lecture 3 - Tactile and Haptic Perception

Question 1

what are the 3 main groups of receptors and what do they respond to

Answer 1

1. Mechanoreceptors (respond to mechanical stimuli, e.g., pressure
   (stroking), stretching and vibration)
2. Thermoreceptors (respond to temperature)
3. Chemoreceptors (respond to certain types of chemicals, e.g.,
   histamine)

4.Nociceptors –subtypes of chemo- and mechanoreceptors?
(mediate the perception of pain

Question 1

different types of receptors are distributed throughout the skin and respond to _______

Answer 1

different touch related stimuli and events

Question 2

mechanoreceptors
merkel
Meissner’s
Pacinian
Ruffini

Answer 2

merkel
-small receptors with sharp borders
-slow adaptation (how the receptors react to any kind of stimulation)
-pressure and fine texture and shape
-close to skin surface
(slow adapting type 1)

meissner’s
small receptors with sharp borders
-rapid adaptation (only respond to an onset or offset of stimulation - drop firing rate in between)
-indentation motion across skin
-close to skin surface
(fast adapting type 1)

pacinian corpuscle
-large receptors with diffuse borders
-rapid adaptation
-vibration and fine texture
-deeper in the skin
(Fast adapting type 2)

Ruffini Cylinder
-large receptors with diffuse borders
-slow adaptation
-stretching
-deeper in skin
(slow adapting type 2)

Question 3

tactile acuity
-how can we measure sensitivity in the skin

Answer 3

-two point threshold
-grating acuity

Question 4

measure
-two point threshold
-confounds
classical measure in early research

Answer 4

-compass with 2 needles , change the separation between the two needles, and lightly touch the skin
-report what you feel-one needle or 2 ?
- what is Minimum separation between two points on the skin that is recognised as two

-find that on sensitive parts of our bodies like fingers and tongue or lips, you need a very small seperation to know theres 2 needles, however for other body parts you cant tell as easily (back/calf etc)

vulnerable to confounds
(the needles MUST touch at the same time,if you have slight temporal offset the thresholds get smaller)

Question 5

grating acuity

Answer 5

-8 cylinders (mushrooms looking ones) they have a pattern of grooves and ridges . each one gets finer and finer
-press them in the finger vertically or horizontally (orientation)
-more objective than 2 point threshold

• Acuity as the spacing for which orientation can still be accurately judged (75% correct) when we are 75 correct that is the threshold
• More objective measure (i.e., no temporal offset)
• Thresholds tend to be a bit lower than with 2-point
  threshold method (fingertip: ~ 1 mm vs. 2-4 mm

Question 6

what are the receptor mechanisms for tactile acuity

-which receptors respond to grooved stimulus
-which body parts have higher receptor density

Answer 6

Receptor properties determine the perceptual experience when skin is stimulated

• Merkel receptors (close to skin) respond to grooved stimulus patterns → firing of the fibre reflects pattern of grooved stimulus →signal detail and texture

Sensitive body parts have higher receptor density

Question 7

better tactile acuity is associaed with _____ density of merkel receptors

Answer 7

higher

Sensitive body parts have higher receptor density
→ Better tactile acuity is associated with higher
density of Merkel receptors

Question 8

what is the receptive field

Answer 8

The receptive field is a portion of sensory space that can elicit neuronal cortical responses when
stimulated

Question 9

is density the only thing that determines tactile acuity?
-what else determines tactile acuity and how

Answer 9

no
-Higher acuity on index finger pad than on pad of little finger
even though receptor density is identical → cortical mechanisms

Size of receptive fields of cortical neurons also determines the tactile acuity
(discrimination) → cortical neurons representing body parts with higher acuity have smaller receptive fields

Question 11

effects of sensory training on tactile acuity
-what is the correlation between tactile acuity and age

Answer 11

-tactile acuity is not fixed but changes with experience (unlike visual acuity etc-cant train to be better)

Intense Braille reading can produce superior tactile spatial acuity in blind and sighted humans (change in cortical representation)
study
trained over 8 weeks

• Tactile acuity declines with age at a rate of about 1% per year (highest sensitivity in mid teens)

Question 12

explain David Katz duplex theory of texture perception

Answer 12

David Katz (1925/1989):
Perception of texture depends on 2 cues:
1) Spatial cues (available to vision and touch): Determined by the size, shape and distribution of surface elements (e.g., bumps and grooves – e.g., Braille Letters)

2) Temporal cues (specific to touch):
Determined by the rate of vibration as the skin moves across finely texture surfaces (e.g., sandpaper) → perception/discrimination of very fine textures require movements

Question 13

texture perception: temporal cues
exp on rough surfaces and coarse
Hollins and Risner

Answer 13

-give people 2 types of fine sandpaper
-had to give roughness estimation

-allowed them to touch (1) and then move (2) (these were the conditions)
Participants struggle to identify
differences between two fine
textures in static conditions (1) but
improved considerably when they
were able to move their fingers
over the textures (2)

• Coarse surfaces were equally
  distinguishable in moving and
  stationary conditions.

Question 14

holins and Risner
sensing fine texture through temporal cues is mediated by…..

Answer 14

Sensing fine texture through temporal cues is
mediated by perception of vibration

Question 15

hollins and risner
adaptation paradigm experiment
- what does fine texture discrimination depend on

Answer 15

(temporarily inactivating different receptor types)

• Meissner corpuscle (responds to low frequencies)
  – adapted at 10 Hz (for 6 min)
• Pacinian corpuscle (responds to high frequencies)
  – adapted at 250 Hz (for 6 min)
• 2-AFC task: Which texture is finer/rougher?
• After adaption to 250 Hz vibration participants were unable to discriminate the two textures

→ fine texture discrimination depends on Pacinian
corpuscles

Question 16

is visually perceived surface texture influenced by illumination?
vision vs touch

Answer 16

Visually perceived surface texture is strongly influenced by illumination!
yes

Surfaces appear rougher with decreasing illumination angle (no “roughness constancy”

Touch involves direct contact with a
surface – provides a more reliable
assessment of surface texture than vision
* For very fine textures touch becomes more accurate (higher resolution than vision) + access to temporal cues…

Question 17

what is haptic perception mediated by,
-how is it different from tactile perception

Answer 17

Mediated by two afferent subsystems

Cutaneous/tactile input
* Mechano- and thermoreceptors
of the skin
* Mediates tactual experience

Kinesthetic input
* Mechanoreceptors embedded in
muscles, tendons and joints
* Contributes to the human perception of limb position → where are our body parts

tactile perception is a part of haptic perception. Haptics usually involves active manual exploration

Question 18

active touch vs passive touch
Gibson

Answer 18

Passive touch (being touched): Object is passively moved across the skin of an observer (no voluntary movement)
→ focuses perception on sensory/bodily responses (skin sensation) of the observer
to a stimulus

• Active touch (touching objects): Observer moves actively with full control over their movements (engages haptic perception)
  → perception focuses on external object properties

Question 19

haptic perception is complex as it requires close interplay between….

Answer 19

Haptic perception is complex as it requires a close interplay between the sensory system (cutaneous sensations), the motor system (moving the fingers and the
hand) and the cognitive system (thinking about information provided by sensory and motor system

Question 20

the Somatosensory system
‘what’ and ‘where’ ongoing debate

Answer 20

Ongoing debate if the touch/somatosensory system can (also) be divided into a “what” (perception of surface properties) and “where” (perceptual guidance of action, localisation of touch) subsystems

Question 21

the ‘what’ system

Answer 21

Perception of material/surface properties:
a) surface texture – e.g., roughness and smoothness;
b) compliance – i.e., deformability under force;
c) thermal quality;
d) weight and
e) geometric properties – i.e., shape and size

• Systematic relationship between exploration actions and object properties
  (Lederman & Klatzky, 1987)

Question 22

manual exploration for haptic perception
Lederman & Klatzky, 1987, relationship between object properties the actions we use to explore them
-exploration procedures

Answer 22

6 most commonly used exploration
procedures (i.e., stereotyped pattern of manual exploration when humans are asked to identify object properties without vision)

• lateral motion (texture)
• unsupported holding (weight)
• pressure (hardness)
• enclosure (global shape) (volume)
• static contact (temprature)
• contour following (global shape) (exact shape)

Question 23

are the exploration techniques we use optimal?

Answer 23

Exploration procedures human use – tend to be optimal → provides the most precise discrimination for the property in question

gives us optimal activation of neural responses (e.g., lateral motion for texture enhances the response of slow adapting Merkel receptors and creates deep vibrations activating the Pacinian Corpuscles

Question 24

‘where’ system
-localisation can be referred relative to?

Answer 24

a) The sensory organ (where on the skin touch occurs)
→ depends on spatial resolving capacity of the skin

b) The environment (where in space is a stimulus touched, i.e., how do we localise points in space during haptic exploration when vision is unavailable)
→ A number of interesting phenomena have been discovered but no coherent theory yet
→ Reliable observation for this : Haptic space is anisotropic, i.e., varies in magnitude depending on direction

Question 25

haptic space is anisotropic
-what does this mean

Answer 25

Reliable observation: Haptic space is anisotropic, i.e., varies in magnitude depending on direction

Question 26

‘where’ system
haptic perception task
Kappers & Koendering, 1999,2003,2007
-what is the task
-results
-what does this indicate to us

Answer 26

Task: adjust a test bar so that it feels “parallel” to the standard bar. (feel one bar blindly and then try and orientate the bar to the same orientation)

Participants make large
orientation errors in this task
→When judging bar orientation
this is related to the orientation of the hand touching the bar (i.e.,
egocentric frame of reference)
→Orientation of the hand touching the bar differs depending on the location (i.e., rotated clockwise for right side and anti-clockwise
for left side

Indicates use of two competing spatial reference frames:
egocentric and allocentric

Question 27

affective touch
-what is it

Answer 27

Materials elicit different affective responses when we touch them (e.g., pleasant feeling of touching a soft toy and unpleasant feeling of touching sandpaper/flour)

Relatively new research area not much is known about affective responses to touched materials in the relation to perceptual characteristics of those materials
(usually only a few perceptual variations are included)

Question 28

are perceptual dimensions of touched materials associated with affective responses?
-appliaction of this

Answer 28

Perceptual dimensions of touched materials seem to be systematically associated with affective responses

• Understanding affective responses to materials can help to design human- machine interfaces (e.g., assistant robots) or safe work environments/tools
• Study by Drewing and colleagues (2017): 47 solid, fluid and granular materials which participants rated according to 32 perceptual and 20 affective attributes.

Question 29

perceptual and affective material perception study
Study by Drewing et al. (2017)

Answer 29

• 47 solid, fluid and granular materials which participants rated according to 32 perceptual and 20 affective attributes.
• 6 perceptual dimensions:
  Fluidity, Roughness, Deformability,
  Fibrousness, Heaviness, and Granularity
• 3 affective dimensions:
  Valence (pleasantness), Arousal
  (exciting/boring), and Dominance

Rougher materials rated as less pleasant, fluid materials associated with higher arousal (less predictable skin stimulation?), heavy materials
judged as more dominant, and deformable materials as less dominant (correlations)

Range of perceptual and affective responses broader than previously assumed

Question 30

skin to cortex, somatosensory processing

-tactile acuity correlates with the density of ____ receptors

-high acuity and receptor density associated with a _____ area of processing in brain

Answer 30

Tactile acuity correlates with the density of Merkel receptors
* Areas on the body with high acuity and high receptor density are also devoted a larger area of processing in the brain

Question 31

somatosensory cortex (s1)
how is s1 organised

Answer 31

S1 is organised in maps corresponding to locations on the body (Penfield & Rasmussen, 1950)

Question 32

the somatosensory homunculus

Answer 32

Somatosensory homunculus: areas on the body are represented
disproportionally in the brain (similar to magnification in vision)

Homunculus shows the relative amount of cerebral cortex
surface devoted to the processing of certain sensory inputs

• Large areas for hands and lips (most sensitive to touch) vs. small areas for leg and back (less sensitive)
• (Similar topographical representation
  in the motor cortex (M1) where
  movements of the body are
  processed)

Question 33

somatosensory cortex (s1) and plasticity

Answer 33

Homunculus shows the relative amount of cerebral cortex
surface devoted to the processing of certain sensory inputs

-but the map is not fixed Homunculus varies based
on individual experiences (shaped by learning)

• Classic experiments on owl monkeys by Merzenich and colleagues (1987)
  : Intensive stimulation of a skin area (3 months training) causes an expansion of the cortical maps

Question 34

what is plasticity

Answer 34

An adjustment or adaptation of a cortical system to environmental
stimuli - or performance requirements

Question 35

evidence of plasticity in humans

Answer 35

Evidence from Musicians: Elbert et al. (1995): Violinists show enlarged somatosensory cortex of
regions that receive touch from the tips of the left hand

Question 36

maladaptive plasticity : focal dystonia
‘musicians cramp’

Answer 36

-a neurological condition causing muscle spasms
-Neurological disorder characterised by the loss of fine motor control of long practiced skilled movements during
instrumental playing

• The movement disorder is
  task specific (limited to
  instrumental playing)
• Painless muscular
  incoordination

Question 37

there is _____ correlation between instrument group and localisation of the focal dystonia

Answer 37

Strong correlation between instrument group
and localisation of the focal dystonia (hand with higher workload becomes affected) +occurs more often in the dominant hand

Question 38

what is the source problem in focal dystonia

Answer 38

Primary source of the problem is in the brain not in the peripheral
nervous system

• Studies have identified a few key mechanisms related to the disease
  – one of them related to plasticity in somatosensory cortex
• Patients with focal dystonia have abnormalities in the hand/ finger
  representation in the primary sensory cortex

→ Abnormal sensory functions causes issues in motor behaviour

Question 39

abnormal ______ functions cause issues in motor behaviour

Answer 39

sensory

Question 40

focal dystonia
Bara-Jimenez et al. (1998):

Answer 40

-mapped the areas in the brain which respond to input from the thumb and little finger (6 patients)

-for healthy controls these areas are nicely separated by about 12mm in the brain
-one subject with the condition , the areas were not separated at al
–Intracortical distance between
representations of different digits are reduced in patients
Finger representations are randomly organised in patients (usual topography inverted)

Degree of abnormality correlates with the severity of the dystonic symptoms
* Animal studies suggest that repetitive input through training causes enlargement and fusion of representations