Light vergence detection in monocular and monochromatic accommodation

Abstract

Ocular accommodation is the autofocus mechanism of the healthy young eye that allows objects at different distances to be seen clearly. Even in the absence of binocular, monocular, and chromatic cues, most eyes continue to be able to accommodate. How? The fundamental mechanism driving accommodation under these stringent conditions is still unclear. There are three main theories. The first theory is that the visual system is able to use the small differences in retinal images formed when the object is focused behind or in front of the retina. These differences are due to imperfections that exist in the optics of all eyes, such as monochromatic aberrations and irregularly shaped pupils. The second, and most widely accepted theory is that accommodation works as a trial-and-error system, with the goal to minimize retinal image blur, and thus maximize retinal contrast. The third theory is that the eye is able to extract the necessary information to accommodate in the right direction directly from light vergence. The aim of this thesis was to put the aforementioned theories to the test. Three experiments were devised that required the use of a custom-built adaptive optics system. Accommodation was recorded with a Shack-Hartmann wavefront sensor, while participants viewed a monochromatic Maltese cross monocularly through a Badal optical system and while their aberrations were being corrected with the adaptive-optics system. This thesis presents evidence against the first theory (in agreement with previous works) and in support of the second and third theory. Even though the human eye can accommodate using a trial-and-error function to minimize blur and maximize contrast, it is considerably more precise when light vergence is present. Thus, the results obtained here support the less accepted theory that the human eye is able to detect and use light vergence to accommodate.