Purpose Photoreceptor light acceptance is closely tied to the Stiles‐Crawford effect of the first kind (SCE‐I). Whether the SCE‐I plays a role in myopic development remains unclear although a reduction in directionality has been predicted for high myopia. The purpose of this study is to analyse the relationship between foveal SCE‐I directionality, axial eye length, and defocus for emmetropic subjects wearing ophthalmic trial lenses during psychophysical measurements and for myopic subjects with their natural correction. Method A novel uniaxial flicker system has been implemented making use of a Digital Micromirror Device (DMD) to flicker between a 2.3 visual degrees circular reference and a set of circular test patterns in a monocular Maxwellian view at 0.5 Hz. The brightness of the test is adjusted by the duty cycle of the projected light to an upper limit of 22 727 Hz. The wavelength and bandwidth are set by a tuneable liquid‐crystal filter centred at 550 nm. A total of four measurement series for 11 pupil entrance points have been realized for the right eye of 6 emmetropic and 10 myopic subjects whose pupils were dilated with tropicamide. Five of the emmetropic subjects wore ophthalmic trial lenses in the range of −3 to +9 dioptres to mimic hyperopic to highly myopic vision and resulting visibility plots have been fitted to a Gaussian SCE‐I function. In turn, the myopic subjects wore their natural correction during the analysis of the SCE‐I. All subjects had their axial eye length determined with an ultrasound device. Results A SCE‐I directionality parameter in the range of 0.03 to 0.06/mm2 was found for the emmetropic subjects with corrected vision in fair agreement to values in the literature. The results also revealed a marked reduction in directionality in the range from 16% to 30% with every 3 dioptre increase of simulated myopia, as well as a 10% increased directionality in simulated hyperopic eyes. For both emmetropic and myopic subjects, a decrease in directionality with increase in axial length was found in agreement with theoretical expectations. Conclusion The study confirms a clear link between SCE‐I directionality, uncorrected defocus, and axial eye length. This may play a role for emmetropization and thus myopic progression as cone photoreceptors capture light from a wider pupil area in elongated eyes due to a geometrical scaling.
The common Hartmann-Shack wavefront sensor makes use of a lenslet array to sample in-parallel optical wavefronts. Here, we introduce a Hartmann-Shack wavefront sensor that employs a digital micromirror device in combination with a single lens for serial sampling by scanning. Sensing is analyzed numerically and validated experimentally using a deformable mirror operated in closed-loop adaptive optics with a conventional Hartmann-Shack wavefront sensor, as well as with a set of ophthalmic trial lenses, to generate controllable amounts of monochromatic aberrations. The new sensor is free of crosstalk and can potentially operate at kilohertz speed. It offers a reconfigurable aperture that can exclude unwanted parts of the wavefront.
A gradual drop in visibility with obliquely incident light on retinal photoreceptors is namely described by the Stiles-Crawford effect of the first kind and characterized by a directionality parameter. Using a digital micromirror device in a uniaxial flicker system, here we report on variations of this effect with luminance levels, wavelengths within the visible and near-infrared spectrum and retinal regions ranging from the fovea to 7.5° parafoveal. Results show a consistent directionality in mesopic and photopic conditions. Higher directionality is measured for longer wavelengths, and a decrease with retinal eccentricity is observed. Results are discussed in relation to an absorption model for the visual pigments taking the outer-segment packing and thickness of the neural retina into account. Good correspondence is found without enforcing photoreceptor waveguiding.
The Hartmann–Shack wavefront sensor is widely used to measure aberrations in both astronomy and ophthalmology. Yet, the dynamic range of the sensor is limited by cross-talk between adjacent lenslets. In this study, we explore ocular aberration measurements with a recently-proposed variant of the sensor that makes use of a digital micromirror device for sequential aperture scanning of the pupil, thereby avoiding the use of a lenslet array. We report on results with the sensor using two different detectors, a lateral position sensor and a charge-coupled device (CCD) scientific camera, and explore the pros and cons of both. Wavefront measurements of a highly aberrated artificial eye and of five real eyes, including a highly myopic subject, are demonstrated, and the role of pupil sampling density, CCD pixel binning, and scanning speed are explored. We find that the lateral position sensor is mostly suited for high-power applications, whereas the CCD camera with pixel binning performs consistently well both with the artificial eye and for real-eye measurements, and can outperform a commonly-used wavefront sensor with highly aberrated wavefronts.
Purpose The retinal circulation regulates blood flow through various internal and external factors; however, it is unclear how locally these factors act within the retinal microcirculation. We measured the temporal and spatial variability of blood velocity in small retinal vessels using a dual-beam adaptive optics scanning laser ophthalmoscope. Methods In young healthy subjects ( n = 3), temporal blood velocity variability was measured in a local vascular region consisting of an arteriole, capillary, and venule repeatedly over 2 days. Data consisted of 10 imaging periods separated into two sessions: (1) five 6-minute image acquisition periods with 30-minute breaks, and (2) five 6-minute image acquisition periods with 10-minute breaks. In another group of young healthy subjects ( n = 5), spatial distribution of velocity variability was measured by imaging three capillary segments during three 2-minute conditions: (1) baseline imaging condition (no flicker), (2) full-field flicker, and (3) no flicker condition again. Results Blood velocities were measurable in all subjects with a reliability of about 2%. The coefficient of variation (CV) was used as an estimate of the physiological variability of each vessel. Over 2 days, the average CV in arterioles was 7% (±2%); in capillaries, it was 19% (±6%); and, in venules, it was 8% (±2%). During flicker stimulation, the average capillary CV was 16% during baseline, 15% during flicker stimulation, and 18% after flicker stimulation. Conclusions Capillaries in the human retina exhibit spatial and temporal variations in blood velocity. This inherent variation in blood velocity places limits on studying the vascular regulation of individual capillaries, and the study presented here serves as a foundation for future endeavors.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.