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12Touch
Chapter 12
12 Touch
• Introduction
• Touch Physiology
• Tactile Sensitivity and Acuity
• Haptic Perception
12 Introduction
Proprioception: Perception mediated by kinesthetic and vestibular receptors.
Somatosensation: A collective term for sensory signals from the body.
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12 Touch Physiology
Touch receptors: Embedded on outer layer (epidermis) and underlying layer (dermis)
– Multiple types of touch receptors– Each touch receptors has three attributes:
1. Type of stimulation the receptor responds to.
2. Size of the receptive field.3. Rate of adaptation.
12 The Four Types of Mechanoreceptors
12 Touch Physiology
Tactile receptors (four): Mechanoreceptors-respond to mechanical stimulation or pressure.
– Meissner corpuscles.– Merkel cell neutrite complexes.– Pacinian corpuscles.– Ruffini endings.
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12 Mechanoreceptors
12 Touch Physiology
Each receptor: different range of responsiveness
12 Touch Physiology
Other types of mechanoreceptors within muscles, tendons, and joints:
– Kinesthetic receptors: Play important role in sense of where limbs are, what kinds of movements are made.
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12 Touch Physiology
Spindles: Convey the rate at which the muscle fibers are changing in length
– Receptors in tendons provide signals about tension in muscles attached to tendons.
– Receptors in joints react when joint is bent to an extreme angle.
12 Muscle Spindle
12 Touch Physiology
Importance of kinesthetic receptors:
– Strange case of neurological patient Ian Waterman.
– Cutaneous nerves connecting Waterman’s kinesthetic mechanoreceptors to brain destroyed by viral infection.
– Lacked kinesthetic senses, dependent on vision to tell limb positions.
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12 Touch Physiology
Thermoreceptors:
– Sensory receptors that signal information about changes in skin temperature.
– Two distinct populations of thermoreceptors: warmth fibers, cold fibers.
– Body is consistently regulating internal temperature.
– Thermoreceptors kick into gear when you make contact with object warmer or colder than your skin.
12 Touch Physiology
Nociceptors:
– Sensory receptors that transmit information about noxious stimulation that causes damage or potential damage to the skin.
– Two groups of nociceptors: A-delta fibers, C fibers.
12 Touch Physiology
Benefit of pain perception:
– Sensing dangerous objects.– Case of “Miss C”: Born with insensitivity to
pain, could not protect herself.
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12 Touch Physiology
Touch sensations travel as far as 2 meters to get from skin and muscles of feet to brain!
– Information must pass through spinal cord.– Axons of various tactile receptors combine
into single nerve trunks.– Several nerve trunks from different areas
of body.– Once in spinal cord: Two major pathways:
Spinothalamic (slower); dorsal-column-medial-lemniscal (faster).
12 Touch Physiology
– Spinothalamic pathway synapses within spinal cord.
– DCML: Synapse in medulla, near base of brain, then ventral posterior nucleus of thalamus, then somatosensory area 1 (S1), somatosensory area 2 (S2).
12 Pathways from Skin to Cortex (Part 1)
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12 Pathways from Skin to Cortex (Part 2)
12 Touch Physiology
Touch sensations are represented somatotopically: Analogous to retinotopy found in vision
– Adjacent areas on skin: Connected to adjacent areas in brain, called homunculus.
– Brain contains several sensory maps of body, different subareas of S1, secondary areas as well.
12 Primary Somatosensory Receiving Areas in the Brain
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12 The Sensory Homunculus (Part 1)
12 The Sensory Homunculus (Part 2)
12 Touch Physiology
Phantom limb:
– Perceived sensation 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.
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12 Touch Physiology
Pain:
– Pain sensations triggered by nociceptors.– Responses to noxious stimuli can be
moderated by anticipation, religious belief, prior experience, watching others respond, excitement.
– Example: Wounded soldier in battle who did not feel pain till after battle.
12 Touch Physiology
Analgesia:
– Decreasing pain sensation during conscious experience.
– Soldier in above example: Experienced effect because of endogenous opiates, chemicals released in body to block release or uptake of neurotransmitters transmitting pain sensations to brain.
– Externally produced substances have similar effect: Morphine, heroin, codeine.
12 Touch Physiology
Gate control theory:
– Description of the system that transmits pain that incorporates modulating signals from the brain.
– Feedback circuit located in substantia gelatinosa of dorsal horn of spinal cord.
– Gate neurons block pain transmission can be activated by extreme pressure, cold, other noxious stimulation applied to another site distant from source of pain.
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12 Gate Control Theory
12 Touch Physiology
Pain sensitization:
– 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 result of damage to or dysfunction of nervous system: Neuropathic.
– No single pain medication will alleviate all types of pain.
12 Touch Physiology
Cognitive aspects of pain
– Pain: Generally subjective experience, two components: Sensation of painful source, emotion that accompanies it.
– Areas S1 and S2: Responsible for sensory aspects of pain.
– Recently: Researchers identified areas of brain that correspond to more cognitive aspects of painful experiences.
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12 Touch Physiology
– Hypnosis: Experiment with lukewarm and hot water: Anterior cingulate cortex responded differentially to two hypnotic suggestions, by increasing or decreasing its activity.
– Secondary pain effect: Emotional response associated with long-term suffering, (e.g., cancer patients undergoing chemotherapy, associated with prefrontal cortex).
12 Tactile Sensitivity and Acuity
How sensitive are we to mechanical pressure?
– Max von Frey (Nineteenth century) developed elegant way to measure this, using carefully calibrated stimuli: Horse and human hairs.
– Modern researchers: Use nylon monofilaments of varying diameters.
12 Sensitivity to Pressure
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12 Tactile Sensitivity and Acuity
How finely can we resolve spatial details?
– Two-point touch threshold: The minimum distance at which two stimuli (e.g., two simultaneous touches) are just perceptible as separate.
12 Two-Point Threshold
12 Tactile Sensitivity and Acuity
Correspondence between pattern of two-point thresholds across body (Show Figure 12.13) and relative distortion of different body parts in sensory homunculus (Show Figure 12.8).
– Sufficient concentration of receptors at the skin, each with small enough receptive field, that two contact points will elicit different responses.
– Drawback of two-point threshold.
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12 Two-Point Thresholds on the Hand
12 Tactile Sensitivity and Acuity
How finely can we resolve temporal details?
– Sound pressure changes of low-frequency notes translate into vibratory skin pressure changes.
– Higher-frequency notes cannot be felt.
12 Minimally Detectable Displacement
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12 Haptic Perception
Haptic perception:
– Knowledge of the world that is derived from sensory receptors in skin, muscles, tendons, and joints, usually involving active exploration.
12 Haptic Perception
Action for perception:
– Exploratory procedure: Stereotyped hand movement pattern used to contact objects in order to perceive their properties.
– Optimal for obtaining precise details about one or two specific properties, (e.g., to find out how rough object is: Lateral motion).
12 Exploratory Procedures
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12 Haptic Perception
The What system of touch:
– Geometric properties of objects: Most important for visual recognition.
– Haptic search.
12 Objects Easy to Recognize by Vision, but not by Touch
12 Feature Detection
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12 Haptic Perception
Perceiving patterns with the skin:
– Braille alphabet: raised dots.– Loomis: Touch acts like blurred vision
when the fingertips explores a raised pattern.
12 Character Recognition Sets Used by Loomis
12 Haptic Perception
Tactile agnosia:
– The inability to identify objects by touch.– Example: Patient documented by Reed
and Caselli (1994).
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12 Haptic Perception
The Where system of touch:
– Frame of reference: Coordinate system used to define locations in space.
– Egocenter: Center of a reference frame used to represent locations relative to the body.
12 Haptic Perception
Interactions between touch and other modalities:
– Experiments that study competitions between sensory modalities.
12 Studying Competition between Sensory Modalities
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12 Testing Integration of Sensory Modalities
12 Haptic Perception
Virtual haptic environments:
– Video games.– Tadoma method of speech perception for
deaf and blind people.
12 The Tadoma Method
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