lecture 3- tactile and haptic perception Flashcards
what is tactile perception?
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.
sense of touch is..
crucial for survival
what are the 3 main groups of receptors?
- 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)
state- different types of receptors are distributed throughout the skin and respond to different touch-related stimuli and events
mechanoreceptors: types
- merkel receptor
- meissners corpuscle
- pacinian
- fuffini cylinder
merkel receptor:
- small receptors with sharp borders
- slow adaption
- pressure fine texture and shape
- close to skin surface
meissners corpuscle:
- small receptors with sharp borders
- rapid adaptation
- indentation motion across skin
- close to skin surface
pacinian corpuscle:
- large receptors with diffuse borders
- rapid adaptation
- vibration and fine texture
- deeper in skin
ruffini cylinder:
- large receptors with diffuse borders
- slow adaptation
- stretching
- deeper in skin
Tactile acuity: how to measure I
- 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)
mechanoreceptors: short introduction 2
DO THIS SLIDE AFTER LISTENING TO LEC
tactile acuity: how to measure II
- 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)
receptor mechanisms for tactile acuity
- 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
neural processing of touch
- 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
what is the definition of receptive field
the receptive field is a portion of sensory space that can elicit neuronal cortical responses when stimulated
effects of sensory training on tactile acuity
- 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
duplex theory of texture perception I 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 (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
texture perception: temporal cues (receptor mechanisms)
hollins and Risner (2001):
sensing fine texture through temporal cues is mediated by perception of vibration
texture perception: temporal cues
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
adaption paradigm:
(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
perceiving texture: vision vs touch
- 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…
interim summary: tactile perception
- 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
haptic perception- mediated by two afferent subsystems:
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
state- haptics usually involves active manual exploration
active vs passive touch (gibson, 1992)
gibson, 1962: difference between active and passive tactual experience
passive touch (being touch)
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
why is haptic perception complex?
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
“Where” system: Haptic Space Perception I- what can localisation be referred relative to?
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
Somatosensory system:
“What” and “Where”” (?)
- 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)
Manual exploration for Haptic Perception
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)
“Where” system: Haptic Space Perception II
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)
affective touch
- 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.
perceptual and affective material perception
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
from skin to cortex: somatosensory processing
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…
interim summary: haptic perception
- 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
Somatosensory Cortex (S1)
- 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)
Somatosensory Cortex (S1) - Plasticity
- 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
plasticity in humans
Evidence from Musicians: Elbert et al. (1995):
Violinists show enlarged somatosensory cortex of regions that receive touch from the tips of the left hand
what is the definition of plasticity
An adjustment or
adaptation of a cortical
system to environmental
stimuli - or performance
requirements
maladaptive plasticity: focal dystonia
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
Maladaptive Plasticity: Focal Dystonia cont
- 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
Maladaptive Plasticity: Focal Dystonia - bara- jimenez et al. (1998)
- 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
final summary: somatosensory processing
- 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)
