Estimation of Detection Thresholds for Acoustic Based RedirectedWalking Techniques

Stefania Serafin ∗ Niels C. Nilsson † Erik Sikstrom ‡ Amalia De Goetzen § Rolf Nordahl ¶

Aalborg University CopenhagenA.C. Meyers Vaenge 15, 2450 Copenhagen, DK

ABSTRACT

We describe two psychophysical experiments where we have quan-tified how much humans can unknowingly be redirected by auditorystimuli. The experiments use a two-alternative-forced-choice task.We tested 19 subjects in two different experiments: (E1) discrimi-nation between virtual and physical rotation, and (E2) discrimina-tion of path curvature. In experiment E1 subjects performed ro-tations with different gains, and then had to choose whether theauditory perceived rotation was smaller or greater than the physi-cal rotation. In experiment E2 subjects estimate the path curvaturewhen walking a curved path in the real world while the visual dis-play shows a straight path in the virtual world. Our results showthat users can be turned physically about 20% percent more or 12%less than the perceived virtual rotation, and users can be redirectedon a circular arc while they believe they are walking straight.

Keywords: auditory feedback, redirected walking.

1 INTRODUCTION

The investigation of natural methods for simulating walking in vir-tual environments is an active research topic in the virtual reality(VR) community. Traveling through immersive virtual environ-ments by means of real walking is an important activity to increasenaturalness of virtual reality (VR)-based interaction [7]. One ac-tive research direction consists of redirecting users [4]. One of thebiggest problems faced by developers of immersive VR applica-tions is the potential discrepancy between the size of virtual andphysical environments. If the physical environment is smaller thanthe virtual environment, this may hamper the user experience andeven be dangerous. Redirected walking makes up one possible so-lution to this problem. Redirected walking has been mostly im-plemented using visual feedback, with auditory feedback used asdistractors but without any formal evaluation of their possibilities[2].

In this paper we present a preliminary investigation on the role ofauditory feedback for redirection. To achieve this goal, we adaptedthe experiments described in [5, 6], to be used only with auditoryfeedback.

2 EXPERIMENTAL SETUP

We performed both experiments in a darkened room, where a sur-round sound system with 16 BM5A Dynaudio speakers was placed.The speakers were positioned at approximatively 1.60m from thefloor, around a circle with a diameter of 7.1 meters. Subjects wereasked to wear a nVisor SX head- mounted display (HMD), which

∗e-mail: sts@create.aau.dk†e-mail:ncn@create.aau.dk‡e-mail:es@create.aau.dk§e-mail:ago@create.aau.dk¶e-mail: rn@create.aau.dk

was turned off during the experiments. In both experiments, sub-jects were asked to navigate in a darkened room, where the onlyaudible auditory feedback was an alarm clock sound played fromdifferent locations in the room. This sound was chosen since it wasa sound emitted by an object that normally does not move. More-over, it made sense to hear the sound while being in a quiet anddark room. During the introduction subjects were asked to imagineexactly this scenario. The sound was delivered to the speakers andspatialized using the vector based amplitude panning (VBAP) algo-rithm. This algorithm allows to precisely place and move a soundin space [3].

19 subjects (13 males and 6 females) participated in both exper-iments. They were aged between 19 and 32 years (mean = 24.6years, standard deviation = 3.6). The two experiments lasted about30 minutes, and subjects were rewarded with a movie ticket at theend. As done in [6], for both experiments we used the method ofconstant stimuli in a two-alternative forced-choice (2AFC) task.

3 EXPERIMENT 1: DISCRIMINATION BETWEEN VIRTUALAND PHYSICAL ROTATION

Figure 1: Results of the discrimination between virtual and physi-cal rotation. The x-axis shows the applied rotation gain, the y-axisshows the probability of estimating a virtual rotation smaller than thephysical counterpart.

In this experiment we investigated subjects’ ability to discrim-inate whether a physical rotation was smaller or greater than thesimulated virtual rotation. Therefore, we instructed the subjects torotate on a physical spot until a distinct alarm sound was perceived

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as being in front of them, at which point they were supposed to clicka button on a Wii remote to indicate that they thought they were fac-ing the virtual alarm clock. Subsequently they were asked whetherthey believed the real world rotation to be smaller or greater thanthe virtual rotation. This rotation was mapped to a correspondingvirtual rotation to which 11 different gains were applied. Each sub-ject experienced each gain twice, for a total of 22 trials per subject.

3.1 Results of experiment 1Figure 1 shows the mean detection thresholds together with thestandard error over all subjects for the tested gains. The x-axisshows the applied rotation gain, the y-axis shows the probabilityfor estimating a physical rotation greater than the mapped virtualrotation. The solid line shows the fitted psychometric function ofthe form f = 1

1+eax+b , with real numbers a and b. From the psy-chometric function we determined a bias for the point of subjectiveequality (PSE), i.e., the point at which the subject perceives thephysical and the virtual movement as identical. This point was de-termined as PSE = 1. Detection thresholds of 75 % were reached atgains of 0.82 for greater responses and at 1.2 for smaller responses,suggesting that gain differences within this range cannot be reli-ably estimated, i. e., subjects have serious problems to discriminatebetween a 90◦ virtual and real rotation ranging from 75◦ to 109◦.Hence, subjects can be turned physically about 20% more or 12%less than the perceived virtual rotation. This range is smaller thanthe one measured for the same task using visual feedback [6]. Thismay imply that participants are less likely to notice that they havebeen redirected when the redirection is done with visual feedbackcompared to spatialized sound. Thus, it would seem that vision ismore likely to dominate proprioception and vestibular cues com-pared to audition which in turn may be explained by fact visiongenerally is considered superior to audition when it comes to theestimation of spatial location of objects [1].

4 EXPERIMENT 2In this experiment we analyzed the sensitivity to curvature gainswhich enforce the user to walk on a curve in order to stay on astraight path (see Section 3.4). Subjects were instructed to walkalong a straight line in the VE, but because the path was manipu-lated they physically had to walk along a curved path in order for thevirtual path to stay straight. We asked whether subjects were ableto discriminate the direction of bending of the physical path, and, ifso, at which threshold they start to do so reliably. We applied in to-tal 10 curvatures and each subject experienced each rotation twice,for a total of 20 trials per subject.

4.1 Results of experiment 2Figure 2 shows the mean detection thresholds together with thestandard error over all subjects for the tested curvatures. The x-axis shows the applied curvature gain, the y-axis shows the prob-ability for estimating a physical rotation greater than the mappedvirtual rotation. The solid line shows the fitted psychometric func-tion as in the previous experiment. From the psychometric functionwe determined a bias for the point of subjective equality (PSE), asPSE = −5. Detection thresholds of 75 % were reached at gains of−25 for greater responses and at 30 for smaller responses.

5 CONCLUSIONS

In this paper we investigated detection threshold for auditory basedredirected walking techniques by running two experiments. In thefirst experiment we investigate the detection threshold for physicalversus virtual rotation, while in the second experiment we investi-gated sensitivity to curvature gains. These results contribute to thedevelopment of future locomotion interfaces that take advantage ofother types of feedback such as auditory feedback. The experi-ments were run by providing only virtual auditory feedback. In

Figure 2: Results for the discrimination of path curvature. The x-axisshows the applied curvature gain which bends the walked path eitherto the left or the right, the y-axis shows the proportion of subjects leftresponses.

the future, we are interested in combining auditory and visual feed-back, in such a way to examine whether known results on redirec-tion using visual feedback can be enhanced by the use of auditoryfeedback.

REFERENCES

[1] E. Goldstein. Sensation and perception. Wadsworth Publishing Com-pany, 2010.

[2] T. Peck, H. Fuchs, and M. Whitton. Improved redirection with distrac-tors: A large-scale-real-walking locomotion interface and its effect onnavigation in virtual environments. In Virtual Reality Conference (VR),2010 IEEE, pages 35–38. IEEE, 2010.

[3] V. Pulkki. Virtual sound source positioning using vector base amplitudepanning. Journal of the Audio Engineering Society, 45(6), 1997.

[4] S. Razzaque, Z. Kohn, and M. Whitton. Redirected walking. In Pro-ceedings of EUROGRAPHICS, volume 9, pages 105–106. Citeseer,2001.

[5] F. Steinicke, G. Bruder, J. Jerald, H. Frenz, and M. Lappe. Anal-yses of human sensitivity to redirected walking. In Proceedings ofthe 2008 ACM symposium on Virtual reality software and technology,pages 149–156. ACM, 2008.

[6] F. Steinicke, G. Bruder, J. Jerald, H. Frenz, and M. Lappe. Estimationof detection thresholds for redirected walking techniques. Visualizationand Computer Graphics, IEEE Transactions on, 16(1):17–27, 2010.

[7] M. Usoh, K. Arthur, M. Whitton, R. Bastos, A. Steed, M. Slater, andF. Brooks Jr. Walking, walking-in-place, flying, in virtual environ-ments. In International Conference on Computer Graphics and In-teractive Techniques: Proceedings of the 26 th annual conference onComputer graphics and interactive techniques, volume 1999, pages359–364, 1999.

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