Embed Size (px)
Citation preview
1
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.
2
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.
3
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.
4
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.
5
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.
6
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)
7
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
8
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.
9
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.
10
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.
11
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
12
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.
13
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
14
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
15
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
16
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).
17
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
18
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
