Touch
description
Transcript of Touch
Touch
Functions:Functions:
Provides tactile information.
Warns us of damaging stimuli.
Contains body fluids and organs.
Protects against bacteria.
Regulates body temperature.
The Skin
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 pressure-sensitive 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.
Touch acuity (pattern acuity) is measured using the two-point threshold test.
Measuring sensitivity and acuity of touch
Tactile contrast sensitivity.
Spatial frequency
Sens
itiv
ity
Linked to centre/surround receptive fields in the thalamus.
Braille.
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 thalamusthalamus have centre-surround organization.
Cortical receptive fields (left)Cortical receptive fields (left) are smaller in the fingers and larger on the hand and forearm.
Neural Processing for TouchTwo-point threshold: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:Warm Fibers:
- increased responding with increasing temperature- sustained firing- decreased firing when temperature decreases- do not respond to mechanical stimulation
Cold Fibers:Cold Fibers:
- 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 FibersEach 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
Spatial PropertyPunctate Diffuse
Tem
por
al P
rop
erty
RA
SA
Meissnercorpuscles
(transient stimulation)
Paciniancorpuscles
(very sensitive with large RFs)
Merkeldisks
(steady pressure of small object)
Ruffiniendings
(steady pressure and stretching)
ReceptorsFrequency
RangePerception Fiber RF Size
MerkelMerkel0.3-3 Hz Pressure SA1 Small
MeissnerMeissner3-40 Hz Flutter RA1 Small
RuffiniRuffini15-400 Hz Stretching SA2 Large
PacinianPacinian10-500 Hz Vibration RA2 Large
Properties of Mechanoreceptors
Ascending pathways for touch
Fibers (receptors) spinal cord
interneurons muscle
Fibers (receptors)
lemniscal neurons brainstem
spinal cord
Reflex (OUCH!!!!)
Sensory Analysis
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
Haptic perception: Haptic perception: exploration of 3D objects with the hand
Exploratory proceduresExploratory procedures (Lederman & Klatzky)
Passive Touch Passive Touch vs. Active Touch Active Touch
Exploratory Procedures
http://psyc.queensu.ca/~cheryl/labpage.htmlhttp://psyc.queensu.ca/~cheryl/labpage.html
Visual and Haptic Object Recognition
Amedi, Jacobson, Hendler, Malach and Zohary (2002). Cerebral Cortex, 12(11), 1202-1212
Haptic perception:Haptic perception: involves coordination of motor, sensory, and cognitive systems
Somatosensory Cortex
Certain cortical neurons respond selectively to orientation and direction.
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.
Disorders related to somatosensory cortex
Phantom Limbs
Flor, Elbert, Wienbruch, Pantev, Knecht, Birbaumer, Larbig & Taub (1995).
Amputees often report that they can still feel their missing limb, and sometimes this is painful!
Referred sensation: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.
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: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
The amount of cortical magnification was correlated with the age at which the person began to play.
Elbert, Pantev, Wienbruch, Rockstroh & Taub (1995)
String instrument players have larger representation in primary sensory cortex for their left hands than normal controls.
Cross-Modal Plasticity
Cohen, Celnik, Pascual-Leone, Corwell, Faiz, Dambrosia, Honda, Sadato, Gerloff, Catala & Hallett (1997).
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!
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.