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Mon, April 26

Speaker: Avi Barliya

Title: Control strategies revealed through representations

Abstract: In this talk I will present two different studies I conducted in the last few years. The first study deals with human locomotion, specifically with the law of intersegmental coordination (Borghese et al. 1996). The law states that when plotting the elevation angles of the thigh, shank and foot with respect to each other, a pattern of covariation appears to reside on a plane.  The orientation of the plane was correlated to gait speed and energy expenditure (Bianchi et al. 1998). This phenomenon, being highly robust, does not occur when one examines instead the joint/anatomical angles. I will present an analytical model which describes the properties and conditions for the emergence of the planar covariation (Barliya et al. 2009).

The second study focuses on the kinematic redundancy problem existing for the human arm and particularly at the level of the joint.versus the hand spaces. The inverse-kinematics problem which is widely discussed in the literature investigates how for a desired trajectory described in terms of hand-space coordinates a suitable unique trajectory is produced at the joint-space. I will show results of using a novel method developed by Omlor & Giese (2007) for dimensionality reduction, applied to both the hand-space and joint-space trajectories. Interestingly, when a particular representation for the joint-space is chosen, a striking similarity between the set of sources (basis functions) used at the hand and joint spaces appears. Furthermore, I will demonstrate the functionality of this observation by claiming that once these two spaces share the same set of sources, one can go back and forth between the spaces. Thus, the shared set of sources may serve as a mediator to go from the hand to joint-space trajectories.

Finally, the thread that connects these two studies is the fact that the respective phenomenon emerges only in one representation. Moreover, the results for both cases are consistent with each other with respect to the representation for which the respective rule/law emerges.  In this representation the configuration space is described in terms of orientation (absolute) angles, using an external frame of reference. Implications and questions related to this observation will be discussed.