Download Touch is complicated

Touch
Hold on!!!
This gets complicated…
Touch is complicated: 2 overlapping & interacting “systems”
Somatosensory
 afferent neural system designed to get information to the brain
 projections to the motor cortex are organized homotopically
Somatomotor
• Efferent neural
system designed
to produce
movement.
 projections to the
motor cortex are
organized
homotopically.
 Somatotopic organization:
 Both pathways are precisely
organized by function and body
location – remember contralaterality!
Neural representation
is such that
neighboring neurons
process tactile signals
from adjacent locations
on the body surface.
Parallel processing of
somatosensory signals
is superimposed upon
a serial arrangement of
cortical areas.
 The somatosensory cortex:
 Columnar organization (see
Figure 3.11 in text):
 Neurons in all six layers respond
to the same modality in any given
part of area S-I.
 Responses of the different types
of receptors are organized in
columns that run vertically from
the cortical surface to the white
matter.
Touch the only modality for which the receptive fields
that are attached to a part of the body that is also used
to manipulate the environment!
So different touch uses of the
different parts of the hand
 Attention/focus & detail
perception receptors must be
protected by use patterns
 Parts of hand used for “hard
work” – requiring thicker skin &
calluses – can’t be depended
on for attention/focus & detail,
but need to provide perception
of amount of “harder contact”
Touch the only modality for which the receptive fields
that are attached to apart of the body that is also used
to manipulate the environment!
This means that “touching” can change the
stimulus being perceived!
 Which is easier to perceive the shape of…
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Wood ?
Foam rubber ?
Jello ?
Rigid vs. flexible
Firm vs. compressable
Touch is complicated: (another) 2 overlapping & interacting “systems”
 Somatosensory perceptions
 Located throughout body - especially “surface”
 Provide information about



Location of touch
Classification and identification of the object
Pain
 Proprioception


Located within muscles & joints
Provides information about


Location of spine, limbs, tongue, eyes (ears, nose,
tail, whiskers, skin – in other animals)
Perception of motion (change in location of above)
Proprioception and Kinesthesis
 Kinesthesis / Proprioception =
the sensation of the movement
of the body, muscles, tendons,
and joints
 Proprioceptor = sensory
receptors involved in
proprioceptive signaling that
reside in deeper structures
such as the muscles, tendons,
and joints:
 Important to the motor
system in guiding
movement through the
environment
Starts out simply enough…
 Mechanoreceptors are terminals of modified bipolar
neurons:
 Single axon with only one dendrite (most neurons have many)


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Peripheral branch receives input from skin.
Central branch carries touch signals to the spinal cord.
The peripheral and central branches of the DRG neuron
that form a continuous cable, carrying somatosensory
signals from the periphery to the spinal cord
Better for “transferring” than “gathering” information
Law of Specific Nerve Energies
Different sets of receptors and nerve fibers are dedicated
to each sensory modality
• different kinds of receptors for different modalities
• different kinds of energy triggers for different modalities
• same kinds of neural signal used for all modalities
• different parts of the brain for different modalities
• same kinds of object information supplied from
multiple modalities
Will this also apply within modalities??? For different “kinds” of touch????
Touch is complicated: many different kinds of receptor organs
 Encapsulated
receptors with a
specialized capsule
that surrounds the
nerve ending
 Sensory nerve fibre in
conjunction with a
separate accessory
structure
 Various types of free
nerve endings that do
not have any
specialized terminal
structures or other
associations
1-to-1 relationship between kinds of
receptor & kinds of information???
Two different ideas proposed to account for the way touch
signals are generated and transmitted to the brain:
 The theory of receptor specificity:



Specific receptors exist for heat, cold, pressure, and pain
They also indicate location of stimulation occurred.
Requires “each type of receptor everywhere”
 The pattern theory:
 Specificity of touch sensations arises from the overall
pattern of activity across a broad spectrum of receptors.
 If different combinations can signal the same sensation,
then differing patterns of
 Today, we accept the theory receptor specificity,
although most forms of touch usually involve
stimulation of multiple types of receptors.
Consider the attributes involved in …
 Catching a hot potato
 Grabbing wet rag from the garage workbench
 Plunging your hand into sudsy dishwater
Touch is complicated: lots of spatial information to convey!
Spinal nerves and ganglia:
 31 pairs of spinal nerves, one
member of each pair innervating
each side of the body
 Each referred to as a “dermatome”
 Two roots make up each spinal
nerve:
 Dorsal root = carries sensory
signals into the spinal cord
 Ventral root = carries motor
signals out to the muscles
“2-point discrimination” is the ability to
detect if you are being touched by 2
contact points or one
Initial research found huge variation –
different labs found wildly different
results !!!!!
Consider (think dermatomes!)
When will 2-point discrimination
performance be best?
When will 2-point discrimination
performance be worst?
How do you “do the research right”?
Touch is complicated: different body parts are differently sensitive for
different kinds of perceptual task!
Different body parts are differentially sensative to different
touch attributes
Absolute threshold
2-point threshold
Touch is complicated: different kinds of “touching”
(please note- this does not quite match how the book uses these terms)
 Static Tactile = undirected & unmodified passive contact of
object with the body surface (skin)

Tactual or Static Passive Touch
 Dynamic Tactile = undirected but modified passive contact
of object with the body surface (skin)
 Object is “moving itself” or being “moved by another”
 Active Tactile = directed & dynamic contact of body
surface (skin) with object surface
 Movement intended to produce or enhance sensations
by changing part of body making contact or making
contact with adjacent areas of body – usually 1dimensional pressing or 2-dimensional sweeping
 Haptic perception = combination of tactile and
proprioceptive perceptions
 Movement intended to produce or enhance sensations
by contact the surface and non-surface attributes of the
object in 3 dimensions
 Can be “exploratory” or “confirmatory”
Tactile (passive) & Haptic (active) Touch – which
energies/attributes are perceived better via active vs.
passive touch?
Better via active touch
Better via passive touch