Attentional and oculomotor capture by onset, luminance and colorsingletons
David E. Irwin *, Angela M. Colcombe, Arthur F. Kramer, Sowon HahnDepartment of Psychology, The Beckman Institute, Uni6ersity of Illinois, 603 E. Daniel St., Champaign, IL 61820, USA
Received 3 May 1999; received in revised form 4 January 2000
Keywords: Attention; Saccadic eye movements; Visual search; Reaction time
www.elsevier.com/locate/visres
1. Introduction
A key issue in attention research concerns the extentto which novel but irrelevant stimulus events involun-tarily capture attention. For example, Yantis and col-leagues (e.g. Yantis & Jonides, 1984, 1990; Yantis &Hillstrom, 1994) have conducted a number of visualsearch studies in which they have found that stimuluscharacters that appear abruptly (i.e. as sudden onsets)in a display are processed first, even if they are no morelikely to be the target of the search than any othercharacter in the display. This suggests that abruptonsets (or new objects, since abrupt onsets signal theappearance of new objects) capture attention. Similarresults were obtained by Theeuwes (1994), who hadsubjects search for a uniquely-colored item (i.e. a colorsingleton target) among other items in a display. Onsome trials a new item abruptly appeared in the displayat the same time as the color change which defined thelocation of the color singleton target. Although theabruptly-appearing new item never served as the target,search performance was slower on trials in which it was
present than on trials in which it was absent. Theeuwes(1994) suggested that this was the result of the abruptonset capturing attention which subsequently needed tobe reoriented to the color singleton target. Perhapsmost impressively, Remington, Johnston and Yantis(1992) found that visual search was slowed by thepresentation of an abrupt onset even when subjectswere told that onsets should be ignored because theynever cued the location of the target.
Although these findings suggest that novel items in adisplay capture attention in an involuntary or obliga-tory fashion, there is controversy regarding the general-ity of the effect and the nature of the processinginvolved. For example, some researchers have arguedthat attentional capture is determined solely by stimulussalience, with little or no contribution from top-down(or conceptually-driven) factors (e.g. Koch & Ullman,1985; Theeuwes, 1991, 1992, 1994, 1996). Others, how-ever, have argued that attentional capture is limited toonly some stimulus properties, such as abrupt visualonsets that define the presence of new objects in theenvironment (e.g. Yantis, 1993, 1996; Yantis & Hill-strom, 1994). Some researchers have argued that atten-tional capture is not purely stimulus-driven, however,but rather is contingent on conceptually-driven (top-
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down) attentional control settings (e.g. Folk, Reming-ton & Johnston, 1992, 1993; Folk & Annett, 1994;Folk, Remington & Wright, 1994; Folk & Remington,1996). According to this account, unique or novel itemsin a display will capture attention only if they areconsistent with the subject’s search goals. For example,Folk et al. (1992) found that abrupt onset stimulicaptured attention when subjects were searching forabrupt onset targets but not when they were searchingfor color targets, and Folk and Remington (1998)found that an irrelevant color singleton captured atten-tion only when it was the same color as the searchtarget. This ‘attentional control setting’ hypothesisseems inconsistent with the findings reviewed abovethat suggest that irrelevant abrupt onsets capture atten-tion, but Folk and Remington (1998) suggest that thosefindings might be due to ‘filtering costs’ or generaldistraction effects rather than to shifts of spatialattention.
Because filtering costs and shifts of spatial attentionare covert in nature, it is difficult to discriminate be-tween these various hypotheses based only on reactiontime (RT) and accuracy data. We have recently con-ducted several eye movement studies which provideadditional evidence regarding this issue, however; theresults of these studies indicate that irrelevant onsetsnot only capture covert attention, but they captureovert attention (i.e. the eyes) as well.
In our first study (Theeuwes, Kramer, Hahn & Irwin,1998), subjects viewed displays containing six gray cir-cles (3.7° in diameter) spaced equally around an imagi-nary circle whose radius was 12.6°. Centered withineach circle was a small (0.4×0.2°) figure-eight pre-mask. After 1 s, all of the circles except for one changedcolor (from gray to red) and the premasks inside thecircles were converted to small letters by removingsome of their line segments. Subjects were instructed tomove their eyes to the remaining gray circle (a colorsingleton) and to determine whether the letter inside itwas a C or a reversed C. They pressed one of twobuttons to indicate their response, and their responselatency and accuracy were recorded. Because the targetletter was so small, it could be identified accurately onlyif it was foveated. On half of the trials, an additionalred circle (an abrupt onset or new object) was added tothe display at the same time as the color singleton andthe stimulus letters were revealed. This onset stimulusalso contained a small letter, but it was never the target;thus, it was irrelevant to the subject’s task. Despite this,on nearly half the trials subjects made a saccade to-wards the new object before moving their eyes to thecolor singleton; in other words, the eyes were capturedby the appearance of a sudden onset in the display eventhough subjects intended to move their eyes to the colorsingleton. This pattern of results was obtained regard-less of whether the onset distractor appeared close to
the target or on the opposite side of the visual display.Fixations on the new object were very brief (median=100 ms), even though a complete change in the direc-tion of the eye movement was required to redirect theeyes toward the color singleton (see McPeek, Skavenski& Nakayama, 1996, for similar results). Furthermore,subjects were generally unaware that onsets (new ob-jects) were presented on half of the trials and theyreported being completely unaware of making saccadesto them, even though they did so on half of the trials.Other experiments showed that saccades to the newobject were eliminated if the location of the colorsingleton was precued for 400–600 ms prior to thecolor change which defined the target location; delayingthe presentation of the new object so that it appeared150 ms after the color change also eliminated saccadesto the new object (Theeuwes, Kramer, Hahn, Irwin &Zelinsky, 1999); and making the onsets more salient (sothat subjects were more aware of their presence) re-duced the number of saccades that young subjects madeto the new object (Kramer, Hahn, Irwin & Theeuwes,in press).
We interpreted these findings as arising from theparallel programming of two saccades: one voluntary,goal-directed eye movement toward the color-singletontarget and one stimulus-driven eye movement reflex-ively and unconsciously elicited by the appearance ofthe task-irrelevant new object. Depending on the finish-ing times of the two eye movement programs, the eyesmoved either toward the target or toward the onset;fixations on the onset were brief if the program to thetarget finished a short time after the program to theonset. Making the onsets more salient increased sub-jects’ ability to consciously inhibit saccades to them. Insum, these results suggest that the effects of abruptonsets or new objects on task performance are not duesimply to filtering costs, as attentional control settingproponents might argue, but rather are due to thecapture of overt (i.e. the oculomotor system) and covertspatial attention.
Our studies to date have investigated only the effectsof abrupt onsets (new objects) on goal-directed move-ments of the eyes. In the present paper we report threeexperiments that investigated the effects of other kindsof distractors on task performance, to see whetherattentional (and oculomotor) capture occur only whenabrupt onsets which define new objects are presented,as Yantis and colleagues have hypothesized, or whetherother salient stimuli also capture attention and the eyeseven when they do not constitute new objects.
2. Experiment 1
Experiment 1 compared search performance in twocomplementary target/distractor conditions. The first
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condition was similar to that used by Theeuwes et al.(1998): Subjects were instructed to identify a targetcharacter that appeared inside a color singleton stimu-lus while an irrelevant onset distractor was presentedon some trials. In the second condition subjects wereinstructed to identify a target character that appearedinside an onset stimulus while an irrelevant color single-ton distractor was presented on some trials. The firstcondition was essentially a replication of Theeuwes etal. (1998), while the second condition allowed a deter-mination of whether irrelevant color singleton distrac-tors also capture attention and the eyes.
2.1. Methods
2.1.1. SubjectsThe subjects in all three experiments ranged in age
from 18 to 30 years and they were paid for theirparticipation. All had normal visual acuity as measuredby Snellen charts and normal color vision as measuredby the Ishihara Color Blindness Test. Nine individuals(six female, three male) participated in experiment 1.
2.1.2. ApparatusA Gateway Pentium 133 MHz computer with a 19
inch SVGA color monitor was used to present thestimuli, control the timing of the experimental events,and record subjects’ reaction times. Eye position wasrecorded with an Eyelink eye tracker (SR ResearchLtd.) with 250 Hz temporal resolution and 0.2° spatialresolution. The system uses infrared video-based tech-nology to compute the center and size of the pupils inboth eyes and an infrared head motion system to trackhead position. This apparatus was used in all threeexperiments.
2.1.3. StimuliEach subject completed an experimental session in
which the target character was presented in a uniquely-colored stimulus (color target, onset distractor condi-tion) and an experimental session in which the targetcharacter was presented in a sudden-onset stimulus(onset target, color distractor condition). Half of thetrials in each session were control trials in which nosudden onset distractor or no uniquely-colored distrac-tor appeared. The properties of the stimuli in thesedifferent experimental and control conditions are de-scribed next.
2.1.3.1. Color target, onset distractor condition. Eachtrial began with the presentation of a visual display thatcontained a central fixation mark (a 0.3×0.3° graystar) and either four (on distractor trials) or five (oncontrol trials) gray circles (see Fig. 1). Four of thecircles always appeared at clock positions 1, 5, 7 and11; on control trials the position of the fifth circle wasdetermined in a manner described below. The graycircles were 2.5° in diameter and they were presented onan imaginary circle with a radius of 8.9°. Each graycircle contained a small (0.3×0.2°) gray figure-8 pre-mask. After 1500 ms the central fixation star changedinto a cross, all of the circles but one changed to red,and line segments were removed from the figure-8premasks to reveal target and distractor letters. Thesubject’s task was to determine whether the letter insidethe remaining gray circle was either a C or a reversedC. Because the letters were very small, subjects had tomake a saccade to the gray circle in order to identifythe target letter (this was confirmed in pilot testing).The subjects responded by pressing the ‘z’ or ‘/’ key onthe computer keyboard. The letters inside the red circleswere distractor letters randomly sampled without re-placement from the set S, H, E, P, F and U. The redand gray circles were matched for luminance (24 cd/m2). The stimuli remained present until a response wasmade by the subject.
On distractor trials, an additional red circle (a sud-den onset) with a distractor letter inside was added tothe display simultaneously with the color change whichdefined the color singleton target. The additional redcircle appeared abruptly at one of two possible dis-tances from the color singleton target, either three clockpositions away (describing an angle of 90° of arc) orfive clock positions away (describing an angle of 150°of arc). In Euclidean terms, these distances corre-sponded to 12.3 and 17.4° of visual angle, respectively.On control trials (i.e. those in which the trial startedwith five circles on the display) the ‘extra’ circle likewiseappeared either 90 or 150° away from the color single-ton target. Of interest was whether the subject’s oculo-motor behavior and reaction time (RT) to identify thetarget letter inside the color singleton would be differ-
Fig. 1. Graphic illustration of the temporal sequence of displayspresented in the color target conditions of experiment 1. Gray circlesare represented by dashed lines and red circles are represented bysolid lines.
D.E. Irwin et al. / Vision Research 40 (2000) 1443–14581446
Fig. 2. Graphic illustration of the temporal sequence of displayspresented in the onset target conditions of experiment 1. Gray circlesare represented by dashed lines and red circles are represented bysolid lines.
not to make any large head movements. The infraredsource and the eye camera were adjusted until there wasa clear corneal reflection in both eyes. After setting thethreshold for detecting the pupil, the Eyelink systemwas calibrated. Subjects fixated nine calibration targetsthat were presented serially in a 3×3 grid in a randomorder across the monitor. Once the calibration proce-dure was successfully completed the experiment began.
Each subject completed two experimental sessionslasting 1 h each. One session corresponded to the colortarget, onset distractor condition described above whilethe other session corresponded to the onset target,color distractor condition. Order was balanced acrosssubjects. Each session consisted of one practice andfour experimental blocks of 64 trials each (32 controland 32 distractors trials in each block). Distractordistance and trial type were sequenced randomly acrosstrials. Subjects began each trial by fixating a centralfixation mark, and pressed any key on the computerkeyboard to initiate the trial. On each trial the eyeposition was automatically recalibrated to the centerposition so that reliable eye movement measurementscould be made. Subjects were instructed to move theireyes to the appropriate target circle as soon as the colorchange occurred, and to respond to the C or reversed Cby pressing the appropriate response key. Subjects wereprovided with feedback regarding their accuracy oneach trial, and feedback regarding speed and accuracyof responding following each block of trials.
2.2. Results
Preliminary analyses indicated that the order inwhich the subjects completed the two tasks had noeffect on the results. Thus, order is not included in theanalyses reported below. To increase the sample size ineach cell of the design, distractor distance was notincluded as a factor either. Trial data were excludedfrom analysis if a manual response error was made(2.4% of all trials), if the manual RT was less than 100ms or greater than 1500 ms (1.6% of all trials), or if aneye movement artifact occurred (8.7% of all trials).
2.2.1. Manual reaction timeThe mean RTs to make the C versus reversed C
judgment are presented in Table 1 as a function oftarget type (color vs onset) and condition (distractorpresent vs control). When the target appeared in anonset stimulus, manual RT was statistically the sameregardless of whether a color singleton distractor waspresent or absent. However, when the target appearedin a color singleton stimulus, manual RT was slowerwhen an onset distractor was present than when it wasabsent. In other words, a task-irrelevant sudden onsetdisrupted task performance but a task-irrelevant colorsingleton did not.
ent when an irrelevant sudden onset appeared in thedisplay compared to when it did not.
2.1.3.2. Onset target, color distractor condition. Thedimensions of the display items were the same as in thecolor target, onset distractor condition, but the se-quence of events and subject instructions were some-what different (see Fig. 2). On each trial four graycircles (each containing a small gray figure-8 premask)were presented at clock positions 1, 5, 7 and 11. After1500 ms, on half of the trials all but one of the circleschanged to red (distractor trials) while on the other halfof the trials all of the circles changed to red (controltrials); on all trials, line segments were removed fromthe figure-8 premasks when the circles changed color toreveal distractor letters. Simultaneously, on all trials anew red circle (sudden onset) containing the targetstimulus was presented. The subject’s task was to deter-mine whether the target stimulus inside the suddenonset was a C or a reversed C. The sudden onsetappeared either 90 or 150° of arc away from the colorsingleton distractor (i.e. the remaining gray circle) ondistractor trials or from another red circle on controltrials. Of interest was whether the subject’s oculomotorbehavior and reaction time (RT) to identify the targetletter inside the onset stimulus would be different whenthe gray circle remained in the display (distractor trials)compared to when all circles were the same color(control trials).
2.1.4. ProcedureThe same procedure was followed in all three experi-
ments. Before starting an experiment, the head band ofthe Eyelink tracker with the infrared light source andcamera were strapped tightly on the subject’s head. Achin-rest was used to stabilize the head at a distance of80 cm from the display monitor. Subjects were asked
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An additional analysis was performed to examine theinfluence of initial scan path on manual RT. Subjectsfrequently misfixated the onset distractor when thecolor singleton was the intended target, but they almostnever misfixated the color distractor when the onsetstimulus was the intended target (see Section 2.2.2.1).The target letter was so small that it could be identifiedonly when it was foveated; thus, it was of interest todetermine whether the RT increase found above is dueentirely to the eyes going first toward the onset distrac-tor on some trials before arriving at the color singletontarget. To that end, the RT data for the color singletontarget trials were sorted on the basis of whether the eyeswent directly to the color singleton target or insteadwent first toward the onset distractor. The difference inmanual RT between the no-distractor control conditionand the distractor-present trials in which the eyes wentdirectly to the color-singleton target was sizeable (37ms) but not statistically significant. Manual RT wasstatistically slower when the eyes went toward the onsetdistractor before going to the color singleton targetcompared to when the eyes went directly to the colorsingleton target, however (see Table 2). Thus, the effectof the onset distractor on manual RT was largely dueto those trials in which the eyes were drawn to theonset, but there is some suggestion that the onsetdistractor slowed manual RT even when the eyes wentdirectly to the target.
2.2.2. Oculomotor beha6iorTo obtain additional information about the effects of
onset and color distractors on task performance, we
examined three aspects of oculomotor behavior: Sac-cade path, saccade latency, and fixation duration fol-lowing the first saccade.
2.2.2.1. Saccade path. Three thresholds were used forsaccade detection: Movement distance, velocity, andacceleration. An eye movement was considered a sac-cade either when the movement distance exceeded 0.2°and velocity exceeded 30 deg/s, or when the movementdistance exceeded 0.2° and the acceleration exceeded8000 deg/s. Saccade paths toward the onset weredefined as eye movements that moved from centralfixation to within 30° of arc toward the left or the rightof the onset (i.e. within a 60° cone which extended fromfixation to the onset, centered on the onset). Saccadepaths to the target were defined using the samecriterion.
Fig. 3 presents information about the direction of theinitial saccade as a function of target type, condition,and distractor location. This figure shows the distribu-tions of the angular deviation of the initial saccadefrom the center of the relevant target (i.e. the colorsingleton target or the onset target) in the control(no-distractor) condition and separated in the distractorcondition by whether the distractor appeared 90 or 150°of arc away from the target. Fig. 3 shows that in thecontrol condition subjects’ initial saccades generallymove directly towards the target. This was true foronset targets when color distractors were present aswell (i.e. the three panels on the right side of the figurelook very similar). In contrast, a very different patternis present when the color singleton was the target and
Table 1Results of experiment 1 by target type (color vs onset) and condition (distractor present vs distractor absent)a
Onset targetColor target
Onset Singleton
AbsentPresent PtAbsentPresentPt
901 850 3.28Manual RT (ms) for all correct trials 0.011 745 765 −2.08 0.0710.046−2.360.0612.18846 764883 745Manual RT (ms) when eyes went directly to target
1.2 0.8%Saccade path (% saccades toward distractor/extra stimulus) 0.8419.4 0.4263.9 5.84 B0.001212Saccade latency (ms) when eyes went directly to target 217301 −1.71 0.126284 2.72 0.026
a The t-values are the results of two-tailed paired t-tests (df=8) for distractor present versus distractor absent trials.
Table 2Results of experiment 1 for trials in which the eyes went directly to the color target compared to trials in which the eyes went to the onsetdistractor before going to the color targeta
PTo target To distractor t
883Manual RT (ms) for all correct trials 963 −2.38 0.0440.0063.76247Saccade latency (ms) 301
197 123First fixation duration (ms) 9.20 B0.001
a The t-values are the results of two-tailed paired t-tests (df=8).
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Fig. 3. Histograms for the control and distractor conditions for color targets and for onset targets illustrating the maximal angular deviation froma straight line path from fixation to the position of the target on each of the initial saccades in experiment 1, averaged across all subjects.
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Fig. 4. Fixation durations (ms) after the first saccade for those saccades that went toward the onset distractor in experiment 1.
an onset distractor was presented. In those cases, afairly substantial number of saccades initially wenttoward the onset distractor, as can be seen by theclump of fixations around 90° in the middle panel ofthe left column and the clump of fixations around 150°in the bottom panel of the left column.
We quantified these observations by calculating thepercentage of trials on which the eyes went initiallytowards the distractor in each condition. The results areshown in Table 1. When the target was the colorsingleton and an onset distractor was present, the eyesmoved to the onset distractor on 19.4% of the trials; inthe color singleton control condition, however, the eyesmoved to the location of the ‘extra’ control stimulus ononly 3.9% of the trials. When the target was the onsetstimulus and a color-singleton distractor was present,the eyes moved to the color distractor on only 1.2% ofthe trials; in the onset control condition the eyes movedto the ‘extra’ control stimulus on only 0.8% of thetrials. In sum, a task-irrelevant sudden onset capturedthe eyes quite frequently, but a task-irrelevant colorsingleton did not.
2.2.2.2. Saccade latency. Saccade latency was defined asthe time that it took the eyes to start moving from thecenter fixation mark to one of the peripheral stimuli.The timing began with target presentation (i.e. presen-tation of the onset in the onset target condition orpresentation of the color change which defined thecolor target in the color singleton condition) and endedas soon as the eyes moved away from fixation (i.e. a 1°circular area around the central fixation mark).
Considering only those trials in which the eyesmoved directly to the target, saccade latency to thecolor target was slower when an onset distractor was
present than when one was not; in contrast, there wasno effect of a color singleton distractor on saccadelatency to an onset target. Thus, in addition to captur-ing the eyes on a substantial proportion of the trials,presentation of an onset distractor appears to haveslowed saccade initiation to a color target even whenthe eyes went directly to the color target.
A second analysis examined whether saccade latencydiffered as a function of whether the initial saccadewent to the onset distractor as opposed to the color-sin-gleton target (Table 2). Mean saccade latency was 247ms when the eyes moved to the onset distractor, com-pared to 301 ms when the eyes moved to the target.These data suggest that onsets elicit fast eye move-ments, causing the eyes to go to the onset distractorinstead of to the intended target.
2.2.2.3. Fixation duration following the first saccade. Thedistribution of fixation durations for those trials inwhich the eyes went initially to the onset distractor arepresented in Fig. 4. This figure shows that the greatmajority of the fixations were too brief to enable theprogramming of another saccade to the target (whichtypically takes 150 ms; see Becker, 1991; Findlay, 1997).The mean fixation duration for trials in which the eyeswent initially to the onset distractor was 123 ms; incontrast, mean fixation duration was 197 ms for trialsin which the eyes went directly to the color target(Table 2).
2.3. Discussion
The results of the color target, onset distractor condi-tion are very consistent with those of Theeuwes et al.(1998). Presentation of an irrelevant onset distractor
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captured the eyes on a substantial portion of the tri-als and it slowed manual RT to identify the targetcharacter. This supports the hypothesis that abruptonsets (new objects) capture attention in an involun-tary fashion, regardless of the subject’s top-down at-tentional control settings. Saccades to the onset had ashorter latency than saccades to the target and theduration of the fixation on the onset was generallytoo short to allow for the programming of a newsaccade to the target; this supports the hypothesisthat two eye movements were programmed in paral-lel, a voluntary, goal-directed saccade to the targetand an involuntary, reflexive saccade to the onset.Depending on the finishing times of the two eyemovement programs, the eyes move either toward theonset or directly toward the target.
In contrast, presentation of an irrelevant color sin-gleton distractor when goal-directed saccades weremade to an abrupt onset stimulus had no effect onmanual RT or on any aspect of oculomotor behavior.This finding supports the hypothesis that abrupt on-sets are special in their ability to capture attention;not all unique stimuli are able to attract attention inthe same way. It is possible that the color singletonstimulus that we used in this experiment was notsalient enough to capture attention, however. It wasthe only stimulus in the display that did not change(i.e. the color singleton was defined by changes in thecolors of the other display items), and ‘sustained’stimuli are generally considered to be less salient than‘transient’ ones (e.g. Yantis & Jonides, 1990). In sup-port of this possibility, a comparison of the no-dis-tractor control conditions shows that manual RT tocolor targets (850 ms) was considerably slower thanmanual RT to onset targets (765 ms). Thus, in experi-ment 2 we examined the hypothesis that a moresalient color singleton stimulus, one defined by a
transient color change, would be more effective incapturing attention.
3. Experiment 2
In one condition subjects were instructed to identifya target character that appeared inside a color singletonstimulus (defined by a transient color change) while anirrelevant onset distractor was presented on some trials;in a second condition subjects were instructed toidentify a target character that appeared inside an onsetstimulus while an irrelevant color singleton distractor(defined by a transient color change) was presented onsome trials. It has often been suggested that stimulustransients automatically attract attention (e.g. Posner,1980; Jonides, 1981; Yantis & Jonides, 1984; Muller &Rabbitt, 1989; Nakayama & Mackeben, 1989).Recently, however, some investigators (e.g. Yantis &Hillstrom, 1994) have argued that stimulus transientscapture attention automatically only when they arecreated by the presentation of a new object. Accordingto Yantis and Hillstrom (1994), changing the color ofan existing object does not constitute the creation of a‘new’ object, so by this account transient colorsingletons should be no more effective than sustainedcolor singletons in attracting attention.
3.1. Methods
Fourteen individuals (nine female, five male) partic-ipated in this experiment. In the color target, onsetdistractor condition each trial began with the presen-tation of a visual display that contained a centralfixation star and either four (on distractor trials) orfive (on control trials) red circles (see Fig. 5). After1500 ms the central fixation star changed into a cross,one of the circles changed to gray, and line segmentswere removed from the figure-8 premasks to revealtarget and distractor letters. As in experiment 1, thesubject’s task was to determine whether the letter in-side the gray circle was either a C or a reversed C.On distractor trials, an additional red circle (suddenonset) with a distractor letter inside was added to thedisplay simultaneously with the color change whichdefined the color singleton target. In contrast, in theonset target, color distractor condition four red circleswere presented at the beginning of each trial (see Fig.6). Then, after 1500 ms one of the circles changed togray on half of the trials (distractor trials) while onthe other half of the trials none of the circles changedcolor (control trials). Simultaneously, on all trials anew red circle (sudden onset) containing the targetstimulus was presented. The subject’s task was to de-termine whether the target stimulus inside the suddenonset was a C or a reversed C.
Fig. 5. Graphic illustration of the temporal sequence of displayspresented in the color target conditions of experiment 2. Gray circlesare represented by dashed lines and red circles are represented bysolid lines.
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Fig. 6. Graphic illustration of the temporal sequence of displayspresented in the onset target conditions of experiment 2. Gray circlesare represented by dashed lines and red circles are represented bysolid lines.
3.2.1. Session 1Analyses of the session 1 data provide between-sub-
jects comparisons of performance in the color target,onset distractor condition and the onset target, colordistractor condition, uncontaminated by previous expe-rience with either version of the task. The results areshown in Tables 3 and 4.
Manual RT to make the C versus reversed C judg-ment when it appeared in an onset target was notaffected by the presence of a color singleton distractor.However, when the target appeared in a color singletonstimulus, manual RT was slower when an onset distrac-tor was present than when it was absent. As in experi-ment 1, the onset distractor slowed manual RT evenwhen the eyes went directly to the color singletontarget. Thus, a task-irrelevant sudden onset disruptedtask performance (presumably by capturing attention)but a task-irrelevant color singleton did not. Note thatthe mean RTs in the two control conditions (colortarget, onset absent and onset target, singleton absent)were very similar (743 vs 754 ms), so the differentialeffectiveness of the sudden onset can not be attributedto differences in stimulus salience.
Analyses of the oculomotor behavior showed that atask-irrelevant sudden onset was more likely to capturethe eyes than was an irrelevant color singleton distrac-tor. However, in experiment 1 the onset distractorcaptured the eyes on 19.4% of the trials, as compared toonly 5.7% of the trials in experiment 2; this suggeststhat the transient color singleton target used in experi-ment 2 was more resistant to distraction by an abrupt
3.2. Results
Unlike experiment 1, preliminary analyses indicatedthat the order in which the subjects completed the twotasks had large effects on task performance. Thus, theresults of each experimental session are reported sepa-rately below. The same analyses were conducted as inexperiment 1. Trial data were excluded from analysis ifa manual response error was made (2.4% of all trials),if the manual RT was less than 100 ms or greater than1500 ms (4.3% of all trials), or if an eye movementartifact occurred (11.5% of all trials).
Table 3Results of session 1 of experiment 2 by target type (color vs onset) and condition (distractor present vs distractor absent)a
Color target Onset target
Onset Singleton
Present Absent t PPresent Absent t P
747 754 −0.44 0.674Manual RT (ms) for all correct trials 776 743 7.53 B0.0010.624−0.52754747B0.001Manual RT (ms) when eyes went directly to target 7.37743770
2.48 0.048 3.3 1.6 1.77 0.127Saccade path (% saccades toward distractor/extra stimulus) 0.95.71962040.0054.32233 0.061245 2.31Saccade latency (ms) when eyes went directly to target
a The t-values are the results of two-tailed paired t-tests (df=6) for distractor present versus distractor absent trials.
Table 4Results of session 1 of experiment 2 for trials in which the eyes went directly to the target compared to trials in which the eyes went to thedistractor before going to the targeta
Color target Onset distractor t P Onset target Color distractor t P
0.664786 −0.47Manual RT (ms) for all cor- 7967680.022−3.28877rect trials
a The t-values are the results of two-tailed paired t-tests (df=5 for color target condition; df=4 for onset target condition).
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Table 5Results of session 2 of experiment 2 by target type (color vs onset) and condition (distractor present vs distractor absent)a
Color target Onset target
Onset Singleton
Absent t P PresentPresent Absent t P
Manual RT (ms) for all correct trials 834 766 3.80 0.009 758 705 2.43 0.050Manual RT (ms) when eyes went directly to target 809 766 4.66 0.003 732 705 1.81 0.121
0.3 2.78 0.032 11.021.9 4.1Saccade path (% saccades toward distractor/extra stimulus) 2.10 0.080240Saccade latency (ms) when eyes went directly to target 221 5.56 0.001 209 195 4.03 0.007
a The t-values are the results of two-tailed paired t-tests (df=6) for distractor present versus distractor absent trials.
Table 6Results of session 2 of experiment 2 for trials in which the eyes went directly to the target compared to trials in which the eyes went to thedistractor before going to the targeta
Onset distractor tColor target P Onset target Color distractor t P
Manual RT (ms) for all cor- 809 930 −5.16 0.002 722 929 −5.22 0.003rect trials
a The t-values are the results of two-tailed paired t-tests (df=6 for color target condition; df=5 for onset target condition).
onset than was the sustained color singleton target usedin experiment 1.
Saccade latency to the color target was slower whenan onset distractor was present than when one was not;in contrast, the effect of a color singleton distractor onsaccade latency to an onset target was nonsignificant.Thus, in addition to capturing the eyes on some trials,presentation of an onset distractor slowed saccade ini-tiation to a color target even when the eyes wentdirectly to the color target. Saccade latency differed asa function of whether the initial saccade went to theonset distractor as opposed to the color-singletontarget, however (Table 4). Mean saccade latency was207 ms when the eyes moved to the onset distractor,compared to 245 ms when the eyes moved to the colortarget. As in experiment 1, these data suggest that fasteye movements are more likely to be made to the onsetdistractor. In contrast, there was no difference in meanlatency for the onset target trials in which the eyesmoved to the color singleton distractor as opposed tomoving directly to the target.
As in experiment 1, the great majority of the fixationsmade to onset distractors were too brief to enable theprogramming of another saccade to the target. Themean fixation duration for trials in which the eyes wentinitially to the onset distractor was 93 ms; in contrast,mean fixation duration was 232 ms for trials in whichthe eyes went directly to the color target. This suggestsparallel programming of a voluntary saccade to thecolor target and a reflexive saccade to the onset distrac-tor. A different pattern was apparent in the condition
in which the onset stimulus was the target and fixationswere made on the color singleton distractor, however.In this case, there was no difference in fixation durationbetween trials in which the eyes went initially to thecolor singleton distractor and trials in which the eyeswent directly to the onset target. Thus, it seems unlikelythat parallel programming of two saccades (i.e. one tothe onset target and the other to the color distractor)was occurring in this condition; rather it appears thatsubjects had programmed a single saccade to the‘wrong’ stimulus, the distractor as opposed to thetarget.
3.2.2. Session 2Subjects who participated in the color target, onset
distractor condition in session 1 completed the onsettarget, color distractor condition in session 2 (and viceversa). Thus, analyses of the session 2 data reveal howeffectively subjects were able to ignore a distractorwhich had been the target in the previous experimentalsession. The results are shown in Tables 5 and 6.
When the target appeared in an onset stimulus, man-ual RT was slower when a color singleton distractorwas present than when it was absent. Likewise, whenthe target appeared in a color singleton stimulus, man-ual RT was slower when an onset distractor waspresent than when it was absent. Thus, unlike session 1,both task-irrelevant sudden onsets and task-irrelevantcolor singletons disrupted performance.
The difference in manual RT between the no-distrac-tor control condition and the distractor-present trials in
D.E. Irwin et al. / Vision Research 40 (2000) 1443–1458 1453
which the eyes went directly to the color-singleton wasalso significant, indicating that the onset distractorslowed manual RT even when the eyes went directly tothe color singleton target. In contrast, the difference inmanual RT between the no-distractor control conditionand the distractor-present trials in which the eyes wentdirectly to the onset was not significant. These resultssuggest that the onset distractor captured covert atten-tion even when it did not capture the eyes, but the colorsingleton distractor slowed performance significantlyonly when it captured the eyes.
Analyses of the oculomotor behavior showed thatfrom session 1 to session 2 there was an increase in thenumber of trials in which subjects moved their eyestoward the distractor before fixating the target andexecuting their response. This was true for both thecolor singleton and onset distractors, but the onsetdistractors captured the eyes much more often. Thus, asin the previous analyses, a task-irrelevant sudden onsetwas more likely to capture the eyes than was an irrele-vant color singleton distractor. Apparently it washarder for subjects to ignore an onset distractor thathad been the target in a previous experimental sessionthan it was to ignore a color singleton distractor thathad been the target in a previous experimental session.
Analysis of the trials in which the eyes went directlyto the saccade target showed that saccade latency to thecolor target was slower when an onset distractor waspresent than when one was not; similarly, saccadelatency to an onset target was slower when a colorsingleton distractor was present than when it was ab-sent. Thus, in both conditions, presentation of a dis-tractor appears to have slowed saccade initiation to thetarget even when the eyes went directly to the target.
Saccade latency also differed as a function of whetherthe initial saccade went directly to the target as opposedto toward the distractor (Table 6). Mean saccade la-tency was 207 ms when the eyes moved to the onsetdistractor, compared to 239 ms when the eyes moved tothe color singleton target. In contrast, mean latency forthe onset target trials in which the eyes moved to thecolor singleton distractor was slower compared to whenthe eyes moved directly to the target. This suggests afundamental difference between saccades elicited by anonset distractor as opposed to those elicited by a color-singleton distractor; this is explored further below.
As in session 1, the great majority of the fixationsmade to onset distractors were too brief to enable theprogramming of another saccade to the target. Whenthe onset stimulus was the target and fixations weremade on the color singleton distractor, however, therewas no difference in fixation duration between trials inwhich the eyes went initially to the color singletondistractor and trials in which the eyes went directly tothe onset target. Thus, as in session 1, it seems unlikelythat parallel programming of two saccades (i.e. one to
the onset target and the other to the color distractor)was occurring in this condition; rather it appears thatsubjects had simply programmed a single saccade to the‘wrong’ stimulus, the distractor as opposed to thetarget.
3.3. Discussion
The results of the first experimental session, whichreflects performance uncontaminated by previous expe-rience with either the onset or color target, replicatethose of experiment 1 in almost every respect. Thisprovides further support for the hypothesis that abruptonsets (new objects) sometimes capture attention in aninvoluntary fashion, regardless of the subject’s top-down attentional control settings. The irrelevant onsetdistractor also captured the eyes on a significant num-ber of trials, though less often than in experiment 1.This difference suggests that a transiently-defined colorsingleton is a better saccade target (i.e. it is moreresistant to distraction) than is a color singleton definedby the absence of change. As in experiment 1, saccadesto the onset had a shorter latency than saccades to thetarget and the duration of the fixation on the onset wasgenerally too short to allow for the programming of anew saccade to the target; this supports the hypothesisthat two eye movements were programmed in parallel,a voluntary, goal-directed saccade to the target and aninvoluntary, reflexive saccade to the onset.
The results of the onset target, color distractor condi-tion in session 1 also replicate those of the first experi-ment. Presentation of an irrelevant color singletondistractor when goal-directed saccades were made to anabrupt onset stimulus had no effect on manual RT oron any aspect of oculomotor behavior. This findingprovides further support for the hypothesis that abruptonsets are special in their ability to capture attention;transient color changes apparently do not (see also,Yantis & Jonides, 1984). It seems unlikely that thedifferential ability of abrupt onsets and color transientsto attract attention in this experiment were due todifferences in stimulus salience, because manual RT inthe onset target and color target control conditions wasvery similar.
The results of session 1 are inconsistent with theattentional control setting hypothesis of Folk and col-leagues, because an irrelevant onset captured attentioneven when subjects were set to search for a colorsingleton target. The results are also inconsistent withtheories that claim that attentional capture is deter-mined solely on the basis of stimulus salience (e.g.Koch & Ullman, 1985; Theeuwes, 1991, 1992, 1994,1996), however, because abrupt onsets captured atten-tion and color transients did not, even though theywere matched in terms of stimulus salience. The resultsof session 1 thus seem most consistent with the ‘new
D.E. Irwin et al. / Vision Research 40 (2000) 1443–14581454
object’ account of Yantis and Hillstrom (1994), becauseabrupt onsets (which did constitute new objects in thedisplay) captured attention while transient colorchanges did not.
The results of session 2 show that prior experiencewith a stimulus as a search target increases its ability toattract attention in a subsequent experimental session.McPeek et al. (1996) reported similar priming effectsacross trials in a visual search task (see also, Maljkovic& Nakayama, 1994). In some ways this is reminiscentof the classic findings on automaticity by Schneider andShiffrin (1977) and Shiffrin and Schneider (1977), whofound that when subjects needed to ignore a well-prac-ticed target they had a great deal of difficulty doing so(i.e. when a consistently mapped target became a visualsearch distractor in their paradigm). One difference isthat subjects in the Schneider and Shiffrin studies expe-rienced thousands of trials with their search targets,whereas our subjects experienced only 320. Anotherdifference is that Schneider and Shiffrin did not exam-ine whether some stimulus characteristics (i.e. onsets/new objects) were more powerful than others (i.e. color)in this regard. Our session 2 results show that whensubjects have responded to and fixated onset and colortargets in session 1, it becomes difficult to ignore thesestimuli in session 2 when they are no longer taskrelevant — however, this was especially true for onsetstimuli as opposed to color stimuli. Onset stimuli insession 2 were also much more likely to attract the eyesand the saccades made to onset stimuli exhibited thecharacteristics of reflexive, parallel programming (i.e.short latencies and short fixations afterwards). Colorstimuli in session 2 captured attention (as shown by themanual RT data) but were less likely to capture theeyes and saccades made to them did not exhibit thecharacteristics of parallel programming. The fact thatcolor stimuli could, with practice, become capable ofcapturing attention seems inconsistent with the ‘newobject’ account of attentional capture proposed byYantis and Hillstrom (1994), because changing thecolor of an existing object is not equivalent to creatinga new object.
4. Experiment 3
In order to generalize our results to stimulus charac-teristics other than color and to provide further tests oftheories of attentional capture, in experiment 3 12subjects (eight female, four male) completed conditionssimilar to those used in experiment 2 but in which allstimuli were monochromatic (gray) and the singletontarget was defined by a luminance increment (to 24 vs16 cd/m2 for all other circles) rather than by a transientcolor change. As in the previous experiments, searchperformance in two complementary target/distractor
conditions was compared. In one condition subjectswere instructed to identify a target character that ap-peared inside a ‘luminance singleton’ stimulus (definedby a luminance increment) while an irrelevant onsetdistractor was presented on some trials. In the secondcondition subjects were instructed to identify a targetcharacter that appeared inside an onset stimulus whilean irrelevant luminance increment was presented else-where on some trials. Half of the trials in each sessionwere control trials in which no sudden onset distractoror no luminance increment distractor appeared. Thelayout of the stimuli and the nature of the conditionswere the same as in experiment 2, except that all stimuliwere gray in color and a luminance increment appearedinstead of a color singleton. According to Yantis andHillstrom (1994), changing the luminance of an existingobject does not constitute the creation of a ‘new’ object,so by their account onset distractors should captureattention but luminance increments should not.
4.1. Results
Preliminary analyses indicated that the same patternof results was obtained in the two experimental ses-sions. Order had an influence in the sense that someeffects were larger in session 2 than in session 1. Be-cause there were no differences in which effects weresignificant, however, we averaged across sessions in theanalyses reported below. Trial data were excluded fromanalysis if a manual response error was made (2.2% ofall trials), if the manual RT was less than 100 ms orgreater than 1500 ms (1.4% of all trials), or if an eyemovement artifact occurred (2.8% of all trials).
The results are shown in Tables 7 and 8. Analysis ofthe manual RTs to make the C versus reversed Cjudgment showed that subjects responded faster to eachtarget type when no distractor was present than when adistractor was present. Thus, the onset distractorslowed RT to the luminance target, and the luminancedistractor slowed RT to the onset target. This was trueeven when only the trials in which the eyes wentdirectly to the target were considered. In sum, thesedata suggest that both irrelevant onsets and irrelevantluminance increments capture attention. The differencein manual RT between the luminance target controlcondition (774 ms) and the onset target control condi-tion (769 ms) was not significant, t(11)=0.3, P\0.7,indicating that the luminance and onset targets werematched for salience.
Analyses of the oculomotor behavior showed thatboth kinds of distractors attracted the eyes, but suddenonset distractors captured the eyes more frequentlythan luminance distractors. This pattern of results wasobtained (and was significant) in both experimentalsessions, but the magnitude of the effects increasedfrom session 1 to session 2. In session 1 the onset
D.E. Irwin et al. / Vision Research 40 (2000) 1443–1458 1455
distractor captured the eyes on 10.3% of the trials,while the luminance distractor captured the eyes on5.5% of the trials. In session 2 these percentages rose to43.0 and 11.7%, respectively. Thus, as in experiment 2,subjects found it more difficult to ignore an irrelevantdistractor if it had served as the saccade target in theprevious experimental session. This was especially truefor sudden onset stimuli.
Analysis of the trials in which the eyes went directlyto the saccade target showed that saccade latency to theluminance target was slower when an onset distractorwas present than when one was not; similarly, saccadelatency to the onset target was slower when a lumi-nance distractor was present than when it was absent.Thus, in addition to capturing the eyes on a substantialproportion of the trials, the presence of a distractorappears to have slowed saccade initiation to the targeteven when the eyes went directly to the target.
Saccade latency also differed as a function of whetherthe initial saccade went to the distractor as opposed tothe target (Table 8). Mean saccade latency was 212 mswhen the eyes moved to the onset distractor, comparedto 230 ms when the eyes moved to the target. Incontrast, there was no difference in mean latency forthe onset target trials in which the eyes moved to theluminance distractor as opposed to moving directly tothe target.
As in the first two experiments, the great majority ofthe fixations made to onset distractors were too brief to
enable the programming of another saccade to thetarget. Unlike experiments 1 and 2, the same patternwas apparent in the condition in which the onset stimu-lus was the target and fixations were made on theluminance distractor. Thus, it appears that both suddenonsets and luminance increments allow parallel pro-gramming of two saccades (i.e. one to the target andthe other to the distractor) whereas color singletondistractors do not.
4.2. Discussion
The results of the luminance target, onset distractorcondition replicate the results of the color target, onsetdistractor conditions in experiments 1 and 2 in almostevery respect. Presentation of an irrelevant onset dis-tractor captured attention, as shown by the significantdifference in manual RT between the distractor andcontrol conditions when subjects were searching for aluminance target. The irrelevant onset distractor alsocaptured the eyes on a significant number of trials. Asin the previous experiments, saccades to the onset hada shorter latency than saccades to the luminance targetand the duration of the fixation on the onset wasgenerally too short to allow for the programming of anew saccade to the target. In sum, the results of thiscondition provide additional evidence that abrupt on-sets (new objects) capture attention in an involuntaryfashion, in part by eliciting involuntary, reflexive sac-
Table 7Results of experiment 3 by target type (luminance vs onset) and condition (distractor present vs distractor absent)a
Onset targetLuminance target
SingletonOnset
Present PtAbsentPresentPtAbsent
769820B0.0014.89774871 0.001Manual RT (ms) for all correct trials 4.75Manual RT (ms) when eyes went directly to target 837 774 3.09 0.010 799 769 3.65 0.004
4.66 0.0010.5 4.40Saccade path (% saccades toward distractor/extra stimulus) 0.00126.7 8.6 0.73.75 0.003205Saccade latency (ms) when eyes went directly to target 7.30230 B0.001 215 201
a The t-values are the results of two-tailed paired t-tests (df=11) for distractor present versus distractor absent trials.
Table 8Results of experiment 3 for trials in which the eyes went directly to the target compared to trials in which the eyes went to the distractor beforegoing to the targeta
t POnset distractor t P Onset target LuminanceLuminancedistractortarget
a The t-values are the results of two-tailed paired t-tests (df=11).
D.E. Irwin et al. / Vision Research 40 (2000) 1443–14581456
cades to the onset that compete with voluntary, goal-di-rected saccades to the target.
Unlike the previous experiments, the results of theonset target, luminance distractor condition indicatethat luminance increments (unlike color singletons) alsocapture attention and the eyes in an involuntary fash-ion. Presentation of an irrelevant luminance singletondistractor when goal-directed saccades were made to anabrupt onset stimulus slowed manual RT, slowed sac-cade latency to the target, and resulted in oculomotorcapture on a significant number of trials. Althoughsaccades made to the luminance distractor did notdiffer in terms of latency to saccades made to the onsettarget, the duration of the fixation on the luminancedistractor was generally too short to allow for program-ming of a new saccade to the target. Thus, luminanceincrements also seem to elicit involuntary, reflexivesaccades that compete with voluntary, goal-directedsaccades to a defined target. Thus, abrupt onsets arenot unique in their ability to capture attention; lumi-nance increments capture attention (both covert andovert) as well. Manual RT in the onset target andluminance target control conditions was very similar, soit is unlikely that these results are somehow due todifferences in stimulus salience.
The results of experiment 3 are inconsistent with the‘new object’ account of attentional capture proposed byYantis and Hillstrom (1994), because luminance incre-ments captured attention even though they did notdefine the appearance of new objects. Abrupt onsetswere more likely to capture the eyes than were lumi-nance increments, however, which suggests that abruptonsets were especially effective in capturing attention.Stimulus salience theories (e.g. Koch & Ullman, 1985;Theeuwes, 1991, 1992, 1994, 1996) and attentional con-trol setting theories (Folk & Annett, 1994; Folk &Remington 1996; Folk et al., 1992, 1993, 1994) ofattentional capture can explain why both luminanceincrements and abrupt onsets captured attention in thisexperiment (stimulus salience was matched; attentionalcontrol setting theory treats abrupt onsets as luminanceincrements) but they can not explain why abrupt onsetswere more effective than luminance increments in cap-turing the eyes.
5. General discussion
The purpose of the present paper was to investigatewhether attentional (and oculomotor) capture occuronly when abrupt onsets which define new objects areused as distractors in a visual search task, or whetherother salient stimuli also capture attention and the eyeseven when they do not constitute new objects. Theresults of three experiments showed that abrupt onsets(new objects) were especially effective in capturing at-
tention and the eyes, but that luminance incrementsthat did not accompany the appearance of new objectscaptured attention as well. Color singletons, whethercreated by changes in the colors of all objects but oneor by a transient color change in a single object, did notcapture attention unless subjects had experience withthe color singleton as a search target in a previousexperimental session.
Both abrupt onsets and luminance increments ap-peared to elicit reflexive, involuntary saccades (as wellas covert attention shifts), whereas transient colorchanges did not. This is consistent with the hypothesisthat two parallel pathways are involved in saccadegeneration: a subcortical pathway (dependent on thesuperior colliculus) that is responsible for generatingreflexive, orienting saccades, and a cortical pathway(headed by the frontal eye fields) that is responsible forgenerating voluntary, goal-directed saccades (e.g.Schall, 1995). Neurons in the superior colliculus appearnot to discriminate color (Marrocco & Li, 1977), socolor distractors should be incapable of eliciting invol-untary, reflexive saccades in our paradigm.
One difference between the results of the presentexperiments and those obtained in our previous studies(e.g. Theeuwes et al., 1998, 1999) is that the eyes werecaptured much less often by the irrelevant onset distrac-tor in the present experiments (approximately 50% ofthe time in our previous studies, compared to 5–40% ofthe time in the present experiments, depending ontarget type and prior experience with the distractor astarget). Two important differences between the presentstudies and our earlier ones is the number of distractorsthat were present in the display and (concomitantly)subjects’ level of awareness of the presence of thetask-irrelevant stimulus. Our previous studies usedlarger sets of distractors (seven, including the onsetdistractor), whereas the present experiments used onlyfive (including the onset distractor). Subjects in ourprevious studies reported (during debriefing) that theywere unaware that onsets were ever present, whilesubjects in our current studies reported being highlyaware of the onsets. In recent work (Kramer et al., inpress) we have found that young adults (like those usedin the present experiments) are able to dramaticallyreduce saccades to an irrelevant onset distractor whenthey are aware of its presence, presumably because oftop-down inhibition from the cortical pathway onto thesubcortical pathway. Such a mechanism could accountfor the lower number of saccades made to irrelevantdistractors in the present experiments, as well.
The results of the current experiments are not en-tirely consistent with any single theory of attentionalcapture. The fact that task-irrelevant abrupt onsetscapture attention even when subjects are instructed tosearch for a color singleton stimulus (experiments 1 and2) is inconsistent with the attentional control setting
D.E. Irwin et al. / Vision Research 40 (2000) 1443–1458 1457
hypothesis of Folk and colleagues. The finding thatabrupt onsets are more effective than color transientsor luminance increments in attracting attention evenwhen stimulus salience is matched is inconsistent withpurely bottom-up theories of attentional capture suchas those proposed by Koch and Ullman (1985) andTheeuwes (1991, 1992, 1994, 1996). However, the factthat practiced color singletons and luminance incre-ments to existing objects capture attention is inconsis-tent with the ‘new object’ theory of attentional captureproposed by Yantis and Hillstrom (1994). Abrupt on-sets (new objects) are clearly the most effective incapturing attention, however, which does support the‘new object’ theory. Considering the attentional captureliterature as a whole, it seems most likely that there aremultiple influences on attentional capture, rather than asingle mechanism that operates under all circumstances.
Acknowledgements
We would like to thank Sarah Poss and Trina Ragainfor their assistance in running subjects for our studies,and two anonymous reviewers for their helpful com-ments. The conduct of the studies was supported bygrants from the Army Research Laboratory (DAAL01-96-2-0003), the National Institute on Aging (AG14966),and the National Science Foundation (NSF SBR9615988).
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