ABSTRACT
It has been commonly accepted that gaze saccade orienting toward a visual target and hand pointing toward the same target are two fundamentally different processes based on specific organization rules. Orientation of gaze is considered to be an open-loop system in which the vestibular signal, through the vestibulo-ocular reflex, mathematically adds to the saccadic command when the head is free to move. By contrast, hand movements have been thought to operate under two sequential modes of control, with a first preprogrammed command distributing the synergies among muscles, followed by a closed-loop process under visual control using the simultaneous vision of hand and target as an error signal.
Two main experiments have been performed, which reveal basic properties common to both systems. It is first shown through passive motion of the head that a gaze saccade is not an open loop process, and that it is very likely controlled by a central feedback loop. In hand pointing, random artificial errors are generated at the onset of the response, by changing the target location unknown to the subject; these errors are automatically corrected by smooth and early modifications, without adding any additional reaction time. These results suggest that continuous internal feedback loops correct unconsciously the dynamic errors throughout the movement.
