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Curs us Doelgericht Handelen (BPSN33). R.H. Cuijpers, J.B.J. Smeets a nd E. Brenner (2004). On the relation between object shape and grasping kinematics. J Neurophysiol , 91: 2598-2606.

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curs us doelgericht handelen bpsn33

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

  • 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
control variables in grasping
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



control variables in grasping1
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)
control variables in grasping2
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

opposing views on visuomotor control
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
p erception vs action
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.
p erception vs action1
perception vs. action
  • Dorsal pathway for guiding movements (should be veridical)
  • Ventral pathway for perception (perception of shape, colour etc.)
p erception vs action2
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.
common source model
Common source model
  • Franz et al (2000): equal effects of illusion
position vs size
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.
planning vs on line control
Planning vs. on-line control

Glover & Dixon (2001)

  • Relative effect of illusion decreases with time

 Illusion mainly affects planning

position vs orientation
Position vs. orientation

Smeets et al. (2002)

  • Assumption: illusion affects orientation, not position
  • Also explains data of Glover and Dixon
research q uestion how is shape information used for grasping
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
experimental design
Experimental design
  • seven 10cm tall cylinders
  • elliptical circumference with fixed 5cm axis
  • variable axis: 2, 3, 4, 5, 6, 7 and 8 cm
experimental design2
Experimental design
  • Optotrak recorded traces of fingertips
  • 2 distances x 7 shapes x 6 orientations = 84 trials
  • 3 repetitions
  • 10 subjects
which positions
Which positions?
  • Geometry: grasping is stable at principle axes
which posi tions
Which positions?
  • Principle axes preferred. But systematic errors…
which positions1
Which positions?
  • Systematic "errors" depending on orientation.
which positions2
Which positions?
  • Scaling grip orientation  0.7 except for aspect ratios close to 1,  0.5

Scaling grip orientation = slope + 1

comfortable grip
Comfortable grip

Suppose:grip orientation = mixture between cylinder orientation + comfortable grip


Slope a =w-1

Offset b = -(w-1)f0

Thus …
  • Subjects grasp principle axes, but make systematic errors
  • Cannot be explained by comfort of posture
  • Additional effect of deformation of perceived shape
how to get there2
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

Thus …
  • Systematic errors already present in the planning of the movement
  • Maximum Grip Aperture reflects planned size rather than true size
experimental design7
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
constant haptic feedback
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)
constant haptic feedback1
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)
  • 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
veridical haptic feedback
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)
veridical haptic feedback1
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)

With consistent haptic feedback

  • Scalings of grip orientation and grip aperture close to those for real cylinders
  • Less variability between subjects
comparison of experiments
Comparison of experiments

Real Cylinders

Consistent Feedback

Inconsistent Feedback

  • Natural grasping of virtual cylinders requires veridical haptic feedback
  • Grip orientation and grip aperture can be scaled independently
modeling grip planning1
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
modeling grip planning2
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
modeling grip planning3
Modeling grip planning




modeling grip planning4
Modeling grip planning
  • Perceptual errors change the perceived cylinder orientation
  • The comfortable posture may also be uncertain
modeling grip planning5
Modeling grip planning

If distributions are Gaussian with zero mean, we get:

For the circular cylinder w=0, so that:

modeling grip planning6
Modeling grip planning
  • Each grip axis may be grasped in different modes:
  • Model predicts probability of each mode
modeling grip planning7
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
  • 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
  • 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