Neural Foundations of Handedness

Recent findings from our laboratory have revealed substantial differences in coordination between the dominant and non-dominant arms in healthy individuals. Such coordination asymmetries have previously been hypothesized to emerge from differential contributions of each cerebral hemisphere to unilateral arm movements. Indeed, the idea that both hemispheres contribute to unilateral arm movements is well supported by neural activation studies (Kim et al., 1993; Kawashima et al., 1998), as well as previous studies demonstrating motor deficits in the ipsilesional limb of stroke patients (Haaland and Delaney, 1981; Haaland and Harrington, 1994, 1996, 1999; Winstein and Pohl, 1995; Wyke, 1967). It has previously been proposed that the individual contributions of the left and right hemisphere to arm movements reflect the employment of feedforward, and feedback processes, respectively (Haaland and Harrington, 1994, 1996, 1999; Winstein and Pohl, 1995) However, based on our recent findings, we have proposed the dynamic dominance hypothesis, which attributes to the left hemisphere, control of limb and task dynamics, and to the right hemisphere, control of limb stiffness, largely determining the final position of reaching. This hypothesis has been supported by studies that have examined interlimb differences in multijoint reaching (Sainburg and Kalakanis, 2000; Sainburg 2002; Bagesteiro and Sainburg, 2002a), targeted single joint movements (Bagesteiro and Sainburg, 2002b), and studies examining transfer of learning between the limbs (Sainburg and Wang, 2002; Wang and Sainburg, 2003). In contrast to previous hypotheses, we expect that both controllers mediate both feedforward and feedback processes, but that the quality of those processes depends on the characteristics of each controller. In support of our hypothesis, Prestopnik et al., (2002) have reported that patients with left hemisphere stroke show deficits in trajectory control, whereas patients with right hemisphere lesions show deficits in final position accuracy. We expect that further research will provide the link between the feedback/feedforward hypothesis and the dynamic dominance hypothesis of motor lateralization.

Sensory Control of Reaching

The aim of this research program is to discern the neural mechanisms underlying control of multijoint reaching movements in humans. We combine both psychophysical experiments and biomechanical simulations to determine the neural processes underlying control of the complex mechanics of the musculoskeletal system. Because of such dynamics, the relationships between muscle activation and movement kinematics are complex and non-linear. Studies in proprioceptively deafferented patients, who lack sense of joint position and movement, have allowed us to examine the role of different types of sensory information in controlling intersegmental coupling forces (Sainburg et al., 1993, 1995; Ghez and Sainburg, 1995). More recent work, in neurologically intact subjects, has confirmed that the nervous system uses sensory information to develop transient representations, or internal models, of musculoskeletal dynamics, in accord with task specific constraints (Sainburg, Kalakanis, and Ghez, 1999). Computer simulations suggest that such representations are utilized to take advantage of specific mechanical properties of the limb during movement planning (Kalakanis and Sainburg, 1999). Recent findings (Sainburg et. al., 2002; Lateiner et. al., submitted; Brown et al., in revision) suggest that vision and proprioception contribute differentially to the movement planning process. Whereas, accurate proprioceptive information is critical for specifying initial limb conditions, visual information is employed, almost exclusively, for specifying movement direction. In addition, our findings provide further support for the idea that direction and distance are specified through independent neural channels.

Interlimb Transfer of Learning

The tendency for practice of a novel activity with one arm to affect subsequent performance with the other arm has previously been demonstrated for a number of tasks, such as finger tapping (Laszlo, Gaguley, and Bairstow ,1970), keyboard pressing (Taylor and Heilman ,1980), inverted and/or reversed writing (Parlow and Kinsbourne, 1989; 1990; Latash, 1999), figure drawing (Thut et al. 1996), and reaching during coriolis force perturbations (DiZio and Lackner, 1995), and during visuomotor displacements (Elliot and Roy, 1981; Imamizu and Shimojo, 1995). However, the mechanisms underlying this transfer are not well understood. Intermanual transfer of motor adaptation is thought to reflect the sharing of specific learned information between left and right arm control systems. Recent findings from our laboratory support the idea that initial training with one arm can improve subsequent adaptation with the other arm. However, different aspects of control appear to transfer in different directions: Opposite arm training improves the initial direction of dominant arm movements, whereas it only improves the final position accuracy of non-dominant arm movements. This suggests that the direction of transfer depends on the proficiency of the arm controller in question for specifying particular features of movement. Current studies suggest that other types of learning transfer differentially, such that adaptation to novel inertial loads transfers from the dominant to the nondominant arm. The mechanisms of this transfer are currently being probed.

Ipsilesional Motor Deficits in Stroke

Recent studies on motor lateralization have revealed consistent differences in control strategies employed by the dominant and nondominant hemisphere/limb systems that could have implications for hemiplegic stroke patients. Studies in stroke patients have demonstrated deficiencies in the ipsilesional arm that reflect these distinctions, such that patients with right hemisphere damage tend to show deficits in positional accuracy, and patients with left hemisphere damage show deficits in trajectory control. Such deficits have been shown to impede functional performance, a problem amplified in patients who have severe dominant side hemiplegia and must learn to use the non-dominant arm as the primary manipulator for activities of daily living. The purpose of this line of research is to comprehensively examine the coordination deficits in the ipsilesional arm, following unihemispheric brain damage due to stroke. We employ experimental paradigms that have previously demonstrated differences in dominant and non-dominant coordination in healthy subjects Sainburg and Kalakanis, 2000; Sainburg 2002; Bagesteiro and Sainburg, 2002; Sainburg and Wang, 2002; Wang and Sainburg, 2003). In these multidirection reaching tasks, inverse dynamic analysis of segment torques, as well as, electromyography is used to compare differences in trajectory dynamics. Through these studies, we hope to better characterize the motor capacities and impairments in the ipsilesional arm of unilateral lesioned stroke patients. We also hope to better understand the individual contributions of each hemisphere to control of unilateral arm movements.