Anatomy of Touch

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The second approach would be to overload the somatosensory system output and generate artificial signals to the dorsal-column-medial lemniscal system (responsible for touch and proprioception) and the spinocerebellar system (responsible for proprioception) in the spinal cord.

Unfortunately, this method also requires advance connectivity between interaction systems and the human nervous system itself, which may be invasive and essentially dangerous. Although this technique could also be very useful in creating realistic sensory information in the future, enhancing interaction to the next level, however, for now medical science has yet to develop safe and non-invasive tethering mechanisms to facilitate human interaction. Therefore, at this time a non-invasive approach, although rudimentary in its ability to generate and simulate cutaneous information, would be ideal for interaction in virtual environments. This approach is to externally simulate environmental conditions to the somatosensory system and the mechanoreceptors themselves, tricking them into generating the necessary signals to the dorsal-column and in turn the sensory cortex in the brain. As mentioned, this approach has its limitations; however, it can be streamlined into achieving haptic illusions to facilitate delivering haptic information in simulated environment based interaction. Although this technique is fundamentally limited, it been used for the last few decades to produce a rudimentary sense of haptic simulation, with considerable success.

Utilizing sensory information to create tactile illusions

Understanding human responses to various tactile stimuli are useful in environments where artificial stimulation is needed (Kang et al., 2012).

Basically, by overloading the somatosensory information, the human brain can be tricked into perceiving a slightly varied form of reality. Much like other illusions (visual and auditory), tactile illusions can be triggered through various techniques and can be utilized to create or facilitate virtual environments and objects (Bean, C. H., 1938). By understanding and generating specific tactile illusions, it is possible to design interfaces that simulate various forms of touch sensations (Goldstein, 1999). This thesis builds on this core principle and tries to identify various techniques of providing tactile simulation (illusion) using vibration-based stimuli.

This Chapter explores these illusions by summarizing the physiological fundamentals of tactile sense in general and the biological systems that sample channel and decode these signals into what humans perceive as haptic information.

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The somatosensory system is responsible for sampling the environment and relaying information to the brain which is then decoded and attributed as common sensations (i.e. cold, hot, smooth, and rough,

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pressure, tickle, itch, pain, and vibrations). To understand this mechanism it is important to understand the structure and layering of the heaviest organ of the body (skin). The skin is composed of several layers: the epidermis, the dermis and the subcutaneous. Although, the epidermis is generally made of dead skin cells, it is waterproof and provides a shielding affect to the remaining skin layers and the internal organs of the body. The dermis, on the other hand, mostly consists of hair follicles, sweat glands, sebaceous (oil) glands, blood vessels, nerve endings, and a wide range of receptors (Gardner, Martin & Jessell, 2000), which actively protect the body from various environmental conditions. The last layer consists of the subcutaneous tissue that mostly consists of fat and connective tissues and is used as a thermal insulator to helps control body temperature. The layer also acts as a mechanical shock absorber (damper or cushion) to protect underlying tissue from damage and is equipped with sensors or mechanoreceptors that mediate information related to force and proprioception (relative movement of joints and muscles). Due largely to the number and distribution of these receptors at various parts of the body, each part may vary in its sensitivity to tactile and pain stimuli.

It is possible to measure this sensitivity by determining the user’s two-point discrimination threshold, the distance between two two-points on the skin necessary in order for the individual to distinguish two distinct stimuli from just one (see figure 1 below).

Figure 1. Illustration of the acuity of and spatial sensitivity of human skin, adapted from Goldstein, 1999. Sensation and Perception (with VirtualLab Manual CD-ROM), 8E. © 2010

South-Western, a part of Cengage Learning, Inc. Reproduced by permission.

www.cengage.com/permissions; and Charles C Thomas Publishers, Ltd., Springfield Illinois.

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15 Somatosensory System: The Ability to Sense Touch

The human somatosensory system is a complex mechanism which consists of cutaneous (contact based) senses that are supplemented by sensory receptors embedded in various layers of the skin. Essentially, there are four key types of receptors (figure 2): mechanoreceptors, thermoreceptors, pain receptors, and proprioceptors (Goldstein, 1999).

Mechanoreceptors: The mechanoreceptors sample sensations similar to force, torque, pressure, vibrations, and texture. There are primarily four kinds of mechanoreceptors which perceive indentions and vibrations of the skin to generate feedback regarding force, vibrations and textures at the point of contact: Merkel's disks, Meissner's corpuscles, Ruffini's corpuscles, and Pacinian corpuscles.

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Figure 2. Structure and placement of various mechanoreceptors in hairy (a) and glabrous (b) parts of the skin, adapted from Goldstein, 1999. Sensation and Perception (with VirtualLab

Manual CD-ROM), 8E. © 2010 South-Western, a part of Cengage Learning, Inc. Reproduced by permission. www.cengage.com/permissions.

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The highest sensitive in mechanoreceptors can be seen in the Merkel's disks and Meissner's corpuscles, as they can be found in the outer layers of the dermis and epidermis and are generally found in non-hairy skin such as the palms, lips, tongue, soles of feet, fingertips, eyelids, and the face.

Merkel's disks are slowly adapting receptors (SA I, SA II) while Meissner's corpuscles are rapidly adapting receptors (RA I, RA II). Utilizing both receptors simultaneously generates the ability to sample information regarding both texture and shape of objects (Swenson 2006, see figure 2).

The ridge based structure of the fingertips further enhances the ability to sense textural variation while rapid sampling (moving the hand over a textured object or surface).

Subsequently, the Ruffini's corpuscles and Pacinian corpuscles are located lower down in the dermis and along joints, tendons, and muscles. These mechanoreceptors sample external vibration migrating down bones and tendons as well as collect information regarding rotational movements of limbs, and the stretching of the overlaying skin. These receptors are ideally suited to sample environments with sub-textural information, such as orientation structure and balance. The receptors provide supporting or secondary information to the already available signals sampled via the Merkel’s disks and Meissner’s corpuscles, essentially, enhancing motor functions in complex tasks (Gardner et al., 2000).

Thermoreceptors: The thermoreceptors perceive sensations regarding variations in temperature sampled by the skin. They are situated within the dermis and can be activated through contact. Fundamentally, two different types of thermoreceptors are used to distinguish between hot and cold. The ‘Cold receptors’ start to sample sensations of cold once the skin temperature drops below ~35° C and are most sensitive near ~25° C.

However, their sensitivity is greatly reduced below ~5° C (Gardner et al., 2000, Swenson 2006). The ‘Hot receptors’, on the other hand can sample environmental conditions once the skin temperature rises above ~30° C and can be most sensitive around 45° C. However, as with ‘Cold receptor’, beyond this stage, their efficiency is greatly reduced and over ~55° C pain receptors are activated to limit harm to the skin and underlying tissues.

Although, both types of thermoreceptors can be found throughout the body, the density of cold receptors is much higher than heat receptors. The highest concentration of thermoreceptors can be found in the face and ears (Gardner et al., 2000).

Pain receptors or nociceptors: "Noci-" in Latin means "injurious" or "hurt"

and these receptors are responsible for sampling pain or stimuli which may cause damage to the skin or the underlying tissue. There are over three million pain receptors throughout the body, which are situated in skin, muscles, bones, blood vessels, and some organs (Swenson 2006).

These receptors can sense pain caused through mechanical stimuli (cuts or

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scrapes), thermal stimuli (burns), or chemical stimuli (poison from an insect sting etc.). Some of these receptors are decoded to yield feelings of sharp pain to encourage avoiding harmful stimulus, such as broken pieces of glass or a hot stove. Other receptors yield dull pain in infected or injured areas of the body to discourage contact and ensure proper healing.

Although most of these receptors react to either thermal, chemical or mechanical stimuli specifically, some of them may be activated by more than one mode of stimuli and these are classified as polymodal. Typically, each nociceptor may have different threshold levels, and may be triggered by chemical, thermal or mechanical stimuli. However, some nociceptors, also known as “sleeping nociceptors” may actually respond to none of these stimuli but may only be triggered due to injury / inflammation to the surrounding tissue.

Proprioceptors: These receptors sense the position and orientation of the various segments of the human body. The receptors achieve this by co-relating positional information amongst each other within various parts of the body and the surrounding environment. Proprioceptors are found in tendons, muscles, and joint capsules. Due to their various locations, the receptors are also able to perceive variations within muscles such as muscle length and tension to generate a comprehensive understanding of any physical interaction. Therefore, proprioceptors play an essential role in providing real-time feedback to the brain during interaction with the surrounding environment.

Although many of the receptors mentioned above have predefined functions to assist the body in sensing environmental information, they generally sample a wide ranging of information concurrently. For example, when drinking a beverage from a cold glass bottle, the hand may sense a number of dissimilar sensations simply by picking up the bottle.

Thermoreceptors in the fingers may sample information regarding the temperature of the bottle and compare it to surrounding temperature variations. Similarly, the mechanoreceptors in your hand may sample the texture and contours of the bottle itself, and possibly also perceive the tiny vibrations within the bottle produced by the bubbles rising to the surface of the beverage. Mechanoreceptors located deeper in the hand may sense muscular state, i.e. hand is stretching around the can, as well as the pressure being exerted to hold the can in place. Furthermore, proprioceptors may also sample the movement and stretching of the fingers and the palm in order to grasp the bottle.

This huge volume of information is continuously being relayed to the brain to provide a real-time information feedback loop, ensuring appropriate actions are undertaken to support the specific interaction.

Therefore, ideally, to simulate the exact perception of ‘holding a metal can full of soda’ all the mentioned receptors need to be activated during the

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artificial stimulation. Though, even partial stimulation of the mechanoreceptors and proprioceptors can trick the brain into perceiving the illusion of the said environment, as the brain can superimpose the partial receptor information onto previous somatosensory experience, inducing ‘haptic imagination’. Nevertheless to engage haptic imagination, it is important to stimulate as much of the needed receptors as possible and supplement this information with supporting sensory (visual and auditory) information.