Spatial codes for movement coordination do not depend on developmental vision

When people make oscillating right–left movements with their two index fingers while holding their hands palms down, they find it easier to move the fingers symmetrically (i.e., both fingers towards the middle, then both fingers to the outside) than parallel (i.e., both fingers towards the left, then both fingers towards the right). It was originally proposed that this effect is due to concurrent activation of homologous muscles in the two hands. However, symmetric movements are also easier when one of the hands is turned palm up, thus requiring concurrent use of opposing rather than homologous muscles. This was interpreted to indicate that movement coordination relies on perceptual rather than muscle-based information (Mechsner et al., 2001). The current experiment tested whether the spatial code used in this task depends on vision. Participants made either symmetrical or parallel right–left movements with their two index fingers while their palms were either both facing down, both facing up, or one facing up and one down. Neither in sighted nor in congenitally blind participants did movement execution depend on hand posture. Rather, both groups were always more efficient when making symmetrical rather than parallel movements with respect to external space. We conclude that the spatial code used for movement coordination does not crucially depend on vision. Furthermore, whereas congenitally blind people predominately use body-based (somatotopic) spatial coding in perceptual tasks (Roder et al., 2007), they use external spatial codes in movement tasks, with performance indistinguishable from the sighted.