Study of accommodation dynamics and defocus fluctuations in the human eye

Laburpena

The quality of the retinal image is the first, physical limiting factor of visual quality. Defocus is the most common source of blur leading to retinal image quality loss. It depends on the interrelationship between the eye's axial length, optical power, and distance to the object. Until it is lost with age, the eye has the ability to modify its optical power (i.e., to accommodate) to produce focused retinal images. Although this process is not instantaneous, accommodation is a fast and fairly accurate mechanism in most young subjects, that results in a clear vision. However, it has been suggested that myopia onset and/or progression may be related to alterations in the accommodative process that could upset the emmetropization process. On the other hand, even when steadily looking at an object at a fixed distance, the optical power of the eye fluctuates more or less randomly. It is unclear if this fluctuation is an undesired inability of the eye to keep a constant focus or may serve a purpose in the accommodative process. In any case, from an optical point of view, fast fluctuations of defocus would be expected to produce some kind of blurring in the retinal images. In those circumstances, a short integration time may allow the visual system to select the best focused position in the sequence to maximize visual quality. In this context, this thesis studies the effects of changes in focus, both discrete and progressive, aiming to discern how the visual system copes with them. Two separate experiments were carried out with an open-view Hartmann-Shack sensor measuring refraction and high-order aberrations in real time. First, the dynamics of the accommodative response was analyzed in realistic binocular viewing conditions, both for emmetropic subjects and myopes, when the fixation abruptly changed from far to near. In a second experiment, we studied the effect on contrast sensitivity of fast oscillations of defocus with different magnitudes and temporal frequencies, generated with a tunable lens attached to the system. During the accommodation mechanism, convergence of the eyeballs and miosis of the pupils accompany the change in optical power of the crystalline lens. There is extensive literature on these processes but relatively few studies simultaneously measuring all three of them in binocular vision. To the best of our knowledge, this is the first study of their combined dynamics in real time under realistic viewing conditions. Furthermore, it was performed in both myopic and emmetropic young individuals. Eighteen young subjects participated in the first experiment, with an average refractive error of -2.3 D and a range from -7.5 D to 0 D. Cylinder was below 2 D in all cases. Excluding refractive errors, no subject had a history of visual problems and all of them reached 20/20 VA or better in both eyes. They were corrected during the measurements. The near stimulus, located at 2.8 D, and far target, at 0.36 D, were both black-on-white Maltese crosses with 1.3° width. Each subject underwent 3 cycles of 6 target switching (far-near-far-near-far-near). The data was analyzed with a threshold method consisting of calculating the initial and final states for each studied variable and considering the central 80% of the variation. Several far-to-near response parameters were calculated, including accommodation speed and amplitude, convergence speed and amplitude, pupil miosis speed, and amplitude, high-order aberration RMS, spherical aberration, lag of accommodation, and duration of accommodation, convergence, and pupil miosis. Correlation analysis between refractive error and accommodation speed and of these two variables with various far-to-near response parameters was performed. The correlation analysis of refraction (spherical equivalent, SE) with accommodation dynamics parameters suggests that myopia mildly affects or is affected by accommodation. The lag of accommodation was found to be linked to refractive error (R = -0.57, p = 0.01). Moreover, the correlation between miosis speed and refractive error also had a p-value below 0.05 (R = -0.49, p = 0.04). In other words, myopes may tend to have less precise accommodation and slower pupil constriction. The correlation coefficients between SE and the rest of accommodation-related parameters were small, with p-values well above 0.05. A substantial, low-p-value correlation was found between accommodation speed and convergence speed (R = 0.48, p = 0.04). To the best of our knowledge, this finding has not been previously reported. Furthermore, the correlation was stronger between accommodation speed and convergence duration (R = 0.57, p = 0.01), which may reflect the differences in the dynamics of these two processes. In addition, there may be a correlation between accommodation speed and miosis amplitude since the p-value was below 0.05 (R = 0.47, p = 0.049). These analyses showed that slower accommodation might be a function of slow convergence and more evident pupil miosis. For the second part of the thesis, a faster HS sensor with a refresh rate of 60 Hz and higher sensitivity to 1050 nm IR light was developed. This sensor was employed to characterize an optically tunable lens both in the typical static mode and, for the first time to our knowledge, in dynamic mode. After calibration, the tunable lens was used to apply defocus oscillations during contrast sensitivity measurements. Different amplitudes and frequencies were induced in 5 young emmetropes with 20/20 or better VA and no previous history of visual troubles. The visual stimulus was a 12 c/deg Gabor patch of 1º angular diameter located at 3 m. It was tilted 10 degrees left or right and a two-choice forced-choice protocol was used to determine the contrast threshold for each oscillation condition. The measurements were carried out in monocular mode, and the subjects viewed the stimulus through the tunable lens with their right eye. The sinusoidal waves induced included combinations of 3 temporal frequencies, 5, 15, and 25 Hz, and 8 peak-to-valley defocus values, ranging from 0.15 to 3 D, presented in fully random order. To the best of our knowledge, the effect of this kind of fast fluctuations of defocus on visual quality has not been previously studied. Visual performance, in the form of contrast threshold, was found resilient to induced defocus oscillations. The data showed that only for fast, large variations (25 Hz, ± 1.5 D), there was a noticeable reduction in contrast sensitivity. This indicates that for the eye to clearly perceive visual stimuli, the retinal image only needs to be in focus for a short time. A quantitative model was developed for predicting the deterioration in retinal image quality due to periodic defocus fluctuations. For the amplitudes and frequencies of oscillation used in the experiment, the average PSF was calculated for several integration times and the loss in the ensuing MTF was computed. Comparison between experimental results and simulated data suggests that the eye may be integrating defocus blur at 10 to 20 ms intervals.