Up-shifted decision criteria in Attentional Blink and ... - Andrei Gorea

This article was downloaded by: [Universite Rene Descartes Paris 5]On: 26 February 2010Access details: Access Details: [subscription number 786636382]Publisher Psychology PressInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UKVisual CognitionPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713683696Upshifted decision criteria in attentional blink and repetition blindnessFlorent Caetta a ; Andrei Gorea aaParis Descartes University and CNRS, Paris, FranceFirst published on: 01 July 2009To cite this Article Caetta, Florent and Gorea, Andrei(2010) 'Upshifted decision criteria in attentional blink and repetitionblindness', Visual Cognition, 18: 3, 413 — 433, First published on: 01 July 2009 (iFirst)To link to this Article: DOI: 10.1080/13506280902884402URL: http://dx.doi.org/10.1080/13506280902884402PLEASE SCROLL DOWN FOR ARTICLEFull terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdfThis article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.

VISUAL COGNITION, 2010, 18 (3), 413433Upshifted decision criteria in attentional blinkand repetition blindnessFlorent Caetta and Andrei GoreaParis Descartes University and CNRS, Paris, FranceDownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010A large number of ‘‘unawareness’’ phenomena have been explained and quantifiedin terms of sensitivity (d?) fluctuations, with very few attempts at addressing analternative putative cause, i.e., fluctuations of subjects’ response criteria (c).Response criteria fluctuations are particularly likely under dual-task paradigmswith unbalanced sensitivities (Gorea & Sagi, 2000) such as those used in evidencingattentional blink (AB) and repetition blindness (RB) phenomena. The present studyinquires into whether AB and RB are indeed prone to a deviant decisionalbehaviour. AB and RB were studied with a yes/no task allowing the assessment of d?and c for the detection (presence/absence) of a target letter T2 as a function of itstemporal lag relative to the presentation of another (AB) or of the same (RB) letter,T1 (Experiment 1). A significant criterion increase was observed in both cases.Additional experiments demonstrate that this criterion effect is typical of thesedual-task AB and RB paradigms as it is not observed in a standard contrastdetection task with mixed contrasts (Experiment 2), in a ‘‘control’’ AB designstripped off its first task T1 (Experiment 3), or in a metacontrast experiment(Experiment 4). We propose that the observed criterion shifts are the consequenceof the inherent dual-task AB and RB designs (where observers have to judge twoevents of unequal saliencies) and that they entail an enhancement of the AB andRB effects as long as these effects are assessed via subjective (yes/no or matching)procedures.Keywords: Dual task; Decision; Sensitivity; Signal Detection Theory;Attentional blink.Among the numerous ‘‘unawareness’’ visual phenomena reported forotherwise highly salient stimuli (such as binocular rivalry, neglect, blindsight,change blindness, invisible priming, etc.), many require an implicit orexplicit dual-task design to be evidenced. This is definitely the case withattentional blink (AB; Raymond, Shapiro, & Arnell, 1992) and repetitionPlease address all correspondence to Florent Caetta, Paris Descartes University and CNRS,45 rue des Saints Pères, 75006 Paris, France. E-mail: f.caetta@gmail.comWe thank our observers who have patiently spent many hours out of sheer altruism, andPedro Cardoso-Leite for useful comments.# 2009 Psychology Press, an imprint of the Taylor & Francis Group, an Informa businesshttp://www.psypress.com/viscog DOI: 10.1080/13506280902884402

414 CAETTA AND GOREADownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010blindness (RB; Kanwisher, 1987) designs. AB is the phenomenon where, in arapid serial visual presentation display, subjects fail to report the second of apair of highly salient targets (T1, T2) when it is delayed with respect to thefirst by about 200500 ms. Critically, AB occurs only if subjects are requiredto report both targets, that is, a dual-task design. Likewise, RB is thephenomenon where, within roughly the same time interval range, subjectsfail to detect the second target in the sequence if it is a repetition of the firstbut not otherwise. Although very similar at face value, AB and RB showrather different sensitivity functions of time-lag (U-shaped and monotonic,respectively) and have been attributed to distinct (e.g., Chun, 1997) thoughdebated processes. Globally speaking, these processes have been related toattentional or short-term memory limitations/bottlenecks or, most frequently,a combination of the two (for recent reviews see Bowman & Wyble,2007; Olivers & Meeter, 2008).It is not the purpose of this study to disentangle or confront these rathernumerous AB and RB theoretical accounts. Instead, we ask whether such‘‘unawareness’’ visual phenomena, like many of those mentioned erlier,might also proceed from other processes than a sensitivity depletion. Bydesign and by definition, AB and RB are dual-task paradigms where the twotargets to be reported are of unequal saliencies with the second in thesequence much less visible than the first. Here we aim to show that, inaddition to a sensory depletion, the ‘‘invisibility’’ of this second target maybe enhanced by a change in observers’ decision strategy (an upward criterionshift) entailed by the inherent dual-task nature of these paradigms. Upwardcriterion shifts have been positively tested or invoked to account for somepathological unawareness conditions such as neglect (Klein, 1998; Ricci &Chatterjee, 2004), extinction (Gorea & Sagi, 2002a), and blindsight(Azzopardi & Cowey, 1997, 1998; Campion, Latto, & Smith, 1983).Criterion shifts have been recently demonstrated (Caetta, Gorea & Bonneh,2007) for the case of motion induced blindness (MIB; Bonneh, Cooperman& Sagi, 2001), but discarded as a putative cause of a response time reductiondue to stimulus repetition in a search task (Sigurdardottir, Kristjánsson, &Driver, 2008).One may think of such decisional changes as being specific to high-levelprocesses. Gorea and Sagi (2000, 2001, 2002a, 2002b, 2005) have argued;however, that they are typical of any testing condition where seen/not seenjudgements bearing on a feebly salient target are implicitly or explicitly madewith reference to a more salient stimulus. According to Signal DetectionTheory (SDT; Green & Swets, 1966), the placement of a decisional criterionby an optimal observer slides along the internal response axis inproportion with the observer’s sensitivity or, equivalently, with stimulussalience. Accordingly, two unequally salient targets (like in AB or RB)should yield, in principle, unequal criteria. Gorea and Sagi have shown that

UPSHIFTED DECISION CRITERIA 415Downloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010this is not the case and that the two criteria tend to ‘‘attract’’ each other tothe point of merging into one. As a consequence, the least salient target willbe reported less and the more salient one more than if either of them werepresented in isolation. As this attraction effect occurs even for sequentiallypresented targets (Gorea & Sagi, 2005), it should be also present in AB andRB paradigms. This is definitely not to say that criterion shifts are the causeof AB or RB effects, but that they may add to the sensory causedperformance depletion. In fact, criterion shifts entailed by dual-tasks withunbalanced stimulus saliencies should not contribute at all to thesephenomena when assessed with criterion-free (such as forced-choice)paradigms. It remains that about 35% of the AB and RB literature we’vereviewed (including the pioneering studies of Kanwisher, 1987, and ofRaymond et al., 1992) uses subjective measurements.In addition to the subjective dual-task paradigms, criterion shifts areknown to occur in any subjective single-task experiment involving therandom presentation (across trials) of different saliency stimuli. In such yes/no paradigms, false alarms cannot be attributed to a particular stimulus sothat their frequency is computed over the whole saliency range, henceyielding one single number. In SDT, the z-score of a given false alarmfrequency has been coined by Gorea and Sagi (2000) the ‘‘absolutecriterion’’, c?. It is the decision variable computed along the internalresponse axis (or sensory continuum) with respect to the mean of theinternal noise distribution. As explained in the next section (see also Figure 1),this ‘‘unique’’ c? derived from mixed saliency designs will stand between thec?-values that would be computed in blocked designs for the lower and thehigher saliency stimuli. As a consequence, the lower and higher saliencystimuli used in mixed subjective designs will be reported respectively less andmore frequently than when tested in blocked subjective designs. This is thecase for all subjective AB and RB assessments as they have all been madewith randomized T1T2 lags (hence different T2 saliencies). Under suchexperimental conditions the main AB or RB effects (i.e., performance dropfor specific lags) are enhanced by necessity.The use of a unique c? is not necessarily avoided in either mixed blockswith ‘‘tagged’’ temporal lags or in experimental designs blocked by lag. Thisshould be so because humans are poor in tagging temporal delays within therange effective for AB or RB effects (i.e., 0200 ms; e.g., Cardoso-Leite,Gorea, & Mamassian, 2007). Subjects may well report false alarmsoriginated at arbitrary moments so that these false alarms are erroneouslyattributed to the nominal (experimentally tagged or blocked) lag. It followsthat the corresponding c?-values will be smeared (or averaged) across thedifferent (tagged or blocked) lags the consequence of which is, once again, aperformance undervaluation for the less visible stimuli.

416 CAETTA AND GOREAac’d’4.03.0bd’2.01.0ABNc 0S0.01.00.5cDownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010p(z)Nuc’S 1S 2SSensory continuum (z)cc’0.0-0.5-1.02.01.51.00.5dAB mixedAB blockedAB mixedAB blocked0 1 2 3 4 5 6 7 8 9LagFigure 1. Noise (N) and signal (S) internal response distributions, sensitivities (d?) and optimalcriteria (c and c?) presented within the SDT framework (a) and derived from hypothetical AB and RBdata (b, c, and d). (a). SDT framework for single and dual-task designs (top and bottom panels,respectively). Gaussian are the probability density functions of the internal response (represented as z-scores on the sensory continuum) for noise-alone (N) and for signal-plus-noise (S) events. Sensitivitycorresponds to the separation between the mean of the S and N distributions (top panel). Whenmeasured with respect to the crossing point of the S- and N-distributions (c 0 ), decision criteria arereferred to as relative (c); when measured with respect to the mean of the N-distribution, they arereferred to as absolute (c?). When S and N are equiprobable, an optimal observer places his responsecriterion at the crossing point of the N and S distributions, whether tested with one (heavy verticalsolid line in the top panel) or two different signals strengths (S1 and S2; dashed vertical lines in thebottom panel). In the latter case observers might not be able to keep the S1 and S2 distributions(dotted Gaussians) apart and merge them into a global S-distribution (heavy Gaussian labelled S in thebottom panel). If so, they will entertain a unique absolute criterion (uc?; heavy vertical line in thebottom panel) that is ‘‘optimal’’ with respect to this global S-distribution but suboptimal with respectto S1 and S2 taken independently. (b) A typical U-shaped d? function of target (T1T2) lag in an ABdesign (c and d). Optimal c- andc?-values in AB for the case where each lag is tested in isolation(dashed lines) or randomized across trials (solid lines).In short, the present study is aimed at testing criterion shifts in AB andRB tasks due both to their dual-task format and to the standard procedureused in these paradigms of randomizing temporal lags. In relation to thislatter point, the study also addresses the issue of lag uncertainty from the

UPSHIFTED DECISION CRITERIA 417Downloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010observer’s standpoint, whether these lags are blocked or experimentallytagged within mixed designs. To make our point, we compare criteriameasured in AB and RB with those assessed in three single-task designs: Astandard contrast detection experiment with mixed contrasts, an AB designwhere observers are asked to ignore the first (and easiest) task in thesequence of two, and a metacontrast task. Metacontrast is the phenomenonwhereby a highly visible target is rendered invisible by a spatially adjacentsuprathreshold mask presented up to a few hundred ms after the target (for arecent review see Hermens, Luksys, Gerstner, Herzog & Ernst, 2008). Themetacontrast control experiment was chosen for three reasons. First, like ABand RB, metacontrast is a time dependent invisibility phenomenon. Second,contrary to AB and RB, it is mostly thought of as due to low-level inhibitoryeffects (e.g., Hermens et al., 2008; Ogmen, Breitmeyer, & Melvin, 2003).Third and most critically, it involves a single (here detection) task and shouldtherefore not be prone to a criterion shift.A FEW DEFINITIONSPerformance in all the present experiments is assessed in terms of sensitivity,d?, and of response bias or criterion. Like d?, the criterion (heavy verticalsolid lines in Figure 1a, top panel), is expressed in units of the internal noise,that is, z-scores. When measured with reference to the value of the sensorycontinuum (or internal response) where the noise (N) and signal (S)distributions cross (noted c 0 in the top panel of Figure 1a), it is referredto as the relative criterion, c.5(zHzFA), with zH and zFA the hit andfalse alarm z-scores (Green & Swets, 1966; Macmillan & Creelman, 1991).For equally probable N and S events, the optimal cc 0 0, whatever the d?(dashed horizontal line in Figure 1c). Another way of measuring this samedecision variable is with reference to the mean of the N-distribution in whichcase it is referred to as the absolute criterion, c?zFA (see Gorea & Sagi,2000). The optimal c (0) is at the mid-distance between the means of theN and S distributions, so that an optimal c?d?/2 (dashed curve in Figure1d). Although c and c? are directly derivable from each other (provided thehit rate is known, i.e., c?2czH), c? is a more convenient index forrevealing the use by observers of a unique (internal response) decisionvariable (as conceptualized by Gorea & Sagi, 2000) across stimuli yieldingdifferent sensitivities (solid horizontal line in Figure 1d). Under suchconditions, standard SDT predicts different optimal c?-values (given thatthe optimal c? equals d?/2; dashed curve in Figure 1d). The assessment of a d?independent c? would be direct evidence of observers using a unique decisionvariable, uc?, across different stimulus saliencies. Such behaviour translatesinto different c-values (solid curve in Figure 1c) that would hinder its graphic

418 CAETTA AND GOREADownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010demonstration. For this reason the measured and theoretical criteriapresented in the Results section are given in c?-units. Because c-units are d?independent, they were used in the only case where we compared thedecision variables across distinct experiments yielding different sensitivities.Gorea and Sagi (2000) have shown that when observers cannot ‘‘tag’’ theinternal distributions evoked by two (or more) different saliency stimuli(dotted Gaussians labelled S1 and S2 in Figure 1a, bottom panel), theirchoice of a uc? is compatible with them merging these distributions into asingle one (heavy Gaussian labelled S in Figure 1a, bottom panel) andpicking up their uc? as the internal response point where the N and themerged S distributions cross. Assuming that observers do indeed complywith this modelling, their uc? should approximately equal half of the d?averaged over all n different saliency stimuli, i (/ða n i d 0iÞ=2n): This should bealso the case when false alarms in a mixed stimuli design cannot beattributed by the experimenter to a specific stimulus among more than two;this is so because c?zFA.Figure 1b illustrates an idealized (but typical) AB d? function of target(T2) lag. Figure 1c shows the optimal c0 to be expected if each lag is testedin isolation (horizontal dashed line) and the expected c-values for the casewhere the lags are mixed and observers use an uc? according to the strategydescribed earlier (/uc 0 ða n i d 0iÞ=2n; solid curve). Figure 1d displays theequivalent c?-values when each lag is tested separately (optimal c 0 i d0i =2;dashed curve) and when they are interleaved and observers use a uc?(horizontal solid line).GENERAL METHODIn all experiments, the stimuli were presented on a 19-inch gamma correctedscreen (Philips Brilliance 109P, 1024768 pixels, 100 Hz refresh rate) andgenerated by a PC running Matlab software with the Psychophysics Toolbox(Brainard, 1997; Pelli, 1997). They were presented on a uniform grey field(35.3 cd/m 2 ). Subjects viewed the display binocularly from a distance of 50cm in a dark room.Experiment 1: Attentional blink (AB) andrepetition blindness (RB)This experiment includes three conditions: ‘‘Mixed’’ AB, ‘‘mixed’’ RB, and‘‘blocked’’ AB (see Figure 2). As noted in the introduction, the lattercondition is meant to check our conjecture that observers may not relatetheir judgements to a specific time lag despite it being blocked and that, as aconsequence, they would use a unique decision criterion across lags. If so,

UPSHIFTED DECISION CRITERIA 419ABRB500 ms450 -..1170 ms+20 msA+ADownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010Timetheir ‘‘not seen’’ rate for lags yielding the largest sensitivity depression shouldbe enhanced.Method70 ms70 -..700 msObservers. Five volunteers (age range 2226), naïve to the purpose ofthis study and with normal or corrected-to-normal vision, participated in allthree experiments.Stimuli. Stimuli were Arial font letters (30 points with the largest letters,‘‘M’’ and ‘‘W’’ subtending 1.158 0.808) randomly chosen from the entirealphabet (with the exception of letter ‘‘X’’ in the AB condition; see theProcedure section), with the constraint that no letter was presented twicewithin a trial. Each trial consisted of a rapid serial visual presentation(RSVP) of 25 letters displayed foveally, each lasting 20 ms with an interletterinterval of 70 ms (i.e., 11.11 letters/s; see Figure 2). Two letters in the RSVPsequence were ‘‘targets’’, with the first target (T1) presented in white font(142.40 cd/m 2 ) and different from all the remaining letters (including thesecond target, T2) that were black (0.01 cd/m 2 ). T1 was presented on eachtrial, whereas T2 was presented on 50% of the trials only. Fixation beforeand after the RSVP sequence was ensured by a black (0.01 cd/m 2 ) fixationcross (0.48 0.48).BT 1T 2Figure 2. The RSVP sequence used in Experiment 1. In the AB condition, T2 was an ‘‘X’’, whereasit was the same letter as T1 in the RB condition. The illustration is for T2-lag1.XBB

420 CAETTA AND GOREADownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010Procedure. The RSVP sequence started after a 500 ms presentation ofthe fixation cross. At the end of the sequence, subjects were presented withtwo letters, one of which was the T1 target and the other chosen randomlyfrom the alphabet (with the exception of ‘‘X’’ in the AB sessions). Subjectshad to choose the T1 target (a two-alternative forced choice task) andsubsequently decide whether or not T2 (an ‘‘X’’ in AB and a T1 repeat inRB) was present (a yes/no task). T1 was randomly preceded by 715 letters.Incorrect responses were signalled by a tone. When present (half of thetrials), T2 was pseudorandomly presented at one of eight temporal lags(‘‘mixed’’ condition), ranging from immediately after T1 (lag 1) to sevenletters after T1 (lag 8).‘‘Mixed’’ AB and RB sessions (50 trials/lag, with no intermixing betweenAB and RB trials) were randomized and repeated four times. In case of a T1identification failure, the whole RSVP sequence was repeated later in thesession so as to reach a constant number of 200 T1-correct trials per T2 lag.The ‘‘blocked’’ condition was run with the AB paradigm only. Theprocedure was the same as in the ‘‘mixed’’ condition except that only one lag(out of four, i.e., lags 1, 3, 5, and 7) was presented per experimental block (50trials). Blocks were repeated thrice in a random order (i.e., 150 T1-correcttrials per lag). Note that while the mixed condition compels the measure of aunique c? shared by all T2-lags, the blocked condition allows for c? measuresspecific to each T2-lag.ResultsThe overall T1 identification performance was 95% across all threeexperiments and subjects. Figure 3 shows T2 group mean sensitivity (Figure3a) and absolute criteria in the mixed (Figure 3b) and blocked (3c)conditions as a function of the T1-T2 lag. Squares and circles pertain toexperiments AB and RB, with solid and open symbols showing data for themixed and blocked conditions, respectively. For the mixed conditions, c?-sare by necessity unique so that they are represented in Figure 3b as black(measured) and white (optimal) bars for the AB and RB experiments. Forthe blocked AB condition (Figure 3c), c? should, but does not appear to belag (and hence d?) dependent (open squares and continuous lines). Insteadthe optimal c? computed for each lag independently does show a lagdependency.AB and RB exhibit the typical d? functions of the lag: U-shaped for ABand monotonically increasing for RB (note that the first two d?-values of theRB function are not statistically different). The maximum AB and RBeffects (in d? units) in the mixed condition are both 1.31 s. The maximumAB effect in the blocked condition is 0.92 s (but note that lag 8 was absent inthis condition). The measured c?-values are in all cases well above the

UPSHIFTED DECISION CRITERIA 421Downloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010Figure 3. d? (a) and c? (b and c) dependency on the T1T2 lag for the AB and RB experiments withmixed (solid symbols and bars; a and b) and blocked (open symbols; a and c). Squares and circles arefor the mixed and blocked conditions, respectively. White bars in (b) and the dashed line in (c) showoptimal c?-values. All datum-points are averages over the five subjects with the vertical lines showing91SE.predicted optimal values; the upward shifts for the mixed AB, mixed RB,and blocked AB conditions are respectively .48, .45 and an average of.59 s units. Taken together, these d? and c? data clearly establish that AB andRB are both sensitivity- and decision-related phenomena.A two-factor (condition*AB and RB mixed*and lag) repeated measuresANOVA on the d? data yields a significant lag effect, F(7, 28)22.97,pB.001. Neither the condition factor nor its interaction with the lag factoryield significant effects. Hence, contrary to previous reports (Chun, 1997),AB and RB effects are statistically undistinguishable. It should be noted,however, that partial comparisons of the d?-values for lags 1 and 2 yield asignificant difference in the AB case, F(1, 4)11.56, pB.05, but not in theRB case.Another two factors (condition*AB-mixed and AB-blocked and lag)repeated measures ANOVA with the mixed lag d?-values restricted to those

422 CAETTA AND GOREADownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010used in the blocked condition (i.e., 4) shows both a mixed/blocked, F(1, 4)7.29, p.05, and a lag, F(3, 12)3.62, pB.05, effect with no interactionbetween the two. It remains that the lag effect in the AB-blocked conditionappears to be attenuated as it is only marginally significant when tested byitself, F(3, 12)2.79, p.08. Quite likely this attenuation in the blockeddesign is due to the global d? increase (presumably entailed by a reduction intemporal uncertainty) the consequence of which could be a sensitivity ceilingeffect for lags 1 and 7.The upward c? shift was tested separately for the mixed (AB and RB) andblocked conditions. In the first case the measured and theoretical (i.e.,optimal) unique c?s were compared via a two-way ANOVA with condition(AB, RB) and c? (measured, optimal) as factors. The ANOVA shows asignificant c? effect, F(1, 4)11.25, pB.05, but no condition or interactioneffects. Another two factors (c?*measured/optimal*and lag) repeatedmeasures ANOVA was run for the AB blocked condition. It yields asignificant measured/optimal c? effect, F(1, 4)18.37, pB.05, but no lageffect and a marginally significant interaction, F(3, 12)3.26, p.06.Because the interaction between measured and optimal c? fails to reachsignificance, the issue of whether or not observers use a unique criterion (inthe blocked lag condition) remains undetermined.Experiment 2: Standard contrast detection (CD)The possibility exists that the upward criterion shift observed in the previousAB and RB experiments are accidental and due to a natural conservativedecisional behaviour of the five observers. To settle the issue we run thesesame observers in a standard yes/no detection experiment with Gaborpatches yielding one out of four contrasts intermixed within an experimentalblock. This experimental format should yield once again a unique c?.MethodObservers. They were the same as in Experiment 1.Stimuli. They were 2 c/deg vertical Gabor patches (s18) with one outof four contrast levels equally spaced on a log scale in-between 0.25 and 0.45.When presented (50% of the trials), they were superimposed on an additiveGaussian noise (s.75, rms contrast of 59%) and displayed for 20 ms at thecentre of the screen, indicated by a black (0.01 cd/m 2 ) fixation cross (0.480.48). ‘‘Noise’’ trials consisted in the Gaussian noise alone. Based onpreliminary experiments, the lowest and highest contrasts of the Gaborpatches were chosen so as to yield a d? difference of at least 1.

UPSHIFTED DECISION CRITERIA 423Downloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010Procedure. Sensitivity and response criterion were assessed via a yes/noprocedure. Observers had to maintain fixation on the central cross for500 ms, with its removal signalling the beginning of the trial. Stimuli (noiseor target plus noise) were displayed 500 ms later with the target contrastpseudorandomly chosen across trials. Subjects had to decide whether atarget was present. Incorrect responses were signalled by a tone. One sessionconsisted of 200 trials (50 trials/contrast) and was repeated four times so thatd? and criteria were computed out of 200 trials/contrast. Notice again thatthis mixed-contrast procedure does not allow for stimulus (here contrast)specific c? assessments.ResultsFigure 4 shows the group mean sensitivity (d?) as a function of contrast(Figure 4a), together with the group mean absolute measured (black bar)and optimal (white bar) criterion (c?, Figure 4b). The major observation isthat the measured and theoretical c?s are very much alike. Repeatedmeasures ANOVA performed on the d? data show a significant effect ofcontrast, F(3, 12)46.84, pB.0001. A paired t-test comparing the measuredc? and the optimal unique c? failed to reach significance, implying thatobservers’ decision behaviour is close to optimal.Comparison between Experiment 1 and Experiment 2As noted in the introduction, a comparison of the absolute criteria (c?)across experiments is not warranted as these indices depend on d?, whichcannot be precisely matched across different experimental setups. Instead,relative criteria (c) are d? independent and do allow for such a comparison.Remember that the optimal relative criterion is by definition 0. Figure 5 showsFigure 4. Mean d? (over five observers) as a function of contrast (a) and absolute measured (blackbar) and optimal (white bar) unique criteria (b) in Experiment 2. Vertical bars are 91SE acrossobservers.

424 CAETTA AND GOREADownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010Figure 5. Measured c-values averaged across observers and lags in the AB and RB conditions(Experiment 1) and across observers and contrasts (Experiment 2). Note that the optimal c equals 0.Vertical bars are 91SE across observers.the measured c-values averaged across observers and lags in the AB and RBconditions of Experiment 1 and across observers and contrasts in Experiment2. AB and RB yield a large upward criterion shift (about 0.45 s, i.e., one-thirdof the d? variation observed in those experiments). In the standard contrastdetection experiment these same observers are much less conservative thoughnot quite optimal (their upward criterion shift is about 0.18, i.e., only oneeighthof the tested d? range). A repeated measures ANOVA (with condition*AB, RB, and MC*as the only factor) shows that c-values differ significantlyacross conditions, F(2, 8)4.64, pB.05. Planned comparisons do not show asignificant difference between AB and RB, and both AB and RB differsignificantly from MC, AB, and MC, F(1, 4) 7.73, pB.0.5; RB and MC,F(1, 4)9.31, pB.05. Overall, the comparison between Experiments 1 and 2demonstrates that the unique up-shifted criterion observed in Experiment 1 isnot due a natural decisional bias of the observers.Experiment 3: Attentional blink (AB)*single taskThe AB (and also RB) phenomenon is by definition bound to the presenceof an ancillary task, T1, during the RSVP presentation. The exclusion of the

UPSHIFTED DECISION CRITERIA 425Downloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010T1 task (but not stimulus) leaves the RSVP presentation unchanged butturns the dual-task AB format into a single-task format. Inasmuch as thecriterion shift observed in the standard AB paradigm (Experiment 1) resultsfrom this task involving a dual (T1 and T2) decision, the exclusion of T1should abolish the criterion shift. The persistence of the criterion shift in theabsence of T1 would exclude this possibility. Unfortunately, the idea oftesting this hypothesis has occurred to us at a late stage in our study so thatnot all the observers run in Experiment 1 could be run in the presentexperiment. As a consequence, the criteria assessed in these two experimentscannot be directly compared.MethodObservers. They were four volunteers*two of whom also participated inExperiments 1 and 2*naïve to the purpose of the study and the first author.Their ages ranged from 25 to 29 years. Observers had normal or correctedto-normalvision.Stimuli. They were the same as in Experiment 1 (AB condition) exceptthat the RSVP sequence was presented on a white Gaussian noise (s.57,rms contrast of 29%) to prevent for 100% correct detection. Indeed, the ABpresentation format without the T1 task eliminates the AB effect (Raymondet al., 1992).Procedure. The procedure was the same as in the Experiment 1 exceptthat observers had only to detect the presence of an ‘‘X’’ (T2). This targetwas (or was not) presented at four lags (1, 3, 5, and 7) with respect to theT1 stimulus. Lags were randomly interleaved across trials with 200 trialsper block (50 trials/lag). Blocks were repeated four times yielding 200trials/lag.ResultsFigure 6 shows the group mean sensitivity (d?, Figure 6a) as a function ofT1T2 lag, together with the absolute measured and optimal criteria (c?,Figure 6b). As expected for this single-task format, d?s remain constantacross lags (mean d? of 1.25). The measured and optimal criteria are close toidentical indicating that subjects are optimal. A repeated measures ANOVAdid not show a significant d? difference across lags. A paired t-test comparingthe measured and optimal c? also failed to reach significance. Overall, thepresent data point to the fact that reducing the dual-task AB format to asingle-task format eliminates both the AB effect and the upward criterionshift.

426 CAETTA AND GOREADownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010Figure 6. Mean d? (over five observers) as a function of lag (a) and absolute measured (black bar)and optimal (white bar) unique criteria (b) in Experiment 3. Vertical bars are 91SE across observers.Experiment 4: Metacontrast (MC)It remains that the highly conservative behaviour observed with the AB andRB paradigms might not be exclusively caused by their dual-task design, butrather be the consequence of them being presumably high-level (attentional?)phenomena. This possibility was tested in a standard yes/no metacontrastexperiment, which is of the single-task type. MC has been assimilated bysome to a perceptual ‘‘consciousness deficit’’ despite it being modelled inmost cases as low-level lateral inhibition process (see Hermens et al., 2008).Here the different sensitivities, also mixed within an experimental block, wereobtained by modulating the targetmask stimulusonset asynchrony (SOA)so that the experimental format is close to the one used in Experiment 1.MethodObservers. They were four volunteers naïve to the purpose of the studyand the first author, ranging in age from 25 to 32 years, with normal orcorrected-to-normal vision. One of the volunteers also participated inExperiments 1 and 2.Stimuli. The target was a vertical 1 c/deg Gabor patch (s18) spatiallygated by a circular aperture 2.48 in diameter. It was superimposed on a largerGabor mask (s28) with an identical vertical 1 c/deg carrier in phase withthe target. The target, presented on 50% of the trials, was spatiallyoverlapping with a horizontal 3 c/deg Gabor patch with the same s as thetarget (resulting in a plaid stimulus) also confined by the 2.48 circularaperture (see Figure 7). This additional stimulus was referred to as the ‘‘tag’’as it identified the targetmask SOA (0, 20, 40, 60, and 80 ms) whether the

UPSHIFTED DECISION CRITERIA 427Downloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010Figure 7. Spatial and temporal layout of the stimuli used in the metacontrast experiment. Note thattarget (i.e. ‘‘tag’’ or ‘‘tagtarget’’) could be presented simultaneously with the ‘‘mask’’; in this the caseboth the duration of the second ‘‘blank’’ and the targetmask SOA are null.target was presented or not. It was added to introduce an additionaltransient, hence preventing subjects from basing their detection responses onthe targetmask transient cues that are known to vary with SOA (Kahneman,1968). Target, tag and mask were centred at 78 to the left or right of ablack (0.01 cd/m 2 ) fixation cross (0.480.48) and had contrasts of 0.20, 0.25,and 0.25, respectively. All three were displayed for 20 ms with the mask onsetfollowing the simultaneous target (when present) and tag onsets by 0, 20, 40,60, or 80 ms. Based on preliminary experiments, the contrasts, spatialfrequencies and orientation of the stimuli were chosen so as to maximize themasking of the target (when present) and to minimize the masking of the tag(e.g., Ishikawa, Shimegi, & Sato, 2006), which remained highly visible for allSOAs. The presence of the tag allowed the attribution of each false alarm toa specific SOA and hence the assessment of SOA-specific criteria.Procedure. The beginning of each trial was signalled by a decrease in thesize of the central cross; after a variable delay ranging from 500 to 2000 ms,the stimuli were flashed for 20 ms with the (target ) tagmask SOArandomized across trials. Subjects had to decide whether the target was

428 CAETTA AND GOREApresent. Incorrect responses were signalled by a tone. One session consistedof 250 trials (50 trials/SOA) and was repeated four times.Downloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010ResultsFigure 8 shows the mean (over the five observers) d? (Figure 8a) andabsolute criteria (Figure 8b) as a function of SOA. Optimal criteria in Figure7b are shown as dashed lines. Sensitivity follows a B-type (U-shape) functionof SOA with a minimum at 40 ms. However, with a mean d? maximum dropof about 0.5 s, the strength of the masking remains relatively weak (despiteall our efforts to enhance it). Repeated measures one-way (lag) ANOVAsseparately performed on the d? and c? data show respectively a significant,F(4, 16)3.02, pB.05, and a nonsignificant lag effect with the lattersuggesting the use by observers of a unique c?. However, this conclusioncannot be sustained in view of a two-way ANOVA comparing the measuredand optimal c?s across SOAs. This analysis fails to show significant effects ofeither the c? type (measured vs. theoretical) or SOA factors, or for theirinteraction. As the theoretical c?s were independently computed for eachSOA, they necessarily depart from a unique c?. The absence of a theoreticalmeasuredc? difference together with the absence of a c? SOA interactionleaves therefore the issue of whether or not observers use a unique c?undecided. It remains that the main conclusion to be drawn from thisexperiment as well as from Experiments 2 and 3 (but not from Experiment 1)is that, observers use a close to optimal decisional behaviour. The presentexperiment also points to the fact that metacontrast, although methodologicallyrelated with the AB and RB paradigms, is not associated with thelarge upward criterion shift observed with the latter two.Figure 8. Mean (over five observers) d? (a) and c? (b) as a function of SOA in Experiment 4(metacontrast). The dashed line in (b) shows optimal c?-values. Vertical bars are 91SE acrossobservers.

UPSHIFTED DECISION CRITERIA 429Downloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010GENERAL DISCUSSIONVisual (un)awareness as well as attentional phenomena are typically referredto and quantified in terms of sensitivity fluctuations. Response biases ordecisional factors may, however, be equally relevant causal factors. On manyoccasions the sensitivity and decisional substrates of such phenomena havebeen methodologically melded due to the use of subjective measurementtechniques. Under such conditions (about 35% of the AB and RB literaturewe’ve reviewed), ignoring the decisional factor entails by necessity anoverestimation of the sensory cause. Building up on previous studies thatillustrated the significant participation of the decisional factor to phenomenasuch as neglect (Klein, 1998; Ricci & Chatterjee, 2004), extinction(Gorea & Sagi, 2002), blindsight (Azzopardi & Cowey, 1997, 1998; Campionet al., 1983), or MIB (Caetta et al., 2007), the present study demonstrates itsinvolvement in both AB and RB inasmuch as they are assessed via subjectiveprocedures.Experiment 1 has shown that, in both AB and RB, the decision criterionfor detecting the second (suppressed) target in the sequence is in average 0.51s above the criterion expected from SDT. In the AB condition, the upwardcriterion shift was observed whether the different time lags were randomlyinterleaved or blocked. Experiments 2 and 3 demonstrated that these upwardcriterion shifts are not due to observers’ natural conservative behaviour astheir criteria were not significantly different from those predicted by SDTboth in the simple detection task with intermingled contrasts (and hence d?-values; Experiment 2), and in the AB presentation format where observershad to identify the second target only (thereby eliminating the ABphenomenon; Experiment 3). Finally, Experiment 4 showed thatmetacontrast*a presumably low-level ‘‘disappearance’’ phenomenon*with interleaved but ‘‘tagged’’ SOAs is also immune to the upward criterionshift.In the process of unveiling the strong criterion modulation under AB andRB, we also asked the question of whether observers use a unique absolutecriterion (c?; see Figure 1) under conditions where, theoretically, they couldhave used distinct c?s for each of the different stimulation conditions (i.e.,different lags in the blocked AB condition in Experiment 1 and different‘‘tagged’’ SOAs in Experiment 4). The absence of a lag/SOA effect on the c?measured in the blocked and tagged experiments supports the use of aunique criterion. In turn, such decisional behaviour enhances the ‘‘not seen’’reports for the less visible targets and therefore the AB and RB effects.However, the use of a unique c? cannot be confidently asserted from theseexperiments (with temporal tags or blocked lags) as they showed little d? andtherefore, by necessity, c? variability across lags.

430 CAETTA AND GOREADownloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010It remains that the use of a unique c? even under conditions where thedifferent stimulations are clearly tagged should be expected from Gorea andSagi’s work (2000, 2001, 2002a, 2002b) suggesting that subjects cannotentertain more than one internal distribution at a time. According to theseauthors, when multistrength stimuli are tested jointly (be they tagged or not),subjects merge the corresponding internal distributions and use one singlecriterion that is optimal given a unique internal representation butnonoptimal with respect to the distinct internal representations of each ofthe two (or more) stimuli. Specifically, the unique c? is located somewhere inbetweenthe optimal criteria for each stimulus (but closer to the most salientstimulus) so that the less and the more salient events are respectively moreand less frequently ignored. As a consequence, the use of subjective methodsin assessing AB or RB deceitfully enhances these effects.At first sight the account above does not explain the significant elevationof the unique c? observed in Experiment 1 (0.51 s for both AB and RB), butnot in Experiments 2, 3, and 4. This apparent incongruence disappears if oneconsiders that AB and RB formats are the only ones in the present series offour involving an explicit dual-task design. Indeed, the exclusion of the‘‘secondary’’, T1 task (Experiment 3) not only eradicates the AB and RBphenomena (as originally shown; Kanwisher, 1987; Raymond et al., 1992),but it also abolishes the upward c? shift. As the secondary T1 task is designedto yield a high d? (typically too high to be measured), it also yields a high c?.Our conjecture is that criterion associated with the less salient (T2) stimulusis ‘‘attracted’’ by the more salient (T1) stimulus as a consequence ofobservers’ (at least partly) merging the two underlying internal distributions.Because T1 performances were measured here via a two-alternative forcedchoice procedure, the prediction that the associated c? should be lowered inthe presence of T2 could not be tested.As noted by Gorea and Sagi (2000), criterion shifts do not follow fromthe mere presence of two different saliency targets, but rather from subjectshaving to make a decision on each of them. This should explain the absenceof a criterion shift for an AB format where the T1 task (but not stimulus) isexcluded (Experiment 3). So, in addition to the many, yet unresolvedaccounts of AB and RB (see introduction), we showed that, when assessedwith subjective methods these phenomena are enhanced by subjects’‘‘deviant’’ decisional behaviour entailed by the very nature of the dual-taskconditions under which they are obtained.On the reasonable assumption that a dual-task needs not be explicit toentail a merged internal representation of two distinct events, the criteriaattraction account may also explain the upward criterion shift (of about0.7 s) observed by Caetta et al. (2007) in an MIB task. In that experiment,sensitivities to small target-luminance increments were measured in

UPSHIFTED DECISION CRITERIA 431Downloaded By: [Universite Rene Descartes Paris 5] At: 11:57 26 February 2010independent experimental blocks for the visible and suppressed MIB states.For this to be feasible, observers must continuously keep track of the MIBtarget fading in and out of ‘‘awareness’’. As a consequence, the internalrepresentations of the two stimulus states may have been merged and aunique criterion used to both partition the internal states and to perform thedetection task (see Gorea & Sagi, 2001).Ever since the formulation of SDT (Green & Swets, 1966), the relationshipbetween decision criteria and consciousness has been vigorouslydebated. According to Macmillan and Creelman (2005, p. 47), ‘‘thedistinction between consciousness and its lack has nothing to do with eitherthe existence or location of a criterion’’. The inescapable fact remains that,inasmuch as consciousness is by definition a strictly subjective construct, itsassessment is necessarily related to subject’s assertion of ‘‘being or not beingconscious’’ and, therefore, to his decisional behaviour. This line of thinkingremains is embraced in a number of recent studies (e.g., Lau, 2008; Lau &Passingham, 2006; Wilimzig, Tsuchiya, Fahle, Einhäuser, & Koch, 2008). Wetherefore conclude that, in addition to a pure sensory depletion, AB and RBalso proceed from a significant upward criterion shift presumably caused bythe inherent dual-task format required for their observation.It is possible that such criterion shifts are also caused by the coreprocess(es) accounting for the sensory depletion in AB and RB. As noted byOlivers and Meeter (2008, p 836), ‘‘all currently active theories of theattentional blink attribute it to a limited-capacity processing stage, orbottleneck’’. These theories share the notion that processing T1 draws on theattentional and/or storage resources needed for the encoding of T2. Goreaand Sagi (2005) and more recently Schneider and Komlos (2008) havepointed to the fact that, in addition to depleting sensitivity, attentionallimitations may also entail a more conservative decisional behaviour.Reminiscent of the ‘‘temporary loss of control theory’’ (Di Lollo, Kawahara,Ghorashi, & Enns, 2005; Kawahara, Kumada, & Di Lollo, 2006), Oliversand Meeter offered an alternative AB account whereby the ‘‘blink’’ for T2 iscaused by the distractor following T1 (i.e., lags1). This distractor wouldtrigger an inhibitory response that blocks its access to visual short termmemory rather than depleting its sensory effect. This proposal does notmatch our account of the presently observed criterion shifts being caused bythe AB (or RB) dual-task format with unequally salient stimuli. It remainsthat the inhibitory process proposed by Olivers and Meeter may as wellentail a more conservative decisional behaviour bearing on the detection ofT2. Be it as it may, systematic studies assessing the modulation of decisioncriteria by transient attentional, mnemonic, and/or selective inhibitioncauses remain to be carried out.

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