The long-term goal of these studies is to understand how the brain and the extraocular muscles generate eye movements in three-dimensions (3D). Using a combined approach, consisting of both single unit recordings and electrical microstimulation, we have provided strong neurophysiological evidence in support of the `active pulley hypothesis', which proposes that motoneuron commands are two-dimensional (i.e., horizontal/vertical) and that extraocular muscle pulleys are responsible for providing for an eye position-dependent muscle pulling direction, thus implementing the `half-angle rule'of 3D eye rotations during visually-guided eye movements. In this application, we propose a systematic series of experiments in macaques that focus on the following questions: How do eye movements that violate Listing's law and the half-angle rule (e.g., the rotational vestibulo-ocular reflex, RVOR) work with an eye plant that mechanically implements the half-angle rule? Specifically, how do the eye position-dependent muscle pulling directions become modified during the RVOR and other conditions that deviate from Listing's law and the half-angle rule? We propose to use electrical microstimulation of the abducens nerve and single unit recordings from extraocular motoneurons as tools to understand the particular configuration of the extraocular muscle pulleys during these movements. Specifically, we propose the following aims/experiments: (1) We will use electrical microstimulation of the abducens nerve to characterize extraocular muscle configuration at tilted head/body orientations (static ocular counter-rolling). (2) We will use electrical microstimulation of the abducens nerve to investigate extraocular muscle configuration during convergence. (3) We will test the hypothesis that a coordinated rotation of the pulleys in the same direction as eyeball torsion is responsible for the deviations from the half-angle rule during the roll RVOR by electrically stimulating the abducens nerve during steady-state sinusoidal and transient roll head rotations. (4) As in aim 3, we will investigate the extraocular muscle pulley configuration during combined rotations and translations by evaluating the eye position dependence of the stimulation-evoked eye velocity during combinations of translational VOR (TVOR) and yaw RVOR, conditions that result in diverse eye position dependencies. (5) Finally, once the organization of the oculomotor plant during the VOR is established, we will next characterize and compare neural activities of superior/inferior oblique and superior/inferior rectus as a function of gaze during the roll RVOR and combined RVOR/TVOR. Under these conditions, the eye position dependence of eye velocity deviates significantly from the half-angle rule (default arrangement of the oculomotor plant). We expect to find a correlate of such RVOR/TVOR-driven torsion in the firing rates of cyclovertical motoneurons. Results from these experiments will substantially expand our current understanding of pulley and extraocular muscle function during eye movements that violate Listing's law.
Strabismus, the misalignment of the two eyes, is a disorder of unknown etiology, caused by problems related to either the central innervation sent to the eye muscles and/or the eye muscles and orbital tissues themselves. Improving our understanding of the central and peripheral aspects of eye movement control is an important and necessary step for an effective clinical diagnosis and treatment of strabismus patients. Results from the proposed experiments are also fundamental in resolving on-going controversies and providing a comprehensive understanding of oculomotor function in health and disease.
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