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Cursus Doelgericht Handelen(BPSN33)
R.H. Cuijpers, J.B.J. Smeets and E. Brenner (2004). On the relation between object shape and grasping kinematics. J Neurophysiol, 91: 2598-
2606. R.H. Cuijpers, E. Brenner and J.B.J. Smeets (2006). Grasping reveals visual misjudgements of shape. Exp Brain Res 175:32-44
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Topics• 1st hour: Control Variables in Grasping
– Opposing views on visuomotor control– Research question
• 2nd hour: Grasping elliptical cylinders– Real cylinders
• Which positions?• How to get there?
– Virtual cylinders• Constant haptic feedback• Veridical haptic feedback
– If time permits: Modeling grip planning– Conclusions
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Control variables in grasping
Many levels of description:• Activity motor neurons• Muscle activity (EMG)• Posture (Joint angles)• Kinetics (Forces, torques)• Kinematics (Position, speed
etc.)• Task level
Degrees of Freedom
(DoF)
low
high
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Control variables in grasping
How does the brain ‘plan’/compute the desired motor neuron output?
• If movements are planned in task space:– little computational power needed for planning stage
• But …– Need to solve DoF-problem (Motor primitives)– Cannot control everything (Stereotypic movements)– Need low-level on-line control (e.g. stiffness control)
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Control variables in grasping
• What is/are the correct level(s) of description for movement planning and visuomotor control?
Method of research in visuomotor control:• Manipulate visual information / haptic feedback /
proprioceptive feedback• Measure effect on motor output
• Variables that have an effect are ‘controlled’• Variables that have no effect are redundant
Haptic = by touch Proprioceptor = sensory receptor in muscles, tendons or joints
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Opposing views on visuomotor control
Fingertip positions and object size• Milner & Goodale: perception vs. action• Franz et al: common source model• Smeets & Brenner: position vs. size
Fingertip positions and object orientation• Glover & Dixon: planning vs. on-line control• Smeets & Brenner: position vs. orientation
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perception vs. action
Goodale (1993); Milner, Goodale (1993)
• RV: lesions in occipito-parietal cortex (dorsal).
• DF: damage in ventrolateral occipital areas due to CO poisoning.
Grasping DiscriminationRV DF
Occipito-Parietal
Ventrolateral oocipital
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perception vs. action
• Dorsal pathway for guiding movements (should be veridical)
• Ventral pathway for perception (perception of shape, colour etc.)
Dorsal = Action
Ventral = Perception
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perception vs. action
Agliotti, De Souza, Goodale (1995):
• Grip aperture NOT influenced by size-illusion.
• Due to separate processing of information for perception and action.
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Common source model
• Franz et al (2000): equal effects of illusion
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Position vs. size
Brenner, Smeets (1996):
• Size-illusion does not affect grip aperture, but does affect the initial lifting force.
• Explanation: not size information is used but position information. They are inconsistent.
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Planning vs. on-line control
Glover & Dixon (2001)
• Relative effect of illusion decreases with time
Illusion mainly affects planning
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Position vs. orientation
Smeets et al. (2002)• Assumption:
illusion affects orientation, not position
• Also explains data of Glover and Dixon
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Research Question: How is shape information used for
grasping?
• The visually perceived shape is deformed• Shape (ventral) determines where it is best
to grasp an object (dorsal)– Grip locations not veridical
• Shape information could be used during planning (ventral) or on-line control (dorsal)– Grip errors arise early or late in the movement
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Grasping elliptical cylinders:real cylinders
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Experimental design
• seven 10cm tall cylinders
• elliptical circumference with fixed 5cm axis
• variable axis: 2, 3, 4, 5, 6, 7 and 8 cm
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Experimental Design
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Experimental design• Optotrak recorded traces of fingertips• 2 distances x 7 shapes x 6 orientations = 84 trials• 3 repetitions• 10 subjects
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Experimental Design
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Example
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Which positions?
• Geometry: grasping is stable at principle axes
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Which positions?• Principle axes preferred. But systematic errors…
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Which positions?• Systematic "errors" depending on orientation.
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• Scaling grip orientation 0.7 except for aspect ratios close to 1, 0.5
Which positions?
Scaling grip orientation = slope + 1
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Comfortable grip
Prediction:
Slope a = w-1
Offset b = -(w-1)0
0)1()1( ww 0)1( ww
0ab
Suppose: grip orientation = mixture between cylinder orientation + comfortable grip
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Thus …
• Subjects grasp principle axes, but make systematic errors
• Cannot be explained by comfort of posture
• Additional effect of deformation of perceived shape
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How to get there?
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How to get there?
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How to get there?
•High correlation despite errors!
•Sudden drop at end: Grip aperture automatically corrected
•Correlation much higher for max. grip aperture than final grip aperture
•Gradual increase: grip errors were planned that way
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Thus …
• Systematic errors already present in the planning of the movement
• Maximum Grip Aperture reflects planned size rather than true size
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Grasping virtual cylinders
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Experimental design
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Experimental Design
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Experimental design
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Experimental design
• Constant haptic feedback:– Real cylinder is always circular– Virtual cylinders: 15 aspect ratios, 3 orientations
• Veridical haptic feedback:– Virtual and real cylinders are the same,
7 aspect ratios and 2 orientations
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Constant haptic feedback
• Only half of the subjects scale their grip orientation
• If they do, the scaling of grip orientation is similar to real objects (0.42)
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Constant haptic feedback
• Subjects hardly scale their max. grip aperture
• Scaling of max. grip aperture is much smaller than for real objects (0.14 instead of 0.57)
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Thus
• Inconsistent haptic feedback reduces scaling gains
Possible cause:• All subjects scale their grip aperture based
on the felt size • Scaling of grip orientation based on seen
orientation for only half of the subjects, and the felt orientation for the other half
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Veridical haptic feedback
• Similar pattern of grip orientations for all subjects
• Scaling of grip orientation (0.58) close to those for real objects (0.60)
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Veridical haptic feedback
• All subjects adjust their maximum grip aperture
• Scaling of max. grip aperture (0.39) much higher and closer to real objects (0.57)
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Thus
With consistent haptic feedback
• Scalings of grip orientation and grip aperture close to those for real cylinders
• Less variability between subjects
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Comparison of experiments
Real Cylinders
Consistent Feedback
Inconsistent Feedback
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Thus
• Natural grasping of virtual cylinders requires veridical haptic feedback
• Grip orientation and grip aperture can be scaled independently
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Modeling grip planning
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Modeling grip planning
• Physical constraints– Grip force through centre of mass– Grip force perpendicular to surface– Optimal grip along major or minor axis
• Biomechanical constraints– For a given cylinder location there is a most
comfortable grip– Evident when grasping circular cylinder
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Modeling grip planning
• Assumptions:– The planned grip orientation is a weighted
average of the optimal and the comfortable grip orientation
– The weights follow from the expected cost functions for comfort and mechanical stability
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Modeling grip planning
maximal is 0)(
)|()(
*)(
))(1()(
)(22
EGEG
dpG
ondistributiGEG
ww
GGG
planned
planned
planned
planned
comfort
comfortoptimal
comfortplanned ww )1(
If
Then (required)
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Modeling grip planning
• Perceptual errors change the perceived cylinder orientation
• The comfortable posture may also be uncertain
)|ˆ(~ˆ p
)|ˆ(~ˆ0000 p
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Modeling grip planningIf distributions are Gaussian with zero mean, we get:
For the circular cylinder w=0, so that:
22
22222
))(1(
)1(var
)1(
comfortmotor
motor
comfortplanned
wwMEG
ww
wwE
comfort
22varcomfortmotorcircular
comfortcircularE
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Modeling grip planning
• Each grip axis may be grasped in different modes:
• Model predicts probability of each mode
kk 21
axismajor
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Modeling grip planning
• The model describes the relative costs for grip comfort and mechanical stability
• It predicts the relative probability of choosing the major or minor axis
• We can incorporate biases in the perceived cylinder orientation
• We can extend to more general shapes
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Conclusions
• Subjects plan their grasps to suboptimal locations based on the perceived shape and the anticipated (dis)comfort
• Upon touching the surface the errors are corrected• Haptic feedback is necessary for natural grasping• With our model we can identify relative
contributions of comfort, stability and perceptual errors
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Conclusions
• Visual shape information (slant, curvature) is used for planning suitable grip locations (position information)– Perceptual bias– Bias due to comfort of posture
• No substantial on-line corrections On-line control uses position information
• When inconsistent, haptic and visual shape information is combined differently for the planning of grip aperture and grip orientation
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The end
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Stable grip of an ellipse