Electrocutaneous (EC) stimulation is a promising modality for noninvasive tactile feedback. Increasing the pulse frequency of EC stimulation has been shown to increase the perceived intensity in a nonlinear way. However, it is unclear why the relationship is nonlinear and how the nonlinearity impacts the theoretical number of discriminable sensory gradations that can be evoked by EC stimulation, as would be necessary to effectively convey the magnitude of tactile interactions. To address this, we systematically quantify just-noticeable differences (JNDs) and corresponding Weber fractions (WFs) through three psychophysical experiments in healthy adults to elucidate the perceptual resolution and mechanisms for frequency-based sensory encoding. First, we quantify the JNDs and WFs at 25-, 50-, 75-, and 100-Hz reference points and provide quantitative data supporting prior work that suggested intensity discrimination is worse at higher pulse frequencies. Second, we demonstrate that perceptual learning is frequency dependent; intensity discrimination at 50 Hz improved with greater exposure through additional JND experiments, whereas no improvement was observed with repeated exposure at 100 Hz. Third, by isolating frequency discrimination from intensity cues using pseudo-randomized pulse amplitudes, we show that temporal discrimination is impossible at higher frequencies. This suggests that temporal cues present at lower EC pulse frequencies contribute to enhanced perceptual sensitivity at lower frequencies. These findings serve as a quantitative benchmark for intensity discrimination across the working range of EC pulse frequencies, and provide new evidence that enhanced perceptual sensitivity at lower frequencies is likely due to the learnability of supplemental temporal cues. These findings highlight the importance of documenting the experience level of participants when reporting discriminability and may be used to optimize EC haptic feedback systems.

Inducing human motion through haptic feedback is a long-standing objective in haptics research. Traditionally, such interaction has relied on mechanical systems that physically apply forces to the body using motors, linkages, or grounded devices. However, recent studies published in IEEE Transactions on Haptics have increasingly explored alternative approaches that evoke force or motion perception through relatively small physical stimuli. These techniques leverage perceptual characteristics of the human sensorimotor system to create compelling sensations of pulling, torque, or movement without requiring large mechanical actuation. This article reviews recent contributions in IEEE Transactions on Haptics related to technologies that "compel" users to move. In particular, we focus on three representative approaches: skin deformation-based feedback, asymmetric vibration-induced pulling illusions, and kinesthetic illusions induced by tendon vibration. We discuss representative studies, application domains such as rehabilitation, navigation, and virtual reality, and the limitations inherent to perceptual force feedback systems. Through this overview, we highlight emerging design directions for lightweight and wearable haptic interfaces capable of influencing human motion.

As wearable devices such as smart watches, bands, and jewelry become more advanced and widespread, it is essential to address the challenges of text entry on their small interaction surfaces. This work introduces ForceKeyboard, a QWERTY soft keyboard for ultra-small devices, which utilizes touch location and force data to improve typing accuracy and speed. In a study involving 26 participants, we compared Force Keyboard with a Baseline keyboard that uses only tapping location data. The evaluation assessed performance across key sizes of 2.50, 1.75, and 1.00 mm in sitting and walking conditions. Participants performed eyes-free text entry tasks to reflect the occlusion and multitasking challenges of text entry on small wearable devices. Participants who used Force Keyboard achieved entry rates of 3.92, 3.57, and 2.77 words per minute, outperforming participants who used the baseline keyboard with 21%-168% higher speed and 16%-68% greater accuracy.

Wearable haptic devices face a trade-off between providing rich feedback and preserving natural fingertip sensation. Current multi-DOF systems encumber the fingerpad, interfering with fingerpad sensory capabilities, while lightweight devices offer limited 1-DOF feedback. To address this, we present a 5.24g, three-degree-of-freedom (3-DOF) haptic device that stimulates mechanoreceptors around the fingernail, leaving the fingerpad unobstructed. It uses two string-pulling motors for distal-proximal feedback and an arc-shaped pin motor for radial-ulnar stimulation to generate directional force vectors. This approach is grounded in the physiology of directionally-sensitive slowly adapting type 2 (SA2) mechanoreceptors. Experimental results demonstrate that the proposed 3-DOF approach improves weight and friction discrimination over single-DOF pressure feedback. Moreover, it delivers directional cues during static contact, a capability absent in vibrotactile systems, and achieves higher overall user satisfaction. By preserving fingertip sensation, the proposed device enables simultaneous interaction with virtual and physical objects, making it suitable for mixed reality applications.

The hanger reflex, in which skin shear deformation elicits a directional force sensation, provides a basis for wearable haptic guidance on the shoulders. In this study, we employ a wearable haptic device that uses four pneumatic airbags to apply controlled skin-shear deformation to both shoulders. Previous work on the hanger reflex has demonstrated that shear deformation of the skin can elicit directional force sensations, and such illusory forces are reliably perceived at the shoulder. However, the strength of these perceived forces and their potential influence on upper-body posture have not yet been quantitatively examined. To enable future applications in posture correction during manual or visually demanding tasks, the present study investigates both the perceived force magnitude and the postural effects elicited by shoulder skin-shear deformation. In Experiment 1, we quantified the magnitude of the illusory forces using a psychophysical comparison between forces elicited by skin-shear deformation and real traction forces. The device generated substantially larger perceived forces than typical illusion-based haptic methods, and the relationship between pressure and perceived force was well approximated by a linear model within the tested range. In Experiment 2, we evaluated how these deformation-elicited forces affect upper-body posture. We measured forward-backward trunk inclination under varying initial postures and stimulation directions. Consistent with the perceptual findings, the device produced consistent posture adjustments, particularly in the backward direction, with significant effects of initial posture and stimulus intensity on angular changes. Together, these findings demonstrate that shoulder hanger-reflex device provides quantifiable force sensations while also inducing measurable posture modification. The results offer essential information for individualized calibration and control-parameter design in wearable posture-support and rehabilitation systems, and they provide a foundation for future integration into ergonomic and assistive applications.

Multimodal haptic feedback that combines electrical muscle stimulation (EMS) and vibrotactile signals can create richer, more immersive experiences than those using a single modality. EMS delivers kinesthetic feedback by inducing muscle contractions, simulating force sensations that complement tactile stimuli from mechanical vibrations. However, presenting these stimuli concurrently can lead to perceptual interference, where one modality masks or alters the perception of the other. Temporal alignment between stimuli is also critical, as asynchrony can affect the perceived quality of haptic sensations. To investigate these phenomena, we conducted three user studies with a total of 40 participants (12, 12, and 16, respectively), focusing on mutual masking effects and temporal order perception between EMS and vibration. Our findings suggest that vibration can alleviate the tingling and discomfort commonly associated with EMS, effectively mitigating these unwanted sensations. Conversely, the presence of EMS increases the Just Noticeable Difference (JND) in vibration frequency discrimination, indicating a decrease in sensitivity to vibratory changes. Additionally, participants generally perceived the stimuli as simultaneous when EMS preceded vibration by 100 to 200 milliseconds. We discuss these findings and present four design guidelines for multimodal haptic rendering with EMS and vibrations in user applications.

When deaf and hard of hearing (DHH) individuals watch videos, captions are essential for them to understand the linguistic content. Current captions, however, are not suitable for conveying non-verbal sound information, such as background music, sound effects, or speech nuances. In this paper, we designed a multimodal system, Motion Caption Haptic System (MCHS), that enables DHH individuals to encounter sounds in videos through animated caption and spatiotemporal vibration patterns, supporting a more vivid and immersive experience. We elaborately designed motion captions and spatiotemporal haptic patterns for representative sound effects and spoken emotions to work well together through surveys from 27 DHH and 64 hearing participants. An evaluation with 19 DHH individuals demonstrated the capabilities and potential of the MCHS to improve their video viewing experience, along with a discussion of important issues that need to be addressed when designing multimodal captioning systems for the DHH viewers.

The haptic communication of languages by engaging wearable displays has recently attracted much attention because of the continuous technological improvements (e.g., miniaturized hardware, and software). Currently, one of the primary research goals towards the haptic communication of language is to develop training protocols that reduce the time of learning by making the learning experience less cumbersome. This study provides a novel training protocol by separating learning into two sections, that is, offline, and online training. During offline training, the technique is based on visual learning, i.e., the spatial-temporal information of the haptic patterns is obtained by sight without stimulating the skin. During online training, the technique is based on kinesthetic learning, i.e., the knowledge of the patterns is obtained by hands-on experience. The learning of English letters is used to illustrate the proposed technique. The results show that the proposed two-section protocol lowers the time of online learning in the state-of-the-art methods.

This review focuses on the interactions between the cutaneous senses, and in particular touch and temperature, as these are the most relevant for developing skin-based display technologies for use in virtual reality (VR) and for designing multimodal haptic devices. A broad spectrum of research is reviewed ranging from studies that have examined the mechanisms involved in thermal intensification and tactile masking, to more applied work that has focused on implementing thermal-tactile illusions such as thermal referral and illusory wetness in VR environments. Research on these tactile-thermal illusions has identified the differences between the senses of cold and warmth in terms of their effects on the perception of object properties and the prevalence of the perceptual experiences elicited. They have also underscored the fundamental spatial and temporal differences between the tactile and thermal senses. The wide-ranging body of research on compound sensations such as wetness and stickiness has highlighted the mechanisms involved in sensing moisture and provided a framework for measuring these sensations in a variety of contexts. Although the interactions between the two senses are complex, it is clear that the addition of thermal inputs to a tactile display enhances both user experience and enables novel sensory experiences.

Haptics user studies are often restricted to a set, physical location and use methods that do not captivate the user. Applying game design elements can create an entertaining environment and increase user engagement. Using ubiquitous tools, like smartphones, to conduct haptics user studies could allow researchers to access larger participant groups while a gamified approach could facilitate the data collection by making the experiment more enjoyable. To explore this concept, this work presents a gamified version of an existing psychophysical experiment that investigates response time to multisensory cues using a smartphone based on "Whac-A-Mole". We conducted a user study to compare our gamified interface with an existing psychophysical interface with thirteen participants exploring the response time from eighteen combinations of auditory, haptic, and visual stimuli at different levels of intensities and participant preferences for both interfaces. The results demonstrate that the gamified interface successfully captured similar trends in response times and significantly elevated participant enjoyment ($p < 0.003$), but did not result in equivalent response times to the original interface. This work shows the benefits and drawbacks of following a gamification approach when designing haptics user studies and discusses factors and trade-offs to consider when gamifying studies.