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to use such interfaces under these conditions: Recognition of its texture, roughness,
temperature or sliding of the object on skin:
For recreational applications;
Predominantly for manual training (medical, handicraft, etc.);
For product design applications, where ergonomics of touch are important.
This type of application is however not common. The use of such interfaces is less
evident for applications where the use of tactile interfaces at the level of the functional
I 2 is not required. First of all, because we can easily “mobilise'' ourselves in a virtual
world without tactile contact and temperature variation. Although vision is necessary,
touch is not indispensable to “live'' in a virtual world. We could perhaps suggest the
same rule for hearing, but it is so easy to introduce sound in a virtual world, with
interfaces that are less expensive and non-invasive, that we do not hesitate to do it.
Conversely, there is a major constraint of the cutaneous sensitivity interfaces for the
tactile sense: it is necessary to equip the skin of the user. At the level of the cognitive
and sensorimotor I 2 , we can consider the question differently: what are the advantages
of the efforts to add cutaneous stimuli?
If it simply involves informing the subject that the contact has taken place between
the object and his hand, the sensorimotor substitution metaphors are generally effi-
cient: change of colour, flashing of the object, sudden variation of force on the force
feedback interface or characteristic noise at the time of contact. But these tactile inter-
faces are useful if we want the subject to be well-informed about the contact or correctly
catch an object in his hand. For example, the sliding surface information aids the
sensation of grasping an object.
Various key points resulting from the physiological study of the human tactile device
(see chapter 3) must be underlined. First, we distinguish between three main qualities
in the cutaneous mechanical sensitivity (Caldwell et al., 1996): Pressure sensitivity,
vibration sensitivity and touch, strictly speaking, or sense of touch (speed or shear-
ing sensitivity). Sensitivity to temperature variation is supported by two types of
thermoreceptors (hot and cold).
A tactile interface must be capable of stimulating all or one of these various bio-
logical sensors. In other words, a tactile interface is an interface that should be able to
reproduce one or more modalities of tactile stimulations. Each of the tactile stimulation
modalities follows the working ranges in terms of frequencies and amplitudes of the
stimulation signal (Figure 10.1). In fact, if the tactile stimulation modality is vibratory,
the working frequency can then vary from around ten hertz to several hundred hertz
(vibratory case) (Howe & Cutkosky, 1992). One could also be led to operate contin-
uously (case of pressure). The amplitude of the deformation of the skin will depend
on the tactile stimulation modality and can vary from a few microns (vibratory case)
to a few millimetres (case of pressure). It must be noted that in case of shearing tac-
tile stimulation modality, the frequency and amplitude variation ranges are very close
to those of the vibratory case (Hayward, 2000). Biggs and Srinivasan (2001) studied
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