The precise bifoveation of objects in three dimensional space requires the coordinated action of the vergence, version and accommodative systems. Spatial geometry dictates that relative accommodation of the two eyes and the eye movements required to maintain alignment of the visual axes vary with different distances and directions of gaze. Spatiotopically organized adaptation is required for continual recalibration throughout life in response to developmental and pathological changes and adaptation allows for accurate eye movements in situations where immediate visual error feedback (e.g. disparity and defocus) is unavailable, such as during large changes of gaze. This project will investigate the degree to which binocular oculomotor responses (accommodation and vergence) are pre-programmed by adaptation in response to the geometry of 3-D space. Cross-coupled (associative) responses between accommodation and convergence, accommodation and versional eye position, and vertical vergence and horizontal convergence will be investigated to estimate the precision of human spatial representation in the control of accommodation, convergence and vertical vergence during such tasks as large shifts of gaze from one distance to another or into tertiary eye positions. Perceptual and oculomotor cues to 3-D space will be identified that facilitate adaptation of these cross-coupled responses. We anticipate that the motor responses will conform to the normal demands of 3-D space and they will also be capable of adapting to unusual demands like those produced by pathological states, such as oculomotor paresis. Spatially selective vertical vergence aftereffects are modeled with two approaches: a cross-coupling model in which aftereffects are determined by an association of vertical vergence with information about direction and distance of gaze, and an orbital mechanics model in which after effects are determined by actions of individual extraocular muscles. Proposed experiments on vertical phoria adaptation will discriminate between these models by examining the spatial spread of adapted oculomotor responses to unadapted directions and distances of gaze and by measuring ocular torsion to monitor activity of specific eye muscles during and after training to disparate stimuli. The knowledge gained from these experiments will enhance our understanding of how sensory-motor binocular functions develop and may lead to refined therapeutic techniques for some forms of strabismus by recognizing the limitations of biological adaptive processes.
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