Stress Fluctuations in Sliding of Textured Objects and the Sense of Touch
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Georges Debrégeas - Alexis Prevost
R. Candelier, J. Scheibert, S. Leurent
Laboratoire de Physique Statistique – ENS Paris
Patrice Rey (CEA-LETI)Joël Frelat (LMM, Paris 6)
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Information transduction in tactile perception
Object + motion
SKIN DEFORMATIONS &VIBRATIONS
NERVOUSSIGNALS
REPRESENTATION
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Glabrous skin mechanoreceptors
Merkel's cellcomplex
Meissner'scorpuscule
Pacinian corpuscule
Ruffiniending
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Glabrous skin mechanoreceptors
Stimulus
t
Slow Adaptation
Fast Adaptation
Merkel's cellcomplex
Meissner'scorpuscule
Pacinian corpuscule
Ruffiniending
Stimulus
t
Bolanowski et al., 1988
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Roughness perception: the duplex theory
Coding of coarse roughness> ~200µm
SA I (Merkel) channel Resolution limited by the small receptive
field (few hundred µm) Spatial coding (static) Fairly independent of finger's motion
Coding of fine roughness< ~200µm
Mediated by Pacinian corpuscules exclusively
Requires active tactile exploration Intensity coding
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Hollins and Bensmaia, 2008
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Questions :
1 – How can one relate the physical properties of the object and exploratory conditions to the mechanical signals experienced by mechanoreceptive nerve endings.
2 – What are the consequences of this filtering process on the transduction and neural encoding of tactile information.
Outline
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1 – Biomimetic tactile sensing – design and calibration.
2 – Dynamic impulse response.
3 – Response to randomly rough substrates.
4 – A possible role for fingerprints.
5 – Conclusions and perspectives.
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The biomimetic approach
Realfinger
Artificial finger
Sensitive area Sensor deth
Contact diameter
Skin elastic modulus
Humanfingertip
0.5 -10 mm 2-3 mm ~13mm (P~0.5N)
1-4 MPa
Artificial fingertip
2 mm 2.5 mm ~6mm(P~1.5N)
2.2±0.1Mpa
MEMS sensor
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Response to a localized force
Indentation protocol:Apply a ponctualforce at on the surface with a rod.
Receptive fields measured by our MEMS sensorsPredicted receptive field for a ponctual sensor in a perfectly elastic material
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Response to a localized force
Indentation protocol:Apply a ponctualforce at on the surface with a rod.
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Response to a localized force
Indentation protocol:Apply a ponctualforce at on the surface with a rod.
Without exploration: roughly the same response for the 10 sensors
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A linear model for tactile transduction
+Coulomb law:
Green function for a ponctual force at the surface:
Hertz contact
+
The stress felt by the sensor is given by:
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A linear model for tactile transduction
+Coulomb law:
Green function for a ponctual force at the surface:
Hertz contact
+
The stress at the sensor location reads:
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Dynamic impulse response.
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Dynamic impulse response.
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Dynamic impulse response.
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Dynamic impulse response
Perturbation in force signal associated with a small, isolated defect :
The modification in stress profile at the interface reads
Perturbation in force signal for a sensor at :
The response highly depends on the sensor's position within the contact zone
)ux()x(ss
)ux()xx(G)x()u(s s 0
u
0xx
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Dynamic impulse response – normal stress
Experiment
Model
Middle RightLeft
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Dynamic impulse response – tangential stress
Experiment
Model
Middle RightLeft
Midline profiles
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Experiment
Model
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Receptive field variability in cortical neurons
DiCarlo et. al., 1998The journal of Neuroscience
« The shape, area and strength of exitatory and inhibitory receptive fields regions ranged widely. »
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Response to randomly rough substrates
Scanning over a binary patterned substrate
?
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The Volterra kernels give a mapping from to .
The Volterra decomposition
The Volterra series is the analog of the Taylor series, but for functionals:
NB: it is hard to extract the Volterra kernels ...
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For Gaussian white noise inputs, the Wiener kernels are orthogonal.
The Wiener decomposition
They can be computed through correlations:
...
Extracting the linear kernel
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1g
measured
Predicted
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Consequences of skin patterning (e.g. fingerprints)
Artificial fingerprintsSquare-wave gratings
(period 220mm)on the skin's surface
Smooth skinFingerprinted skin
Linear model of mechanical transduction
dx)ux(T).xx(G)x(s)u(s s 0
)x(g1
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Square wave gratings:
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)ux(T).x()u( ss
1)x(T
Interfacial stress profile:
Force signal :
Stimulus- signal response function
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Stimulus- signal response function
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Numerical illustration of the filtering process
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Numerical illustration of the filtering process
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Numerical illustration of the filtering process
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Numerical illustration of the filtering process
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Numerical illustration of the filtering process
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2 mm
• Typical inter-ridge distance l ~ 500 µm• « Natural » exploratory finger/substrate velocity V ~ 10 cm/s
• Frequency f = V / l ~ 200 Hz • Order of the best frequency of Pacinian fibers• Pacinian fibers = mediate the coding of fine texture
Scenario Fingerprints select one spatial frequency Velocity chosen to match Pacinian best response
Consequence of fingerprints for fine texture perception
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Conclusions
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Biomimetic approach allowed to characterize the linear mechanical transduction of texture information, and clarify the roles played by intrinsic sensor’s response, interfacial contact stress field and skin topography.
But :- Limited to binary topography.- Non-linear effects should be important (stress coupling within the contact zone, normal stress dependence of the friction coefficient, etc.) Reverse correlation should allow to probe that.
Important open question :How does the tactile system deal with such context dependent variability of
individual sensors’ response. What encoding strategies may yield a stable representation of the probed surface.
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Comparing biomimetic and human touch
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JP Roll - LNH – Marseille
Can one relate the subcutaneous stress field measured with the biomimetic sensor with actual neurographic data ?
Comparing whisker and digital touch
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Dan Shulz – Yves BoubenecUNIC - Gif-sur-Yvette
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Daniel Shulz (Gif-CNRS)
A 1A 2A 3A 4
Rodents whisker touch
Wolfe & Feldman, ‘08
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Johnson & Phillips, 1981
F
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The cochlea
Georg von Békésy, ‘47
inner hair cellsouter hair cells
Nobili, Mammano and Ashmore, ‘98
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…Back to the actual finger
Can we see this effect on a real finger ?
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C D
Communicative & Integrative Biology, 2009
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