Cursus Doelgericht Handelen (BPSN33)

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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 17532-44
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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

high
Degrees of Freedom (DoF)
low
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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.

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perception vs. action

• Dorsal pathway for guiding movements (should be
  veridical)
• Ventral pathway for perception (perception of
  shape, colour etc.)

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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 cylindersreal 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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Which positions?

• Scaling grip orientation ? 0.7 except for aspect
  ratios close to 1, ? 0.5

Scaling grip orientation slope 1
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Comfortable grip
Suppose grip orientation mixture between
cylinder orientation comfortable grip

• Prediction
• Slope a w-1
• Offset b -(w-1)f0

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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?

• Gradual increase grip errors were planned that
  way

• High correlation despite errors!

• Sudden drop at end Grip aperture automatically
  corrected
• Correlation much higher for max. grip aperture
  than final grip aperture

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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
If
Then
(required)
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Modeling grip planning

• Perceptual errors change the perceived cylinder
  orientation
• The comfortable posture may also be uncertain

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Modeling grip planning

• If distributions are Gaussian with zero mean, we
  get
• For the circular cylinder w0, so that

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Modeling grip planning

• Each grip axis may be grasped in different modes
  
• Model predicts probability of each mode

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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