Task-dependent mixtures of coordinate systems in visuomotor transformations.

In two experiments the involvement of relative and fixed coordinate systems in visuomotor transformations was examined. The experimental task required the successive performance of two movements in each trial, which had to "correspond" to different visual stimuli. One kind of visual display indicated target positions by way of different horizontal positions of a vertical line on a monitor (position mode), while the other indicated movement amplitudes by way of different lengths of a horizontal line (amplitude mode). Formal analysis of variances and covariances of successive individual movements led to the conclusion that in the position mode visuomotor transformations were based on a mixture of relative and fixed coordinate systems, while in the amplitude mode only a relative coordinate system was involved. Thus, visuomotor transformations can be characterized as mixtures of different coordinate systems, and their respective weights in the mixtures are task-dependent.