Auditory self-motion illusions ("circular vection") can be facilitated by vibrations and the potential for actual motion

Transcription

1 Submitted to ACM APGV 28 cference Auditory self-moti illusis ("circular vecti") can be facilitated by vibratis and the potential for actual moti Bernhard E. Riecke Vanderbilt University, USA (now at Sim Fraser University, Canada) Daniel Feuereissen Vanderbilt University, USA John J. Rieser Vanderbilt University, USA Abstract 1 Introducti It has lg been known that sound fields rotating around a statiary, blindfolded observer can elicit self-moti illusis ("circular vecti") in 2-6% of participants. Here, we investigated whether auditory circular vecti might depend whether participants sense and know that actual moti is possible or impossible. Although participants in auditory vecti studies are often seated moveable seats to suspend the disbelief of self-moti, it has never been investigated whether this does, in fact, facilitate vecti. To this end, participants were seated a hammock chair with their feet either solid ("movement impossible" cditi) or suspended ("movement possible" cditi) while listening to individualized binaural recordings of two sound sources rotating synchrously at 6 /s. In additi, hardly noticeable vibratis were applied in half of the trials. Auditory circular vecti was elicited in 8/16 participants. For those, adding vibratis enhanced vecti in all dependent measures. Not touching solid increased the intensity of self-moti and the feeling of actually rotating in the physical room. Vecti set latency and the percentage of trials where vecti was elicited were ly marginally significantly (p<.1) affected, though. Together, this suggests that auditory self-moti illusis can be strger when e senses and knows that physical moti might, in fact, be possible (even though participants always remained statiary). Furthermore, there was a benefit of both adding vibratis and having e s feet suspended. These results have important implicatis both for our theoretical understanding of self-moti percepti and for the applied field of self-moti simulatis, where both vibratis and the cognitive/perceptual framework that actual moti is possible can typically be provided at minimal cost and effort. CR Categories: H.1.2 [Models and Principles]: User/Machine Systems Human factors, Human informati processing H..1 [Informati Interfaces and Presentati]: Multimedia Informati Systems Artificial, augmented, and virtual realities J.4 [Social and Behavioral Sciences]: Psychology Keywords: self-moti illusis, circular vecti, auditory vecti, self-moti simulati, human factors, psychophysics, virtual reality, cue-integrati, vibratis, higher-level/cognitive influences ber1[at]sfu(dot)ca feuereissen[at]gmail.com j.rieser[at]vanderbilt.edu While modern virtual reality simulatis can have stunning photorealism, they are typically unable to provide a life-like, compelling experience of actually moving through the simulated world. This might limit perceived realism, behavioral effectiveness, user acceptance, and thus commercial success. We propose that investigating and exploiting self-moti illusis might be a lean and elegant way to overcome such shortcomings and provide a truly moving experience in computer-mediated envirments. 1.1 Self-moti illusis Probably e of the most compelling illusis are self-moti illusis ( vecti ), where moving visual stimuli can induce a powerful sensati of self-moti, even though e never physically moves [Dichgans and Brandt 1978; Mach 187; Warren and Wertheim 199]. Many people know this phenomen as the train illusi : When sitting in a statiary train waiting to depart from the train stati, watching a train departing from the adjacent track can induce a strg (but erreous) sensati that e s own train is departing in the opposite directi. Moving auditory stimuli can elicit similar, albeit less compelling, self-moti illusis in 2-6% of blindfolded participants. Although auditory vecti has been described a lg time ago [Dodge 1923] and later been replicated several times [Gekhman 1991; Hennebert 196; Lackner 1977; Marmekarelse and Bles 1977], there has ly recently been an increased interest in this interesting phenomen [Larss et al. 24; Riecke et al. 2c; Sakamoto et al. 24; Väljamäe et al. 24]. For recent reviews auditory vecti, see [Riecke et al. 28a; Väljamäe 2; Väljamäe 27]. While earlier studies typically used an array of speakers to present sound fields rotating around the statiary blindfolded listener, more recent studies demstrated that headphe-based, real-time spatialized audio rendering can also be employed to induce both circular and linear vecti. In the current study, we addressed two open questis that will be outlined in the following subsectis. 1.2 Does the possibility of actual self-moti facilitate illusory self-moti? Recently, there has been an increasing number of studies showing that vecti is not ly affected by low-level, bottom-up factors like the physical parameters of the vecti-inducing stimuli, but also by cognitive, higher-level factors like the interpretati and meaning of the vecti-inducing stimulus [Palmisano et al. 2; Palmisano et al. 23; Riecke et al. 2c; Schulte-Pelkum and Riecke 28]. Here, we were interested in investigating whether e s percepti, pre-knowledge, and assumptis about whether actual moti is possible or not might affect circular auditory vecti Visual vecti Lepecq and colleagues demstrated that children of 7 or 11 years perceive visually induced linear forward vecti earlier when they 1

2 Submitted to ACM APGV 28 cference were seated a chair that could potentially move [Lepecq et al. 199]. However, the probability of obtaining vecti remained unaffected by this manipulati. Later studies showed that adult observers perceive linear up-down ( elevator ) vecti as more compelling when they knew that actual body displacement was possible [Wright et al. 26]. Note that neither the vecti set latency nor the perceived distance traveled was affected. Several others linear vecti studies also seated participants movable platforms [Andersen and Braunstein 198; Berthoz et al. 197; Pavard and Berthoz 1977], but without quantifying whether this manipulati affected vecti respses. We are not aware of any study that demstrated the influence of the possibility of actual self-movement circular vecti. Riecke and colleagues showed that about 2/3 of participants can be fooled into believing that they physically rotate if they know that this is, in fact, possible [Riecke et al. 2c; Schulte-Pelkum et al. 24]. Neither vecti set latency, intensity, or compellingness were affected by whether or not actual moti was possible Auditory vecti Here, we investigated whether auditory circular vecti induced by a rotating sound field in blindfolded adult participants might be facilitated if they knew and sensed that actual rotati was possible. To this end, participants were seated in a hammock chair, and either put their solid ( moti impossible cditi) or a footrest attached to the hammock chair ( moti possible cditi). Even though participants are often seated a potentially rotating chair in auditory circular vecti studies [Lackner 1977; Väljamäe 2; Väljamäe 27], we are not aware of any study that investigated whether this does, in fact, facilitate circular vecti. If this should turn out to be true, however, it would not ly be of theoretical interest, but also relevant for many self-moti simulati applicatis, where it is often desired to provide a natural and compelling experience of the simulated envirment and e s movement through that envirment: Actual movement in the real world are typically accompanied by a strg sense of self-moti, suggesting that all virtual reality or multi-media simulatis that do not elicit a similar feeling of self-movement might be severely limited and might not enable natural, effortless behavior and spatial orientati in particular [Riecke et al. 2b]. 1.3 Influence of vibratis vecti Albeit vibratis being frequently used in moti simulati and multi-media applicatis, there has ly recently been experimental evidence that providing vibro-tactile stimulati of the participants seat can in fact increase both visually induced linear/circular vecti [Riecke et al. 2b; Schulte-Pelkum et al. 24; Schulte- Pelkum 28] and auditorily induced linear vecti [Väljamäe et al. 26]. Note that in the study by Väljamäe and colleagues linear, auditorily induced vecti, adding vibratis facilitated vecti ly when accompanied by an engine sound and/or when ly e rotating sound source was used. In a recent study auditory circular vecti, however, vibratis did not facilitate the self-moti illusi [Väljamäe 27, paper C]. This negative result might, in part, be related to the low amplitude to the vibrotactile stimulati: Only e of the 16 participants apparently noticed the vibratis when debriefed. Here, we assessed whether vibratis that are still quite subtle, but above the percepti threshold, can facilitate auditory circular vecti. If so, this would extend our understanding of multi-modal cue integrati for self-moti simulati. Furthermore, it would provide important evidence for self-moti simulati applicatis, where seat vibratis can be readily applied with affordable, door 4 birds sound vecti treadmill moti circular treadmill auditory moti 27 ape 31 river sound 1.2m radius mat 13 beer 22 Figure 1: Left: Experimental setup showing the circular treadmill and a participant seated the hammock chair with blindfold, noise-canceling headphes, and a pointing device used for a sound localizati pre-test. Note that throughout the test phase, the treadmill was switched and the hammock chair was fixed to prevent actual chair rotatis. Right: Top-down schematic view of the experimental setup (not drawn to scale). Note that treadmill moti was ly used during the vecti demstrati phase. the-shelf hardware. We propose that vibratis might facilitate vecti by reducing the percepti and assumpti of statiarity. 2 Methods Participants All 16 Participants in this experiment had normal or corrected-to-normal visi, normal, binaural hearing, and no signs of vestibular dysfuncti, as determined by a Romberg test [Khasnis and Gokula 23]. Only eight of these 16 participants experienced any auditory vecti. The remaining eight participants thus had to be excluded from the data analysis. This ratio is in accordance with the literature, where auditory vecti typically occurs ly in about 2-6% of blindfolded observers [Lackner 1977; Väljamäe 27]. The eight participants (five female) who perceived auditory vecti were between 18 and 32 years old (mean: 22.7) and had various occupatial backs. Participati was voluntary and paid at standard rates. The experiment was approved by the IRB and cducted in accordance with ethical standards laid down in the 1964 declarati of Helsinki. Participants gave their written informed csent prior to the experiment. 2.1 Stimuli and apparatus Hammock chair and circular treadmill Throughout the experiment, participants were seated a hammock chair that was hanging from a swivel joint centered above a m circular treadmill, as illustrated in Figure 1. A detailed descripti of the setup can be found in [Feuereissen 28]. The treadmill was used to rotate participants during the binaural recordings and thus provide the experience that the hammock chair can, in fact, rotate. The circular treadmill was switched for the rest of the experiment. Vibratis To provide barely noticeable vibratis in half of the trials, a small eccentric motor (a modified USB fan) that rotated at about 7 Hz was mounted the horiztal cross-bar of the hammock chair, as illustrated in Figure 2 (Right). The set and set of the vibratis was synchrized with the set and set of the auditory moti, respectively, as this was expected to enhance the 2

3 Stimulus velocity [ /s] Vecti intensity [%] Submitted to ACM APGV 28 cference Figure 2: Left: Side view of the experimental setup, showing a participant with his feet suspended in the footrest attached to the hammock chair ( movement possible cditi). Center: Miniature microphes positied at the entrance of the ear canals during the binaural recordings Right: A USB fan was modified to act as an eccentric motor that provided barely noticeable vibratis to the hammock chair. sensati of a csistent moti metaphor, which has been shown to be essential for auditory vecti [Väljamäe 27]. Sound sources and target objects As illustrated in Figure 1 (right), two speakers were placed directly to the frt (, 2.3m away) and the right (27, about 3.3m away), respectively, of the participant seated in the hammock chair. During the binaural recordings (and ly then), the 27 speaker displayed a custommade mix of several river and waterfall noises, and the speaker displayed a custom-made mix of 14 different bird sounds. The stimuli were chosen because they could be well localized, easily disambiguated, and were much less disturbing than the white/pink noise stimuli used in many studies. As sketched in Figure 1, the room ctained four target objects positied at 4 (door), 13 (mat), 22 (beer), and 31 (ape) with respect to the observer seated in the default orientati. Binaural recordings Binaural recordings served to generate the sound files that were later used to induce auditory circular vecti. To this end, participants were seated the hammock chair facing, and were passively rotated either counterclockwise or clockwise, at the velocity profile described in Figure 3 and a maximum velocity of 6 /s. To ensure that participants moved in sync with the treadmill, they were asked to keep their feet statiary the platter of the rotating treadmill without stepping alg. The speakers located at and 27 displayed the bird and river sound mixes, respectively, throughout the recording. For the binaural recordings, we used miniature microphes (Core Sound Binaural Microphe Set) mounted at the entrance of the ear canal, as illustrated in Figure 2. An external high-quality analog-to-digital audio cverter (DigiDesign MBox2) attached to a laptop mounted the hammock chair was used for the binaural recordings as well as the audio playback during the experiment. Participants were instructed to rest their elbows the armrest while placing the head the back of their fists in order to stabilize their head in an unobtrusive manner. Note that participants always listened to their own, individualized recordings during the subsequent vecti experiment in order to improve spatializati fidelity. Hence, the binaural recording sounded just like what it would sound like to rotate in the lab, and the recording naturally included all reflectis, reverberatis, and ambient sound of the room. Audio playback During the main experiment, participants were seated the hammock chair, blindfolded, and equipped with active noise canceling headphes (Audiotechnica AT-7ANC) displaying the individualized binaural recordings. Note that the binaural recording acted not ly as a vecti-inducing stimulus, but also as a masking sound for the noise and ambient sound present in the actual lab. stimulus velocity (max: 6 /s) 6 9s 9s 4 3s 2 1s vecti set time 3s Time [s] vecti time-course Figure 3: Time-course of the rotating vecti-inducing sound field (solid black line) and hypothetical vecti intensity (dashed gray line). 2.2 Procedure The experiment lasted a total of about 2h per pers and csisted of the following parts described in chrological order. Demstrati of possibility of movement To demstrate that physical moti is possible, participants were seated the hammock chair, and the chair and treadmill were rotated. We hypothesized that this experience and knowledge that actual rotati is possible might facilitate experiencing vecti later ( suspensi of disbelief ). Vecti demstrati phase To explain the ccept of vecti and demstrate what compelling vecti should feel like, participants were exposed to four trials where the auditory moti was accompanied with synchrized treadmill rotati (see Fig. 3), thus resulting in a combinati of auditory and biomechanical vecti [Riecke et al. 28b]: While being blindfolded, wearing headphes, and seated the statiary hammock chair, participants were asked to step alg with the platform ce it started rotating [Bles 1981]. As expected, this procedure elicited compelling circular vecti in all participants. Calling out target objects Throughout the vecti demstrati phase and main experiment, participants were instructed to ctinuously keep track of their orientati with respect to the lab and the four targets positied at 4, 13, 22, and 31. To quasi-ctinuously assess which directi participants perceived themselves facing ce they perceived actual self-moti, they were asked to call out the name of the respective target object whenever they believed they were actually facing it. This allowed us to assess whether vecti is decoupled of the surrounding envirment or whether vecti resulted in an updating of e s mental representati of the surrounding lab. Furthermore, this procedure allowed us to estimate the time course of the perceived self-rotati velocity. The mean perceived vecti velocity was estimated by dividing the total angle turned (estimated by the adding the relative angles between all the passed target objects) by the total durati of the vecti experience (estimated by subtracting the vecti set time from the total stimulus moti time of 12s). Binaural recording phase Three binaural recordings of 11s were taken for each participant as described above, e for clockwise moti, e for counterclockwise moti, and e statiary recording. Auditory moti directi percepti pre-test A pre-test using 2s sound clips showed that all participants could correctly determine whether or not a presented binaural recording was statiary or moving. When asked to indicate the rotati direc

4 Vecti intensity at end of trial [%] Overall vecti intensity [%] Estimated vecti set time [s] Vecti intensity at set [%] Percentage of trials with vecti [%] Realism of actually rotating in room [%] Submitted to ACM APGV 28 cference ti (left/right) of a binaural recording, they correctly respded in 8.7% of the trials. The remaining 14.3% of erreous respses can probably be ascribed to frt-back cfusis: When the presented sound is csistently perceived as frt-back mirrored, the resulting moti directi should be left-right reversed as well. Main vecti experiment For the main vecti experiment, participants were seated in the chair that was fixed in the positi. They were wearing headphes and blindfold and had their feet resting either the floor ( moti impossible cditi) or the footrest attached to the chair ( moti possible cditi, see Fig. 2). During each trial, participants were asked to verbally report as so as they sensed vecti ( vecti set ). Furthermore, they were asked to keep track of their orientati in the lab and in particular their orientati with respect to the four target objects, and call out the object s name whenever they believed they were facing e. The experimenter used a custom-written stop watch program to record these events. After each trial, participants were asked to take the headphes and blindfold and put their the to re-anchor themselves within the physical lab and to reduce potential after-effects and moti sickness. To familiarize participants with the experimental procedure and demands, we performed four practice trials ( for each stimulus combinati) prior to the main experiment Handling of trials where no vecti was perceived As most participants experienced trials where they did not perceive any vecti at all (this was particularly true in the no--- cditi), we used the following procedure to avoid discarding those trials and thus biasing the results: Whenever no vecti occurred, we assigned a fictitious estimated vecti set time of 12s to those trials, which was the whole durati of the moti phase. Note that this is a cservative estimate of the vecti set time in the following sense: If participants would have perceived vecti for lger stimulus presentati (as is not unlikely), the resulting vecti set times would all be beyd 12s. Hence, any statistical result should hold true if we would have used a lger stimulus presentati. The percentage of trials where any vecti was experienced was used as an additial measure of the vectiinducing power of the respective experimental stimuli Post-trial debriefing At the end of each trial, participants were verbally asked the following questis to quantify their vecti experience: (1) How intense was the set of vecti? (2) How intense was the sensati of self-moti towards the end? (3) How intense was the sensati of self-moti overall?, and (4) Did you really feel like you were rotating in the physical room? Participants respded verbally using a ctinuous scale from - 1%. Although some of these measures might be highly correlated, we decided to use several different vecti measures to test if the experimental manipulati would affect the various aspects of the self-moti experience differently. 2.3 Experimental design For the main experiment, each participant completed 16 vecti trials in e sessi of about 4 minutes. These trials csisted of a factorial combinati of 2 moti directis (clockwise vs. counterclockwise; alternating) 2 vibratis cditis (jitter vs. ;) 2 feet cditis (, moti impossible vs. feet suspended the footrest, moti possible ) 2 repetitis per cditi. All cditis were balanced to avoid order effects. Figure 4: Vecti data for experiment 1. The bars represent the arithmetic mean, the whiskers depict e standard error of the mean. 3 Results The data from the various dependent measures were analyzed using repeated measures within-subject 2 2 ANOVAs for the different vecti measures and the independent variables jitter (/) and feet ( floor/suspended). The ANOVA results are summarized in Table 1, and the data are graphically represented in Figure 4. Auditory vecti was perceived in % of all trials, depending the experimental cditi (see Fig. 4, top left). While 3/8 participants always experienced vecti, others perceived ly occasial vecti, and 2/8 never perceived any vecti in the cditi without jitter and. Overall vecti intensity ranged between % and was thus rather low. This is in agreement with the literature, where auditory vecti is always found to be much less compelling than visually induced vecti. 4

5 Submitted to ACM APGV 28 cference Jitter Feet Interacti / /suspended jitter feet F(1,7) p ηp 2 F(1,7) p ηp 2 F(1,7) p ηp 2 Percentage of trials with vecti.81.47* 4.4%.12.8m 42.2%.73.48* 4.% Estimated vecti set time * 1.1% m 41.2% % Realism of actually rotating in room ** 66.1%.6.* 44.3% % Vecti intensity at set * 8.1% m 34.6% % Vecti intensity at end of trial ** 67.8% * 48.%. 1..% Overall vecti intensity ** 64.4% m 41.3% % Table 1: Analysis of variance results for the different dependent variables. The asterisks indicate the significance level (α = %, 1%, or.1%), marginally significant effects (α 1%) are indicate by an m. Significant and marginally significant effects are typeset in bold and italics, respectively. The effect strengths partial η 2 p indicates the percentage of variance explained by a given factor. 3.1 Influence of adding jitter to auditory vecti Table 1 shows that adding barely noticeable jitter to the participants seat significantly enhanced auditory circular vecti in all dependent measures. That is, the percentage of trial where vecti was perceived was increased, and the intensity of the selfmoti sensati was higher throughout the trial whenever vibratis accompanied the auditory moti. Furthermore, vecti was perceived earlier, and participants had a more realistic sensati of actually rotating in the physical lab. The effect strength (partial η 2 p ) ranged between 4 and 68%, indicating that between 4-68% of the observed variance in the data can be explained by the experimental manipulati of adding jitter Influence of suspending feet When participants were asked to put their the solid, nmoving (instead of having them suspended with the hammock chair), there was an overall tendency towards reduced vecti in all dependent measures. This trend reached significance (α %) for the realism of actually rotating in the physical lab and the vecti intensity at the end of the trial. All other dependent measures reached marginal significance (α 1%). Between ηp 2 = 3% and ηp 2 = 49% of the variability in the data could be ascribed to the feet being suspended or not. These effect sizes can be described as quite large [Cohen 1988], even though some of the effects reached ly marginal significance. This suggests that more reliable effects might be expected if more participants were to be tested, which we plan to do in the near future. Csidering the small number of participants tested, these results are already quite substantial, though. 3.3 Interacti jitter feet The percentage of trials where vecti was perceived showed a significant interacti between the independent variables jitter and feet. Figure 4, top left, suggests that this interacti might be due to a ceiling effect: Adding jitter and suspending e s feet at a time increased the percentage of vecti trials from 3.1% to 84.4%, which is already close to the ceiling level of 1%. Combining both measures raised the percentage of vecti trials to 96.9%, indicating that vecti was almost always perceived when vibratis were present and e s feet did not touch the. Ne of the other dependent measures showed any significant interactis, suggesting independent (e.g., additive) facilitati of vecti: Both adding 1 The effect strength (partial η 2 p ) is a statistical measure that quantifies what proporti of the observed variance of a dependent measure (e.g., vecti intensity) can be accounted for by a given independent variable (e.g., adding vibratis) [Cohen 1988]. Mean perceived vecti velocity [ /s] jitter, 33. jitter, Figure : Mean estimated perceived vecti velocity, plotted as in Figure 4. jitter and not touching solid facilitated vecti, and combining both measures enhanced vecti even further. 3.4 Perceived vecti velocity All but e participant experienced actual self-moti with respect to the lab, and thus called out the target name whenever they believed they were facing e of the four target objects. The data from these seven participants were used to estimate their perceived vecti velocity (see Figure ). Although there was a tendency towards higher perceived self-rotati velocities when jitter was added and e s feet did not touch solid, ne of these effects reached significance (F(1,6)=2.62, p=.17, η 2 p = 3.4% and F(1,6)=1.84, p=.224, η 2 p = 23.4%, respectively). Mean perceived vecti velocities per cditi varied csiderably between participants, and ranged from /s to 48.7 /s. Note that even the highest reported mean vecti velocities were still slightly below the stimulus velocity of 6 /s, suggesting that participants were typically not locked to the auditory rotati velocity. This is an interesting difference to visually induced vecti, where providing a naturalistic visual envirment can lead participants vecti velocity to be locked to the stimulus velocity [Riecke et al. 26a, informal observatis]. Furthermore, naturalistic visual stimuli can induce obligatory spatial updating, such that e s mental spatial representati is always aligned with the orientati of the presented visual scene [Riecke et al. 2d]. Interestingly, this did not seem to happen for the auditory stimuli used here. Nevertheless, participants clearly updated the surrounding lab while being blindfolded and perceiving vecti albeit at a lower speed than the auditory rotati velocity.

6 Submitted to ACM APGV 28 cference 4 Discussi and cclusis 4.1 The possibility of actual self-moti can facilitate vecti One main goal of this study was to investigate whether auditory circular vecti might be affected by participants either having their feet touch solid, such that they sensed and knew that actual moti was impossible, or by having their feet suspended while sitting a hammock chair, such that they had no direct ctact to any statiary object and actual moti might seem more plausible. Participants are often seated movable chairs in auditory vecti studies to facilitate vecti [Lackner 1977; Väljamäe 2; Väljamäe 27], but it has never been shown that this procedure actually does facilitate auditory vecti. The current study provides first evidence that such facilitati does indeed exist: Vecti was enhanced when participants had no direct ctact with the floor or any other obviously earth-statiary object. That is, not having e s feet touch the showed significant and or at least marginally significant facilitati of vecti in all six vecti measures used. It is important to csider that this facilitati could have occurred both via perceptual and/or cognitive mechanisms. That is, having e s feet touch the provides the e hand lower-level, perceptual informati like biomechanical, inertial, and deep pressure cues that specify statiarity, in the sense that we sense/feel that we are statiary. On the other hand, e s feet standing solid also provides us with higher-level, more cognitive informati that e cannot move in the sense that we know that we cannot possibly be moving. Cversely, sitting a hammock chair with e s feet suspended with the chair and having experienced that the chair can, in fact, be moved, might help to cognitively prime participants to anticipate vecti or at least believe that actual self-moti is not absolutely impossible, similar to a suspensi of disbelief often employed in arts and entertainment. The literature has cvincingly shown that both lower-level, perceptual and higher-level, more cognitive influences vecti exist [Dichgans and Brandt 1978; Hettinger 22; Lepecq et al. 199; Riecke et al. 2c; Schulte-Pelkum and Riecke 28; Wright et al. 26]. Thus, it seems possible that both influences might have ctributed in the current study, although further experiment would be needed to disambiguate between them. 4.2 Vibratis can enhance vecti Adding vibratis has been shown to facilitate visual circular and linear vecti [Riecke et al. 2b; Schulte-Pelkum 28] as well as auditory linear vecti [Väljamäe et al. 26], but not auditory circular vecti [Väljamäe 27, paper C]. Here, we investigated whether barely noticeable jitter might enhance auditory circular vecti, in particular in situatis where actual moti might seem possible. Adding jitter was found to facilitate auditory circular vecti in all dependent measures, and the effect sizes were all quite high. This finding came somewhat as a surprise, as Väljamäe found no such facilitati for circular auditory vecti [Väljamäe 27, paper C]. Apart from minor differences in the experimental methodology, there are three main factors that might have ctributed to these seemingly ctradicting findings: First, while Väljamäe used vibratis that were just below the percepti threshold for most people, the current study used vibratis that were just above the percepti threshold and thus slightly strger. Due to differences in the seating and vibrati procedure, and the difficulty of assessing how the applied vibratis are transduced through the chair and the participants body, it is quite difficult to quantitatively compare the actual amount of vibrati exerted to different parts of the human body. Secd, while Väljamäe seated participants a solid chair mounted a small turntable that could be rotated by a computer but was otherwise perfectly statiary, participants in our study were sitting in a hammock chair that was held in place using soft cnectis, such that minimal swinging was still possible (as intended), whether or not e s feet were touching the. Albeit such swinging motis being possible, their amplitudes during the vecti trials were sufficiently small to pass unnoticed by the experimenter s naked eye. Thus, it is cceivable that very subtle swinging motis might be sufficient to facilitate auditory vecti and enhance the influence of jitter. Third, while Väljamäe employed n-individualized HRTF rendering of the stimuli (i.e., participants were listening through somebody else s ears ), we used individualized binaural recordings (i.e., participants were listening through their own ears ). We are currently running experiments to test whether individualizati of the binaural recordings might affect auditory circular vecti, but so far found no evidence supporting this hypothesis. This is in agreement with earlier findings by Väljamäe and colleagues where individualizing HRTFs did not facilitate auditory circular vecti, although it successfully reduced the occurrence of perceptual artifacts (e.g. distorted trajectories of the rotating sound objects or in-head localizati) and even increased auditory spatial presence [Väljamäe et al. 24]. It is interesting to note that jitter enhanced vecti irrespective of whether or not participants feet touched the. This suggests that a cognitive/perceptual framework of actual self-movement being possible is not absolutely essential for the vecti-facilitating effect of adding jitter. Nevertheless, lifting e s the such that actual self-moti might seem more likely enhanced vecti, even when vibratis were present. 4.3 Cclusis The current data suggests a clear cross-modal benefit for auditory vecti, especially if multiple cues are combined. This ctributes to the growing interest in multi-modal ctributis and interactis, and is in agreement with recent studies showing that auditory vecti can (at least under some circumstances) benefit from adding infrasound, vibratis, or engine sound [Väljamäe 2; Väljamäe 27] as well as apparent stepping-around a circular treadmill [Riecke et al. 28b]. Similarly, visual vecti can be facilitated by adding vibratis [Riecke et al. 2b; Schulte-Pelkum 28], small jerks that coincide with the visual moti set [Riecke et al. 26b; Schulte-Pelkum 28; Wg and Frost 1981], or spatialized auditory cues that rotate in sync with the visual stimulus [Riecke et al. 2a; Riecke et al. 28a]. In cclusi, the current study provides first evidence that adding vibratis can enhance auditorily induced circular vecti. Furthermore, providing a perceptual/cognitive framework of movability was found to facilitate auditory vecti, irrespective of whether or not vibratis were present. These findings have potentially interesting theoretical as well as applied implicatis: On the e hand, understanding how different perceptual and cognitive factors influence vecti fosters our theoretical understanding of human multimodal percepti and cue integrati, a field that receives growing research interest. On the other hand, the current findings have several applied implicatis. In terms of designing auditory vecti setups, care should be taken to allow participants to sense and believe that actual moti is possible. This extends previous findings that found such influences visually induced vecti [Lepecq et al. 199; Riecke et al. 2c; Wright et al. 26]. Furthermore, many applicati that involve simulated movements of 6

7 Submitted to ACM APGV 28 cference the observer might benefit from the current findings, as both vibratis and a perceptual/cognitive framework of movability can often be provided cost-effectively and with little effort. Such potential applicatis include driving/flight simulatis, first-pers computer/arcade games, movies, architecture walk-throughs, virtual travel, and other tele-presence applicatis. Finally, spatialized sound of compelling fidelity can nowadays be provided with relatively little effort and costs, and has been shown to induce selfmoti illusis as well as facilitate visually or biomechanically induced vecti [Riecke et al. 28a; Riecke et al. 28b]. The current study adds to the growing body of evidence highlighting the importance of csistent multi-modal simulati embedded in a coherent perceptual and cognitive framework. Acknowledgments Bernhard Riecke was funded NIMH Grant 2-R1-MH7868; the research was funded by NSF Grant 7863, the Vanderbilt University, and the Max Planck Society. References ANDERSEN, G. J., AND BRAUNSTEIN, M. L Induced self-moti in central visi. Journal of Experimental Psychology Human Percepti and Performance 11, 2, BERTHOZ, A., PAVARD, B., AND YOUNG, L. R Percepti of linear horiztal self-moti induced by peripheral visi (linearvecti) - basic characteristics and visual-vestibular interactis. Exp. Brain Res. 23,, BLES, W Stepping around: circular vecti and coriolis effects. Attenti and performance, COHEN, J Statistical Power Analysis for the Behavioral Sciences. Hillsdale, NJ: Erlbaum. DICHGANS, J., AND BRANDT, T Visual-vestibular interacti: Effects selfmoti percepti and postural ctrol. In Percepti, R. Held, H. W. Leibowitz, and H.-L. Teuber, Eds., vol. VIII of Handbook of Sensory Physiology. Springer, DODGE, R Thresholds of rotati. J. Exp. Psychol. 6, FEUEREISSEN, D. 28. VR: Getting the Reality Part Straight Does Jitter and Suspensi of the Human Body Increase Auditory Circular Vecti? Bachelor s thesis, Department of Computer Science in Media, Furtwangen University, Germany. Available: spacestudies/uploads/uploads/vr-vecti-t1.pdf. GEKHMAN, B Audiokinetic nystagmus. Sensornye Sistemy, 2, (in Russi). HENNEBERT, P. E Audiokinetic nystagmus. Journal of Auditory Research 1, 1, HETTINGER, L. J. 22. Illusory self-moti in virtual envirments. In Handbook of Virtual Envirments, K. M. Stanney, Ed. Lawrence Erlbaum, ch. 23, KHASNIS, A., AND GOKULA, R. M. 23. Romberg s test. J Postgrad Med 49, 2, Available: LACKNER, J. R Inducti of illusory self-rotati and nystagmus by a rotating sound-field. Aviati Space and Envirmental Medicine 48, 2, LARSSON, P., VÄSTFJÄLL, D., AND KLEINER, M. 24. Percepti of self-moti and presence in auditory virtual envirments. In Proceedings of 7th Annual Workshop of Presence, Available: LEPECQ, J. C., GIANNOPULU, I., AND BAUDONNIERE, P. M Cognitive effects visually induced body moti in children. Percepti 24, 4, MACH, E Grundlinien der Lehre v der Bewegungsempfindung. Engelmann, Leipzig, Germany. MARMEKARELSE, A. M., AND BLES, W Circular vecti and human posture ii: Does the auditory-system play a role. Agressologie 18, 6, PALMISANO, S., GILLAM, B. J., AND BLACKBURN, S. G. 2. Global-perspective jitter improves vecti in central visi. Percepti 29, 1, PALMISANO, S., BURKE, D., AND ALLISON, R. S. 23. Coherent perspective jitter induces visual illusis of self- moti. Percepti 32, 1, PAVARD, B., AND BERTHOZ, A Linear accelerati modifies perceived velocity of a moving visual scene. Percepti 6,, RIECKE, B. E., SCHULTE-PELKUM, J., CANIARD, F., AND BÜLTHOFF, H. H. 2. Influence of Auditory Cues the visually-induced Self-Moti Illusi (Circular Vecti) in Virtual Reality. In Proceedings of 8th Annual Workshop Presence 2, RIECKE, B. E., SCHULTE-PELKUM, J., CANIARD, F., AND BÜLTHOFF, H. H. 2. Towards lean and elegant self-moti simulati in virtual reality. In Proceedings of IEEE Virtual Reality 2, RIECKE, B. E., VÄSTFJÄLL, D., LARSSON, P., AND SCHULTE-PELKUM, J. 2. Top-down and multi-modal influences self-moti percepti in virtual reality. In Proceedings of HCI internatial 2, 1 1. RIECKE, B. E., VON DER HEYDE, M., AND BÜLTHOFF, H. H. 2. Visual cues can be sufficient for triggering automatic, reflex-like spatial updating. ACM Transactis Applied Percepti (TAP) 2, 3, RIECKE, B. E., SCHULTE-PELKUM, J., AVRAAMIDES, M. N., VON DER HEYDE, M., AND BÜLTHOFF, H. H. 26. Cognitive factors can influence self-moti percepti (vecti) in virtual reality. ACM Transactis Applied Percepti 3, 3, RIECKE, B. E., SCHULTE-PELKUM, J., AND CANIARD, F. 26. Visually induced linear vecti is enhanced by small physical acceleratis. In 7th Internatial Multisensory Research Forum (IMRF). Available: RIECKE, B. E., VÄLJAMÄE, A., AND SCHULTE-PELKUM, J. 28. Moving Sounds Enhance the Visually-Induced Self-Moti Illusi (Circular Vecti) in Virtual Reality. ACM Transactis Applied Percepti. (accepted). RIECKE, B. E., FEUEREISSEN, D., AND RIESER, J. J. 28. Ctributi and interacti of auditory and biomechanical cues for self-moti illusis ("circular vecti"). In Proceedings of CyberWalk workshop. Available: SAKAMOTO, S., OSADA, Y., SUZUKI, Y., AND GYOBA, J. 24. The effects of linearly moving sound images selfmoti percepti. Acoustical Science and Technology 2, SCHULTE-PELKUM, J., AND RIECKE, B. E. 28. An integrative approach to presence and self-moti percepti research. In Immersed in Media Experiences: Presence Psychology and Design (Handbook of Presence). Lawrence Erlbaum. in print. SCHULTE-PELKUM, J., RIECKE, B. E., AND BÜLTHOFF, H. H. 24. Vibratial cues enhance believability of ego-moti simulati. In Internatial Multisensory Research Forum (IMRF). Available: SCHULTE-PELKUM, J. 28. Percepti of self-moti: Vecti experiments in multisensory Virtual Envirments. PhD thesis, Ruhr-Universität Bochum. (submitted). VÄLJAMÄE, A., LARSSON, P., VÄSTFJÄLL, D., AND KLEINER, M. 24. Auditory presence, individualized head- related transfer functis, and illusory ego-moti in virtual envirments. In Proceedings of 7th Annual Workshop of Presence, Available: VÄLJAMÄE, A., LARSSON, P., VÄSTFJÄLL, D., AND KLEINER, M. 26. Vibrotactile enhancement of auditory induced self-moti and spatial presence. Journal of the Acoustic Engineering Society 4, 1, VÄLJAMÄE, A. 2. Self-moti and presence in the perceptual optimizati of a multisensory virtual reality envirment. Licentiate dissertati, Chalmers University of Technology, Göteborg, Sweden. Available: chalmers.se/~sasha/aleksander_valjamae_licentiate.pdf. VÄLJAMÄE, A. 27. Sound for Multisensory Moti Simulators. PhD thesis, Chalmers University of Technology, Göteborg, Sweden. WARREN, R., AND WERTHEIM, A. H., Eds Percepti & Ctrol of Self- Moti. Erlbaum, New Jersey, Ld. WONG, S. C. P., AND FROST, B. J The effect of visual-vestibular cflict the latency of steady-state visually induced subjective rotati. Percepti & Psychophysics 3, 3, WRIGHT, W. G., DIZIO, P., AND LACKNER, J. R. 26. Perceived self-moti in two visual ctexts: dissociable mechanisms underlie percepti. J Vestib Res 16, 1-2,