Touch Hold on!!!

This gets complicated…

Touch is complicated: 2 overlapping & interacting “systems”

Somatomotor •Efferent neural system designed to produce movement.projections to the motor cortex are organized homotopically.

Somatosensory afferent neural system designed to get information to the brainprojections 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.

So different touch uses of the different parts of the handAttention/focus & detail perception receptors must be protected by use patternsParts 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 a part 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… Wood ? Foam rubber ? Jello ? Rigid vs. flexible Firm vs. compressable

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!

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)

Touch is complicated: (another) 2 overlapping & interacting “systems”

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

Mechanoreceptors are terminals of modified bipolar neurons: Single axon with only one dendrite (most neurons have many)

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

Starts out simply enough…

Law of Specific Nerve EnergiesDifferent 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????

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

Touch is complicated: many different kinds of receptor organs

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

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

Touch is complicated: lots of spatial information to convey!

“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”?

Different body parts are differentially sensative to different touch attributes

Absolute threshold 2-point threshold

Touch is complicated: different body parts are differently sensitive for different kinds of perceptual task!

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”

Touch is complicated: different kinds of “touching”(please note- this does not quite match how the book uses these terms)

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 1-dimensional 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