NEW MODELS OF THE OCULOMOTOR MECHANICS BASED ON DATA OBTAINED WITH CHRONIC MUSCLE FORCE TRANSDUCERS

Several phenomenological models of the oculomotor mechanics that produ ce saccadic eye movements have been developed. These models have been based on measurements of macroscopic muscle and orbital tissue propert ies and measurements of eye kinematics during saccades. We recorded th e forces generated by the medial and lateral recti during saccades in an alert, behaving monkey using chronically implanted force transducer s. With this new data, we tested the ability of the classic saccade mo dels to generate realistic muscle force profiles. Errors in the predic tions of the classic saccade models led to a reexamination of the curr ent models of extraocular muscle. Both a phenomenological, Hill-type m uscle model and an approximation to Huxley's molecular level muscle mo del based on the crossbridge mechanism of contraction (distribution mo ment model) were derived and studied for monkey extraocular muscle. Si mulations of the distribution moment model led to insights suggesting (i) specific modifications in the lumped force/velocity relationship i n the Hill-type model that resulted in this type of phenomenological m odel being able to generate realistic dynamics in extraocular muscle d uring saccades; (ii) the distribution of activity in the different fib er types in extraocular muscle may be central to the characteristics e xhibited by the muscle during saccades; (iii) the transient properties of lengthening muscle such as yielding are not significant during sac cades; and (iv) the series elastic component in active muscle may be p redominantly generated by the elastic properties of the cross-bridges.