Somatic senses

somatic: 1: of, relating to, or affecting the body [...]; 2. of or relating to the wall of the body [217]

The main keywords related to somatic senses are tactile and haptic which are both related to the sense of touch (see 1.1.3). Concerning this sense, there is a lot more than only touch itself. For example, [120] distinguishes five ``senses of skin'': the sense of pressure, of touch, of vibration, of cold, and of warmth.

[120] indicates two more senses, called ``sense of position'' and ``sense of force'', related to the proprioceptors. The proprioceptors are receptors (special nerve-cells receiving stimuli) within the human body. They are attached to muscles, tendons, and joints. They measure for example the activity of muscles, the stressing of tendons, and the angle position of joints. This sense of proprioception is called kinesthesis and [120] calls the accompanying modality kinesthetical:

Kinesthesis (perception of body movements): (physiology, psychology) Kinesthesis is the perception that enables one person to perceive movements of the own body. It is based on the fact that movements are reported to the brain (feedback), as there are:

· angle of joints

· activities of muscles

· head movements (reported by the vestibular organ within the inner ear)

· position of the skin, relative to the touched surface

· movements of the person within the environment (visual kinesthesis)

Kinesthesis supports the perception of the sense organs. If some informations delivered by a sense organ and by kinesthesis are contradictory, the brain will prefer the information coming up from the sense organ.

Human computer interaction makes use of kinesthesis, e.g. if a key has a perceptible point of pressure or if the hand performs movements with the mouse to position the mouse-pointer on the screen, and so on [67].

For a multimodal system, kinesthesis is not as relevant as it is for a VR system, because aspects like experiencing an outer influence to the sense of balance, e.g. when wearing a head-mounted display (HMD), will not be a major topic here. Therefore, the most relevant somatic sense is the sense of touch, which can be addressed by special output devices either with tactile or with force feedback (see 2.2.4).

According to [309], four different types of touch receptors (or mechanoreceptors) are present at the human hand . They have different characteristics which will not be addressed within the scope of this report. As tests have shown, the output response of each receptor decreases over time (called stimulation adaptation) for a given input stimulus. The 2-point discrimination ability is also very important. The index finger pulp is able to sense all points with a distance of more than 2 mm whereas in the center of the palm two points that are less than 11 mm apart feel like only one. Other relevant factors are the amplitude and the vibration frequency of the contactor.

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