun (2007) found that it wasnot sensitive to shape grouping. However, because an event-relatedfMRI design was used in that study, which has weaker statisticalpower than a blocked fMRI design, small effects in LOC may havebeen overlooked. The present blocked fMRI design should increasestatistical power and allow us to reexamine the grouping effect in theLOC.

This study used a region of interest (ROI) approach andextracted averaged fMRI responses from functionally definedinferior IPS and LOC ROIs, as was done previously (Xu and Chun,2006, 2007). Time courses were then extracted from these ROIs toexamine grouping-related responses.

Experimental procedures

Participants

Ten paid observers (3 females) participated in the experiment.Theywere recruited from the Yale University campus, were all right-handed, had normal or corrected to normal vision and normal colorvision. Informed consent was obtained from all observers, and thestudy was approved by the Human Investigation Committee of theYale University School of Medicine.

Design and procedure

The experiment followed the blocked fMRI experimental designof Kourtzi and Kanwisher (2000; see also Downing et al., 2001).Specifically, observers viewed a sequential presentation of four dif-ferent types of images. The images were presented in blocks, con-taining either connected shapes (Fig. 1A), disconnected shapes(Fig. 1B), white noise (Fig. 1C), or phase scrambled shapes (Fig. 1D).These images served both as the main stimuli for the experiment andthe stimuli for the ROI localizer (see below). Because orthogonalcomparisons were made in the experiment and in localizing the ROIs,this design afforded a compact experimental design withoutsacrificing the advantages of an independent ROI-based approach(see Friston et al., 2006; Saxe et al., 2006).

Each image block contained 20 different images of the same type.Each image appeared for 200 ms and was followed by a 600 ms blankinterval before the next image appeared. The brief image presentationtime was used to minimize eye movements. Each blocked lasted 16s.Besides the stimulus blocks, therewere also 16-s blank fixation periodsin which only a fixation dot was present. Each experimental runconsisted of five 16-s fixation periods sandwiched between four 64-sstimulus periods. Each 64-s stimulus period contained one 16-sstimulus block from each of the four stimulus condition as in Kourtziand Kanwisher (2000). Observers fixated at the center of the displaysand, to ensure that their attention was focused on the displays, theydetected a slight motion jitter of the entire display occurring randomlyin 1 out of every 10 displays. During eachmotion jitter, the displaywaspresented sequentially in 4 spatial locations for 50 ms each followingthis order: center→ right→ left→ center, with right to left displace-ment being 0.2° of visual angle. Each observer was tested with tworuns (balanced for presentation order following Kourtzi andKanwisher, 2000). All displays subtended 11.7°×11.7° of visualangle and were presented on a light gray background. Each runcontained a total of 80 images from each stimulus condition and lasted5 min and 40s.

Twenty unique images were used for each stimulus condition. Forthe connected-shape condition, 20 different imageswere created, eachcontaining three connected shapes (see Fig. 1A for some examples).For the disconnected-shape condition, the three shapes that hadappeared in the connected-shape condition were detached from eachother and rearranged to form 20 new images (see Fig. 1B). Thearrangement of the shapes in this condition was such that the shapesused in the disconnected- and the connected-shape conditions wereequally dispersed. This was assessed by first finding the center ofgravity for all the shape pixels in a given image and then calculatingthe mean standard deviation between each shape pixel and this centerof gravity. With this measure, there was no difference in shapedispersion between the two shape conditions (F(1,19)=1.82, pN0.19;if anything, the connected shapes were slightly more dispersed thanthe disconnected shapes). The white noise images were included tolocalize the inferior IPS and the LOCROIs as in Xu and Chun (2006).Phase scrambled shape imageswere included to preserve the low levelimage statistics of the shape images and to provide a different way tolocalize the LOC (e.g., Op de Beeck et al., 2006). Half of the phasescrambled images were generated from the connected shapes, and theother half were generated from the disconnected shapes. Each displayappeared four times in a given run and each time in a different spatialorientation to increase stimulus novelty.

fMRI methods

Observers lay on their back inside a Siemens Trio 3Tscanner andviewed, through a mirror, the displays projected onto a screen at thehead of the scanner bore by an LCD projector. Stimulus presentationand behavioral response collection were controlled by an ApplePowerbook G4 running Matlab with Psychtoolbox extensions(Brainard, 1997; Pelli, 1997). Standard protocols were followed toacquire the anatomical images. To acquire the functional images, agradient echo pulse sequence (TE 25 ms, flip angle 90°, matrix64×64) with TR of 2.0 s was used, and 24 5-mm-thick (3.75 mm×3.75 mm in-plane, 0 mm skip) axial slices parallel to the AC–PC linewere collected.

Data analysis

fMRI data collected in the experiment were analyzed usingBrainVoyager QX (www.brainvoyager.com). Data pre-processingincluded slice acquisition time correction, 3D motion correction,linear trend removal and Talairach space transformation (Talairachand Tournoux, 1988).

Following Xu and Chun (2006), the LOC and the inferior IPSROIs were defined as regions in the ventral and lateral occipitalcortex and in the inferior IPS, respectively, whose activations werehigher for the connected- and the disconnected-shape images thanfor the white noise images (false discovery rate qb0.05, correctedfor serial correlation; Fig. 2). LOC was also defined by localizingregions in the ventral and the lateral occipital cortex whoseactivations were higher for the connected- and the disconnected-shape images than for the scrambled shape images (false discoveryrate qb0.05, corrected for serial correlation; Fig. 2B). Thus, therewere a total of three individually localized ROIs from each observer:an inferior IPS ROI and a LOC ROI defined by intact shapes andwhite noise (LOC-wn) and a LOC ROI defined by intact shapes andphase scrambled shapes (LOC-ps). Across observers, the LOC-wnwas in general larger than the LOC-ps, with the LOC-wn and theLOC-ps overlapping to a great extent (see Fig. 2B).

Fig. 2. (A) Inferior IPS ROIs from an example observer. Mean Talairach coordinates for the inferior IPS ROIs are the following: right 27, −76, 28 and left −21,−77, 25. (B) LOC ROIs from the group analysis (pb0.001) showing the overlap between the LOC-wn and the LOC-ps.

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These ROIs were overlaid onto the data from the experiment, andtime courses were extracted from each observer. As in previousstudies (e.g., Kourtzi and Kanwisher, 2000), these time courses wereconverted into percent signal changes for each stimulus condition bysubtracting the corresponding value for the fixation periods and thendividing by that value. Peak fMRI responses were derived bycollapsing the time courses from the connected- and the disconnected-shape conditions and determining the eight continuous time points(totaling 16 s) of greatest signal amplitude in the averaged response.This was done separately for each observer in each ROI. The resultingpeak responses were then averaged across observers.

Results

Behavioral results

Behavioral motion jitter detection accuracies for the connectedshapes, the disconnected shapes, the white noise, and the phasescrambled shapes were 93%, 94%, 97%, and 93%, respectively.

There was no main effect of the stimulus condition, F(3,27)=1.30,pN0.29. In pairwise comparisons, the difference between the twoshape conditions was not significant (Fb1), but the differencebetween the two non-shape conditions was significant (F(1,9)=7.36, pb0.05).

Averaged fMRI peak results

Peak fMRI responses from the three ROIs examined here(averaged over both hemispheres) are plotted in Fig. 3. Theseresponses were averaged over eight continuous time pointscorresponding to the 16-s stimulus presentation. In the inferior IPS,response was higher for the disconnected than for the connectedshapes (F(1,9)=5.39, pb0.05). In both the LOC-wn and the LOC-ps,however, the opposite was true: response was lower for thedisconnected than for the connected shapes (F(1,9)=6.03, pb0.05,and F(1,9)=10.11, pb0.05, respectively). This resulted in significantinteractions between stimulus condition and inferior IPS and LOCROIs (F(1,9)=37.47, pb0.001, between stimulus condition and the

Fig. 3. Peak fMRI responses from the three ROIs examined (averaged overboth hemispheres). These responses were averaged over 8 continuous timepoints corresponding to the 16-s stimulus presentation. While responseswere higher for the disconnected than for the connected shapes in the inferiorIPS ROI, the opposite was true in the two LOC ROIs. Error bars indicatewithin-subject standard errors.

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inferior IPS vs. the LOC-wn, and F(1,9)=48.46, pb0.001, betweenstimulus condition and the inferior IPS vs. the LOC-ps). Theinteraction between stimulus condition and the two LOC ROIs wasalso significant (F(1,9)=12.33, pb0.01), showing a bigger stimulusdifference in the LOC-ps than in the LOC-wn.

For the two non-shape conditions, response was significantly ormarginally significantly lower for the white noise than for thephased scrambled shape condition in all three ROIs examined (inthe inferior IPS, F(1,9)=9.21, pb0.05; in the LOC-wn, F(1,9)=14.95, pb0.01; and in the LOC-ps, F(1,9)=3.29, p=0.10). Thiscould be because fuzzy shape blobs were still visible in the phasedscrambled shape images, and this may have increased response inthe inferior IPS and the LOC.

fMRI results for the first and the second halves of the image block

Fig. 4 plots the time courses from the three ROIs examinedaveraged over both hemispheres. While responses for theconnected shapes in all three ROIs were similar during the firstand the second halves of the stimulus block, responses for thedisconnected shapes in all three ROIs were higher in the first thanin the second half of the stimulus block. This observation wasconfirmed by pairwise statistical tests comparing responses fromthe first half of the block (time points 8 and 10) with those from thesecond half of the block before the drop in response started (timepoints 14 and 16).

In the inferior IPS, the response was higher in the first than in thesecond half of the block for the disconnected shapes (F(1,9)=5.57,pb0.05) but not for the connected shapes (Fb1). Consequently, thedifference between the connected and the disconnected shapes wassignificant in the first half (F(1,9)=6.82, pb0.05) but not in thesecond half of the block (Fb1). There was an overall marginallysignificant interaction between the two stimulus conditions and thetwo halves of the block (F(1,9)=4.79, p=0.056). Similarly, in theLOC-wn, the response was higher in the first than in the second halfof the block for the disconnected shapes (F(1,9)=5.30, pb0.05) butnot for the connected shapes (Fb1). This resulted in no differencebetween the connected and the disconnected shapes in the first half

(Fb1) but a significant difference between these two conditions inthe second half of the block (F(1,9)=45.05, pb0.001). There wasalso an overall significant interaction between the two stimulusconditions and the two halves of the block (F(1,9)=8.70, pb0.05).Like the LOC-wn, in the LOC-ps, the response was marginallysignificantly higher in the first than in the second half of the block forthe disconnected shapes (F(1,9)=3.90, p=0.08) but not for theconnected shapes (Fb1). This resulted in no difference between theconnected and the disconnected shapes in the first half (F(1,9)=1.13, pN0.31) but a significant difference between these two con-ditions in the second half of the block (F(1,9)=77.44, pb0.001).There was also an overall significant interaction between the twostimulus conditions and the two halves of the block (F(1,9)=10.70,pb0.05).

Comparing the different brain regions, the interactions betweenthe two stimulus conditions and the inferior IPS and the LOC-wnweresignificant for both halves of the block (for the first half, F(1,9)=47.22, pb0.001; and for the second half, F(1,9)=9.71, pb0.05).Similarly, the interactions between the two stimulus conditions andthe inferior IPS and the LOC-ps were also significant for both halvesof the block (F(1,9)=46.55, pb0.001; and F(1,9)=14.85, pb0.01,respectively). The interactions between the two stimulus conditionsand the two LOC ROIs were marginally significant for the first halfand significant for the second half of the block (F(1,9)=4.06,p=0.075; and F(1,9)=18.49, pb0.01, respectively). Three-wayinteractions between the inferior IPS and either of the LOC ROIs,the two stimulus conditions, and the two halves of the block were notsignificant (Fsb1). The three-way interaction between the two LOCROIs, the two stimulus conditions, and the two halves of the blockwas marginally significant (F(1,9)=3.53, p=0.093).

Time courses from the left and right hemispheres are also plottedseparately in Fig. 4. The response patterns of the two hemisphereswere very similar. The three-way interaction of connected vs. dis-connected shapes, the left vs. the right hemisphere, and the twohalves of the block was not significant in any of the ROIs examined(Fsb1).

Whole brain group analysis

At the pb0.001 threshold, whole brain group analyses bothacross the stimulus block and just the first half of the stimulusblock did not reveal any brain area showing a higher response forthe disconnected than for the connected shapes or the reverse. Evenafter Talairach transformation, the precise location of the IPS wasmore medial for some observers and more lateral for others. At theTalairach coordinate of y=−73 and z=33, the x coordinate of theIPS varied from 21 to 40 in the right hemisphere and varied from−28 to −20 in the left hemisphere across the 10 observers tested.This variability in IPS location together with the size of thegrouping effect may explain why no area around the IPS could beidentified in the whole brain group analyses.

Discussion

Although human lesion data have indicated the importance ofthe parietal cortex in object-based representations, our under-standing of these parietal object grouping and selection mechan-isms in normal observers remains largely incomplete. This studymanipulated grouping between shapes and found that inferior IPSfMRI response was higher for the disconnected (ungrouped) thanfor the connected (grouped) shapes in a task in which observers

Fig. 4. fMRI time courses from the three ROIs examined, averaged over ROIs from both hemispheres, from the left hemisphere, and from the right hemisphere,respectively. Responses for the disconnected shapes in all three ROIs were higher in the first than in the second half of the stimulus block, while those for theconnected shapes in all three ROIs did not differ between the two halves of the stimulus block. Error bars indicate within-subject standard errors.

1854 Y. Xu / NeuroImage 40 (2008) 1849–1856

simply watched the displays and performed a simple image motionjitter detection task. These results replicated similar findings from aprevious study by Xu and Chun (2007) but used a different Gestaltgrouping cue and a different experimental paradigm that did not

impose a VSTM encoding demand. Assuming that a lower re-sponse corresponds to a greater ease of representation, the presentfinding may explain why grouped visual elements are easier toperceive than ungrouped ones for patients with bilateral occipital–

1855Y. Xu / NeuroImage 40 (2008) 1849–1856

parietal brain lesions (e.g., Balint, 1909; Coslett and Saffran, 1991;Mattingley et al., 1997; Gilchrist et al., 1996; Ward et al., 1994).

Because identical shapes were used and the overall shapedispersion was matched between the connected and the discon-nected shape conditions, the inferior IPS grouping effect cannot beattributed to differences in the amount of attentional spread in thetwo conditions. Neither can the effect be attributed to differences intask difficulty. This is because behavioral performance accuraciesdid not differ between the two conditions. In a follow-up analysis,when observers were separated into two groups according towhether their behavioral performance accuracy was higher or lowerfor the connected than for the disconnected shapes, the inferior IPSresponses were lower for the connected than for the disconnectedshapes for both groups of observers. Although reaction times (RTs)were not recorded in this experiment, in a separate follow-upbehavioral study using the same displays and paradigm, no RTdifferences were found between the connected and the disconnectedshapes (F(1,5)=1.24, pN0.31). If anything, RTswere slightly longerfor the connected than for the disconnected shapes. Lastly, thegrouping effect observed in the inferior IPS was not observed in theLOC—another brain region involved in shape processing. Thus, theinferior IPS grouping effect observed in the present study cannot beaccounted for by either differences in attention spread or differencesin task difficulty between the connected and the disconnectedshapes. Rather, the present finding indicates that the inferior IPSplays an important role in tracking the grouping between visualelements during visual perception.

Close examination of the time courses of the fMRI responsesrevealed that, while responses for the connected shapes did not differbetween the two halves of the stimulus block, those for thedisconnected shapeswere higher in the first than in the second half ofthe stimulus block in all three ROIs examined. A set of very simpleshapes (circles, squares, triangles and curved lines) was usedrepeatedly in the different displays. When shapes were connected,each shape assembly formed a unique three-part object, and shapeconfiguration became an important property of each display.Differences in the individual shapes and shape configurations thusmade the 20 connected-shape displays all distinct from each other.However, when shapes were disconnected, if the placement of thethree shapes in each display was perceived to be accidental, thenobservers might have viewed shape configuration as nonessential inshape encoding and viewed the different disconnected-shapedisplays as containing the same set of shapes placed at differentspatial locations. As a result, brain responses might have becomehabituated to the repeated presentation of these shapes in thedisconnected-shape displays. Indeed, repeated presentation of thesame visual stimulus has been shown to result in decreased fMRIresponse due to priming or simply attention withdrawal or neuralfatigue in blocked-design fMRI experiments (e.g., Wiggs andMartin, 1998; Henson, 2003; Grill-Spector et al., 2006). This mayexplain response pattern differences between the connected and thedisconnected shapes across the two halves of the stimulus block inthe three ROIs.

Because inferior IPS response was higher for the disconnectedthan for the connected shapes in the first half of the stimulus block,fMRI response habituation for the disconnected shapes in thesecond half of the stimulus block does not invalidate the mainresult of this study regarding the representation of grouping in thisbrain area. In contrast, although overall LOC response was higherfor the connected than for the disconnected shapes, the effect solelycame from response habituation for the disconnected shapes in the

second half of the stimulus block, and there was no responsedifference between the two conditions in the first half of thestimulus block. Taken together, these results suggest that LOCresponse amplitude was not sensitive to the grouping betweenvisual elements, in line with what was reported in Xu and Chun(2007), although LOC response amplitude did track the number ofshapes present in VSTM tasks (Xu and Chun, 2006, 2007; Xu, inpress). Thus, while the inferior IPS response amplitude is sensitiveto both the total number of visual elements present and thegrouping between them, LOC response amplitude seems to be onlysensitive to the total number but not to the grouping between thevisual elements.

Further studies are needed to understand response decline for thedisconnected shapes over the two halves of the stimulus block bysystematically varying the identity of the individual shapes and theshape configurations. Nonetheless, this result is a novel and inter-esting finding on its own and suggests that our brain may representconnected and disconnected shapes in qualitatively different man-ners. Thus, not only response amplitude, but also response declineover time may provide us with important clues regarding theformation of visual objects in the brain.

Acknowledgment

This research was supported byNSF grants 0518138 and 0719975to Y.X.

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