Do muscles matter for coordinated action?

This article investigates coordination stability when 2 fingers of each hand periodically tap together. The main question concerns the functional origin of the symmetry tendency, which has widely been conceived as a bias toward coactivation of homologous fingers and homologous muscular portions. In Experiment 1, the symmetry tendency was independent of finger combination. In Experiment 2, virtually identical stability characteristics were revealed under full vision and no vision. In Experiment 3, symmetrical and parallel visual labels on the fingers neither stabilized nor destabilized symmetrical and parallel tapping patterns. In Experiment 4, in which the relative position of the hands was varied, it revealed that the observed stability characteristics are to be defined in a hand-centered reference frame. Because the symmetry tendency was always independent of finger combination, the authors suggest that it is not a bias toward coactivation of homologous muscle portions but instead originates on a more abstract, functional level

Perceptual basis of bimanual coordination

Periodic bimanual movements are often the focus of studies of the basic organizational principles of human actions1±25. In such movements there is a typical spontaneous tendency towards mirror symmetry. Even involuntary slips from asymmetrical movement patterns into symmetry occur, but not vice versa. Traditionally, this phenomenon has been interpreted as a tendency towards co-activation of homologous muscles, probably originating in motoric neuronal structures. Here we provide evidence contrary to this widespread assumption. We show for two prominent experimental models – bimanual four-finger oscillation and bimanual four-finger tapping – that the symmetry bias is actually towards spatial, perceptual symmetry, without regard to the muscles involved. We suggest that spontaneous coordination phenomena of this kind are purely perceptual in nature. In the case of a bimanual circling model, our findings reveal that highly complex, even `impossible' movements can easily be performed with only simple visual feedback. A `motoric' representation of the performed perceptual oscillation patterns is not necessary. Thus there is no need to translate such a `motoric' into a `perceptual' representation or vice versa, using `internal models' (ref. 29). We suggest that voluntary movements are organized by way of a representation of the perceptual goals, whereas the corresponding motor activity, of sometimes high complexity, is spontaneously and flexibly tuned in