Touch Flashcards
What processing system is the perception of touch part of?
The somatic sensory (somatosensory) processing system.
What does touch provide information about?
The nature of surfaces and objects in the world that are in direct contact with the skin.
How is touch vital for survival?
It informs us of potentially useful and harmful stimuli.
In what ways is touch distinct from the other 4 sense modalities?
- receptors are varied and distributed throughout the body (rather than specific localised structures)
- perceptual apparatus (receptors, pathways etc.) mediating touch responds tod different types of stimulation, with diverse sensation qualities produced.
What does the fact that different types of stimulation cause different sensations suggest?
That there are multiple touch pathways, rather than a unitary pathway.
List some of the sensations touch can evoke.
Mechanical pressure (contact, vibration, roughness, etc.), temperature, itch, and pain.
What do receptors in the muscles and joints encode?
The postures, locations and movements of the body - proprioception.
What is proprioception important for?
Active touch.
What are the dead layers and living cells called in mammalian skin?
Epidermis and dermis, respectively.
What are the sensory receptors in mammalian skin?
Meissner’s corpuscle, basket cell, Merkel’s disc, Free nerve ending, Kraus end bulb, Ruffini ending, and Pacinian corpuscle.
Which of the sensory receptors in mammalian skin is studied the most?
Pacinian corpuscles.
Which sensory receptors in mammalian skin respond only to pressure?
Meissner’s corpuscle, basket cell, Merkel’s disc, Ruffini ending, and Pacinian corpuscle.
What do free nerve endings in mammalian skin respond to?
Pain, temperature, and pressure.
What do Kraus end bulbs respond to?
Temperature (possibly).
What did Iggo (1976) propose?
That different sensory qualities are mediated by different specialised receptors within the skin layers.
What are mechanoreceptors?
Touch receptors that respond to pressure or indentation of the skin.
What is glabrous skin?
The smooth, hairless skin found on the palms, fingers, and soles of the feet.
What 4 kinds of mechanoreceptors does glabrous skin contain?
Pacinian corpuscles, Meissner corpuscles, Merkel discs and Ruffini endings.
What did Johansson and Vallbo (1983) state?
That there are about 17000 mechanoreceptors on the glabrous skin on the hand.
Does stimulation of a particular type of receptor exclusively evoke a specific touch sensation?
It’s not clear - natural stimuli activate multiple types of mechanoreceptors.
How do the four types of mechanoreceptors send information to the brain?
Via afferent touch fibres.
How can afferent fibres be classified?
According to their properties (temporal and spatial).
How do slowly and rapidly adapting afferent fibres differ?
Slowly adapting fibres respond continuously to a persistent tactile stimulus, whereas rapidly adapting fibres respond only to the onset and termination (i.e. change) of a stimulus.
Describe the receptive fields in the skin.
They’re organised concentrically, e.g. with an excitatory centre and inhibitory surround.
How do afferent fibres vary spatially?
Some fibres have large receptive fields and some have small ones.
What did Greenspan and Bolanowski (1996) create?
The four-channel model of mechanoreception, differentiated mechanoreceptors by their adaptational property, receptive field size, location in skin, and sensation evoked.
Describe Pacinian corpuscles in terms of adaptational property, receptive field size, location in skin, and sensation evoked.
Rapid, large, subcutaneous and vibration.
Describe Meissner corpuscles in terms of adaptational property, receptive field size, location in skin, and sensation evoked.
Rapid, small, superficial and flutter.
Describe Merkel discs in terms of adaptational property, receptive field size, location in skin, and sensation evoked.
Slow, small, superficial and pressure.
Describe Ruffini endings in terms of adaptational property, receptive field size, location in skin, and sensation evoked.
Slow, large, subcutaneous, buzz-like.
What is the problem with the four-channel model of mechanoreception?
The sensations evoked are difficult to operationalise. E.g. what is meant by a ‘buzz-like’ sensation is unclear.
What is the difference between efferent and afferent fibres?
Efferent fibres travel from the nervous system, afferent fibres travel to it.
What is the most well-studied mechanoreceptor?
The Pacinian corpuscle - the largest, least numerous, and most deeply located.
How is the Pacinian corpuscle typically investigated in the lab?
Using large vibrotactile stimuli that vibrate at high frequencies with variable amplitude (strength).
What is it possible to compare when investigating Pacinian corpuscles (Verillo, 1966; Bolanowski & Verillo, 1982)?
Neural thresholds of isolated Pacinian corpuscles with detection thresholds measured psychophysically.
How do neural (Pacinian corpuscle) and psychophysical thresholds for vibrations appear on a graph?
For both, vibration sensitivity is a U-shape function of stimulus frequency. Pacinian corpuscle threshold is about 20dB lower.
Where is sensitivity greatest for both neural and psychophysical vibration thresholds?
In the region of 250Hz, where the threshold is about a ten thousandth of a millimetre.
How well can skin containing no Pacinian corpuscles detect vibrations?
Not very well - straight line on graph between 10 and 15dB.
What, specifically, is encoded by Pacinian corpuscles?
Rapid changing tactile stimulation.
What mediates vibration detection at frequencies too low for Pacinian corpuscles?
Other receptors - Meissner corpuscles and Merkel discs.
How are absolute thresholds for passive touch often measured?
By recording minimum force necessary to detect fine nylon filament on skin.
Under ideal conditions, what pressure level can evoke a sensation of displacement (Verillo, 1975)?
Less than 0.001mm.
Are all regions of the skin equally sensitive?
No - absolute sensitivity varies over skin surface.
Where is absolute sensitivity the best?
On the face (Weinstein, 1968).
What is a two-point localisation threshold?
The smallest separation between two points that are just resolvable.
What are two-point localisation thresholds a measure of?
Localisation accuracy - acuity.
Where are two-point localisation thresholds lowest?
The hands, feet and face.
Describe the nerve fibres that carry information about touch from corpuscular mechanoreceptors.
Large, fat, myelinated afferent fibres that enter the dorsal side of the spinal cord.
What are the ascending pathways for somatosensory information?
Lemniscal pathway for touch, the spinothalamic tract for pain and temperature.
How does touch information travel through the brain?
Lemniscal pathway axons project to the brainstem, cross the midline, then to the thalamus on the contralateral side of the brain. Thalamic neurons send axons to the somatosensory cortex in the parietal lobe.
Where is touch information processed in the brain?
In the somatosensory cortex - topographic map of skin locations of the body.
What is meant by the statement that the most sensitive skin areas have more neural hardware?
The distribution of neural tissue is accentuated for those body parts that are most important for survival - they are represented by larger areas of the somatosensory cortex on the topographic map.
Which areas of the somatosensory cortex homunculus are the most accentuated?
Hands, lips and tongue.
What is active touch?
The active exploration of objects in the immediate environment.
In active touch, what information is combined?
Information from mechanoreceptors and proprioceptors - tactile and proprioceptive information (‘Haptics’).
What are proprioceptors?
Receptors in the muscles and joints which encode the postures, locations and movements of the body.
What did Darian-Smith et al. (1982) find?
That some cells in the primate somatosensory cortex respond to both movement if the body and tactile features of surfaces that are touched - evidence supporting active touch.
According to Lederman and Klatsky (1990), what are we adept at gauging through active touch?
Size, shape, texture, weight, hardness, and curvature of objects.
What did Klatsky, Lederman and Metzger (1995) find?
When blindfolded, subjects could identify 100 common objects with 95% accuracy within 5 seconds by ‘feeling’ them.
What system utilises our ability to discriminate and recognise complex kinds of information using active touch?
The Braille reading system - tactile pattern alphabet designed by Louis Braille in the 19th Century.
How fast can experienced Braille readers read?
100+ words per minute according to Foulke and Berla (1984).
Why is it easier to read Braille than embossed Roman letters using the fingers?
Loomis (1981) because of the limited ability of the skin to resolve fine spatial details - there is mechanical blurring, as tactile stimuli deform neighbouring regions as well as the touched area. Braille is made from coarse elements (dots) which are changed little by blur.
Why is pain a difficult sensory experience to study?
- It’s defined exclusively in terms of subjective experience.
- It’s difficult to assess the quality and intensity of pain.
- Individual differences make it impossible to identify a particular evoking stimulus.
- Psychological factors may influence the intensity and quality of pain.
What kinds of psychological factors can influence pain experienced?
Expectation, attitude, attention, suggestion, emotional state and cognitive processes.
What is the principal (evolutionary) function of pain?
The adaptive avoidance of harmful environments - although tissue injury is neither necessary nor sufficient for pain, most stimuli that produce pain are potentially damaging.
Give examples of harmful stimuli that produce pain.
Prolonged exposure to very intense light, sound, pressure, and temperature.
What does pathological pain insensitivity lead to?
Serious self-inflicted injury or death (Melzack, 1973).
What changes to pain experiences can brain damage cause?
- a failure to recognise stimuli as painful (Ploner et al., 1999)
- a lack of emotion and withdrawal from stimuli identified as ‘painful’ (Berthier et al, 1988).
What is a nociceptor?
A specialised class of receptor that results in pain when stimulated.
What is a prime candidate for nociceptors?
The free nerve endings found almost everywhere within both hairy and glabrous skin.
Why are free nerve endings a prime candidate for nociceptors?
They’re the most common type of skin receptor, also found in muscles, tendons, joints and internal organs, and are found wherever ‘pain spots’ are located on skin.
Describe the fibres that carry neural messages from nociceptors.
High-threshold, small diameter, slow-conducting, thinly or unmyelinated afferent fibres. Large receptive fields.
Why is the slow conduction velocity of pain fibres unimportant?
Because once tissue damage occurs it’s already too late.
Why do pain fibres have large receptive fields?
Because the amount of pain is more important than fine discrimination.
Where do the fibres that carry neural messages from nociceptors carry information from and to?
The skin to the spinothalamic tract of the spinal cord.
What does primate electrophysiology and human brain imaging (PET, MEG, fMRI) show about the neurones in the somatosensory cortex?
Some of them respond selectively to noxious stimuli, i.e. pain (e.g. Talbot et al., 1991; Ploner et al., 2000).
What comprehensive theory of pain has generally received wide support?
Melzack and Wall’s (1965, 1988) gate-control theory.
What is the main premise of the gate-control theory of pain?
Fast touch fibres and slow pain fibres connect with substantia gelatinosa (SG) and transmission (T) cells in the spinal cord.
According to the gate-control theory of pain, what do T cells do?
Send information to the brain.
According to the gate-control theory of pain, what do SG do?
Act as a gate to allow or inhibit T cells.
According to the gate-control theory of pain, what controls the opening or closing of the gate?
Activity in fast fibres (touch) closes it, whereas activity in slow fibres (pain) opens it.
Why does rubbing the skin to alleviate pain work?
A light touch accompanying the noxious stimuli partially closes the gate.
