Transformation from head- to shoulder-centered representation of target direction in arm movements.

In a previous study (Soechting and Flanders 1990a) we suggested that subjects used a coordinate system centered at the shoulder while pointing to targets in extrapersonal space. In particular, we suggested that this coordinate system was used to define target location in terms of its distance and the direction from the shoulder. In this paper we examine this suggestion in more detail. We show that when subjects make errors in the distance of a pointing movement, the computed errors in direction will depend on the origin of the coordinate system chosen to measure direction. From an analysis of the computed error, we estimate the origin of each subject's coordinate system. We artificially induced large errors in pointing distance by asking subjects to point half-way to a target on a line from the shoulder or from the head, that is, in directions from two possible centers. The subjects' performance on both these tasks was comparable to the performance of subjects asked to point directly to the target. From this finding we argue that there exists both a head-centered and a shoulder-centered representation of target location within the central nervous system.