Brief introduction to Perceptual process: Human sense of touch, Somatosensory system, Mechanoreceptors, Tactile acuity, Receptive field.

1. Tactile perception
Mariacarla Memeo
Robotics, Brain and Cognitive Science(RBCS)
Multisensory mapping group
Istituto Italiano di Tecnologia (IIT), Italy

2. ● Perceptual process
● Human sense of touch
Outline
1. Somatosensory system
2. Mechanoreceptors
3. Tactile acuity
4. Receptive field
● Electric metaphor of mechanoreceptors

3. Perceptual process

4. Skin
Skin - heaviest organ in the body
Functions:
Provides tactile information.
Warns us of damaging stimuli.
Contains body fluids and organs.
Protects against bacteria.
Regulates body temperature.

5. Why touch is important?
Haptic senses—or “touch”—is the most basic of senses; we learn this before
vision and smell.
Everyday Tasks:
● Grasping objects;
● Dialing a phone;
● Playing a guitar or piano;
● Finding a light switch;
● Feeling your pulse.
Touch is complex (e.g. tying a shoelace): bi-directional communication channel
– both input & output

6. Human sense of Touch
Touch helps us identify objects and provides unique
information (e.g., texture, friction)

7. Human Sense of Touch
Skin structure and
mechanoreceptorsu
Somatosensory system

8. Somatosensory system
The 'somatosensory system' is a sensory that detects
experiences labelled as touch or pressure, temperature(warm
or cold), pain(including itch and tickle) and those that belong
to proprioception(muscle movement and joint position).

9. Somatosensory Cortex
Damage to somatosensory cortex destroys ability to
recognize objects by touch.

10. Somatosensory Cortex
The body is mapped topographically onto somatosensory cortex, but
body parts are not represented equally.

11. Somatosensory Pathway
ANTEROLATERAL SYSTEM
This system is involved in
the perception of touch,
temperature, and sharp
pain. Next instead is
selectively involved in the
perception of deep, chronic
pain.

12. Mechanoreceptors
These are the
tissue “filter”
between stimulus
and nerve ending .
Each type encodes
a different aspect
of deformation.
(Stimulus)
Mechanical
filter
Transducer
(mechanical energy
→
bioelectric signals)
Merkel receptors
Encoder
Ruffini cylinder
Pacini corpuscole
Meissner
corpuscole

13. Mechanoreceptors
Two types located close to surface of the skin
Merkel receptor fires continuously while stimulus
is present.
Responsible for sensing fine details
Meissner corpuscle fires only when a stimulus is
first applied and when it is removed.
Responsible for controlling hand-grip

14. Mechanoreceptors

15. Mechanoreceptors
Two types located deeper in the skin
Ruffini cylinder fires continuously to stimulation
Associated with perceiving stretching of
the skin
Pacinian corpuscle fires only when a stimulus is
first applied and when it is removed.
Associated with sensing rapid vibrations
and fine texture

16. Mechanoreceptors

17. Mechanoreceptors – temporal features

18. Mechanoreceptors – spatial features

19. Tactile acuity
Tactile acuity thresholds are determined by Merkel receptors
(SA1).
● Measuring tactile acuity
● Two-point threshold - minimum separation needed between
two points to perceive them as two units

20. Tactile acuity
● Measuring tactile acuity
● Grating acuity - placing a grooved stimulus on the skin and
asking the participant to indicate the orientation of the
grating

21. Tactile acuity
Merkel receptors are densely packed on the fingertips - similar to cones
in the fovea.
Both two-point thresholds and grating acuity studies show these results

22. Receptive Field
The receptive field of an
individual sensory neuron is the
particular part of the body
surface in which a stimulus will
trigger the firing of that neuron.
The two-point threshold for any
part of the body is determined by
the size of the receptive fields
and the extent of overlap.

23. Receptive Field
Tactile acuity is determined by how close the mechanoreceptors are to
each other and by the size of the receptive field
Inhibitory region
Excitatory
region

24. Receptive Field
Mechanoreceptors found in areas of the body with less tactile
acuity tend to have larger receptive fields.

25. Electrical metaphor
Ruffini cylinder
Pacini corpuscole
Merkel receptors
Meissner
corpuscole

27. Outline
● Perceptual process (continue)
● Meaning
1. Psychophysics
2. Absolute threshold
3. Differential threshold (Just Noticeable Difference)
● Tactile feedback
● Tactile technologies
● Simulate tactile perception (our robot)

28. Perceptual process

29. Psychophysics
Science that focuses on how the physical environment is integrated into
our personal, subjective world.
● Ernst Weber & Gustav Fechner -- psychophysicists

30. Absolute threshold
● Weber published “De tactu” describing the minimum amount of
tactile stimulation needed to experience a sensation of touch – the
absolute threshold.
● Using weights he found that holding versus lifting them gave different results
(due to muscles involved).
● He used a tactile compass to study how two-point discrimination
varied across the body.
● On the fingertip .22 cm, on the lips .30 cm,
on the back 4.06 cm.
Aesthesiometric compass

31. Just Noticeable Difference
(JND)
● Weber studied how much a stimulus must change in order for a
person to sense the change.
● How much heavier must a weight be in order for a person to notice that it is
heavier?
● This amount is called the just noticeable difference JND

32. Just Noticeable Difference
(JND)
1)A stimulus is presented at a
starting quantity Q.
1)Once the subject is adapted to
this stimulus, the attribute is
increased …
…each time the subject is asked…
yes or no
if there is a difference in the
stimulus.

33. Just Noticeable Difference
(JND)
After repeated trials the
increment T that produces a
perceptible difference
is identified:
75% of right difference in
threshold identified.

34. Just Noticeable Difference
(JND)
After repeated trials the
increment T that produces a
perceptible difference
is identified.
It is calculated when the
75% of right difference in
threshold identified.

35. Just Noticeable Difference
(JND)
When the starting quantity is the
stimulus zero value (complete
absence of the stimulus), then
the first jnd value T is called
… absolute threshold Q0…
This is the minimum stimulus
quantity necessary to produce
any sensation.

36. Just Noticeable Difference
(JND)
● JND can be expressed as a ratio:
where (Q + Q0) is stimulus magnitude and k is a constant and DQ means the
change in R (D usually means change).
or
k
T
Q Q

(  ) 0
k
Q
D
( Q Q
) 0


● Weber's law asserts that the ratio k is constant across a wide range of stimulus
quantities, although the value of k is different for different sensory domains or
types of stimulus.

37. Fechner Contribution
● Fechner related the physical and
psychological worlds using
mathematics.
● Fechner (1860) said:
“Psychophysics, already related to
physics by name must on one hand be
based on psychology, and [on] the other
hand promises to give psychology a
mathematical foundation.” (pp. 9-10)
● Fechner called Weber’s finding
about the JND “Weber’s Law.”

38. Fechner Contribution
● Fechner's Law allows the estimation of the sensory intensity (S) from
the stimulus quantity:
log( ) 0 S  k Q Q
where S is sensation, k is Weber’s constant and R is the magnitude of a stimulus
● The larger the stimulus magnitude, the greater the amount of
difference needed to produce a JND.

39. Relationship of JND with
Stimulus
log( ) 0 S  k QQ

40. Fechner Contribution
● His methods are still used in psychophysics.
● Ideas from signal detection theory have been applied to a wide
variety of other topics.
● Threshold for criminal behavior, scenic beauty.
● Scaling techniques, including rating scales, were placed on a sound
scientific basis, especially by S.S. Stevens modified Fechner’s Log
Law to a Power Function in the early 1950’s.

41. Tactile Feedback
Goal is to stimulate the skin in a
programmable manner to create
a desired set of sensations.
Tactile feedback is generated by a
tactile device, sometimes called a
tactile displays or tactile
interfaces.
Tactile feedback aims to create a
metaphor to communicate
specific information.

42. Tactile Technologies
Pins or other mechanical vibrating elements, such as servo motors - either alone or
in an array, as in devices for Braille display
● typically used for fingertip stimulation
Wagner & Howe 2002
Kyung et al. 2006
Brayda et al. 2010
Cannella 2009

43. TActile MOuse (TAMO)
Tactile device, able to deliver simple tactile maps of virtual objects by
means of heights information.

44. Tactile Technologies
Cushions of air can be inflated or deflated to vary pressure on skin
Okamura et al. 2014
Chih-Hung King 2008
Siarhei Vishniakou et al. 2013

45. Tactile Technologies
● Skin stretch to simulate moving stimuli.
Provancher et al. 2013 Hayward et al. 2003-7
● Electro-tactile devices.
Kajimoto laboratory 2014