tactile inputs distort perception of relative fingertip...
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Neural control of Movement Laboratory
This work has been partially supported by a NSF Collaborative Research grant BCS-1455866
Tactile input alone contributes to the biased estimation of relative fingertip position. However, the relative contribution of tactile input and voluntary motor commands onto the biased estimation of relative fingertip position remains to be determined. The extent to which motor commands alone may be used to estimate relative fingertip position is currently being investigated.
Tactile Inputs Distort Perception of Relative Fingertip Position Daisuke Shibata,1 Francesco Chinello,3,4 Domenico Prattichizzo,3,4 Marco Santello2
1Athletic Training Education Program, Department of Health, Exercise and Sports Sciences, University of New Mexico, Albuquerque, New Mexico, USA 2School of Biological and Health Systems Engineering, Arizona State University, Tempe, Arizona, USA
3Department of Information Engineering, University of Siena, Siena, Italy 53100 4Department of Advanced Robotics, Istituto Italiano di Tecnologia, Genova, Italy 16163
Dexterous manipulation relies on the ability to coordinate digit forces to positions, and therefore sensing the relative position of fingertips.1-6 Our recent work examined the contribution of voluntary motor commands on the accuracy with which subjects could reproduce the vertical distance between the center of pressure of thumb and index finger (dy). Although dy was always zero, subjects consistently overestimated dy by positioning the thumb higher or lower than the index finger - but only when digit forces were exerted in opposite directions.7 This perceptual-motor illusion might have been caused by a) motor commands responsible for digit forces, b) tactile afferent signals arising from the deformation of the finger pads, or c) a combination of motor commands and tactile inputs. The present study addressed the extent to which tactile inputs alone may bias the estimation of relative fingertip position.
Experimental protocol
Skin deformation exert a directional bias on matching fingertip vertical distance
Horizontal fingertip distance matching was accurate across all conditions
Role of tactile inputs on dy estimation bias Forces applied to the fingertips consistently biased fingertip vertical distance matching. The present bias is 50% smaller than reported for tasks in which subjects generate digit forces,7 therefore both tactile inputs and efference copy of digit forces are likely to be responsible for the matching bias.
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Contact information: Daisuke Shibata: [email protected], Francesco Chinello: [email protected], Domenico Prattichizzo: [email protected], Marco Santello: [email protected]
1. Crajé C, Lukos JR, Gordon AM, Ansuini C, Santello M. (2011). Planning of multi-digit contact points as a function of task. Exp Brain Res. 212: 119-124.
2. Fu Q, Zhang W, Santello M. (2010). Anticipatory planning and control of grasp positions and forces for dexterous two-digit manipulation. J Neurosci. 31: 9117-9126.
3. Fu Q, Hasan Z, Santello M. (2011). Transfer of Learned Manipulation following Changes in Degrees of Freedom. J Neurosci. 31: 13576-13584.
4. Lukos J, Ansuini C, Santello M. (2007). Choice of contact points during multidigit grasping: effect of predictability of object center of mass location. J Neurosci 27:3894-3903.
5. Lukos J, Ansuini C, Santello M. (2008). Anticipatory control of grasping: independence of sensorimotor memories for kinematics and kinetics. J Neurosci 28:12765-12774.
6. Zhang W, Gordon AM, Fu Q, Santello M. (2010). Manipulation after object rotation reveals independent sensorimotor memory representations of digit positions and forces. J Neurophysiol. 103: 2953-2964.
7. Shibata D, Kapper AML, Santello M. (2014). Digit forces bias sensorimotor transformations underlying control of fingertip position. Front Hum Neurosci. 8: 564.
Skin deformation alone biases the estimation of vertical fingertip distance but only when the force vectors experienced at the fingertips have opposite directions.
Matching error Test dy – Reference dy
time
Relax Active match Hold
5 s 5 s < 5s 5 s
Sense and memorize
Passive dy adjustment
Reference dy Test dy
<5 s
Reference Test Reference Test Reference Test
thumb
index finger
Ftan: 2.1 to 3.1 N
Ftan: –2.1 to –3.1 N
Fn: 4.1 to 4.6 N
Fn: < 0.1 N
Ftan: 0 ± 0.1 N *
Forces exerted by the wearable device
Same Opposite Control
IDOWN-TUP
Fn only
No Ftan/Fn
IUP-TDOWN IUP-TUP
IDOWN-TDOWN
* *
* *
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Nor
mal
ized
ver
tical
mat
chin
g er
ror (
mm
)
IUP-TUP IDOWN- TDOWN
IDOWN- TUP Fn only IUP-
TDOWN No Ftan/Fn
Thumb CoP higher than index finger CoP
Thumb CoP lower than index finger CoP
Nor
mal
ized
hor
izon
tal m
atch
ing
erro
r (m
m)
Shorter horizontal distance between
the fingertips
The bias on matching relative fingertip position reflected the direction of force vectors applied to the fingertips, but only when force were produced in opposite directions (Opposite condition), i.e., subjects positioned the thumb lower than the index finger when the skin deformation pointed upward and downward, respectively, and vice versa.
Same Opposite Control
n.s.
P < 0.05
P < 0.05 *
*
P <
0.0
01
IUP-TUP IDOWN- TDOWN
IDOWN- TUP Fn only IUP-
TDOWN No Ftan/Fn
Same Opposite Control
S
A
P
A: local reference frame S: global coordinates of the tracking system PA: coordinates of P related to A : rotation matrix between the homonyms reference frames dSA: distance between S and A
SAR
Wearable haptic device
Longer horizontal distance between
the fingertips
Reference collinear contacts dy: 0 mm, dz: 64 mm
y
z
IUP-TDOWN IDOWN-TUP
(Shibata et al., 2014) -20
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n.s.
IUP-TUP IDOWN-TDOWN
IDOWN-TUP
IUP-TDOWN Fn only No Ftan/Fn
Nor
mal
ized
ver
tical
mat
chin
g er
ror (
mm
)
P < 0.001
P < 0.001 *
*
P <
0.0
01
Predicted Sensory
Feedback
Internal Forward Models
Motor Commands
Skin Receptors
Joint Receptors
Muscle Receptors Vision
muscles ±10 mm error in perception of relative digit position
Predicted Sensory
Feedback
Internal Forward Models
Motor Commands
Skin Receptors
Joint Receptors
Muscle Receptors Vision
muscles ±5 mm error in perception of relative digit position
SP = SAR0
SAd1
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟• AP
Opposite condition x
y
Screen
Skin deformation induced by external forces