1 IntroductionRecently, researchers have been exploring a wide variety of failures of visual awareness,ranging from failures to detect visual changes or failures to detect unexpected stimuli,even when they are bizarre or noxious (eg Simons and Chabris 1999; Wayand et al2005). One of these failures is referred to as change blindness' (CB), and it occurswhen people are unaware of visual changes that occur across views (for reviewssee Simons and Levin 1997, 2003; Rensink 2002). For example, CB can occur whensubjects view a short film in which visual properties, such as the actors' clothing,change between shots. Even though the changes appear salient once they are pointedout, almost all subjects miss them, especially when they are not looking for changes(Levin and Simons 1997).

The key to producing CB is to somehow block the perceptual transient that wouldnormally call attention to a change. Often this is done by adding another broadertransient such as a flicker (Rensink et al 1997), or a set of object onsets (O'Regan et al1999), simultaneous with the change. Thus, the change-detection task can inform researchthat explores exogenous capture of attention in a situation where two transients compete.Typically, attentional capture is revealed as a transient-imposed cost, or benefit to aprimary task such as visual search or probe detection. One characteristic of these tasksis that they typically focus the subject's attention on a very specific well-defined targetdimension or category, and this focus may cause similar restrictions on the transient thatcan capture attention (Simons 2000). In contrast, a change-detection task, particularlyone which involves natural scenes, requires subjects to have a very broad focus, becauseany of a large number of properties or objects may change. This makes the change-detection task particularly suitable for studying the breadth of transients that may captureattention. In these experiments, we first explored the degree to which the sudden additionor removal of color, and changes in overall luminance, could induce CB.

2 How can one block transients to produce CB?CB has been produced in a wide variety of ways. Initial research demonstrating CBinvolved changes that were synchronized with saccades. Thus, subjects' eye movements

Color onsets and offsets, and luminance changescan cause change blindness

Perception, 2006, volume 35, pages 1665 ^ 1678

James G Arrington, Daniel T Levinô, D Alexander VarakinDepartment of Psychology and Human Development, Vanderbilt University, Peabody College,Box 512, 230 Appleton Place, Nashville, TN 37203-5701, USA; e-mail: daniel.t.levin@vanderbilt.eduReceived 16 February 2005, in revised form 29 December 2005

Abstract. It has recently been demonstrated that people often fail to detect between-view changesin their visual environment. This phenomenon, called change blindness' (CB), occurs wheneverthe perceptual transient that usually accompanies a change is somehow blocked, or made lesssalient. In the well-known flicker paradigm, the transient is blocked by inserting a blank screenbetween the original and changed scenes. We tested whether transients that do not involve theappearance or disappearance of visual objects would also produce CB. Therefore we testedwhether the appearance or disappearance of color information, and increments or decrements inluminance, could cause CB. In three experiments, subjects searched for changes in natural scenes.We found that both color transients and luminance transients significantly reduced change detec-tion (by �30%) relative to a no-transient condition.

DOI:10.1068/p5599

ôAuthor to whom all correspondence should be addressed.

were monitored while they viewed text or natural scenes, and visual changes wereintroduced after the initiation of a saccade (McConkie and Zola 1979; Grimes 1996;Hollingworth and Henderson 2000). This technique is highly effective at masking thechanges because the retinal `smear' that accompanies a saccade effectively competeswith the change-induced transient. However, the technique is quite demanding technically,so researchers interested in exploring CB have designed a variety of ways to mask thechange without the need for eye tracking.

The first, and most widely used, of these methods has been to insert sudden visualonsets and/or offsets at the same time as the change. In the `flicker paradigm' (Rensinket al 1997), subjects view two different versions of a scene. For example, one versionof the scene might have a building in the background which is edited out of the otherversion. The versions are switched back and forth, separated by a blank grey screen.The fact that the grey screen causes the scene to disappear and reappear allows thetransient caused by the change to be lost in the more global transient that occurs inthe disappearance and reappearance. Moreover, these onsets and offsets do not needto entail the appearance and disappearance of the entire image. O'Regan et al (1999)found that the sudden addition of a set of spots to the screen at the time of the changeis sufficient to cause CB, even though none of the spots obscures the change itself.

Another means of creating a sufficient transient is to simply move the image diag-onally simultaneous with the change (Blackmore et al 1995), or, as in motion-picture edits,to make the change simultaneous to the change in viewpoint (Levin and Simons 1997).Even simpler, change detection can be disrupted by briefly occluding the changing objectsduring the change (Simons and Levin 1998). Finally, changes can be made gradually bythe use of `fade-ins' that occur over the course of several seconds (Simons et al 2000).

Recently, Turatto et al (2003) reported that CB could be induced by sudden contrastinversions that do not include flickers. In their experiments, subjects viewed black-and-white images of natural scenes that were alternately presented in their normal andcontrast-reversed forms. Contrast inversions are different from previous methods ofinducing CB in that they involve no onsets or offsets of objects (which one could argueare necessary to attract attention away from the change; see Jonides and Yantis 1988) anddo not require that the scene be off-screen when the change is made. However, contrastinversion still represents a substantial and sudden transformation in the apparent shapeof objects. For example, faces are more difficult to recognize when contrast-inverted, whichsuggests that the inversion interferes with the subject's ability to extract form or configuralinformation from the face (Kemp et al 1996).

Although it is plausible that Turatto et al's inversion disruption was successfulbecause it involved form-based or shape-based transients, there are several reasons tobelieve that this is not the case. For example, it is well known that quick changes inluminance strongly activate both ventral/parvocellular and dorsal/magnocellular partsof the visual system (Livingstone and Hubel 1988), and a number of studies havedemonstrated that these transients can powerfully capture attention (see Simons 2000for a review). However, the attentional-capture capability of other non-form transientssuch as sudden changes in color is less clear. Some studies have reported that thesechanges do attract attention (Gellatly et al 1999; Snowden 2002; Lu and Zhou 2005),whereas other studies have provided evidence that they do not. For example, Cole et al(2005) argued that Snowden (2002) had observed color-based attentional capture becausethe color transient in his study was confounded with an object onset. When Cole et alremoved the object onset, color changes did not capture attention.

One limitation of many of these findings is that they often start from the assumptionthat a given cue either does or does not attract attention across the board. In contrast,other researchers have argued that attentional capture instead reflects the match bet-ween the nature of the target-detection task and the nature of the irrelevant stimulus.

1666 J G Arrington, D T Levin, D A Varakin

For example, Folk et al (1992) found that when a color singleton was the irrelevantstimulus in an onset-detection task it did not attract attention, but when the same single-ton was presented in the context of a color-target detection task, it did attract attention.On the basis of this and other findings, they argued that attentional capture occurswhen task parameters focus subjects' attention on the specific feature dimension thatchanges. This is particularly important in settings where the task requires searching for achange in a natural scene. Given that the task of detecting these changes may requiresubjects to search for changes in any one of a range of features, such as form, luminance,or color, it is possible that the transients capable of interfering with change detection will besimilarly broad.

The present experiments were designed to test the hypothesis that color onsets andoffsets are sufficient to reduce change detection below a no-transient baseline. In experi-ment 1, we compared a color-onset condition with a no-flicker condition, and with ano-transient condition. In experiment 2, we added a color-offset condition, along witha no-transient grey-scale condition to test the degree to which gray-scale images in them-selves reduce change detection. In experiment 3, we tested luminance changes to comparetheir ability to reduce change detection.

3 Experiment 1Experiment 1 was designed to determine whether the onset of color alone could maska change. We therefore compared change detection under four conditions: (i) the changewas masked both by a color onset (the pre-change scene was presented in grey scale,and the post-change scene was presented in color) and by a 100 ms flicker between thepre-change and post-change scenes; (ii) the change was masked by the onset of color, withno flicker; (iii) the change was masked by traditional means, with both pre-change andpost-change scenes presented in color, and separated by a flicker; and (iv) the changewas not masked by flicker or by color onset. We therefore sought to determine the degreeto which the color onset would cause CB relative to a condition that produced notransient and to one that contained a traditional large-scale disruption.

3.1 Method3.1.1 Participants.Twenty-nine undergraduate students from Kent State University partic-ipated in this experiment. All were enrolled in General Psychology, and none of themhad previous experience with other CB experiments. Two of the participants wereexcluded because they did not follow instructions when making their responses (theyindicated that the color transients themselves were changes). Three of the participantswere excluded because they did not have normal or corrected-to-normal vision basedon self-report. The analyses were conducted with the remaining twenty-four participants.

3.1.2 Apparatus and stimuli. Stimuli for this experiment were created from thirty-twocolor pictures of natural scenes, which were collected from The Big Box of Art(Hemera Technologies, Hull, Quebec). Pictures were selected based on the presence ofan object, objects, or part of an object, that could be easily altered so that the changedversions would look as natural as the original versions. Three versions of each scenewere created with Adobe PhotoShop 4.0 (Adobe Systems, San Jose, CA): pre-changescenes presented in 16-bit color, post-change scenes presented in 16-bit color, and pre-change scenes presented in 8-bit grey-scale. We created a variety of changes but didnot include any color changes. Of the 32 changes, 9 were additions or deletions ofparts, 7 were changes to the height or width of objects, 5 were additions or deletionsof whole objects, 2 were exemplar substitutions (eg replacement of an object withanother object from the same basic-level category), 6 were changes to the surfacepattern of an object, and 3 were rotations of an object (see figure 1). The conversionfrom color to grey-scale was done with the change mode' command in Photoshop.

Color and luminance changes 1667

Addition/delection of object parts (9 scenes)

Changes in height/width of objects or their parts (7 scenes)

Addition/deletion of objects (5 scenes)

Exemplar substitutions (2 scenes)

Changes to surface pattern (6 scenes)

Rotation changes (3 scenes)

Figure 1. Sample changes.

1668 J G Arrington, D T Levin, D A Varakin

This command achieves the change by computing a weighted mean of R, G, and Bbit values. The weights reflect the Rec. 709 standard based on the typical computermonitor (Y709 � 0:2125R� 0:7154G� 0:721B ; see Poynton 1998). All pictures werepresented on Mac OS computers with 15-inch Sony fx100 CRT monitors set at aresolution of 10246768 pixels (75 Hz). Luminance of the color and grey-scale versionsof each scene was also measured directly from one of the monitors with a MinoltaLS110 luminance meter. Each scene was sampled in four locations, one in each quad-rant, and the average difference between the color and black-and-white scenes acrossall scenes was 0.20 cd mÿ2 (SD � 0:55 cd mÿ2), and the average absolute differencebetween the versions was 0.355 cd mÿ2 (SD � 0:421 cd mÿ2). Each scene measured17.64 cm611.29 cm, and was presented at the center of the 21 cm628 cm screen at anunconstrained viewing distance of �60 cm. Thus, each scene subtended approximately16.47 deg (horizontal)610.74 deg (vertical) of visual angle.

3.1.3 Design and procedure. The two factors of interestöcolor onset versus no coloronset and flicker versus no flickeröwere crossed, yielding four experimental conditions.The four conditions were: (i) color onset/flicker: grey-scale pre-change scene to acolor post-change scene, separated by a flicker; (ii) color onset only: grey-scale pre-change scene to a color post-change scene, without a flicker; (iii) flicker only: colorpre-change scene to a color post-change scene, separated by a flicker; (iv) no transient:color pre-change scene to a color post-change scene, without a flicker. For conditions(i) and (iii), the pre-change scene was presented for 2000 ms, followed by a 100 ms `flicker'to mask the change, and then by the post-change scene. For conditions (ii) and (iv), thepost-change scene immediately replaced the pre-change scene. For all four conditions,the post-change scene remained present until the participant made a response and wasready to move on to the next trial. The original thirty-two pictures were randomly dividedinto four sets of eight to serve in the four different conditions (the assignment of picturesto conditions was rotated across subjects so that all pictures served in all conditions).

Participants sat approximately 61 cm from the computer screen, and were run insmall groups ranging in size from 1 to 4. Participants were instructed that there wouldbe a change on every trial, and that their job would be to detect what had changedin a variety of scenes. On each trial, they were instructed that they would see an initialversion of the scene before the change, which would occur after a few seconds. Theywere then instructed that on some trials there would be a brief flicker before thechange, and that on other trials there would be no flicker before the change. They werealso instructed that either both scenes would be in color or the first scene would bein black-and-white and the second scene would be in color. Participants were explicitlyinformed that switching the image from black-and-white to color did not count as achange in the scene. After each trial, participants indicated whether or not theydetected the change: first on the computer, and then on a response sheet. On thecomputer, participants were told to press the `Y' key if they detected the change or topress the `N' key if they did not. This response cue appeared simultaneously with thepost-change scene. The response sheet was numbered 1 to 32 with a `Y' and `N' nextto each number; each number also had a blank space labeled ` Pre'' and ` Post''. Partic-ipants were instructed to circle `Y' if they detected the change or to circle `N' if theydid not. In addition, if they detected the change, the participants used the responsesheet to write a description of the pre-change object and of the post-change object.This use of the keyboard response in addition to the response sheet was intended toallow for reaction-time measurements (which proved unilluminating, and are notreported here), to push subjects to make their change-detection responses withoutextensive deliberation, and to ensure accurate coordination between the computerizedstimulus order files and the response sheets.

Color and luminance changes 1669

3.2 Results3.2.1 Detected changes. Two independent raters scored the response sheets to verifythat `Y' responses corresponded to correct change detections. The three criteria forscoring responses were: (i) the location of the changing object, more specifically thatindicating the top/bottom or right/left side of the screen; (ii) the name of the changingobject; and (iii) the correct type of change (appearance, disappearance, or movement).For each trial, change detection was scored as correct if the location and/or nameof the changing object were correctly identified. If the type of change was correctlyidentified, but the location and/or name of the changing object were not.

For the color-onset flicker condition, changes were detected on 14% (SD � 14%) ofthe trials. For the color-onset-only condition, changes were detected on 35% (SD � 14%)of the trials. For the flicker-only condition, changes were detected on 17% (SD � 13%) ofthe trials. For the no-transient condition, changes were detected on 68% (SD � 16%)of the trials. In a 2 (color onset versus no color onset)62 (flicker versus no flicker) within-subjects ANOVA, there was a main effect both for color (F1 23 � 58:03, MSE � 1:3%,p 5 0:001) and for flicker (F1 23 � 137:35, MSE � 2:2%, p 5 0:001). The interactionwas also significant (F1 23 � 58:06, MSE � 0:9%, p 5 0:001; see figure 2). Pairwise com-parisons confirmed that significantly fewer changes were detected in the color-onset-onlycondition than in the no-transient condition (t23 � 9:559, p 5 0:001), whereas signif-icantly more changes were detected in the color-onset-only condition than in eitherof the flicker conditions (color onset only versus flicker only: t23 � 4:813, p 5 0:001;color onset only versus color onset/flicker: t23 � 6:244, p 5 0:001). The flicker conditionsdid not differ significantly from each other (t23 � 1:187, p 4 0:10).

3.3 DiscussionIn experiment 1, color onsets clearly increased the prevalence of CB, but not asmuch as a flicker. The color onsets reduced change detection by 33% relative to the no-transient baseline, whereas the more traditional flickers reduced change detection by 51%.

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Experiment 1 Experiment 2 Experiment 3

coloronset/flicker

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Figure 2. Percentage change detection in experiments 1 ^ 3.

1670 J G Arrington, D T Levin, D A Varakin

One unusual aspect of the data is that change detection with no transient was only 68%.According to most accounts, these conditions should produce very high rates of changedetection. One plausible explanation for the relatively low rates of change detectionmight be that at least some identification of the changing objects was required in orderfor a response to be counted as a hit. Therefore, we may be measuring change identi-fication rather than change detection, and some authors have argued that the twoare different (Fernandez-Duque and Thornton 2000, 2003). On the other hand, otherresearchers have observed similar patterns of results both for change identification andfor change detection (Mondy and Coltheart 2000), and so it may be that change detectionis simply the result of a more liberal coding criterion than change identification.

In any case, the pattern of results for experiment 1 is similar even when all positiveresponses are counted as hits: color onset/flicker (M � 33%; SD � 24%); color onset only(M � 52%, SD � 18%); flicker only (M � 28%, SD � 20%); no transient (M � 79%,SD � 14%). The increased rate of change detection with this more liberal criterion couldbe caused by a number of factors. Most likely, subjects may sometimes have experienceda near-threshold change detection but were mistaken about its location. This possibilityis reinforced by findings that changes can be detected by means of temporary, volatilerepresentations that are partially disrupted by the post-change scene such that they remain,but lack strong information about specific changing properties or locations (Beck andLevin 2003). As mentioned above, the alternative is that the additional change detectionsare simply false alarms, and without no-change catch trials there is no good way ofdistinguishing these possibilities.

One potential problem with experiment 1 is that the response cue may have actedas a distractor on each trial. On the computer, the response cue, ` Did you see anythingchange? Press the `Y' key if you did, or the `N' key if you did not'', appeared at thesame time as the post-change scene and may have lowered change-detection rates overall.This issue will be eliminated in experiments 2 and 3.

4 Experiment 2In experiment 2, we replicated and extended the results of experiment 1 by testing bothcolor onsets and color offsets, and by adding an all-grey-scale no-transient condition.The color transient effect in experiment 1 was large, but it is possible that it occurredbecause subjects found it relatively difficult to search and/or parse the grey-scale images.Therefore, we added the all-grey-scale no-transient condition to provide a comparisonwith an all-color non-transient condition. If the grey-scale images were simply moredifficult to process, then there should be a large reduction in change detection rela-tive to color scenes when they are presented under no-transient conditions as well.In addition, we added a color-offset condition to determine whether onsets are specialin attracting attention away from the change, or if offsets will serve the same purpose.

4.1 Method4.1.1 Participants.Thirty-seven undergraduate students from Kent State University partic-ipated in the experiment. All were enrolled in General Psychology, and none of themhad previous experience with other CB experiments. Three participants were excludedbecause they did not follow instructions when making their responses. The analyseswere conducted with the remaining thirty-four participants.

4.1.2 Apparatus and stimuli. Stimuli were similar to those used in experiment 1, withthe exception that a set of 8-bit grey-scale post-change scenes was created to allowpresentation of color offsets.

4.1.3 Design and procedure. Procedures were similar to those followed in experiment 1,with the exception that a color-offset condition and an all-grey-scale condition were

Color and luminance changes 1671

added, and the flicker conditions were eliminated. Experiment 2 was therefore a 262cross of pre-change scene color (black-and-white versus color) and post-change scenecolor (black-and-white versus color). The four conditions were: (i) color onset: grey-scalepre-change scene to color post-change scene; (ii) color offset: color pre-change scene togrey-scale post-change scene; (iii) no transient color: color pre-change scene to color post-change scene; (iv) no transient grey-scale: grey-scale pre-change scene to grey-scalepost-change scene. For all four conditions, the pre-change scene was presented for2000 ms, followed immediately by the post-change scene. In experiment 2, participantswere again asked the question ` Did you see anything change? Press the `Y' key if youdid, or the `N' key if you did not''. However, the question appeared 2000 ms after thepost-change scene appeared, instead of simultaneously as in experiment 1. For all fourconditions, the post-change scene was present until the participant made a response andwas ready to move on to the next trial.

Participants were given a more detailed set of instructions for experiment 2. Partic-ipants were again instructed that there would be a change on every trial, that theirjob would be to detect changes in a variety of scenes, and that switching the imagefrom black-and-white to color, or from color to black-and-white, did not count as achange in the scene. In addition, participants were given some examples of possiblechanges to a post-change scene, such as additions, deletions, size, and/or changes tothe location of an object, objects, or parts of an object. These examples were providedto reinforce that changes in overall color did not count as a change. Participants werealso instructed that there would not be any changes in color to the object, objects, orparts of an object in a post-change scene. They were told that there would be trialswhere: (i) both the pre-change and the post-change scenes would be in color; (ii) boththe pre-change and post-change scenes would be in black-and-white; (iii) the pre-changescene would be in black-and-white and the post-change scene would be in color; and(iv) the pre-change scene would be in color and the post-change scene would be inblack-and-white. Change detection was measured in the same way as in experiment 1.

4.2 Results4.2.1 Detected changes. Two independent raters scored the response sheets from thisexperiment by means of the same criteria as those employed in experiment 1. For thecolor-onset condition, changes were detected on 40% (SD � 20%) of the trials. Forthe color-offset condition, changes were detected on 49% (SD � 20%) of the trials.For the no-transient-color condition, changes were detected on 83% (SD � 13%) ofthe trials. For the no-transient-grey-scale condition, changes were detected on 79%(SD � 19%) of the trials. The pattern of results for experiment 2 is similar, even whenall positive responses are counted as hits: color onset (M � 51%, SD � 23%); coloroffset (M � 66%, SD � 23%); no transient color (M � 89%, SD � 13%); no transientgrey-scale (M � 88%, SD � 14%). A 262 within-subjects ANOVA was conducted withpre-change scene color (black-and-white versus color) and post-change scene color(black-and-white versus color) as factors (note that this ANOVA is possible becausethere were no flicker conditions, so the transients were embedded in a 262 cross ofthe presence of color in the pre-change scene and the post-change scene). The maineffect for the color of the pre-change scene was significant (F1 33 � 4:452, MSE � 3%,p 5 0:05), but the main effect for the color of the post-change scene was not(F1 33 � 1:848, MSE � 1:6%, ns). The main effect for the pre-change scene color reflectsbetter change detection when the pre-change scene was in color (66% detection) thanwhen it was in grey-scale (60% detection). The interaction between pre-change scene colorand post-change scene color was significant (F1 33 � 131:691, MSE � 3:4%, p 5 0:001;see figure 2). Pairwise comparisons revealed that both color onsets and offsets disruptedchange detection relative to no-transient conditions. Change detection in the color-onset

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1672 J G Arrington, D T Levin, D A Varakin

condition was significantly worse than in the no-transient-grey-scale condition (t33 � 9:862,p 5 0:001), and change detection in the color-offset condition was worse than in theno-transient-color condition (t33 � 9:082, p 5 0:001). Change detection in the color-offsetcondition was better than in the color-onset condition (t33 � 9:082, p 5 0:012), and therewas no difference between the no-transient conditions (t33 5 1, ns). All comparisonsbetween color-transient conditions and no-transient conditions in experiments 1 ^ 3 aresignificant with Bonferroni a posteriori tests.

4.3 DiscussionRelative to both the no-transient-grey-scale baseline and to the no-transient-color base-line, both the onset and the offset conditions reduced change detection by at least 30%.Although there was an attenuation of change detection when the pre-change scene wasin grey-scale, the effect was much smaller (6%). Therefore, it is clear that our colortransients caused CB, and that the effect cannot be attributed solely to the difficulty ofprocessing the grey-scale images.

We should note that overall the base rate of change detection in this experimentdoes appear to be higher than that in experiment 1. This could have been caused bydelaying the response cue, but we are hesitant to make between-experiment comparisonsbecause the difference is relatively small, and not the main focus of this experiment.Thus, differences in subject populations recruited at different times during the semestercould have easily caused this. In addition, we point out that the rate of change detectionin the flicker-only condition of experiment 3 was just as low as that in experiment 1,despite the delayed response cue in that experiment.

5 Experiment 3In experiments 1 and 2 we have argued that color onsets and offsets may be part ofa relatively broad set of visual transients capable of causing CB. In experiment 3, wetest whether luminance changes can also cause CB. Clearly, changes in luminance con-stitute a large part of the signal for many changes, and much research has explored visualchannels dedicated to processing sudden changes in luminance (eg the magnocellularpathway; for a review see Shapley 1995). Furthermore, early research on attentionalcapture used changes in the luminance of an existing object as a cue (Posner et al 1980),and in certain circumstances sudden luminance changes may be even more powerfulin attracting attention than color changes (Theeuwes 1995; but see Gellatly et al 1999).Therefore, luminance changes would also appear to be a good candidate to mask changes.In experiment 3, we directly compared the degree to which luminance changes and flickerscause CB in color images.

5.1 Method5.1.1 Participants. Twenty-four undergraduate students from Kent State University partic-ipated in this experiment. Sixteen of the students were enrolled in General Psychologyand received extra course credit for their participation. The remaining eight studentssigned up through flyers and were paid US$5.00 for their participation. None of thetwenty-four students had previous experience with other CB experiments. One participantwas excluded because he/she did not follow the instructions when making his/her responses.The analyses were conducted with the remaining twenty-three participants.

5.1.2 Apparatus and stimuli. Stimuli were similar to those in experiment 1 and experi-ment 2, with the exception that sets of bright color pre-change and post-change sceneswere created by increasing the luminance by 11.7%. This level of adjustment wasselected to avoid significant loss of highlight detail. As in experiment 1, the luminancedifference between the versions of each scene was also measured with a luminance meter.The mean difference was 9.80 cd mÿ2 (SD � 3:30 cd mÿ2).

Color and luminance changes 1673

5.1.3 Design and procedure.The procedures were similar to those used in experiment 1 andexperiment 2, with the exception that a luminance-increase condition and a luminance-decrease condition were added, both without a flicker. All scenes were presentedin color. Therefore, the four conditions were: (i) luminance increase: color pre-changescene to color post-change scene with a luminance increase; (ii) luminance decrease:color pre-change scene with a luminance increase to color post-change scene;(iii) flicker only: color pre-change scene to color post-change scene, separated by aflicker; (iv) no transient: color pre-change scene to color post-change scene, withouta flicker. For condition (iii), the pre-change scene was presented for 2000 ms, followedby a 100 ms `flicker' to mask the change, and then by the post-change scene. Forconditions (i), (ii), and (iv), without a flicker between, the post-change scene imme-diately replaced the pre-change scene. As with experiment 2, participants were againasked the question ``Did you see anything change? Press the `Y' key if you did, or the`N' key if you did not''. However, the question appeared 2000 ms after the post-changescene appeared, instead of simultaneously as in experiment 1. For all four conditions,the post-change scene was present until the participant made a response and was readyto move on to the next trial.

Participants were given a set of instructions for experiment 3 that were very similarto those in experiment 2. Participants were again given examples of the kinds ofchanges that would occur to a post-change scene, and were explicitly informed thatswitching the brightness of the image did not count as a change in the scene. Theywere instructed that there would be trials where: (i) the pre-change scene would bebrighter than the post-change scene; (ii) the post-change scene would be brighter thanthe pre-change scene; and (iii) both the pre-change and the post-change scenes would beof equal brightness. Change detection was measured in the same way as in experiments 1and 2.

5.2 Results5.2.1 Detected changes. Two independent raters scored the response sheets from this exper-iment by using the same criteria as those employed in experiment 1 and experiment 2.For the luminance-increase condition, changes were detected on 59% (SD � 17%)of the trials. For the luminance-decrease condition, changes were detected on 51%(SD � 16%) of the trials For the flicker-only condition, changes were detected on14% (SD � 11%) of the trials. For the no-transient condition, changes were detectedon 85% (SD � 13%) of the trials. The pattern of results for experiment 3 is similar,even when all positive responses are counted as hits: luminance increase (M � 64%,SD � 20%); luminance decrease (M � 57%, SD � 18%); flicker only (M � 26%,SD � 16%); no transient (M � 86%, SD � 13%). A within-subjects one-way ANOVA ofthe four conditions was significant (F3 66 � 108:64, MSE � 1:8, p 5 0:001; see figure 2).Pairwise comparisons revealed that change detection was worse in the flicker conditionrelative to luminance-increase (t22 � 9:898, p 5 0:001), and luminance-decrease condi-tions (t22 � 8:770, p 5 0:001). However, change detection was better in the no-transientcondition relative to the luminance-increase (t22 � 8:321, p 5 0:001) and luminance-decrease conditions (t22 � 8:853, p 5 0:001). There was also a difference between theluminance-increase and luminance-decrease conditions (t22 � 2:091, p � 0:048), reflectingbetter change detection in the luminance-increase condition.

5.3 DiscussionLuminance increases and luminance decreases clearly increased the prevalence of CB,but not as much as a flicker. The luminance increases and luminance decreasesreduced change detection by 26% and 34%, respectively, relative to the no-transientbaseline, whereas the more traditional flickers reduced change detection by 71%.

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1674 J G Arrington, D T Levin, D A Varakin

Given that luminance transients clearly produced CB, we need to look back atthe results from experiments 1 and 2 to ensure that the effects of color changes werenot entirely due to luminance transients. Although we confirmed that the overallluminance was matched between the color and grey-scale versions of the scenes, thisdoes not ensure that all of the regions in each scene were luminance matched.Indeed, it would be extraordinarily difficult to demonstrate that color and grey-scaleversions of natural scenes were matched for luminance throughout. Therefore, insteadof attempting to equalise the stimuli, we sought to determine the degree to whichdifferent scenes are more or less luminance equalized, and then correlated this withthe change-detection results. To do this, we assessed the amount of rapid flickerpresent in each scene when the color and grey-scale versions of the scene were alter-nated at 15 Hz. Under these circumstances, equiluminant regions should not appearto flicker and instead should look less saturated (for review see Livingstone andHubel 1988).

An informal review of the flicking scenes verified that many regions in each scenedid not flicker, although some did. Further, scenes did vary with regard to the propor-tion of each that had strong flicker (the strength of the flicker also appeared to vary).Therefore, we asked a total of seven judges (including the three authors) to rate eachof the 32 scenes for flicker (both for grey-scale ^ color transients, and for luminancetransients) on a 7-point scale ranging from ` no flicker'' (rating of 1) to ` maximumflicker'' (rating of 7). The mean between-judge rating correlation was 0.500, and alljudges except one were significantly correlated with all other judges. Without the judgewho showed non-significant correlations (4 of 7 of this judge's correlations werenon-significant), the mean intercorrelation was 0.587. Averaged ratings for each ofthe 32 images from the remaining six judges were uncorrelated with change detectionfor color changes for each of these images (r � ÿ0:045), and were also uncorrelatedwith the difference in change detection between color changes and no-transient changesfor each scene (r � 0:052). Change detection for color changes was correlated withchange detection without transients (r � 0:579, p 5 0:01), supporting the validity ofthe change-detection scores for each scene. (Note that this comparison contrastsperformance across different subjects because a given scene was in the color-changecondition for some subjects and in the no-transient condition for others.)

These results suggest that luminance changes were not solely responsible for reducedchange detection in experiments 1 and 2, and reinforce the hypothesis that a relativelybroad category of transients can induce CB. This conclusion is further reinforced bythe fact that the luminance changes did not overall produce more CB (when the resultsof experiment 3 are compared with those of experiments 1 and 2) than the color changes,despite the fact that the luminance changes produced much stronger flickering in thecontrol experiment (judges rated the luminance-induced flicker to be stronger thanthe color-induced flicker of 31 or the 32 scenes).

6 General discussionIn the three experiments we consistently observed that luminance and color transientsresulted in a 26% ^ 39% reduction in change detection relative to a no-transient base-line. In experiment 1, a color onset reduced change detection by 33% relative to ano-transient baseline, whereas a flicker reduced change detection by 51%^ 54%.Similarly, in experiment 2, color onsets and offsets reduced change detection by30% ^ 39% relative to the no-transient baseline. Finally, in experiment 3, luminancetransients produced a similar effect, reducing change detection by 26%^ 34% relative tothe no-transient baseline.

These findings clearly demonstrate that substantial CB can be induced by a broadrange of transients, even by those that do not involve the appearance of new objects,

Color and luminance changes 1675

or changes to object forms. However, it is important to note that the transients wetested were not as powerful as flickers in masking changes. This stands in contrast tothe results of Turatto et al (2003), who found that contrast inversion and flicker causedequivalent levels of CB. There are at least two possible reasons for this difference. First,contrast inversion might be more powerful than either luminance or color transientsin producing CB. Alternatively, the difference might be accounted for by differences instimulus presentation between the studies. Turatto et al's contrast inversions actuallyintroduced two transients before each change. In their paradigm, the contrast-invertedimage replaced each flicker such that the original image was replaced by a contrast-inverted image with no change for 100 ms, which in turn was replaced by a changedimage with normal contrast relations. Thus, each change was associated with twocontrast reversals (positive! negative and negative! positive). This is similar to theflicker paradigm in which each change is preceded by a disappearance and occurssimultaneously with a re-appearance. In contrast, the method we chose involved only asingle transient, simultaneous with the change.

Our single transients might have produced less CB than Turatto et al's two transientsfor two reasons. The most intuitive hypothesis is that two transients summate andtogether are more powerful than a single transient. Another possibility is that the tran-sients are most powerful in blocking change detection when they precede the changeby 80 ^ 100 ms. This might occur if awareness of the transient momentarily blocks anattentional bottleneck while the change might have been perceived or elaboratedupon. Accordingly, the offset transient in the flicker paradigm, or the positive ^ negativetransient in Turatto et al's experiment, would be the primary cause of CB. Recentfindings do suggest that a pre-change transient [in this case the onset of new objectssimilar to O'Regan et al's (1999) mudsplash paradigm] can be powerful when subjectsmust identify the change by indicating the relative orientation of a rotated Gabor patchbut not when they only locate the change (Watanabe 2003). However, even in thiscase, the transient is not more powerful than one simultaneous to the change and it isnot clear whether the current paradigm is more comparable to Watanabe's locationcondition or to change-identification conditions.

More generally, these results demonstrate that any of a broad category of transientsis sufficient to induce CB. Accordingly, sudden appearances of objects, or transformationsof form, are not unique in producing transients sufficient to block change detection.Most likely, any transient that either leads attention away from a change transient, orthat reduces its salience, will be effective, and researchers have recently speculated thatany of a variety of salient changes to objects may attract attention (Cole et al 2005).The ability of a relatively broad category of transients to capture attention may have beenenhanced by the natural scene stimuli and changes we used. These required subjects tofocus their attention on a relatively broad range of subtle form, texture, and orientationchannels that may overlap with the color channels that would be activated by suddenchanges in the presence of color information. Thus, the more general debate over therelationship between attentional focus and attentional capture may have another dimen-sionöthe breadth of attention across features that a given task setting requires. This kind ofvariation is particularly salient in natural change-detection tasks for which the target isnot pre-specified on a given dimension but rather is a change to many of the dimensionsof natural variation in a scene.

Acknowledgments. We would like to thank Bonnie Angelone for helpful comments on earlierversions of this manuscript, along with Triniti Anderson, Kate Barker, and Jessica O'Dea forassisting in the running and scoring of the experiments. This research was supported by NSFgrant #SES-0214969 to DTL.

1676 J G Arrington, D T Levin, D A Varakin

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