Estudio de la óptica ocular del bovino de Lidiaun modelo animal de ojo con tolerancia al desenfoque

Abstract

The bovine is an animal model often use for academic purposes, in particular during ocular anatomy teaching classes of intermediate levels. Due to the size of the eye, the different physiological elements can be easily manipulated and identified. However, unlike other mammal or bird eye models, existing literature hardly provider accurate details on the bovine's ocular optics or data about the visual function. The ocular globe of bovine animals presents a property similar to that known as prebyopia in humans. Accommodation is almost absent and the refractive state is, in general, hyperopic. This implies an overall lack of (or reduced) capacity to see objects up close. However, it is known that these animals are able to response to stimuli located at both far and closer distances. In this sense, the present PhD Thesis is centered on the detailed optical analysis of the cornea and the crystalline lens of the bovine eye (as well as the combination of both physiological components in a unique optical system). The study of the geometry and optical properties of both ocular components will help to understand how the natural configuration of the eye of the bovine might (partially) compensate for the lack of accommodation and contribute to an improved vision of objects placed at different distances. Along this work, the eye of the fighting bull has been used as a model. This is a particular bovine breed (known as "toro bravo" in Spanish) bred free range on extensive farms (i.e. open-air facilities), with minimum human contact and a very strict age control, which visual function is one of the keys for an optimized performance during the fight ("lidia") at the arena face to face with the "torero" or bullfighter. In order to study the lens, an instrument based on a shadow photography technique has been developed. From the acquired images the geometry of the anterior and posterior surfaces was determined. A commercial topographer was used to explore the corneal first surface. A ray-tracing procedure was implemented to calculate the aberrations of those individual components and the combination of both as a whole optical system. Results show that the radii of curvature of the lens surfaces as well as the thickness increase with age as a results of a general change in size of the ocular globe. The horizontal meridian of the cornea is flatter than the vertical one. In addition, whereas lens asphericity is mainly positive (oblate), that of the cornea is strongly negative (prolate). The dominant aberrations are astigmatism and spherical aberration. Ocular refraction is larger along the horizontal meridian than the vertical meridian, being both hyperopic in most cases. Moreover, the presence of an elliptical-shaped pupil (instead of a circular one) leads to a noticeable higher contribution of spherical aberration within the whole eye. This is closely associated with an increase in depth of focus. As defocus is increased (i.e. objects are located closer to the eye), the retinal image for an elliptical pupil degrades at a lower ratio than that corresponding to an equivalent circular pupil. Thus, this increasing tolerance to defocus might be interpreted as a natural tool of the bovine eye to partially compensate a markedly presbyopia.