Adaptive optics visual simulators to study the impact of aberrations on perception

Resumo

Prevalence of myopia has been steadily increasing worldwide. The optical effects of myopia have been intensively studied. However, the neural component of vision has been scarcely considered. Adaptive optics visual simulators (AOVS) allow to understand the effect of optical changes on vision, but most of them are not suitable to high myopes due to their limited measurement and modulation range. An AOVS with enhanced capabilities has been developed in this thesis. The system allowed to expand the application of adaptive optics to highly myopic patients. An electrically tunable lens was used for defocus manipulation, while a liquid-crystal-on-silicon spatial light modulator (LCoS-SLM) was used for high-order aberrations (HOA) correction. A tunable lens was placed in a conjugated plane before a Hartmann-Shack sensor, allowing pre-compensation of defocus. A digital video-projector was used as a stimulus generator providing photopic conditions for the visual testing. Pupil diameter control was implemented by a motorized diaphragm. A detailed description of the developed AOVS is provided in the thesis, followed by a description of calibration for individual components and the complete system itself. Proof-of-concept measurements were done in young adults with high myopia. The visual acuity (VA) was compared using refraction obtained with AOVS and manifest refraction provided by the subjects. VA values when correcting AOVS-obtained refraction was consistently higher than with correction of manifest refraction. HOA of high myopes were also analyzed, being similar to the values found in emmetropic eyes. VA was also obtained for some of the subjects with full wavefront correction, showing a moderate increase. Separation of defocus modulation from the rest of phase modulation has allowed to use diffractive phase masks for longitudinal chromatic aberration (LCA) control. Some aspects of the visual effect of modified LCA are not yet completely understood. As the LCA value is predictable, a number of studies have demonstrated its correction. Some of them found an improvement of VA, while the others did not find it. VA was evaluated through-focus under different modified LCA conditions. Correction of LCA did not provided an increase in through-focus VA, while doubling of the LCA further degraded it. Ray-tracing simulations of the studied LCA conditions coupled with a chromatic eye model was done to better understand the results. A semi-empirical equation predicted VA values from the optical quality parameters. Simulations predicted the behavior of VA in the natural LCA case, showcasing the feasibility of the method. In modified LCA conditions, the correlation between predicted and experimental VA values dropped. A more drastic drop was found in the doubled LCA case. These results suggest that neural and visual compensation phenomena can exist for the natural LCA case. For a deeper understanding of potential neural compensation mechanism for LCA of the eye, an additional experiment was done. VA was measured continuously for a period of time in low and high contrast for natural and corrected LCA. Analysis of the data showed that in the high contrast condition there was no evolution of VA through time, with VA being lower for corrected LCA. In low contrast, VA increased in corrected LCA case after some time, despite an initial drop compared to natural LCA. These findings suggest that some neural adaptation to LCA exists, affecting vision in modified LCA conditions. The instrument extends the benefits of adaptive optics for visual simulation to high myopes, allowing to evaluate their visual performance. Experiments done in modified LCA conditions showed an existence of neural adaptation to different LCA states, resulting in practical implications for the design of optical aids for the eye.