Extraocular muscle forces in alert monkey

We describe an extraocular muscle (EOM) force transducer that provides low-noise signals from an alert animal for several months, is implanted without disinserting the muscle, and is well-tolerated by the body, and present results obtained with the device. The transducer can be used to study orbital statics and dynamics, and oculomotor control signals undiminished by orbital low-pass filtering and antagonistic pairing of muscles. It may provide an index of effective EOM innervation, useful in studies of orbital tissue healing and plasticity, and oculomotor (OM) signal adaptation. During horizontal saccades transducers implanted in the lateral rectus (LR) and medial rectus (MR) of a monkey trained to fixate revealed an agonist muscle tension waveform corresponding to the "pulse-slide-step" pattern of saccadic innervation, and an antagonist waveform that was similar within a scale factor. We never observed transient increases in antagonist force at the ends of saccades (active braking) or at the beginnings. Onset of saccadic force in LR preceded that in MR by 1.6 msec for abducting saccades, and lagged that in MR by 1.1 msec for adducting saccades. During vertical saccades, transient force changes were found in LR and MR, which were likely due, at least in part, to globe translation. LR and MR forces during fixation tended to be largest with the eye about 10 degrees in elevation, and smallest in depression, indicating that effective total innervation was a function of vertical gaze, or that there was variation in the elastic component of muscle force related to orbital geometry, with LR and MR innervation independent of vertical gaze. An exponential decrease in fixation force, having a time constant of about 10 days, was observed after implantation. This may have reflected adaptive muscle lengthening or post-surgical healing.