|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 <0.001). The current dissertation makes four major contributions to the field of accommodation and myopia: first, a motor deficit such as a sluggish or rigid plant cannot explain the abnormal behavior of accommodation in myopes. This was established based on the findings of a similar first and second order main sequence relationship between myopes and non-myopes (p > 0.05). Also, CA/C measures were not found to be attenuated as would be expected with a rigid plant. Novel simulations with reduced blur sensitivity coupled with a motor recalibration of AC gain did predict the myopic behavior. Second, atypical response patterns of accommodation such as the dynamic overshoots and double step responses were reported to exist with blur-driven accommodation. These patterns were suggested to occur due to an incorrect initiation of the response based on the similar main sequence findings between typical and atypical responses. Refractive error had no influence on the frequency of the atypical response in children. However, the influence of age reflects a developmental pattern in these dynamic atypical responses. Third, young progressive myopes showed large DOF and reduced accommodative sensitivity to lens defocus compared to non-myopes and adults. However, the ability to detect blur was similar between myopes and non myopes. Finally, depth of focus was found to increase with an increase in the stimulus demand. This change in the depth of focus was strongly correlated to a change in the microfluctuations but was independent of changes in the pupil size.
Discussion: These findings confirm that school aged myopic children exhibit an altered pattern of accommodation. The pattern appears to be associated to progressive myopia but cannot be readily explained by current motor models of accommodation and vergence. Empirical data and model simulations suggest that a sluggish or rigid motor plant (lens and ciliary muscle) cannot predict this altered behavior completely. The current dissertation speculates a reduced sensory gain (or reduced blur sensitivity) as the prime factor coupled with a compensatory motor recalibration in the accommodative convergence cross link. The large lags of accommodation reflect a large objective depth of focus in myopes. However, the capacity to detect blur was found to be independent of the retinal defocus present in the myopic eyes. The elongating eye of a progressive myopes appears to compensate the persistent retinal defocus on the retina by some form of a perceptual adaptation.