The processes which develop to coordinate eye and hand movements in response to motion of a visual target were studied in young children and adults. We have shown that functional maturation of the coordination control between eye and hand takes place as a result of training. We observed, in the trained child and in the adult, that when the hand is used either as a target or to track a visual target, the dynamic characteristics of the smooth pursuit system are markedly improved: the eye to target delay is decreased from 150 ms in eye alone tracking to 30 ms, and smooth pursuit maximum velocity is increased by 100%. Coordination signals between arm and eye motor systems may be responsible for smooth pursuit eye movements which occur during self-tracking of hand or finger in darkness. These signals may also account for the higher velocity smooth pursuit eye movements and the shortened tracking delay when the hand is used as a target, as well as for the synkinetic eye-arm motions observed at the early stage of oculo-manual tracking training in children. We propose a model to describe the interaction which develops between two systems involved in the execution of a common sensorimotor task. The model applies to the visuo-oculo-manual tracking system, but it may be generalized to other coordinated systems. According to our definition, coordination control results from the reciprocal transfer of sensory and motor information between two or more systems involved in the execution of single, goal-directed or conjugate actions. This control, originating in one or more highly specialized structures of the central nervous system, combines with the control processes normally operating in each system. Our model relies on two essential notions which describe the dynamic and static aspects of coordination control: timing and mutual coupling.

Oculo-manual tracking of visual targets: control learning, coordination control and coordination model

The study was aimed at defining the role of hand (and arm) kinaesthetic information in coordination control of the visuo-oculo-manual tracking system. Baboons were trained to follow slow-moving and stepping visual targets either with the eyes alone or with the eyes and a lever moved by the forelimb about the vertical axis. A LED was attached to the lever extremity. Four oculo-manual tracking condidtions were tested and compared to eye-alone tracking: Eye and hand tracking of a visual target presented on a screen, eye tracking of the hand, and eye tracking of an imaginary target actively moved by the arm. The performance of the animals evaluated in terms of latency, and velocity and position precision for both eye and hand movements was seen to be equivalent to that of humans in similar situations. After dorsal root rhizotomy (C1-T2) the animals were unable to produce slow arm motion in response to slow-moving targets. Instead, they produced successions of ballistic-like motions whose amplitude decreased as retraining proceeded. In addition, the animals could not longer respond with smooth pursuit eye movements to an imaginary target actively displaced by the animal's forelimb. It was concluded that the absence of ocular smooth pursuit after lesion results from the disruption of a signal derived from arm kinaesthetic information and addresses to the oculomotor system. This signal is likely to be used in the control of coordination between arm and eye movements during visuo-oculo-manual tracking tasks. One cause of the animal's inability to achieve slow arm movement in response to slow target motion is thought to be due to a lesion-induced alteration of the spinal common pathway dynamics which normally integrate the velocity signal descending from the arm movement command system.

F. Mussa Ivaldi

Papers

Etude des troubles oculomoteurs affectant certains enfants infirmes moteurs cerebraux: semiologie instrumentale et possibilite de reeducation