# A Review of Recent Literature on Audio-Based Pseudo-Haptics
## 2 The Review
### Visual-Auditory-Based Pseudo-Haptics
Visual cues included motion speed (either slow or fast), while auditory cues included frequency manipulation (no change, low-pass, or high-pass filters). Under active touch, both visual and auditory cues influenced roughness perception, with objects displaying slower motion and low-frequency accentuated sounds leading to higher perceived roughness ratings. Moreover, post hoc tests for auditory cues indicated significant differences between the three frequencies considered. In the passive-touch, auditory-only cues, particularly low-frequency accentuated sounds were perceived as stiffer. Post hoc tests revealed significant differences between high and medium sound levels and between high and low sound levels. The manipulation of visual and auditory cues can influence participants' perception of the physical characteristics of virtual objects, suggesting multisensory information processing, and enable the perception of haptic sensations. [PERSON] et al. (2018) developed a simulation to enable medical students to train during the COVID-19 pandemic by performing procedures on a virtual patient. Using a Leap Motion Controller mounted on an HMD, they tracked and estimated the user's body position in real time and used the body's gestures as input to control the user's avatar. To further deepen immersion, they integrated percussion sounds to enable the perception of the outline of the virtual patient's internal organs (e.g., heart, lungs, and liver), and determine the shape and size of the organs. Additionally, they included normal organic sounds such as heart percussion sounds and sounds typically emanating from the chest cavity. When the user's hand collides with an organ, a percussion sound plays, and the intensity of this sound varies according to the collision speed. To enhance realism, they incorporated visual-based pseudo-haptics to create the impression of the sense of touch when the user was interacting with the virtual patient's organs. Although no formal experimentation was conducted, the authors believe this approach has significant potential for teaching students during the pandemic. [PERSON] et al. (2018) explored the embodied social touch illusion that provides the ability to \"touch\" another in a virtual environment, using pseudo-haptics in VR. In their prototype, two users had their corresponding particle-based avatar and were able to interact with each other via their avatars. More specifically, the avatars were able to hold hands or high-five each other. All of the haptic interactions were conducted using pseudo-haptics and included (i) proximity interaction such as visual warmth where the particles comprising the avatar become more red to indicate warmth, particle attraction, where the two particles of each avatar are attracted to each other the closer in proximity the avatars get, and tremolo flute, an auditory response that plays as the avatars get closer together, (ii) contact interaction where visual (small fireworks and sparks lighting) and auditory feedback (ping sound) are represented when the two avatars make contact with each other, and (iii) resistance that prevents the avatars from passing through each other by incorporating physics colliders and audio force feedback, and more specifically, a hum that grew louder with increased force. It was observed that although pseudo-haptics cannot replace ordinary touch, it was able to provide a subtle alternative to represent the perception of touch during social interaction in a virtual environment. [PERSON] et al. [64] examined the simulation of a vibrating virtual object using visual and auditory cues in an AR environment. The task of the 18 participants was to touch a virtual white plate to activate different conditions. These conditions included: (i) visual (different levels of blurring of the virtual object), (ii) auditory (different auditory frequency), and (iii) audiovisual (a combination of the two stimuli in ascending order).