Exp Brain Res

DOI 10.1007/s00221-009-1734-4

RESEARCH ARTICLE

Crossmodal exogenous orienting improves the accuracy of temporal order judgments

Valerio Santangelo · Charles Spence

Received: 18 September 2008 / Accepted: 2 February 2009© Springer-Verlag 2009

Abstract Although many studies have demonstrated thatcrossmodal exogenous orienting can lead to a facilitation ofreaction times, the issue of whether exogenous spatial ori-enting also aVects the accuracy of perceptual judgments hasproved to be much more controversial. Here, we examinedwhether or not exogenous spatial attentional orientingwould aVect sensitivity in a temporal discrimination task.Participants judged which of the two target letters, pre-sented on either the same or opposite sides, had been pre-sented Wrst. A spatially non-predictive tone was presented200 ms prior to the onset of the Wrst visual stimulus. In twoexperiments, we observed improved performance (i.e., adecrease in the just noticeable diVerence) when the targetletters were presented on opposite sides and the auditorycue was presented on the side of the Wrst visual stimulus,even when central Wxation was monitored (“Experiment2”). A shift in the point of subjective simultaneity was alsoobserved in both experiments, indicating ‘prior entry’ forcued as compared to uncued Wrst target trials. No such JNDor PSS eVects were observed when the auditory tone waspresented after the second visual stimulus (“Experiment3”), thus conWrming the attentional nature of the eVectsobserved. These Wndings clearly show that the crossmodalexogenous orienting of spatial attention can aVect the accu-racy of temporal judgments.

Keywords Exogenous orienting · Attentional capture · Visual · Auditory · Accuracy · Temporal order judgment

Introduction

The capacity of abrupt peripheral onsets to capture a per-son’s spatial attention (usually referred as exogenous, orreXexive, orienting) has been documented frequently incognitive neuroscience research (e.g., see the chapters inFolk and Gibson 2001, for a review). Typically, attentionalcapture has been studied using variants of the Posner cuingparadigm (Posner and Cohen 1984). In cuing studies, visualtargets are normally presented at one of two or more possi-ble locations at which an abrupt onset cue (usually visualand non-predictive with regard to the location of the forth-coming target) may or may not be presented shortly before-hand. Response latencies are usually faster when targetsoccur at the cued position than when they appear elsewhereat cue-target intervals of up to 200–300 ms (e.g., Klein2000). Exogenous spatial cuing eVects have also been dem-onstrated when the cue and target stimuli are presented indiVerent sensory modalities (e.g., auditory cues have beenshown to facilitate visual target discrimination latenciesand the reverse has sometimes also been reported), which isthought to reXect shared (or at the very least linked) mecha-nisms of spatial attention (see Spence 2001; Spence et al.2004, for reviews).

Although such reaction time (RT) facilitation (i.e., exog-enous orienting) eVects have been demonstrated in a verywide range of experimental settings, the issue of whether ornot exogenous spatial orienting also aVects the accuracy ofperceptual judgments has proven to be a somewhat morecontroversial issue. In fact, certain studies have observedthat the exogenous orienting of spatial attention can aVect

V. Santangelo · C. SpenceDepartment of Experimental Psychology, University of Oxford, Oxford, UK

V. Santangelo (&)Department of Human and Educational Sciences, University of Perugia, Piazza G. Ermini, 1, 06123 Perugia, Italye-mail: v.santangelo@gmail.com

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the accuracy of participants’ performance (Luck et al.1996; Luck and Thomas 1999; Prinzmetal et al. 1997; Shiuand Pashler 1994; Warner et al. 1990). For instance, Lucket al. (1996) conducted several spatial cuing experiments,including peripheral non-predictive cuing experiments, inwhich target discrimination was more accurate at theattended as compared to the unattended location.

By contrast, other recent studies repeatedly found nosuch eVect of exogenous orienting on the accuracy of par-ticipants’ responses (Prinzmetal et al. 2005a, 2005b). Forinstance, Prinzmetal et al. (2005a) conducted a number ofexperiments in which their participants had to discriminatewhich target letter (‘F’ vs. ‘T’) had been presented in one offour locations (note that in a control experiment, theyensured that their participants were near 100% correct inindicating the location, in which the target letter had beenpresented, i.e., there was no target–location uncertainty). Avisual cue was sometimes presented at the location, wherethe target letter would appear soon afterward [at stimulusonset asynchronies (SOAs) of 0 or 300 ms]. The visual cuewas spatially non-predictive (25% validity for each of thefour possible target locations). While in some of theseexperiments, the participants were encouraged to respondas rapidly and accurately as possible (RT experiments); inothers, the participants were encouraged to take their timeto respond as accurately as possible (accuracy experi-ments).

Prinzmetal et al. (2005a) observed exogenous orientingeVects (i.e., faster RTs for cued than for uncued trials) inthe RT experiments, thus showing that their uninformativespatial cues had eVectively captured their participants’exogenous spatial attention. Crucially, however, they didnot observe any diVerence between the percentages of cor-rect responses for cued versus uncued trials in any of theiraccuracy experiments. Prinzmetal et al. therefore concludedthat spatially uninformative visual (or auditory, see theirExperiment 8) cues do not enhance the visual representa-tion at the cued location, and that the exogenous orientingof spatial attention does not aVect the accuracy of partici-pants’ responses to subsequently presented target stimuli in‘genuinely-unspeeded’ tasks.

Prinzmetal et al. (2005a) went on to argue that the onlyreason why the earlier studies (Luck et al. 1996; Luck andThomas 1999; Prinzmetal et al. 1997; Shiu and Pashler1994; Warner et al. 1990) may have observed an eVect ofexogenous orienting on performance accuracy measures isthat in their paradigms, there was a degree of uncertaintyregarding the target–location (i.e., Prinzmetal et al. arguedthat participants in these earlier studies may well have beensomewhat uncertain as to which location had actually con-tained the target). What is more, these studies have neverbeen conducted in a truly unspeeded response setting. Thatis, although the participants had to respond accurately, a

deadline to respond was also given, which clearly does notprovide the best situation in which to assess the accuracy ofperformance (cf. Prinzmetal et al. 2005a).

For these reasons, Prinzmetal et al. (2005a) outlined fourcriteria that they argued needed to be met in order to con-vincingly demonstrate an improvement of performance inan accuracy experiment involving the presentation of whatthey called ‘automatic’ (i.e., noninformative) cues: Wrst,participants should be told to take their time and to respondas accurately as possible. That is, they should not beencouraged to be ‘fast and accurate’, and what is more, nodeadline (either explicit or implicit) to respond should beimplemented (that is, it must be a ‘true’ accuracy experi-ment; McDonald et al.’s 2000, study was criticized on thisscore); second, the participants should be near 100% accu-rate in knowing where the target stimulus was presented(i.e., there should be no target–location uncertainty).Indeed, they argued that under conditions of target locationuncertainty, accuracy eVects in spatial cuing paradigms canoccur, because an observer may be uncertain as to the loca-tion where the target has been presented (Prinzmetal et al.2005b); third, cue validity should not be confounded withany other factor (such as, for instance, in Klein and Dick’s2002 study, where same-stream targets appearing immedi-ately after the cue, in valid trials, occurred on average laterin the experiment than other-stream targets on the invalidtrials), but should be orthogonal relative to all other factors;fourth, the eye-movements should be monitored in order toensure that any cuing eVects that are observed cannotsimply be accounted for participants potentially makingcue-directed eye-movements (this criticism was shown byPrinzmetal et al. to confound the interpretation of Dufour’s1999, studies).

While we believe that Prinzmetal et al.’s (2005a)hypothesis may, in principal, be correct, we do not agreethat it necessarily follows the orienting of a participant’sexogenous spatial attention will not aVect the accuracy of aparticipant’s judgments in all tasks. In particular, whilePrinzmetal et al.’s claim may be valid in certain domains(such as, for example, in the visual discrimination of theletters ‘F’ and ‘T’ used in their experiments), it may not becorrect in other domains. For example, in tasks requiringtemporal discriminations, for example, as in temporal orderjudgment (TOJ) or simultaneity judgment (SJ) tasks (e.g.,Santangelo and Spence 2008a). We therefore designedtwo experiments that matched the criteria outlined byPrinzmetal et al. (2005a) in order to further investigate thisissue (note, however, that eye-movements were onlyexplicitly monitored in “Experiment 2”). In particular, weassessed whether crossmodal attentional capture wouldaVect perceptual sensitivity in a TOJ task, in which twostimuli are presented at diVerent SOAs and the participantsjudge which stimulus appears to have been presented

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Wrst (e.g., Kanai et al. 2007; McDonald et al. 2005; Shoreet al. 2001; Spence et al. 2001). In order to elicit the exogenousorienting of spatial attention, a peripheral non-predictive(and modality-irrelevant) auditory cue was presented200 ms before the onset of the Wrst visual stimulus, on eitherthe left or rightside (e.g., Dufour 1999; Frassinetti et al.2002; McDonald et al. 2000, 2000; McDonald and Ward2000; Spence and Driver 1997). The visual stimuli couldeither be presented on the same side or opposite sides withrespect to a central Wxation point (cf. Shore et al. 2001).

This design therefore enabled us to investigate the inter-play between the exogenous orienting of spatial attentionand TOJs. If the exogenous orienting of crossmodal spatialattention aVects the sensitivity of participants’ responses,one might expect to observe a decrease in the just noticeablediVerence (JND; which provides a standardized measure ofthe accuracy of participants’ responses; in particular, an esti-mate of the interval between two stimuli needed in order forparticipants to judge their temporal order of occurrence cor-rectly on 75% of the trials) for cued than for uncued targettrials. Moreover, one would also expect to observe a shift inthe point of subjective simultaneity (PSS; which, in thiscase, would provide an estimate of the amount of time bywhich the uncued target has to lead the cued target in orderfor synchrony to be perceived, or rather, for the tworesponses to be chosen equally often by participants) as afunction of changing the focus of participants’ exogenousattention. This would be in line with previous Wndingsshowing a ‘prior entry’ eVect for attended relative to unat-tended stimuli (e.g., Neumann et al. 1993; McDonald et al.2005; Shimojo et al. 1997; Shore et al. 2001; Spence et al.2001; Titchener 1908; Vibell et al. 2007; Zampini et al. 2005).In particular, a PSS shift would account for a speeding-up ofthe relative time of arrival of the visual stimulus presented atthe attended, relative to the unattended location. Finally, iftemporal sensitivity is related to the amount of informationcoming from a certain spatial location, we would expect toobserve diVerent JND and PSS values in the same versusopposite sides conditions. This result would be consistentwith other recent Wndings showing an impairment of tempo-ral resolution as a function of the amount of informationpresented from a given spatial location (Nicol and Shore2007; see also Yeshurun and Levy 2003).

Experiment 1

Methods

Participants

Data were collected from 15 volunteers from OxfordUniversity, who reported normal or corrected-to-normal

vision and were naïve as to the purpose of the study. Theyall received a £5 UK sterling gift voucher in return for takingpart in the study, which lasted for approximately 40 min.Prior to the start of each of the experiments reported here,participants had to perform a pre-test, in which they had toindicate the side of presentation (left vs. right) of the audi-tory cue. Participants had to score at least 95% correct inthis pre-test in order to participate in the subsequent experi-mental session. None of the participants were excluded atthis stage, thus showing that they could localize the audi-tory cue without any problem.

Apparatus and materials

The stimuli and procedure are illustrated schematically inFig. 1. The two visual target stimuli (an ‘X’ and an ‘O’,2.9° £ 1.7°, black boldface, <0.01 cd/m2, 13 ms in dura-tion; i.e., one screen refresh) were displayed against a lightgrey background (8.2 cd/m2) on a 17� computer monitor(refresh rate = 75 Hz) located in a soundproof booth, 45 cmfrom the participant’s head. A spatially non-predictiveauditory cue (a white noise burst, SPL = 75 dB(A), 50 msin duration) was presented 200 ms before the onset of the

Fig. 1 Schematic representation of the sequence of events in the oppo-site sides and same side blocks. Each trial started with the onset of aWxation point, which remained on the screen until the ‘respond now’signal was presented. After 1,000 ms, the auditory cue was presentedequiprobably on either the left or right. 200 ms after the onset of theauditory cue, either an X or an O was presented on either the left orright. Following a SOA of 13, 27, 54, or 120 ms, the second visualstimulus was presented on either the same (same side condition) oropposite side (opposite sides condition) as the Wrst visual stimulus. Avisual pattern mask was presented for 1,000 ms following the oVset ofthe second visual stimulus. After the oVset of the visual mask, a‘respond now’ signal replaced the Wxation point

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Wrst visual stimulus on either the left or right side (bymeans of two loudspeaker cones, one located on each sideof the computer monitor). A visual pattern mask, consistingof scrambled X and O letter parts, was presented for1,000 ms coincident with the oVset of the second visualstimulus.

Procedure

Two diVerent conditions (same side and opposite sides)were presented in four separate blocks of experimental tri-als (two blocks per condition), with the order of presenta-tion of the two same side and two opposite sides blockscounterbalanced across participants, who were allowed torest between the blocks of trials. Each trial started with theonset of a Wxation point (a 0.8° £ 0.8° black cross), whichremained on the screen until a signal to respond was pre-sented (i.e., a ‘respond now’ display; see Fig. 1). The audi-tory cue was presented equiprobably on either the left orright after 1,000 ms. 200 ms after the onset of the auditorycue, either an X or an O was presented (for 13 ms; i.e., forone frame) on either the left or right. Following a SOA of13, 27, 54, or 120 ms, the other visual stimulus was pre-sented (for 13 ms) on the same (same side condition) oropposite side (opposite sides condition) as the Wrst visualstimulus. A visual pattern mask was presented for 1,000 msfollowing the oVset of the 2nd visual stimulus1 (e.g., Schar-lau and Neumann 2003). Immediately after the oVset of thevisual mask, a ‘respond now’ signal replaced the Wxation

point. This instruction remained on the screen until the par-ticipant’s response was detected. The auditory cue couldeither be presented on the side where the Wrst visual targetwas presented (i.e., ‘‘cued target Wrst’’ trials) or else on theopposite side (i.e., ‘‘uncued target Wrst’’ trials). The partici-pants were told not to pay any particular attention to theauditory cue, given that it was entirely uninformative withrespect to their task (note that our auditory cue was not onlytask-irrelevant, but also modality-irrelevant; see Santangeloand Spence 2008b, on this point). Each target letter (i.e., the‘‘X’’ and the ‘‘O’’) was presented Wrst equally often.

The participants were encouraged to take their timewhen judging which stimulus had been presented Wrst(i.e., they were informed that their task was completelyunspeeded) and to wait until the ‘respond now’ signalbefore making their response. If they responded prior theonset of this signal, they were forced to repeat theirresponse, so that the optimal strategy (as became apparentto participants during their practice section) was to waituntil the ‘respond now’ signal before initiating theirresponse. Participants initially completed 12-trials of train-ing in both the same side and opposite sides conditionsbefore starting the main experiment. The participantsresponded by pressing one of two response keys (either the‘X’ or ‘O’ key) on a computer keyboard. A new trial started1000 ms after a response was given on the preceding trial.

There were 128 trials [i.e., cue side (2) £ visual stimulusside (2) £ SOA (8) £ trial repetition (4)] in each of the fourblocks (two per experimental condition, same and oppositesides), which lasted for approximately 8 min. The data werecollapsed across the side of presentation (left vs. right).

Results and discussion

Mean responses for each participant for each condition atthe various SOAs tested were Wtted using a sigmoidfunction (see Fig. 2a) and then converted to their equivalentz-scores under the assumption of a cumulative normaldistribution (Finney 1964). These data were then used tocalculate the slope and the JND for each participant in eachexperimental condition. Finally, the data were collapsedacross the target type (‘X’ vs. ‘O’) in order to calculate theintercept and the PSS for each participant in each experi-mental condition as a function of ‘cued target Wrst’responses (see Fig. 2b). Greenhouse–Geisser adjustmentwas applied to all F tests in order to correct for possibleviolations of the assumption of sphericity. Five participantswere excluded from the data analyses, due to the fact thatthey had JND or PSS values that deviated more than twostandard deviations from the group average. The analyseswere therefore performed on the data from the remainingten participants (9 males, mean age 22.4 years, rangingfrom 18 to 32 years).

1 Note that the presentation of the visual mask was necessary in orderto make the TOJ task more diYcult. In fact, the results of a pilot studyconducted in the absence of visual masks showed a ‘ceiling eVect’, i.e.,performance that was near-perfect, thus showing that the task was tooeasy, and making it diYcult to meaningfully discriminating betweenperformance in the various experimental conditions. Note that al-though other methods might have been chosen in order to avoid ceilingeVects (e.g., using shorter SOAs between the two target stimuli, ormanipulating the contrast of the stimuli), masking was chosen since italso has the advantage that it reduces the impact of iconic memory onperformance, therefore allowing for a better evaluation of our partici-pants’ temporal sensitivity. However, we also conducted a controlstudy (N = 8 participants), in which we used only the shortest SOA(§13.3 ms), in order to check that participants were able to discrimi-nate the identity of the second visual stimulus, regardless of the presen-tation of the visual pattern mask just after its oVset. The task wasidentical to the one described in the main text, with a crucial exception:the identity of the two visual stimuli was made independent (i.e., boththe 1st and 2nd stimulus were equally likely to be an ‘X’ as an ‘O’).Consequently, our control participants could not identify the 2nd stim-ulus on the basis of the identity of the 1st stimulus. The results of thiscontrol study showed that the participants could discriminate the iden-tity of the second visual stimulus at a level that was signiWcantly abovechance, regardless of the presence of the mask in both same side andopposite sides conditions (Kruskal–Wallis test: �2 values > 12.9; pvalues < .001). IdentiWcation performance for the second visual stimu-lus did not change signiWcantly in the same versus opposite sides con-ditions (75.8 and 77.8%, respectively; �2s = 0.7; ps < 0.401).

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A two-way within-participants ANOVA on the JND datawith the factors of Side (same vs. opposite) and Cuing(cued vs. uncued) revealed a main eVect of side [F(1,9) = 12.0, MS = 7,960.0, p = 0.007], with a signiWcantlysmaller JND (i.e., higher sensitivity) being reported in theopposite sides condition (M = 39 ms) than in the same sidecondition (67 ms). This result is consistent with otherrecent Wndings showing an impairment of temporal resolu-tion as the amount of information presented from a certainspatial location increases (Nicol and Shore 2007; see alsoYeshurun and Levy 2003). JND values reported for thecued (49 ms) and the uncued (57 ms) target Wrst trials didnot diVer signiWcantly (main eVect of cuing [F(1, 9) = 3.7,MS = 662.7, p = 0.085]). Crucially, however, the analysisrevealed an interaction between these two factors [F(1,9) = 6.6, MS = 1194.9, p = 0.030], with a signiWcantlysmaller JND reported in the cued (30 ms) as compared tothe uncued (49 ms) target Wrst trials in the opposite sidescondition (as revealed by Bonferroni-corrected comparison,

p = 0.014), but not for the same side condition (69 and66 ms, respectively; p = 0.606), as one might expect.

Visual inspection of Fig. 2b suggests a PSS shift for theopposite sides condition, but not for the same sidecondition. Two two-tailed t tests (test value = 0, 95% conW-dence interval) on the PSS data conWrmed this result. Inparticular, they indicated that in the opposite sides condi-tion, the target on the uncued side had to have beenpresented 21 ms before the target on the cued side(t = ¡4.8, p < 0.001, ¡31.3 < PSS < ¡11.2 ms), for theparticipants to make both responses equally often, whilethere was no such diVerence between cued and uncuedtarget Wrst stimuli in terms of the PSS for the same sidecondition (mean diVerence = 0.2 ms; t = 0.1, p = 0.897,¡4.2 < PSS < 4.7 ms).

These results demonstrate that the presentation of a task-irrelevant spatially non-predictive auditory cue in “Experi-ment 1” led to an improvement in our participants’ abilityto judge the temporal order of pairs of visual stimuli whenthey were presented on opposite (i.e., diVerent) sides (andthe sound was presented on the side where the Wrst visualstimulus was presented). On the other hand, the presenta-tion of an exogenous peripheral auditory cue did not lead toany improvement in TOJ performance (i.e., no reduction ofthe JND was observed) when the to-be-judged stimuli werepresented from the same side (see Shore et al. 2001 for sim-ilar results in their unimodal visuospatial cuing study).

The shift in the PSS observed in the opposite sides con-dition of “Experiment 1” is consistent with previous Wnd-ings showing a ‘prior entry’ eVect for attended relative tounattended stimuli (e.g., McDonald et al. 2005; Neumannet al. 1993; Shimojo et al. 1997; Shore et al. 2001; Shoreand Spence 2005; Spence et al. 2001; Titchener 1908;Vibell et al. 2007; Zampini et al. 2005). The absence of asimilar shift in the PSS as a function of cuing in the sameside condition is also consistent with the available uni-modal literature (see Shore et al. 2001), since under suchconditions, the cue presumably speeds up the perception ofeither both stimuli (in the cued trials) or else neither stimu-lus (in the uncued trials).

The Wrst conclusion to be drawn from the results of“Experiment 1” is therefore that the presentation of a com-pletely task-irrelevant peripheral auditory cue can inXuencethe sensitivity of participants’ visual performance (seeMontagna and Carrasco 2006; Pestilli et al. 2007). Thishappens even in a genuinely unspeeded psychophysicaljudgment task, at least when the participants have to judgethe temporal order of two visual stimuli presented fromdiVerent spatial locations. The most parsimonious explana-tion for this result is in terms of the exogenous capture ofour participants’ spatial attention by the sudden onset of theauditory cue (see Spence et al. 2004 for a review). Partici-pants’ perception of any visual target that is subsequently

Fig. 2 a Mean proportion of ‘O’ Wrst responses as a function of theSOA and inset the JND for the same side and opposite sides conditionsin “Experiment 1”. b The mean proportion of ‘Cued target’ Wrstresponses as a function of the SOA, and inset, the PSS for the sameside and opposite sides conditions in “Experiment 1”. The psychomet-ric functions represent the sigmoid Wts of participants’ mean responsesfor each condition at the various SOAs tested

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presented on the cued side will then have beneWted from aprior entry eVect. Alternatively, however, it could also beargued that the crossmodal attentional capture eVectsreported here were not due to a shift in participants’ covertattention toward the side of the cue, but rather were simplydue to an overt shift of their eyes in the direction of theauditory cue on some proportion of the trials. In order toaddress this potential criticism, we therefore conducted asecond experiment in which we also controlled for any eye-movements that participants may have made.

Experiment 2

Methods

Participants

Data were collected from 14 volunteers from OxfordUniversity, who reported normal or corrected-to-normalvision and were naïve as to the purpose of the study. Theyreceived a £10 UK sterling gift voucher in return for takingpart in the study, which lasted for approximately 60 min.

Apparatus and materials

The apparatus and materials were identical to those used in“Experiment 1”, with the sole exception that horizontaleye-movements (of each participant’s right eye) were mon-itored using the IRIS, eye-movement measurement system(SKALAR, Cambridge Research Systems). An adjustablechinrest was used to minimize any head movements. Partic-ipants were instructed to avoid making eye-movements (orblinking), while the Wxation cross was presented.

Procedure

The procedure was the same as for “Experiment 1”, with thesole exception that an additional pair of SOAs (§240 ms;negative values indicate that the ‘X’ was presented Wrst,whereas positive values indicate that the ‘O’ was presentedWrst) was included in order to (1) extend our Wndings to alarger range of SOAs tested, and (2) make the task slightlyeasier to reduce the number of participants that had to beexcluded from the data analysis; each of the four test blockstherefore included 160 trials [cue side (2) £ visual stimulusside (2) £ SOA (10) £ trial repetition (4)].

Results and discussion

Two participants were excluded from the data analysesbecause their JND and/or PSS values deviated more thantwo standard deviations from the average of the group,

leaving 12 participants (7 males, mean age 23.9 years,ranging from 18 to 29 years). Trials in which horizontaleye-movements (>2.5°) or blinks occurred while the Wxa-tion cross was presented (M = 2.6% of all trials) wereremoved from further analyses. As in “Experiment 1”, sig-moid Wts of participants’ mean responses for each conditionat the various SOAs tested were computed and used to cal-culate the slope and JND values for each participant in eachcondition (see Fig. 3a).

A two-way within-participants ANOVA on the JND datawith the factors of Side (same vs. opposite) and Cuing(cued vs. uncued) revealed no main eVect of Side [F(1, 11)< 1, MS = 345.6, n.s.] or Cuing [F(1, 11) = 3.3, MS =6020.0, p = 0.101]. The interaction between these factorswas, however, once again, signiWcant [F(1, 11) = 6.5,MS = 3873.3, p = 0.029]. As in our previous experiment,Bonferroni-corrected comparisons revealed that the JNDwas signiWcantly smaller for cued (35 ms) as compared to

Fig. 3 a Mean proportion of ‘O’ Wrst responses as a function of theSOA and inset the JND for the same side and opposite sides conditionsin “Experiment 2”. b Mean proportion of ‘Cued target’ Wrst responsesas a function of the SOA, and inset, the PSS for the same side andopposite sides conditions in “Experiment 2”. The psychometric func-tions represent the sigmoid Wts of participants’ mean responses foreach condition at the various SOAs tested

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uncued (77 ms) target Wrst trials in the opposite sides condi-tion (p = 0.049), but not in the same side condition (59 and64 ms, respectively; p = 0.994). As in “Experiment 1”,analysis of the PSS data (calculated after collapsing the ‘XWrst’ and ‘O Wrst’ responses; see Fig. 3b) revealed that theuncued stimulus had to lead the cued stimulus by 28 ms forthe PSS in the opposite sides condition (t = ¡3.1,p = 0.011, ¡47.3 < PSS < ¡7.8 ms), but not in the sameside condition (mean diVerence = ¡0.5 ms; t = ¡0.1,p = 0.911, ¡9.5 < PSS < 8.6 ms). The null eVects observedfor the same side condition once again appear to indicate animpairment of temporal sensitivity when more stimuli arepresented from the same spatial location (Nicol and Shore2007; see also Yeshurun and Levy 2003).

Overall then, the results of “Experiment 2” conWrm theWndings obtained in “Experiment 1”, while at the same timeruling out any potential overt orienting (i.e., eye-movementrelated) account of our earlier Wndings. However, it couldbe argued that the eVects reported so far are not attentionalin nature, but more related to a ‘crossmodal temporalcapture’ eVect (e.g., Keetels and Vroomen 2007; Morein-Zamir et al. 2003; Parise and Spence 2008; Scheier et al.1999), or to a ‘crossmodal perceptual grouping’ eVect(Spence et al. 2007). That is, the auditory cue might simplyhave pulled the subjective perception of the two visualstimuli apart in time (thus improving performance in cuedrelative to uncued trials). For instance, Morein-Zamir et al.investigated whether irrelevant sounds can inXuence theperception of lights in a visual TOJ task, where participantshad to judge which of two lights appeared Wrst. Theyshowed that presenting one sound before the Wrst light andanother after the second light improved performance rela-tive to baseline (sounds appearing simultaneously with thelights), as if the sounds pulled the perception of lights fur-ther apart in time. Alternatively, participants might havebuilt up a ‘crossmodal perceptual group’ that included theauditory cue and the closest visual stimulus (i.e., the Wrstone).

In order to rule out these alternative hypotheses, weperformed a Wnal experiment in which the auditory tonewas now presented after the second visual stimulus in theTOJ task. If the crossmodal temporal capture (or percep-tual grouping) account were to be correct, we shouldobserve a bias toward the second visual stimulus thistime. More speciWcally, we would expect an increase inthe JND (and a change in the PSS) when the retro-cue andthe second visual stimulus were presented on diVerentsides as compared to when the retro-cue was presented onthe same side, where the second visual stimulus hadrecently been presented. Conversely, the absence of anysigniWcant modulations of the JND (or PSS) by the retro-cue would rule out a crossmodal temporal capture accountof our data.

Experiment 3

Methods

Participants

Data were collected from 12 volunteers (5 males, mean age25.6 years, ranging from 18 to 31 years) from the Univer-sity of Rome ‘La Sapienza’. They reported normal or cor-rected-to-normal vision and were naïve as to the purpose ofthe study, which lasted for approximately 60 min.

Apparatus and materials

The apparatus and materials were identical to those used in“Experiment 2”. The participants were instructed to avoidmaking any eye-movements (or blinking), while the Wxa-tion cross was presented. However, we no longer monitoredthe eye-movements of our participants, given that thepotential overt orienting account of our Wndings had beeneVectively ruled out by the results of “Experiment 2”.

Procedure

The procedure was the same as for “Experiment 2”, withthe sole exception that the auditory cue was presented75 ms after the onset of the second visual stimulus (ratherthan 200 ms prior to the onset of the Wrst visual stimulus),equiprobably on either the left or right side. Importantly,the 75 ms interval was chosen because Morein-Zamir et al.(2003) showed crossmodal temporal capture eVects forauditory retro-cues presented in the temporal range from 75to 225 ms after the presentation of their second visual stim-ulus. The auditory cue could therefore either be presentedon the side, where the second visual target was presented(i.e., ‘‘cued second stimulus’’ trials) or else on the oppositeside (i.e., ‘‘uncued second stimulus’’ trials). Just as in theprevious experiment, each of the four test blocks included160 trials [cue side (2) £ visual stimulus side (2) £ SOA(10) £ trial repetition (4)].

Results and discussion

As in the previous experiments, sigmoid Wts of participants’mean responses for each condition at the various SOAstested were computed and used to calculate the slope andJND values for each participant in each condition (seeFig. 4a). A two-way within-participants ANOVA on theJND data with the factors of Side (same vs. opposite) andRetro-cuing (cued vs. uncued) revealed no main eVects (forSide, [F(1, 11) < 1, MS = 833.0, n.s.]; and for Retro-cuing[F(1, 11) = 1.7, MS = 109.3, p = 0.220]), nor any interac-tion [F(1, 11) < 1, MS = 0.858, n.s.], indicating similar

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JND values in all of the experimental conditions (cuedsecond stimulus, same side = 69 ms; uncued secondstimulus, same side = 65 ms; cued second stimulus, oppo-site side = 59 ms; uncued second stimulus, opposite side =56 ms; see Fig. 4a). The analysis of the PSS data (calcu-lated after collapsing the ‘X Wrst’ and ‘O Wrst’ responses;see Fig. 4b) also failed to reveal any signiWcant eVect ineither the same side condition (mean diVerence = 1.2 ms;t = 0.333, p = 0.746, ¡7.5 < PSS < 10.1 ms), or in theopposite sides condition (mean diVerence = ¡1.5 ms;t = ¡0.398, p = 0.700, ¡10.0 < PSS < 7.0 ms).

Taken together, the results of this Wnal experimentclearly show that when the auditory tone is presented afterthe second visual stimulus, there were no substantial diVer-ences between cued and uncued conditions in the partici-pants’ TOJ performance (i.e., no modulation of the JND orPSS was observed), thus ruling out a crossmodal temporalcapture (or crossmodal perceptual grouping) view of ourWndings.

General discussion

The experiments reported in the present study were designedto examine whether or not the crossmodal orienting of exog-enous spatial attention would aVect the accuracy of partici-pants’ judgments in a ‘genuinely unspeeded’ perceptualtask, when the relative timing of the two events was thedimension along which participants had to make their dis-crimination responses. The participants in our experimentshad to judge which of two visual stimuli (an X or O), pre-sented on the same or opposite sides of a computer monitor,had been presented Wrst. Crucially, a completely task-irrele-vant, and spatially non-predictive, auditory cue was shownto improve the sensitivity of participants’ TOJ responses (asreXected by a decrease in the JND) when it happened to bepresented on the side on which the Wrst visual stimulus waspresented in the opposite sides condition (while the samecue was irrelevant when presented after the second visualstimulus; see the null eVects reported in “Experiment 3”).This Wnding demonstrates that the exogenous orienting ofspatial attention can aVect the accuracy of participants’ gen-uinely-unspeeded responses, at least in tasks that involve thejudgment of the temporal order in which two sequentially-presented and spatially-separated (see the null eVect for thesame side condition) visual stimuli were presented.

The eVectiveness of task-irrelevant exogenous auditorycues in inXuencing visual discrimination performance whentemporal order was the dimension along which participantshad to make their discrimination responses was also sup-ported by the observed shift in the PSS. The fact that thevisual stimulus presented on the uncued side had to lead atthe PSS suggests that the orienting of spatial attention elicitedby the presentation of the peripheral auditory cue may havesped-up the elative time at which participants became awareof visual stimuli subsequently presented from the cued loca-tion. In fact, the PSS shift observed in “Experiments 1 and 2”is consistent with the existence of the phenomenon of multi-sensory prior entry (e.g., McDonald et al. 2005; Shore andSpence 2005; Spence et al. 2001, 2004; Vibell et al. 2007;Zampini et al. 2005). That is, the exogenous auditory cuesmay have sped-up the relative time of arrived of the visualstimulus presented at the cued location relative to any stimulipresented elsewhere (i.e., from an uncued location).

These results appear to be in line with the proposal madeby Prinzmetal et al. (2005a) that the involuntary (exogenous)orienting of spatial attention aVects a process they call channelselection (or decision). They argued that this process involvesthe decision (or selection) as to which location to respond to,but they argued it does not aVect the perceptual representationof the stimuli at that (or at any other) location (by contrast,they argue that endogenous orienting leads to channelenhancement instead, which is thought to aVect the percep-tual representation of the stimuli). In the present series of

Fig. 4 a Mean proportion of ‘O’ Wrst responses as a function of theinterstimulus interval (ISI), and inset, the JND for the same side andopposite sides conditions in “Experiment 3”. b Mean proportion of‘Cued second stimulus’ as a function of the ISI, and inset, the PSS forthe same side and opposite sides conditions in “Experiment 3”. Thepsychometric functions represent the sigmoid Wts of participants’ meanresponses for each condition at the various SOAs tested

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experiments, we measured both the JND and PSS. WhileJNDs have been typically used to measured changes in sensi-tivity, PSSs have been usually used to measure timing toaccess awareness (Spence et al. 2001). The experimentsreported here show a clear improvement in the sensitivity ofparticipants’ TOJ performance (JND results, “Experiments 1and 2”) when an auditory tone directed their exogenous spatialattention to the location, where the Wrst visual stimulus wassubsequently presented. In principle, our JND results mightsuggest that some form of perceptual facilitation (i.e., a ‘per-ceptual enhancement’ in Prinzmetal et al.’s terms) took placeat cued as compared to uncued target locations in our experi-ments. However, a change in sensitivity for cued as comparedto uncued trials can be seen as the logical consequence of thespeeding-up of the processing in the cued side/channel (ourPSS results, “Experiments 1 and 2”), which aVected the per-ceived interval between the two visual stimuli, thus explainingthe observed JND diVerences between cued and uncued trials.

It is, however, worth noting that the present results are alsorelevant to Prinzmetal et al.’s (2005a, b) conclusions. Whilethe channel selection hypothesis provides the most parsimoni-ous account of the present data, it should be noted that thisdoes not necessarily imply that involuntary attention cannotalso aVect performance in a genuine accuracy experiment.Such a possibility was, in fact, hinted at by Prinzmetal et al.(2005a): “We cannot claim that a noninformative cue couldnever aVect performance in a pure accuracy experiment” (p.89). As pointed out in the “Introduction”, Prinzmetal et al.(2005a) outlined four criteria that, they argued, one needs tomeet in order to convincingly demonstrate an improvement ofaccuracy elicited by the presentation of exogenous cues. Thesecond experiment reported in this study, we believe, matchesall these criteria (while “Experiment 1” meets only the lastthree criteria). In particular: (1) the possibility that participantsmade eye-movements in the direction of the cue was ruledout; (2) it was a genuine ‘accuracy’ experiment, in which nodeadline to respond was given, and where participants wereprevented from making a response until at least a second afterthe presentation of the two target stimuli (note that from amethodological point of view, Prinzmetal et al. 2005a, didnot use a delay to prevent participants from respondingearly in their accuracy experiments); (3) the resultsobtained in the opposite sides condition cannot be attrib-uted to location uncertainty2; and (4) the cue validity factor

was not confounded with any other factor in our exogenouscuing design (see also Prinzmetal et al. 2005b, on this point).Here we therefore provided the Wrst empirical demonstration(at least, to our knowledge) that an exogenous cue can indeedaVect performance in a pure accuracy experiment (though it ispossible that our Wndings may only apply to the domain oftemporal judgments, such as TOJs; this remains an interestingquestion for future studies).

To conclude, the experiments reported in the presentstudy show that the exogenous orienting of auditory spatialattention can inXuence accuracy judgments (at least forjudgments of temporal order) by speeding-up the time ofaccess to visual stimuli at the cued location (or ‘channel’,relative to stimuli at the uncued ‘channel’; see “Experiments1 and 2”). All previous attempts to show that an auditoryperipheral cue facilitates visual temporal performance at aperceptual level (using an accuracy measure; e.g., Dufour1999; Frassinetti et al. 2002; McDonald et al. 2000; see alsoSpence et al. 2004) suVer from the fact that they did notmeet Prinzmetal et al.’s (2005a) criteria of convincinglydemonstrating an eVect in an accuracy experiment involvingthe presentation of automatic—noninformative—cues; seePrinzmetal et al. for a discussion of this point). The currentstudy therefore represents the Wrst study (including bothintramodal and crossmodal cuing paradigms) to demonstratethat exogenous orienting of attention facilitates the accuracyof visual perception, while at the same time conforming toall four of Prinzmetal et al.’s criteria.

Acknowledgments We would like to thank Alberto Gallace andArgiro Vatakis for their helpful suggestions with regard to the dataanalysis.

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