Skin, tactile receptors

In human skin, tactile receptors generate elementary sensations such as touch, pressure, vibratory sensation, pain, temperature sense, and so on. A tactile impression is an integrated sensation of these elementary sensations. To present tactile feel arbitrarily, stimuli applied to these receptors should be controlled selectively and quantitatively. As mentioned previously, tactile receptors cannot sense the physical factors of environments directiy. They detect only the skin deformation caused by contacting objects. A tactile illusion can be provided by the reproduction of the activities of tactile receptors, regardless of the inner deformations. 

The most characteristic feature of tactile sensation is the variety of perceptual contents reflected, from physical factors such as rigidity, elasticity, viscosity, friction and the surface shape of the target material. It is interesting that tactile receptors in human skin cannot sense the physical factors directly. They only detect the inner skin deformation caused by contact with the object. This fact suggests that a tactile display does not have to reproduce the same physical factors of the material to represent virtual touch. In other words, virtual touch needs only to reproduce the internal deformations in the skin. Furthermore, reproduction of only the nervous activities of the tactile receptors can provide the virtual touch regardless of the inner... 

Wide frequency range A tactile display can stimulate several tactile receptors selectively by ehanging frequency ranges because eaeh taetile receptor has different time response eharaeteristies for vibratory stimulation [9]. The required frequency range is from 5 to 200 Hz to stimulate all kinds of tactile reeeptors. The response speed of an IPMC is fast enough to make a vibratory stimulation on a skin higher than 200 Hz. This means that an IPMC ean stimulate all receptors seleetively. 

In actual fact, tactile sensation over the skin surface is highly dependent on finger pressure. The slow adapting receptors (RufRni and Merkell) probably determine this pressure, which then generates information relative to skin firmness. 

The answer to this question apparently comes from the sensitivity of the receptors themselves. Each receptor has a broad range of sensitivity, being most sensitive to one particular input value and less sensitive to other input values, with the degree of sensitivity becoming smaller as the difference from the most sensitive input value becomes greater. With several of these receptors, each with a different maximum sensitivity, perception can be related to the combination of outputs from different receptors. Thus, we may see different colors that do not correspond to the maximum sensitivities of any of the color receptors. Tactile localization could occur as long as the output frequencies of the skin touch sensors vary with distance from the receptor loeation. 

Humans receive and combine two types of perceptual information when touching and manipulating objects. Kinesthetic information describes the relative positions and movements of body parts as well as muscular effort. Muscle and skin receptors are primarily responsible for kinesthesis joint receptors serve primarily as protective Hmit switches [Rabischong, 1981]. Tactile information describes spatial pressure patterns on the skin given a fixed body position. Everyday touch perception combines tactile and kinesthetic information this combination is called tactual or haptic perception. Loomis and Lederman [1986] provide an excellent review of these perceptual mechanisms. 

The concepts of the selective stimulation method are illustrated in Figure 8.6. There are four types of mechanoreceptors embedded in human fingers FA I (Meissner s corpuscle), SA I (Merkel corpuscle), FA II (Pacinian corpuscle) and SAII (Ruffini endings) [18]. It is known that each receptor has temporal response characteristics for mechanical stimulation and causes subjective sensation corresponding to its responsive deformation. For example, SA I detects static deformations of skin and produces static pressure sensation, and FA I detects the velocity of the deformation and produces the sense of fluttering vibration. Tactile impression is an integrated sensation of these elementary sensations. To present tactile feel arbitrarily, stimuh applied to these receptors should be controlled selectively. 

Asamura, N., Yokoyanra, N. and Shinoda, H. (1998) Selectively Stimulating Skin Receptors for Tactile Display, IEEE Computer Graphics and Applications, 18 (6), 32-7. 

---