Perceptual basis of bimanual coordination

Periodic bimanual movements are often the focus of studies of the basic org anizational principles of human actions(1-25). In such movements there is a typical spontaneous tendency towards mirror symmetry. Even involuntary sli ps from asymmetrical movement patterns into symmetry occur, but not vice ve rsa. Traditionally, this phenomenon has been interpreted as a tendency towa rds co-activation of homologous muscles, probably originating in motoric ne uronal structures. Here we provide evidence contrary to this widespread ass umption. We show for two prominent experimental models-bimanual finger osci llation(1) and bimanual four-finger tapping(2)-that the symmetry bias is ac tually towards spatial, perceptual symmetry, without regard to the muscles involved. We suggest that spontaneous coordination phenomena of this kind a re purely perceptual in nature. In the case of a bimanual circling model, o ur findings reveal that highly complex, even 'impossible' movements can eas ily be performed with only simple visual feedback. A 'motoric' representati on of the performed perceptual oscillation patterns is not necessary. Thus there is no need to translate such a 'motoric' into a 'perceptual' represen tation or vice versa, using 'internal models' (ref. 29). We suggest that vo luntary movements are organized by way of a representation of the perceptua l goals, whereas the corresponding motor activity, of sometimes high comple xity, is spontaneously and flexibly tuned in.