Cognitive Psychology: Touch lecture notes (C82COG)

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

Sensory Receptors

 Epidermis (dead skin layers)
 Dermis (living skin)
 Meissners corpuscle (pressure)
 Basket cell (pressure – feels pressure when hair cell moves)
 Merkel’s disk (pressure)
 Free nerve ending (pain, temperature and pressure)
 Ruffini ending (pressure)
 Pacinian corpuscle (pressure)
 (corpuscle = swelling)

Characteristics

 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.