Binocular alignment must be maintained in the horizontal, vertical and torsional planes to ensure binocular sensory fusion. Normal development ensures binocular alignment during fixation and binocular coordination during eye movements. Unfortunately, abnormal visual experience during development usually leads to ocular misalignment (strabismus). In fact, various studies have reported the incidence of strabismus to be about 2-5% of the infant population. Data from strabismic humans and from strabismic monkeys in our laboratory have shown that ocular misalignment is accompanied by a lack of conjugate eye movements. Though strabismus is most often associated with a horizontal misalignment, often a combined horizontal, vertical and torsional misalignment is observed. Along with the static horizontal, vertical and torsional misalignment, there appears to be substantial dynamic cross-talk between the principal eye movement planes. In the clinical literature these apparent cross-axis interactions are usually described as 'A' and 'V' patterns of strabismus. Unfortunately, there is a lack of understanding of the neural or mechanical bases for these cross-axis movements, the putative relationship or lack thereof to the neural control of horizontal, vertical or torsional eye movements and the relationship to the etiology of the strabismus. Competing hypotheses include static malpositioning of extraocular muscle pulleys, sideslip of extraocular muscles and muscle pulleys, torsional control of eye movements gone awry leading to apparent muscle dysfunction and finally simply unexplained overaction/underaction of individual extraocular muscles. The goal of our studies is to clarify static and dynamic properties of cross-axis movements and examine its source in animals with a sensory induced strabismus. Our approach will include structural imaging of extraocular muscle to determine role of muscle pulleys; behavioral experiments to examine control of torsion and Listing's laws; neurophysiological experiments to examine the role of motor and pre-motor structures in the brain and biomechanical modeling of extraocular musculature to simulate experimental data. Completion of our studies will be of benefit to the understanding and treatment of certain types of strabismus.
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