Lecture 21. Tues, 11/17
Jonathan Pillow
Perception (PSY 323), Fall 2009
The University of Texas at Austin

Touch receptors : Embedded on outer layer (epidermis) and underlying layer (dermis) of skin
Each touch receptor can be categorized by three criteria:
1. Type of stimulation to which the receptor responds
2. Size of the receptive field
3. Rate of adaptation (fast versus slow)

• Called “ mechanoreceptors ” because they respond to mechanical stimulation: Pressure, vibration, or movement
Meissner corpuscles
Merkel cell neurite complexes
Pacinian corpuscles
Ruffini endings
• Each has a different range of responsiveness

4 types of mechanoreceptors, de fi ned by:
1. Receptive fi eld size (type I vs. II) big or small

4 types of mechanoreceptors, de fi ned by:
1. Receptive fi eld size (type I vs. II)
2. Temporal response properties (FA vs. SA)
Touch Stimulus
Fast Adapting (FA)
Slow Adapting (SA)

Cross section of the human hand illustrating locations of the four types of mechanoreceptors and the two major layers of skin
(FA1)
(SA1)
(SA2)
(FA2)

- found within muscles, tendons, and joints:
• Kinesthetic receptors : sense of where limbs are, what kinds of movements are made
• Muscle spindle : A sensory receptor located in a muscle that senses its tension
(Also known as a “stretch receptor”)
Receptors in tendons signal tension in muscles attached to tendons
Receptors in joints react when joint is bent to an extreme angle

Figure 12.3 A muscle spindle embedded in main muscle fi bers contains inner fi bers

Strange case of neurological patient Ian Waterman:
• Cutaneous nerves connecting kinesthetic mechanoreceptors to brain destroyed by viral infection
• Lacks kinesthetic senses, dependent on vision to tell limb positions
“Lacking kinesthetic senses, Waterman is now completely dependent on vision to tell him about the positions of his limbs in space. If the lights are turned off, Waterman cannot tie his shoes, walk up or down stairs, or even clap his hands, because he has no idea where his hands and feet are! Caught in an elevator when the lights went out, he was unable to remain standing and could not rise again until the illumination returned.” http://www.youtube.com/watch?v=FKxyJfE831Q

• Sensory receptors that signal information about changes in skin temperature
• Two distinct populations of thermoreceptors: warmth fibers , cold fibers
• Respond when you make contact with an object warmer or colder than your skin

• Sensory receptors that transmit information about noxious stimulation that causes damage or potential damage to skin
• Two groups of nociceptors:
A-delta fibers : Intermediate-sized, myelinated sensory nerve fibers. (Faster)
C fibers : Narrow-diameter, unmyelinated sensory nerve fibers (Slower)

Benefit of pain perception: Sensing dangerous objects
Case of “Miss C” (Melzack & Wall 1973):
• Born with insensitivity to pain
“Not only diid Miss C lack pain sensation, but she did not sneeze, cough, gag, or protect her eyes re fl exively. She suffered childhood injuries from burning herself on a radiator and biting her tongue while chewing food. As an adult, she developed problems in her joints that were attributed to lack of discomfort, for example, from standing too long in the same position. She died at age 29 from infections that could probably have been prevented in someone who was alerted to injury by painful sensations.”

Touch sensations travel as far as 2 meters to get from skin and muscles of feet to brain!
• Two major pathways from spinal cord to brain:
Spinothalamic pathway : Carries most of the information about skin temperature and pain
(slower)
Dorsal column–medial lemniscal (DCML) pathway: Carries signals about touch, pressure, position (fast)

1. Spinothalamic pathway 2. DCML pathway
(Dorsal Column Medial Lemniscal)

• Primary somatosensory cortex: S1
• secondary somatosensory cortex: S2
Somatotopic organization - cortex has topographic map of body surface
(Compare with “retinotopic” and “tonotopic organization of V1 and A1).
Homunculus : Maplike representation of regions of the body in the brain

Phantom limb: Sensation perceived from a physically amputated limb of the body.
• Parts of brain listening to missing limbs not fully aware of altered connections, so they attribute activity in these areas to stimulation from missing limb

Ramachandran s mirror box: http://www.youtube.com/watch?v=h0QiGj9eOOw
• idea is that tricking the brain into “seeing” the phantom limb relax can relieve pain
• related approach: virtual reality display where phantom limb mirrors intact limb.

• Pain sensations triggered by nociceptors
• Responses to noxious stimuli can be moderated by anticipation, religious belief, prior experience, watching others respond, and excitement
Example: Wounded soldier in battle who does not feel pain until after battle

Dorsal Horn of
Spinal Cord substatia gelatinosa
T cells
• extreme pressure, cold, or other noxious stimulation can stimulate “gate” neurons in SG that prevent the T cells from transmitting pain signals
• signals from central NS (eg. brain) can also stimulate gate neurons

• Nociceptors provide signal when there is impending or ongoing damage to body’s tissue: “Nociceptive” pain
• Once damage has occurred, site can become more sensitive: Hyperalgesia
• Pain as a result of damage to or dysfunction of nervous system: Neuropathic

Newly uncovered fifth component of touch: Pleasant touch
(contrast with: “Discriminative touch” - classic touch sensations of tactile, thermal, pain, and itch experiences)
• Mediated by unmyelinated peripheral C fibers known as
“C tactile afferents” (CT afferents)
• CT afferents not related to pain or itch
• Respond best to slowly moving, lightly applied forces
(e.g., petting)
• Processed in orbitofrontal cortex rather than S1 or S2

Perception for action: Using somatosensation to control our impressive ability to grasp and manipulate objects in a stable and highly coordinated manner and to maintain proper posture and balance
Movement is necessary for accurate touch perception!

Our sensors report
• Afferent information:
• Re-afferent information: changes in outside world changes we cause
Wolpert 2004
+ =
Internal source
External source
Claim: sensory percept is given by the difference between actual sensory stimulation and the amount predicted to arise from our movements

Wolpert 2004

Does prediction underlie tactile cancellation in tickle?
Wolpert 2004

Wolpert 2004
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Wolpert 2004

Wolpert 2004
• naive subjects
• separately instructed
• your task: “try to deliver exactly the same force that was delivered to your fi nger by the other subject”
Force escalates under rules designed to achieve parity: Increase by ~40% per turn
(Shergill, Bays, Frith & Wolpert, Science, 2003)

Results
Wolpert 2004

Wolpert 2004
70% overestimate in force

Wolpert 2004

Wolpert 2004
failures to predict the consequences of motor behavior may lead to the illusion that movements are not our own!

Wolpert 2004
Patients
Controls
• The CNS predicts sensory consequences
• Sensory cancellation in
Force production
• Defects may be related to delusions of control