Shear shock waves mediate haptic holography via focused ultrasound.

Emerging holographic haptic interfaces focus ultrasound in air to enable their users to touch, feel, and manipulate three-dimensional virtual objects. However, current holographic haptic systems furnish tactile sensations that are diffuse and faint, with apparent spatial resolutions that are far coarser than would be theoretically predicted from acoustic focusing. Here, we show how the effective spatial resolution and dynamic range of holographic haptic displays are determined by ultrasound-driven elastic wave transport in soft tissues. Using time-resolved optical imaging and numerical simulations, we show that ultrasound-based holographic displays excite shear shock wave patterns in the skin. The spatial dimensions of these wave patterns can exceed nominal focal dimensions by more than an order of magnitude. Analyses of data from behavioral and vibrometry experiments indicate that shock formation diminishes perceptual acuity. For holographic haptic displays to attain their potential, techniques for circumventing shock wave artifacts, or for exploiting these phenomena, are needed.

Rendering Spatiotemporal Haptic Effects Via the Physics of Waves in the Skin.

A major challenge in haptic engineering has been to design practical methods to efficiently stimulate distributed areas of skin. Here, we show how to use a single actuator to generate vibrotactile stimuli which cause sensations of temporally varying spatial extent. Through optical vibrometry methods, we show that vibrational stimuli applied at the fingertip elicit waves in the finger that propagate proximally toward the hand and show how the frequency-dependent damping behavior of skin causes propagation distances to decrease rapidly with increasing frequency of stimulation. Utilizing these results, we design haptic stimuli applied through a single actuator that produces wavefields that expand or contract in size. In a perception experiment, participants accurately (median $>$95%) identified these stimuli as expanding or contracting without prior exposure or training. As a potential application, we used these effects as haptic cues for interactions in virtual reality. We show through a second experiment that the spatiotemporal haptic stimuli were rated as significantly more engaging than conventional vibrotactile stimuli. These findings demonstrate how the physics of waves in skin can be utilized to excite spatiotemporal tactile effects over large surface areas with a single actuator, and inform methods to utilize the effects in practical applications.

Proprioceptive Localization of the Fingers: Coarse, Biased, and Context-Sensitive.

The proprioceptive sense provides somatosensory information about positions of parts of the body, information that is essential for guiding behavior and monitoring the body. Few studies have investigated the perceptual localization of individual fingers, despite their importance for tactile exploration and fine manipulation. We present two experiments assessing the performance of proprioceptive localization of multiple fingers, either alone or in combination with visual cues. In the first experiment, we used a virtual reality paradigm to assess localization of multiple fingers. Surprisingly, the errors averaged 3.7 cm per digit, which represents a significant fraction of the range of motion of any finger. Both random and systematic errors were large. The latter included participant-specific biases and participant-independent distortions that evoked similar observations from prior studies of perceptual representations of hand shape. In a second experiment, we introduced visual cues about positions of nearby fingers, and observed that this contextual information could greatly decrease localization errors. The results suggest that only coarse proprioceptive information is available through somatosensation, and that finer information may not be necessary for fine motor behavior. These findings may help elucidate human hand function, and inform new applications to the design of human-computer interfaces or interactions in virtual reality.