Accurate planning of manual tracking requires a 3D visuomotor transformation of velocity signals

Humans often perform visually guided arm movements in a dynamic environment. To accurately plan visually guided manual tracking movements, the brain should ideally transform the retinal velocity input into a spatially appropriate motor plan, taking the three-dimensional (3D) eye-head-shoulder geometry into account. Indeed, retinal and spatial target velocity vectors generally do not align because of different eye-head postures. Alternatively, the planning could be crude (based only on retinal information) and the movement corrected online using visual feedback. This study aims to investigate how accurate the motor plan generated by the central nervous system is. We computed predictions about the movement plan if the eye and head position are taken into account (spatial hypothesis) or not (retinal hypothesis). For the motor plan to be accurate, the brain should compensate for the head roll and resulting ocular counterroll as well as the misalignment between retinal and spatial coordinates when the eyes lie in oblique gaze positions. Predictions were tested on human subjects who manually tracked moving targets in darkness and were compared to the initial arm direction, reflecting the motor plan. Subjects spatially accurately tracked the target, although imperfectly. Therefore, the brain takes the 3D eye-head-shoulder geometry into account for the planning of visually guided manual tracking.