Touch
The Skin
Functions:
Provides tactile information.
Warns us of damaging stimuli.
Contains body fluids and
organs.
Protects against bacteria.
Regulates body temperature.
Touch
Touch helps us identify objects and provides
unique information (e.g., texture)
Touch is important in development and social
interaction (e.g., Harlow’s monkeys).
The skin on the hand contains thousands of pressuresensitive mechanoreceptors.
Touch acuity is best when fingers move over the object
of interest.
Measuring sensitivity and acuity of touch
Pressure sensitivity is greatest for fingertips and lips (the
lightest touch). The back is least sensitive to pressure.
Females more sensitive than males.
Vibrotactile sensitivity is greatest for palms. Females are
more sensitive than males.
Measuring sensitivity and acuity of touch
Touch acuity (pattern acuity) is measured using the twopoint threshold test.
Tactile contrast sensitivity.
Linked to centre/surround receptive
fields in the thalamus.
Braille.
Spatial frequency
Neural Processing for Touch
Receptive field sizes of the
different mechanoreceptors
determines our ability to
discriminate fine details.
Sensitive body parts have
higher density of RA1 and
SA1 fibers.
Receptive Fields
Receptive fields in the
thalamus have centre-surround
organization.
Cortical receptive fields (left)
are smaller in the fingers and
larger on the hand and
forearm.
Neural Processing for Touch
Two-point threshold: the smallest discriminable
distance between two points
Localization of tactile stimulation
Localization ability enables judgment of where
stimulus has been applied to the skin.
Tactile judgments of relative position are highly
accurate.
Perception of surface texture
Surfaces have unique “texture signatures” (e.g., coarse vs.
fine).
Gratings of different spatial frequencies measure tactile
acuity.
Tactile sensitivity (temperature)
Touch temperature is the perception of surface
temperature.
Objects differ in thermal conductivity.
Touch temperature is based on temperature gradient
between object and skin.
Thermoreceptors
Located just below the skin.
Continuous nerve impulses at a certain temperature.
Small receptive fields (less than 1mm2).
There are spaces between receptive fields (“blind spots”).
Two classes: (1) cold receptors
(2) warm receptors
Responds to CHANGES in temperature!!!!
Perceived temperature depends on the state of the receptors.
Thermoreceptors
Warm Fibers:
Cold Fibers:
- increased responding with
increasing temperature
- sustained firing
- decreased firing when
temperature decreases
- do not respond to mechanical
stimulation
- increased responding with
decreasing temperature
- sustained firing
- decreased firing when
temperature increases
- do not respond to mechanical
stimulation
Mental set and tactile sensitivity
Uncertainty makes tactile discrimination more difficult.
Advance information improves the identification of a
tactile stimulus.
Practice also improves tactile discrimination.
Touch Fibers
Each nerve fiber signals touch to a specific area of the
skin (the fiber’s receptive field).
Temporal properties
Slowly adapting (SA) fibers respond to initial stimulation and
continue responding (perception of light, uniform pressure).
Rapidly adapting (RA) fibers respond only to start and stop
points of stimulation (perception of buzzing vibration).
Spatial properties
Punctate fibers have small receptive fields with sharply
defined boundaries.
Diffuse fibers have large receptive fields with fuzzy
boundaries.
The four-channel model: Mechanoreceptors
Temporal Property
Spatial Property
Punctate
Diffuse
RA
SA
Meissner
corpuscles
Pacinian
corpuscles
(transient
stimulation)
(very sensitive with
large RFs)
Merkel
disks
Ruffini
endings
(steady pressure
of small object)
(steady pressure
and stretching)
Properties of Mechanoreceptors
Receptors
Merkel
Meissner
Ruffini
Pacinian
Frequency
Range
Perception
Fiber
RF Size
0.3-3 Hz
Pressure
SA1
Small
3-40 Hz
Flutter
RA1
Small
15-400 Hz
Stretching
SA2
Large
10-500 Hz
Vibration
RA2
Large
Ascending pathways for touch
Reflex (OUCH!!!!)
Fibers (receptors)
spinal cord
interneurons
muscle
Sensory Analysis
Fibers (receptors)
spinal cord
lemniscal neurons
brainstem
Somatosensory Cortex
Damage to somatosensory cortex destroys ability to
recognize objects by touch.
The body is mapped topographically onto somatosensory
cortex, but body parts are not represented equally.
Homoculus
Homunculi
Tactile Object Recognition
Passive Touch vs. Active Touch
Haptic perception: exploration of 3D objects with the hand
Exploratory procedures (Lederman & Klatzky)
Exploratory Procedures
http://psyc.queensu.ca/~cheryl/labpage.html
Visual and Haptic Object
Recognition
Haptic perception: involves coordination of motor,
sensory, and cognitive systems
Amedi, Jacobson, Hendler, Malach and Zohary (2002).
Cerebral Cortex, 12(11), 1202-1212
Somatosensory Cortex
Certain cortical neurons respond selectively to
orientation and direction.
Disorders related to somatosensory cortex
A person with unilateral neglect denies ownership of
limbs on one side of the body.
A person with a phantom limb experiences sensation
from a limb that no longer exists.
Phantom Limbs
Amputees often report that they can still feel their
missing limb, and sometimes this is painful!
Referred sensation: stimulation of one part of the body results in
a sensation on another part of the body (i.e. the phantom limb).
The amount of functional
cortical reorganization is
positively correlated with
the degree of phantom
limb pain.
Flor, Elbert, Wienbruch, Pantev, Knecht, Birbaumer, Larbig & Taub (1995).
Pain perception
Nociceptors respond to painful stimuli.
Two categories:
(1) mechanical receptor
- severe pressure on skin
- tearing
(2) thermal receptor
- responds to very high and very low
temperatures
Pain
Nociceptors: receptors in
the skin that respond to
intense pressure, extreme
temperature, or burning
chemicals.
http://www.sfn.org/content/Publications/BrainBriefings/pain.html
The perception of pain can be modulated by cognitive factors:
expectation, placebo, shifting attention, emotional distraction,
individual differences
Endorphins, Opiates and Pain
Relief
The “reward pathway” contains opioid receptors for exogenous and
endogenous substances.
- neurotransmitters (dopamine)
- opiate drugs (morphine, heroin, cocaine)
- endorphins
Somatosensory cortex plasticity
Cortical reorganization occurs in monkeys when
fingers are surgically connected.
PET studies reveal differences in the brains of
musicians that suggest somatosensory cortical
changes occur in humans.
Cortical Plasticity
String instrument players have larger representation in primary
sensory cortex for their left hands than normal controls.
The amount of cortical
magnification was correlated
with the age at which the person
began to play.
Elbert, Pantev, Wienbruch, Rockstroh & Taub (1995)
Cross-Modal Plasticity
People who have been
blind from a very young
age show activity in
visual cortex during
Braille reading.
A TMS pulse to the
visual cortex impaired
Braille reading.
Evidence of functional
reorganization of the
brain!
Cohen, Celnik, Pascual-Leone, Corwell, Faiz, Dambrosia, Honda, Sadato, Gerloff, Catala & Hallett (1997).
Summary
• The homuculus describes the amount of
cortex devoted to processing sensory and
motor information from the different parts
of the body.
• Haptic perception results from active touch
and exploratory hand movements.
• Haptic and visual object recognition share
an overlapping region in the ventral “what”
visual stream.
• Nociceptors provide information about
painful stimuli.
• Central neural mechanisms, such
as emotional state and drugs,
modulate our perception of pain.
• The somatosensory system can
undergo substantial reorganization
after intensive practice and injury.