Where Should I Look? Comparing Reference Frames for Spatial Tactile Cues Erik Pescara, Anton Stubenbord, Tobias Röddiger, Likun Fang, Michael Beigl {pescara,stubenbord,roeddiger,fang,michael}@teco.edu Karlsruhe Institute of Technology Karlsruhe, Germany ABSTRACT When designing tactile displays on the wrist for spatial cues, it is crucial to keep the natural movement of the body in mind. Depend- ing on the movement of the wrist, diferent reference frames can infuence the output of the wristband. In this paper, we compared two possible reference frames, one where spatial cues are fxed in a wrist-centered frame of reference, and an allocentric frame of reference which fxes spatial cues in the global coordinate system. We compared both conditions in terms of reaction time, achievable accuracy and cognitive load. Our study with 20 participants shows that utilizing the allocentric reference frame reduces cognitive load (avg. 38% reduction) and reaction time (avg. 240ms reduction), with no statistically signifcant diference in accuracy. CCS CONCEPTS · Human-centered computing → Haptic devices; Laboratory experiments; User studies. KEYWORDS Haptic Interfaces, Spatial Cues, Reference Frames ACM Reference Format: Erik Pescara, Anton Stubenbord, Tobias Röddiger, Likun Fang, Michael Beigl. 2021. Where Should I Look? Comparing Reference Frames for Spatial Tactile Cues. In 2021 International Symposium on Wearable Computers (ISWC ’21), September 21–26, 2021, Virtual, USA. ACM, New York, NY, USA, 5 pages. https://doi.org/10.1145/3460421.3478822 1 INTRODUCTION Never before were digital screens as prevalent as they are today. Be it while driving a new car, taking a walk or at work ś the sheer food of primarily visual information that users are confronted with poses an enormous challenge to the ability to flter and process in- formation selectively. Tactile displays allow communicating spatial information without further contributing to the already overloaded and heavily demanded visual system. While numerous places on the body surface are suited for mounting a tactile display [22], they are most efective when placed at parts of the body with lots of otherwise unused skin like the back [5] or anatomical points of Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proft or commercial advantage and that copies bear this notice and the full citation on the frst page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specifc permission and/or a fee. Request permissions from permissions@acm.org. ISWC ’21, September 21–26, 2021, Virtual, USA © 2021 Copyright held by the owner/author(s). Publication rights licensed to ACM. ACM ISBN 978-1-4503-8462-9/21/09. . . $15.00 https://doi.org/10.1145/3460421.3478822 reference like the wrist and elbows [4]. However, no matter where the wearable is located on the body, chances are that the limb is not in line with the body’s natural posture, causing ambiguous inter- pretations of tactile stimuli, particularly when spatial information is involved. Active research into reference frames for vibrotactile interfaces aims to synthesize spatial mappings of corresponding events onto tactile cues. The main objective is to provide frame- works that are intuitive and easy to understand when building tactile displays. Therefore understanding diferent reference frames and their efects on the user gives valuable insights into designing real-world tactile applications, such as monitoring applications for time critical systems [15]. As humans tend to move and stretch even sedentary positions, incorporating the natural movement of the user into tactile displays is very important [5]. In this paper, we conducted a controlled lab study with 20 partic- ipants to evaluate two reference frames in terms of their reaction time, accuracy and cognitive load in spatial localization tasks using a wrist-worn vibrotactile bracelet. The frst reference frame encodes spatial coordinates with respect to a wrist-centered frame of refer- ence, while the second reference frame is set in an allocentrically anchored coordinate system to represent spatial directions. 2 RELATED WORK Research into haptic and tactile displays often aims to expand the perception of our subjective reality. Whether the expansion is mak- ing us feel things that we would otherwise see [1], or make us feel things we would not be able to see [16, 20]. Over the years, a multitude of diferent tactile systems with varying functionalities were developed. Brewster and Brown used structured tactile cues - Tactons - to communicate abstract information non-visually. Im- portant factors for designing a Tacton are frequency, amplitude, duration, rhythm, and placement on the body [2]. Lee and Starner presented a series of experiments to investigate the perceptibility of a wearable tactile device on the wrist as well as the efects of the four parameters intensity, starting point, temporal pattern and direction on the distinguishability of the tested patterns [8]. Although some research suggests that reacting to a directional haptic cue comes with a reduced reaction time compared to a visual cue [10], this fnding is debated elsewhere [17], taking the position that a multimodal interface with haptic and visual cues has the most benefts [17]. Spatial vibrotactile cues can help pilots during long fights keeping the aircraft in balance by decreasing the attention needed to perform specifc tasks like maintaining an aircraft’s alti- tude and take corrective action when the autopilot goes of bound [3]. Tan et al. used a 3 × 3 grid of vibrotactile actuators mounted to the back of a chair to signifcantly decrease reaction time in a change detection task. Particularly wrist-worn devices, such as 68