TOUCH
PSYCHOPHYSIOLOGY
L. Négyessy
PPKE, 2010
Haptic exploration of local shape
Static stimuli
1-2 mm
2,8 mm
min. 0,5 mm
: 3%
0,17 mm
2
Thresholds I
Braille dots’ height: 500 µm
3
Vibrating stimuli
OPTACON
6 x 18 = 108 blunt pins
230 Hz
4
Thresholds II
d’
Discriminabitlity (d’) of complex waveforms
for low frequencies
5
The 4 channel model
- psychophysics
Threshold
6
The 4 channel model
- neurophysiology
Sensory Afferents
Conduct.: 35-70 m/s
(Hand scan: 60-80 mm/s)
7
Receptive field organization (finger pad)
SAI
RA
P
SAII
?
1 mm2
0.82 mm2
diffuse
diffuse
Aff. denz.
100 cm-2
150 cm-2
350/ finger
? (low)
Diverg.
(RF area)
4-16
(5 mm2)
4-16
(5 mm2)
1:1
1:1
Converg.
1:1
2-7
1:1
1:1
Adequ. stim.
Strain energy
density (point,
edge, curve)
Slip,
load force
High freq.
vibration
Skin
stretch
function
Form, texture
Grip
control, fine
discr.
Distant
events,
tool use
Hand
shape
RF size
8
Form perception:
dots, edges, curves
Braille reading
Aperiodic grating
SAI channel
9
Receptive field characteristics relevant to
form perception
RAI & SAI channels
10
Surround supression
skin mechanics
11
Perception of texture:
roughness
12
Fine texture perception:
SAI spatial variation*
*Mean absolute difference in firing rates between
SAI afferents with RFs separated by ~2mm
13
SAI spatial variation code for fine textures (0.2-1mm)
14
Summary of form & texture perception
coding of spatial features
Evidences of SA1 specialization for the representation of spatial
information:

SA1 responses to stimulus elements on a surface are independent of
the force of application.

SA1-receptive fields grow minimally (relative to RA receptive fields) with
increasing indentation depth.

SA1 afferents possess a response property, surround suppression,
which confers response properties similar to those produced by
surround inhibition in the central nervous system. This response
property is a consequence of sensitivity to strain energy density, not a
synaptic mechanism.

SA1 spatial resolution is affected minimally by changes in scanning
velocity at velocities up to at least 80 mm s–1.

SA1 afferents are at least ten times more sensitive to dynamic than to
static stimuli.

SA1 responses to repeated skin indentation are practically invariant:
the variability is about one impulse per trial regardless of the number of
action potentials evoked.

The RA system has greater sensitivity but poorer spatial resolution and
15
limited dynamic range.
Vibrotactile perception:
flutter, vibration
16
RA channel
Periodic Stimulus
Firing rate
Periodic St
Aperiodic St
Periodicity (IS interval) Firing rate
monkey
ideal obs.
Periodic+aperiodic St
Only firing rate
firing rate
Thresholds ratios: psychometric/neurometric thresholds
periodicity
17
P channel
ideal obs.
of St motion
10 nm skin motion at 200 Hz
18
Response to vibrating stimulus
19
Summary of vibrotactile perception
- coding of temporal features
P channel

intense filtering (at nearly 60 dB per decade) of low-frequency stimuli

respond to stimuli less than 100–150 Hz with a phase-locked, Poisson
discharge, therefore a whole population firing randomly but at a rate
proportional to the instantaneous stimulus amplitude can represent the
stimulus waveform accurately
RA channel

RA neurons of S1, like their afferent fibers, fire periodically, in phase
with mechanical oscillations

RA neurons modulate their firing rates as a function of the stimulus
frequency

Flutter is encoded by firing rate of RA neurons
20
Adaptation
Peripheral mechanisms
21
Time course of adaptation and recovery
22
RA interference in spatial processing
23
Tool use
24
Coding object size
25
Grasping and manipulation
26
Grasping and manipulation
27
SUMMARY
•The 4 channel model of vibrotactile discrimination
•RF correlates of the 4 channel model
•Elements of form perception
•Texture (roughness) perception
•Vibrotactile perception
•Object manipulation
28