Understanding the sensory and motor behavior of accommodation in progressive myopic children

Introduction: Accommodation and vergence, in unison, help in maintaining a clear and single binocular vision, a linchpin for normal vision development. Under natural viewing conditions, accommodation and vergence systems mutually interact with each other through a unique cross-link mechanism. Based on the empirical data from visually normal adults, several control theory models were proposed to predict the behavior of accommodation and vergence. Progressive myopic children, however, exhibit a transient but consistent abnormal accommodative behavior which is not predicted by the currently accepted models. Progressive myopes exhibit high accommodative adaptation, elevated AC output coupled with high accommodative lags. Several predictions, both sensory and motor origin, have been proposed to explain this behavior however they failed to predict this behavior completely. Furthermore, empirical conclusions on the impact of myopia over accommodation and blur sensitivity were mostly based on data obtained from adult population. Accordingly, this dissertation examined the sensory and motor mechanism of accommodation in young children with and without progressive myopia. Data from children were also compared to adults. Methods: 12 children (8-13 years), 6 myopic and 6 non-myopic, and 6 naïve adults (25-33 years) were recruited for the purpose of this dissertation. Varying demands of accommodation were stimulated using a Badal optical system and recorded using custom-built dynamic photorefraction system (DPRS). Theoretical and empirical designs were developed to identify the ideal sampling rate necessary to measure a dynamic accommodative response. First and second order main sequence relationship and strength of the cross-link (CA) response was tested in the first experiment. In the second experiment, blur discrimination thresholds were tested psychophysically using simulated blur targets at two different stimulus demands. Furthermore, objective DOF and sensitivity of accommodative system to blur were examined using known demands of lens defocus and compared across different subject groups. Results: The new custom built high speed photorefractor (DPRS) was found to be superior in measuring dynamic accommodation compared to the commercially available Power Refractor (p