Analysing the impact of myopia on the Stiles-Crawford effect of the first kind using a digital micromirror device

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 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.