Precision and limits of human vision

Abstract

The human ability to perceive fine spatial details, known as visual resolution, requires combined capacities of both the eye and the brain. Research in anatomy, physiology, and psychophysics has extensively studied the biological structures responsible for visual resolution and identified numerous optical, retinal, and cortical factors that affect its limits. However, the direct observation of small retinal structures is obstructed by optical aberrations, and postreceptoral connectivity in humans in vivo remains unexamined. By integrating psychophysical experiments, advanced imaging techniques, and retinal physiology, this thesis investigated the factors that define the limits of human vision and the mechanisms that optimize visual performance. I focused on the interplay between optical properties of the human eye, the anatomical structure and transient signals due to fixational eye movements which result in neural processes that shape our ability to perceive fine spatial details. <br/> Inherent imperfections of the eye's optics such as defocus, astigmatism (known as LOA), and higher-order aberrations (HOA), affect the quality of the retinal image. Corrective measures, like spectacles and contact lenses address the LOA, but HOA remain more challenging to compensate for, thereby influencing the ultimate limits of optical image quality in natural vision. The first study evaluated the impact of habitual HOA on visual resolution and discrimination thresholds. The results showed that resolution acuity decreased significantly with stronger image degradation, while hyperacuity was not affected by HOA of the eye. <br/> A key element for a deepened understanding of the underlying mechanisms within the visual system is the knowledge of the individual cell structure within the foveola, the central 1 degree of the retina. The foveola is densely packed with cone photoreceptors, whose spacing and organization are crucial for resolving fine spatial details. By correcting the individual optical aberrations in real-time, adaptive optics scanning laser ophthalmoscopy (AOSLO) achieves cellular-level image resolution in the living human retina. By employing AOSLO retinal imaging and micro-stimulation with a fixation target, the second study revealed a systematic displacement of the retinal locus preferably used for fixation and the topographical center of cone distribution. Given the high precision with which the eye comes back to the same few hundreds of cones in a fixational task over a period of multiple years and the highly individual cell topography of the central retina, this demonstrated a close interplay during developmental processes of the foveolar photoreceptor arrangement and visual behavior. <br/> In the third study, the visual demand was changed from the fixation task to a resolution task to study the impact of photoreceptor packing density and fixation behavior on visual acuity. The spacing between foveolar cones has long been assumed to be the limiting factor for visual acuity when optical aberrations are bypassed. This study showed for the first time that resolution acuity is highly correlated with the cone density at the retinal location that samples the stimulus. By precisely recording the eye motion across the cone resolved foveola, the extend of ocular drift as well as it's direction could be shown to be finely tuned to the task within only a few hundreds of milliseconds. The fixation behavior approached stimulus sampling with the most densely packed cones. This combination of quickly adaptable fixational eye motion and precise AO corrected stimulus display allowed for resolution thresholds about 18 % below the theoretical static sampling limit. <br/> In summary, by integrating high-resolution imaging with psychophysical testing, the study results offer detailed insights into the role of the foveolar photoreceptor mosaic and the dynamics of fixational eye movements. These findings deepen the understanding of the human visual system's spatiotemporal processing and contribute to the broader fields of vision science and ophthalmology.