Training of voluntary torsion.

By means of a visual feedback technique, human subjects were trained to make large conjugate cyclorotary eye movements at will. The range of movement increased with training at a rate of approximately 0.8 degrees per hour of practice, reaching 30 degrees at the end of training. Photographs recorded the ability to make voluntary cyclofixations at any amplitude within the subject's range. Cyclotorsional pursuit was also trained, with ability increasing with greater amounts of fisual feedback. In addition, torsional saccadic tracking was trained, showing a magnitude vs. peak velocity relationship similar to that seen for normal saccades. Control experiments indicate that all of these movements were voluntary, with no significant visual induction. With extended practice, large torsional movements could be made without any visual stimulation. The emergence of voluntary torsion through training demonstrates that the oculomotor system has more plasticity than has generally been assumed, reopening the issue as to whether other movements could also be trained to alleviate the symptoms of strabismus.