curs us doelgericht handelen (bpsn33)

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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. - PowerPoint PPT Presentation


  • 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

  • Topics1st hour: Control Variables in GraspingOpposing views on visuomotor controlResearch question2nd hour: Grasping elliptical cylindersReal cylindersWhich positions?How to get there?Virtual cylindersConstant haptic feedbackVeridical haptic feedbackIf time permits: Modeling grip planningConclusions

  • Control variables in graspingMany levels of description:Activity motor neuronsMuscle activity (EMG)Posture (Joint angles)Kinetics (Forces, torques)Kinematics (Position, speed etc.)Task levelDegrees of Freedom(DoF)lowhigh

  • Control variables in graspingHow does the brain plan/compute the desired motor neuron output?

    If movements are planned in task space:little computational power needed for planning stageBut 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 graspingWhat 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 feedbackMeasure effect on motor output

    Variables that have an effect are controlledVariables that have no effect are redundant

    Haptic = by touch Proprioceptor = sensory receptor in muscles, tendons or joints

  • Opposing views on visuomotor controlFingertip positions and object sizeMilner & Goodale: perception vs. actionFranz et al: common source modelSmeets & Brenner: position vs. size

    Fingertip positions and object orientationGlover & Dixon: planning vs. on-line controlSmeets & Brenner: position vs. orientation

  • perception vs. actionGoodale (1993); Milner, Goodale (1993)

    RV: lesions in occipito-parietal cortex (dorsal).DF: damage in ventrolateral occipital areas due to CO poisoning.










    Ventrolateral oocipital

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

    Dorsal = Action

    Ventral = Perception

  • perception vs. actionAgliotti, De Souza, Goodale (1995):Grip aperture NOT influenced by size-illusion.Due to separate processing of information for perception and action.

  • Common source modelFranz et al (2000): equal effects of illusion

  • Position vs. sizeBrenner, 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 controlGlover & Dixon (2001)

    Relative effect of illusion decreases with time Illusion mainly affects planning

  • Position vs. orientationSmeets et al. (2002)Assumption: illusion affects orientation, not position

    Also explains data of Glover and Dixon

  • Research Question: How is shape information used for grasping?The visually perceived shape is deformedShape (ventral) determines where it is best to grasp an object (dorsal)Grip locations not veridicalShape information could be used during planning (ventral) or on-line control (dorsal)Grip errors arise early or late in the movement

  • Grasping elliptical cylinders:real cylinders

  • Experimental designseven 10cm tall cylinderselliptical circumference with fixed 5cm axisvariable axis: 2, 3, 4, 5, 6, 7 and 8 cm

  • Experimental Design

  • Experimental designOptotrak recorded traces of fingertips2 distances x 7 shapes x 6 orientations = 84 trials3 repetitions10 subjects

  • Experimental Design

  • Example

  • Which positions?Geometry: grasping is stable at principle axes

  • Which positions?Principle axes preferred. But systematic errors

  • Which positions?Systematic "errors" depending on orientation.

  • Which positions?

    Scaling grip orientation 0.7 except for aspect ratios close to 1, 0.5Scaling grip orientation = slope + 1

  • Comfortable gripPrediction:Slope a = w-1Offset b = -(w-1)f0 Suppose: grip orientation = mixture between cylinder orientation + comfortable grip

  • Thus Subjects grasp principle axes, but make systematic errorsCannot be explained by comfort of postureAdditional effect of deformation of perceived shape

  • How to get there?

  • How to get there?

  • How to get there?High correlation despite errors!Sudden drop at end: Grip aperture automatically correctedCorrelation much higher for max. grip aperture than final grip apertureGradual increase: grip errors were planned that way

  • Thus Systematic errors already present in the planning of the movement

    Maximum Grip Aperture reflects planned size rather than true size

  • Grasping virtual cylinders

  • Experimental design

  • Experimental Design

  • Experimental design

  • Experimental designConstant haptic feedback:Real cylinder is always circularVirtual cylinders: 15 aspect ratios, 3 orientations

    Veridical haptic feedback:Virtual and real cylinders are the same, 7 aspect ratios and 2 orientations

  • Constant haptic feedbackOnly 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 feedbackSubjects hardly scale their max. grip aperture

    Scaling of max. grip aperture is much smaller than for real objects (0.14 instead of 0.57)

  • ThusInconsistent 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 feedbackSimilar pattern of grip orientations for all subjectsScaling of grip orientation (0.58) close to those for real objects (0.60)

  • Veridical haptic feedbackAll subjects adjust their maximum grip aperture

    Scaling of max. grip aperture (0.39) much higher and closer to real objects (0.57)

  • ThusWith consistent haptic feedbackScalings of grip orientation and grip aperture close to those for real cylinders

    Less variability between subjects

  • Comparison of experimentsReal CylindersConsistent FeedbackInconsistent Feedback

  • ThusNatural grasping of virtual cylinders requires veridical haptic feedback

    Grip orientation and grip aperture can be scaled independently

  • Modeling grip planning

  • Modeling grip planningPhysical constraintsGrip force through centre of massGrip force perpendicular to surfaceOptimal grip along major or minor axisBiomechanical constraintsFor a given cylinder location there is a most comfortable gripEvident when grasping circular cylinder

  • Modeling grip planningAssumptions:The planned grip orientation is a weighted average of the optimal and the comfortable grip orientationThe weights follow from the expected cost functions for comfort and mechanical stability

  • Modeling grip planningIfThen(required)

  • Modeling grip planningPerceptual errors change the perceived cylinder orientation

    The comfortable posture may also be uncertain

  • Modeling grip planningIf distributions are Gaussian with zero mean, we get:

    For the circular cylinder w=0, so that:

  • Modeling grip planningEach grip axis may be grasped in different modes:

    Model predicts probability of each mode

  • Modeling grip planningThe model describes the relative costs for grip comfort and mechanical stabilityIt predicts the relative probability of choosing the major or minor axisWe can incorporate biases in the perceived cylinder orientationWe can extend to more general shapes

  • ConclusionsSubjects plan their grasps to suboptimal locations based on the perceived shape and the anticipated (dis)comfortUpon touching the surface the errors are correctedHaptic feedback is necessary for natural graspingWith our model we can identify relative contributions of comfort, stability and perceptual errors

  • ConclusionsVisual shape information (slant, curvature) is used for planning suitable grip locations (position information)Perceptual biasBias due to comfort of postureNo substantial on-line corrections On-line control uses position informationWhen inconsistent, haptic and visual shape information is combined differently for the planning of grip aperture and grip orientation

  • The end

  • Stable grip of an ellipse

    ReadExplain taskExplain starting position handThe positions of the fingertips are recorded with an Optotrak camera-systemTop view of the finger traces when the object is placed at 60cmIndicate subject positionIndicate trace of thumbIndicate trace of index fingerAspect ratio is 1.4

    Comment pick-up position: near short axis

    As the object rotates 30 degrees, hand rotates along with it

    You see that finger end- positions remain close to the short axis

    rotating the object again 30 degrees, we see the same thing


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