Transcript The 4 channel model

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
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Thresholds II
d’
Discriminabitlity (d’) of complex waveforms
for low frequencies
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The 4 channel model
- psychophysics
Threshold
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The 4 channel model
- neurophysiology
Sensory Afferents
Conduct.: 35-70 m/s
(Hand scan: 60-80 mm/s)
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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
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Form perception:
dots, edges, curves
Braille reading
Aperiodic grating
SAI channel
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Receptive field characteristics relevant to
form perception
RAI & SAI channels
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Surround supression
skin mechanics
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Perception of texture:
roughness
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Fine texture perception:
SAI spatial variation*
*Mean absolute difference in firing rates between
SAI afferents with RFs separated by ~2mm
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SAI spatial variation code for fine textures (0.2-1mm)
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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
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limited dynamic range.
Vibrotactile perception:
flutter, vibration
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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
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P channel
ideal obs.
of St motion
10 nm skin motion at 200 Hz
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Response to vibrating stimulus
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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
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Adaptation
Peripheral mechanisms
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Time course of adaptation and recovery
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RA interference in spatial processing
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Tool use
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Coding object size
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Grasping and manipulation
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Grasping and manipulation
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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
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