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which also includes proprioception and kinesthesis (sensory input from joint, muscles, and internal
organs) and pain (tissue damaging high intensity stimuli). Touch is, for the most part, a proximal
sense. In other words, we tend to feel stimuli that are in contact with, or at least in close proximity
to, our body (Cholewiak and Collins, 1991). Touch is also our only bi-directional sense, that is, it sup-
ports both perception and acting on the environment.
We will start our overview of touch by examining its medium, the skin, which is a multi-layered sheet
of 1.8 m
2
in area and approximately 4 kg in weight in an average adult. There are three types of skin: (a)
glabrous skin, that is, hairless skin such as the skin of our palms, (b) hairy skin, and (c) mucocutaneous
skin, that is, skin that borders the entrances to the body's interior (Greenspan and Bolanowski, 1996).
The most active role in tactual perception is played by the glabrous skin, especially in the palmar and
fingertip regions of the hand. The ridges and valleys of the skin in this area have been implied in the per-
ception of texture and in the tactile identification of objects. Most studies on tactual perception have
focused on these regions, and they are most often used to present tactile stimuli in current interfaces
(e.g., CyberTouch, Tactools, and Touchmaster). In general, the skin is composed of the epidermis (its
outer layer) and the dermis (the inner layer), both of which contain several types of receptors (see
Figure 23.12).
In our overview of touch, we will focus on tactile sensations resulting from mechanical stimulation of
the skin, which forms the basis of most current tactile displays. Mechanoreceptors can be divided into
four major types (e.g., Burdea, 1996):
1. Meissner corpuscles: these receptors represent approximately 43% of all tactile receptors in the
hand. They are found only in glabrous skin and are sensitive to stimuli such as velocity and
skin curvature.
2. Merkel's disks: merkel's disks represent 25% of all mechanoreceptors in the hand and sense gentle
localized pressure and vibration information.
3. Pacinian corpuscles: these receptors are located deeper in both hairy and glabrous skin. Approxi-
mately 13% of all mechanoreceptors are Pacinian corpuscles. They sense rapid variations of
deformation, acceleration, and vibration.
4. Ruffini corpuscles: 19% of all mechanoreceptors in the hand are Ruffini corpuscles. They are
located deep under the skin and are sensitive to vibrations, stretching of the skin, and thermal
changes.
The distribution of these receptors varies considerably across different body regions. For example,
there is a total of approximately 17,000 mechanoreceptors in the human hand (Johansson and Vallbo,
1983), which exceeds by far the number of these receptors in other body regions. Also, certain types
of receptors are not represented in some body regions. For example, there are no Pacinian corpuscles
in the skin of the cheek (Cholewiak and Collins, 1991).
The four types of mechanoreceptors can be classified according to the following two criteria (Konta-
niris and Howe, 1995; Johansson and Vallbo, 1983):
1. The receptor's active area: small well-defined receptive fields (Type I units) and larger receptive
fields with obscure borders (Type II).
2. The receptor's response to static stimuli: approximately 45% of all mechanoreceptors respond to
static stimuli with a sustained discharge and are called slowly adapting (SA). The remaining recep-
tors respond to the onset and offset of stimuli with bursts of impulses and are called fast or rapidly
adapting (FA or RA).
Table 23.2 summarizes important characteristics of the four receptor types.
For the purpose of this chapter, it is not necessary to examine in detail the process of transformation of
mechanical stimuli into neural events. This process is still poorly understood and, more importantly, it is
of limited relevance for human factors practitioner. Ultimately, all tactile information is relayed to the
somatosensory cortex, which is laid out in the form of a homunculus representing the opposite side
of the body. In this representation, areas of greater sensitivity occupy larger cortex areas. This suggests
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