SELF-MOVED TARGET EYE TRACKING IN CONTROL AND DEAFFERENTED SUBJECTS -ROLES OF ARM MOTOR COMMAND AND PROPRIOCEPTION IN ARM-EYE COORDINATION

1. When a visual target is moved by the subject's hand (self-moved tar get tracking), smooth pursuit (SP) characteristics differ from eye-alo ne tracking: SP latency is shorter and maximal eye velocity is higher in self-moved target tracking than in eye-alone tracking. The aim of t his study was to determine which signals (motor command and/or proprio ception) generated during arm motion are responsible for the decreased time interval between arm and eye motion onsets in self-moved target tracking. 2. Six control subjects tracked a visual target whose motion was generated by active or passive movements of the observer's arm in order to determine the role played by arm proprioception in the arm-e ye coordination. In a second experiment, the participation of two subj ects suffering complete loss of proprioception allowed us to assess th e contribution of arm motor command signals. 3. In control subjects, p assive movement of the arm led to eye latencies significantly longer ( 130 ms) than when the arm was actively self-moved(-5 ms: negative valu es meaning that the eyes actually started to move before the target) b ut slightly shorter than in eye-alone tracking (150 ms). These observa tions indicate that active movement of the arm is necessary to trigger short-latency SP of self-moved targets. 4. Despite the lack of propri oceptive information about arm motion, the two deafferented subjects p roduced early SP (-8 ms on average) when they actively moved their arm s. In this respect they did not differ from control subjects. Active c ontrol of the arm is thus sufficient to trigger short-latency SP. Howe ver, in contrast with control subjects, in deafferented subjects SP ga in declined with increasing target motion frequency more rapidly in se lf-moved target tracking than in eye-alone tracking. 5. The deafferent ed subjects also tracked a self-moved target while the relationship be tween arm and target motions was altered either by introducing a delay between arm motion and target motion or by reversing target motion re lative to arm motion. As with control subjects, delayed target motion did not affect SP latency. Furthermore, the deafferented subjects adap ted to the reversed arm-target relationship faster than control subjec ts. 6. The results suggest that arm motor command is necessary for the eye-to-arm motion onset synchronization, because eye tracking of the passively moved arm was performed by control subjects with a latency c omparable with that of eye-alone tracking of an external target. On th e other hand, as evidenced by the data from the deafferented subjects, afferent information does not appear to be necessary for reducing the time between arm motion and SP onsets. However, afferent information appears to contribute to the parametric adjustment between arm motor c ommand and visual information about arm motion.