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Flexible representations of dynamics are used in object manipulation.

Ahmed AA, Wolpert DM, Flanagan JR - Curr. Biol. (2008)

Bottom Line: These results indicate that object dynamics can be flexibly represented in different coordinate frames by the brain.We suggest that with experience, the representation of the dynamics of a manipulated object may shift from a coordinate frame tied to the arm toward one that is linked to the object.The additional complexity required to represent dynamics in object-centered coordinates would be economical for familiar objects because such a representation allows object use regardless of the orientation of the object in hand.

View Article: PubMed Central - PubMed

Affiliation: Computational and Biological Learning Lab, Department of Engineering, University of Cambridge, Cambridge, United Kingdom.

ABSTRACT
To manipulate an object skillfully, the brain must learn its dynamics, specifying the mapping between applied force and motion. A fundamental issue in sensorimotor control is whether such dynamics are represented in an extrinsic frame of reference tied to the object or an intrinsic frame of reference linked to the arm. Although previous studies have suggested that objects are represented in arm-centered coordinates [1-6], all of these studies have used objects with unusual and complex dynamics. Thus, it is not known how objects with natural dynamics are represented. Here we show that objects with simple (or familiar) dynamics and those with complex (or unfamiliar) dynamics are represented in object- and arm-centered coordinates, respectively. We also show that objects with simple dynamics are represented with an intermediate coordinate frame when vision of the object is removed. These results indicate that object dynamics can be flexibly represented in different coordinate frames by the brain. We suggest that with experience, the representation of the dynamics of a manipulated object may shift from a coordinate frame tied to the arm toward one that is linked to the object. The additional complexity required to represent dynamics in object-centered coordinates would be economical for familiar objects because such a representation allows object use regardless of the orientation of the object in hand.

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Straight-Pulley Condition(A) Locations of the hands (red circles) and four pulleys used in the straight-pulley condition.(B) Each blue cross represents the median force vector generated by a given participant during catch trials after learning. The thick blue lines show mean force vectors, averaged across participants, and the blue ellipses represent the corresponding 50% confidence ellipses. Each green cross represents the median force vector during transfer trials after learning in the training position. The thick green line shows the mean force vector, averaged across participants, and the green ellipse represents the corresponding 50% confidence ellipse.(C) Each red cross represents the median transfer angle for a single participant, and the thick red line shows mean angles averaged across participants. The shaded areas represent ± 1 SE. Object-centered and arm-centered predictions are represented by dashed and dotted lines, respectively.
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fig5: Straight-Pulley Condition(A) Locations of the hands (red circles) and four pulleys used in the straight-pulley condition.(B) Each blue cross represents the median force vector generated by a given participant during catch trials after learning. The thick blue lines show mean force vectors, averaged across participants, and the blue ellipses represent the corresponding 50% confidence ellipses. Each green cross represents the median force vector during transfer trials after learning in the training position. The thick green line shows the mean force vector, averaged across participants, and the green ellipse represents the corresponding 50% confidence ellipse.(C) Each red cross represents the median transfer angle for a single participant, and the thick red line shows mean angles averaged across participants. The shaded areas represent ± 1 SE. Object-centered and arm-centered predictions are represented by dashed and dotted lines, respectively.

Mentions: It is possible that the difference in generalization between the straight-visible and pulley conditions is related to visual complexity rather than mechanical complexity. Therefore, we ran an additional group of participants by using an object with simple mechanics but high visual complexity. We simulated an elastic band running through a set of four pulleys (Figure 5A). Although visually complex, the mechanics in this straight-pulley condition are identical to those in the transverse straight conditions. The results for this condition are shown in Figures 5B and 5C, which correspond to Figures 2 and 4A, respectively. The blue lines in Figure 5B show the average force vectors generated by the left hand during catch trials delivered (after learning) at the training and transfer positions in phases 1 and 2, respectively. The green line shows the average force vectors during transfer trials delivered at the transfer position in phase 1. Figure 5C shows the average transfer angle—the angle of the force vector in transfer trials (phase 1) minus the angle of the force vector in catch trials delivered at the transfer position (phase 2). The results for the straight-pulley are very similar to those obtained for the straight-invisible band in the transverse configuration (see Figure 2). In particular, transfer was intermediate between object-centered and arm-centered coordinates. Thus, although participants could not exploit the complex visual feedback provided in the straight-pulley condition to form an object-centered representation, the complex visual feedback did not lead to encoding in arm-centered coordinates. In other words, the results suggest that the arm-centered encoding seen in the pulley conditions cannot be explained on the basis of visual complexity alone.


Flexible representations of dynamics are used in object manipulation.

Ahmed AA, Wolpert DM, Flanagan JR - Curr. Biol. (2008)

Straight-Pulley Condition(A) Locations of the hands (red circles) and four pulleys used in the straight-pulley condition.(B) Each blue cross represents the median force vector generated by a given participant during catch trials after learning. The thick blue lines show mean force vectors, averaged across participants, and the blue ellipses represent the corresponding 50% confidence ellipses. Each green cross represents the median force vector during transfer trials after learning in the training position. The thick green line shows the mean force vector, averaged across participants, and the green ellipse represents the corresponding 50% confidence ellipse.(C) Each red cross represents the median transfer angle for a single participant, and the thick red line shows mean angles averaged across participants. The shaded areas represent ± 1 SE. Object-centered and arm-centered predictions are represented by dashed and dotted lines, respectively.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2387196&req=5

fig5: Straight-Pulley Condition(A) Locations of the hands (red circles) and four pulleys used in the straight-pulley condition.(B) Each blue cross represents the median force vector generated by a given participant during catch trials after learning. The thick blue lines show mean force vectors, averaged across participants, and the blue ellipses represent the corresponding 50% confidence ellipses. Each green cross represents the median force vector during transfer trials after learning in the training position. The thick green line shows the mean force vector, averaged across participants, and the green ellipse represents the corresponding 50% confidence ellipse.(C) Each red cross represents the median transfer angle for a single participant, and the thick red line shows mean angles averaged across participants. The shaded areas represent ± 1 SE. Object-centered and arm-centered predictions are represented by dashed and dotted lines, respectively.
Mentions: It is possible that the difference in generalization between the straight-visible and pulley conditions is related to visual complexity rather than mechanical complexity. Therefore, we ran an additional group of participants by using an object with simple mechanics but high visual complexity. We simulated an elastic band running through a set of four pulleys (Figure 5A). Although visually complex, the mechanics in this straight-pulley condition are identical to those in the transverse straight conditions. The results for this condition are shown in Figures 5B and 5C, which correspond to Figures 2 and 4A, respectively. The blue lines in Figure 5B show the average force vectors generated by the left hand during catch trials delivered (after learning) at the training and transfer positions in phases 1 and 2, respectively. The green line shows the average force vectors during transfer trials delivered at the transfer position in phase 1. Figure 5C shows the average transfer angle—the angle of the force vector in transfer trials (phase 1) minus the angle of the force vector in catch trials delivered at the transfer position (phase 2). The results for the straight-pulley are very similar to those obtained for the straight-invisible band in the transverse configuration (see Figure 2). In particular, transfer was intermediate between object-centered and arm-centered coordinates. Thus, although participants could not exploit the complex visual feedback provided in the straight-pulley condition to form an object-centered representation, the complex visual feedback did not lead to encoding in arm-centered coordinates. In other words, the results suggest that the arm-centered encoding seen in the pulley conditions cannot be explained on the basis of visual complexity alone.

Bottom Line: These results indicate that object dynamics can be flexibly represented in different coordinate frames by the brain.We suggest that with experience, the representation of the dynamics of a manipulated object may shift from a coordinate frame tied to the arm toward one that is linked to the object.The additional complexity required to represent dynamics in object-centered coordinates would be economical for familiar objects because such a representation allows object use regardless of the orientation of the object in hand.

View Article: PubMed Central - PubMed

Affiliation: Computational and Biological Learning Lab, Department of Engineering, University of Cambridge, Cambridge, United Kingdom.

ABSTRACT
To manipulate an object skillfully, the brain must learn its dynamics, specifying the mapping between applied force and motion. A fundamental issue in sensorimotor control is whether such dynamics are represented in an extrinsic frame of reference tied to the object or an intrinsic frame of reference linked to the arm. Although previous studies have suggested that objects are represented in arm-centered coordinates [1-6], all of these studies have used objects with unusual and complex dynamics. Thus, it is not known how objects with natural dynamics are represented. Here we show that objects with simple (or familiar) dynamics and those with complex (or unfamiliar) dynamics are represented in object- and arm-centered coordinates, respectively. We also show that objects with simple dynamics are represented with an intermediate coordinate frame when vision of the object is removed. These results indicate that object dynamics can be flexibly represented in different coordinate frames by the brain. We suggest that with experience, the representation of the dynamics of a manipulated object may shift from a coordinate frame tied to the arm toward one that is linked to the object. The additional complexity required to represent dynamics in object-centered coordinates would be economical for familiar objects because such a representation allows object use regardless of the orientation of the object in hand.

Show MeSH
Related in: MedlinePlus