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Vohnsen, Brian
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Vohnsen, Brian
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Vohnsen, Brian
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- PublicationAnalysing the impact of myopia on the Stiles-Crawford effect of the first kind using a digital micromirror devicePurpose: 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 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 1 Hz. The brightness of the test is adjusted by the duty cycle of the projected light to an upper limit of 22727 Hz. The wavelength and bandwidth are set by a tuneable liquid-crystal filter centred at 550 nm. A total of 4 measurement series for 11 pupil entrance points have been realized for the right eye of 5 emmetropic and 8 myopic subjects whose pupils were dilated with tropicamide. 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 length determined with an ultrasound device. Results: A SCE-I directionality parameter for well-corrected vision 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 axial length was found in agreement with theoretical expectations. Conclusion: The study confirms a clear link between SCE-I directionality, uncorrected defocus, and axial 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.
582Scopus© Citations 25 - PublicationMonocular foveal, parafoveal, and perifoveal accommodation response to random defocus step changesAccommodation of the human eye relies on multiple factors, including – object size, monochromatic and chromatic aberrations, and vergence, and corrects defocus even in monocular conditions. Previous studies have been done to understand whether the retina can decode the sign of defocus as this may play a role for emmetropization and possibly also accommodation. Yet, findings have not been unambiguous and questions remain. Thus, in this study we tried to understand how accommodation makes use of defocus blur to detect the sign of defocus by performing experiments using a fast wavefront sensor in a vision testing system while eliminating other visual cues that may otherwise confound the analysis. A new automated method has been introduced to study monocular accommodation by using a currentdriven tunable lens (TL) to induce a random sequence of defocus step changes within the accommodative range of each observer. The response was captured in real time using a Hartmann-Shack wavefront sensor (HS-WFS) operating at 20 Hz while detecting aberrations and Zernike coefficients until 4th radial order across a 3 mm limited pupil. Foveal, parafoveal and perifoveal accommodation has been studied for young emmetropes and myopes to determine until which eccentricity accommodation is triggered. Our findings show that the accommodative range diminishes with eccentricity and at 14° (diameter) and beyond it becomes largely absent.
72 - PublicationDefocus-corrected analysis of the foveal Stiles-Crawford effect of the first kind across the visible spectrumThe Stiles-Crawford effect of the first kind describes a gradually diminished visibility of light that enters the eye towards the pupil rim. Although of retinal origin, it is commonly described by a Gaussian pupil apodization whose width is determined by a directionality parameter that depends on retinal eccentricity, wavelength and spatial coherence of the light. As the measurements are done psychophysically they are prone to subjective variations and difficult to obtain across the visible spectrum. In this work, requirements for accurate refractive correction when determining the directionality parameter at any given wavelength are discussed and we show that a current-controlled tunable liquid-polymer lens provides a convenient means to accomplish this without requiring mechanical readjustments. This may be the most convenient way to combat defocus across the visible spectrum in the analysis of the Stiles-Crawford effect as demonstrated through experiments and with a detailed Zemax eye-and-system analysis. The results obtained are discussed in relation to myopia and a reduced directionality for highly myopic eyes. © 2013 IOP Publishing Ltd.
71Scopus© Citations 8 - PublicationUnderstanding the role of retinal cone photoreceptors in color perception, blur, and emmetropizationThe photoreceptors are responsible for the conversion of optical images into neural signals that are conveyed to the visual cortex where vision is triggered. Rod photoreceptors provide night vision whereas cone photoreceptors provide daylight vision and color perception. The photoreceptors have commonly been represented as discrete but dense array of pixels despite of their elongated cellular structure. Earlier studies have suggested that they act as biological waveguides transmitting images from the inner to the outer segments. However, this understanding may not fully encompass their role in vision which is more related to that of optical antennas organized in such a way that optical image contrast and resolution is optimized. Here, we discuss the role of the photoreceptors analyzed as three-dimensional adaptable detectors of light (voxels) using electromagnetic principles. We show that this understanding is compatible with how light is perceived when being incident onto the retina at different angles in the effect commonly known as the StilesCrawford effect. We discuss how this can explain the reduced sensitivity to aberrations and chromatic blur of the threedimensional retina when compared to the common two-dimensional understanding of image formation in the eye. We show how the same principles may impact on emmetropization and ultimately how it may play a key role to prevent the onset or progression of myopia.
84 - PublicationGeometrical scaling of the developing eye and photoreceptors and a possible relation to emmetropization and myopiaIn this study the role of vergence in relation to age-dependent scaling of eye and photoreceptor parameters is studied. The underlying hypothesis is that the size and packing of outer segments is matched to the pupil size outdoors in photopic conditions. Vergence is analysed in relation to the angular spectrum of waves being incident using age-dependent data from the literature for the actual geometry and density of photoreceptor cones and rods. This approach is used to derive simple relations for the angular confinement of light along outer segments. Only with a small photopic pupil can leakage and crosstalk for both central and peripheral photoreceptors be entirely ruled out due to the finite length of the outer segments. A limiting 3 mm pupil size is found for children in the school age. Larger pupils will increase the likelihood of leakage and crosstalk that may therefore impact on emmetropization. This study has introduced a new paradigm in myopia research by considering vergence across the 3-D retina as being matched to the angular spectrum of waves being incident from the eye pupil. Emmetropization suggests a delicate balance between photoreceptor outer segment length and density in relation to pupil size. Only when balanced will leakage and crosstalk between adjacent outer segments be effectively suppressed thereby ensuring the highest possible light capture efficiency by visual pigments in the outer segments whether an image is formed on the retina or not.
83Scopus© Citations 7