Three parallel feedback streams communicate the spatial and temporal behavior of the end-point. The z' distance to target (a) is the shortest physical distance the end-point must travel from rest position (z' = 0) to reach the target (z' = 1). Based on distance traveled, the end-point's location along z' prime controls the (b) size of the image formation (image progression) as well as which (c) chords are played (harmonic progression). The rhythmic shape or (d) rhythmic progression is controlled by the end-point's speed. These simultaneous feedback streams can form an integrated descriptor of the (e) velocity profile.
Lehrer et al. Journal of NeuroEngineering and Rehabilitation 2011 8:54 doi:10.1186/1743-0003-8-54