Full lecture notes from Cognitive Psychology (C82NAB) - Touch. Includes Sensory Receptors, Neural and Psychophysical Thresholds, Somatosensory Information, Braille, Pain,
Touch
The perception of touch is part of the somatosensory system
Touch is distinct from the other 4 sense modalities in a number of ways
- Receptors for touch are varied and distributed throughout the entire body not just confined
to specific localised structures.
- The perceptual apparatus mediating touch responds to many different types of stimulation
and the quality of the sensations produced are extremely diverse
Receptors in the muscles and joints also encode the postures, locations and movements of the
body (proprioception) – important for active touch
It has been proposed that the different sensory qualities are mediated by different specialised
receptors within the skin layers (Iggo)
Mechanoreceptors are touch receptors that responds to pressure or indentation of the skin
The smooth, hairless portions of the skin (“glabrous skin”) found on the palms, fingers and soles
of the feet contains 4 main types of mechanoreceptors
- Pacinian corpuscles, meissner corpuscles, Merkel disks and Ruffini endings
- 17,000 mechanoreceptors on the hairless skin on the hand
A given cutaneous pressure sensation may thus arise from activation of several different
specialised mechanoreceptors rather than a single one
It is not clear that stimulation of a particular type of receptor exclusively evokes a specific touch
sensation. Complex natural stimuli probably activate multiple types of mechanoreceptors.
Afferent fibres = from outside world to CNS
Efferent fibres = fro CNS to outside world
The 4 types of mechanoreceptors send information to the brain via afferent fibres that can be
classified according to their properties.
Temporal properties
- Slowly adapting fibres respond continuously to a persistent stimulus and rapidly adapting
fibres respond only to the onset and termination of a stimulus
Spatial properties
, - Receptive fields in the skin have a concentric organisation (excitary centre and inhibitory
surround) like the visual system.
Pacinian corpuscle:
- rapidly changing,
- large receptive field
- Subcutaneous
- Vibration
Meissner Corpuscle:
- Rapidly changing
- Small receptive field
- Superficial
- Flutter
Merkel Disk:
- Slow changing
- Small receptive field
- Superficial
- Pressure
Ruffini Ending:
- Slow
- Large receptive field
- Subcutaneous
- Buzz-like
Comparisons of neural and psychophysical thresholds
The most well studied receptor is the Pacinian corpuscle.
It is extremely sensitive to touch and typically investigated in the laboratory using large
vibrotactile stimuli that vibrate at high frequencies with variable amplitude.
It’s possible to compare directly neural thresholds of isolated Pacinian corpuscles with detection
thresholds measured psychophysically.
Vibration sensitivity is a U shaped function of stimulus frequency
Sensitivity greatest in the region of 250Hz(thresholds – 0.0001mm)
Crosses show thresholds for skin containing no Pacinian corpuscles
Rapid changing tactile stimulation encoded by Pacinian corpuscles
At lowest frequencies detection mediated by other receptors
Are all areas of the skin equally sensitive?
Studied using thin bristles and poking parts of the body – used forced choice task – which arm?
Under ideal conditions skin displacement < 0.001 mm can evoke a sensation of pressure
Absolute sensitivity varies over skin surface, but is best for face, hands and feet.
Two point localisation threshold (smallest separation between two points – are you being poked
by one bristle or two bristles? Keep moving closer till can’t tell the difference between the two
stimuli.
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