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Se presentan cinco estudios relacionados con los efectos que producen las aberraciones oculares monocromáticas en distintos aspectos del funcionamiento del sistema ocular. En el primero se hace uso de un sistema de óptica adaptativa para simular la óptica ocular durante la acomodación en un ojo con la acomodación paralizada y poder así estimar la profundidad de campo del ojo durante la acomodación. Los resultados muestran que dicha profundidad varía poco durante la acomodación, y el sistema visual hace uso de ella para acomodar lo menos posible para estímulos cercanos. En el segundo se realiza un estudio teórico para ver cómo afecta la apodización en el cambio de foco que maximiza la intensidad de un sistema óptico en presencia de aberración esférica. El estudio indica que, en ojos normales, el efecto Stiles-Crawford no debe cambiar apenas la refracción ocular. En el tercero se realizan cálculos teóricos y se hace uso de un programa de trazado de rayos para comprobar en dos modelos de ojo que existe una dependencia entre la acomodación medida desde el plano corneal y la longitud axial del ojo. Este efecto se presenta tanto si se consideran las aberraciones de alto orden como si no y se demuestra que es debido a la distancia no nula entre el plano corneal y el plano principal del ojo, así como al movimiento de los planos principales al acomodar. Se propone una fórmula para poder evitar estos efectos. En el cuarto se diseña, construye y testea un nuevo ojo artificial para poder realizar estudios de acomodación dinámica. Las dos novedades que presenta respecto a los modelos existentes son que la luz es originada en la retina y por tanto los rayos solo atraviesan una vez el ojo en su camino de salida, y que es capaz de emular la acomodación del ojo humano gracias al uso de una lente de focal variable. Los resultados muestran que el ojo construido puede emular bien el comportamiento del ojo humano al acomodar. Finalmente, en el último trabajo se desarrolla un nuevo sistema de óptica adaptativa que es aplicado al estudio del efecto de las aberraciones en la acomodación dinámica. El principal resultado obtenido tras un estudio en varios sujetos es que las aberraciones monocromáticas analizadas no parecen influir en la acomodación dinámica del ojo. Five studies related to the effects of monochromatic ocular aberrations on different aspects of ocular system functioning are presented. In the first, an adaptive optics system is used to simulate the ocular optic during the accommodation in an eye with the accommodation paralyzed and thus to be able to estimate the depth of field of the eye during the accommodation. The results show that such depth varies little during accommodation, and that the visual system makes use of it to accommodate as little as possible for nearby stimuli. In the second, a theoretical study is performed to see how apodization affects the focus change that maximizes the intensity of an optical system in the presence of spherical aberration. The study indicates that, in normal eyes, the Stiles-Crawford effect should hardly change the ocular refraction. In the third, theoretical calculations are made and a ray-tracing program is used to check in two eye models that there is a relationship between the accommodation measured from the corneal plane and the axial length of the eye. This effect occurs both when high-order aberrations are considered and when they are not, and is shown to be due to the non-zero distance between the corneal plane and the first principal plane of the eye, as well as the movement of the principal planes when accommodating. A formula is proposed to avoid these effects. In the fourth, a new artificial eye is designed, constructed and tested to perform dynamic accommodation studies. The two novel aspects that it presents regarding the existing models are that the light originates in the retina, and therefore the rays only cross the eye once in their exit path, and that it is able to emulate the accommodation of the human eye thanks to the use of a variable focal lens. The results show that the constructed eye can well emulate the behaviour of the human eye when accommodating. Finally, in the last work a new adaptive optics system is developed that is applied to the study of the effect of the aberrations in the dynamic accommodation. The main result obtained after a study in several subjects is that the monochromatic aberrations analysed do not seem to influence the dynamic accommodation of the eye.

PhD Thesis in Sciences - Specialty in Physics, Myopia affects approximately 25% of the World population, being a public health concern due to the socioeconomic impact and to the risk of vision loss related to other co-morbidities. If current trends continue, half the world’s population (almost 5 billion) will be short-sighted in just over three decades, with one-fifth of those expected to have a significantly increased risk of blindness. Clinical evidence from animal models and human clinical trials seems to indicate that the peripheral refraction pattern plays an important role in the regulation of eye growth. Lower progression rates have been reported over the last years in myopic children wearing orthokeratology (ortho-k) or special design contact lenses, when compared with those wearing traditional ophthalmic lenses. To date, the only justification for this effect seems to lie in the significant myopization effect induced by these alternative forms of correction beyond the foveal area, but despite the moderate results obtained researcher’s still lack knowledge of the exact mechanism behind this effect and why does it work better in some subjects than others. In this thesis a frame work was developed to model the possible impact of the eye’s posterior shape and the optical changes produced by ortho-k in myopia progression. Optical modeling and biometric eye length measures were used to calculate the retinal contour in 55 myopic subjects, with an accuracy of tenths of a micron. The results show that there is large inter-subject variability in the shape of the posterior pole, even among subjects with similar refractive errors. An exhaustive characterization of the ortho-k cornea was also conducted to analyze the main morphological, topographical and optical changes induced by these treatments and their possible implications in the peripheral refractive error and accommodative response. The results suggest that the reported effects in the retention of eye growth, supposedly due to the peripheral myopization produced by ortho-k treatments, might be dependent on pupil size. Optical quality analysis revealed that although the increase in positive spherical aberration is the main cause of the loss of retinal image quality in the unaccommodated eye after ortho-k, it also seems to have a positive effect, as it extends the depth of field of the eye and may contribute to a better image quality in subjects with accommodative lag during high contrast near vision tasks., A actual prevalência mundial da Miopia (25%) é já considerada um problema de saúde pública devido ao impacto sócio-económico e ao risco de perda de visão relacionada com outras co-morbidades. Se as tendências actuais se mantiverem, metade da população mundial (quase 5 mil milhões) será míope daqui a pouco mais de três décadas, e cerca de um quinto deverá ter um aumento significativo do risco de cegueira. Evidências clínicas baseadas em modelos animais e ensaios clínicos com pacientes humanos parecem indicar que o padrão da refracção periférica desempenha um papel importante na regulação do crescimento axial do olho. Níveis mais baixos de progressão têm sido reportados ao longo dos últimos anos em grupos de crianças míopes corrigidas com ortoqueratologia (orto-k) ou lentes de contacto com geometrias especiais, em comparação grupos de controle compensados com lentes oftálmicas tradicionais. Até à data, a única justificação plausível para estes resultados parece residir no efeito miopização periférica induzido por essas formas alternativas de correção para além da área foveal, mas apesar dos resultados moderados obtidos ainda falta conhecimento do exacto mecanismo por trás deste efeito e porque o efeito é maior em alguns indivíduos que em outros. Nesta tese foi desenvolvido um quadro de trabalho com o objectivo de modelizar o possível impacto da forma do polo posterior do olho e a das alterações estruturais induzidas pela ortoqueratologia na progressão da miopia. O contorno da retina de 55 indivíduos míopes foi calculado com recurso a modelização óptica e medidas biométricas do comprimento do olho, com uma precisão de décimos de micras. Os resultados demonstram que existe uma grande variabilidade inter-individual na forma do pólo posterior do olho, mesmo entre indivíduos com erros refractivos semelhantes. Foi também realizada uma caracterização exaustiva da córnea pós orto-k, com o objectivo de analisar as principais alterações morfológicas, topográficas e ópticas induzidas por estes tratamentos e as suas possíveis implicações no erro refractivo periférico, assim na resposta acomodativa. Os resultados sugerem que os relatos de uma menor taxa de progressão da miopia em olhos tratados com ortok, supostamente devido à miopização periférica produzida por estes tratamentos, pode ser dependente do tamanho da pupila. A análise da qualidade óptica revelou que, embora o aumento da aberração esférica positiva após orto-k seja a principal causa da diminuição da qualidade da imagem retiniana no olho desacomodado, também aparenta ter um efeito positivo na extensão da profundidade de campo, o que poderá contribui para um aumento da qualidade da imagem retiniana em indivíduos com atraso acomodativo durante tarefas de alto contraste em visão próxima.

Objetivos. La acomodación en el ojo humano va acompañada de convergencia ocular y miosis pupilar. Ambas pueden afectar la calidad óptica y la iluminación de las imágenes retinianas, cambiando la máxima capacidad de acomodación del ojo: amplitud acomodativa (AA). Se presentan cuatro estudios en los que se analizan los efectos de esos dos parámetros en la AA. Metodología. En el primero se evaluó objetivamente la acomodación de 15 ojos con un aberrómetro Shack-Hartmann (SH), en cuatro demandas de convergencia monocular (0º, 5º, 10º, 15º). Se analizaron los cambios de AA, astigmatismo, aberraciones de tercer, cuarto y sexto orden. El segundo experimento consistió en evaluar en 15 ojos con SH la media de AA, tamaños de pupila relajada y contraída, desenfoque y Aberración Esférica (AE) de cuarto orden. Se hizo en condiciones de luz alta y baja lo que generó dos tamaños pupilares para todas las demandas acomodativas (DA). El tercero surgió de la necesidad de utilizar en nuestra investigación el fármaco fenilefrina. Existe controversia sobre si ésta tiene efecto en la AA. Para dilucidarlo, calculamos en 8 ojos la curva estímulo/respuesta obtenida a diferentes vergencias con SH. Lo hicimos en pupilas naturales antes y a los 30 minutos de instilar fenilefrina al 10%. En el segundo experimento encontramos diferencias de 1 D en la AA paraxial entre alta y baja iluminación. En el último experimento buscábamos saber cómo afecta a la acomodación un cambio de imagen retiniana en el que la iluminación de la imagen retiniana al acomodar fuera independiente del tamaño pupilar. Lo hicimos en cuatro diferentes iluminaciones, lo que permitió comprobar el efecto de las mismas en la AA subjetiva. Participaron 20 personas divididas en dos grupos: jóvenes y présbitas. Se evaluó la AA del ojo izquierdo antes y tras 40 minutos de instilar 2 gotas de fenilefrina, con un optómetro Badal y un foróptero con pupila artificial de 5mm. Resultados y Conclusiones. La AA objetiva cambia levemente con distintas convergencias monoculares. Este aumento es significativo en jóvenes en la convergencia de 5º, y en présbitas en convergencia de 15º. Las aberraciones oculares de alto orden no presentan diferencias en sus cambios al acomodar en distintas convergencias monoculares. El astigmatismo cambia ligeramente al acomodar: aumenta la potencia y el eje gira contra la regla, especialmente en jóvenes y en convergencia monocular de 15º. La AA objetiva depende del tamaño pupilar. Si se utiliza para evaluarla una medida de refracción que incluya la AE, el cambio de tamaño pupilar variará la AE y la medida de AA. Si el tamaño pupilar aumenta al disminuir la luz, la AA objetiva puede reducirse más de 1 D, aunque la luminancia de la imagen foveal del estímulo acomodativo sea constante. La fenilefrina no parece afectar, o afecta poco, la RA. Su efecto se debe al cambio de la formación de imágenes en la retina por la midriasis pupilar y por tanto de las aberraciones de alto orden. Hemos encontrado un aumento significativo de AA subjetiva obtenido con un mismo tamaño pupilar cuando la iluminación retiniana aumenta, atribuible al cambio de contraste percibido en esas diferentes iluminaciones y que sería la señal que activaría el músculo ciliar. Este aumento es en jóvenes (2 D) y présbitas (0,6 D) y su origen es sensorial. Un control de las condiciones de luz ambiental debería ser un factor importante en la clínica optométrica., Purpose. Accommodation of the human eye is accompanied by binocular convergence and pupil myosis, both of which may affect optical quality and retinal luminance. This, in turn, changes the maximum capacity of accommodation of the eye or accommodative amplitude (AA). Four studies are presented where the effects are studied of convergence and myosis on AA. Methods. In the first experiment, objective accommodation was evaluated on 15 eyes with a Shack-Hartmann aberrometer on four demands of monocular convergence at 0º, 5º, 10º, 15º. Changes in AA, astigmatism, and third-, fourth-, and sixth-order aberrations were analyzed. The second experiment consisted on evaluating in 15 eyes the average AA, pupil size when relaxed and contracted, defocus and fourth-order spherical aberration (SA). These evaluations were made on high and low luminance conditions, which generated 2 pupil sizes for all accommodative demands. In the third experiment the effects of phenylephrine, used on the previous studies, was studied and there is controversy on whether it affects accommodation. To answer this question, the accommodative demand/response curves were obtained objectively before and 30 minutes after instillation of phenylephrine. The results of the second experiment, that there are differences of about 1 D in AA high and low luminance conditions, lead to the fourth and last experiment. In paraxial optics, high-order aberrations and depth-of-field do not affect calculated accommodation. The last experiment, the effect of retinal luminance on accommodation ensuring that pupil size remained constant over accommodative response and illumination conditions. Four illumination conditions were considered and subjective AA recorded. Twenty subjects, divided in two groups, young and presbyopic, participated in the experiment. Subjective AA was measured in the left eye before and 40 minutes after instillation of 2 drops of phenylephrine with a Badal optometer and a phoropter with an artificial pupil of 5 mm of diameter. Results and Conclusions. Monocular objective AA changes slightly with eye convergence. A statistical significance increase in young subjects was obtained at convergences of 5º and in presbyopic subjects at 15º. High-order ocular aberrations did not present differences following changes in accommodation at different convergences. Astigmatism did change slightly with accommodation: power increased and axis rotated against the rule, specially in young subjects and convergence of 15º. Objective AA depended on pupil size. If SA is included in the calculation of objective accommodation, and because it changes with pupil size, such calculated accommodation would vary with pupil size. If pupil size increases as light dims, objective AA can be reduced up to 1 D, even if retinal luminance remained constant. Phenylephrine did not seem to affect the eye’s ability to accommodate. Its effect on accommodative response is an indirect result of mydriasis, which increases high-order aberrations and retinal luminance. A statistically significant increase in subjective AA was obtained when pupil size was set to be constant and retinal luminance increased. This increase was attributed to an increase in retinal contrast as room illumination was increased. Increased contrast may be the signal that activates the ciliary muscle. The increase in subjective AA was observed in young subjects (by about 2 D) and in presbyopic subjects (by about 0.6 D) and its origin is sensorial. Control of room lighting conditions should be an important factor to be considered in optometric clinic.