Biomedical Engineering Reference
In-Depth Information
phantom sample containing simulated tumors. However, it was not possible to extract any
data from this article regarding the size and depth of any lump that was ostensibly found.
1.9 Tactile Sensing in Humans
As previously mentioned, touch is one of the five human senses, and, although all of the
senses are important, touch is often subordinated as being one of the lesser. This is per-
haps not surprising given the fact that sight and hearing are usually considered as being
the two most important in our everyday lives. Although the senses of smell and taste
have their deserved place in our cognitive senses, it is the sense of touch that has always
been very important to our existence because it has enabled us to use tools with great
dexterity. It therefore follows that a paramount step in the realm of tactile sensing would
be to develop a mechanical device that would duplicate and replace the sense of touch in
the human hand. Designing hardware for this purpose, however, poses a significant chal-
lenge, primarily because our hands (especially the fingers) are extremely sensitive to even
the smallest vibrations over a range from 10 to 100 Hz [41]. To help define the design
requirements of such hardware, it is important to understand the fundamentals of tactile
sensing mechanisms, since this knowledge will greatly assist in improving current designs.
In addition, it will aid our understanding of which signals are important to communicate
and the extent to which they should be communicated. Although this knowledge is a pre-
requisite, there is more to consider than simply the neurophysiology of touch, which is,
in reality, a combination of tactile and kinesthetic information. The combination of cuta-
neous and kinesthetic sensing is referred to as haptic perception and it is this, in addition
to the interpretation of these signals at a higher level, which ultimately interests us.
1.10 Haptic Sense
Two groups of physiological sensations are involved in the wider meaning of haptic sense.
The first is the tactile sense where the receptors are located under the dermis and are known
as mechanoreceptors. Tactile sense detects the information from the skin surface, such as
contact pressure or vibration. The other is the proprioceptive sense, where the receptors
exist in the muscle or in the tendon.
1.10.1 Mechanoreception
The human hand contains a complex array of specialized receptors that are rugged enough
to survive repeated impacts, while still retaining the ability to detect faint vibrations and
the softest touch.
Four main types of tactile mechanoreceptors have thus far been identified [42, 43],
of which each is associated with a specific phenomenon: pressure, shear, vibration, and
texture [43]. The sensing element of each of these mechanoreceptors is very similar in that
they possess physical packaging and position within the skin that is exclusively adapted
to its purpose. Figure 1.2 shows a cross-sectional view of the skin on a human fingertip
and the position of specialized touch receptors underneath the skin surface [44, 45]. The
distribution of the mechanoreceptors on the palm is shown in Figure 1.3 [46 - 48].
