THE SENSE OF TOUCH
Jukka Raisamo
Multimodal Interaction Group
Tampere Unit for Computer-Human Interaction
Department of Computer Sciences
University of Tampere, Finland
Outline for the talk
• About the sense of touch
• Physiology of touch
• Haptic perception
1
The sense of touch 1/2
• Touch is our oldest, most primitive and
pervasive sense
– in general, the first sense we experience in the
womb and the last one we lose before death
• Touch is a proximal sense, i.e. we feel
things close to us or actually contact us
– some exceptions occur, e.g. heat radiation &
deep bass tones
– touch can be extended with special tools (e.g. a
long cane provides vibratory and pressure
information for a blind)
2
The sense(s) of touch 2/2
• Synonym to tactual perception
• Touch often considered one of five human senses
defined by Aristotle..
..however, when a person touches various feelings
from pressure to temperature and pain are evoked
• Thus the term “touch” is actually a combined term
for several sub-modalities
– in medicine touch is usually replaced with the term
somatic senses to better reflect the variety of sersory
mechanisms involved
• The senses of touch are mediated by the
somatosensory system
3
”The little man inside the brain”
• Sensory homunculus
for touch
– shows what a man's
body would look like if
each body part would
grow in proportion to
the area of the brain
concerned with its
sensory perception
Natural History Museum, London
http://www.nhm.ac.uk/
4
• Lips, tongue, hands,
feet and genitals are
considerably more
sensitive than other
parts of the body
Somatosensory system 1/2
• Sensory systems associated with the body
• Concerned with sensory information from the skin,
joints, muscles and internal organs
– the sensory information is highly sensitive to
temperature
• Four main modalities:
– the tactile senses (touch)
– the kinesthetic senses (proprioception)
– temperature
– pain
5
Somatosensory system 2/2
• Each somatosensory modality has its own receptors
or nerve endings
• Basic somatosensory pathway in short:
– if stimulus is larger than the threshold of its receptor, a
response is triggered
– electrical discharge is carried by the afferents to the
peripheral nerves
– impulses travel through spinal cord to the brains
– sensations are registered at the somatosensory cortex
• The greater the stimulus the more the receptor
discharges & the larger amount of receptors
discharge
6
Receptor classifications 1/2
• Location based classification
– skin receptors (exteroceptors) are located close
to the skin surface (touch-pressure, vibration,
temperature, pain)
– muscle and joint receptors (proprioceptors) are
located in tendons, muscles and joints (position
& movement)
– visceral receptors (interoceptors) are
associated with the internal organs (heart rate,
blood pressure)
7
Receptor classifications 2/2
• Transduction mechanism based
classification
– mechanoreceptors are responsive to any kind of
mechanical deformation
– thermoreceptors are responsive to changes in
temperature
– chemoreceptors are responsive to substances
produced within the skin
– nociceptors are specialized for detecting
painful stimuli
8
The tactile senses
tactile [from Latin tactilis, to touch]:
1 : perceptible by touch
2 : of, relating to, or being the sense of touch
(Merriam-Webster Online Dictionary )
9
Video: The Power of Touch (2:57)
http://www.teachersdomain.org/resources/tdc02/sci/life/reg
/touch/assets/tdc02_vid_touch/tdc02_vid_touch_56.mov
10
Skin 1/2
• The largest & heaviest organ in human body (~1,8
m2, 4 kg)
–
–
–
–
viscoelastic tissue (stretches & maintains its shape)
protects the body from dehydration & physical injury
regulates body temperature & blood pressure
contains structures responsible for ability to feel
• When we feel embarrassed, touched, selfconscious, or extremely sensitive we often feel it
directly as a charge at skin level
– psychological characteristic of the openness of skin may
be vulnerability, pleasure or excitement
11
Skin 2/2
• Layers of the skin
– epidermis (0.15-1.5 mm): outermost protective layer,
renews fast, contains e.g. pigment cells and keratin
– dermis (0.3-3.0 mm): beneath epidermis, contains e.g.
most of the skin receptors, nerve endings, blood vessels
– subcutaneous tissue (thickness varies greatly): for
insulation and storage of energy, contains e.g. fat, nerves
and blood vessels
12
Skin receptors 1/3
• Tactile sensations are experienced with the
entire skin surface
– different areas of skin have different qualities (e.g. hairy
skin has “soft touch” channel that is found to be
associated with emotions)
• Tactile sensing plays an important role in object
discrimination and manipulation
– contact detection
– tool manipulation
– tactile ability to sense vibration & skin deformation is
critical for determining surface texture
13
Skin receptors 2/3
• The receptors function optimally with light contact
• Skin receptors are divided into two categories
based on their speed of adaptation
– slowly adapting (SA) receptors detect constant stimulus
(e.g. pressure & skin stretch)
– rapidly adapting (RA) ones detect only short pulses (e.g.
initial contact & vibration)
RA
SA
stimulus
stimulus
14
Skin receptors 3/3
• Mechanoreceptors have different spatial and
temporal resolutions
– spatial resolution depends on location of the skin (i.e.
what and how many receptors are found in the locus)
– the size of the receptive field depends on how deep in the
skin the particular receptor type lies
– type I receptors have large receptive field (low spatial
resolution); type II receptor have small field (good
resolution)
– temporal resolution depends on the stimuli, not receptor
type
Small
receptive
field
Large
receptive
field
15
Tactile senses
• Skin can be stimulated by tactile feedback
(discussed more on Lecture 2)
16
The kinesthetic senses
(proprioception)
proprioception [from Latin proprius, one’s own]:
the reception of stimuli produced within the organism
(Merriam-Webster Online Dictionary )
17
Proprioception 1/2
• Tells us what happens below the body
surface
– stretch receptors within muscles with rapidlyadapting and slowly-adapting components
– receptors to monitor tensions and forces at the
tendons and joints
– associates with the neural signals derived from
motor commands
18
Proprioception 2/2
• Mediated by receptors located in muscles,
tendons, and joints
– stimulated by bodily movements and tensions
• Receptors register different kinds of information
– cutaneous mechanoreceptors (skin stretch)
– muscle spindles (muscle stretch),
– Golgi tendon organs (tendon stretch)
– joint receptors (joint stretch)
• Proprioception can be stimulated by force
feedback (discussed more on Lecture 3)
19
Temperature and pain
20
Pain and temperature 1/2
• Pain and temperature system does not have
specialized receptor organs
– the changes in the body state is perceived via
free nerve endings found throughout skin,
muscles, bones, and tissues
– most pain is a result of substances released by
damaged tissues
21
Pain and temperature 2/2
• Temperature receptors (thermoreceptors)
– around 30 cold receptors per each hot receptor
– warm receptors are maximally responsive at
45°C, cold receptors at 27°C
– more responsive to a change in temperature
than to a constant temperature
• Pain receptors (nociceptors)
– nerve endings sensitive for mechanical, thermal
or chemical stimuli
– highly important for avoiding accidents
22
Haptic perception
haptic [from Greek haptesthai to touch]:
1 : relating to or based on the sense of touch
2 : characterized by a predilection for the sense of
touch
(Merriam-Webster Online Dictionary )
23
Haptic perception 1/2
• Vision vs. touch
– vision more capable of providing geometric
information & general picture
– touch more effective at providing material
information & fine surface details
• Different strategies for touching
– active touch (focus on the object)
– passive touch (focus on the sensation
experienced)
24
Haptic perception 2/2
• Haptic perception integrates somatosensory
information
– touch mediates material properties (texture, hardness,
temperature etc.)
– proprioception provides spatial and motor information ()
• The perceived frequency of the
grating depends on both the
physical frequency of stimulation
and information about how fast the
finger is being moved across the
surface.
25
Active vs. passive touch
• The great cookie-cutter
experiment by Gibson (1962)
– experimenter pushes cookie cutter
onto subject’s palm Ö 49% correct
identification
– subject actively feels cookie cutter
with the palm Ö 95% correct
identification
• Demonstrated that active
exploration is essential in the our
ability to perceive the objects in
the physical world
26
Exploratory procedures
• Exploratory procedures defined by Lederman & Klatzky
(1987)
– stereotypical ways of touching
– enhances the relevant perceptual information by heightening
the responses of the specific mechanoreceptors
27
Real-world exploration
• Blind paleobiologist: Geerat Vermeij, Professor of Geology,
UC Davis
– Born with a childhood form of glaucoma, had both eyes
removed in 1950 because of constant pain and in danger of
brain damage.
– Vermeij does all his work with his hands, using his exquisitely
trained sense of touch.
28
Questions, please?
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