New motor lateralization model verified through virtual reality experiments could revolutionize post-stroke rehabilitation
Comparison of hand-paths between (A) representative left healthy (LHC) and right healthy control (RHC) subjects, and (B–E) patients with left (LHD) and right hemisphere damage (RHD) across severity of hemiparesis (Fugl-Meyer score). Credit: Sainburg Lab.
By: Seth Palmer
This article is the third and final of a series exploring the diversity of interests and variety of experimental approaches represented by neuroscientists at the Huck Institutes.
Investigating subjects ranging from neural response and habituation to alcohol, to mechanisms of motor control, to effects of neural injury, these researchers are devising unique methodologies employing a wide variety of technologies and techniques, and they are making discoveries with the potential to change the way we experience our world.
Since the 1860s, neuroscientists have known that the human brain is organized into two hemispheres, each of which is responsible for different functions; known as neural lateralization, this functional division has significant implications for the control of movement, and is, in fact, familiar to all of us in the phenomenon of handedness.
Understanding the connections between neural lateralization and motor control is crucial to many applications — among them, the rehabilitation of stroke patients — but while most people intuitively understand handedness, the neural foundations underlying motor asymmetry have, until recently, remained elusive.
New research, though — published in the journal Brain by Huck Institutes affiliate Robert Sainburg and his colleagues in the Center for Motor Control — has revealed a new model of motor lateralization that accounts for the neural foundations of handedness, and this discovery could change the way post-stroke rehabilitation is designed from the ground up.
“Each hemisphere of the brain is specialized for different aspects of motor control, and thus, each arm is 'dominant' for different features of movement,” explained Sainburg, a professor of kinesiology and neurology at Penn State who participates in the Huck Institutes' neuroscience and physiology graduate programs. “The ‘dominant’ arm is used for applying specific force sequences — such as when slicing a loaf of bread with a knife — and the other arm is used for impeding forces to maintain stable posture, such as holding the loaf of bread; together these specialized control mechanisms are seamlessly integrated into every day activities.”
“Our research has shown that this integration breaks down in neural disorders such as stroke, which produces different motor deficits depending on whether the right or left hemisphere has been damaged,” Sainburg continued. “Traditionally, physical rehabilitation professionals have used the same protocols to practice movements of the paretic arm, regardless of the hemisphere that has been damaged. Our research shows that each arm should be treated for different control deficits, and it also indicates that therapists should directly retrain patients in how to use the two arms together in order to recover function.”
In preparing to test their model, Sainburg and his team needed to precisely select study participants based on very specific criteria in order to accurately distinguish the motor control mechanisms specific to each brain hemisphere.
“We selected patients from a large pool of potential participants from two stroke research centers, at the New Mexico Veteran’s Administration Hospital and at the Hershey Medical Center,” said Sainburg. “In order to examine our specific ideas about brain mechanisms, we used structural MRI to match both the size and location of brain lesions between right and left hemisphere stroke patients, and to rule out any bilateral lesions and lesions of subcortical structures in the base of the brain or the brainstem. In addition, patients were matched for demographic factors, such as age, education, and gender, as well as for extent of motor impairment, measured by clinical assessments.”
Once the participants had been selected, they were asked to perform a series of tasks on a virtual reality interface, programmed and designed by Sainburg, which allowed the researchers to record detailed 3D position and motion data.
Each participant sat with the arms supported over a tabletop by an air-jet system, which was used to eliminate the potentially fatiguing effects of gravity and friction, and sensors were placed at various points on the participant's arm and on the fingertip. A high-definition television (HDTV) was used to project a start point, target(s), and a cursor representing the participant's fingertip position onto a mirror that was positioned parallel to the tabletop and above the participant's arm so it would appear to be in the same horizontal plane as the participant's fingertip, and the participant was then asked to move the cursor from the start point to the target in a single rapid motion. Stroke patients performed the task using their contralesional arm (opposite the side of the lesion), and control subjects used either their left or right arm, depending on their categorization as right- or left-hemisphere healthy, respectively.
Results and implications
The data for all the participants' hand trajectories and final positions were then aggregated to compare the effects of left versus right hemisphere damage on different aspects of control.
“Our results indicated that while both groups of patients showed similar clinical impairment in the contralesional arm, this was produced by different motor control deficits,” Sainburg explained. “Right hemisphere damaged patients were able to make straight movements that were directed toward the targets, but were unable to stabilize their arms in the targets at the end of motion. In contrast, left hemisphere damaged patients were unable to make straight and efficient movements, but had no difficulty stabilizing their arms at the end of motion. These results confirmed that each hemisphere contributes unique control to its contralesional arm, verifying why our arms seem different when we use them for the same tasks.”
The results mirror those of Sainburg's prior studies of motor deficits in unilateral stroke patients — focused on the ipsilesional arm — which formed the basis for his model of lateralization. Now that the same deficits have been observed on both sides of the body, it appears that Sainburg's model has been undeniably confirmed.
“Because both arms in stroke patients show motor deficits that are specific to the hemisphere that was damaged, we have concluded that the left arm is not simply controlled with the right hemisphere and vice versa,” Sainburg said. “This is a revolutionarily new perspective on sensorimotor control: each hemisphere contributes different control mechanisms to the coordination of both arms, regardless of which arm is considered 'dominant'.”
Sainburg and his colleagues are currently designing follow-up studies that will aid the development of new rehabilitation protocols addressing the specific motor deficits associated with each hemisphere; these studies involve using robotics and virtual reality to selectively require the specialization of either the right or left hemisphere during motor tasks, and to determine how the two hemispheres are integrated when both arms are used together.
“Bilateral activities comprise most activities of daily living,” Sainburg explained, “and our new research suggests specific neural mechanisms are required for this integration, which might be selectively improved through motor training in rehabilitation. By exploiting virtual reality, we can require both arms to work together to manipulate virtual objects on the computer screen — a type of motor practice which is often impossible in the real world, due to the fact that finger movements often experience the most profound weakness and paresis following stroke. We are hoping that by integrating these cutting-edge technologies with our new understanding of neural control, we can introduce novel treatment approaches that will revolutionize physical rehabilitation for stroke patients.”
This work was supported by grants from the National Institutes of Health, National Institute for Child Health and Human Development, Department of Veterans Affairs Research and Development Medical Merit Review, and Rehabilitation Research and Development.
More about Dr. Sainburg
Robert Sainburg is a co-funded faculty member of the Huck Institutes. His academic appointment is split between the Department of Kinesiology at University Park and the Department of Neurology in Hershey, and he maintains active laboratories on both campuses.
- Mani S
- Mutha PK
- Przybyla A
- Haaland KY
- Good DC
- Sainburg RL
- Contralesional motor deficits after unilateral stroke reflect hemisphere-specific control mechanisms
- Brain 136 (4): 1288-1303