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.
