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Cognition xxx (2006) xxx–xxx

Brief article

Perceptual and decisional contributionsto audiovisual interactions in the perceptionof apparent motion: A signal detection study

Daniel Sanabria a,*, Charles Spence a, Salvador Soto-Faraco b

a Department of Experimental Psychology, University of Oxford, UKb ICREA and Parc Cientıfic de Barcelona – Universitat de Barcelona, Spain

Received 30 May 2005; revised 23 November 2005; accepted 8 January 2006

Abstract

Motion information available to different sensory modalities can interact at both per-ceptual and post-perceptual (i.e., decisional) stages of processing. However, to date,researchers have only been able to demonstrate the influence of one of these componentsat any given time, hence the relationship between them remains uncertain. We addressedthe interplay between the perceptual and post-perceptual components of information pro-cessing by assessing their influence on performance within the same experimental paradigm.We used signal detection theory to discriminate changes in perceptual sensitivity (d 0) fromshifts in response criterion (c) in performance on a detection (Experiment 1) and a classi-fication (Experiment 2) task regarding the direction of auditory apparent motion streamspresented in noise. In the critical conditions, a visual motion distractor moving either left-ward or rightward was presented together with the auditory motion. The results demon-strated a significant decrease in sensitivity to the direction of the auditory targets in thecrossmodal conditions as compared to the unimodal baseline conditions that was indepen-dent of the relative direction of the visual distractor. In addition, we also observed signif-icant shifts in response criterion, which were dependent on the relative direction of thedistractor apparent motion. These results support the view that the perceptual and

0010-0277/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.cognition.2006.01.003

* Corresponding author. Tel.: +44 1865 271307; fax: +44 1865 310447.E-mail address: daniel.sanabria@psy.ox.ac.uk (D. Sanabria).

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decisional components involved in audiovisual interactions in motion processing can coex-ist but are largely independent of one another.� 2006 Elsevier B.V. All rights reserved.

Keywords: Crossmodal interactions; Motion perception; Apparent motion; Audition; Vision

1. Introduction

Recent research has revealed substantial crossmodal links in the processing ofmotion information (Soto-Faraco, Kingstone, & Spence, 2003). Multisensory inter-actions in motion perception have often been addressed using intersensory conflictsituations (Welch & Warren, 1980) where participants judge a particular feature ofmotion in one sensory modality (e.g., sound direction) while trying to ignoremotion information presented in another modality (e.g., vision). Given that theirrelevant distractors can potentially affect perceptual (Soto-Faraco, Spence, &Kingstone, 2005) as well as decisional (post-perceptual) mechanisms (Wuerger,Hofbauer, & Meyer, 2003), the level of processing at which these interactions takeplace remains somewhat controversial (Bertelson & de Gelder, 2004; Soto-Faracoet al., 2003).

Dissociating perceptual from post-perceptual processes is critical to many areas ofcrossmodal research (Aschersleben, Bachmann, & Musseler, 1999; Bertelson & deGelder, 2004; de Gelder & Bertelson, 2003). Perceptual processes affect the combina-tion of multisensory cues prior to response selection/execution, whereas post-percep-tual processes influence general response selection and/or execution mechanismsinstead. Several studies addressing audiovisual interactions in motion processinghave reported post-perceptual influences in the absence of any perceptual effects,supporting the claim that motion cues are processed independently in each sensorymodality at a perceptual level, with any interactions occurring only at decisionalstages (Alais & Burr, 2004; Meyer & Wuerger, 2001; Wuerger et al., 2003). By con-trast, other findings support the notion that auditory and visual motion informationcan interact at a perceptual level (Kitagawa & Ichihara, 2002; Mateeff, Hohnsbein, &Noack, 1985; Soto-Faraco et al., 2005; Vroomen & de Gelder, 2003). These latterstudies have used methodologies such as psychophysical staircases (Soto-Faracoet al., 2005) or adaptation after-effects (Kitagawa & Ichihara, 2002; Vroomen &de Gelder, 2003), that are designed to minimize potential cognitive and/or responsebiases that affect decisional stages of processing. Overall, this pattern of results sug-gests that both perceptual and post-perceptual influences might play a significantrole in explaining the interactions between auditory and visual motion.

We investigated the relationship between these two components within the sameexperimental paradigm by using signal detection theory (SDT; Macmillan & Creel-man, 1991). Participants had to detect auditory apparent motion streams moving ina predefined (target) direction (left or right). In one condition, auditory streams (thatcould move in the target or the non-target direction) were combined with visual

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distractor motion that always moved in the direction of the pre-defined target (tar-get-compatible block). In the other condition, the visual distractors always movedin the direction opposite to the pre-defined target auditory motion (target-incompat-ible block). Both conditions were physically equivalent (with directionally congruentand incongruent audiovisual motion displays being equiprobable), the only differ-ence being that the direction of the visual distractor was either compatible or incom-patible with the pre-defined auditory target direction (see Fig. 1). Participants alsoperformed a unimodal block where they detected the direction of sound streams inthe absence of visual distractors.

If visual motion can capture the perceived direction of auditory motion (Soto-Faraco et al., 2005), then we should find considerable lower sensitivity in crossmodalblocks than in the unimodal baseline. That is, participants should find it more diffi-cult to discern the direction of sounds because of the influence exerted by the visualdistractors, which always move in one direction. Since target-compatible and target-incompatible blocks contain physically equivalent trials, they should be no differentin terms of participants’ sensitivity. Any influence of the visual distractor on partic-ipants’ decisions about sound direction should manifest itself as a change in theresponse criterion, that would shift as a function of compatibility of the directionof visual motion.

To ensure that participants in Experiment 1 performed a pure detection task(as opposed to a discrimination task), we used a go/no-go procedure. InExperiment 2, we used a two-alternative forced-choice (2AFC) procedure toassess the influence of the task at hand on participants’ response criterion,as the processes involved in the decision and response execution have some-times been shown to differ between these two tasks (Chmiel, 1989; Perea, Rosa,& Gomez, 2002).

Fig. 1. Example of the type of audiovisual (directionally congruent and incongruent) trials present in thetwo crossmodal blocks (target-compatible and target-incompatible). In the example shown here, thedirection of target auditory motion was toward the right. Note that when participants were instructed todetect leftward auditory motion the auditory and visual streams moved in opposite directions to thosedepicted in the figure. AM and VM refer to the direction of auditory and visual motion, respectively.

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2. Experiment 1

2.1. Methods

2.1.1. ParticipantsForty-eight participants (34 women, 19–40 years) took part. All reported normal

hearing and normal or corrected-to-normal vision.

2.1.2. Apparatus and stimuli

Four loudspeakers (10 cm in diameter) positioned at eye-level were used to pres-ent the auditory stimuli. The visual stimuli consisted of the illumination of fourorange LEDs (1 cm in diameter), each situated directly above each loudspeaker(see Fig. 2).

The apparent motion streams consisted of four events (white noise burst 65 dB[A]for the sounds, and LED flashes for the lights) sequentially presented (50 ms on,50 ms off) from each of four correlative spatial locations from left-to-right or viceversa. An auditory mask consisting of four 50 ms white noise bursts (65 dB[A],50 ms off time) originating from the centre (elicited by simultaneously presentingsounds to the loudspeakers located on either side) before and after the auditory stim-ulus was used to lower performance.

2.1.3. Procedure

Participants sat in a dark roomandwere asked to detect either leftward or rightwardauditory apparent motion by pressing a key on a keyboard, and withholding theirresponse when the sound moved in the non-target direction (target direction wasmanipulated across participants). The sounds moved in the target direction on 50%of trials and in the opposite direction on the remaining trials. The next trial commenced1500 ms after response or after 3500 ms on trials where no response was made.

The session began with a practice block in which 12 auditory streams were pre-sented in the absence of visual distractors (this was repeated if performance was

Fig. 2. Illustration of the experimental set-up used in the present study.

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below 83%). Then a threshold block was run to establish the mask-target-interval(MTI) at which the participant’s performance without visual distractors fell between65% and 85% (using the method of limits). The MTI was continually adjustedaccording to the performance in the last 10 trials throughout the rest of the experi-ment. Next, in the baseline block participants completed 60 trials of auditory streams(50% in the target direction) presented in the absence of visual distractors. Finally,two crossmodal blocks (order counterbalanced) were run. Each consisted of 60 trials(50% in the target direction) in which a visual apparent motion stream was presentedin synchrony with the auditory stream, and in a fixed direction. In the ‘‘target-com-patible’’ block, the visual distractor direction matched that of the predefined targetauditory motion (if the task was to detect leftward auditory motion, the visualmotion always moved toward the left). In the ‘‘target-incompatible’’ block, the direc-tion of the visual distractor was always opposite to that of the pre-defined targetauditory motion. Note that crossmodal blocks contained 50% audiovisually congru-ent and 50% audiovisually incongruent trials, regardless of the auditory target direc-tion to which participants were instructed to respond to (see Fig. 1) and thecompatibility of the visual distractor with this target direction.

2.2. Results

Sensitivity (d 0) and criterion (c) were assessed in the unimodal baseline, and inthe target-compatible and target-incompatible crossmodal blocks for each partici-pant based on their hit and false-alarm rate. Repeated-measures analyses of vari-ance (ANOVA) revealed no significant differences in either d 0 or c (both Fs < l) asa function of absolute target direction (left or right), thus the data were pooledacross this variable in subsequent analyses. d 0 was significantly higher in the base-line unimodal block (M = 1.76) than in either crossmodal block (target-compatible,M = 1.24, |t| (47) = 4.10, p < .001; target-incompatible, M = 1.36, |t| (47) = 2.95,p = .004). The difference in d 0 between the two crossmodal blocks was not signif-icant, |t| < 1.

For the criterion data, in the baseline condition c was virtually zero (there was nopreference toward responding or not), whereas c was negative in the target-compat-ible block (M = �0.24, revealing a bias toward responding) and positive in the tar-get-incompatible block (M = 0.22; people tended not to respond). The differences inc between the baseline block and the two crossmodal blocks, and the differencebetween the two crossmodal blocks themselves, were all significant (all ps < .01;see Fig. 3).

The results of Experiment 1 revealed that visual motion information had a signif-icant influence on the perceived direction of auditory motion (i.e., on the ability ofparticipants to distinguish a leftward from a rightward moving sound). This decreasein d 0 was independent of the compatibility effect arising between the direction of to-be-detected auditory target and the direction of the visual distractor. However,response criterion shifted as a function of the compatibility between target directionand the distractor direction. This provides a dissociation between the perceptual anddecisional components responsible for audiovisual interactions in motion perception.

Fig. 3. (a) Mean d 0 (+SE) as a function of the Experiment (1 or 2) and condition (baseline, target-compatible, and target-incompatible). (b) Mean c (+SE) as a function of the experiment and condition.

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In Experiment 1, we used a go/no-go task to ensure that participants per-formed a detection task. However, it has been argued that different response-related processes might be involved in 2AFC discrimination tasks (Pereaet al., 2002), thus potentially affecting the conclusions of our study. In Exper-iment 2, we used a 2AFC task in which participants responded to both thetarget direction (‘‘yes’’ response) and non-target direction (‘‘no’’ response)auditory streams. Another factor that might have affected the criterion mea-sures in Experiment 1 was the particular auditory masking procedure used.As the MTI was adjusted continually on the basis of each participant’s per-formance, it is possible that the difficulty of the task changed within eachexperimental block, perhaps eliciting shifts in response criterion and interactingwith the distracting effects induced by the irrelevant visual information. InExperiment 2, constant white noise was presented instead of the auditorymask to maintain a constant level of difficulty throughout the experimentalsession.

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3. Experiment 2

3.1. Methods

3.1.1. ParticipantsTwenty-eight new participants (18 women, 16–32 years) took part. The data from

two participants were removed as they performed at chance levels in the baselinecondition.

3.1.2. Apparatus and stimuli

Participants were instructed to press ‘‘h’’ or ‘‘b’’ on the keyboard whenever thesound appeared to move in the target or the non-target direction (respectively;response mapping reversed for half of the participants). The next trial began onlyafter participants had responded (1500 ms intertrial interval). White noise(77 dB[A]) was presented continuously from a loudspeaker placed 20 cm behindthe participant to bring performance off ceiling (performance in a pilot study withthis noise level was below 80%).

3.2. Results

d 0 was significantly higher in the baseline block (M = 1.26) than in either the tar-get-compatible, |t| (25) = 2.55, p = .02, or target-incompatible crossmodal blocks,|t| (25) = 2.32, p = .03 (M = 0.77 and 0.88, respectively), whereas the two crossmodalblocks did not differ, |t| < 1. Criterion (was close to zero in the unimodal baselineblock (M = 0.02), negative in the target-compatible block (M = �0.33) and positivein the target-incompatible block M = 0.29). All comparisons reached statistical sig-nificance (all ps < .001; see Fig. 3).

We performed a between-experiments analysis with Experiment (1 and 2) andcondition (unimodal, target-compatible, and target-incompatible) as factors. Therewas no interaction between condition and experiment in d 0 F (1,72) < 1, though par-ticipants performed worse in Experiment 2 than in Experiment 1 (M = 1.45 and 0.97,respectively; F (l, 72) = 17.51, p < .0001), indicating that the white noise mask(Experiment 2) proved more effective than the mask procedure used in Experiment1. Critically, the main effect of condition, F (2,72) = 13.91, p < .0001, revealed thesame pattern observed in each of the individual experiments. The analysis of c onlyshowed a significant main effect of condition, F (2,72) = 46.33, p < .0001, again sup-porting the results of each individual experiment.

In Experiment 2, participants performed a 2AFC task regarding the direction ofauditory apparent motion (Soto-Faraco, Lyons, Gazzaniga, Spence, & Kingstone,2002), allowing us to analyse the data from the crossmodal blocks in terms of thedirectional congruency between the two modalities. d 0 was higher for directionallycongruent trials (M = 1.46) than for incongruent ones (M = 0.19; t (25) = 6.39,p < .001). Participant’s were also more sensitive on congruent trials than in the uni-modal baseline (M = 1.26), although this difference was not statistically significant,t (25) = 1.09, p = .28. These results suggest that the decrement in d 0 reported in the

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crossmodal blocks was primarily caused by a decrease in participants’ sensitivitycaused by directional incongruence between visual and acoustic cues to motion, sup-porting our earlier prediction. The results regarding participants’ sensitivity inExperiments 1 and 2 cannot be solely accounted for by an interference effect dueto the presence of irrelevant visual information (which was present in both the con-gruent and incongruent trials). On the contrary, it appears that the perceived direc-tion of the auditory apparent motion stream was captured by the direction of thevisual apparent motion stream on incongruent trials, resulting in an overall reduc-tion of perceptual sensitivity.

The results of Experiment 2 reinforce those of Experiment 1 in showing that theperceptual and post-perceptual influences on audiovisual interactions are indepen-dent. Importantly, there were no significant changes in criterion between the twoexperiments, suggesting that the same response mechanisms were invoked by partic-ipants in the 2AFC and go/no-go tasks used here (Gomez, Perea, & Ratcliff, submit-ted). Furthermore, Experiment 2 confirmed that the shifts in c were caused by thedirect influence of the visual information and not by the particular masking proce-dure used in Experiment I1.

4. General discussion

These findings provide the first empirical evidence for the independent co-exis-tence of both perceptual and post-perceptual influences on audiovisual interactionsin motion processing2. Our results confirm the existence of interactions at a percep-tual level in the processing of motion information (Soto-Faraco et al., 2005), consis-tent with previous data obtained using static events (Bertelson & de Gelder, 2004).Additionally, the reported shifts in c, independent of changes in d 0, confirmed thatthe congruence of visual motion information can also bias decisions about the direc-tion of sounds.

Previous studies of audiovisual interactions in motion perception have arrived atapparently divergent conclusions regarding the level of processing at which theseinteractions occur. A number of studies have suggested that auditory and visual

1 A further control experiment (N = 16) was run with constant white noise (as in Experiment 2) and ago/no-go task (as in Experiment 1). The results were equivalent to those of Experiments 1 and 2. Namely,we found significant differences in d 0 between the unimodal baseline block (M = 2.41) and bothcrossmodal blocks (M = 1.47; p < .01, for target-compatible; and M = 1.62; p < .01, for target-incompatible), whereas the two crossmodal blocks did not differ (|t| < 1). The difference in c betweenthe target-compatible and target-incompatible crossmodal blocks was significant (M = �0.10 and 0.63,respectively, p < .01). There were also significant differences in c between the unimodal baseline block(M = 0.39) and both crossmodal blocks (ps < .05).2 It is worth noting that audiovisual interactions in the perception of motion are asymmetrical. While

vision has been shown to exert a considerable influence over the perception of moving auditory stimuli,visual motion perception is usually little influenced by audition (Soto-Faraco, Spence, & Kingstone, 2004).Therefore, one might expect that, unless the visual signal is presented at or near threshold (cf. Meyer &Wuerger, 2001), visual motion processing would not be influenced by distracting auditory information.

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motion information can interact prior to response selection/execution (Soto-Faracoet al., 2005), whereas others have argued against the existence of perceptual interac-tions in the perception of audiovisual motion (Wuerger et al., 2003). The presentstudy demonstrates the existence of both perceptual and post-perceptual influenceson audiovisual interactions in the perception of motion within the same experimentalprotocol. Importantly, these influences were shown to be largely independent of oneanother.

Meyer and Wuerger (2001) studied the effect of irrelevant auditory motioninformation on visual motion processing, and reported large shifts in responsecriterion consistent with the direction of distractors, together with a small dec-rement in perceptual sensitivity to visual motion in congruent audiovisual dis-plays. This sensitivity shift is an intriguing result, given that it contrasts withother studies that have demonstrated perceptual enhancement (and not inhibi-tion) for co-localized (and congruent) audiovisual signals (Meyer, Wuerger,Rohrbein, & Zetzsche, 2005). Indeed, as noted by the authors themselves, it isunlikely that the changes in the sensitivity index in their study were causedby interactions taking place at a perceptual level of processing (cf. Soto-Faraco& Kingstone, 2004).

It seems likely that the decrement in d 0 reported in the crossmodal blocks (ascompared to the unimodal baseline block) of Experiments 1 and 2 reflects theexistence of a perceptual illusion whereby the perceived direction of the auditorystream was, in many trials, ‘‘captured’’ by the direction of motion of the visualstream. That is, on trials where the auditory and visual streams moved in oppo-site directions, observers may have perceived the sounds moving in the directionof the visual stream, thereby resulting in a reduced ability to distinguish theseincongruent trials from congruent ones (when sounds actually moved in the direc-tion of visual distractors). This interpretation is supported by phenomenologicalreports from early studies (Zapparoli & Reatto, 1969) as well as by recent psy-chophysical data (Soto-Faraco et al., 2005). However, the decrease in d 0 reportedin the crossmodal blocks might also reflect a cost of dividing attention betweenthe visual and the acoustic information (Bonnel & Hafter, 1998). We do notfavour this interpretation because, using a similar stimulus set-up to thatdescribed here, Soto-Faraco et al. (2002, Experiment 3) failed to observe any dec-rement in performance when the visual and auditory streams moved in orthogonal

directions (i.e., the mere presence of visual motion information seemed insufficientin-and-of-itself to influence the perception of auditory motion in the mannerobserved here). Moreover, a decrement in d 0 was only found on incongruentaudiovisual trials, while a numerical (albeit not statistically significant) increasein perceptual sensitivity was found on congruent audiovisual trials, supportingthe ‘capture’ account, rather than the distraction account.

Visual motion information not only affected the perceptual processing of auditorymotion, but also participants’ decisions regarding the direction of the auditorystreams. The fact that the feature to be judged (left–right direction) was the sameas the dimension in which the visual distractors moved introduced potential stimu-lus-response compatibility effects (Simon, 1990; see Bosbach, Prinz, & Kerzel,

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2004, 2005, for examples of such effects elicited by moving stimuli). At decisionalstages of information processing, the response code for the direction of motionwould have been primed by the direction of the salient visual distractor stimulus,accounting for the tendency toward ‘‘yes’’ responses in target-compatible blocksand toward ‘‘no’’ responses in target-incompatible blocks.

The results reported here go beyond previous research that has investigated mul-tisensory motion processing by demonstrating the independent co-existence of bothperceptual and post-perceptual influences on performance, while at the same timeproviding solid evidence regarding the perceptual nature of these crossmodal inter-actions. Thus, the present results stands in contrast with recent studies claiming thatvisual and auditory motion information interact only at decisional stages (Alais &Burr, 2004; Meyer & Wuerger, 2001; Wuerger et al., 2003). We believe that method-ological factors that might have promoted sensory segregation rather than sensoryintegration (see Sanabria, Soto-Faraco, Chan, & Spence, 2005, for empirical evi-dence) may account for the null results reported in these earlier studies. For example,the use of random dots kinetograms and inter-aural differences for the visual andauditory stimuli (respectively) in these previous studies means that the auditoryand visual cues to motion were different in number and were not necessarily spatiallyco-localised. In contrast, we used discrete visual and auditory stimuli that were bothspatially and temporally matched (Soto-Faraco et al., 2002, 2004), and were of anequal number (Sanabria et al., 2005; Spence, Sanabria, & Soto-Faraco, in press).As has been shown for the case of static events (Morein-Zamir, Soto-Faraco, &Kingstone, 2003; Spence & Driver, 2004; see also Welch, 1999), it would seem likelythat in order to find the most pronounced crossmodal interactions at early levels ofprocessing, auditory and visual inputs need to be spatiotemporally aligned andmatched in number as far as possible.

Acknowledgements

This study was supported by a Network Grant from the McDonnell-Pew Centrefor Cognitive Neuroscience in Oxford to Salvador Soto-Faraco and Charles Spence.

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