Firm foundation in the main hci principles, the book provides a working

Figure 3.14 A high-quality natural-language-based interaction through the cloud. The smart-
media client devices would send the captured sentence (in voice or text), and a correct and intel-
ligent response is given back in real time. (From http://www.research.att.com/project/WATSON.)

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4 8
H U M A N – C O M P U T E R I N T E R A C T I O N 
haptic is defined to be the modality that takes advantage of touch by 
applying forces, vibrations, or motions to the user [17]. Thus haptic 
refers to both the sensation of force feedback as well as touch (tactile). 
For convenience, we will use the term haptic to refer to the modal-
ity for sensing force and kinesthetic feedback through our joints and 
muscles (even though any force feedback practically requires contact 
through the skin) and the term tactile for sensing different types of 
touch (e.g., texture, light pressure/contact, pain, vibration, and even 
temperature) through our skin.
3.2.3.1 Tactile Display Parameters
• Tactile resolution: The skin sensitivity to physical objects is dif-
ferent over the human body. The fingertip is one of the most 
sensitive areas and is frequently used for HCI purpose. The 
fingertip can sense objects as small as 40 μm in size [18].
• Vibration frequency: Rapid movement such as vibration is 
mostly sensed by the Pacinian corpuscle, which is known to 
have a signal-response range of 100–300 Hz. Vibration fre-
quency of about 250 Hz is said to be the optimal for comfort-
able perception [16].
• Pressure threshold: The lightest amount of pressure humans 
can sense is said to be about 1000 N/m
2
. For a fingertip, 
this amounts to about 0.02 N for the fingertip area [19]. The 
maximum threshold is difficult to measure, because when the 
force/torque gets large enough, the kinesthetic senses start to 
operate, and this threshold will greatly depend on the physi-
cal condition of the user (e.g., strong vs. weak user).
As mentioned previously, there are many types of tactile stimula-
tion, such as texture, pressure, vibration, and even temperature. For 
the purposes of HCI, the following parameters are deemed important, 
and the same goes for the display system providing the tactile-based 
feedback. Physical tactile sensation is felt by a combination of skin 
cells and nerves tuned for particular types of stimulation, e.g., the 
Meissner’s corpuscle for slight pressure or slow pushing (stimulation 
signal frequency of 3–40 Hz), Merkel cells for flutter and textured/
protrusion surfaces (0.3–3 Hz), the Pacinian corpuscle for more rapid 

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4 9
H U M A N FA C T O R S A S H C I T H E O R I E S
vibratory stimulation (10–500 Hz), and Ruffini endings for skin 
stretch (Figure 3.15).
While there are many research prototypes and commercial tactile 
display devices, the most practical one is the vibration motor, mostly 
applied in a single actuator configuration. Most vibration motors do 
not offer separate controllability for amplitude and frequency. In addi-
tion, most vibrators are not in direct contact with the stimulation tar-
get (e.g., the hand), making the signal somewhat muffled through the 
casing. Thus additional care is needed to set the right parameter values 
for the best effects under the circumstances.
Another way to realize vibratory tactile display is to use thin and 
light piezoelectric materials that exhibit vibration responses according 
to the amounts of electric potential supplied. Due to their flat form 
factor, such materials can be embedded, for instance, into flat touch 
screens. Sometimes sound speakers can be used to generate indirect 
vibratory feedback with high controllability (responding to wide 
ranges of amplitude and frequency signals) (Figure 3.16).
3.2.3.2 Haptic and Haptic Display Parameters 
Along with tactile feed-
back, haptic feedback adds a more apparent physical dimension to 
interaction. Force feedback and movement is felt by the cells and 
Free nerve
ending
Meissner’s
corpuscle
Pacinians
corpuscle
Ruffini
ending
Hair follicle
nerve ending
Merkel's
disk