(Verbatim from application) Handedness, the manual asymmetry characterized by the tendency to favor one hand for performance of skilled unimanual tasks, is a prominent feature of human motor performance that is believed to result from differences in the neural control of each limb. However, the precise mechanisms responsible for handedness remain controversial. The proposed studies build on our current findings, which indicate interlimb disparities in the control of intersegmental dynamics. Previous research indicates that reaching movements are initially planned in terms of task relevant variables, such as hand movement direction and amplitude (Krakauer and Ghez, 1999), and that this plan must be transformed into muscle activations in order for movement to take place. This transformation relies on internal representations of musculoskeletal and task specific dynamics (Gandolfo, et at, 1996; Goodbody and Wolpert, 1998; Jordan and Rumelhart, 1992; Lackner and Dizio, 1994; Sainburg, et al, 1999; Shadmehr and Mussa-Ivaldi, 1994). We hypothesize that the dominant arm controller is specialized for developing and updating such neural representations. To test this hypothesis, we employ a unique experimental paradigm that we previously developed to investigate learning of novel intersegmental dynamics with the dominant arm (Sainburg et al., 1999). We will analyze movement strategies following adaptation to altered inertial dynamics, imposed by attaching a mass to an outrigger, either medial or lateral to the forearm. Because this manipulation specifically alters the amplitude of interaction torques acting between the segments, we can investigate the extent to which the Central Nervous System (CNS) represents these dynamics and, in turn, utilizes such representations for planning and executing subsequent movements. We will compare interlimb differences in adaptation to novel visual-motor transformations and to novel inertial dynamics, to determine the level of the motor control process at which handedness is expressed. We will investigate differences in both anticipatory mechanisms, as well as, visual and somatosensory based error correction mechanisms. By specifically manipulating the characteristics of movement targets, we will determine whether transfer of learning is greater for movements in which either interaction torques or net torques remain constant. We will then examine interlimb differences in the extent to which learning transfers across changes in the relevant torque. These studies will provide a more thorough understanding of the neural mechanisms underlying handedness, which is critical for clinical rehabilitation applications that address motor learning in patients with unilateral movement deficits.
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