New Findings Help Explain the Dynamics Between The Dominant and Non-Dominant Arm

The phrase, “the right hand doesn’t know what the left hand is doing,” has its roots in a passage of the Bible (Matthew 6:3). If there is truth to this old saying, the reasons may have as much to do with the way the brain obtains information from the arms as it does from the observations of ancient scribes.

Background

Most individuals are either left- or right-handed. How the skills they have learned from the dominant arm (or hand) are transferred to the non-dominant arm have long intrigued physiologists and neurologists.

The transfer of a skill learned in one hand to the other hand has been used as evidence for the role of the brain’s hemispheres in controlling that skill. The movement of knowledge from the dominant to the nondominant arm (D ->ND) has been interpreted as confirmation of the brain’s ability to encode an experience in the dominant hemisphere with the dominant hand and to influence the performance of the nondominant hand. Many researchers believe that this process is accomplished either through connections across both hemispheres or through the same side of the brain. Other scientists believe that transfer in the opposite direction reflects a dominance of the right hemisphere (in right-handers) for some aspects of motor control, so both directions of transfer can be explained with a single model.

Little is known about the involvement of the body’s subcortical structures (such as the cerebellum, and spinal cord) in this process. While it is possible to get some indication of the role of the cerebral hemispheres through the study of subjects with a sectioned corpus callosum, this has rarely been pursued in the case of motor learning and transfer. Accordingly, a team of researchers wondered whether learning a force field with one arm generalizes to the other arm.

Previous observations have found that since learning generalizes in a muscle-like, intrinsic coordinate system for the trained arm, there was little expectation that there would be generalization to the contralateral arm. The scientists found the very surprising result that there was not only strong generalization, but also that it seemed to be with respect to an extrinsic coordinate. To investigate the neural basis of this generalization, they examined an individual who had undergone a complete section of the corpus callosum. Their results provide a significant challenge to current models of how the brain learns reaching movements.

The authors of “Learned Dynamics of Reaching Movements Generalize From Dominant to Nondominant Arm,” are Sarah E. Criscimagna-Hemminger, Opher Donchin, and Reza Shadmehr, from the Laboratory for Computational Motor Control, Department of Biomedical Engineering, Johns Hopkins School of Medicine, Baltimore, MD; and Michael S. Gazzaniga, at the Center for Cognitive Neuroscience, Dartmouth College, Hanover, NH. Their findings appear in the January 2003 edition of the Journal of Neurophysiology.

Methodology

Quantifying inter-arm generalization allowed testing of the sensitivity of these elements to the other arm. Two possible coordinate systems were considered: (1) an intrinsic (joint) representation should generalize with mirror symmetry reflecting the joint’s symmetry and (2) an extrinsic representation, which should preserve the task’s structure in extrinsic coordinates. Both coordinate systems of generalization were compared with a naïve control group.

The researchers tested transfer in right-handed subjects both from dominant to nondominant arm (D ->ND) and vice versa (ND ->D). This led to a 2 × 3 experimental design matrix: transfer direction (D ->ND/ND ->D) by coordinate system (extrinsic, intrinsic, control). Generalization occurred only from dominant to nondominant arm and only in extrinsic coordinates. To assess the dependence of generalization on callosal inter-hemispheric communication, the researchers tested commissurotomy (brain surgery) patient JW. JW showed generalization from dominant to nondominant arm in extrinsic coordinates.

Results

This study produced three main findings.

• First, learning to compensate for dynamics of reaching movements in right-handed individuals generalizes from dominant arm to the nondominant arm (D ->ND) but not vice versa.
• Second, D ->ND generalization in the workspace that we tested (near the midline) is in an extrinsic, Cartesian-like coordinate system.
• Third, generalization of this motor skill does not depend on transfer of information between the hemispheres via the corpus callosum.

Conclusions

The results suggest that when the dominant right arm is used in learning dynamics, the information could be represented in the left hemisphere with neural elements tuned to both the right arm and the left arm. In contrast, learning with the nondominant arm seems to rely on the elements in the nondominant hemisphere tuned only to movements of that arm.

Source: January 2003 edition of the Journal of Neurophysiology.

The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.