It is common knowledge that movements of the arms are naturally coupled. For instance, certain modes of temporal coupling are more natural for the system than others. In-phase (clapping) and anti-phase (walking) rhythms can be maintained at higher frequencies than other phase relationships; further, in-phase rhythms are more stable than anti-phase rhythms (Tuller and Kelso 1989). Spatial coupling of movements also exists: subjects easily produce circles or lines with both hands simultaneously, but when they draw lines with one hand and circles with the other, they make ovals with both hands (references in 5, 15).
To a neurophysiologist, these aspects of bimanual movements raise questions. Where is the coupling produced? What neural mechanisms produce it? For instance, does one side of the nervous system control the other, or is there a cooperative interaction of the two sides? What mechanisms accomplish decoupling? Since many bimanual synergies can be achieved with practice—from playing the drums to typing—there must be neural mechanisms that modify default coupling. What are these mechanisms, and where are they localized?
