Quantifying spatial acuity of frequency resolved midair ultrasound vibrotactile stimuli.

Spatial acuity is a fundamental property of any sensory system. In the case of the somatosensory system, the two-point discrimination (2PD) test has long been used to investigate tactile spatial resolution. However, the somatosensory system comprises three main mechanoreceptive channels: the slowly adapting channel (SA) responds to steady pressure, the rapidly adapting channel (RA) responds to low-frequency vibration, and the Pacinian channel (PC) responds to high-frequency vibration. The use of mechanical stimuli in the classical 2PD test means that previous studies on tactile acuity have primarily focussed on the pressure-sensitive channel alone, while neglecting other submodalities. Here, we used a novel ultrasound stimulation to systematically investigate the spatial resolution of the two main vibrotactile channels. Contrary to the textbook view of poor spatial resolution for PC-like stimuli, across four experiments we found that high-frequency vibration produced surprisingly good spatial acuity. This effect remained after controlling for interchannel differences in stimulus detectability and perceived intensity. Laser doppler vibrometry experiments confirmed that the acuity of the PC channel was not simply an artifact of the skin's resonance to high-frequency mechanical stimulation. Thus, PC receptors may transmit substantial spatial information, despite their sparse distribution, deep location, and large receptive fields.

It Sounds Cool: Exploring Sonification of Mid-Air Haptic Textures Exploration on Texture Judgments, Body Perception, and Motor Behaviour.

Ultrasonic mid-air haptic technology allows for the perceptual rendering of textured surfaces onto the user's hand. Unlike real textured surfaces, however, mid-air haptic feedback lacks implicit multisensory cues needed to reliably infer a texture's attributes (e.g., its roughness). In this paper, we combined mid-air haptic textures with congruent sound feedback to investigate how sonification could influence people's (1) explicit judgment of the texture attributes, (2) explicit sensations of their own hand, and (3) implicit motor behavior during haptic exploration. Our results showed that audio cues (presented solely or combined with haptics) influenced participants' judgment of the texture attributes (roughness, hardness, moisture and viscosity), produced some hand sensations (the feeling of having a hand smoother, softer, looser, more flexible, colder, wetter and more natural), and changed participants' speed (moving faster or slower) while exploring the texture. We then conducted a principal component analysis to better understand and visualize the found results and conclude with a short discussion on how audio-haptic associations can be used to create embodied experiences in emerging application scenarios in the metaverse.

RecHap: An Interactive Recommender System For Navigating a Large Number of Mid-Air Haptic Designs.

Designing haptics is a difficult task especially when the user attempts to design a sensation from scratch. In the fields of visual and audio design, designers often use a large library of examples for inspiration, supported by intelligent systems like recommender systems. In this work, we contribute a corpus of 10,000 mid-air haptic designs (500 hand-designed sensations augmented 20x to create 10,000), and we use it to investigate a novel method for both novice and experienced hapticians to use these examples in mid-air haptic design. The RecHap design tool uses a neural-network based recommendation system that suggests pre-existing examples by sampling various regions of an encoded latent space. The tool also provides a graphical user interface for designers to visualize the sensation in 3D view, select previous designs, and bookmark favourites, all while feeling designs in real-time. We conducted a user study with 12 participants suggesting that the tool enables people to quickly explore design ideas and experience them immediately. The design suggestions encouraged collaboration, expression, exploration, and enjoyment, which improved creativity support.

"I See Where This is Going": A Psychophysical Study of Directional Mid-Air Haptics and Apparent Tactile Motion.

Mid-air haptic technology can render a plethora of tactile sensations including points, lines, shapes, and textures. To do so, one requires increasingly complex haptic displays. Meanwhile, tactile illusions have had widespread success in the development of contact and wearable haptic displays. In this article, we exploit the apparent tactile motion illusion to display mid-air haptic directional lines; a prerequisite for the rendering of shapes and icons. We present two pilot studies and a psychophysical study that contrasts a dynamic tactile pointer (DTP) to an apparent tactile pointer (ATP) in terms of direction recognition. To that end, we identify optimal duration and direction parameters for both DTP and ATP mid-air haptic lines and discuss the implications of our findings with respect to haptic feedback design, and device complexity.

UltraButton: A Minimalist Touchless Multimodal Haptic Button.

We present UltraButton a minimalist touchless button including haptic, audio and visual feedback costing only $200. While current mid-air haptic devices can be too bulky and expensive (around $2 k) to be integrated into simple mid-air interfaces such as point and select, we show how a clever arrangement of 83 ultrasound transducers and a new modulation algorithm can produce compelling mid-air haptic feedback and parametric audio at a minimal cost. To validate our prototype, we compared its haptic output to a commercially-available mid-air haptic device through force balance measurements and user perceived strength ratings and found no significant differences. With the addition of 20 RGB LEDs, a proximity sensor and other off-the-shelf electronics, we then propose a complete solution for a simple multimodal touchless button interface. We tested this interface in a second experiment that investigated user gestures and their dependence on system parameters such as the haptic and visual activation times and heights above the device. Finally, we discuss new interactions and applications scenarios for UltraButtons.

"I Can Feel It Coming in the Hairs Tonight": Characterising Mid-Air Haptics on the Hairy Parts of the Skin.

Ultrasound mid-air haptics has received much attention from both academic and industrial groups, however, such investigations have almost exclusively focused on the tactile stimulation of glabrous (hairless) skin of our hands. Meanwhile, the non-glabrous (hairy) part of the skin covers the largest area of our body, yet remains largely untouched and unexplored by this haptic technology. 1) We study acoustic streaming and the 2) acoustic radiation force associated with a mid-air haptic stimulus. 3) We characterise the perceived strength, temperature, and definition of the stimulus through a user study. 4) Finally, in a second user study we explore the possibility of conveying affective (pleasant) touch. These objective and subjective experiments provide the first deep understanding of how mid-air haptics can affect tactile perception through stimulating the hairy skin. To that end, we discuss how researchers and haptic designers can leverage mid-air haptic technology to vary the perceived touch intensity, temperature, and deliver affective touch.