lecture 3- tactile and haptic perception

Question 1

what is tactile perception?

Answer 1

Tactile perception refers to the ability of the brain to understand (perceive) the information that comes from the skin, especially the skin of the hands. The hands are used to record sensory information, and then the brain uses this information to guide the hands during an activity.

Question 2

sense of touch is..

Answer 2

crucial for survival

Question 3

what are the 3 main groups of receptors?

Answer 3

• mechanoreceptors (respond to mechanical stimuli eg pressure (stroking), stretching and vibration)
• thermoreceptors (respond to temperature)
• chemoreceptors (respond to certain types of chemicals eg histamine)

note- this one is not certain
- nociceptors, subtypes of chemo-and mechanorecptors? (mediate the perception of pain)

Question 4

state- different types of receptors are distributed throughout the skin and respond to different touch-related stimuli and events

Question 5

mechanoreceptors: types

Answer 5

• merkel receptor
• meissners corpuscle
• pacinian
• fuffini cylinder

Question 6

merkel receptor:

Answer 6

• small receptors with sharp borders
• slow adaption
• pressure fine texture and shape
• close to skin surface

Question 7

meissners corpuscle:

Answer 7

• small receptors with sharp borders
• rapid adaptation
• indentation motion across skin
• close to skin surface

Question 8

pacinian corpuscle:

Answer 8

• large receptors with diffuse borders
• rapid adaptation
• vibration and fine texture
• deeper in skin

Question 9

ruffini cylinder:

Answer 9

• large receptors with diffuse borders
• slow adaptation
• stretching
• deeper in skin

Question 10

Tactile acuity: how to measure I

Answer 10

1. two-point threshold
   - classical measure in early touch research (but vulnerable to cofounds)
   - minimum separation between two points on the skin that is recognised as two
   => smallest discriminable distance between two points (JND)

Question 10

mechanoreceptors: short introduction 2

Answer 10

DO THIS SLIDE AFTER LISTENING TO LEC

Question 10

tactile acuity: how to measure II

Answer 10

2. grating acuity
   - 2-AFC task: horizontal vs. vertical orientation
   - Acuity as the spacing for which orientation can still be accurately judged (75% correct)
   - 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 11

receptor mechanisms for tactile acuity

Answer 11

• 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 desity
=> better tactile acuity is associated with higher density of Merkel receptors

Question 12

neural processing of touch

Answer 12

• But not the whole story: 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 13

what is the definition of receptive field

Answer 13

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

Question 14

effects of sensory training on tactile acuity

Answer 14

• Intense Braille reading can produce
  superior tactile spatial acuity in blind and
  sighted humans (change in cortical
  representation)
• Tactile acuity declines with age at a rate of
  about 1% per year

Question 15

duplex theory of texture perception I David Katz (1925/1989)- perception of texture depends on 2 cues:

Answer 15

1) spatial cues (available to vision and touch): determined by the size, shape and distribution of surface elements (eg., bumps and grooves- eg braille letters)

2) temporal cues (specific to touch):
determined by the rate of vibration as the skin moves across finely texture surfaces (eg sandpaper) => perception/ discrimination of very fine textures require movements

Question 16

texture perception: temporal cues (receptor mechanisms)

Answer 16

hollins and Risner (2001):
sensing fine texture through temporal cues is mediated by perception of vibration

Question 16

texture perception: temporal cues

Answer 16

hollins and risner (2000):

• participants struggle to identigy differences between two fine textures in static conditions but improved considerably when they were able to move their fingers over the textures
• coarse surfaces were equally discriminable in moving and stationary conditions

Question 17

adaption paradigm:

Answer 17

(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?
• After adaption to 250 Hz vibration participants were unable to discriminate the two textures
  → fine texture discrimination depends on Pacinian corpuscles

Question 18

perceiving texture: vision vs touch

Answer 18

• Visually perceived surface texture is
  strongly influenced by illumination!
• 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 19

interim summary: tactile perception

Answer 19

• Four types of mechanoreceptors with differently sized receptive fields
  and different responses to stimulation (slow vs. rapidly adapting) that mediate different types of tactile perception
• How to measure tactile acuity
• Mechanisms underlying tactile acuity
• Duplex theory of texture perception: Tactile perception of coarse textures is determined by spatial cues and perception of fine textures by temporal (vibration) cues

Question 20

haptic perception- mediated by two afferent subsystems:

Answer 20

cutaneous/tactile input
- mechano- and thermoreceptors of the skin
- mediates tactual experience

kinaesthetic input
- mechanoreceptors embedded in muscles, tendons and joints
- contributes to the human perception of limb position => where are our body parts

Question 21

state- haptics usually involves active manual exploration

Question 22

active vs passive touch (gibson, 1992)

Answer 22

gibson, 1962: difference between active and passive tactual experience

Question 23

passive touch (being touch)

Answer 23

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

Question 24

active touch (touching objects)

Answer 24

observer moves actively with full control over their movements (engages haptic perception) => perception focuses on external object properties

Question 25

why is haptic perception complex?

Answer 25

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 26

“Where” system: Haptic Space Perception I- what can localisation be referred relative to?

Answer 26

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 ie 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: haptic space is anisotropic ie varies in magnitude depending on direction

Question 26

Somatosensory system: “What” and “Where”” (?)

Answer 26

* 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 (similar as vision, see Lecture 2) “What”-system: * 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 27

Manual exploration for Haptic Perception

Answer 27

Lederman & Klatzky, 1987 - 6 most commonly used exploration procedures (i.e., stereotyped pattern of manual exploration when humans are asked to identify object properties without vision) -Exploration procedures human use – tend to be optimal → provides the most precise discrimination for the property in question - 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 28

“Where” system: Haptic Space Perception II

Answer 28

Kappers and Koendering, 1999, 2003, 2007- Task: adjust a test bar so that it feels “parallel” to the standard bar. 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)

Question 29

affective touch

Answer 29

* 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) * 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 30

perceptual and affective material perception

Answer 30

study by drewing et al. (2017) * 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

from skin to cortex: somatosensory processing

Answer 30

Reminder: Two-Point Thresholds * 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

interim summary: haptic perception

Answer 31

* Differences between active and passive touch * Common exploration procedures for haptic identification (and their optimality) * Anisotropy of haptic space * Affective touch and the relation between perceptual and affective material dimensions

Question 32

Somatosensory Cortex (S1)

Answer 32

* S1 is organised in maps corresponding to locations on the body (Penfield & Rasmussen, 1950) * Somatosensory homunculus: areas on the body are represented disproportionally in the brain (similar to magnification in vision) * Large areas for hands and lips (most sensitive to touch) vs. small areas for leg and back (less sensitive) note * (Similar topographical representation in the motor cortex (M1) where movements of the body are processed)

Question 33

Somatosensory Cortex (S1) - Plasticity

Answer 33

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

plasticity in humans

Answer 34

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 35

what is the definition of plasticity

Answer 35

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

Question 36

maladaptive plasticity: focal dystonia

Answer 36

Definition: Musician’s Cramp * 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 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 37

Maladaptive Plasticity: Focal Dystonia cont

Answer 37

- 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 38

Maladaptive Plasticity: Focal Dystonia - bara- jimenez et al. (1998)

Answer 38

* Mapping of finger representations in 6 patients with focal dystonia * 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

Question 38

final summary: somatosensory processing

Answer 38

* Touch is processed in Somatosensory Cortex that maps the areas of the body disproportionally (larger processing areas for body sites with high sensitivity) * Body Map is plastic and adapts to sensory experience and learning (within limits) * Plasticity can be adaptive but also maladaptive (dystonia)