The exquisite precision with which the eyes acquire, pursue and fixate visual targets appears to stand in contrast to the more gross methods used to correct abnormalities in the system. Extraocular muscles (EOMs) may be surgically shortened or repositioned to compensate for inappropriate motor activity. Botulinum toxin type A can be injected in order to weaken a particular muscle so that it may perform better in relation to other muscles, although force changes after such injections have not been systematically studied. The predicted outcome of these measures can be unreliable and the interventions may need to be repeated in the same patient because it's often difficult to obtain the proper alignment of the eyes with a single procedure. While the clinical effectiveness of these strategies is unquestioned, there is obvious need to improve their precision and predictability. This proposal, using cats and monkeys, will primarily focus on two related aspects of eye movement control exposed by perturbing the normal system. I) How does EOM contractile force change from O-2 months post botulinum toxin injection and do those changes directly relate to eye displacement changes? 2) How precise are motoneuron MN firing patterns during repeated identical movements and how might that precision be altered after botulinum toxin injection? Additionally, is there a relationship between VIth nerve branching and orbital and global layer of the lateral rectus muscle? Studies of EOM electromyography (EMG), muscle immuno-histochemistry and myosin expression will be carried out concurrently with the examination of these questions. The correlative evaluation of MN firing, whole muscle plus motor unit force, muscle cytology and EMG measures with eye displacement is unique and unavailable in either normal or botulinum toxin treated motor systems. It is hoped that these studies will provide information critical to clinicians seeking to improve patient outcomes as well as basic researchers who want to understand the complex dynamics of eye movements.